Downloadable programs
  • #SolFuel19 - Solar Fuel Synthesis: From Bio-inspired Catalysis to Devices 
  • #SolCat19 - (Photo)electrocatalysis for sustainable carbon utilization: mechanisms, methods, and reactor development
  • #Sol2D19 - Two Dimensional Layered Semiconductors
  • #Exciup19 - Excitonic up-downconversion
  • #NCFun19 - Fundamental Processes in Semiconductor Nanocrystals
  • #CharDy19 - Charge Carrier Dynamics
  • #PERInt19 - Interplay of composition, structure and electronic properties in halide-perovskites
  • #PERFuDe19 - Halide perovskites: when theory meets experiment from fundamentals to devices
  • #OPV19 - Organic Photovoltaics: recent breakthroughs, advanced characterization and modelling
  • #MapNan19 - Mapping Nanoscale Functionality with Scanning Probe Microscopy
  • #RadDet19 - Radiation Detection Semiconductors Materials, Physics and Devices
Program
 
Mon Nov 04 2019
Plenary Session 1
Chair: Roel van de Krol
09:00 - 09:30
1-K1
Nocera, Daniel
Harvard University
Sustainable and Renewable Carbon and Nitrogen Cycles for Fuel and Crop Production
Daniel Nocera
Harvard University, US

Daniel G. Nocera is the Patterson Rockwood Professor of Energy at Harvard University. He is widely recognized in the world as a leading researcher in renewable energy. His group has pioneered studies of the basic mechanisms of energy conversion in biology and chemistry with a particular focus on multielectron transformations and the coupling of protons to electron transfer (i.e., proton-coupled electron transfer). A recent focus in the group has been to exploit this mechanistic knowledge for the generation of solar fuels.He has accomplished the solar process of photosynthesis – the splitting of water to hydrogen and oxygen using light from neutral water, at atmospheric pressure and room temperature at efficiencies of greater than 10%. This discovery, called artificial leaf, was named by Time magazine as Innovation of the Year for 2011. He has since elaborated this invention to accomplish a complete artificial photosynthetic cycle. To do so, he created the bionic leaf, which is a bio-engineered bacterium that uses the hydrogen from that artificial leaf and carbon dioxide from air to make biomass and liquid fuels. The bionic leaf, which was named by the World Economic Forum as the Breakthrough Technology for 2017, performs an artificial photosynthesis that is ten times more efficient than natural photosynthesis. Extending this approach, Nocera has achieved a renewable and distributed synthesis of ammonia (and fertilizer) at ambient conditions by coupling solar-based water splitting to a nitrogen fixing bioorganism, which is powered by the hydrogen produced from water splitting. These science discoveries set the stage for a storage mechanism for the large scale, distributed, deployment of solar energy and distributed food production and thus are particularly useful to the poor of the world, where large infrastructures for fuel and food production are not tenable.Other areas of interest in the group include the development of proton-coupled electron transfer and its application to radical enzymology, the development of new cancer therapies by creating nanocrystal chemosensors for metabolic tumor profiling, the creation of spin frustrated materials, which has culminated in the discovery of the quantum spin liquid, and the invention of molecular tagging velocimetry technique for the measurement of highly turbulent fluid flows.

Authors
Daniel Nocera a
Affiliations
a, Harvard University, 12 Oxford Street, Cambridge, 0, US
Abstract

Hybrid biological | inorganic (HBI) constructs have been created to use sunlight, air and water (as the only starting materials) to accomplish carbon and nitrogen fixation, thus providing a path to a sustainable nitrogen and carbon cycle for distributed and renewable fuels and crop production.

The carbon and nitrogen fixation cycles begin with the artificial leaf, which was invented to accomplish the solar process of natural photosynthesis – the splitting of water to hydrogen and oxygen using sunlight – under ambient conditions. To create the artificial leaf, an oxygen evolving complex of Photosystem II was mimicked, the most important property of which was the self-healing nature of the catalyst. Self-healing catalysts of the artificial leaf permit water splitting to be accomplished under benign conditions and thus the system may be easily interfaced with bioorganisms. To this end, using the tools of synthetic biology, a bio-engineered bacterium converts carbon dioxide from air, along with the hydrogen produced from the catalysts of the artificial leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. The HBI, called the Bionic Leaf, operates at unprecedented solar-to-biomass (10.7%) and solar-to-liquid fuels (6.2%) efficiencies, greatly exceeding the 1% efficiency of natural photosynthesis.

Extending this approach, we have discovered a renewable and distributed synthesis of ammonia (and fertilizer) at ambient conditions by coupling solar-based water splitting to a nitrogen fixing bioorganism in a single reactor. Nitrogen is fixed to ammonia by using the hydrogen produced from water splitting to power a nitrogenase installed in a bioorganism. Nitrogen reduction reaction proceeds at a turnover number of 1010 per cell and operates without the need for a carbon feedstock (other than the CO2 provided from air). This nitrogen fixing HBI can be powered by distributed renewable electricity, enabling sustainable crop production with a large and negative carbon budget.

The science that will be presented will show that using only sunlight, air and water, distributed and renewable systems may be designed to produce fuel (carbon neutral) and food (carbon negative) within sustainable cycles for the biogenic elements.

Plenary Session 2
Chair: Doron Naveh
09:00 - 09:30
2-K1
Lifshitz, Efrat
Technion - Israel Institute of Technology
The Effect of Magnetism on the Optical Properties of Bulk and Confined Perovskite Structures
Efrat Lifshitz
Technion - Israel Institute of Technology, IL
Authors
Efrat Lifshitz a, Maksym Kovalenko b, Andrew Rappe c
Affiliations
a, Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute and the Helen Diller Quantum Information Center; Technion, Haifa, Israel
b, ETH, Zurich, 8093
c, Philadelphia University, US
Abstract

The perovskite materials are at the forefront of scientific and technological interest more than a decade, due to their intriguing physical properties and extraordinary performance in a few different opto-electronic applications.  The lecture focuses on the influence of photo-induced magnetic fields on the optical properties of the nominally non-magnetic MAPbBr3 bulk and nanocrystal perovskites.  Although the electronic band structure of MAPbBr3 is based on the inorganic network, the MA counter induce an inversion of symmetry breaking and consequence development of a Rashba effect.1  The last is associated with the creation of an exotic spin-orbit magnetic field which split both valence and conduction band in k-space into two valleys with opposing spin helicities, with the anomalous exciton fine-structure with a bright triplet state below a dark singlet state,2 and with the relatively long spin lifetime  (~1nsec) and coherence time (~80 spec).3, 4

The lecture includes a description of magneto-photoluminescence experiments which monitored the degree of circular polarization (DECP) of bulk- or single nanocrystals, under the influence of an external magnetic field (B) and by mounting them onto a confocal microscope immersed in cryo-magnetic system.  The plot DCP versus B exposed non-Zeeman behavior with a few interesting features: The quenching of polarization at external field close to zero, followed by a rebuilt of polarization via a non-linear trend.  The quenching events are associated with creation of carrier-to-nuclear spin coupling creating an internal nuclear (Overhauser) magnetic field, with a significant influence on the carrier polarization and coherence.  The non-linear behavior at B>>0 is related to the influence of a Rashba field. Furthermore, the results exposed three type of recombination processes, associated with neutral exciton, trapped exciton and charged exciton, depending on the excitation history  (resonant/non-resonant excitation and fluence, direction of the excitation beam with respect to a unique crystallographic axis), each of which characterized by typical DCP(B) pattern.  Overall, the photo-induced Rashba and nuclear internal fields have a paramount influence on the optical properties of halide perovskites.

 

1.       Isarov, M.; Tan, L. Z.; Bodnarchuk, M. I.; Kovalenko, M. V.; Rappe, A. M.; Lifshitz, E., Rashba Effect in a Single Colloidal CsPbBr3 Perovskite Nanocrystal Detected by Magneto-Optical Measurements. Nano Letters 2017, 17 (8), 5020-5026.

2.       Becker, M. A.; Vaxenburg, R.; Nedelcu, G.; Sercel, P. C.; Shabaev, A.; Mehl, M. J.; Michopoulos, J. G.; Lambrakos, S. G.; Bernstein, N.; Lyons, J. L.; Stöferle, T.; Mahrt, R. F.; Kovalenko, M. V.; Norris, D. J.; Rainò, G.; Efros, A. L., Bright triplet excitons in caesium lead halide perovskites. Nature 2018, 553, 189.

3.       Akkerman, Q. A.; Rainò, G.; Kovalenko, M. V.; Manna, L., Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nature Materials 2018, 17 (5), 394-405.

4.       Belykh, V. V.; Yakovlev, D. R.; Glazov, M. M.; Grigoryev, P. S.; Hussain, M.; Rautert, J.; Dirin, D. N.; Kovalenko, M. V.; Bayer, M., Coherent spin dynamics of electrons and holes in CsPbBr3 perovskite crystals. Nature Communications 2019, 10 (1), 673.

CharDy 1.1
Chair: Marcus Scheele
09:30 - 10:00
1.1-I1
Talapin, Dmitri
University of Chicago
Electronic Coupling and Transport in Non-covalent Inorganic Nanoscale Assemblies
Dmitri Talapin
University of Chicago, US
Dmitri Talapin is a Professor of Chemistry at University of Chicago. His research interests revolve around inorganic nanomaterials, spanning from synthetic methodology to device fabrication, with the desire of turning colloidal nanostructures into competitive materials for electronics and optoelectronics. He received his doctorate degree from University of Hamburg, Germany in 2002 under supervision of Horst Weller. In 2003 he joined IBM Research Division at T. J. Watson Research Center as a postdoctoral fellow to work with Chris Murray on synthesis and self-assembly of semiconductor nanostructures. In 2005 he moved to Lawrence Berkeley National Laboratory as a staff scientist at the Molecular Foundry and finally joined faculty at the University of Chicago in 2007. His recent recognitions include MRS Outstanding Young Investigator Award (2011); Camille Dreyfus Teacher Scholar Award (2010); David and Lucile Packard Fellowship in Science and Engineering (2009); NSF CAREER Award (2009) and Alfred P. Sloan Research Fellowship (2009).
Authors
Dmitri Talapin a
Affiliations
a, Department of Chemistry, University of Chicago, Chicago, Illinois 60637, USA
Abstract

In recent years, progress in the synthesis, and, especially, surface chemistry of colloidal nanomaterials led to steadily improved electronic coupling, conductivity and charge mobility in nanocrystal films. This has important implications for performance of nanomaterials in various devices, where high mobility helps to collect charges in photodetectors and solar cells, and maximizes brightness of LEDs and lasers. We discuss a set of experimental studies for quantum dot (QD) and metal nanocrystal arrays which show unprecedentedly good electronic properties and provide a new platform for in-depth studies of charge transport through a network of discrete quantum-confined electronic states. To the best of our knowledge, we demonstrate the first instance of resolved charge filling of quantum states in solid state QD devices. This has been achieved by applying novel surface passivation strategies that efficiently eliminated surface states, thus allowing for reversible charging of QDs, switching between n-type and p-type transport, and air-stable tunable doping. The metal nanocrystal superlattices showed metallic transport, characteristic of the extended electronic states, or minibands. For QD films casted at room temperature without any post treatments, we observe “band-like” transport at temperatures above 70K. Furthermore, the measured FET mobilities up to ~8 cm2V-1s-1 are similar to the measured Hall mobility. This opens up a host of exciting opportunities for QD technologies and also raises important questions about the degree of electron delocalization and transport in QD solids. These questions are vital to the prospects of colloidal nanomaterials for competitive electronic and optoelectronic technologies.

10:00 - 10:30
1.1-I2
Lhuillier, Emmanuel
Sorbonne Universités
Designing Photovoltaic Devices Using HgTe Nanocrystals for SWIR and MWIR Detection
Emmanuel Lhuillier
Sorbonne Universités, FR

Emmanuel Lhuillier has been undergraduate student at ESPCI in Paris and then followed a master in condensed matter physics from university Pierre and Marie Curie. He was then PhD student under the mentorship of Emmanuel Rosencher at Onera in the optics department, where he work on transport in quantum well heterostructure. As post doc he moved to the group of Philippe Guyot-Sionnest in the university of Chicago, and start working on infrared nanocrystal. Then he moved back to ESPCI for a second post in the group of Benoit Dubertret working on optoelectronic properties of colloidal nanoplatelets. Since 2015 he is a CNRS researcher at Institute for nanoscience of Paris at Sorbinne université. His research activities are focused on optoelectronic properties of confined Nanomaterial with a special interest on infrared system. He receive in 2017 an ERC starting grant to investigate infrared colloidal materials.

Authors
Bertille martinez a, Clement Livache a, Charlie Greboval a, Audrey Chu a, nicolas goubet a, Emmanuel Lhuillier a
Affiliations
a, Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75005 Paris, France.
Abstract

HgTe nanocrystals offer a unique combination of broadly tunable optical absorption in the infrared from 1 to 100 µm and photoconductive properties.

However the maturity of the current device remains far weaker what have been achieve for solar cell using PbS nanocrystals. Current limitations are the result of limited material development and poor understanding of the electronic structure of the material under colloidal form.

In this talk, I will discuss some of the recent developments relative to HgTe nanocrystal integration into photodiodes operating in the short wave and mid wave infrared. I will present how the concepts of unipolar barrier has been transferred from III-V semiconductor to colloidal HgTe nanocrystal film [1].

Another important limitation to address is tradeoff between the absorption depth (> 1 µm) and the carrier diffusion length (< 100 nm) which make that most current devices have a weak light absorption (10 % of the incident light typically). This issue can be addressed by two strategies which are the development of ink to achieve thick high mobility film [2] and by enhancing the light matter coupling thanks to the introduction of plasmonic resonator.

Finally, I would say a few words about how it is possible to take advantage of intraband transitions in self doped nanocrystal to design mid wave infrared detector [3]

MapNan 1.1.
Chair: Lukas M. Eng
09:30 - 10:00
1.1.-I1
Kim, Yunseok
Sungkyunkwan University, Republic of Korea
Fast Local Probing of Polarization Charge
Yunseok Kim
Sungkyunkwan University, Republic of Korea, KR

Dr. Yunseok Kim is an associate professor in the School of Materials Science and Engineering, Sungkyunkwan University (SKKU), Korea. He received his M.S. and Ph.D. degrees in Materials Science and Engineering from Korea Advanced Institute of Science and Technology (KAIST), Korea, in, respectively, 2004 and 2007. From 2008 to 2010, he was awarded the Humboldt research fellowship from the Alexander von Humboldt foundation which allowed him to work as a postdoctoral researcher at Max Planck Institute of Microstructure Physics, Germany. Then, from 2011 to 2012, he was a postdoctoral researcher at Oak Ridge National Laboratory, USA. In 2012, he joined the School of Materials Science and Engineering, SKKU, Korea. His research interests include scanning probe microscopy studies of electromechanical, ferroelectric, transport, and ionic phenomena at the nanoscale.

Authors
Yunseok Kim a
Affiliations
a, Sungkyunkwan University, Republic of Korea, 2066 Seobu-ro, Jangan-gu, Suwon, KR
Abstract

Ferroelectric materials possess spontaneous polarization that can be switched by an electric field and can be used for multiple applications such as information technologies and energy harvesting devices. The existence of the ferroelectricity has been macroscopically examined by measuring polarization charge based on the detection of switching current. However, a local probing of polarization charge at the nanoscale is necessary for further applications of the ferroelectric materials because smaller and thinner materials and devices have been of significant interest. Although piezoresponse force microscopy (PFM) has been used extensively for this purpose, it was recently revealed that non-ferroelectric effects can additionally contribute to the PFM signal. In this presentation, I will summarize our recent effort on the local probing of the polarization charge based on the conductive atomic force microscopy (AFM) by combination of positive-up-negative-down method, so called AFM-PUND. In particular, fast local probing of the polarization charge will be discussed because the amount of switching current is increasing according to the increase of the frequency, indicating that smaller polarization charge can be measured by faster probing. These results could provide a new guideline for examining the existence of the ferroelectricity at nanoscale. Moreover, the present results could be further extended for probing other electrical properties such as capacitance and dielectric constant.

10:00 - 10:30
1.1.-O1
Leonhard, Tobias
Karlsruhe Institute of Technology KIT
Ferroelectricity in methylammonium lead iodide perovskite solar cells
Tobias Leonhard
Karlsruhe Institute of Technology KIT
Authors
Tobias Leonhard a, b, Holger Röhm a, b, Alexander Schulz a, b, Susanne Wagner c, Michael J. Hoffmann c, Alexander Colsmann a, b
Affiliations
a, Light Technology Institute, Karlsruhe Institute of Technology, Germany
b, Material Research Center for Energy Systems, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany
c, Institute for Applied Materials – Ceramic Materials and Technologies, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany
Abstract

Among the emerging photovoltaic technologies, perovskite solar cells stand out with remarkable power conversion efficiencies (PCEs) and low-cost solution processability, rivaling established technologies. Currently, the scientific community controversially discusses the importance of the ferroic properties for the exceptional performance of MAPbI3 light-harvesting layers.

In this work, we performed a comprehensive AFM study including Piezoresponse Force Microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM). On large flat crystals, we find 90 nm wide ferroelectric domains of alternating in-plane polarization. Electron Backscattered Diffraction (EBSD) mapping allowed for the spatially resolved correlation of the ferroelectric patterns and the crystal orientation within the MAPbI3 thin-films. Electrical simulations provide insight into the working principle of ferroelectric MAPbI3 solar cells. Poling experiments elucidate the impact of the ferroelectric microstructure on macroscopic device properties.

Altogether, these investigations provide micro-structural target properties for MAPbI3 thin-film deposition and outline pathways forward for more efficient, eco-friendly and lead-free perovskite solar cells.

PERInt 1.1
Chair: Antonio Abate
09:30 - 10:00
1.1-O1
Juarez-Perez, Emilio J.
ARAID Foundation
Release of Sym-triazine and HCN During the Thermal Degradation of FA Based Hybrid Perovskites at Low T Conditions
Emilio J. Juarez-Perez
ARAID Foundation, ES
Authors
Emilio J Juarez-Perez a, b
Affiliations
a, ARAID Foundation, Zaragoza, ES
b, Institute of Nanoscience of Aragon (INA) and Department of Chemical Engineering and Environmental Technology, University of Zaragoza., C/ Mariano Esquillor, s/n. Ed I+D. Campus Río Ebro, 50018, Zaragoza., ES
Abstract

Controlled thermal degradation experiments of formamidinium-based perovskite and their halide precursors were carried out in helium and vacuum atmosphere under pulsed illumination/dark conditions to simulate working temperature conditions of photovoltaic devices.

The identification of decomposition gas products based on the quadrupole mass spectrometry technique uncovered the release of sym-triazine, formamidine, and hydrogen cyanide (HCN). Meanwhile, sym-triazine was obtained as thermal product of degradation for temperatures above 95 ºC. Below this temperature, only formamidine and HCN generation routes were observed.

Experimental results supported by DFT calculations indicated that formamidinium is more resilient to thermal degradation and release of irreversible decomposition products compared to methylammonium cation because a larger enthalpy and activation energy for the decomposition reactions.

The HCN instantaneous concentration observed during low temperature heating tests and maximum release achievable per meter-square of FA based perovskite based solar cell estimations are compared to acute exposure guideline levels of airborne HCN concentration.

References

E. J. Juarez-Perez, L. K. Ono, Y. Qi, Thermal degradation of formamidinium based lead halide perovskites into sym-triazine and hydrogen cyanide observed by coupled thermogravimetry - mass spectrometry analysis, J. Mater. Chem. A 2019, DOI: 10.1039/C9TA06058H

10:00 - 10:30
1.1-O2
Jacobsson, Jesper
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
The Power of the Crowd. What Could be Learned by Collective Pooling of all the World’s Perovskite Device Data, and How do We Get There?
Jesper Jacobsson
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
T. Jesper Jacobsson a, b, Eva Unger a, c
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, Department of Chemistry, Ångström Laboratory, Uppsala University, Lägerhyddsvägen, 1, Uppsala, SE
c, Department of Chemistry & NanoLund, Lund University, Sweden
Abstract

Much research related to perovskite solar cells follows a familiar pattern. A group of postdocs and PhD students enter the lab where they with skill and determination meticulous handcraft a staggering number of solar cells, where after they carefully measure the device characteristics of the cells. This process triggers the writing of papers in which insights are summarised, and where a subset of the generated data is used for plotting figures and filling tables. Over the last ten years, this has been repeated about 10000 times. Sadly, much of the data presented in figures are non-trivial to extract, and much of the raw data newer leaves the hard drives of the students doing the work, and when they move on, the data is many times forever lost in the growing heap of unconsidered data. We firmly believe that much could be gained if more of this data could be stored in one place where it is easily accessible for the research community. So is also an increasing number of funding agencies. Such a collection of data could lead to new insights that are hard to get when the data is scattered over thousands of articles, and it could thus be a way to accelerate the pace of development. In this project, we are working on creating a platform for what could be described as a Wikipedia of perovskite device data. We will in this presentation describe what we have created, what can be conclude from the data so far accumulated, as well as the potential benefits for the perovskite community and beyond if this approach to data sharing reach full penetration in the field.

RadDet 1.1
Chair: Mahshid Ahmadi
09:30 - 10:00
1.1-I1
Choy, Wallace
University of Hong Kong
A new kind of Cuboid CH3NH3PbI3 Single Crystals for Highly Performed X-ray and Photon Detectors
Wallace Choy
University of Hong Kong, HK

is currently a professor of Department of Electrical and Electronic Engineering, the University of Hong Kong (HKU). Dr. Choy has published over 175 internationally peer-reviewed journal papers, contributed to one book and five book chapters, as well as a number of US and China patents. Among his publications, 12 papers have been featured as cover-story articles such as Adv. Mater., Adv Energy Mater., and Chem Comm., and 14 articles have been highlighted in research new/scholarly articles. Details of publication can be found in http://scholar.google.com.hk/citations?user=GEJf9dAAAAAJ. He was the recipient of the Sir Edward Youde Memorial Fellowship, the Croucher Foundation Fellowship, and the Outstanding Achievement Award from National Research Council of Canada and HKU Research Output Prize. He received overseas visiting fellowships from HKU to take a sabbatical leave at George Malliaras’s Group, Cornell University in 2008, a visit to Prof. Yang Yang, UCLA in summers of 2009 and 2011, Prof. Karl Leo, Institut fuer Angewandte Photophysik (IAPP), Technische Universitaet Dresden, Germany in the summer of 2010, and Prof./Sir Richard Friend, Cavendish Lab, Cambridge University, UK.

Wallace Choy is a fellow of OSA and senior member of IEEE. He has been recognized as Top 1% of most-cited scientists in Thomson Reuter’s Essential Science Indicators (ESI) three years in a row 2014, 2015 and 2016. He has been recognized as prolific researcher on organic solar cells in the index (WFC in physical sciences) in Nature Index 2014 Hong Kong published by Nature. He has been serving a technical consultant of HK-Ulvac (a member of stock-listed Ulvac Corp) since 2005. He has served as editorial board member for Nature Publishing Group of Scientific Reports and IOP Journal of Physics D, senior editor of IEEE Photonics Journal, topical editor of OSA Journal of the Optical Society of America B (JOSA founded in 1917), and guest editor of OSA Journal of Photonic Research, and Journal of Optical Quantum Electronics. He has delivered over 60 invited talks and served as a committee member in internationally industrial and academic conferences organized by various organizations such as IEEE, OSA and Plastic Electronics Foundation.

Authors
Fei Ye a, Wallace C.H. Choy a
Affiliations
a, Department of Electrical and Electronic Engineering, The University of Hong Kong
Abstract

Crystalline perovskite materials are promising materials for X-ray and photon detection due to their superior optoelectronic properties. Particularly, single-crystal perovskites have drawn a lot attention recently due to their substantially low crystal defects, which contribute to improving the figures of merit of the devices. However, cuboid CH3NH3PbI3 SGC with the naturally favorable geometry for device fabrication is rarely reported in X-ray and photon detection application. In this work, we propose the concept of seed dissolution-regrowth (SDR) to improve crystal quality of cuboid CH3NH3PbI3 SGC and provide fundamental understanding of the nucleation and growth thermodynamically [1]. Our results show that the approach improves crystal quality by improving the structural matching between the seed and the subsequently deposited crystal to lower the required Gibbs free energy for nucleation. c-MAPbI3 synthesized with SDR had a long carrier lifetime of 497 ns, and its X-ray detector yielded a sensitivity of 968.9 μC/Gy/cm2 under -1 V bias, higher than that of MAPbBr3 SGCs counterpart. This is the first report of c-MAPbI3 SGCs application in X-ray detection with the promising sensitivity. By comparing the X-ray and photon detection performances of c- and d-MAPbI3 SGCs, we found that c-MAPbI3 performs better than d-MAPbI3 due to its conducive preferred crystal orientation for charge carrier diffusion and collection originated from the high quality of c-MAPbI3 SGCs by SDR. Overall, our work paves the way to synthesize high-quality perovskite SGCs and demonstrate the advantages of MAPbI3 SGCs with preferred crystal orientation for device applications.

10:00 - 10:30
1.1-I2
Cao, Lei (Raymond)
The Ohio State University
Acquisition and Evaluation of Gamma-ray Energy Spectrum with CsPbBr3
Lei (Raymond) Cao
The Ohio State University

Dr. Lei R. Cao is Professor in the Nuclear Engineering Program at The Ohio State University (OSU) and the Director of OSU-Nuclear Reactor Lab. Dr. Cao received his BS in Experimental Nuclear Physics from Lanzhou University in 1994, MS degree in Nuclear and Particle Physics in 2002, and PhD degree in Nuclear and Radiation Engineering Program, the Department of Mechanical Engineering at University of Texas at Austin in 2007. Prior to joining OSU, Dr. Cao was a research associate at the Center for Neutron Research, U.S. National Institute of Standards and Technology (NIST) and also received a short-term training at the Positron Emission Tomography Laboratory at Harvard Medical School. At OSU, Dr. Cao founded the Nuclear Analysis and Radiation Sensor laboratory (NARS) in 2010.  

Dr. Cao's major research interests focus on applied nuclear physics and radiation science, including nuclear instrumentation and radiation detection, sensor development, radiation effects, and nuclear methods (PGAA, NDP, neutron radiography/tomography) for advanced materials characterization. Dr. Cao has published 110+ peer-reviewed journal articles and conference proceedings. Dr. Cao serves as Associate Editor for IEEE Transactions on Nuclear Science.

Authors
Lei R. Cao a, Lei Pan a, Yuanxiang Feng b, Praneeth Kandlakunta a, Jinsong Huang b
Affiliations
a, 1. Nuclear Engineering Program, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, Ohio, 43210, USA
b, Department of Applied Physical Sciences, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27514, USA
Abstract

The all-inorganic CsPbBr3 perovskite has advantages over the organic-inorganic perovskites for X- and gamma-rays detection due to the structural stability. It also has a series of desirable properties for ionizing radiation detection such as high attenuation coefficient, a wide band gap energy, and a large mobility-lifetime (mt) product. We have grown and fabricated CsPbBr3 single crystals into gamma detector and was able to acquire gamma energy spectrum from Cs-137 and Co-57 sources. The surface condition and metal contacts are proved to be critical factors in improving the performance of the detector for gamma ray detection. The energy resolution, in full-width-at-half-maximum, of the gamma spectra is 5.5% at 662 keV and 13.1% at 122 keV, respectively. The 59.5 keV gamma-rays from Np-237 nuclear decay in a Am-241 source is also clearly distinguishable with resolution of 28.3% when the detector is exposed to an Am-241 source. Electron-hole averaged mobility-lifetime mt product is evaluated to be 7.91´10-4 cm2 V-1 by Hecht equation fitting. It has found that the hole mobility is higher than electron mobility, which subsequently affects the resolution of the full energy peak. We demonstrated the effects of a better hole transport properties compared to that of electron at spectroscopy level by acquiring gamma-rays’ spectra, in other words, the overserved lower energy tailing of a full energy peak is attributed to low electron mobility. The small pixel and digital pulse processing (DPP) are suggested as possible solutions to improve the energy resolution of gamma spectrum acquired by CsPbBr3. A DPP algorithm is also developed to process the preamplifier signal with potentially long rise time (in the order of tens of micro seconds) in perovskite detectors, which ensures the elimination of ballistic deficit in the charge collection as well as in pulse shaping for distortion-free energy histogram reconstruction.

 

 

Sol2D 1.1
Chair: Doron Naveh
09:30 - 10:00
1.1-I1
Delerue, Christophe
IEMN - UMR 8520
Theory of optical absorption in semiconductor nanocrystals: From single nanocrystals to superlattices
Christophe Delerue
IEMN - UMR 8520, FR
Authors
Christophe Delerue a, Maryam Alimoradi Jazi b, Tim Prins b, Niall Killilea c, Wiel Evers d, Peter Geiregat e, Zeger Hens e, Wolfgang Heiss c, Arjan Houtepen d, Daniel Vanmaekelbergh b
Affiliations
a, IEMN, UMR-CNRS 8520, Villeneuve d'Ascq, France
b, Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands
c, Friedrich Alexander University Erlangen-Nuremberg, Erlangen, DE
d, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
e, Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium
Abstract

Recent progress in colloidal chemistry has enabled the fabrication of epitaxially-connected superlattices of PbX (X=Se, S, Te) nanocrystals with square or honeycomb geometry [1-2]. CdX nanocrystal superlattices can be derived using cation exchange process. In parallel, it is also possible to form hexagonal lattices in which the nanocrystals are still separated by their ligand shell. In this presentation, I discuss different aspects on the optical absorption process in these materials. It is shown that the absorptance of superlattices is considerably enhanced due to the epitaxial connections between neighbor nanocrystals. The resulting value is close to the quantum absorptance found in epitaxial two-dimensional films (quantum wells). I discuss the physics governing the variations of the aborptance from the case of isolated nanocrystals to closely-packed situations, i.e., hexagonal lattices of nanocrystals [3], epitaxial superlattices, up to the quantum well limit.In the case of superlattices in which the nanocrystals are epitaxially connected, I discuss the evolution of the local-field factor with the necking strength between neighbor nanocrystals.

 

 

 

10:00 - 10:30
1.1-I2
Fiori, Gianluca
University of Pisa
On the Perspectives of Graphene and Related Materials for Nanoelectronics Applications
Gianluca Fiori
University of Pisa
Authors
Gianluca Fiori a
Affiliations
a, Dipartimento di Ingegneria dell’Informazione, Universita’ di Pisa, Via Caruso 16, 56122, Pisa, Italy
Abstract

Abstract:

 

Despite the huge expectations and high hopes coming out of two-dimensional materials (2DMs), graphene and related materials still have to find their place in every-day life, since a product leveraging the extraordinary intrinsic electrical, mechanical and optical properties of such new material is unfortunately lacking.

In this talk, I will try to give some perspectives regarding the applications where two-dimensional materials could represent an enabling technology for new applications in the electronic field, while assessing their ultimate performance through numerical simulations.

I will also address the topic of printable electronics, since 2DMs have recently demonstrated their potential to obtain deposited materials through inkjet technique, with insulating, semiconducting and metallic properties, that are the main ingredients to obtain printed electronic devices. The ability to stack them one on top of the other forming heterostructures, is an adding additional degree of freedom, that could pave the way towards working devices for medium-scale level of integration.

 

Acknowledgment

ERC PEP2D project (Contract Number 770047), WASP project (Contract Number 825213)

 

SolFuel 1.1
Chair: Erwin Reisner
09:30 - 10:00
1.1-I1
Robert, Marc
Laboratoire d'Electrochimie Moléculaire, Université Paris Diderot
Running the Clock: Catalytic Reduction of CO2 with 2, 6 and 8 Electrons Using Co and Fe Molecular Complexes
Marc Robert
Laboratoire d'Electrochimie Moléculaire, Université Paris Diderot, FR
Authors
Marc Robert a
Affiliations
a, Laboratoire d'Electrochimie Moléculaire, Université Paris Diderot, 15, rue Jean-Antoine de Baïf, Paris, 75013, FR
Abstract

Reduction of carbon dioxide has as main objective the production of useful organic compounds and fuels - renewable fuels - in which solar energy would be stored. Molecular catalysts can be employed to reach this goal. One route consists in first converting sunlight energy into electricity that could be further used to reduce CO2 electrochemically. Another approach is to directly use the visible photons and photo-stimulate the electrochemical reduction of the gas in the presence of an appropriate sensitizer and a sacrificial electron donor. Molecular catalysts may provide excellent selectivity but usually with less durability and more complex processability than solid materials. Hybrid systems in which a robust molecular catalyst is associated to a porous carbon material as conductive support may combine the advantages of both homogeneous and heterogeneous catalysis.

Using Fe and Co complexes (porphyrins, phthalocyanines and quaterpyridines), our recent results for CO, HCOOH, CH3OH and CH4 production will be discussed, illustrating the synergy between electrochemical and photochemical approaches and the rich potential of molecular catalysts to generate fuels from CO2 used as a renewable feedstock.

10:00 - 10:15
1.1-O1
Karadas, Ferdi
Bilkent University, Turkey
An Iron Chromophore-sensitized Photoanode for Water Oxidation
Ferdi Karadas
Bilkent University, Turkey, TR
Authors
Ferdi Karadas a
Affiliations
a, Bilkent University, Turkey, Ankara, TR
Abstract

The concept of dye-sensitized photoelectrosynthesis cells (DS-PECs) has recently been proposed as an alternative water splitting cell. The DS-PEC architecture involves an oxide semiconductor sensitized by an organic/inorganic chromophore and a water oxidation catalyst (WOC) connected either to the chromophore (a dyad assembly) or directly to the semiconductor. Recently, several dye-sensitized photoanodes, all of which consist of Ru chromophores, have been developed [1,2]. The rarity of ruthenium, however, forces the scientific community to introduce earth abundat chromophores. Being in the same group, iron bipyridyl complexes appear to be the obvious candidates for this purpose. Due to primogenic effect, the MLCT state in iron bipyridyl complexes is much less favored compared to that in ruthenium bipyridyl complexes [3]. Therefore, new iron complexes should be investigated to overcome this significant challenge.

We recently found that chromophore-Prussian blue assemblies can be constructed utilizing Fe(CN)5- group as a bridging as well as a relay group [4]. Herein, we take this idea to a new level by demonstrating an iron chromophore based photoanode. The preparation of the donor-acceptor iron chromophore and the dye-sensitized photoanode will be presented. Moreover, photoelectrochemical, transition absorption and quantum mechanical calculations will be presented to explain the charge transfer dynamics of the photoelectrode.

10:15 - 10:30
1.1-O2
Alfano, Antonio
Center for Nano Science and Tecnology, Istituto Italiano di Tecnologia
Towards Stable and High Efficiency Hybrid Organic Photoelectrochemical Cell Based Artificial Leaf: the Role of Materials and Interfaces
Antonio Alfano
Center for Nano Science and Tecnology, Istituto Italiano di Tecnologia, IT
Authors
Antonio Alfano a, b, Alessandro Mezzetti a, Francesco Fumagalli c, Chen Tao a, Annamaria Petrozza a, Fabio Di Fonzo a
Affiliations
a, CNST, Istituto Italiano di Tecnologia, Milano
b, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci, 32, Milano, IT
c, European Commission, Joint Research Centre (JRC), Via Enrico Fermi, 2749, Ispra (Va), IT
Abstract

Molecular hydrogen produced via solar energy is emerging as a prominent way to convert and store the conspicuous, yet intermittent, amount of energy that the Sun daily irradiates on Earth. A plethora of different approaches  is building up, creating an heterogenous picture of viable technologies which can tackle the need of promoting sustainable Solar Fuel production.
Among many, Hybrid Organic Photoelectrochemical (HOPEC) water splitting is gaining momentum in this field, and various approaches are currently being developed to realize hybrid tandem systems to perform unbiased water splitting. By taking advantage of the organic semiconductors properties such as low cost, stability, tuneable electronic properties and ease of large area production, these materials can help overcoming the limitations of standard inorganic photoelectrochemical water splitting. The potential of hybrid organic systems has been proven by our previous works[1]. Indeed, excellent photocurrent performances[2] or extended operational lifetime[3] have been obtained through careful optimization of hybrid photocathodes (PC) architecture.
Through materials research and device engineering, we are working toward the realization of a robust and high performing architecture. To meet this goal, in depth characterization and optimization of each layer has been performed. To this extent, through Intensity Modulated Photocurrent/Photovoltage spectroscopy (IMPS/IMVS) and Electrochemical Impedance Spectroscopy (EIS) we collected valuable insights on the charge transport mechanisms both from the Bulk Heterojunction to the charge selective contacts and from the catalyst layer to the electrolyte.

Various materials have been tested to maximize the performances. Charge selective contacts potential candidates have been selected from the classes of metal oxides, Transition Metal Dichalcogenides (TMD) and Small Molecules. Furthermore, to maximize the photovoltage and photocurrent of the device it is of paramount importance to tune the properties of hybrid organic PC acting on their photoactive layer, taking advantage of the latest advancements in the field of organic photovoltaic (OPV). Promising materials from OPV are, among many, the high-performance photo-absorbers PCE11 and PCDTBT, and the non-fullerene acceptors IDTBR and IDFBR, which were found to be responsible of a sharp increase in the open circuit voltage in OPV devices[4]. Their improved electronic properties and optimized band gap are here exploited to realize hybrid PC specifically designed to be coupled in a tandem configuration with a high performing perovskite[5] , realizing a full water-splitting system with cheap, easily processable and suitable for large area production materials. By modifying the hybrid PC, it was possible to extend the absorption range of the stack. Taking advantage of the high Voc of the perovskite and the additional photovoltage coming from the PC, the Perovskite-HOPEC system efficiently performs the full water splitting reaction without the application of any external bias. The results clearly indicate that the Solar To Hydrogen (STH) efficiency of the system increases sensitively when proper design of the tandem system is achieved, with STH above 2% for the best performing case. These results prove that a tandem hybrid organic perovskite-photocathode stack can be used to realize efficient photoelectrochemical systems for solar fuels production, relying on highly tuneable materials and architectures.

10:30 - 11:00
Coffee Break
CharDy 1.2
Chair: Iwan Moreels
11:00 - 11:30
1.2-I1
Schreiber, Frank
University of Tuebingen, Germany
Coupled Organic-Inorganic Nanostructures (COINs): From Complex Structure Formation to Advanced Functional Properties
Frank Schreiber
University of Tuebingen, Germany, DE
Authors
Frank Schreiber a
Affiliations
a, University of Tuebingen, Germany, Auf der Morgenstelle, 18, Tübingen, DE
Abstract

We discuss the emerging field of coupled conjugates of quantum dots and organic semiconductors, referred to as ‘‘coupled organic–inorganic nanostructures’’ (COIN). We indicate some aspects of their optical properties and highlight suitable descriptions of their electrical transport behavior. In particular, we discuss the key role of the electronic structure at the interface of COINs and the impact of structural / morphological features on the optoelectronic properties [1].

For a detailed structural characterization, we demonstrate the use of nanobeams for diffraction from microscopically small parts of a sample [2]. Exploiting this simultaneously for small angle as well as wide angle X-ray scattering (SAXS and WAXS) on a microscopic grain of a COIN, we show how for a single grain X-ray cross correlation analysis (XCCA) can be employed. We obtain a complete map of correlations between the atomic lattice order and the mesoscopic superlattice order, with the opportunity to quantify even small levels of angular disorder. Finally, we comment on the implications for device applications.

Financial support by the DFG and contributions by numerous students and collaborators are gratefully acknowledged, including in particular M. Scheele and I. Vartanyants and their groups.

 

References

[1]  M. Scheele et al., Phys. Chem. Chem. Phys. 17 (2015) 97

[2]  I. A. Zaluzhnyy et al., ACS Nano Lett. 17 (2017) 3511

11:30 - 12:00
1.2-O1
Marchioro, Arianna
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland
Surface Characterization of Colloidal Nanoparticles by Second Harmonic Scattering: Surface Potential and Interfacial Water Order
Arianna Marchioro
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, CH
Authors
Arianna Marchioro a, Marie Bischoff a, Sylvie Roke a
Affiliations
a, Laboratory for fundamental BioPhotonics, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
Abstract

The microscopic description of the interface of colloidal nanoparticles (NPs) in solution is essential to understand and predict the stability of such systems, as well as their chemical and photochemical reactivity. However, the details of such an interface are still poorly understood and often rely on the use of simple electrostatic mean field models. Besides, surfaces are challenging to probe selectively as the bulk contribution often dominates, and in many cases the surface properties of a colloidal system cannot be inferred from studies of the corresponding planar surface.

Here we show that the electrical double layer of NPs in aqueous solution can be probed with polarimetric angle-resolved second harmonic scattering (AR-SHS). This nonlinear optical technique selectively probes the interfacial region and offers an all-optical alternative to surface‑sensitive techniques that usually require more sophisticated resources, as for example X-ray photoelectron spectroscopy.[1] Furthermore, AR-SHS gives access to quantities such as surface potential and water molecular orientation at the interface, two parameters not easily obtained experimentally, without the use of any labeling molecule nor a priori models for the structure of the interface. [2]

In this work, AR-SHS is applied to model SiO2 NPs as a proof of concept, and to semiconductor NPs such as TiO2. We are able to monitor surface changes as a function of pH and salt concentration, and show results under illumination for TiO2. An extension of AR-SHS to the time domain is discussed. This work provides a description of the structure and energetics of the NPs interface in different aqueous environments and highlights some of the electronic specificities of the semiconductor/aqueous interface.

MapNan 1.2
Chair: Laura Fumagalli
11:00 - 11:30
1.2-I1
Collins, Liam
Oak Ridge National Laboratory
Multiscale Functional mapping in Hybrid Organic Inorganic Perovskites: Linking Device Hysteresis with Local Functionality
Liam Collins
Oak Ridge National Laboratory
Authors
Liam Collins a
Affiliations
a, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, EE. UU., Oak Ridge, US
Abstract

A defining property of hybrid organic inorganic perovskites (HOIP) is the ability to conduct both ionic and electronic charge carriers. This interplay between conduction channels offers a rich physical and chemical landscape, with potential opportunities for many existing and emerging technologies from optoelectronic to ionic memory and energy storage devices.  Like all aspects of nanoscience and nanotechnology, electrochemical functionalities often emerge on the scale of individual micron and nanometer scale defects and defect assemblies, functional interfaces and artificial elements. The characteristic length scale and relevant energy scales of these phenomena would seem to benefit from a scanning probe microscopy (SPM) approach.

In the first part of my talk I will discuss investigations of ferroelectricity in HOIP using interferometric detection sensing-(IDS) piezoresponse force microscopy (PFM). IDS-PFM enables intrinsically calibrated measurements of a materials ferro/piezo-response, as well as the ability to decouple (unwanted) cantilever motion from tip-sample motion of interest. Using IDS-PFM, combined with multimodal chemical imaging, we demonstrate that twin domains in MAPbI3 are associated with chemical segregation.[1] We see no detectable (<0.4 pm/V) evidence of piezo or ferroelectric response in these films and describe how the observed twin domain structures are due to changes in viscoelastic properties. I will describe how traditional PFM can be misinterpreted in the presence of cantilever crosstalk. I will further demonstrate how the commonly used switching spectroscopy PFM technique is ill equipped to study ferroelectric behavior in the presence of ionic motion and cantilever coupling.[2] I hope this aspect of my talk acts as a cautionary tale for scientists who intend to use PFM as a tool to study ferroic properties in mixed ionic electronic conductors.

In the second part of my talk I will discuss opportunities to uncouple multiscale charge dynamics by time resolved Kelvin probe force microscopy (KPFM). With this aim in mind we develop and implement a multiscale multimodal mapping approach (M3) combining multitemporal (102-10-6 S) macroscopic and nanoscale time resolved KPFM measurements on a single device. I will use this approach to successfully decouple hysteretic device behavior in a series of ternary blended HOIP (i.e. (FA)x(MA)1-x(Cs)0.5 PbI3 where x=0,15,75%) devices. Backed by MD simulations, our results demonstrate that insertion of the larger FA cation leads to a more dynamic lattice and a greater propensity for dipole reorientation and ion migration. Overall, we demonstrate that ion migration plays only a small role in the underlying hysteresis, and that the inclusion of the larger FA cation into the organic lattice results in the activation of an ionic subsystem which is not observed in the parent MAPbI3 composition. In this aspect of my talk I hope to demonstrate that the M3 approach enables the ability to (near-) simultaneously link device characteristics with local charge dynamics, which may begin to alleviate controversies regarding the exact nature of different phenomena including hysteresis, ion migration, ferroelectricity, charge carrier trapping, redox processes, etc.

[1] Liu, Yongtao, et al. "Chemical nature of ferroelastic twin domains in CH 3 NH 3 PbI 3 perovskite." Nature materials 17.11 (2018): 1013.

[2] Collins, Liam, et al. "Quantitative Electromechanical Atomic Force Microscopy." arXiv preprint arXiv:1904.06776 (2019). (just accepted ACS Nano)

 

11:30 - 12:00
1.2-O1
Giridharagopal, Rajiv
Department of Chemistry, University of Washington
Advances in Multimodal Scanning Probe Microscopy at the Nanoscale
Rajiv Giridharagopal
Department of Chemistry, University of Washington
Authors
Rajiv Giridharagopal a, David Ginger a
Affiliations
a, University of Washington, Department of Chemistry, Seattle, WA 98195-1700
Abstract

The recent combination of functional scanning probe microscopy with data science techniques has started to yield interesting new insights into a wide range of materials. Large multimodal datasets comprising many different measurements on the same area can reveal structure-function relationships beyond that of any single technique. As an example, combining photoluminescence mapping with electrical current measurements in conductive atomic force microscopy shows how the perovskite-electrode contact affects functional solar cells by correlating optical properties with electrical function. Such multidimensional approaches are particularly attractive given the recent emergence of techniques like photoinduced force microscopy (PiFM) that can measure vibrational spectroscopy with nanometer spatial resolution, thereby combining chemical and electrical information. In this talk I will discuss our recent work in functional imaging of materials using data-driven scanning probe methods, from sub-microsecond time-resolved electrostatic force microscopy (trEFM) to multimodal analysis using hyperspectral PiFM. These methods allow for investigation into spatially-dependent properties of nanostructured photovoltaics, such as the nature of grain boundaries in perovskite active layers. While I will focus primarily on hybrid organic-inorganic perovskites, I will also discuss how these techniques reveal new insight into mixed ionic-electronic conductors and nanostructured polymers.

12:00 - 12:30
1.2-I2
Hermes, Ilka
Max Planck Institute for Polymer Research, Mainz, Germany
Anisotropic Charge Carrier Diffusion Correlated to Ferroelastic Twin Domains in MAPbI3 Perovskite
Ilka Hermes
Max Planck Institute for Polymer Research, Mainz, Germany, DE
Authors
Ilka M. Hermes a, Andreas Best a, Julian Mars a, b, Sarah M. Vorpahl c, Markus Mezger a, b, Hans-Jürgen Butt a, David S. Ginger c, Kaloian Koynov a, Stefan A. L. Weber a, b
Affiliations
a, Max Planck Institute for Polymer Research, Mainz, Germany, Ackermannweg, 10, Mainz, DE
b, Institute of Physics, Johannes Gutenberg University Mainz
c, University of Washington, Department of Chemistry, Seattle, WA 98195-1700
Abstract

Since the introduction of the perovskite compound methylammonium lead iodide (MAPbI3) as absorber material for photovoltaic (PV) devices in 2009,[1] studies continue to reveal fascinating material properties, including switchable ferroelastic twin domains. [2, 3, 4] However, the relation between a structural phenomenon like crystal twins and the electronic transport properties in the PV absorber remains unclear. Here, we present the results of a correlative piezoresponse force microscopy and photoluminescence study aiming to resolve whether and how twin domains influence the photocarrier diffusion in micrometer-sized MAPbI3 crystallites. We observed a distinct anisotropy in the photocarrier diffusion times that correlates to the arrangement of the twin domains, with a faster diffusion parallel to the domains and a slower diffusion perpendicular to the domains. This anisotropy could originate from domain walls acting as energy sinks or barriers for electronic charges, which leads to favorable diffusion times parallel to the domain structure. In combination with the switchability of the domains under mechanical stress, this anisotropic charge carrier diffusion promises the improvement of charge transport in MAPbI3 for PV and other optoelectronic applications via controlled crystal growth and strain engineering.

PERInt 1.2
Chair: Antonio Abate
11:00 - 11:15
1.2-O1
Muscarella, Loreta Angela
Institute AMOLF
Crystal Orientation and Grain Size: Do They Matter for Optoelectronic Properties of MAPbI3 Perovskite?
Loreta Angela Muscarella
Institute AMOLF
Authors
Loreta A. Muscarella a, Eline M. Hutter a, Sandy Sanchez b, Christian D. Dieleman a, Tom J. Savenije c, Anders Hagfeldt b, Michael Saliba d, Bruno Ehrler a
Affiliations
a, Center for Nanophotonics, AMOLF, Science Park 104, The Netherlands
b, Laboratory of Photomolecular Science (LSPM), Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
c, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
d, Adolphe Merkle Institute, Chemin des Verdiers 4, Fribourg, Switzerland
Abstract

It is thought that growing large, oriented grains of perovskite can lead to more efficient devices. We compare two model systems, randomly oriented, small grain MAPbI3 films fabricated via antisolvent dripping (AS) and MAPbI3 films fabricated via Flash Infrared Annealing (FIRA) consisting of highly oriented, large grains. We measure the grain size, crystal structure and grain orientation using Electron Back-Scattered Diffraction (EBSD) and compare these to the optoelectronic properties as characterized by local photoluminescence and time-resolved microwave conductivity measurements. For the perovskites grown with FIRA, we find a spherulitic growth yielding large (tens of µm), highly oriented grains along the (112) and (200) planes in contrast to randomly oriented, smaller (400 nm) grains in the AS films. We observe a local enhancement and shift of the photoluminescence emission at different regions of the FIRA clusters, but these can be fully explained with variations in thickness, light-outcoupling, and self-absorption. We observe no effect of crystal orientation. Additionally, despite a substantial difference in grain size, we find that grain size does not play a major role in charge carrier mobilities and lifetime for the FIRA and AS films. These findings show that the orientation and size of crystalline domains in perovskite films are not necessarily related to their optoelectronic quality.

11:15 - 11:30
1.2-O2
Flatken, Marion
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Structural Properties of Perovskite Layers in High-Performance Solar Cells
Marion Flatken
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Marion Flatken a, Nga Phung a, Antonio Abate a, c, Armin Hoell b, Robert Wendt b
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), BESSYII, Institut für Nanospektroskopie, Albert-Einstein-Straße 15, 12489 Berlin, Germany
c, Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Piazzale Tecchio 80, 80125 Fuorigrotta, Naples
Abstract

In the past ten years, Halide Perovskites gained enormous attention due to the rapidly increasing performance of perovskite solar cells. Starting with firstly reported power conversion efficiencies of 3.8% in 2009 researchers now present groundbreaking results of 24.2%[1]. Driven by their fascinating properties, such as long charge carrier diffusion length and large absorption coefficients, halide perovskites became one of the most interesting materials for photovoltaic applications[2], [3]. Based on the general ABX3 structure of perovskites, where the A-side cation is commonly occupied by methylammonium (MA), formamidinium (FA) or cesium (Cs), the B side cation by lead (Pb) or tin (Sn) and the X side anion by halide anions such as iodine or bromine, many different compositions exist showing varying degrees of the above described features. Still, it is not determined why the observed differences are present. One approach to give an explanation might lay in a deeper knowledge of perovskites structural properties. 

Structural modifications induced by adding small amounts of specific elements lead to an adjustment of the electronic structure and properties of perovskites. A deeper knowledge of these doped perovskite systems compared to undoped perovskites may lead to a better understanding of crystal formation and further to control the properties of different perovskite compositions. In our work, we focus on the characterization of methylammonium lead iodide perovskite (MAPbI3) compared to a strontium (Sr) doped perovskite system (MAPbI3).  Using X-Ray Fluorescence Spectroscopy, we are able to identify even small amounts (0.2 at%) of strontium in the perovskite thin films. Moreover, in this work, we propose the use of resonant elastic X-ray spectroscopy such as anomolous small angle X-ray scattering (ASAXS) as a comprehensive methodology to obtain a more detailed characterization of halide perovskites using synchrotron radiation at BESSYII. ASAXS provides valuable information about the crystal growth and grain size distribution in the perovskites thin films. In addition to measuring the thin films, we also performed SAXS experiments on the precursor solutions corresponding to the thin films. First results confirm a cluste  r formation starting already in the liquid precursor phase. Interestingly, different features are observed for each investigated perovskite system, which in the end also leads to differences in the perovskite film and stability of the perovskites. A combination of the results from spectroscopy with full device perovskite solar cell performance may lead to a controlled adjustment of the processing of stable perovskites.

11:30 - 12:00
1.2-I1
Olthof, Selina
Universität zu Köln
Determination of the Electronic Structure of Lead and Tin based Perovskites
Selina Olthof
Universität zu Köln, DE
Dr. Selina Olthof studied Physics at the University Stuttgart (Germany) and wrote her master thesis in the group of Klaus Kern at the Max Planck Institute for Solid State Physics. In 2010, Dr. Olthof received her Ph.D. from the University of Dresden (Karl Leo), followed by a two year postdoctoral research stay at Princeton University with Antoine Kahn. Currently, she is head of a Junior Research Group at the University of Cologne in the Department of Chemistry. Her research is centered around enhancing the understanding of the electronic structure of novel semiconducting materials, with a focus on organic semiconductors and hybrid perovskites.
Authors
Selina Olthof a
Affiliations
a, University of Cologne, Institute for Physical Chemistry, Luxemburgerstrasse 116, Köln, 50939, DE
Abstract

In recent years, the interest in halide perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaic while other fields, like light emitting diodes, show great potential as well. One intriguing property of this material class is the wide tunability of the band-gap that can be induced by changing the perovskite composition. While changes in band gap are regularly reported, it is unclear how the respective conduction and valence band positions change and what the underlying origins of these changes are. Knowing the band positions is however crucial for device design, i.e. ensuring efficient charge transport across the various interfaces.

In this talk, I will discuss recent findings regarding the variations in ionization energy and electron affinity, covering the complete library of lead and tin based halide perovskite systems. [1] Using a combination of photoelectron spectroscopy, density functional theory, and a tight binding model we are able to reliably extract the relevant energy level positions. Furthermore, we are able to explain the origin of these changes based on changes in hybridization strength, atomic level positions, and lattice distortion.

RadDet 1.2
Chair: Eric Lukosi
11:00 - 11:30
1.2-I1
Matt, Gebhard
I-Meet Lehrstuhl für Werkstoffe der Elektronik- und Energietechnik, FAU University
High Performance X-ray to Current Converters Fabricated Via Sintering or Melting of a Metal-halide Perovskite
Gebhard Matt
I-Meet Lehrstuhl für Werkstoffe der Elektronik- und Energietechnik, FAU University, DE

-2005, Ph.D LIOS (Linz institute for organic solarcells), J. Kepler University,Linz, Austria. Head: Prof. N.S. Sariciftci 2006-10, Post-doc, Institute for Semiconductor and Solid-State Physics, J. Kepler University, Austria, Head: Prof. G. Bauer 2011- Senior researcher, I-Meet, Erlangen, Germany. Head. Prof. C.J. Brabec.

Authors
Gebhard J. Matt a, Ievgen Levchuk b, Judith Knüttel a, Shreetu Shrestha a, Johannes Dallmann c, Rainer Hock c, Wolfgang Heiss a, Christoph J. Brabec a
Affiliations
a, I-Meet Lehrstuhl für Werkstoffe der Elektronik- und Energietechnik, FAU University, Martensstraße 7, Erlangen, 91058, DE
b, Siemens Healthineers GmbH, CT Division, Siemensstrasse 1, 91301 Forchheim, Germany.
c, Institute for Crystallography and Structural Physics, FAU University, Staudtstrasse 3, 91058 Erlangen, Germany
Abstract

The search for an ideal X-ray sensitive photo-conductor is an ongoing task since the most semiconductors do not absorb high energy radiation effectively. However, the latter is an intrinsic property of the lead-halide perovskites semiconductors due to high radiation attenuation by the lead and halide ions combined with there good charge transport properties.

In this presentation we will review our latest efforts utilizing organic-inorganic (MAPbI3) as well as a pure inorganic metal-halide perovkite (CsPbBr3) for X-ray to current converters. We demonstrate a sintering as well as a melting process which leads to the formation of several hundred µm thick and crystalline samples. Most notable for these samples are the good charge transport properties and unusual high mobility- lifetime products (~1E-3 cm^2/V).

The achieved X-ray to current conversation rate up to 2500 µC/Gycm2 is on par to the current state-of-the-art Cd(Zn)Te detector technology and our findings inform on a low-cost scaleable technology for the next generation of high energy detector technology.

11:30 - 12:00
1.2-O1
Gros-Daillon, Eric
CEA-LETI
Investigation on Chromium and PEDOT-PSS Electrodes on CH3NH3PbBr3 Single Crystals: Impact on Dark Current and X-ray Photocurrent
Eric Gros-Daillon
CEA-LETI, FR
Authors
Eric Gros-Daillon a, Jean-Marie Verilhac b, Oriane Baussens a, Smail Amari b, Julien Zaccaro c, Alain Ibanez c, Pierre Rohr d
Affiliations
a, University Grenoble Alpes, CEA-LETI
b, University Grenoble Alpes, CEA-LITEN
c, Institut Néel, CNRS and Université Grenoble-Alpes /Grenoble
d, Trixell
Abstract

Thanks to their high X-ray absorption coefficient and good charge carriers transport properties, hybrid (organic-inorganic) halogenoplumbate perovskites exhibit interesting properties for direct X-ray detection. Among the different perovskite compositions, CH3NH3PbBr3 could be a promising candidate for solution-process, low-cost and large-area flat-panel X-ray detector.

In this work, monocrystalline CH3NH3PbBr3 perovskite crystals were prepared by using the inverse temperature crystallisation (ITC) method using dimethylformamide (DMF) as solvent. Symmetric contacts were deposited by evaporation of chromium or brushing of PEDOT-PSS in opposite side of the singles crystals, and the dark current and X-ray induced photocurrent were compared.

Dark current-voltage characteristic (J-V curves) were carried out at room temperature, by sweeping the voltage at low scan rates of 200 mV.s-1. Resistivity measurements indicate values around 108 Ω.cm. Symmetrical contacts would entail symmetric J-V curves. However, devices with PEDOT-PSS electrodes show non-symmetric J-V curves with large hysteresis and high temporal variations. On the other hand, devices with chromium contacts show symmetric J-V curves without hysteresis and with reduced temporal variations.

The samples were irradiated by X-ray using medical radiography settings (70 kV, 23.5 mm of aluminum filtering, dose rate: 100 µGyair/s, 100 ms per shot with 4 Hz repetition rate). The X-ray to electron conversion rate was measured with respect to the applied voltage. Devices with PEDOT-PSS electrodes present a photocurrent with large rise time and fall time, along with photoconduction gain which depends on the device voltage history. By comparison, devices with chromium contacts have lower conversion rate without gain, associated with shorter rise time and fall time.

The non-stable behaviour of devices with PEDOT-PSS contacts will results in undesired temporal artefacts such as ghosting in dynamic X-ray imaging such as angiography. The devices with chromium contacts will be preferable for this application.

Sol2D 1.2
Chair: Christian Klinke
11:00 - 11:30
1.2-I1
MARIE, Xavier
University of Toulouse - INSA - CNRS
Control of the Exciton Radiative Lifetime in van der Waals Heterostructures
Xavier MARIE
University of Toulouse - INSA - CNRS
Authors
Xavier MARIE a
Affiliations
a, Université de Toulouse, INSA-CNRS-UPS, Toulouse, France
Abstract

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strength [1,2,3].

Encapsulation of monolayers such as MoSe2in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogeneous exciton linewidth [4,5]. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect [6].

The time-resolved photoluminescence measurements together with cw reflectivity and photoluminescence experiments show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizeable enhancement of the exciton radiative decay rate.

Understanding the role of these electrodynamical effects allow us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

[1] G. Wang, A. Chernikov, M. M. Glazov, T. F. Heinz, X. Marie, T. Amand, B. Urbaszek, Rev. Mod. Phys. 90, 021001 (2018)

[2] D. Lagarde, L. Bouet, X. Marie, C. R. Zhu, B. L. Liu, T. Amand, P. H. Tan, B. Urbaszek, Phys. Rev. Lett. 112, 047401 (2014)

[3] C. Robert, D. Lagarde, F. Cadiz, G. Wang, B. Lassagne, T. Amand, A. Balocchi, P. Renucci, S. Tongay, B. Urbaszek, X. Marie, Phys. Rev. B 93, 205423 (2016)

[4] F. Cadiz, E. Courtade, C. Robert, G. Wang, Y. Shen, H. Cai, T. Taniguchi, K. Watanabe, H. Carrere, D. Lagarde, M. Manca, T. Amand, P. Renucci, S. Tongay, X. Marie, B. Urbaszek, Phys. Rev. X7, 021026 (2017)

[5] G. Wang, C. Robert, M. M. Glazov, F. Cadiz, E. Courtade, T. Amand, D. Lagarde,T. Taniguchi, K. Watanabe, B. Urbaszek, and X. Marie, Phys. Rev. Lett. 119, 047401 (2017)

[6] H.H. Fang, B. Han, C. Robert, M.A. Semina, D. Lagarde, E. Courtade, T. Taniguchi, K. Watanabe, T. Amand, B. Urbaszek, M.M. Glazov, and X. Marie, ArXiv 1902.00670 (2019)

11:30 - 12:00
1.2-I2
Bar Sadan, Maya
Ben-Gurion University of the Negev, Israel
Transition Metals Dichalcodenides: Growth mechanism, Structure and Catalytic Activity
Maya Bar Sadan
Ben-Gurion University of the Negev, Israel, IL
Authors
Maya Bar Sadan a
Affiliations
a, Department of Chemistry, Ben Gurion University, Beer sheva, Israel
Abstract

The ability to dope 2D layered materials by substituting with various transition metals make transition metals dichalcogenides (TMDs) an interesting system for tailor-made applications. Here, the formation of ternary compounds of TMDs by wet chemistry will be described. Specifically, the doping of TMDs with other transition metals and the impact it has on their catalytic properties for hydrogen production will be presented. The growth mechanism of nanoflowers nanostructures of TMDs was revealed using electron tomography. This growth mechanism allows for facile doping of the materials by adding the dopants either at the beginning or at the end of the reaction, thus forming a homogenous material or a graded one. We have used this approach to dope MoS2 and WSe2 with Ru, Co, Fe, Ni and significantly improved their catalytic activity towards hydrogen production.

In addition, we have surveyed the structural features of various hybrids in order to correlate them with the catalytic activity. This work aims at correlating the atomic-scale structures with the catalytic activity, and for that goal to be achieved, there is a need to understand the dopant sites and the atomic scale arrangement within the MoS2 lattice. The use of high-resolution electron microscopy with other characterization methods allows first the understanding of the structural features of the materials and thereafter it will serve to understand the origin of catalytic activity.

FIG. 1. Functional materials based on TMDs are produced by using various synthetic routes

12:00 - 12:30
1.2-I3
Platero, Gloria
Materials Science Factory, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid E-28049, Spain.
Simulation of Chiral Topological Phases in Driven Low Dimensional Systems
Gloria Platero
Materials Science Factory, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid E-28049, Spain.
Authors
Gloria Platero a
Affiliations
a, Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, ES
Abstract

Recently, there is a big effort to implement long arrays of semiconductor quantum dots. The high tunnability of these systems allows to control the different parameters of the system, as the tunneling rates between dots. Therefore, they
become good quantum simulators of real molecules, as dimerized molecular chains.
A one-dimensional tight-binding model with alternating tunnel matrix elements, the SSH model, represents a simple description of a dimerized polymer. It is characterized by a topological invariant, the Zak phase which has been measured in cold atom systems[1]. For finite chains in the topologically nontrivial phase, a pair of exponentially decaying edge states emerges. In this talk I will discuss the dynamics of interacting electrons in a 1D chain of dimers and the role of edge states in the dynamics[2]. We extend our result for two strongly interacting electrons propagating in two dimensional lattices under the action of a periodic electric field both with and without a magnetic flux[3]. Finally we extend the analysis to include long range hopping in a dimer chain[4,5]. We show that a quantum simulator for 1D chiral topological phases can be obtained by periodically driving an array of quantum dots with long-range hopping. We propose a driving protocol which enables us to imprint bond-order in the lattice, while also offers tunability of the long-range hoppings[5]. This control can be used to tune the hopping amplitudes to configurations that would be unreachable otherwise, while preserving the fundamental symmetries which guarantee topological features. Thus, the driving protocol triggers topological behaviour in a trivial setup, opening the door to the simulation of different chiral topological phases. Furthermore, we also study the exact time-evolution for the case of two interacting electrons and show that the dynamics of different edge states modes can become highly correlated. This allows to discriminate between different topological phases and also opens up new possibilities for quantum state transfer protocols.


[1] M. Atala, et al. , Nat. Phys. 9, 795–800 (2013).
[2] M. Bello et al., Sci. Rep. 6, 22562, (2016).
[3] M. Bello et al., Phys Rev B , 95, 094303 (2017)
[4] B. Pérez-González et al., Phys. Rev. B, 99, 035146 (2019)
[5] B. Pérez-González et al., submitted, arXiv:1903.07678v1

SolFuel 1.2
Chair: Erwin Reisner
11:00 - 11:30
1.2-I1
Ishitani, Osamu
Tokyo Institute of Technology
Photocatalytic and Electrocatalytic Reduction of Low Concentration of CO2
Osamu Ishitani
Tokyo Institute of Technology, JP
Authors
Osamu Ishitani a
Affiliations
a, Tokyo Institute of Technology, 2-12-1 Oookayama, Meguro-ku, Tokyo, JP
Abstract

Exhaust gases from such as thermal power plants and iron manufactures contained several % - 20% of CO2. Although several methods such as adsorption and desorption methods using amines and membrane filtration have been already developed for enrichment of CO2, these procedures require high energy consumption. If low concentration of CO2 can be directly reduced by artificial methods, it should give a new direction of researches for artificial photosynthesis.

In this presentation, I report such systems using CO2-capturing properties of metal complexes. It has been reported that fac-[ReI(diimine)(CO)3X]n+ can work as both photocatalyst and electrocatalysts for CO2 reduction. We found that the Re complexes also have CO2-capturing properties with the assistance of triethanolamine (TEOA) as shown in the eq.1. Although this reaction is in equilibrium, the equilibrium constant is very large (K = 1.7 x 103 M-1). This means that the Re complex can efficiently capture CO2 from gases containing low concentration of CO2 such as 1% CO2. This type of CO2 adducts work as a catalyst for photochemical reduction of CO2. A Ru(II)-Re(I) supramolecular photocatalyst consisting of a [Ru(diimine)3]+ type photosensitizer unit and the Re complex as a catalyst unit efficiently photocatalyzed reduction of low concentration of CO2 (even 1% CO2).

11:30 - 11:45
1.2-O1
Sahm, Constantin
University of Cambridge - UK
Surface Modification of ZnSe Nanocrystals with a Ni(cyclam) Catalyst Enables Visible Light-driven Photochemical CO2 Reduction in Water
Constantin Sahm
University of Cambridge - UK, GB
Authors
Constantin D. Sahm a, Moritz F. Kuehnel a, Gaia Neri b, Jonathan R. Lee b, Katherine Orchard a, Alex J. Cowan b, Erwin Reisner a
Affiliations
a, Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of Chemistry, University of Cambridge, Lensfield Road, GB
b, University of Liverpool, UK, University of Liverpool, Liverpool, GB
Abstract

Artificial photosynthesis represents a hopeful strategy to overcome the global dependence on fossil fuels because it allows for the storage of solar energy and simultaneous mitigation of CO2 emissions. Here, we present a noble metal and Cd-free photocatalyst system for CO2 reduction in water. Ligand-free ZnSe quantum dots (QDs) are used as a visible-light photosensitiser and combined with a phosphonic acid-functionalised Ni(cyclam) catalyst, NiCycP. This hybrid assembly exhibits high activity towards CO2 to CO reduction (Ni-based TON CO > 120), whereas a freely diffusing (anchor-free) catalyst, Ni(cyclam), evolves significantly less CO. Surface modification of ZnSe QDs with 2-mercapto-ethyl dimethylammonium chloride (MEDA) partly suppresses H2 production and increases CO evolution leading to a TONNi CO of > 280 and 33% CO-selectivity after 20 h of visible light irradiation (λ > 400 nm, AM 1.5G, 1 sun). The external quantum efficiency was determined to be 3.4 ± 0.3 % under 400 nm monochromatic light irradiation. Ultrafast transient absorption spectroscopy rationalises the high photocatalytic activity of this catalyst. Band-gap excitation of ZnSe QDs is followed by rapid hole scavenging through the sacrificial electron donor. Excited electrons are subsequently trapped below the ZnSe conduction band which enables efficient charger transfer to NiCycP on the ps time scale. In conclusion, we present ZnSe QDs as promising material for the generation of solar fuels.1

11:45 - 12:00
1.2-O2
Garcia de Arquer, F. Pelayo
University of Toronto
Catalyst Management for Multiampere Gas-Phase Electrolysis
F. Pelayo Garcia de Arquer
University of Toronto, CA
Authors
F. Pelayo Garcia de Arquer a, Cao-Thang Dinh a, David Sinton a, Edward Sargent a
Affiliations
a, University of Toronto, King's College Road, 10, Toronto, CA
Abstract

Electrochemical reduction of CO2 is attractive to store renewable electricity in the form of carbon‐based fuels enabling a carbon-neutral development. Efficient electrochemical reduction of CO2 requires catalysts that combine high productivity, high selectivity, and low overpotential. Programming catalysts to achieve these metrics is challenging, as they can undergo extensive surface reconstruction during operation modifying their physicochemical properties. I will present different approaches to program this reconstruction, in-situ monitoring catalyst transformation, leading to CO2 electroreduction to carbon monoxide or formate at high efficiency. The productivity of gas-phase electrolysis is today curtailed by the limited gas diffusion through the electrolyte to catalyst’ active sites, shrinking the volume over which reactants and electrons overlap at the catalyst. I will present a materials strategy that decouples gas, ion and electron transport breaking this tradeoff. The catalysts achieve CO2 electroreduction towards multicarbon products at selectivities > 80% and partial current densities beyond 1.3 A/cm2 at ~50% cathodic energy efficiency - a sixfold increase over the best previously reported CO2 reduction catalysts.

12:00 - 12:30
Abstract not programmed
12:00 - 13:30
Lunch
12:30 - 14:00
Lunch
CharDy 1.3
Chair: Frank Schreiber
13:30 - 14:00
1.3-I1
Disch, Sabrina
Universität zu Köln
Spatially Resolved Magnetization and Spin Disorder in Magnetic Nanoparticles
Sabrina Disch
Universität zu Köln, DE
Authors
Sabrina Disch a
Affiliations
a, Department für Chemie, Universität zu Köln
Abstract

Magnetic nanoparticles reveal unique magnetic properties which make them relevant for data storage, electronic and mechanical engineering, and biomedical applications[1,2]. With regard to these applications, one of the main aspects of fundamental interest is the magnetic anisotropy and the related magnetization distribution in individual nanoparticles. However, it is challenging to isolate surface-related effects from the effective magnetic anisotropy using macroscopic, integral magnetization methods. A size dependence of surface effects is well established, and non-saturation magnetization behavior in magnetic nanoparticles even at high fields is commonly attributed to surface spin canting or formation of a magnetically dead layer.  

Polarized small-angle neutron scattering (SANS) is a versatile technique, which allows investigating the nanoparticle magnetization with spatial resolution[3]. In this presentation, I will give an overview of our studies of ferrite nanoparticles aiming at their intraparticle spatial magnetization distribution as investigated using polarized SANS[4,5]. I will introduce the capabilities of small angle scattering to determine chemical and magnetic nanoparticle morphologies and highlight the different contributions of particle core and surface spin disorder to the macroscopically observed magnetization.

14:00 - 14:30
1.3-I2
Moreels, Iwan
Department of Chemistry, Ghent University
Silver Doping in Cadmium Chalcogenide Colloidal Nanoplatelets
Iwan Moreels
Department of Chemistry, Ghent University, BE

I obtained my PhD degree in applied physics at Ghent University in 2009, studying near-infrared lead salt quantum dots. This was followed by a postdoc on quantum dot emission dynamics at Ghent University in collaboration with the IBM Zurich research lab. In 2012 I joined the Istituto Italiano di Tecnologia, where I led the Nanocrystal Photonics Lab in the Nanochemistry Department. In 2017 I returned to Ghent University as associate professor, focusing mostly on 2D and strained nanocrystals. The research in our group ranges from the synthesis of novel fluorescent nanocrystals to optical spectroscopy and photonic applications.

Authors
Ali Khan a, Valerio Pinchetti b, Ivo Tanghe c, Zhiya Dang d, Beatriz Martín-García d, Zeger Hens a, Dries Van Thourhout c, Pieter Geiregat a, Sergio Brovelli b, Iwan Moreels a
Affiliations
a, Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium
b, Dipartimento di Scienza dei Materiali, Universitá degli Studi di Milano-Bicocca, Via Cozzi 55, 20125 Milano, Italy
c, Photonics Research Group, INTEC Department, Ghent University-IMEC, Technologiepark-Zwijnaarde 15, 9052 Zwijnaarde, Belgium
d, Istituto Italiano di Tecnologia (IIT), Genova, Italy, Via Morego, 30, Genova, IT
Abstract

Colloidal semiconductor nanoplatelets have a band-edge emission that by now can be tuned from the UV to the near infrared.[1-3] To further steer their optical properties, we can introduce heterovalent dopants. This leads to a broadened emission combined with an enlarged Stokes shift, so that reabsorption of the emitted light can be minimized. In this presentation, I will discuss how we synthesized CdS:Ag, CdSe:Ag, and CdSe:Ag/CdS nanoplatelets with emission peaks ranging from 570 nm to 880 nm, and fluorescence quantum efficiencies exceeding 50%.[4] Optical characterization of the samples with time-resolved fluorescence and transient absorption spectroscopy, as well as spectro-electrochemical measurements, yielded a detailed picture of dopant distribution and carrier dynamics in CdSe:Ag nanoplatelets. In particular, at low Ag concentration the dopant mainly passivates surface traps, and only at higher concentration all CdSe nanoplatelets in the ensemble show deep defect emission. By independent tuning of the electron and hole energy levels via quantum confinement and doping, respectively, we obtained a strongly red shifted fluorescence with large Stokes shift that, combined with an enhanced absorption cross section in doped 2D nanoplatelets, should benefit applications such as near-infrared light-emitting diodes or luminescent solar concentrators.

14:30 - 15:00
1.3-O1
Wächtler, Maria
Leibniz Institute of Photonic Technology
Charge-separation in Ni-tipped CdSe@CdS Nanorods for Hydrogen Evolution
Maria Wächtler
Leibniz Institute of Photonic Technology, DE
Authors
Maria Wächtler a, b
Affiliations
a, Leibniz Institute of Photonic Technology, Albert-Einstein-Straße 9, Jena, DE
b, Institute of Physical Chemistry, Friedrich Schiller University Jena
Abstract

Colloidal nanostructured semiconductor materials, e.g. dot-in-rod heteronanostructures, gained a lot of interest over the past decade as sensitizers for the light-driven water-reduction half reaction by coupling such nanostructures with potential catalysts, e.g. metal nanoparticles. The electronic and optical properties of these nanostructures, which are tunable via their dimensions, render them perfect model systems to study fundamental aspects of their function spectroscopically.

Recently, one of the essential steps in the photocatalytic scheme, the charge separation at the semiconductor/catalyst interface, was studied for a series of CdSe@CdS nanorods tipped with Ni nanoparticles by applying time-resolved transient absorption and photoluminescence spectroscopy. The results indicate that the efficiency of this step depends sensitively on the size of the metal tip, which can be explained based on a model comprising two opposing trends: a size dependent Coulomb blockade and Schottky barrier.[1,2] Further, indications for the need to put charge-carrier transport between the spot of exciton generation and the separating interface into consideration, especially for anisotropic nanostructures with one dimension in the 10s of nm, are currently under closer investigation.

PERInt 1.3
Chair: Pablo P. Boix
13:30 - 14:00
1.3-I1
Padture, Nitin
Brown University
The Materials Science of Halide Perovskites and Solar Cells
Nitin Padture
Brown University
Authors
Nitin Padture a
Affiliations
a, Brown University, 184 Hope Street, Box D, Providence, RI 02912
Abstract

Thin-film perovskite solar cells (PSCs), where the record efficiency has rocketed from under 4% to over 24% (comparable to silicon solar cells) in just ten years, offer unprecedented promise of low-cost, high-efficiency renewable electricity generation. Pb-containing organic-inorganic halide perovskite (OIHP) materials at the heart of PSCs have unique structures, which entail rotating organic cations inside inorganic cages, imparting them with desirable optical and electronic properties.To exploit these properties for PSCs application, the reliable deposition of high-quality OIHP thin films of varrious compositions and structures over large areas is critically important. The microstructures and grain-boundary networks in the resulting polycrystalline OIHP thin films are equally important as they control the PSC performance and stability. Fundamental phenomena pertaining to synthesis, crystallization, coarsening, microstructural evolution, and grain-boundary functionalization involved in the processing of OIHP thin films for PSCs will be discussed with specific examples. In addition, the discovery of new classes of Pb-free halide perovskites (all-inorganic, organic-inorganic, low-dimensional), together with the demonstration of viable PSCs based on these new materials, will be presented. Furthermore, the unique mechanical behavior of halide perovskites, and its implication on the reliability of PSCS, will be discussed. The overall goal of our research is to have deterministic control over the scalable processing of tailored halide perovskite thin films with desired compositions, phases, dimensionalities, microstructures, and grain-boundary networks for scalable, efficient, stable, and reliable PSCs of the future.

14:00 - 14:15
1.3-O1
Aversa, Pierfrancesco
LSI, CEA/DRF/lRAMIS, Ecole Polytechnique, CNRS, lnstitut Polytechnique de Paris, F-91128 Palaiseau, France
Radiative Recombination in Quadruple Cation Organic-Inorganic Mixed Halide Perovskite Layers: Electron Irradiation Induced Ageing Effects
Pierfrancesco Aversa
LSI, CEA/DRF/lRAMIS, Ecole Polytechnique, CNRS, lnstitut Polytechnique de Paris, F-91128 Palaiseau, France
Authors
Pierfrancesco Aversa a, Senol Öz b, f, Eunhwan Jung b, Olivier Plantevin c, Olivier Cavani a, Nadège Ollier a, Bernard Geffroy d, e, Sanjay Mathur b, Catherine Corbel a
Affiliations
a, LSI, CEA/DRF/lRAMIS, Ecole Polytechnique, CNRS, lnstitut Polytechnique de Paris, F-91128 Palaiseau
b, Institute of Inorganic and Material Chemistry, University of Cologne, Greinstraße 6, 50939 Cologne, Germany
c, CSNSM, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay Campus
d, LPICM, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, Route de Saclay, 91128 Palaiseau, France
e, LICSEN, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif-sur-Yvette Cedex,France
f, Graduate School of Engineering, Toin University of Yokohama, 1614, Kurogane-cho, Aoba, Yokohama, Kanagawa, Japan 225-8503
Abstract

Hybrid organic-inorganic halide perovskites attract much attention for their application in optoelectronic devices. However, the performance in domain such as photovoltaics still strongly depends on the quality of the active layers and their capacity to withstand device operation without irreversible damage. For example, applying a bias in dark in CH3NH3PbI3 (MAPI) based solar cells results in ion migration [1]. This questions the generation and role of defects under bias and light illumination [2] on photovoltaics performance.

Most recent works are oriented towards the study of multi cation organic-inorganic mixed halide perovskites, 2/3/4APbX3, where the A sites can be occupied by a distribution of 2-4 metallic/organic ions - Cs+ (cesium ion), GA+ (guanidinium)/MA+ (methylammonium), FA+ (formamidinium) and X sites halide ions I- (iodide), Br- (bromide). The reason is that these mixed perovskites appear more stable than MAPI under PV operation [3] [4] [5]. 

In this work, electron irradiation is used as a tool for the introduction of point defects in a controlled way in polycrystalline 4APbX3 layers spin coated on glass. The created point defects may introduce energy levels and modify electronic and light emitting properties of the material.  The defect production has a strong effect on the photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra. The results illustrate how the PL and TRPL properties depend on the layer history before and after electron irradiation.

 

[1] Lee et al., Direct Experimental Evidence of Halide Ionic Migration under Bias in CH3NH3PbI3-xClx Based Perovskite Solar Cells using GD-OES Analysis, ACS Energy Lett., 2017 DOI: 10.1021/acsenergylett.7b00150

[2] DeQuilettes et al., Photo-induced halide redistribution in organic–inorganic perovskite films, Nature Communications, 2016 DOI: 10.1038/ncomms11683

[3] M. Shahbazi and H. Wang, Progess in reseach on the stability of organometal perovskite solar cells, Solar Energy, 123 (2016) 74-87

[4] Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.-P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; Grätzel, M. Energy Environ. Sci., 9 (2016) 1989–1997

[5] https://www.nrel.gov/pv/assets/pdfs/pv-efficiencies-07-17-2018.pdf

14:15 - 14:30
1.3-O2
Kerremans, Robin
Swansea University, Department of Physics, Swansea, United Kingdom.
On the Electro-optics of the Carbon Stack Perovskite Solar Cells
Robin Kerremans
Swansea University, Department of Physics, Swansea, United Kingdom.
Authors
Robin Kerremans b
Affiliations
a, Swansea University, Department of Physics, Swansea, United Kingdom.
b, Sêr Cymru Sustainable Advanced Materials
Abstract

The mesoporous carbon stack architecture is attracting considerable interest as a potential candidate for scalable, environmentally stable and low-cost perovskite solar cells amenable to high throughput manufacturing processes. These cells are characterised by microns-thick mesoporous titania and zirconia layers capped by a non-selective carbon top electrode with the whole stack being infused with a perovskite semiconductor. Although the architecture does not yet deliver the >20% power conversion efficiencies characteristic of some perovskite planar and mesoporous structures, it does appear to produce cells with respectable efficiencies >16% which is unexpected due to the carbon electrode being far from an ideal anode and the active layers being so thick. Full optimization of these cells requires a detailed understanding of the coupled efficiencies of light absorption, charge generation and extraction but the mode of operation is not yet understood. In this communication, we report a combined experimental-simulation study which elucidates the photogeneration and extraction of charge. By determination of the optical constants of the individual components of cell and using effective medium approximation, we determine the internal quantum efficiency (IQE) in both the titania and zirconia layers to be equally ~85%. Our numerical drift-diffusion simulations indicate that this high IQE together with a respectable open circuit voltage is a consequence of the thick junctions in play – reducing minority carrier concentrations at the electrodes and thereby decreasing surface recombination which is otherwise present in thinner cells with a non-selective contact.  This insight can now be used to further tune the carbon stack for efficiency and simplicity.

14:30 - 14:45
1.3-O3
Choulis, Stylianos
Cyprus University of Technology
The Role of Interfaces on the Device Performance of Inverted Perovskite Photovoltaics
Stylianos Choulis
Cyprus University of Technology, CY
Authors
Stelios Choulis a
Affiliations
a, Cyprus University of Technology, Kitiou Kyprianou, 45, Limassol, CY
Abstract

Many of the physical and engineering aspects that govern the behavior of perovskite photovoltaics occur at interfaces. Especially, the metal oxide-perovskite interface is of great importance for inverted perovskite PV operation, and control of the interface materials properties is critical for high performance photovoltaics [1,2]. We have recently reported the synthesis of a low-temperature solution-processable monodispersed nickel cobaltite (NiCo2O4) as a hole transporting layer for inverted perovskite PVs [1]. We have shown that metal oxide interface materials influence the perovskite grain boundaries formation [2] and that control of metal oxides interfacial energetics through doping (please see TOC figure) are beneficial for inducing a desired PV device functionality [3]. Hence, the development of intimate interfaces through their fundamental understanding and manipulation is expected to be crucial to the continued progress of perovskite PVs. Up to now the main efforts of the perovskite research community have been focused on improving the power conversion efficiency (PCE). There are fewer reports on the degradation of perovskite PVs. We have recently reported hear improved stability of inverted perovskite photovoltaics by incorporation of fullerene-based diffusion blocking layer [4].  However, the effect of metal-oxide interfacial layers on lifetime performance remain unclear. The presentation aims in covering a range of electronic materials for high performance perovskite photovoltaics. A systematic understanding of the relationship between interface materials [1-3] and inverted perovskite photovoltaic power conversion efficiency [2-3] and lifetime performance [4] will be presented.

14:45 - 15:00
1.3-O4
Pockett, Adam
SPECIFIC, Swansea University
Utilizing Optoelectronic Characterization Techniques in the Development of Triple Mesoporous Perovskite Solar Cells
Adam Pockett
SPECIFIC, Swansea University
Authors
Adam Pockett a, Matt Carnie a
Affiliations
a, SPECIFIC, Swansea University, Baglan Bay Innovation and Knowledge Centre, Baglan, SA12 7AX
Abstract

Triple mesoporous layer devices containing a TiO2 electron transport layer, a ZrO2 insulating layer and carbon as the hole transporting contact show great promise for scale-up and widespread implementation. To improve these devices and begin to challenge inorganic PV record efficiencies a deeper understanding of their operation, and in particular sources of performance loss, is needed.

Our previous work has highlighted the extremely slow dynamic response of these devices under illumination and its links to interfacial recombination and ion migration.[1] One striking feature of these devices is the thickness of the layers – the mesoporous TiO2 and ZrO2 layers are each a few microns thick. The operation of these devices is therefore extremely sensitive to charge separation, transport and recombination properties. Added to this is the poor selectivity of the carbon electrode. Nevertheless, despite these unusual device properties, efficiencies in excess of 15% have been achieved. The one noticeable deficiency relative to other perovskite device architectures is a lower open-circuit voltage – typical maximum of around 0.9 V.

We have probed the recombination behaviour in these devices, specifically focusing on spectral dependence to study the impact of light penetration depth (where charges are generated), using a range of techniques including transient photovoltage spectroscopy (TPV), impedance spectroscopy (EIS) and time-resolved photoluminescence (TRPL). By varying the thickness of the TiO2 and ZrO2 layers we have begun to understand the main source of recombination loss in these devices, which has allowed us to optimize the device architecture.

15:00 - 15:30
1.3-I2
Garcia-Belmonte, Germà
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
Capacitive Response of Photovoltaic Perovskites: Defects, Interfaces and Contact Reactivity
Germà Garcia-Belmonte
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES

Germà Garcia-Belmonte (1964) received his Ph.D. degree at UNED, 1996. He worked (1988-1992) at CIEMAT, Madrid, on experimental and theoretical research in the area of digital processing of nuclear signal. He joined the Universitat Jaume I, Castelló, in 1992 and currently works as a Full Professor of Applied Physics (2010) at the Institute of Advanced Materials. He published 198 papers in research journals, and has 11.000 citations and h-index 52 (WOS). He is recognized as 2018 Highly Cited Research (Clarivate Analytics) in the cross-field category. He studied intercalation processes in oxides and polymer films by impedance methods. He follows researches in various areas within the field of Organic Electronics and photovoltaics as electronic mechanisms in organic light-emitting diodes, organic photovoltaics, and plastic and thin-film solar cells. He is currently conducting researches in the topic of perovskite-based solar cells. Also of interest is the electrochemical kinetics of electrodes for batteries. Device physics using impedance spectroscopy (including modeling and measuring) is his main subject.

Authors
Germà Garcia-Belmonte a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

Hybrid lead halide perovskite-based solar cells have rapidly attained very large solar to electricity power conversion efficiencies. Despite reaching efficiency records, there are several open issues concerning the operating mechanisms and, in relation to this, the selection of proper probing techniques able to extract electronic parameters in a reliable manner. One of these techniques, capacitance analysis, may convey information about several electronic and ionic mechanisms such as dielectric bulk polarization, space-charge depletion zones, electronic bulk chemical storage, defect contribution to the capacitance steps, and interfacial accumulation processes. In general, electrical response of perovskite solar cells are influenced by several capacitive and resistive mechanisms, which are also modulated by light. This variety of capacitive effects may induce wrong interpretations and produce misleading outcomes when uncritically connected to the bulk polarization or defect responses [1,2].  

Capacitive effects occurring at outer interfaces are by far much more elusive than those taking place at the absorber bulk. In practical terms, it is necessary to distinguish between capacitive currents originated from the charging−discharging dynamics of capacitors of dielectric nature linked with the ionic polarization of outer electrodes [3] and also light-induced electronic accumulation surface layers [4] confined within the Debye length in the vicinity of the contacts, from chemical interactions between mobile ions within the absorber perovskite and the contact layers [5]. It has been recently demonstrated that electrical biasing induces contact reaction and produces modifications of the current level by favoring the ability of perovskite/Au interfaces to inject electronic carriers [6]. It is then mandatory to pay attention at the reactivity of perovskite contacts if this technology aims to reach long-term stability.

RadDet 1.3
Chair: Sergii Yakunin
13:30 - 14:00
1.3-O1
Panzer, Fabian
Soft Matter Optoelectronics, University of Bayreuth, Germany
A Solvent free Route for Halide Perovskite Film Processing Based on Pressure Treatment of Perovskite Powders
Fabian Panzer
Soft Matter Optoelectronics, University of Bayreuth, Germany, DE
Authors
Maximilian Schultz a, Nico Leupold b, Ralf Moos b, Fabian Panzer a
Affiliations
a, Soft Matter Optoelectronics, Department of Physics, University of Bayreuth, Bayreuth 95440, Germany
b, Functional Materials, University of Bayreuth, Universitätsstraße 30, Bayreuth, 95440, DE
Abstract

Even though hybrid perovskites have undergone a remarkable development within the last years, state of the art processing approaches such as solution processing or evaporation suffer from an intrinsically high complexity, as the actual perovskite crystallization and its film processing happen simultaneously and are inextricably interconnected.

Here we present an alternative, entirely dry processing approach, which decouples perovskite crystallization and film formation, by using readily prepared perovskite powders and produce films by appropriate mechanical pressure treatment. We show how a mechanochemical synthesis approach by ball milling allows to produce a wide range of phase pure and exceptionally stable hybrid perovskite powders with a high flexibility in processing and address the impact of milling parameters on the powder properties. Using these powders, we demonstrate how the used pressure and the powder microstructure, i.e. particle size and stoichiometry affect the mechanical stability, compactness and surface roughness of the pressed layers. We further address how specific temperature treatment during the pressing step can improve the properties of the pressed layer, and show their capability to be used in perovskite based optoelectronic devices, such as X-Ray detectors.

14:00 - 14:30
1.3-O2
Grozema, Ferdinand
Delft University of Technology, The Netherlands
Radiation-induced conductivity in 3D and 2D hybrid perovskites
Ferdinand Grozema
Delft University of Technology, The Netherlands, NL
Authors
Ferdinand Grozema a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
Abstract

Hybrid halide perovskites are promising materials for application in solar cells, but also for a range of other applications such as in photodetectors, light-emitting diodes and lasers. In this work we have studied the conductive properties of 3D and 2D halide perovskites by using high energy radiation in the so-called pulse radiolysis time-resolved microwave conductivity technique. In this technique, materials are irradiated with high enery electrons and the change in conductivity is monitored on a nanosecond time-scale by microwave absorption. In this work we show that pulsed irradiation of perovskites leads to a high, long-lived conductivity response that is a result of the large concentration of charge generated due to the heavy Pb component, a large dielectric screening and a high charge carrier mobility. This makes these materials potentials candidates for application in fast sensitive radiation detectors. We show that the radiation induced conductivity has a significant dependence on the composition of the material, both is magnitude and in lifetime, offering insight in the design principles to optimize these materials for radiation detection applications.

14:30 - 15:00
1.3-I1
Du, Mao-Hua
Oak Ridge National Laboratory
Self-Activated Low-Dimensional Metal Halide Phosphors and Scintillators
Mao-Hua Du
Oak Ridge National Laboratory

Dr. Mao-Hua Du is a Senior R&D Staff in the Materials Sciences and Technology Division at Oak Ridge National Laboratory. He received his B.S. in Physics at Fudan University, China, in 1998 and Ph. D in Physics at the University of Florida in 2003. He was a postdoctoral associate at National Renewable Energy Laboratory (Golden, Colorado, 2004-2006) and a National Research Council Research Associate at Naval Research Laboratory (Washington, DC, 2006-2007). He joined Oak Ridge National Laboratory in 2007. His research focuses on electronic structure, optical properties, and defect physics in electronic and optical materials (with applications in photovoltaics, energy efficient lighting, radiation detection,  etc.). 

Authors
Mao-Hua Du a
Affiliations
a, Oak Ridge National Laboratory
Abstract

Low-dimensional metal halides are a large family of compounds that consist of 2D, 1D, or 0D anionic metal halide framework coupled with organic or inorganic countercations. Due to the quantum confinement in low-dimensional structures, excitons can be localized with sufficiently large binding energies, enabling luminescence at room temperature. In hybrid organic-inorganic metal halides, both the inorganic metal halide anion and the organic cation can be functionalized to act as luminescent centers. The broad structural and compositional flexibilities of low-dimensional metal halides offer tremendous opportunities to optimize optical properties for light emitting applications. High photoluminescent quantum efficiencies (PLQEs) up to 100% have been reported for a number of 0D metal halides [e.g., (C4N2H14Br)4SnBr6 (95%), [(C6H5)4P]2SbCl5 (100%), Cs3Cu2I5 (83%)]. In this talk, we show first-principles calculations of electronic structure and excitonic properties (including excitation/emission energies, exciton self-trapping, etc.) in low-dimensional metal halides, and discuss the emission mechanism, the material chemistry that affects the PLQE, and the prospect of using these materials as phosphors and scintillators.

15:00 - 15:30
1.3-I2
Saliba, Michael
TU Darmstadt, Optoelectronics
Bright and Fast Scintillation of Organolead Perovskite MAPbBr3 at Low Temperatures
Michael Saliba
TU Darmstadt, Optoelectronics, DE
Authors
Michael Saliba a
Affiliations
a, TU Darmstadt, Petersenstrasse 32, Darmstadt, 64287, DE
Abstract

At lower cryogenic temperatures, perovskite crystals show excellent scintillation properties in terms of signal output and quick response time. Such materials have been sought after for many decades and could dramatically impact the entire scintillation field. Importantly, the concept of cryogenic scintillation is underexplored and novel because established scintillators do not exhibit dramatically improved performance with decreased temperature. Perovskites, however, do and prove to be among the best scintillation materials measured to date. Remarkably, our work uses yet unoptimized crystals and thus it is possible that further improvements will be achieved over time. Operation at lower temperatures is already common for medical applications. Therefore, this work has the potential to trigger a new generation of cryogenic scintillators, as well as providing a new direction for the perovskite field. In particular, perovskites are promising for the medical sector because of the potential for improved cancer diagnostics through an increased imaging resolution, e.g. for early-stage brain cancer. Importantly, perovskites contain elements with a high atomic number (Z), e.g. Pb, making perovskites highly relevant scintillation materials.

MapNan 1.3
Chair: Patrick Mesquida
14:45 - 15:00
1.3-O3
Alikin, Denis
University of Aveiro
Correlative Scanning Probe and Confocal Raman Microscopy for the Evaluation of Li-ion Kinetics in LiMn2O4 Cathodes
Denis Alikin
University of Aveiro
Authors
Denis Alikin a, b, Boris Slautin b, Konstantin Romanyuk a, Daniele Rosato c, Alexander Tselev a, Andrei Kholkin a, b
Affiliations
a, Department of Physics & CICECO—Aveiro Institute of Materials, University of Aveiro, 3810–193 Aveiro, Portugal
b, Ural Federal University, School of Natural Sciences and Mathematics, Ekaterinburg, Russia, RU
c, Robert Bosch GmbH, 70839 Gerlingen-Schillerhoehe, Germany
Abstract

Li-ion accumulators (LIA) are extremely important for the development of energy storage systems for transport, portable appliances and mobile electronics. The studies of the LIA materials by microscopic methods play a key role in the understanding of the mechanisms of electrochemical processes at the micro- and nanoscales. Scanning probe microscopy (SPM) methods for studying LIA materials started in the beginning of 90s and evolved from the simple topographic measurements to voltage driven local intercalation-deintercalation studies [1]. Direct methods of ionic conductivity evaluation at the nanoscale, so called scanning electrochemical microscopy, are based on the measurements of ion interaction with a sample in an electrolyte media [2]. These methods require complicated sample preparation, specialized cells and probes, as well as a high homogeneity of the surface [2]. This makes their use complicated for the implementation in practical electrochemical systems.

In this work, a quantitative indirect method based on the strain response to local voltage excitation, so-called electrochemical strain microscopy (ESM), is considered. The method relies on the modulation of ion concentration in the vicinity of SPM tip [1]. To interpret the ESM data quantitatively we measure current and acoustic correlative responses allowing to estimate the voltage drop at the interface and contact stiffness, respectively [3]. Correlative confocal Raman and scanning probe microscopy approach was implemented to find a relation between the structural state and functional electrochemical response in individual micro-scale particles of LiMn2O4 spinel in a commercial Li battery cathode [4,5]. It was shown that the high-frequency ESM has a significant cross-talk with the topography due to the tip-sample electrostatic interaction, while the low-frequency ESM yields a response that can be linked to the distributions of Li ions and electrochemically inactive phases revealed by the confocal Raman microscopy. We conclude that the obtained low frequency ESM image contrast is caused by the Vegard strain and can be used  to map local Li-ion concentration and thus it is controlled by local diffusivity. The observed features of the low-frequency ESM signal distribution across the cathode are used for the interpretation of Li ion intercalation kinetics during battery degradation.

 

15:00 - 15:15
1.3-O1
BENTEN, Hiroaki
Nara Institute of Science and Technology - Japan
Nanoscale Morphology for Charge Transport of Conjugated Polymer Blend Films Studied by Conductive Atomic Force Microscopy
Hiroaki BENTEN
Nara Institute of Science and Technology - Japan, JP
Authors
Hiroaki BENTEN a
Affiliations
a, Division of Materials Science, Nara Institute of Science and Technology - JP, JP
Abstract

Blend films of conjugated polymers (p-type donor polymer and n-type acceptor polymer) have gained increasing attention as a photovoltaic layer for polymer solar cells.[1]  Photovoltaic performances of the blend film critically depend on the charge (hole and electron) transport within the film, which is influenced by the crystallization, aggregation, and phase separation of the constituent conjugated polymers.  The electrical conductivity and mobility of the blend film have been evaluated by macroscopic current density-voltage measurement.  However, there is a large gap between the insights that could be derived from the bulk-averaged conducting properties and those about local conductive features which are subject to the nanoscale morphologies of the blend film.  Thus, in-depth understanding of the local conductive property of the blend films is required for designing materials to give efficient charge transport toward the improvement of the photovoltaic performance.

In this study, we apply conductive atomic force microscopy (C-AFM) as a tool for studying the local conductive properties of conjugated polymer blend films on a scale of nanometers.  First, we employed C-AFM to characterize the local hole conductivity of the donor/acceptor polymer blend film.  We discussed the existence of an intermixed region that was located between donor-rich and acceptor-rich domains, via observed hole current images.[2]  The restricted charge-transport of the intermixed region found to be closely related to the device photovoltaic performance.[3]  Second, we developed a method to measure electron current through acceptor polymers by using air-stable low work function electrode that is prepared by coating ethoxylated polyethyleneimine as a surface modifier on to indium-tin-oxide electrode.  With this electrode, electron-transport network formed in donor/acceptor polymer blends can be successfully observed as well as hole-transport network is characterized.[4]  Our approach based on C-AFM to electrically resolve nanostructures of conjugated polymer blends contributes to further understanding of the mechanisms for excellent charge transport and creation of photovoltaic functions of polymer solar cells composed of donor and acceptor polymers.

15:15 - 15:30
1.3-O2
Alikin, Denis
University of Aveiro
Piezoelectric Response and Polarization-Dependent Conductivity of Grain Boundaries in BiFeO3 Thin Films
Denis Alikin
University of Aveiro
Authors
D.O. Alikin a, Y. Fomichov b, S.P. Reis c, d, A.S. Abramov a, D.S. Chezganov a, V.Ya. Shur a, E. Eliseev e, A. Morozovska f, E.B. Araujo d, A.L. Kholkin a, b
Affiliations
a, Ural Federal University, School of Natural Sciences and Mathematics, Ekaterinburg, Russia, RU
b, Faculty of Mathematics and Physics, Charles University in Prague, Prague 8, 180 00, Czech Republic
c, Department of Chemistry and Physics, São Paulo State University, Ilha Solteira - SP, Brazil
d, Federal Institute of Education, Science and Technology of São Paulo, 15503-110 Votuporanga, Brazil
e, Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine
f, Institute of Physics, National Academy of Sciences of Ukraine, 03028 Kyiv, Ukraine 7Department of Physics & CICECO—Aveiro Institute of Materials, University of Aveiro, 3810–193, Aveiro
Abstract

Many efforts have been devoted so far to achieve the control of interfaces in ferroelectric materials based on their polarization. These efforts resulted in the discovery of a variety of different phenomena such as polarization-dependent tunneling effect, resistive switching, symmetry breaking, etc. [1,2] In particular, domain wall conductivity [3], formation of topological defects [4], phase boundaries[5] and ferroelectric-insulator interfaces [6] have been studied. Charge transport across the interfaces in complex oxides attracts a lot of attention because it allows creating novel functionalities useful for device applications. In particular, it has been observed that movable domain walls in epitaxial BiFeO3 films possess enhanced conductivity that can be used for read out in ferroelectric-based memories [3]. In this work, the relation between the piezoelectric response, polarization and conductivity in sol-gel BiFeO3 films with special emphasis on grain boundaries (GBs) as natural interfaces in polycrystalline ferroelectrics is investigated. The grains exhibit self-organized domain structure in these films, so that the “domain clusters” consisting of several grains with aligned polarization directions are formed. Surprisingly, GBs between these clusters (with antiparallel polarization direction) have significantly higher electrical conductivity in comparison to “inter-cluster” GBs, in which the conductivity was even smaller than in the bulk. As such, polarization-dependent conductivity of the GBs was observed for the first time in ferroelectric thin films. The results are rationalized by thermodynamic modelling combined with finite element simulations of the charge and stress accumulation at the GBs giving major contribution to conductivity.

The existence of low and highly conductive GBs may have an important impact on many macroscopic properties, e.g. dielectric permittivity and leakage current. It becomes even more important for nanosized-grain ceramics, where the influence of multiple GBs is dominant. Conductive GBs may have as well significant effect on the domain wall motion and polarization reversal in polycrystalline materials. The observed polarization-dependent conductivity of GBs in ferroelectrics opens up a new avenue for exploiting these materials in electronic devices.

 

 

Sol2D 1.3
Chair: Daniel Vanmaekelbergh
14:00 - 14:15
1.3-O1
Achtstein, Alexander
TU Berlin
Tunable Emission Fine Structure and Origin of Quadratic TPA in 2D CdSe Nanoplatelets
Alexander Achtstein
TU Berlin, DE

Alexander W. Achtstein recieved a PhD from Technical University of Berlin in 2013. After a postdoc period at TU Delft he returned TU Berlin. His research is focussed on the linear and nonlinear optical properties of II-VI nanosheets and and transition metal dichalcogenides.

Authors
Riccardo Scott a, Judith Specht b, Juan Climente c, Marta Corona-Castro b, Anatol Prudnikau d, Artsiom Antanovich d, Nina Owschimikow a, Sotirios Christodoulou e, Laurens Siebbeles f, Michael Quick a, Guilliaume Bertrand g, Iwan Moreels h, Mikhail Artemyev d, Ulrike Woggon a, Joseph Planelles c, Marten Richter b, Alexander Achtstein a
Affiliations
a, Institute of Optics and Atomic Physics, Technical University of Berlin, Strasse des 17. Juni 135, 10623, Berlin, Germany
b, Technische Universität Berlin, Straße des 17. Juni, 124, Berlin, DE
c, University Jaume I, Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
d, Research Institute for Physical Chemical Problems of Belarusian State University, 220006, Minsk, Belarus
e, ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels (Barcelona), Spain.
f, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
g, CEA Saclay, 91191 Gif-sur-Yvette, France
h, Department of Chemistry, Ghent University, Krijgslaan 281 - S3, 9000 Gent, Belgium
Abstract

We demonstrate a novel emission fine structure of the low temperature emission in II-VI nanoplatelets depending on the lateral size. Nanoplatelets are in an intermediate confinement regime with a rich substructure of excitons, which is neither quantum dot like nor like an ideal 2D exciton. We discuss the observed transition energies and relaxation dynamics of exciton states in CdSe platelets with varying lateral dimensions and compare them with a microscopic theoretical model including exciton-phonon scattering. The interplay of (lateral) confinement and Coulomb coupling in the intermediate regime results in strong changes with respect to simple weak or strong confinement models which are recovered by solving the full four dimensional lateral factorization free exciton wavefunction. A rich substructure of several exciton states is observed. We also demonstrate that the interplay of exciton bright and dark states provides principle insights into the overall temporal relaxation dynamics. Not only the linear properties of nanoplatelets are interesting, but also their nonlinear properties. In a second part we study the origin of the extremely high two photon absorption cross sections and their puzzling quadratic volume scaling. We show that excitonic correlation, wavefunction coherence and the related scaling of inter- and intraband transition dipole moments result in the observed quadratic volume dependence. Our results open up the possibility to engineer two photon absorbers with unprecedented cross sections and nonlinear optical applications at lowered intensities, like high sensitivity two photon autocorrelation.

14:15 - 14:30
1.3-O2
Schimpf, Alina
University of California San Diego
Modulation of Precursor Reactivity for Colloidally Synthesized WSe2 Nanocrystals and Heterostructures
Alina Schimpf
University of California San Diego, US
Authors
Alina Schimpf a, Jessica Geisenhoff a
Affiliations
a, University of California San Diego, Gilman Drive, 9500, San Diego, US
Abstract

Trandisiton metal dichalcogenides (TMDs) can host a variety of phases, each with a unique electronic structure, allowing access to a compositionally and electronically diverse set of 2D materials. Among these materials, the metastable 1T′ phase of WSe2 has recently gained attention due to its potential application as a quantum spin hall insulator operable at room temperature. This metastable 1T′ phase, however, is difficult to access via traditional synthetic methods due to the low barrier for conversion to the thermodynamically favored 2H phase. Colloidal chemistry is uniquely poised for the synthesis of metastable phases because conditions can be chosen to access kinetic growth regimes. We show that control over that size and phase of colloidal WSe2 nanocrystals is achieved by careful choice of ligand, where increasing the coordinating strength of ligands present during synthesis leads to larger nanocrystals with increasing contribution from the 1T′ phase. Specifically, oleic acid is used to coordinate W in solution, slowing down the W reactivity and yielding large 1T′ WSe2 nanocrystals. We can further exploit this modulation of the reactivity to enable one-pot synthesis of colloidal core/shell heterostructured nanocrystals. The core nanocrystals can subsequently be removed by soaking in ethylene diamine and trioctylphosphine, allowing easy access to hollow WSe2 nanocrystals. Overall, these syntheses allow access to heterostructured or hollow nanostructures in just one or two steps and demonstrate a synthetic strategy to ultimately enable facile, solution-phase syntheses of exotic nanostructures.

14:30 - 15:00
1.3-I1
Castellanos-Gomez, Andres
Consejo Superior de Investigaciones Científicas (CSIC)
Strain Engineering in 2D Materials: Towards Strain Tunable Optoelectronic Devices
Andres Castellanos-Gomez
Consejo Superior de Investigaciones Científicas (CSIC), ES

Andres Castellanos-Gomez is a Tenured Scientist in the Spanish National Research Council. He explores novel 2D materials and studies their mechanical, electrical and optical properties with special interest on the application of these materials in nanomechanical and optoelectronic devices. He is author of more than 100 articles in international peer review journals and 6 book chapters. He was awarded an ERC Starting Grant in 2017 and has been selected as one of the Top Ten Spanish Talents of 2017 by the MIT Technology Reviews. He has been also recognized with the Young Researcher Award (experimental physics) of the Royal Physical Society of Spain (2016).

Authors
Patricia Gant a, Riccardo Frisenda a, Andres Castellanos-Gomez a
Affiliations
a, Materials Science Factory, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid E-28049, Spain.
Abstract

 

Strain engineering is an interesting strategy to tune a material’s electronic properties by subjecting its lattice to a mechanical deformation. Conventional straining approaches, used for 3D materials (including epitaxial growth on a substrate with a lattice parameter mis-match, the use of a dielectric capping layer or heavy ions implantation) are typically limited to strains lower than 2% in most cases due to the low maximum strains sustained by brittle bulk semiconducting materials. Bulk silicon, for example, can be strained only up to 1.5% before breaking. Moreover, these straining approaches induce static deformations of the semiconductor materials and therefore they are not suitable for tunable functional devices.

 

2D materials can be literally stretched, folded, bent or even pierced.[1] This outstanding stretchability (and the possibility of using dynamically varying strain) of 2D materials promises to revolutionize the field of strain engineering and could lead to "straintronic" devices – devices with electronic and optical properties that are engineered through the introduction of mechanical deformations.

 

In this talk I will discuss our recent efforts to study strain engineering in 2D materials and to exploit it to fabricate strain tunable functional optoelectronic devices.[2-6]

 

 

 

 

  

15:00 - 15:30
1.3-I2
Ismach, Ariel
Tel Aviv University
Chemical Vapor Deposition of 2D Materials and Heterostructures
Ariel Ismach
Tel Aviv University, IL
Authors
Ariel Ismach a
Affiliations
a, Department of Materials Science and Engineering, Tel Aviv University
Abstract

The ability to synthesize large-area and high quality atomic films is a prerequisite for their successful integration into a wide variety of novel and existing technologies. Here we show the growth of transition metal dichalcogenides (MoS2, WS2 and WSe2) via modified chemical vapor deposition (CVD) methods using volatile precursors [1,2]. The use of high vapor pressure precursors allows for the controlled delivery to the growth sample [2], and therefore, suitable for homogeneous and large-scale synthesis, as required for many applications. However, one of the problems with these precursors is the small domain size usually obtained. In order to address these issues, two different concepts were implemented and will be described: i. Seeded-growth and, ii. Pulsed-growth approaches. In the first, and following the success in growing 0 (QDs, NPs), 1 (NWs, NTs), 2 (films) and 3D crystals, the growth of 2D materials is nucleated at well-defined seeds. The description of the methodology as well as the influence of the seed-material on the grown layered domains will be described. In the second, a modified approach in which the metal and chalcogen precursors are delivered in a pulsed fashion is demonstrated. This approach allows to achieve a ten-fold increase in the domain size, from ~10 nm (or below) to ~10s of microns. Moreover, we demonstrate that the growth kinetics is highly dependent on the surface chemistry and by controlling it, the growth of ad-layers is inhibited and thus, more than 95% monolayer films are obtained.  Another advantage of using volatile precursors is the ability to control the lateral and vertical heterostructures formation, and will be described as well. Following this work and in order to expand our growth capabilities, the growth of monochalcogenides using lessons learned while growing TMDs will be briefly described.

SolFuel 1.3
Chair: Marc Robert
14:00 - 14:30
1.3-I1
Buonsanti, Raffaella
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland
Colloidal Synthesis for Controllable and Tunable Materials in Artificial Photosynthesis
Raffaella Buonsanti
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, CH

Raffaella Buonsanti obtained her PhD in Nanochemistry in 2010 at the National Nanotechnology Laboratory, University of Salento. Then, she moved to the US where she spent over five years at the Lawrence Berkeley National Laboratory, first as a postdoc and project scientist at the Molecular Foundry and after as a tenure-track staff scientist in the Joint Center for Artificial Photosynthesis. In October 2015 she started as a tenure-track Assistant Professor in the Institute of Chemical Sciences and Engineering at EPFL. She is passionate about materials chemistry, nanocrystals, understanding nucleation and growth mechanisms, energy, chemical transformations.

Authors
Raffaella Buonsanti a
Affiliations
a, Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, Lausanne, CH
Abstract

Converting water and carbon dioxide into value-added chemicals, including fuels, with sunlight requires new materials which allow the transformation involved to be efficient and selective. Our work highlights how colloidal chemistry can aid to construct materials and to develop new concepts for storing energy in chemical bonds, namely artificial photosynthesis. [1-8] The first part of this talk will focus on the design of Cu-based heterogeneous catalysts for selective conversion of CO2 while suppressing the hydrogen evolution reaction, which is a big challenge in the field at the moment. We show the importance of size, shape and interfaces to tune the intrinsic activity and selectivity of copper. [1-6] In the second part, I will discuss our recent progress on protection schemes for perovskite nanocrystals to enable their use as light absorbers in artificial photosynthesis, something impeded so far by their instability in polar environments. Our approach consists in using thin shells of metal oxides, deposited via gas-phase or colloidal atomic layer deposition, which still enable charge transfer.[7,8]  Eventually, the two classes of materials will be combined to drive CO2 conversion using sunlight.

14:30 - 15:00
1.3-O1
Marschall, Roland
University of Bayreuth, DE
Nanostructured Spinel Ferrite Materials for Photoelectrochemical Water Splitting
Roland Marschall
University of Bayreuth, DE, DE

Dr. Roland Marschall obtained his PhD in Physical Chemistry from the Leibniz University Hannover in 2008, working on mesoporous materials for fuel cell applications. After a one year postdoctoral research at the University of Queensland in the ARC Centre of Excellence for Functional Nanomaterials, he joined in 2010 the Fraunhofer Institute for Silicate Research ISC as project leader. In 2011, he joined the Industrial Chemistry Laboratory at Ruhr-University Bochum as young researcher. From 07/2013 to 08/2018, he was Emmy-Noether Young Investigator at the Justus-Liebig-University Giessen. Since 08/2018, he is FUll Professor at the University of Bayreuth, Germany. His current research interests are heterogeneous photocatalysis, especially photocatalytic water splitting using semiconductor mixed oxides, and synthesis of oxidic mesostructured materials for energy applications.

Authors
Roland Marschall a
Affiliations
a, University of Bayreuth, DE, Universitätsstraße, 30, Bayreuth, DE
Abstract

We have developed a straightforward microwave synthesis protocol to produce nanocrystals of the earth-abundant cubic spinel ferrites MgFe2O4, NiFe2O4 and ZnFe2O4,[1,2] which are promising materials for both photoelectrochemical and photocatalytic water splitting under visible light irradiation due to their narrow band gaps (~ 2.0 eV) and matching band positions. The crystallite size can be tailored by post-synthetic heat treatment, however the materials are already partly crystalline as-prepared. Samples were characterized employing transmission electron microscopy (TEM), X-ray diffraction (XRD), dynamic light scattering (DLS), Raman spectroscopy and N2 physisorption, indicating highly-crystalline, single phase nanoparticles with specific surface areas of around 200 m²/g and good colloidal stability in non-polar solvents. Phase transfer into aqueous medium has been performed using different organic capping ligands, resulting in stable dispersions with a narrow size distribution. First results of photocatalytic experiments will be presented.

In addition, well-ordered mesoporous ZnFe2O4 and a-LiFe5O8 thin film photoanodes were fabricated by sol-gel synthesis,[3,4] using a polymer-templating approach previously reported by Haetge et al..[5] Ordered mesopores are obtained after dip-coating by evaporation-induced self-assembly followed by heat treatment. Scanning electron microscopy (SEM) confirms the porous morphology with average pore diameters of 12-15 nm. Raman spectroscopy and XRD Rietveld analysis revealed phase pure mesoporous thin films with a crystallite size of 15 nm. Furthermore, photocurrent and Mott-Schottky measurements were performed at different pH values to determine the flat band potential and photocurrent density of the thin film electrodes calcined at various temperatures.

Finally, mesoporous CaFe2O4 photocathodes will be presented showing p-type behavior.[6] For the first time, this material can be prepared at temperatures as low as 700 °C.

15:00 - 15:15
1.3-O2
Volokh, Michael
Ben-Gurion University of the Negev, Israel
Water-splitting Photoelectrochemical Cells Based on Carbon Nitride Materials: Progress through Improved Deposition Techniques
Michael Volokh
Ben-Gurion University of the Negev, Israel, IL

Lab Manager, Researcher, and Teacher at the Department of Chemistry at the Ben-Gurion University of the Negev, Israel.

Authors
Michael Volokh a, Menny Shalom a
Affiliations
a, Ben-Gurion University of the Negev, Israel, Beer-Sheva, IL
Abstract

Water-splitting photoelectrochemical cells based on carbon nitride materials: progress through improved deposition techniques



Carbon nitride materials (CNs) are an emerging class of materials, which exhibit excellent photo- and heterogeneous-catalytic properties for various reactions thanks to their tunable band gap, suitable energy-band position, high stability under harsh chemical conditions, and low cost. However, the utilization of CN in photoelectrochemical (PEC) and other photoelectronic devices is yet to be wide-spread due to the difficulties in depositing high-quality and homogenous CN layer on substrates, the wide band gap of ‘intrinsic’ CN, poor charge-separation efficiency, and low electronic conductivity.[1]

We present recent synthetic progress achieved in our group through several pathways for the preparation of various structures of CN on substrates and their underlying photophysical properties and photoelectrochemical performance. We focus on the ‘doctor-blade’ technique for deposition of a paste of supramolecular assemblies.[2] We show how the incorporation of reduced graphene oxide[3] and graphene oxide[4] at different stages alters the properties of the paste and the final CN-based layer. Furthermore, we show a method to fabricate closely-packed CN film using crystallization of monomers.[5] The main challenges for CN incorporation into PEC cell are described, together with possible routes to overcome the standing limitations toward the integration of CN materials in photoelectronic devices.[1]

15:15 - 15:30
1.3-O3
Mitoraj, Dariusz
University of Ulm
Water-Soluble Polymeric Carbon Nitride Colloidal Nanoparticles for Quasi-Homogeneous Photoredox Applications
Dariusz Mitoraj
University of Ulm, DE
Authors
Igor Krivtsov a, b, Dariusz Mitoraj a, Radim Beranek a
Affiliations
a, University of Ulm, Albert-Einstein-Allee 11, Ulm, DE
b, Department of Organic and Inorganic Chemistry, University of Oviedo-CINN, 33006 Oviedo, Spain
Abstract

The heptazine-based polymeric carbon nitrides are well established as promising photocatalysts for light-driven selective redox transformations. However, the activity of these materials is hampered by their low surface area translating into low concentration of surface active sites accessible for reactants. Herein, we report the synthesis of poly(heptazine imide) materials from melamine at unprecedentedly low temperature of 330 °C. The resulting colloidal nanoparticles are soluble in water, and exhibits a 6.5 times higher reaction rate in selective (up to 100%) photooxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde and simultaneous H2O2 production by O2 reduction as compared to the conventional polymeric carbon nitride. The solubility of this material allows estimation of a quantum yield of the catalytic photochemical reaction, which is usually complicated for heterogeneous systems, giving a value of ca. 10 % at 365 nm. The dissolved photocatalyst can be easily recovered and re-dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity. This work thus establishes a new paradigm of quasi-homogeneous operation for photocatalysis with carbon nitrides.

15:30 - 16:00
Coffee Break
CharDy 1.4
Chair: Maksym Yarema
16:00 - 16:30
1.4-I1
Wuttig, Matthias
RWTH Aachen University
Phase Change Materials by Design: Taming Bond No. 6
Matthias Wuttig
RWTH Aachen University

Matthias Wuttig received his Ph.D. in Physics in 1988 from RWTH Aachen/ Forschungszentrum Jülich. From 1995 to 1997 he worked with a Feodor-Lynen stipend at Bell Labs, Murray Hill, New Jersey. He was a visiting professor at several institutions including Lawrence Berkeley Laboratory, Stanford University, Hangzhou University, IBM Almaden, Bell Labs, DSI in Singapore, CiNAM in Marseilles and the Chinese Academy of Sciences in Shanghai. In 1997, he was appointed Full Professor at RWTH Aachen, where his work focusses on the design of novel functional materials. From 2009 to 2018, he was the speaker of the strategy board of RWTH. Since 2011, he heads a collaborative research centre on resistively switching chalcogenides (SFB 917), funded by the German Science Foundation DFG. In 2013, he received an ERC Advanced Grant to realize novel functionalities by disorder control. He is a member of Acatech and the North Rhine-Westphalian Academy of Sciences and has written about 330 publications (~17.000 citations). In 2019 he was selected as an MRS Fellow for path-breaking contributions to the advancement of phase-change materials, including unraveling their unique bonding mechanism, unconventional transport properties and unusual kinetics.

Authors
Matthias Wuttig a, b
Affiliations
a, RWTH Aachen University - Germany, Aaachen, DE
b, Peter Grünberg Institut, Forschungszentrum Jülich, 52425 Jülich, Germany;
Abstract

It has been a long-time dream of mankind to design materials with tailored properties. In recent years, the focus of our work has been the design of phase change materials for applications in data storage. In this application, a remarkable property portfolio of phase change materials (PCMs) is employed, which includes the ability to rapidly switch between the amorphous and crystalline state. Surprisingly, in PCMs both states differ significantly in their properties. This material combination makes them very attractive for data storage applications in rewriteable optical data storage, where the pronounced difference of optical properties between the amorphous and crystalline state is employed. This unconventional class of materials is also the basis of a storage concept to replace flash memory.  This talk will discuss the unique material properties, which characterize phase change materials. In particular, it will be shown that only a well-defined group of materials utilizes a unique bonding mechanism (‘Bond No. 6’), which can explain many of the characteristic features of crystalline phase change materials. Different pieces of evidence for the existence of this novel bonding mechanism, which we have coined metavalent bonding, will be presented. This insight is subsequently employed to design phase change materials as well as thermoelectric materials. Yet, the discoveries presented here also force us to revisit the concept of chemical bonds and bring back a history of vivid scientific disputes about ‘the nature of the chemical bond’.

 

 

16:30 - 17:00
1.4-I2
Ibáñez, Maria
IST Austria
High-Performance Thermoelectric Nanocomposites from Nanocrystal Building Blocks
Maria Ibáñez
IST Austria, AT

Maria Ibáñez was born in La Sénia (Spain). She graduated in physics at the University of Barcelona, where she also obtained her PhD in 2013, under the supervision of Prof. Dr. Cabot and Prof. Dr. Morante. Her PhD thesis was qualified Excellent Cum Laude and awarded with the Honors Doctorate by the University of Barcelona. Her PhD research was funded by a Spanish competitive grant (FPU) which supported her to conduct short-term research stays in cutting-edge laboratories. In particular she worked at CEA Grenoble (2009), the University of Chicago (2010), the California Institute of Technology (2011), the Cornell University (2012) and the Northwestern University (2013). In 2014, she joined the group of Prof. Dr. Kovalenko at ETH Zürich and EMPA as a research fellow where in 2017 she received the Ružička Prize. In September 2018 she became an Assistant Professor (tenure-track) at IST Austria and started the Functional Nanomaterials group.

Authors
Maria Ibáñez a
Affiliations
a, IST Austria, Am Campus 1, Klosterneuburg, 3400, AT
Abstract

The conversion of thermal energy to electricity and vice versa by means of solid state thermoelectric devices is appealing for a large number of applications. However, its cost-effectiveness is hindered by the relatively high production cost and low efficiency of thermoelectric devices and more specifically of current thermoelectric materials. To overcome present challenges and being able to deploy thermoelectric technology in its wide market range, novel and complex materials with significantly improved performance need to be designed (identified) and engineered (optimized). Current conventional thermoelectric material production technologies are based on purely inorganic compound semiconductors produced by solid-state synthesis of ingots. This synthetic method lacks the level of control required, and alternative strategies need to be developed with superb control over structural and chemical parameters at multiple length scales. One such alternative technologies is the bottom-up engineering of materials by solution processing approaches. Composite materials with precisely tuned electronic properties can be produced by the assembly and consolidation of colloidal nanocrystals.[1] This methodology is facile, scalable, potentially low cost, and extremely versatile. Beyond the synthetic control over crystal domain size, shape, crystal phase, and composition, solution-processed nanocrystals permit exquisite surface engineering. Herein, we present three different strategies that allow to optimize the transport properties at different material production level (TOC Graphic): i) nanoparticle synthesis;[2] ii) nanoparticle surface tuning;[3], [4] and iii) nanoparticle consolidation.[5] These results demonstrate the unique possibilities of the nanocrystal bottom-assembly to produce high performance nanocomposites.

PERInt 1.4
Chair: Pablo P. Boix
16:00 - 16:30
1.4-O1
Mayer, Thomas
Technische Universität Darmstadt
Tapered Cross Section Photo Electron Spectroscopy of a State of the Art Mixed Ion Perovskite Solar Cell: Band Bending Profile in the Dark and Photo-Potential Profile Under Open Circuit Illumination
Thomas Mayer
Technische Universität Darmstadt, DE
Authors
Thomas Mayer a, Michael Wussler a, Cittaranjan Das b, Iwan Zimmermann c, Mohammad Khaja Nazeeruddin c, Wolfram Jaegermann a
Affiliations
a, Institute of Material Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany
b, Light Technology Institute, Karlsruhe Institute of Technology, Germany
c, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Lausanne, CH
Abstract

We use photoemission line scans on small angle tapered cross sections of a wet chemically produced MA0.85FA0.15PbI0.85Br0.15 solar cell of 18% efficiency to reveal the potential distributions across the full device in the dark and operating under open circuit illumination. We find the perovskite absorber to be n-type and the hole extraction contact of p-type spiro-MeOTAD with Au on top photoactive, while the electron extraction contact of meso-porous n-type TiO2 on TiO2 hole blocking layer shows little activity. In addition the perovskite layer formed from a single precursor solution shows self organized depth variations in the chemical analysis. In particular, the bromide concentration is increased while iodide is reduced in front of and within the meso-porous TiO2 layer.  The approach of photoemission analysis on tapered cross sections is generally applicable to different types of microelectronic thin film devices allowing for detailed electronic and chemical analysis under working condition as demonstrated here for operation under open circuit illumination of a perovskite solar cell.

The spectroscopic elementspecific information is used to identify the respective device layers. We found that the absorber is n-type and the device shows a n-n-p structure, in contrast to the generally assumed n-i-p structure. Our measurements reveal that the photovoltage mainly develops at the back contact within the spiro-MeOTAD layer, where in the dark band bending was strongest.

16:30 - 17:00
1.4-O2
Masi, Sofia
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
Interaction between Perovskite and PbS QDs towards an Improved Material
Sofia Masi
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES
Authors
Sofia Masi a, Salim K. M. Muhammed a, Iván Mora-Seró a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

Solution-processed semiconductors, perovskite bulk materials and inorganic quantum dots, are the most competitive materials for the future photovoltaics. Their outstanding properties in terms of photoconversion efficiency (PCE) led to a big progress, reaching an impressive PCE of 24%.[1] The soft nature of perovskite allows the easy combination with other additives that can provide enhanced properties. Different class of additives are employed to control the morphological and optoelectronic properties and a lot of efforts are dedicated in the understanding the chemical interactions and the physical process involved in the stabilization of the perovskite materials.[2] Here, the use of PbS quantum dots (QDs) as additive embedded in the perovskite matrix is presented, aiming to highlight the effect of the inorganic and semiconductor additives in the final properties of the perovskite. In details, the interaction of the perovskite and PbS QDs with different sizes, and in function of the concentration is careful analyzed. The introduction of the PbS QDs of different sizes or the different amount loaded in the perovskite matrix are found to be crucial in influencing the structural and the photo-physical properties of the final nanocomposite and improving the device performances.

RaDet 1.4
Chair: Pablo P. Boix
16:00 - 16:30
Abstract not programmed
Sol2D 1.4
Chair: Efrat Lifshitz
16:00 - 16:30
1.4-I1
Marino, Eduardo
Universidade Federal do Rio de Janeiro
The Exciton Spectrum in Transition-Metal Dichalcogenides: a Quantum-Electrodynamics Approach
Eduardo Marino
Universidade Federal do Rio de Janeiro, BR

Eduardo C Marino graduated as a Bachelor in Physics in 1975. He has got a MSc and a PhD degrees, respectively in 1978 and 1980, both in Quantum Field Theory (QFT). He was a Post-Doctoral Fellow at Harvard University, from 1981 to 1983. By this time he became interested in applicatiions of QFT in Condensed Matter Physics (CMP) and, subsequently, introduced this new area of research in Brazil. He was a visiting Professor at Princeton Universty from 1991 to 1993 and again from 2007 to 2008. He was awarded the State Academic Prize for students graduating in the year of 1975 with the 10 best academic records in all areas of knowledge. In 2000 he became an elected member of the Brazilian National Academy of Sciences. In 2005 he was awarded the National Order of Scientific Merit by the President of Brazil. He has been invited to deliver talks in International Conferences, as well as colloquia and seminars in more than 15 countries. He is Profesor of Physics at the Federal University of Rio de Janeiro, since 1994 and his research interests are still in applications of QFT to CMP, more specifically in graphene, Transition Metal Dichalcogenides, High-Tc Superconductivity, Topological Insulators, Weyl semi-metals, Topological Quantum Computation, Topological effects in CMP, among other subjects.

Authors
Eduardo Marino a, Leandro O. Nascimento a, Van Sérgio Alves a, N. Menezes a, C. Morais Smith a
Affiliations
a, Institute of Physics, UFRJ, Cx.P. 68528, Rio de Janeiro, Brazil
Abstract

Manipulation of intrinsic electron degrees of freedom, such as charge and spin, gives rise to electronics and spintronics, respectively. Electrons in monolayer materials with a honeycomb lattice structure, such as the Transition-Metal Dichalcogenides (TMD’s), can be classified according to the region (valley) of the Brillouin zone to which they belong. Valleytronics, the manipulation of this electron’s property, is expected to set up a new era in the realm of electronic devices. In this work, we accurately determine the energy spectrum and lifetimes of exciton (electron-hole) bound-states for different TMD materials, namely WSe2, WS2 and MoS2. For all of them, we obtain a splitting of the order of 170 meV between the exciton energies from different valleys, corresponding to an effective Zeeman magnetic field of 1400 T. Our approach, which employs quantum-field theory (QFT) techniques based on the Bethe-Salpeter equation and the Schwinger-Dyson formalism, takes into account the full electromagnetic interaction among the electrons. The valley selection mechanism operates through the dynamical breakdown of the time-reversal (TR) symmetry, which originally interconnects the two valleys. This symmetry is spontaneously broken whenever the full electromagnetic interaction vertex is used to probe the response of the system to an external field.[1]

References

[1] E. C. Marino, Leandro O. Nascimento, Van Sérgio Alves, N. Menezes, C. Morais Smith Quantum Electrodynamical Approach to the Exciton Spectrum in Transition Metal Dichalcogenides, 2D Materials 5 (2018) 041006

[2] A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. Li, O. B. Aslan, D. R. Reichman, M. S. Hybertsen, & T. F. Heinz, Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2, Phys. Rev. Lett. 113, (2014). 076802

[3] K. He, N. Kumar, L. Zhao, Z. Wang, K. F. Mak, H. Zhao, & J. Shan, Tightly Bound Excitons in Monolayer WSe2, Phys. Rev. Lett. 113, (2014) 026803

16:30 - 16:45
1.4-O1
Climente, Juan Ignacio
University Jaume I, Spain
Signatures of Molecular Coupling between Semiconductor Colloidal Nanoplatelets
Juan Ignacio Climente
University Jaume I, Spain, ES
Authors
Juan Ignacio Climente a, José Luis Movilla a, Josep Planelles a
Affiliations
a, University Jaume I, Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

Semiconductor nanoplatelets are the colloidal analogous of epitaxial quantum wells. However, molecular coupling between epitaxial quantum wells --which is mediated by quantum tunneling across inter-well barriers and has been key to developing superlattices- has not been clearly demonstrated with platelets.

It is generally acknowledged that the organic ligands passivating the surfaces creates a high potential barrier preventing tunneling of carriers. Yet, recent experiments with CdSe nanoplatelets suggest that stacking of nanoplatelets gives rise to inter-platelet excitonic species revealed in the emission spectrum.[1]

In this presentation, we show theoretically that molecular coupling between colloidal semiconductor nanoplatelets with face-to-face orientation is indeed feasible. Unlike in epitaxial wells, the coupling is not mediated by mechanical tunneling but by Coulomb interaction in a dielectrically inhomogeneous environment. This a weaker, yet longer ranged interaction.

We study the case of two, three, four and five coupled nanoplatelets. The absorption spectra show distinct features and clear trends, with an unusual electronic structure resulting from the high symmetry of the system along the coupling direction. We predict experimental signatures which can be used to confirm the synthesis of molecular species. These point toward the formation of minibands with increasing number of platelets. 

Our theoretical predictions not only set the ground for novel molecular physics behavior -mediated by dielectric confinement- but also hint that colloidal superlattices with coherent carrier transport, reminiscent of epitaxial quantum well superlattices, may be achievable.

 

 

16:45 - 17:00
1.4-O2
Tornatzky, Hans
Friedrich Alexander University Erlangen-Nuremberg
Phonon Dispersion in MoS2 by Inelastic X-ray Scattering
Hans Tornatzky
Friedrich Alexander University Erlangen-Nuremberg, DE
Authors
Hans Tornatzky a, b, Roland Gillen b, Hiroshi Uchiyama c, Janina Maultzsch b
Affiliations
a, Technische Universität Berlin, Straße des 17. Juni, 124, Berlin, DE
b, Department Physik, Friedrich-Alexander Universität Erlangen Nürnberg, 91058 Erlangen, Germany
c, Super Photon Ring 8-GeV (SPring8), JASRI, Hyogo 679-5198, Japan
Abstract

We present the first experimental, full basal plane phonon dispersion, determined by inelastic X-Ray scattering with accompanying van-der-Waals corrected DFT-D3 simulations [1]. The implementation of the D3 vdW-correction in DFT, allows the simulation of both the dispersion and structural properties, not given in commonly used LDA / PBE calculations. From our calculations, we show the displacement patterns of phonons at the K and M points, allowing further considerations regarding, e.g., scattering selection rules.

 

[1]: H. Tornatzky, Roland Gillen, Hiroshi Uchiyama and Janina Maultzsch, Phys. Rev. B, 99, 144309 (2019)

SolFuel 1.4
Chair: Marc Robert
16:00 - 16:15
1.4-O1
Yao, Liang
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland
Catalyst-Free Synthesis of Robust Covalent Polymer Network Semiconducting Films and Application in Photoelectrochemical Water Splitting
Liang Yao
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, CH
Authors
Liang Yao a, Kevin Sivula a
Affiliations
a, Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, Lausanne, CH
Abstract

Photoelectrochemical (PEC) water splitting is an enticing approach to directly convert solar energy into a chemical commodity like hydrogen paving the way for a sustainable carbon-neutral society. So far, discovering efficient, robust and cheap materials to drive solar water splitting stands out as the main challenge in this field.[1] Organic semiconductors (OSs) are based on earth abundant elements, have a tunable optoelectronic properties, and achieved an excellent solar to electricity conversion efficiency. Therefore, a rapidly growing interest has been attracted to employ OSs in PEC water splitting in recent years.[2] Until now, most of the research attention is focused on the development of OS based photocathodes, while the development of OS photoanodes[3] is lacking despite their necessity in PEC tandem devices.

Covalent polymer networks (CPNs) offer a superior robustness among various types of OSs, thus providing a route to satisfy the harsh operation requirements of photoanodes. However, the application of CPNs in optoelectronic devices is severely limited by their poor solubility and processability. Here, an in-situ CPN film preparation approach (based on thermal azide alkyne cycloaddition solid state reaction) is reported to overcome the challenge of CPN film fabrication.[4] The optoelectronic properties of the CPN films are investigated in photovoltaic devices. As a proof of concept for the application in PEC water splitting, n-type CPN semiconducting films with remarkable robustness are used for photoactive materials in photoanodes for oxygen evolution reaction (OER). After optimizing the interfacial layers and OER catalyst, a water oxidation photocurrent with the stability among the best of OS photoanodes is achieved. An outlook on the application of these CPN films toward practical solar fuel generation is finally given.

16:15 - 16:30
1.4-O2
Emmler, Thomas
Helmholtz-Zentrum Geesthacht
Kinetic Aerosol Spray Deposition of BiVO4 Powder for OER Photoelectrodes
Thomas Emmler
Helmholtz-Zentrum Geesthacht
Authors
Thomas Emmler a, Charline Wolpert b, Mauricio Schieda a, Maria T. Villa Vidaller b, Stefen Fengler a, Jun Akedo c, Kentaro Shinoda c, Thomas Klassen a, b
Affiliations
a, Helmholtz-Zentrum Geesthacht, Dep. Sustainable Energy Technology, Institute of Materials Research, Max-Planck-Str. 1, Geesthacht, DE
b, Helmut-Schmidt-University, Functional Materials, Holstenhofweg, 85, Hamburg, DE
c, National Institute of Advanced Industrial Science and Technology (AIST), Advanced Coating Technology Research Center, 305-8565, Japón, Tsukuba, JP
Abstract

 

Hydrogen will play a major role in the necessary transition towards a decarbonized energy economy. Solar water splitting is a renewable hydrogen generation pathway, but its implementation at large scales requires the development of cost efficient electrode fabrication methods.

 

In recent years, we have explored cold-gas-spray deposition of semiconductor particles as a solvent-free, scalable process for the manufacture of water splitting photoelectrodes. While this method works well with a small set of light-absorbing materials, including WO3 or TiO2, it is difficult to implement for semiconductors such as Fe2O3 or BiVO4, for which unfavorable charge carrier dynamics pose severe limitations on the coating thickness.

 

To overcome this problem, we are developing aerosol cold gas spraying (aerosol deposition method) as coating technology. This vacuum-based deposition method enables the use of sub-micrometer particles, resulting in thin semiconductor layers, while expanding the range of applicable supports to include fragile substrates such as FTO-coated glass. Initial test coatings with undoped BiVO4 have produced electrodes with photocurrent densities of up to 2mA/cm2.

 

In this presentation, we describe the experimental setup and the parameter selection process for the FTO/BiVO4 system, and we furthermore discuss the relationship between experimental spraying parameters and the thickness, morphology, and physical and electrochemical properties of the resulting BiVO4 layers.

  

16:30 - 16:45
1.4-O3
Khorashadizade, Elham
Sharif University of Technology
Black Ru-doped TiO2 Nanotubes as Efficient Photoanode in Photoelectrochemical Water Splitting
Elham Khorashadizade
Sharif University of Technology

 I am a PhD student of condensed matter physics at Sharif University of Technology. I was in Patrik Schmuki's group as a visiting research student for nine months last year.

Authors
Elham Khorashadizade a, Shiva Mohajernia b, Seyedsina Hejazi c, Naimeh Naseri d, Omran Moradlou e, Patrik Schmuki f, Alireza Z. Moshfegh g
Affiliations
a, Department of Materials Science, WWIV-LKO, University of Erlangen-Nuremberg, Erlangen, Germany, Department of Physics, Sharif University of Technology, Azadi Avenue, Tehran, Iran
b, Department of Materials Science, WWIV-LKO, University of Erlangen-Nuremberg, Erlangen, Germany
c, Department of Materials Science, WWIV-LKO, University of Erlangen-Nuremberg, Erlangen, Germany
d, Department of Physics, Sharif University of Technology, Azadi Avenue, Tehran, Iran
e, Department of Chemistry, Alzahra University, Vanak Village Street, Tehran, Iran
f, Department of Materials Science, WWIV-LKO, University of Erlangen-Nuremberg, Erlangen, Germany
g, Department of Physics, Sharif University of Technology, Azadi Avenue, Tehran, Iran, Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Azadi Avenue, Tehran, Iran
Abstract

     Hydrogen (H2) production via solar water splitting is one of the most ideal strategies for providing sustainable fuel because the process requires only water and sunlight. In achieving high-yield production of hydrogen as a recyclable energy carrier, the nanoscale design of semiconductor materials plays a pivotal role in both photoelectrochemical (PEC) and photocatalytic (PC) water splitting reactions [1].

     TiO2 due to its chemical stability, low cost and suitable band position for H2 and O2 generation from water, is a promising material for application in PEC cells [2, 3]. However, wide band gap (3.0-3.2 eV) and fast electron–hole recombination are main factors limiting the efficiency using pure TiO2 as a water splitting photoanodes. To overcome these barriers, efforts have been carried out to enhance photoconversion efficiency by band gap engineering and to facilitate charge transfer by increasing the surface area are important issues in practical use and commercialization [4].

     In the present research, we demonstrate a significant improvement of the photoelectrochemical water splitting activity of anodic black Ru-doped TiO2 nanotube layers prepared by anodization on Ti-Ru alloys containing 0.2 at % Ru. To do this, first, the anodization was carried out using a high-voltage potentiostat instrument in a two-electrode configuration with a platinum foil as the counter electrode. Then, the deposited Ru-doped TiO2 nanotubes were reduced under Ar/H2 (90/10 vol%) atmosphere at temperature of 550℃. The resulting black Ru-doped TiO2 photoelectrodes show three-fold improvement in water splitting activity as compared to an undoped black TiO2 nanotubes samples which can be attributed to synergic effect of Ru doping followed by hydrogenation process leading improved conductivity.

16:45 - 17:00
1.4-O4
Naseri, Naimeh
Physics Department, Sharif University of Technology
Solar Water Splitting on Ti-doped Hematite Nanostructured Photoanode Modified with FeOOH Electro-catalysts
Naimeh Naseri
Physics Department, Sharif University of Technology, IR
Authors
Shima Farhoosh a, Behrooz Eftekharinia b, Naimeh Naseri a
Affiliations
a, Department of Physics, Sharif University of Technology, Azadi Avenue, Tehran, Iran
b, School of Physics, Institute for Research in Fundamental Sciences (IPM), Tehran, Iran
Abstract

Depletion of oil and gas as hydrocarbon reservoirs and adverse effects of their combustion necessitate the use of renewable and environmentally friendly resources of energy for human societies [1,2]. Photo-assisted electrochemical water splitting into O2 and H2 is the most promising solution to produce solar hydrogen as a green and sustainable energy carrier [3,4]. To achieve the highest efficiency in this approach, it is vital to design proper photoanode in which high conductivity, enough hole diffusion length and active surface with a reasonable response in visible range could be provided [5].

Here, nanostructured hematite photoanodes have been synthesized in a simple hydrothermal approach and modified with FeOOH as an electrocatalyst for the water oxidation reaction. To facilitate hole transfer at the surface and prohibit charge recombination, photoanodes were doped with Ti while the molar concentration of titanium in the starting solution changed as 0.05, 0.1 and 5% of iron. Using UV-visible spectroscopy, all photoanodes obtained an optical band gap of 2.0 eV in agreement with the expected value for iron oxide. X-ray diffractometry results also revealed the formation of hematite phase of iron oxide crystalline structure with obtained 32.6 nm as an average size for nanocrystals. Field effect scanning electron microscopy illustrated the formation of multi-scale roughness on the surface with a branch like features. Photo-electrochemical measurements obtained the highest photocurrent of 0.52 mA.cm-2 for the photoanode containing 5% Ti modified with FeOOH electrocatalyst, which provided the least charge transfer resistance of 3 kΩ.cm2.

 
Tue Nov 05 2019
Plenary Session 3
Chair: Marcus Scheele
08:30 - 09:00
3-K1
Tisdale, William
Massachusetts Institute of Technology
Nonequilibrium Dynamics of Excitons and Charges in Semiconductor Nanomaterials
William Tisdale
Massachusetts Institute of Technology

Will Tisdale joined the Department of Chemical Engineering at MIT in January, 2012, where he holds the rank of Associate Professor and is currently the ARCO Career Development Professor in Energy Studies.  He earned his B.S. in Chemical Engineering from the University of Delaware in 2005, his Ph.D. in Chemical Engineering from the University of Minnesota in 2010, and was a postdoc in the Research Laboratory of Electronics at MIT before joining the faculty in 2012. Will is a recipient of the Presidential Early Career Award for Scientists and Engineers (PECASE), the DOE Early Career Award, the NSF CAREER Award, an Alfred P. Sloan Fellowship, the Camille Dreyfus Teacher-Scholar Award, the AIChE Nanoscale Science & Engineering Forum Young Investigator Award, and MIT’s Everett Moore Baker Award for Excellence in Undergraduate Teaching.

Authors
William Tisdale a
Affiliations
a, Massachusetts Institute Of Technology, 77 Massachusetts Avenue, Room 2-216, Cambridge, 2139
Abstract

Structure, surface chemistry, and energetic disorder can dramatically affect excited state dynamics in low-dimensional systems. Using a combination of ultrafast laser spectroscopy, time-resolved optical microscopy, and kinetic modeling, I will show how these effects manifest in assemblies of colloidal quantum dots (QD) and atomically thin 2D semiconductors, which are promising components of next-generation photovoltaic and lighting technologies. In particular, I will demonstrate the counterintuitive role of entropy in the nonequilibrium population dynamics of excitons and charge carriers in nanoscale systems.

In semiconductors, increasing mobility with decreasing temperature is a signature of charge carrier transport through delocalized bands. Here, we show that this behavior can also occur in nanocrystal solids due to temperature-dependent structural transformations. Using a combination of broadband infrared transient absorption spectroscopy and numerical modeling, we investigate the temperature-dependent charge transport properties of well-ordered PbS quantum dot (QD) solids. Contrary to expectations, we observe that the QD-to-QD charge tunneling rate increases with decreasing temperature, while simultaneously exhibiting thermally activated nearest-neighbor hopping behavior. Using synchrotron grazing-incidence small-angle X-ray scattering (GISAXS), we show that this trend is driven by a temperature-dependent reduction in nearest-neighbor separation that is quantitatively consistent with the measured tunneling rate.[1]

Plenary Session 4
Chair: Pablo P. Boix
08:30 - 09:00
4-K1
Bisquert, Juan
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
Understanding Time Scales of Ionic and Electronic Phenomena in Perovskite Solar Cells
Juan Bisquert
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES

Juan Bisquert (pHD Universitat de València, 1991) is a Professor of applied physics at Universitat Jaume I de Castelló, Spain. He is the director of the Institute of Advanced Materials at UJI. He authored 360 peer reviewed papers, and a series of books including Nanostructured Energy Devices (1. Equilibrium Concepts and Kinetics, 2. Foundations of Carrier Transport) and 3. Physics of Solar Cells: Perovskites, Organics, and Photovoltaics Fundamentals (CRC Press).  His h-index 82, and is currently a Senior Editor of the Journal of Physical Chemistry Letters. He conducts experimental and theoretical research on materials and devices for production and storage of clean energies. His main topics of interest are materials and processes in perovskite solar cells and solar fuel production. He has developed the application of measurement techniques and physical modeling of nanostructured energy devices, that relate the device operation with the elementary steps that take place at the nanoscale dimension: charge transfer, carrier transport, chemical reaction, etc., especially in the field of impedance spectroscopy, as well as general device models. He has been distinguished in the 2014-2017 list of ISI Highly Cited Researchers.

 

Authors
Juan Bisquert a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

We suggest a classification of the time scales for ionic/electronic phenomena in the perovskite solar cells, based on the results of analysis of kinetic phenomena using frequency modulated techniques and time transient techniques. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases. The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We also show the identification of the impedance of ionic diffusion by measuring single crystal samples. The attachment of ions to the electrode surface appears as the major factor responsible for hysteresis in perovskite solar cells. Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in defects of the structure. We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The combination of IMPS and Impedance Spectroscopy is able to provide a detailed picture that explains low frequency characteristics, influencing the fill factor of the solar cell.

CharDy 2.1
Chair: Maksym Yarema
09:00 - 09:30
2.1-I1
Cahen, David
Weizmann Institute and Bar-Ilan University
Proteins can be “Good” Electronic Conductors !
David Cahen
Weizmann Institute and Bar-Ilan University, IL

Born in the Netherlands,David Cahen studied chemistry & physics at the Hebrew Univ. of Jerusalem (HUJ), Materials Research and Phys. Chem. at Northwestern Univ, and biophysics of photosynthesis (postdoc) at HUJ and the Weizmann Institute of Science, WIS. After joining the WIS faculty he focused on alternative sustainable energy resources, in particular various types of solar cells. In parallel he researches hybrid molecular/non-molecular systems, focusing on understanding and controlling electronic transport across (bio)molecules. He is a fellow of the AVS and the MRS. He heads WIS' Alternative, sustainable energy research initiative.

Authors
David Cahen a
Affiliations
a, Weizmann Institute and Bar-Ilan University, IL
Abstract

Solid state Electron Transport (ETp), electronic conduction, across junctions with an ultra-thin protein film as active layer, can be surprisingly efficient. Length-normalized, their ETp efficiency can be similar or even exceed that of conjugated molecules; on top of that, it is temperature-independent down to 4 K. That is amazing as nature does not seems to need these features at RT, where electron transfer, ET, involving proteins in solution and/or membranes is a central, ion transport-coupled process.  If contacts do not limit ETp which is a challenge in itself to measure and achieve, i.e., transport across the proteins dominates, then we cannot measure a transport barrier. For small proteins we have now  good evidence for tunnelling as the mechanisms, also as result  of fruitful collaborations with computational theory experts. However, behaviour of larger proteins remains a puzzle, which is still unsolved.  I will show experimental data1,2, incl. recent ones (ours and others), which should at least help to define this puzzle.   Note that understanding ETp may be important also for ET, in which coupling to the contacts is replaced by electron injection/extraction.

 

* work done with Mordechai Sheves & Israel Pecht, at the Weizmann Inst. of Science, Rehovot, Israel

  (DC is also at Bar-Ilan Univ., Ramat Gan, Israel).

  1. C. Bostick et al.  Rep. Prog. Phys., 81 (2018) 026601
  2. N. Amdursky et al., Adv. Mater. 42, (2014) 7142
09:30 - 10:00
2.1-I2
Luisier, Mathieu
ETH Zurich, Department of Information Technology and Electrical Engineering
Carrier Transport in 2-D Materials: an Ab Initio Study
Mathieu Luisier
ETH Zurich, Department of Information Technology and Electrical Engineering
Authors
Mathieu Luisier a, Aron Szabo a, Cedric Klinkert a, Christian Stieger a, Martin Rau a, Tarun Agarwal a, Youseung Lee a
Affiliations
a, Swiss Federal Institute of Technology (ETH) Zurich, CH
Abstract

Two-dimensional (2-D) materials beyond graphene, e.g. MoS2, MoSe2, WS2, as well as other transition metal dichalcogenides (TMDs), are attracting a lot of attention from the scientific community due to their excellent transport properties given their sub-nanometer thickness and their possible application as future logic switches at the end of Moore's scaling law. Despite these promising features, 2-D materials are still far from reaching their optimal performance, mainly because of the quality of the underlying crystals and the difficulty to fabricate components with low contact resistances. To address these issues, shed light on the intrinsic potential of 2-D compounds, and support the on-going experimental activity, device simulation can be of great help, provided that tools capturing the physics at play are available. First-principles quantum transport approaches lend themselves perfectly to these tasks as they account for all necessary quantum mechanical, bandstructure, and atomistic effects present in 2-D materials. In this presentation, we will introduce such a state-of-the-art device simulator that relies on density-functional theory (DFT), maximally localized Wannier functions, and the Non-equilibrium Green's Function (NEGF) formalism [1]. It will be applied to the calculation of the carrier mobility and "current vs. voltage" characteristics of conventional TMDs and to the investigation of more exotic 2-D materials whose existance has been recently theoretically postulated [2]. Finally, the origin of the high contact resistance observed in single-layer MoS2 with a Titanium electrode on top of it will be discussed, highlighting the mechanisms responsible for the transfer of carriers from metals into monolayer structures [3]. 

10:00 - 10:15
2.1-O1
Pandya, Raj
Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK.
Ultrafast Long-Range Energy Transport via Strong Light-Matter Coupling in Organic Semiconductor Films
Raj Pandya
Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., GB
Authors
Raj Pandya a, Akshay Rao a
Affiliations
a, Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., J.J. Thomson Avenue, Cambridge, GB
Abstract

The efficient transport of energy in the form of spin-singlet excitons lies at the heart of natural light harvesting for photosynthesis and optoelectronic devices based on synthetic organic semiconductors. For optoelectronic applications, energy transport over long length scales would be highly desirable, but most organic semiconductors exhibit singlet exciton diffusion lengths between 5 – 50 nm [1], with only a handful of systems being suggested to exceed this [2]. This is because the primary mechanism for exciton transport is Förster resonance energy transfer (FRET), which involves site-to-site hopping within a disordered energy landscape, leading to short diffusion lengths.  While there have been reports of higher diffusion lengths in highly ordered one-dimensional organic nanowires, these materials have proved challenging to integrate into optoelectronic devices. Hence, what is required is a new methodology that allows for long range energy transport within thin films of organic semiconductors, which form the basis of current optoelectronic devices.  Here, we show that long-range and ultrafast transport of energy can be achieved at room temperature in a range of chemically diverse organic semiconductor thin films through strong light-matter coupling to form exciton-polaritons. These effects occur despite the absence of an external cavity, metallic or plasmonic structures. We directly visualize energy transport via femtosecond transient absorption microscopy with sub-10 fs temporal and sub-10 nm spatial precision and find energy transport lengths of up to ~270 nm at effective velocities of up to ~5 ×106 m s-1. Evidence of strong light-matter coupling in the films is provided via peak splittings in the reflectivity spectra, measurement of the polariton dispersion and emission from collective polariton states. The formation of these exciton-polaritons states in organic semiconductor thin-films is a general phenomenon, independent of underlying materials chemistry, with the principal requirements being a high oscillator strength per unit volume and low disorder. These results and design rules will enable a new generation of organic optoelectronic and light harvesting devices based on robust cavity-free exciton-polaritons [3].

10:15 - 10:30
2.1-O2
Sandberg, Oskar J.
Swansea University, Department of Physics, Swansea, United Kingdom.
A Theoretical Perspective on Transient Photovoltage and Charge Extraction Techniques
Oskar J. Sandberg
Swansea University, Department of Physics, Swansea, United Kingdom.
Authors
Oskar Sandberg a, Kristofer Tvingstedt b, Paul Meredith a, Ardalan Armin a
Affiliations
a, Swansea University, Department of Physics, Swansea, United Kingdom.
b, Experimental Physics VI, Julius Maximillian University of Würzburg, 97074 Würzburg, Germany
Abstract

The transient photovoltage (TPV) and charge extraction (CE) techniques have been frequently used in the past to determine charge carrier lifetime and density, respectively, in thin-film solar cells (such as organic and perovskite solar cells), allowing for the underlying charge carrier recombination properties in these systems to be clarified [1]. In the ideal case, the lifetime obtained from TPV, using a small perturbation of the carrier density at open-circuit, reflects the bulk recombination of photo-generated charge carriers in the active layer. In organic solar cells, this lifetime is generally strongly dependent on the steady-state light intensity (i.e. the open-circuit voltage). A simultaneous determination of the corresponding charge carrier density with CE at different steady-state light intensities (open-circuit voltages) then allows for the recombination rate and recombination order of the charge carriers to be obtained. Recently, however, the relevance of both the TPV lifetime and the associated recombination order, obtained in combination with CE, as material or device figure of merits for the understanding of the recombination in solar cells and other photoactive material systems has been questioned [2].

In this work, we review and expand the underlying theory of TPV and CE [3]. Based on fundamental electrical transient theory, we derive expressions for the associated TPV lifetime and CE carrier density, allowing for their physical meaning in thin-film solar cells to be assessed. The derived theoretical framework is verified by numerical transient drift-diffusion simulations and demonstrative experiments on organic solar cells. Based on the theoretical findings, different regimes for the extracted lifetime and charge carrier density in TPV and CE, respectively, can be distinguished. At low steady-state light intensities, the determination of the carrier lifetime and the carrier density is strongly distorted by capacitive effects associated with non-uniform carrier profiles. On the other hand, at high light intensities, the CE carrier density is susceptible to incomplete charge extraction. In order to identify the “working dynamic range” for the accurate determination of both the carrier lifetimes and the extracted carrier densities, TPV and CE measurements over a wide range of steady-state light intensities are required.

MapNan 2.1
Chair: Liam Collins
09:00 - 09:30
2.1-I1
Fumagalli, Laura
Manchester University, UK
Probing the Dielectric Constant on the Nanoscale: from Thin Films to DNA and Confined Water
Laura Fumagalli
Manchester University, UK, GB
Authors
LAURA FUMAGALLI b
Affiliations
a, Manchester University, UK, GB
b, National Graphene Institute, Manchester, UK
Abstract

Electric polarizability (or dielectric constant) is a fundamental physical property that plays a crucial role in a variety of phenomena and disciplines, from physics and materials science to chemistry and molecular biology. It is inherently linked to charge storage and transport in energy and electronic devices. It modulates various forces (Coulomb, van der Waals, solvation and hydration) between macromolecules and, therefore, is crucial in macromolecular assembly and interactions. For many decades, dielectric spectroscopy has been one of the main methods used for sample characterization. Yet, despite a massive amount of literature, the dielectric polarization properties at molecular level have essentially remained unknown for great difficulties in measuring an electric response on such a small scale.

In ths talk, first I will review our work in which we developed novel scanning probe approaches that probe the dielectric constant on the nanoscale based on current-sensing [1,2] and electrostatic-force sensing [3]. We determined the local dielectric constants of a variety of nanostructures and biological samples for the first time, from thin oxides and biological membranes to single nanoparticles and bacteria [1-5]. We demonstrated that measurements can be done in electrolytic environment [5]. Noteworthy, we resolved the dielectric constant of DNA [5], a critical parameter for understanding DNA-protein interactions. 

In the second part of my talk, I will present our recent work in which we succeeded to probe the dielectric constant of water confined between atomically thin crystals [6]. For many decades, it has been speculated that the dielectric constant of water near surfaces should be different from that of bulk water. Our experiments revealed the presence of an interfacial layer with vanishingly small polarization. The electrically dead layer was found to be two to three molecules thick, in agreement with the thickness predicted by molecular dynamics calculations. Our results provide much needed feedback for theories describing water-mediated surface interactions and behaviour of interfacial water.

 

09:30 - 10:00
2.1-O1
Borgani, Riccardo
KTH Royal Institute of Technology
Multifrequency AFM Methods for Electrical Characterization at the Nanoscale
Riccardo Borgani
KTH Royal Institute of Technology
Authors
Riccardo Borgani a, David Haviland a
Affiliations
a, Nanostructure Physics, KTH Royal Institute of Technology, Stockholm, Sweden
Abstract

We give an overview of advanced multifrequency techniques for atomic force microscopy (AFM). These techniques exploit the nonlinear tip-surface interaction to perform electrical measurements with high speed and low noise.

Intermodulation electrostatic force microscopy (ImEFM) [1] is an open-loop alternative to Kelvin-probe force microscopy (KPFM), where the surface potential of the surface is obtained from a measurement of four force components within the cantilever resonance frequency. This single-pass technique combines the low noise of AM-KPFM with the high spatial resolution of FM-KPFM, while maintaining the speed and ease of use of an open-loop technique. We have recently developed a time-resolved variant of ImEFM [2], where the intermodulation of the cantilever drive and a series of excitation pulses on the sample produce a force at multiple frequencies around resonance. By measuring these force components we demonstrate the reconstruction of dynamic processes in the material with time resolution of 30 nanoseconds.

Intermodulation conductive AFM (ImCFM) [3] is a technique to acquire the full current-voltage characteristic (IVC) at every pixel of an AFM image. The AFM is operated in contact mode with an AC bias applied to the sample, and the current flowing through the tip is measured. A compensation voltage is used to cancel the effect of the parasitic capacitance arising from the measurement setup, while the frequency-domain analysis allows for the complete separation of the remaining displacement contribution to the current that flows through the tip-sample junction. We demonstrate high-resolution electrical characterization at imaging speeds normal for contact mode, with a speedup of up to four orders of magnitude, compared to the traditional way of slowly ramping the bias at a grid of points. In addition to acquiring the IVC, the technique maps the voltage dependence of the tip-sample capacitance, allowing for the investigation of effects such as quantum capacitance in two-dimensional materials.

We give an introduction to the theoretical and technical foundations of these techniques, and show experimental results on a variety of energy materials.

10:00 - 10:15
2.1-O2
CERRETA, Andrea
Park Systems Europe GmbH
Simultaneous SKPM and Current-Voltage Characterization of Slow Charging Processes in Transistors
Andrea CERRETA
Park Systems Europe GmbH, DE
Authors
Andrea Cerreta a, Florian Stumpf a, Ilka Hermes a, Manfred Madel b, Linh Trinh-Xuan b, Sandra Riedmüller b, Daniel Sommer b, Hervé Blanck b
Affiliations
a, Park Systems Europe GmbH, Janderstraße, 5, Mannheim, DE
b, United Monolithic Semiconductors GmbH
Abstract

Scanning Kelvin Probe Microscopy (SKPM) resolves surface potentials on the nanoscale, which translate into the work function distribution of the sample as well as the distribution of additional charge carriers upon electronic excitation.[1] For quantitative analysis, frequency modulated (FM) SKPM methods have shown the highest accuracy. Heterodyne FM-SKPM in particular allows for fast scanning and exhibits low topographic crosstalk.[2] A quantitative visualization of work function and charge carrier distribution is especially of interest for the semiconductor-based industry (transistors, solar cells, etc.) to locate bottlenecks in device performances.

In this study, we investigated dynamic charging processes on active transistor devices (InAlN/GaN HEMT with LG = 100 nm) via a line-by-line heterodyne SKPM approach to acquire transient potential signals with a time resolution of around one second, while simultaneously recording macroscopic current-voltage (IV) characteristics on the same devices. During switching, some of the transistors exhibited a slow charging over several minutes in their IV response. Via time-resolved SKPM, we located the spatial origin of the slow charging in the gate-drain area of the transistor, where we observed a change in surface potential transients coinciding with the macroscopic IV transients. The observed charging could be caused by slow trapping mechanisms located either at the semiconductor/dielectric interface or the bulk dielectric passivation (PECVD SiOx or SiNx).

10:15 - 10:30
2.1-O3
O'Neill, Katie
Trinity College Dublin
Manipulation of Transition Metal Dichalcogenides: Nanomachining 2D PtSe2 using AFM
Katie O'Neill
Trinity College Dublin, IE
Authors
Katie O'Neill a, Cormac Ó Coileáin a, Jason Kilpatrick b, Max Prechtl c, Niall McEvoy a, Georg S. Duesberg c
Affiliations
a, CRANN/AMBER, School of Physics, Trinity College Dublin, Ireland
b, Adama Innovations Ltd., CRANN, Trinity College Dublin, Ireland
c, Institute of Physics, Universität der Bundeswehr München, Germany
Abstract

For the realisation of 2D-material-based electronic devices, two things in particular are needed; good gate control and low contact resistance. Current issues with realising such devices stem from bulk metal contacts which form poor interfaces with 2D materials leading to high contact resistances. We propose a solution to these issues by way of ‘self-contacting’ 2D-material-based field effect transistors (FETs), resulting in seamless interfaces and low contact resistance.

2D layered materials, such as transition metal dichalcogenides (TMDs), have been heavily studied due to their exciting physical properties and high potential for use in a wide range of future nanoelectronic devices.[1] The band structure of many TMDs changes drastically with thickness. In the case of PtSe2, a little-studied TMD, it goes from semimetallic in bulk to semiconducting in mono and bilayer. [2] This makes PtSe2 a strong candidate for a one-material device, consisting of a thin mono/bilayer channel seamlessly contacted by multilayer regions.

By using novel manipulation techniques, such as nanomachining with an atomic force microscope (AFM), we demonstrate that TMDs can be incrementally machined down, altering their electrical properties. Using Kelvin probe (KPFM) and conductive AFM (C-AFM), the change in surface potential and conductivity can be characterised/monitored. Furthermore, nanomachining of contacted TMD channels is performed while monitoring the device performance with each layer removal down to the monolayer. This paves the way for `self-contacted' devices through the creation of a semiconducting channel via nanomachining, leading to high mobility, low contact resistance and low power. [3]

Figure 1: SEM of nanomachined thermal-assisted conversion (TAC) films of PtSe2

PERInt 2.1
Chair: Juan-Pablo Correa-Baena
09:00 - 09:30
2.1-O1
Khan, Jafar Iqbal
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia
Probing Carrier Extraction from Lead Halide Perovskite to Polymeric Charge Transport Layers by Ultrafast Transient Absorption Spectroscopy
Jafar Iqbal Khan
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, SA
Authors
Jafar Khan a, Esma Ugur a, Erkan Aydin a, Mindaugas Kirkus a, Marios Neophytou a, Stefaan De Wolf a, Iain McCulloch a, Frederic Laquai a
Affiliations
a, King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, 4700 King Abdullah University of Science and Technology (KAUST), Thuwal, SA
Abstract

 

Solution-processed metal-halide perovskite solar cells (PSCs) have received immense attention in the field of photovoltaic research due to their outstanding power conversion efficiency (PCE), which has surpassed 24% in a relative short time. Understanding carrier losses at metal halide perovskite/charge transport layer interfaces is a prerequisite to bring the efficiency closer to the Shockley-Queisser limit. Here, we report the direct observation of hole extraction and carrier recombination dynamics of mixed-cation lead mixed-halide perovskite layers interfacing with a polymeric hole transport layer: PDPP-3T. We employ ultrafast transient absorption spectroscopy and observe the dynamics of the ground state bleach of the polymer, which directly reveals the hole extraction and recombination at the perovskite/polymer interface. The perovskite hole mobility was found to be 3.08 cm2 V-1 s-1. To gain further insight into the hole extraction dynamics, we vary the thickness of the perovskite film. We observe that the hole extraction time is slower with increasing the perovskite thickness following optical excitation from the perovskite side. Mimicking the device architecture via introducing an electron transport layer to the perovskite/PDPP-3T stack resulted in slower carrier recombination dynamics due to decreased charge carrier recombination in the perovskite.  

  

09:30 - 10:00
2.1-O2
Goñi, Alejandro Rodolfo
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC
Equal Footing of Thermal Expansion and Electron-Phonon Interaction in the Temperature Dependence of Lead Halide Perovskite Band Gaps
Alejandro Rodolfo Goñi
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC, ES
Authors
Adrián Francisco-López a, Bethan Charles b, Oliver J. Weber b, M. Isabel Alonso a, Miquel Garriga a, Mariano Campoy-Quiles a, Mark T. Weller b, Alejandro R. Goñi a, c
Affiliations
a, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain
b, Dept. of Chemistry & Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, UK
c, Institució Catalana de Recerca i Estudis Avançats (ICREA), Spain, Passeig Lluis Companys 23, Barcelona, ES
Abstract

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase [1], which speaks against the assumption of negligible thermal expansion effects. Here I will show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling [2]. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts. As an example, I will also present recent results obtained for MA rich FAxMA1-xPbI3 solid solutions, where FA stands for formamidinium.

10:00 - 10:30
2.1-O3
Marquez Prieto, Jose
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Interplay between Composition, Structural Transitions and Optoelectronic Properties in Fully Inorganic CsPbI3 Perovskites
Jose Marquez Prieto
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Jose Marquez Prieto a, Pascal Becker a, Justus Just b, Hannes Hempel a, Chen Li c, d, Charles Hages a, e, Roland Mainz a, Thomas Unold a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, Lund University, Sweden, Kämnärsvägen 10H, Lund, 22645, SE
c, Max Planck Institute for Solid State Research, Stuttgart, Germany
d, EMAT, University Antwerpen, Belgium
e, University of Florida, Gainesville, USA
Abstract

The use of high-throughput experimentation can be of great advantage to explore the compositional-phase landscape of halide perovskites. Here we present a study of a coevaporated CsPbI3 sample with a lateral compositional gradient which varies from Cs-rich conditions to Pb-rich conditions. The composition, crystal structure, grain size, charge carrier mobility, lifetime and photoluminescence external quantum yield (PLQY) were determined by mapping these properties across the samples. Correlation of these results provides complete structure-property relationships and shows that stable high quality g-CsPbI3 can be obtained by Cs-rich low temperature deposition, without need of a high temperature annealing step. 12% efficient solar cells are demonstrated based on these results [1].

 

The phase transitions of CsPbI3 have been further investigated due to the interest of increasing the stability of the solar cells and to allow possible applications of these materials in thermochromic devices [1, 2]. We use several in-situ techniques to reveal the phase transformation relations between the a, b and g  (black) perovskite phases and the d (yellow) non-perovskite polymorph in thin films.  Structural changes were tracked as a function of relative humidity (RH) and temperature by in-situ X-ray diffraction with a liquid-metal jet X-ray source. We find that at room temperature, the transition from the g-black to the d-yellow phase is exclusively triggered by the % of RH whereas O2 is not involved in the mechanism. The use of samples with lateral gradients allowed us to conclude that this transition is slowed down under the Cs-rich regime. Spectrally filtered in-situ optical microscopy reveals that this transformation does not affect the grain structure. An intermediate state is observed, where the optical transmission at 700 nm is significantly reduced during the atomic reordering. The reversed transition from the d (yellow) non-perovskite polymorph to the perovskite-CsPbI3 upon heating happens at the same temperature regardless if the sample is in the Cs-rich or the Pb-rich regime.  This conversion is further explored atomically by in-situ heating using scanning transmission electron microscopy (STEM), revealing the transition from the d- to the a-phase in which a layered structured is observed in an intermediate step.

RadDet 2.1
Chair: Mao-Hua Du
09:00 - 09:30
2.1-I1
Lukosi, Eric
University of Tennessee, Knoxville
Engineering Hybrid Perovskite Materials for Spectroscopic Sensing of Ionizing Radiation
Eric Lukosi
University of Tennessee, Knoxville

Dr. Eric Lukosi received hi PhD in Nuclear Engineering in 2012 from the University of Missouri. He is currently an Associate Professor in the Nuclear Engineering Department at the University of Tennessee and is affiliated with the Joint Institute for Advanced Materials. Dr. Lukosi's expertise is in radiation sensor development and application in fields ranging from high energy physics to nuclear security. Dr. Lukosi specializes in the development of semiconductor detectors, such as lithium indium diselenide, diamond, and methylammonium lead tribromide.

Authors
Eric Lukosi a, b, Jeremy Tisdale b, c, Travis Smith a, b, Ryan Tan a, b, Bogdan Dryzhakov b, c, Andrew Shayotovich a, b, Andrew Naylor a, b, Kate Higgins b, c, Jessica Charest a, b, Bin Hu b, c, Mahshid Ahmadi b, c
Affiliations
a, Department of Nuclear Engineering, University of Tennessee, Knoxville, TN 37996
b, Joint Institute for Advanced Materials, University of Tennessee, Knoxville, TN, 37996
c, Department of Materials Science Engineering, University of Tennessee, Knoxville, TN 37996
Abstract

Methylammonium lead halide hybrid perovskite semiconductors are a potential low-cost option for moderate energy resolution semiconductor detectors for gamma and neutron sensing.  However, their chemical/environmental instabilities and ionic conductivity are significant challenges that must be overcome, and the ability to consistently reproduce sufficient material quality for spectroscopic gamma sensing has not yet been achieved. In this presentation, we will report on several experiments aimed at better understanding the pertinent electronic properties of methylammonium lead halide hybrid perovskites. Methods include direct charge transient measurements using alpha particles, photo-Hall electron spectroscopy, the transient current technique, time-resolved photoluminescence, and the effect of precursor purity used for growth. With these methods, we have identified the drift mobility, trapping time constant, detrapping time constant, and trap cross section for holes in CH3NH3PbBr3. Further, the shallow and deep energy levels within the band gap have been identified, providing insight into the causes of trap-controlled conductivity and non-radiative recombination mechanisms. Finally, it was found that particle size on the nucleation surface increased by as much as a factor of five when using higher purity precursors with a corresponding increase in charge carrier transport properties. Using alpha particles, the signal amplitude increased by as much as 30%, and using time-resolved photoluminescence, the radiative recombination lifetime increased by two orders of magnitude. 

09:30 - 10:00
2.1-I2
Xu, Yadong
Northwestern Polytechnical Universit
The Progress on Solution-Processed Metal Halide Perovskites for Nuclear Radiation Detection in NPU
Yadong Xu
Northwestern Polytechnical Universit, US

Dr. Yadong XU received his PhD in School of Materials Science & Engineering, Northwestern Polytechnical University in 2010 and is currently a Professor in State Key Laboratory of Solidification Processing and Key Laboratory of Radiation Detection Materials and Devices, Northwestern Polytechnical University, China. Dr. XU has received many prestigious awards including “Second-class of National Technological Invention”, P.R. China, 2013, “First- class of Scientific and Technical Awards”, Shaanxi Province,2012, “Youth outstanding talent support program" in Shaanxi, China, (2017), Excellent Talents project in Shaanxi Province, China, 2016. His research interests cover development of new semiconductor materials for X/γ-ray detectors, growth of electro-optical crystals for THz application, optical and electrical properties of the semiconductor materials and defect engineering. Dr XU has published more than 70 SCI papers and documented 16 patents.

Authors
Yadong Xu a, b
Affiliations
a, Key Laboratory of Radiation Detection Materials and Devices,
b, 2State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
Abstract

The recent rapid demand for large-volume X-ray and γ-ray spectrometers and imaging arrays have triggered tremendous opportunities in the field of nuclear medicine, astronomy and high energy physics, industrial on-line monitoring and national security. The large mobility and carrier lifetime of perovskite crystals and the high atomic numbers of heavy metals (Pb/Bi/Te/Sn), I and Br make them ideal materials for X-ray and γ-ray detection [1-2]. Here, we report centimeter-sized detector-grade APbBr3 (A=Methylammonium, Cs) perovskite crystals grown using solution method [3, 4]. The resulting single crystals exhibited high resistivity and mobility lifetime products (μτ). The X-ray sensitivity of 529 μC·Gyair-1cm-2 and 1256 μC Gy-1cm-2 were achieved for MAPbBr3 and CsPbBr3 detectors, respectively, under 80 kVp X-ray at an electric field of 50-200 V·cm-1. Besides, the CsPbBr3 detectors show the capability to detect 241Am @ 5.49 MeV α particles, with an energy resolution of ~3%. Simultaneously, for the 241Am @ 59.5 keV γ-ray response, the full energy peak was resolved with good peak discrimination. In addition, we demonstrate a potential candidate the 0-D perovskite material Cs2TeI6 grown by electrostatic assisted spray (E-spray) deposition, as a sensitive all-inorganic X-ray photoconductor for direct photon-to-current conversion X-ray detectors. The electrospray apparatus can be readily automated and fully integrated with the existing display systems based on TFT or CMOS, which will help to implement and scale up this device for manufacturing next generation of flat panel X-ray imagers.

 

References:

[1] Wei, H.; Fang, Y.; Mulligan, P.; Chuirazzi, W.; Fang, H.-H.; Wang, C.; Ecker, B. R.; Gao, Y.; Loi, M. A.; Cao, L.; and Huang, J. Nat. Photonics, 2016, 10, 333-339.

[2] Stoumpos, C. C.; Malliakas, C. D.; Peters, J. A.; Liu, Z.; Sebastian, M.; Im, J.; Chasapis, T. C.; Wibowo, A. C.; Chung D. Y.; and Freeman A. J. Cryst. Growth Des. 2013, 13, 2722-2727.

[3] Zhang, H.; Liu, X.; Dong, J.; Yu, H.; Zhou, C.; Zhang, B.; Xu, Y. Cryst. Growth Des. 2017, 17, 6426-6431.

[4] Liu, X.; Zhang, H.; Zhang, B.; Dong, J.; Jie, W.; and Xu Y. J. Phys. Chem. C, 2018, 122 (26), 14355-14361.

 

 

10:00 - 10:30
2.1-I3
Yakunin, Sergii
ETH Zurich
Solution-processed Metal Halide Perovskites of Various Dimensionalities for Hard-radiation Detection Using Direct Conversion and Scintillation
Sergii Yakunin
ETH Zurich, CH

Obtained PhD degree from National Academy of Sciences of Ukraine in 2007. 2008 - 2013 years Sergii is a PostDoc in JKU Linz, Austria in Prof. Wolfgang Heiss group. In 2013 he joined the group of Prof. Maksym Kovalenko group in ETH Zurich, Switzerland where he is a Senior Research Associate (Oberassistent)  since 2018. 

The main achievements are for the discovering of perovskite hard radiation and full-colour photo-detectors, optical gain and lasing in perovskite nanocrystal films. 

Authors
Sergii Yakunin b, Maksym Kovalenko b
Affiliations
a, Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, CH-8093 Zürich, Switzerland
b, Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland
Abstract

A decade ago, lead halide perovskites emerged in optoelectronic research as a novel class of solution-processed, low-cost semiconductors with intriguing charge-transport properties. Since then, enormous interest has been directed towards these materials. Aside from the mainstream research focused on perovskite PVs, these materials also promise to benefit other adjacent optoelectronic fields such as photodetectors, light emitting diodes, and lasers. These benefits are not only due to the low cost and easy processability associated with perovskites, but also with the notable charge-transport, defect-tolerance, efficient excitonic dynamics, and band gap tunability that 3D perovskites exhibit. Unlike in PV applications where the heavy metal content of lead halide perovskites is considered a drawback, X-ray detectors instead turn this drawback into an advantage due to their high absorptivity. Additionally, the high carrier mobility, low cost and easy scalability make the lead halide perovskites almost ideal materials for the large scale production of flat panel X-ray radiation detectors that are in high demand in medicine, high-precision control in advanced manufacturing, and security [1].

The numerous advantages of lead halide perovskites were also realized in gamma detectors [2].  While gamma detectors offered several additional challenges in comparison to X-ray detectors, such as achieving extremely low detector noise and high charge collection efficiency, these were surmounted by selecting the proper cation composition in lead halide perovskite single crystals [3].

In contrast to direct conversion by bulk 3D perovskites, 3D perovskite nanocrystals were recently proposed as promising scintillators given their high photoluminescence quantum yield (PLQY), although issues pertaining to the self-absorption of emitted light are expected. The self-absorption is indeed suppressed by low-dimensional (LD) perovskites that are structurally derived from their 3D counterparts with disconnected metal halide octahedra. Given that they still exhibit high X-ray absorption, and that LD perovskites exhibit self-trapped exciton (STE) emission with large Stokes shifts and high PLQY, it was recently demonstrated that LD perovskites act as efficient X-ray scintillators [4]. 

Sol2D 2.1
Chair: Efrat Lifshitz
09:00 - 09:30
2.1-I3
Bacher, Gerd
University of Duisburg-Essen
Growth and Device Integration of 2D Materials for Optoelectronic Applications
Gerd Bacher
University of Duisburg-Essen, DE

Gerd Bacher actually holds the chair of electronic materials and nanostructures at the Faculty of Engineering at Duisburg-Essen University. His research career started at Stuttgart University in the 1990s working on optical spectroscopy on epitaxially grown quantum wells, which was then extended to nanotechnology and nanodevice fabrication for optoelectronic applications at Würzburg University and Tokyo Institute of Technology. Being full professor since 2003, he is currently working on a wide diversity of nanomaterials, including 2D materials and nanocrystals, for applications in optoelectronics, information science and energy science. He is author or co-author of more than 250 articles in peer-reviewed journals.

Authors
Gerd Bacher a
Affiliations
a, Werkstoffe der Elektrotechnik and CENIDE, University Duisburg-Essen
Abstract

Atomically thin layers represent a novel class of materials with unique properties, like e.g. high electrical conductivity combined with high transparency in graphene or efficient light absorption and emission in transition metal dichalcogenides (TMDCs). Originally prepared by mechanical exfoliation for more basic scientific studies, recent developments in large area CVD growth techniques paved the path towards practical applications.

In this contribution some of our recent efforts on 2D materials for optoelectronic applications will be presented. This includes CVD growth of graphene and its application as transparent electrode in blue GaInN/GaN light emitting devices (LEDs). Hereby, we use a plasma-enhanced growth procedure developed by us for Cu substrates [1] and transferred thereafter to GaN-based substrates. Strong lateral current spreading, and a reduced turn-on voltage indicate the suitability of our approach. In addition, large area fabrication approaches of TMDCs via MOCVD growth were developed. High quality films for both, MoS2 as well as WSmonolayers have been realized and were analyzed via confocal optical spectroscopy [2]. A scalable p-n device design was established using inorganic and organic supporting layers for electron and hole injection, respectively [3]. Subsequently, the architecture was adapted for including MOCVD grown WS2 monolayers as active material emitting in the red spectral range. Large area electroluminescence stemming from the TMDC layer with a turn-on voltage as low as 2.5 V has been achieved, demonstrating the potential of 2D semiconductors for optoelectronic devices in a scalable approach.

09:30 - 10:00
2.1-I1
Plochocka, Paulina
Laboratoire National des Champs Magnétiques Intenses, CNRS
Excitons in MoS2/MoSe2 Van der Waals heterostructures
Paulina Plochocka
Laboratoire National des Champs Magnétiques Intenses, CNRS, FR

Paulina Plochocka, Directrice de recherché de 2e classe (DR2) in Laboratoire National des Champs Magnétiques Intenses (LNCMI), CNRS in Toulouse.

P. Plochocka obtained her PhD cum-laude in 2004 at the University of Warsaw working on the dynamics of many-body interactions between carriers in doped semi-magnetic quantum wells (QW). During her first post doc at Weizmann Institute of science, she started working on the electronic properties of a high mobility 2D electron gas in the fractional and integer quantum Hall Effect regime. She continued this topic during second post doc in LNCMI Grenoble, where she was holding individual Marie Curie scholarship. At the same time, she enlarged her interest of 2D materials towards graphene and other layered materials as TMDCs or black phosphorus. In 2012 she obtained permanent position in LNCMI Toulouse, where she created the Quantum Electronics group, which investigates the electronic and optical properties of emerging materials under extreme conditions of high magnetic field and low temperatures. Examples include semiconducting layer materials such as transition metal dichalcogenides, GaAs/AlAs core shell nanowires and organic inorganic hybrid perovskites.

Authors
Paulina Plochocka a
Affiliations
a, Laboratoire National des Champs Magnétiques Intenses, CNRS (FR)
Abstract

Recently, the stacking of atomic monolayers of TMDs has emerged as an effective way to engineer their properties. In principle, the staggered band alignment of such heterostructures should result in the formation of inter-layer excitons with long lifetimes and robust valley polarization. Since single layer TMDs suffer from very short exciton lifetimes and rapid valley depolarization, TMDs heterostructures can circumvent these drawbacks, paving the way for implementation of valleytronic and spintronic concepts. In this talk I will discuss the optical properties of excitons in MoS2/MoSe2 van der Waals heterostructure. First, I will demonstrate a long lived inter-layer exciton emission. Under circularly polarized excitation, the inter-layer exciton emission is intriguingly counter polarized; the emitted light has the opposite helicity compared to the excitation.  This surprising effect could be partially explained by the formation of the Moiré excitons in van der Waals heterostructures. To support this idea I will demonstrate splitting of the intralayer exciton and trion in a monolayer MoSe2 assembled in a heterostructure with MoS2 and encapsulated in hBN. Such a splitting, observed for the first time, is a direct consequence of the Moiré pattern formed between MoSe2 and MoS2. Secondly, I will demonstrate the results of the magneto-photoluminescence spectroscopy of interlayer excitons, which exhibits a non-trivial dependence of the valley polarization as a function of the magnetic field. The measured trends can be accounted for by considering that the valley polarization of energetic levels split by the valley Zeeman effect stems from the interplay between exchange interaction and phonon mediated intervalley scattering

10:00 - 10:30
2.1-I2
Vanmaekelbergh, Daniel
Universiteit Utrecht
Quantum and Dielectric Confinement Effects on the Absorption Strength in Semiconductors
Daniel Vanmaekelbergh
Universiteit Utrecht, NL

Vanmaekelbergh's research started in the field of semiconductor electrochemistry in the 1980s; this later evolved into the electrochemical fabrication of macroporous semiconductors as the strongest light scatterers for visible light, and the study of electron transport in disordered (particulate) semiconductors. In the last decade, Vanmaekelbergh's interest shifted to the field of nanoscience: the synthesis of colloidal semiconductor quantum dots and self-assembled quantum-dot solids, the study of their opto-electronic properties with optical spectroscopy and UHV cryogenic Scanning Tunneling Microscopy and Spectroscopy, and electron transport in electrochemically-gated quantum-dot solids. Scanning tunnelling spectroscopy is also used to study the electronic states in graphene quantum dots. More recently, the focus of the research has shifted to 2-D nano structured semiconductors, e.g. honeycomb semiconductors with Dirac-type electronic bands.

Authors
Daniel Vanmaekelbergh a, T. Prins a, M. Alimoradi Jazi a, A. J. Houtepen b, W. Heiss c, C. Delerue d
Affiliations
a, Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands
b, Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands
c, Debye Institute for Nanomaterials Science, Utrecht University, The Netherlands
d, IEMN-Department of ISEN, UMR CNRS 8520, 59046 Lille, France
Abstract

Recent advances in colloidal synthesis and assembly1-3 allows a comparison of the strength of light absorption of semiconductor nanocrystals in three distinct electronic phases: (i) as non-interacting individual nanocrystals with strong three-dimensional quantum confinement dispersed in solution, (ii) as ligand-separated nanocrystals present in an ordered monolayer, and (iii) as nanocrystals epitaxially connected in a monolayer superlattice. We performed quantitative absorptance measurements on these three different samples, for the case of PbSe (band gap in the IR) and CdSe (band gap in the visible).  The light absorption cross section of PbSe nanocrystals in a hexagonal monolayer is 5-10 fold increased versus nanocrystals in solution; this is due to far-field polar coupling, which reduces the dielectric screening of the electric field in a NC monolayer.  The absorption cross section is further enhanced in superlattices as the epitaxial connection results in a two-dimensional electronic system, and thus complete quenching of dielectric screening. Nanocrystal monolayer superlattices of PbSe and CdSe on quartz show 1.6 % absorptance, a value directly related to the fine structure constant.  This “quantum of light absorption” has been reported for several other two-dimensional systems, from graphene4 to III-V semiconductor quantum wells5, and explained with Fermi’s golden rule and the effective mass approximation5. We calculated the absorptance of two-dimensional II-VI and IV-VI semiconductors and superlattices with atomistic tight-binding theory, resulting in values close to the absorptance quantum.

REFERENCES

1. Boneschanscher, M. P. et al. Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices. Science 344, 1377-1380 (2014).

2. Schliehe, C. et al. Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment. Science 329, 550-553 (2010).

3. Peters, J. L. et al. Mono- and Multilayer Silicene-Type Honeycomb Lattices by Oriented Attachment of PbSe Nanocrystals: Synthesis, Structural Characterization, and Analysis of the Disorder. Chem. Mater. 30, 4831-4837 (2018).

4. Nair, R. R. et al. Fine structure constant defines visual transparency of graphene. Science 320, 1308-1308 (2008).

5.Fang, H. et al. Quantum of optical absorption in two-dimensional semiconductors. P.. Natl. Acad. Sci. USA 110, 11688-11691 (2013).

SolFuel 2.1
Chair: Roel van de Krol
09:00 - 09:30
2.1-I1
Sharp, Ian
Technical University of Munich
Identifying and Overcoming Loss Processes in Transition Metal Oxide Photoelectrodes
Ian Sharp
Technical University of Munich, DE
Authors
Ian Sharp a
Affiliations
a, Walter Schottky Institute and Department of Physics, Technical University of Munich
Abstract

The capture of sunlight and its direct conversion to chemical fuels in artificial photosystems provides a promising route for sustainably meeting future energy demands. However, progress has been hindered by a lack of materials that are simultaneously stable, efficient, and scalable. To address this gap, international research efforts have intensively targeted transition metal oxide semiconductors as potentially stable light harvesting compounds. While recent work has led to the discovery of a remarkable range of new materials, the energy conversion efficiencies from these systems often fall far short of thermodynamic limits. Using bismuth vanadate, copper vanadate, and copper iron oxide as representative materials systems, we discuss how the fundamental electronic structure and defects impact photoexcitation processes, charge transport characteristics, and excited state lifetimes. Insights into dominant loss processes inform efforts to integrate these materials into monolithic photosystems and indicate strategies for improving performance characteristics.  At the device level, we introduce the hybrid  photo-electrochemical and -voltaic cell, which provides a route to overcome the problem of mismatched tandem component performance by adding a third electrical terminal to the bottom sub-cell. This architecture, which allows electrical power to be produced at the same time as chemicals in order to overcome current mismatches, provides one route to creating efficient and functional systems from the existing set of semiconductor light absorbers.

09:30 - 10:00
2.1-O1
Hamann, Thomas
Michigan State University
Electron Dynamics at Copper Tungstate / Catalyst Interfaces
Thomas Hamann
Michigan State University, US
Assistant Professor 2008-present Michigan State University Postdoctoral Fellow 2006-2008 Northwestern University Ph.D., Chemistry 2006 California Institute of Technology Research Interests: Inorganic chemistry, renewable energy technology, investigations of homogeneous and heterogeneous electron-transfer reactions, synthesis of novel nanostructured materials, development and investigations of photovoltaic and photoelectrochemical cells
Authors
Thomas Hamann a
Affiliations
a, Department of Chemistry, Michigan State University, 485 Chemistry, East Lansing, MI, 48823
Abstract

Metal oxides comprise an interesting class of photoanode materials for photoelectrochemical (PEC) water oxidation; a key reaction for the realization of solar fuels. Under PEC water oxidation conditions, many metal oxides have been shown to accumulate holes in surface states which can recombine with conduction band electrons and thus limit the performance. This surface state recombination can be mitigated through introduction of electrocatalysts on the metal oxide semiconductor surface which often leads to improved performance. We have found that some semiconductor/catalyst combinations result in similar or worse performance compared to the bare electrode, however. We will present recent results of copper tungstate (CuWO4) photoanodes coupled with electrocatalysts which deepen the understanding of the energetics and electron-transfer processes at semiconductor/electrocatalyst interfaces that control the performance of such systems. Through rigorous comparison of cyclic voltammetry, current transient and electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy (IMPS), and dual-working electrode experiments measurements, we have been able to gain a significant insight into the role of the catalyst and the electron-transfer at the interface on the performance of PEC water oxidation. General lessons and design rules for efficient PEC water splitting will finally be discussed.

10:00 - 10:15
2.1-O2
Selim, Shababa
Imperial College London
Using Transient Spectroscopic Techniques to Investigate the Effect of Catalyst Overlayers and Morphology on the Water Oxidation Performance of Bismuth Vanadate
Shababa Selim
Imperial College London, GB
Authors
Shababa Selim a, Laia Francas a, Camilo Mesa a, Sacha Corby a, Dongho Lee b, Andreas Kafizas a, Kyoung-Shin Choi b, James Durrant a
Affiliations
a, Department of Chemistry, Imperial College London, South Kensington Campus London, London, GB
b, Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706, USA
Abstract

The photoelectrocatalytic performance (i.e. water oxidation) of semiconducting materials as photoanodes is continually optimised through the implementation of nanostructuring and catalyst overlayers. Yet, the exact origin of this photoelectrocatalytic enhancement can often remain elusive. In this work we present direct spectroscopic evidence that the enhanced performance of FeOOHNiOOH coated BiVO4 results from irreversible hole transfer to the catalyst layer and consecutive hole transfer to the electrolyte to oxidise water. This hole transfer spatially separates the photogenerated charge, thus slowing down recombination. Moreover, despite not directly enhancing the kinetics of the catalytic process, we have demonstrated using steady state spectroscopic measurements that holes accumulated on the catalyst layer are responsible for oxygen generation. Additionally, charge tranport is a crucial factor that needs to be considered when optimsing electrodes for catalysis. Our thermal studies shed light on the importance of oxygen vacancies on electron transport processes that govern the overall photoelectrocatalytic performance of metal-oxide based photoanodes for water oxidation.

10:15 - 10:30
2.1-O3
Gimenez Julia, Sixto
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
Mechanistic Insights on Solar Water Splitting with Metal Oxide Semiconductor Materials
Sixto Gimenez Julia
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES

Sixto Giménez (M. Sc. Physics 1996, Ph. D. Physics 2002) is Associate Professor at Universitat Jaume I de Castelló (Spain). His professional career has been focused on the study of micro and nanostructured materials for different applications spanning from structural components to optoelectronic devices. During his PhD thesis at the University of Navarra, he studied the relationship between processing of metallic and ceramic powders, their sintering behavior and mechanical properties. He took a Post-Doc position at the Katholiek Universiteit Leuven where he focused on the development of non-destructive and in-situ characterization techniques of the sintering behavior of metallic porous materials.  In January 2008, he joined the Group of Photovoltaic and Optoelectronic Devices of University Jaume I where he is involved in the development of new concepts for photovoltaic and photoelectrochemical devices based on nanoscaled materials, particularly studying the optoelectronic and electrochemical responses of the devices by electrical impedance spectroscopy. He has co-authored more than 80 scientific papers in international journals and has received more than 5000 citations. His current h-index is 31. 

Authors
Sixto Gimenez a, Miguel García-Tecedor a, Drialys Cárdenas-Morcoso a, Roser Fernandez-Climent a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

In this presentation, we will address the use of metal oxides as candidate materials for photoelectrodes in water splitting electrochemical cells. Their low-cost, earth-abundance, stability under harsh envoronments and easy synthesis and up-scalability have positioned these materials as interesting candidates for this application.[1] Although in terms of functional performance there are still important challenges to address, these materials can be engineered to significantly decrease the bulk and surface loss mechanisms, with remarkable examples like BiVO4, where photocurrents close to the theoretical maximum have been achieved.[2] In the present contribution, we will describe different examples of the synergistic combination of metal oxides (Fe2O3,[3] WO3,[4] and BiVO4,[4,5]) with catalytic layers (Fe-Co Prussian Blue,[3,6] Ag3PO4, MOF derivatives,[7]…), emphasizing the mechanistic insights leading to enhanced performance. Our studies focus on the correlation of the photoelectrochemical response of the materials with a detailed structural and mechanistic characterization carried out by different microscopic and spectroscopic tools.

10:30 - 11:00
Coffee Break
CharDy 2.2
Chair: Marcus Scheele
11:00 - 11:30
2.2-I1
Nelson, Jenny
Imperial College London
Charge Carrier Dynamics at Molecular Heterojunctions in Organic Photovoltaic and Photocatalytic Systems
Jenny Nelson
Imperial College London, GB

Jenny Nelson is a Professor of Physics at Imperial College London, where she has researched novel varieties of material for use in solar cells since 1989. Her current research is focussed on understanding the properties of molecular semiconductor materials and their application to organic solar cells. This work combines fundamental electrical, spectroscopic and structural studies of molecular electronic materials with numerical modelling and device studies, with the aim of optimising the performance of plastic solar cells. She has published around 200 articles in peer reviewed journals, several book chapters and a book on the physics of solar cells.

Authors
Jenny Nelson a
Affiliations
a, Department of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, UK.
Abstract

In the application of moleculr electronic materials to solar energy conversion or light emission, device performance depends on the competition between interfacial charge transfer processes and charge transport. Carrier dynamics depend strongly on the chemical structure and physical organisation of the molecular components. In this talk we will consider dynamic electronic processes in two types of solar energy conversion system, molecular photovoltaic devices and conjugated polymer photocatalysts. In molecular donor: acceptor solar cells, we use a combination of transient optical spectroscopy, luminescence and device measurements to probe charge separation and recombination dynamics, and to study the dependence of these processes on the energies and other properties of the relevant excited states. We find that the energetic offset, local microsctructure and brightness of the interfacial state all influence charge generation efficiency. In the case of polymer photocatalysts, activity is driven by a photoinduced charge transfer event from a polymer to a hole scavenger. We use a combination of steady-state and time-resolved spectroscopic tools to show that dynamics depend on microstructure both of the polymer – which controls exciton diffusion – and of the local solvent environment – which controls the driving force. We conclude with a comparison of the key factors controlling useful charge generation in the two types of system.

11:30 - 11:45
2.2-O1
Zeiske, Stefan
Swansea University, Department of Physics, Swansea, United Kingdom.
Quantifying Trap-assisted Recombination in Thin Film Solar Cells from Intensity Dependent Photocurrent Measurements
Stefan Zeiske
Swansea University, Department of Physics, Swansea, United Kingdom.
Authors
Stefan Zeiske a, Oskar Sandberg a, Nasim Zarrabi a, Paul Meredith a, Ardalan Armin a
Affiliations
a, Sêr Cymru Chair in Sustainable Advanced Materials Department of Physics, Swansea University, Singleton Park Swansea SA2 8PP
Abstract

Intensity dependent photocurrent (IPC) measurements are a commonly used technique to investigate photocurrent losses in photovoltaic devices as a function of the incident light intensity under different operational device conditions. In particular, such measurements are able to identify recombination processes which are dominant at particular light intensities.[1], [2] Here, we investigate first- and higher-order recombination processes and photocurrent loss mechanisms on different variants of thin film solar cells from sensitive IPC measurements performed over a broad range of light intensities and present a summary of their dependencies and precise identification. We show in particular that in the presence of trap states two linear photocurrent regimes can be identified.[3] Aided by one-dimensional Drift Diffusion (DD) simulations we present a model that can explain their origin based on trap-assisted Shockley-Read-Hall (SRH) recombination and a trap filling that leads to a charge build-up. By relating the light irradiance and open-circuit voltage, at which the transition between the two linear photocurrent regimes occurs, we can estimate the trap depth and density, which is in good agreement with the theoretical, predicted values. In combination, broad dynamic range IPC and DD simulations provide a powerful tool to probe and quantify the mechanistic aspects of photocurrent losses in solar cells.

11:45 - 12:00
2.2-O2
Kolay, Ankita
Indian Institute of Technology Hyderabad
Nickel Oxide Based Photocathode and Selenium Nanowires Coated Photoanode for a Highly Efficient Tandem Quantum Dot Solar Cell
Ankita Kolay
Indian Institute of Technology Hyderabad
Authors
Ankita Kolay a, Melepurath Deepa a
Affiliations
a, Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana, IN
Abstract

Recent research efforts on quantum dot solar cells (QDSCs) have shown that scientific investigations centered on the counter electrode (CE) can potentially yield very high efficiency, of more than 12% [1]. The concept of replacing the conventional electrocatalytic CE with novel photocathode architectures consisting of wide range of visible and NIR light responsive quantum dots is being scrutinized in the quest for increasing power conversion efficiencies. Quantum dots are more promising light-absorbing photovoltaic material over dyes. Engineering a p/n tandem QDSC by assembling both a photoanode (based on n-type TiO2 or ZnO) and photocathode (typically based on p-type NiO semiconductor) in a single device can potentially lead to (i) light harvesting over a broader range of the electromagnetic spectrum, (ii) open-circuit voltage increment and (iii) reduced recombination at various interfaces.

Light harvesting metal chalcogenides were deposited onto NiO nanostructure scaffold and on pairing with the n-QDSC i.e., CdS QD-sensitized TiO2 layer coated with hole conducting selenium nanowires, efficient charge separation occurs and a power conversion efficiency of more than 8% was achieved. In spite of the disparity in performance of the p- and n-QDSC half cells owing to the reverse pathways for electron propagation, the synergism between the half-cells with complementary effects, when juxtaposed, gets reflected in terms of substantial gains in the photovoltaic performance of the tandem device. This work rationalizes the charge propagation dynamics between the photoanode and photocathode elaborately to obtain a hitherto unmatched solar energy conversion in tandem QDSCs.

MapNan 2.2
Chair: Ilka Hermes
11:00 - 11:30
2.2-I1
Eng, Lukas M.
TU Dresden
"SFM-mediated NanoMagnetism & NanoOptics: From Skyrmions to THz Near-field Optics"
Lukas M. Eng
TU Dresden, DE

- PhD in Physics, University of Basel, Switzerland - Post-Doctoral Research Assistant, BASF AG, Ludwigshafen, Germany: Molecular Science - Maitre d’Enseignement et de Recherche, Univ. Geneva, Switzerland - Biological Molecules - Team-leader, Institute of Quantum Electronics, ETH Zurich, Switzerland - Habilitation & vein legendi in Physics, University of Basel, Switzerland, 1998 - Since 1998: Full Professor in Photophysics / Nano-Optics / Nano-Physics at TU Dresden, School of Science Profile: Nanoscale research of quantum nanostructures: magnetic, optical. electronic, molecular; application to magnetic textures, charged domain walls, near-field metamaterials, etc.

Authors
Lukas M. Eng a, b
Affiliations
a, Institute of Applied Physics, TU Dresden, Nöthnitzer Straße, 61, Dresden, DE
b, lukas.eng@tu-dresden.de
Abstract

Ultra-high resolution and quantitative microscopy are the key needs for the successful and unambiguous inspection of functionality at the nanometer length scale. I will present two such hot topics, NanoMagnetism (NM) of modern topological textures and NanoOptics (NO) down to the 1 THz frequency, where scanning force microscopy (SFM)-based mimics are the key in-and-outs to address a multitude of scientific questions.

NM rapidly develops into a prospective field where non-collinear magnetic textures allow applications into novel device concepts such as data storage and sensing. Skyrmions for instance are one of these popular nanomagnetic structures that become stable due to topological protection. I will introduce how SFM-based conservative/non-conservative force sensing can be used to monitor, quantify and manipulate magnetic functionality down to the 1-nm-length scale.

NO, on the other hand, continuously develops into a SFM-based near-field optical scattering method at infrared wavelengths. I will present how we couple such an apertureless setup with the brilliant light source of a free-electron laser that covers the broad range from 75 … 0,1 THz, i.e. wavelengths from 4 µm up to mm. Selected experiments both at room and at liquid-helium temperatures will be presented.

11:30 - 12:00
2.2-I2
Mesquida, Patrick
Kings College London
Kevin-probe Force Microscopy of Biological Materials
Patrick Mesquida
Kings College London, GB
Authors
Patrick Mesquida a
Affiliations
a, Kings College London, Strand, London, GB
Abstract

 

Although almost 30 years old, surface potential determination by Kevin-probe Force Microscopy (KPFM) is surprisingly little used for investigations of biological processes or materials. This could be due to the fact that KPFM, as an electrical measurement mode of AFM, is not considered to be readily amenable to in-water experiments. However, even KPFM performed in air, when conducted correctly, can tell us a lot about properties of biological samples. Here, I will give an overview of the possibilities, requirements and pitfalls of KPFM on such samples. A particular focus will be on sample preparation, the influence of the most relevant functional surface groups [1], and the influence of air humidity on the KPFM signal [2]. An example will be shown, where KPFM is used to study subtle surface charge shifts of collagen extracellular matrix fibres, which could have significant implications in cell adhesion, bone metabolism and the design of novel tissue scaffolds [3]. There are probably no other analytical methods at present to detect such small but significant changes of the electrical properties of biological materials while maintaining a high, spatial resolution. In addition, a brief overview will be given of current activities and challenges to expand AFM-based surface potential measurements to true in-water experiments [4].

  

12:00 - 12:15
2.2-O1
Thelen, Richard
Karlsruhe Institute of Technology KIT
Correlated Characterization: a Realistic Option For Future Metrology ?
Richard Thelen
Karlsruhe Institute of Technology KIT
Authors
Richard Thelen a
Affiliations
a, Karlsruhe Institute of Technology (KIT), Institute of Microstructure Technology (IMT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
Abstract

For many decades metrology tasks have successfully been performed, but subsequently and without precise correlation to other metrology methods. Now that subjects to metrology are getting smaller and smaller many metrology systems come to their limit. This is not new. But what is new is that modelling at the same time is usually based on microscopic cells and small amounts of similar cells where at the same time metrology fails to support these small cells with solid data from different prospective. One reason is that metrology is bridging different scales. It might describe the subject`s behavior on a macroscopic scale or a microscopic scale. Sometimes it does both but the values diverge by an order of magnitude.
To create a digital twin there is a strong need to merge high resolution metrology data from diverse sources and with different magnitude of scales included. This is an easy claim but not that easy to realize.
The author shows how to integrate some major prerequisites that are a must for successful correlated characterization into a line that is build up. It is discussed what type of practical limitations have to be faced when trying to set the path. A typical error using multi scaled metrology process is that a large depiction helps to create an overview and based on that it is determined where to have a closer look. All deviation from regular information embedded into height, texture, mechanical properties etc. can only be judged at the specific scale that is chosen for the depiction. So zooming might reveal useful information but there is no hint on larger scale. What can be done is to have a closer look to detect a possible origin for property mismatch using different metrology tools. That is one of the strength of correlated characterization. How to make this happen if no visible markers are present or if metrology subjects are impossible to reveal using standard microscopes is one more subject to this talk.

An outlook of where all this might lead to is also included. This is definitely not a forecast but a vision.

12:15 - 12:30
2.2-O2
Unger, Miriam
Bruker Nano Surfaces
Latest Advances in Nanoscale IR Spectroscopy and Imaging
Miriam Unger
Bruker Nano Surfaces
Authors
Miriam Unger a, Anirban Roy a, Qichi Hu a
Affiliations
a, Bruker Nano Surfaces, Bruker Corporation
Abstract

For the last few decades the rapid growth in the field of nanoscience and technology has led to the development of new characterization tools for nanoscale materials. Traditional IR and Raman spectroscopy and imaging offers excellent chemical insights; however, the spatial resolution is limited by the optical diffraction limit (~lambda/2). Although, recent Super-resolution microscopy techniques offer superior spatial resolution, they are primarily implemented in fluorescence imaging, hence needs external fluorophore tag for detection.  Alternatively, nanoscale IR spectroscopy/imaging offers a “tag free” spectral detection with high spatial resolution beyond optical diffraction limit (2-5 mm) by exploiting an AFM probe to detect either photothermal expansion force (PTIR) or near field scattered IR light (sSNOM).

 

Recent developments in PTIR and sSNOM technology have significantly augmented the speed and spatial resolution for chemical analysis. One of the new developments (Tapping AFM-IR) allows acquisition of IR images at a specific absorption band simultaneously with sample topography and nano-mechanical properties, providing a complete set of topographical, chemical and mechanical insights with <10 nm spatial resolution. These high-resolution measurements are currently accompanied by high speed tunable laser enabling fast point spectral acquisition (1-2 ms/spectrum) leading to hyperspectral data cube for rigorous statistical analysis similar to Chemometrics applications.

 

In this presentation, we will highlight the technical background and applications of these emerging technologies in different fields, e.g., nanomaterials, life sciences, polymers, microelectronics etc.

 

PERInt 2.2
Chair: Juan-Pablo Correa-Baena
11:00 - 11:30
2.2-I1
Petrozza, Annamaria
Istituto Italiano di Tecnologia
Understanding Defect Physics to Stabilize Metal-halide Perovskite Semiconductors for Optoelectronic Applications
Annamaria Petrozza
Istituto Italiano di Tecnologia, IT

Annamaria Petrozza received her PhD in Physics from the University of Cambridge (UK) in 2008 with a thesis on the study of optoelectronic processes at organic and hybrid semiconductors interfaces under the supervision of Dr. J.S. Kim and Prof Sir R.H. Friend. From July 2008 to December 2009 she worked as research scientist at the Sharp Laboratories of Europe, Ltd on the development of new market competitive solar cell technologies (Dye Sensitized Solar cells/Colloidal Quantum Dots Sensitized Solar cells). Since January 2010 she has a Team Leader position at the Center for Nano Science and Technology -IIT@POLIMI. She is in charge of the development of photovoltaic devices and their characterization by time-resolved and cw Photoinduced Absorption Spectroscopy, Time-resolved Photoluminescence and electrical measurements. Her research work mainly aims to shed light on interfacial optoelectronic mechanisms, which are fundamental for the optimization of operational processes, with the goal of improving device efficiency and stability.

Authors
Annamaria Petrozza a
Affiliations
a, Istituto Italiano di Tecnologia (IIT), Genova, Italy, Via Morego, 30, Genova, IT
Abstract

Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects.

 

Here, first I will summarize our understanding of the nature of defects and their photo-chemistry, which leverages on the cooperative action of density functional theory investigations and accurate experimental design. Then, I will show the correlation between the nature of defects and the observed semiconductor instabilities. Instabilities are manifested as light-induced ion migration and segregation, eventually leading to material degradation under prolonged exposure to light. Understanding, controlling and eventually blocking such material instabilities are fundamental steps towards large scale exploitation of perovskite in optoelectronic devices. By combining photoluminescence measurements under controlled conditions with ab initio simulations we identify photo-instabilities related to competing light-induced formation and annihilation of trap states, disclosing their characteristic length and time scales and the factors responsible for both processes. We show that short range/short time defect annihilation can prevail over defect formation, happening on longer scales, when effectively blocking undercoordinated surface sites, which act as a defect reservoir. Finally, based on such knowledge, I will discuss different synthetic and passivation strategies which are able to stabilize the perovskite layer towards such photo-induced instabilities, leading to improved optoelectronic material quality and enhanced photo-stability in a working solar cell.

11:30 - 12:00
2.2-I2
Deschler, Felix
Technische Universität München
Ultrafast Spectroscopy of Carrier and Spin Dynamics in Hybrid Perovskites
Felix Deschler
Technische Universität München, DE
Authors
Felix Deschler a, b
Affiliations
a, Walter Schottky Institut and Physics Department, Technische Universität München, 85748 Garching, Germany
b, Cavendish Laboratory, Department of Physics, University of Cambridge, UK, JJ Thomson Avenue, Cambridge, GB
Abstract

Metal-halide perovskites have emerged as promising solution-processable semiconductors for optoelectronic applications. These materials show unexpectedly high luminescence yields, long carrier lifetimes under operating conditions. Facile changes in composition during fabrication can be used to control their optical properties, and the nature of electronic states. Recently, the ad-mixture of monovalent cations to the precursor solution has been demonstrated to maximize the luminescence yields and device performance, which harvests photon-recycling effects.

The properties and dynamics of the perovskites’ electronic states are controlled by their crystal structure and symmetry. Strong spin-orbit coupling was predicted to introduce Rashba-type state splitting in the electronic band structure. In combination with the soft crystal structure of the perovskite lattice, it is likely that dynamic changes occur in the electronic states during their lifetime. So far, it is not understood how such effects change after optical excitation and how they proceed during relaxation of electronic states.

In this talk I will present how we use advanced spectroscopic techniques to study the dynamics of electronic states and crystal structure in metal-halide perovskites on ultrafast timescales. I will report results on layered and bulk lead-halide perovskites, but also on sustainable lead-free variants. I will discuss how the crystal structure affects the properties of electronic states, and how we can use these effects to create novel optoelectronic devices.

RadDet 2.2
Chair: Germà Garcia-Belmonte
11:00 - 11:30
2.2-I1
Sumathi, R. Radhakrishnan
Leibniz Institute for Crystal Growth (IKZ)
High Purity Germanium Crystals for Detector Application - the Path Traveled so far and the Way Ahead
R. Radhakrishnan Sumathi
Leibniz Institute for Crystal Growth (IKZ)

Dr. R. Radhakrishnan Sumathi is a Vice-head of volume crystals department at Leibnitz-Institute for Crystal Growth (IKZ), Berlin.  She is leading and responsible for the semiconductor section, which focuses its niche research and development in elemental and compound semiconductor materials (Si, Ge, III-Vs, II-VIs) for various applications.  Dr. Sumathi holds a Ph.D degree (Anna University, Chennai/Madras, India) and also has received a “habilitation” title from Ludwig-Maximilans University (LMU, Munich, Germany), where she is also a faculty at Materials Science and Crystallography institute.  She has about 25 years of expertise in crystal growth/materials sciences field and specialised experience in semiconductor materials and devices.  Her research interest also covers advanced functional materials. She is very active in many professional societies (IACG, DGKK) and has received many awards (Young Scientist, Young Researcher), the recent one being, Young Achiever Award by Indian Science and Technology Association in 2018.  She has over 75 papers in internal journals (peer-reviewed, high impact factor)  and/or conferences and has given invited talks in more than 25 meetings.  She is a guest editor of Results in Materials (Elsevier publications) and serving as a international committee member in many national/international scientific conferences.

Authors
K. P. Gradwohl a, O. Gybin a, J. Janicskó-Csáthy a, N. Abrosimov a, R. R. Sumathi a
Affiliations
a, Leibniz Institute for Crystal Growth (IKZ)
Abstract

High purity Germanium (HPGe) is in the front line of both fundamental research in the field of astrophysics (e.g. dark matter searches) and nuclear physics (radiation detectors). Among its many other attractive detector applications, the use of Ge in Large Enriched Germanium for Neutrinoless double beta Decay (LEGEND) experiments would need 100s of kg of detectors made of HPGe single crystals produced from enriched 76Ge isotope material.  In this talk, the development of in-house constructed equipment and the process steps in the growth of 2-inch diameter single crystals will be presented. The properties that give Ge an advantage for the detector application will be discussed. The method employed to grow these bulk crystals is the Czochralski (Cz) growth technique. Crystals of high purity and a low dislocation density were grown under pure hydrogen atmosphere. These grown crystals were characterised for their suitability to detector fabrication. The challenges remaining - in achieving further “very high” purity - due to the presence of unwanted impurities with different segregation coefficients will be emphasized.  Moreover, the technology for large diameter Ge crystal growth, which is currently under development, will be highlighted, including some proposals for further improvements in the crystal quality to get the detector grade material that can be used for experiments like LEGEND.

Sol2D 2.2
Chair: Maya Bar Sadan
11:00 - 11:30
2.2-I1
Rodina, Anna
Ioffe Physical Technical Institute
Bright and Dark Exciton Emission in Two Dimensional Nanostructures
Anna Rodina
Ioffe Physical Technical Institute, RU

Prof. Anna Rodina is Senior Scientific Researcher in the laboratory of Optics of Semiconductors at Ioffe Institute of Russian Academy of Sciences (St.-Petersburg, Russia). She received her Ph.D. (1993) and Habilitation (2016) degrees in Physics from Ioffe Institute and became the Professor of Russian Academy of Sciences in 2018. The expertise of Prof. Rodina is in the theory of semiconductors and semiconductor nanostructures. The current research interests are focused on the magneto-optical properties and spin-dependent phenomena in colloidal nanocrystals.  

Authors
Anna Rodina a
Affiliations
a, Ioffe Institute, Ioffe Institute,St. Petersburg, RU
Abstract

We present a new theoretical analysis of the possibility to observe the low temperature photoluminescence (PL) stemming from the radiative recombination of bright (optically allowed)  and dark (optically forbidden) excitons. We consider both spin-forbidden and momentum- forbidden (indirect) dark excitons with different activation mechanisms of their radiative recombination. We discuss the cases of pulsed or continues wave resonant and non-resonant optical excitations and derive general expressions for the ratio between bright and dark exciton contributions to the PL intensities. The analysis of the experimental data for the low temperature PL as well as its dependencies on the temperature or external magnetic field based on these expressions allows one to make important conclusions about the exciton relaxation and recombination rates in the system. We apply the developed approach to the low temperature PL studies of CdSe colloidal nanoplateletes [1] and transition metal dichalcogenides (such as MoS2) flakes and nanotubes [2].

11:30 - 12:00
2.2-I2
Klinke, Christian
University of Rostock
Synthesis and Optoelectronic Properties of Two-dimensional Colloidal Nanomaterials
Christian Klinke
University of Rostock, DE

Christian Klinke studied physics at the University of Karlsruhe (Germany) where he also obtained his diploma degree in the group of Thomas Schimmel. In March 2000 he joined the group of Klaus Kern at the Institute of Experimental Physics of the EPFL (Lausanne, Switzerland). Then from 2003 on he worked as Post-Doc at the IBM TJ Watson Research Center (Yorktown Heights, USA) in the group of Phaedon Avouris. In 2006 then he became member of the Horst Weller group at the Universitiy of Hamburg (Germany). In 2007 he started as assistant professor at the University of Hamburg. In 2009 he received the German Nanotech Prize (Nanowissenschaftspreis, AGeNT-D/BMBF). His research was supported by an ERC Starting Grant and a Heisenberg fellowship of the German Funding Agency DFG. Since 2017 he is an associate professor at the Swansea University and since 2019 full professor at the University of Rostock.

Authors
Christian Klinke a
Affiliations
a, University of Rostock, Universitätsplatz 3, Rostock, 18055, DE
Abstract

Two-dimensional colloidal nanomaterials have gained more and more attention as attractive optoelectronic materials in the recent years. They combine good lateral conductivity with solution-processability and geometry-tunable electronic properties. The formation of ordered and densely packed surface layers of amphiphilic ligand molecules on certain crystal facets can drive the normally isotropic into a two-dimensional crystal growth, resulting in semiconducting nanosheets. Due to the strongly reduced height,  such metastable materials are in electronic confinement, which allows tuning their effective bandgap. Other features are reduced charge screening, efficient doping, and solution processablility. First, I will discuss the syntheses of the materials, followed by analyses of their optical properties and the electrical transport characterization through these materials as field-effect and spin transistors. In particlar, I will introduce nanosheets based on lead sulfide, tin sulfide and lead halide perovskites. The materials show surprising effects and demonstrate the versatility and usefulness of this synthesis approach. 

12:00 - 12:30
2.2-O1
Failla, Michele
Delft University of Technology (TU Delft), The Netherlands
Room Temperature Evidence of Multiple Heavy-hole Excitonic States and Biexciton-phonon Coupling in CdSe Nanoplatelets
Michele Failla
Delft University of Technology (TU Delft), The Netherlands, NL
Authors
Michele Failla a, Bastiaan Salzmann b, Daniel Vanmaekelbergh b, Laurens Siebbeles a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
b, Utrecht University, Princetonplein, 1, Utrecht, NL
Abstract

Cadmium selenide nanoplatelets (CdSe-NPLs) are solution processable nanostructures showing narrow absorption and emission excitonic features at room temperature [1]. These result from a giant oscillator strength and weak phonon-coupling [2], offering the opportunity to create robust, long-lived, excitons upon photoexcitation. 

In comparison with quantum dots (strong lateral QC) and wells (absent lateral QC), the characteristic CdSe-NPL lateral dimensions (𝐿𝑥 and 𝐿𝑦), ranging from a few to tens of nm, offer the chance to exploit and study excitons in different lateral confinement regimes. Detailed calculations showed that in the strong QC regime (NPL areas < 40 nm) excitonic states can be 100 meV apart, while in the weak/intermediate QC regime (NPL areas between ~120 and ~500 nm2) this difference can be as small as tens of meV [3]. In optical experiments, resolving multiple excitonic states depends on their relative energy difference as well as on the linewidth of the respective absorption and emission features. To date, the experimental and direct observation of multiple excitonic states in CdSe-NPLs have been reported from photoluminescence measurements at low temperature (200 K) [4]. 

Here, we report the first (direct) room temperature observation of multiple excitonic states in CdSe-NPLs from transient absorption (TA) experiments. As shown in Fig. (a), the differential absorption change 𝛥𝐴 is measured, after pumping resonantly at the HH excitonic peak, as a function of the pump-probe delay, 𝑡. By adding 𝛥𝐴 to the steady state absosorption 𝐴0, see Fig. (b), a clear splitting of the HH excitonic peak is observed at around 𝑡=0 ps, which corresponds to the time when the pump-pulse ends to excite the NPLs. This splitting, altough with different strength, is observed indipendently of the used excitation energy, therefore ruling out any coherent artifact that can eventually happen in pump-probe experiments. 

In addition, fits of the decay kinetics of the TA biexciton (XX) feature with a multi-exponential decay function [Fig. (c)] resolve a 0.9 THz (3.7 meV) oscillation with a decay constant of ~1.5 ps [Fig. (d)]. This frequency does not depends on the excitation energy and well compares with the LA phonon mode reported in similar CdSe-NPLs [5], indicating a weak, but appreciable, XX-phonon coupling. Both frequency and decay constants are not found to change with the NPL area, further confirming the interaction with the LA phonon mode, as the latter is not expected to change along samples with an identical thickness.

 

 

SolFuel 2.2
Chair: Roel van de Krol
11:00 - 11:30
2.2-I1
Starr, David
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Shining Light on Bismuth Vanadate/Aqueous Electrolyte Interfaces
David Starr
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
David E. Starr a, Marco Favaro a, Pip Clark a, Michael J. Sear a, Fatwa F. Abdi a, Ibbi Ahmet a, Marlene Lamers a, Roel van de Krol a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany, Berlin, DE
Abstract

Due to their ease of synthesis, low production cost, and potential long-term stability, multinary, semiconducting metal oxide materials have received much attention for use as photoanode materials in photoelectrochemical (PEC) water splitting devices. Among the multinary oxides investigated, bismuth vanadate (BiVO4) has garnered particularly high interest and remains the highest performing multinary oxide photoanode material to date. Reactions at the BiVO4/electrolyte interface may give rise to, or passivate, surface states. These surface states can act as relay sites for charge injection into the electrolyte, or as electron and hole traps that can enhance recombination rates. A detailed understanding of the chemical composition at the BiVO4/electrolyte interface and its dependence on specific conditions (applied potential and illumination) would provide valuable input for strategies to suppress surface recombination and to further optimize BiVO4-based photoanode materials.

We have used ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) to understand the light induced changes at the BiVO4/aqueous electrolyte interface. AP-HAXPES can be used to directly interrogate a solid surface under a bulk-like electrolyte film that is tens of nanometers thick. Using AP-HAXPES we have studied the open-circuit behaviour of the thin-film BiVO4/aqueous electrolyte interface under dark conditions and when illuminated with a solar simulator. In a phosphate buffer electrolyte and under illumination, we observe spectral changes consistent with the formation of a thin bismuth phosphate layer and the repulsion of anions away from the interface. In a borate buffer solution no chemical changes at the interface are observed.  Furthermore, by studying a series of electrolytes, consisting of a sodium citrate buffer, a sodium phosphate buffer and a combined sodium citrate-phosphate buffer, each with a pH of ~6.2, we show that the addition of citrate suppresses the formation of the thin bismuth phosphate layer.  These results provide fundamental information about the complex chemical behaviour of semiconductor/electrolyte interfaces used in water splitting devices and indicate that judicious choice of electrolyte may provide a means of controlling the interfacial properties.

11:30 - 11:45
2.2-O1
Venugopal, Anirudh
Delft University of Technology (TU Delft), The Netherlands
The Dynamic Nature of the Semiconductor-liquid Junction under Operating Conditions
Anirudh Venugopal
Delft University of Technology (TU Delft), The Netherlands, NL
Authors
Anirudh Venugopal a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
Abstract

Photo-electrochemical (PEC) water splitting has been championed as a prospective way to synthesize “green hydrogen” in an efficient and cheap manner. In that regard, different metal oxide semiconductors have been extensively investigated for their application as photoelectrode materials due to their low cost and supposed high stability in oxidizing environments. In addition, metal oxides have increasingly been used as protective layers to passivate unstable photoactive materials. In both cases, metal oxides are in contact with the electrolyte making a semiconductor-liquid junction (SLJ). While a significant amount of research focuses on the material properties of the metal oxide semiconductors, less attention has been paid to the role of the electrolyte, and how its composition (pH, anions, and cations) can affect the SLJ, especially under illumination. In this talk, we provide new fundamental insights on the electrode/electrolyte interface of metal oxide photoanodes under operating conditions, which can have substantial implications in the performance evaluation and long term stability of these materials. Using a series of characterization techniques, we show how the different components of the electrolyte can either form new layers on the surface under illumination, or can stabilize reaction intermediates enhancing electrocatalytic performance. Both of these effects have implications for practical utilization of PEC water splitting materials, but also for fundamental mechanistic insights of reaction pathways. We would like to emphasize to the PEC community that the dynamic nature of this electrode/electrolyte interface and the effect of co-ions in the solution should be accounted for while making performance analysis or comparisons, and in the testing of new photoanode materials.

Additional reference : Firet, Nienke J., and Venugopal, Anirudh, et al., “Chemisorption of anionic species from the electrolyte alters the surface electronic structure and composition of photocharged BiVO4” (under review in Chemistry of Materials)

11:45 - 12:00
2.2-O2
magnan, helene
SPEC
Time Resolved Photoemission for a Fine Characterization of Oxide Photoanode
helene magnan
SPEC, FR
Authors
Helene Magnan a, Pierre-Marie Deleuze b, Bruno Domenichini b, Antoine Barbier a, Mathieu Silly c
Affiliations
a, Service de Physique de l’Etat Condensé SPEC, UniversitéParis-Saclay, CEA Saclay
b, ICB UMR 6303 CNRS-UBFC, 9 avenue Savary BP 47870, 21078 Dijon Cedex, France
c, Synchrotron SOLEIL, Saint-Aubin, Gif-sur-Yvette, 91191, FR
Abstract

The chemical storage or solar energy in form hydrogen through photoelectrochemical water splitting is a promising method that has the important advantages of being environment friendly and free from carbon dioxide emission. Metal oxides are promising candidates for photoanode but their low efficiency is suspected to be due to strong electron-hole recombination. The carrier lifetime being one of the important limiting processes, its determination appeared as a mandatory step in the understanding of the underlying phenomena and eventually defining strategies to overcome this shortcoming. We have investigated the electron hole recombination dynamics in different oxide photoanodes using time-resolved photoelectron spectroscopy (TR-PES) utilizing a laser-pump / synchrotron – radiation (SR) probe method at different time resolutions. Upon photoexcitation, in the presence of a surface depletion layer (for an n-type semiconductor), electrons are promoted into the conduction band and migrate into the bulk, inducing a positive surface photovoltage (SPV). This surface charge induces a reduction of kinetic energy of photoelectrons measured by PES. The reverse phenomenon (shift toward higher kinetic energy) occurs for p-type semiconductors [1]. Therefore, the time-resolved energy shift of the photoemission spectra caused by the surface photovoltage effect is expected to be linked to the charge dynamics.

We have studied the carriers’ lifetime in Ti-doped Fe2O3 as a function of the Ti doping level. The samples were thin epitaxial films prepared by atomic oxygen plasma assisted molecular beam epitaxy deposited on single crystalline Pt(111) [2]. We will show that the electronic structure and lifetime are strongly influenced by carbon contamination while Ti doping is less determinent.

In a second part we will report on the influence of an internal electric field on carrier’s lifetime in ferroelectric BaTiO3 thin films. The samples were BaTiO3 thin epitaxial films prepared by atomic oxygen plasma assisted molecular beam epitaxy deposited on single crystalline Pt(100) with different orientation of electrical polarisation. We observe that the SPV value and the time decay strongly depends on polarisation orientation.

All these results were correlated with photoelectrochemical measurements on the same samples.

12:00 - 12:15
2.2-O3
Pasquini, Chiara
Freie Universität Berlin
Redox Energetics and Kinetics of Water Oxidation in Neutral versus Alkaline Electrolyte: an In-Operando Time-Resolved X-Ray Absorption Study
Chiara Pasquini
Freie Universität Berlin, DE
Authors
Chiara Pasquini a, Holger Dau a
Affiliations
a, Freie Universität Berlin, Arnimallee 14, Berlin, DE
Abstract

In photoelectrolyzers for storage of solar energy in form of molecular hydrogen, the choice of alkaline environment favors the oxygen evolution reaction (OER) strongly (500 times higher current densities) and stabilizes the catalyst material. On the other hand, it introduces severe disadvantages: it is corrosive and impedes the coupling of OER with CO2 reduction. This increases the importance of development of water-oxidation catalysts active in the neutral pH range. One prominent example is the amorphous Co–based and phosphate containing catalyst (CoCat) proposed in 2008 by Kanan and Nocera [1]. Previous investigations of CoCat films on conducting electrodes revealed metal atoms assembled in fragments of edge-sharing CoO6 octahedra, which are accumulating oxidation equivalents through CoII/III and CoIII/IV oxidation before O2 formation can take place [2].

We investigated the electrochemical performances of this amorphous oxide in both neutral and alkaline pH regimes. At parity of overpotential, we observed a strong increase in catalytic activity in alkaline electrolyte. In order to understand the factors that favors oxygen production at alkaline pH, we performed in-situ time-resolved X-ray absorption spectroscopy (XAS) during electrochemistry, which revealed an improvement in both the energetics and kinetics of Co redox reactions. The lower onset of OER in alkaline electrolyte is caused by a shift in the equilibrium potential of the CoIII ↔ CoIV reaction, which was assigned to easier CoIII-OH deprotonation. The faster oxidation state changes observed where attributed to better electron and proton transport inside the bulk of the material, which are due to a crystallization process.

We found that the amorphous CoCat undergoes a spontaneous increase in order resembling transition from an amorphous to a crystalline material when exposed to alkaline electrolyte; the crystallization is coupled with a major change in Co average oxidation state. Raman spectroscopy and analysis of EXAFS data were employed to follow the structural changes, which resulted in a structure with bigger CoOx clusters, less defects and strong similarities to CoO(OH). The loss of amorphicity improved electron transfer but reduced number of Co active sites. We propose a technique for stopping the crystallization process, by applying a strong oxidative potential immediately after exposing the catalyst to alkaline solution.

12:15 - 12:30
2.2-O4
Corby, Sacha
Imperial College London
Spectroscopic Analysis of NiOx Catalysts for Water Oxidation
Sacha Corby
Imperial College London, GB
Authors
Sacha Corby a, Miguel Garcia-Tecedor b, Laia Francas a, Shababa Selim a, Sven Tengeler c, Dongho Lee d, Camilo Mesa a, Wolfram Jaegermann c, Sixto Gimenez b, Kyoung-Shin Choi d, James Durrant a
Affiliations
a, Department of Chemistry, Imperial College London, South Kensington Campus London, London, GB
b, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
c, Institute of Material Science, TU Darmstadt, 64287 Darmstadt (Germany)
d, Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706, USA
Abstract

Photoelectrochemical water splitting presents a viable means of sustainable fuel generation, but the limiting kinetics of the four-hole water oxidation reaction presents a bottleneck. While significant progress has been made to enhance the performance of transition metal oxide photoanodes through improved synthetic measures and nanostructuring, overall STH efficiencies remain low. The development of electrocatalysts, which may be used in conjunction with a photovoltaic device (PV + electrolysis) or integrated with a photoanode material, is thus an active area of research. NiOx based materials have received a lot of attention recently as efficient, stable water oxidation electrocatalysts. In this presentation, I will discuss our on-going research into a mechanistic understanding of NiOx electrocatalysts. In particular, I will focus on the role of Fe-dopants in these systems, and examine samples of different thicknesses to determine where the oxidised species accumulate in such films. We employ electrochemical impedance spectroscopy, spectroelectrochemistry and transient optical techniques to monitor the oxidative steps involved in the reaction cycle. With these insights, we aim to identify the cause of the high performance seen in these materials and the limiting factors which may be addressed to further improve performance.

12:00 - 13:30
Lunch
12:30 - 14:00
Lunch
CharDy 2.3
Chair: Jenny Nelson
14:00 - 14:30
2.3-I2
Itskos, Grigorios
University of Cyprus
Photophysics and Optoelectronic Applications of Lead Halide Perovskite Nanocrystals
Grigorios Itskos
University of Cyprus, CY

Grigorios Itskos obtained a B.Sc. in Physics in 1997 from University of Thessaloniki, Greece and carried out his PhD studies at SUNY at Buffalo, USA (Ph.D. in Physics 2003), under the supervision of Prof. Athos Petrou within the newly-born field of semiconductor spintronics. He worked as postdoctoral researcher (Imperial College London, 2003-2007) under the supervision of Profs. Donal Bradely and Ray Murray, focusing on photophysical studies of hybrid organic-inorganic semiconductors. In September 2007 he become a faculty member at the Department of Physics, University of Cyprus (Lecturer 2007-2011, Assistant Professor 2011- 2017, Associate Professor 2017- now). His group research activities focus on optical studies of inorganic, organic and hybrid solution-processed semiconductors, with recent emphasis on the characterization and optoelectronic applications of semiconductor nanocrystals.  

Authors
Grigorios Itskos a
Affiliations
a, Experimental Condensed Matter Physics Lab, Department of Physics, University of Cyprus, CY
Abstract

Lead halide perovskite nanocrystals (LHP NCs) have emerged as outstanding light emitters exhibiting luminescence with near unity quantum yield and narrow linewidth that can be tuned across the visible spectrum via facile ion exchange reactions. The talk will discuss recent work on the photophysics and optoelectronic applications of such nanomaterials, with emphasis on the following:

(i) Green-emitting CsPbBr3 and red-emitting FAPbI3 NCs exhibit efficient amplified spontaneous emission with high net modal gain and low threshold making them attractive for optically pumped solution-processed lasers1.

(ii) Intense photoexcitation generates long-lived hot carriers in FAPbI3 NCs2,3, making them promising candidates for hot solar cells. Furthermore the produced hot electron–hole gas appears to influence the energetics and dynamics of the radiative recombination via a competition between the stimulated emission and the emission from non-thermalized carriers3.

(iii) Efficient light emitting structures are demonstrated via two routes: 1. Using LHP NCs as robust downconverters of blue-emitting GaN nanohole array emitters and 2. Employing electronic-functionalized solids of such NCs in the active region of solution-processed electroluminescent devices.

(iv) Encapsulation of LHP NCs into polymer nanofibers yields robust composites exhibiting bright emission, improved air- and water-stability and potential applications in textile-based LED devices4

14:30 - 14:45
2.3-O1
Hempel, Hannes
Helmholtz Zentrum Berlin, Germany
Limits of Charge Carrier Transport in Halide Perovskites Revealed by Optical-Pump Terahertz-Probe Spectroscopy
Hannes Hempel
Helmholtz Zentrum Berlin, Germany
Authors
Hannes Hempel a, Andrei Petsiu a, Martin Stolterfoht b, Pascal Becker a, Dieter Neher b, Rainer Eichberger c, Thomas Unold a
Affiliations
a, Department Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Hahn-Meitner-Platz, 1, Berlin, DE
b, Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476, Potsdam-Golm, Germany
c, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany, Berlin, DE
Abstract

Metal-halide hybrid perovskites exhibit excellent optoelectronic properties – apart from their rather moderate charge carrier mobilities.

The origin of these moderate mobilities has been attributed to several (contradicting) effects, such as the formation of large and small polarons, dynamic disorder due to the soft nature of these materials, slow rotational modes of the organic molecules, as well as to the confinement of charge carriers in grains, ferroelectric domains or nanostructures.

To clarify the nature of the charge carrier transport, we probed different hybrid and inorganic halide perovskites thin films and nano-crystals by Optical-Pump Terahertz-Probe (OPTP) spectroscopy.

Interestingly, we find very similar sum mobilities of approximately 60 cm2/Vs as well as similar phonon spectra for MAPbI3, (Cs,FA)PbI3, CsPbI3 [1] and (Cs,FA,MA)Pb(I,Br)3, thin films [1,2]. This finding excludes a significant impact of the organic cation rotation and supports the dominance of the Pb-I cage on the optoelectronic properties.

Additionally, OPTP reveals the frequency-dependence of the charge carrier mobility at THz-frequencies, which indicates very short scattering times at room temperature. Such large scattering impedes transport and limits the charge carrier mobilities.

Temperature-dependent OPTP measurements on the same materials show a strongly increasing mobility with lower temperature, thus excluding small polaron formation and hopping transport. Instead, this behavior can be modeled by conventional large polaron theory and Fröhlich-type electron-phonon scattering.

Thin films of CsPbI3-nanocrystals exhibit even smaller charge carrier mobilities of < 1cm2/Vs compared to their polycrystalline counterparts. Therefore, their charge transport is limited by the coupling between the nano-crystals and different approaches are shown to improve this coupling.

Further, we discuss the impact of phonon modes on the OPTP signal that overlay the mobility-correlated signal. Such phonon contributions are especially relevant for nano-crystals and at low temperatures.

 

14:45 - 15:00
2.3-O2
Pydzinska, Katarzyna
Adam Mickiewicz University in Poznań
Charge Dynamics, Absorption and Emission Spectra of Triple Cation Perovskite Solar Cells under Different Place of Excitation, Illumination and Applied Potential.
Katarzyna Pydzinska
Adam Mickiewicz University in Poznań
Authors
Katarzyna Pydzinska a, Jesus Idigoras b, Juan Anta b, Victoriia Durshliak a, Marcin Ziółek a
Affiliations
a, Adam Mickiewicz University in Poznan, Faculty of Physics, Uniwersytetu Poznanskiego 2, 61-614 Poznan, Poland.
b, Pablo de Olavide University, Department of Physical, Chemical and Natural Systems, Crta. De Utrera km.1, 41089 Seville, Spain.
Abstract

Perovskite solar cells due to outstanding absorption and charge transport properties are one of the most promising types of photovoltaic devices. The possible ion migration, several ions content and non-mono crystal structure makes them  really intriguing as well as challenging system to investigate by optical techniques. Electrons in perovskite system, after absorption, are promoted from valence to conduction band. First hundreds of femtosecond after absorption are governed by cooling of hot carriers. When the process is finished,  sharp absorption bleach due to the band filling phenomena occurs which decay correlates with photoluminescence kinetics and represents the excited carrier lifetime [1]. This decay proceeds by several paths such as recombination (first-, second- and third-order) and injection to electron transporting material (ETM) or hole (HTM) transporting material.        

We focused on triple cation perovskite FA0.76MA0.19Cs0.05(I0.81Br0.19)3 sandwiched between spiro-OMeTAD and mesoporous TiO2 layers prepared in open air conditions. We performed femtosecond to nanosecond transient absorption studies as well as picosecond to nanosecond time-resolved emission decays measurements of the prepared cells. Different placement of excitation within perovskite material is realized by varying the excitation side (either from TiO2 or spiro-OMeTAD  side) and wavelength. We observed strikingly different dynamical and spectral responses when the excitation localized close to ETM (TiO2) or HTM (spiro-OMeTAD). In the case of ETM interface the carrier lifetime is usually shorter than for HTM interface, and both transient and stationary absorption and emission bands are shifted more to the red. This indicates the possible different properties of the perovskite material close to the contacts with ETM and HTM. We have also observed the correlation of the charge lifetime with photocurrent of the cells, similar to our recent studies on mixed methylammonium and formamidinium solar cells [2].

We also introduced additional bias illumination and bias voltage during the  femtosecond transient absorption experiments to make the measurements under real operating conditions for the solar cell. Additionally, we studied an influence of DMSO concentration (DMF:DMSO ratio) in precursor solution on macroscopic and microscopic cell parameters such as photovoltaic parameters, recombination rate constant, electron lifetime, absorption and aging.

15:00 - 15:30
2.3-I1
Kirchartz, Thomas
FZ Jülich
Combination of Transient and Steady-state Photoluminescence for the Characterization of Halide Perovskite-based Layer Stacks
Thomas Kirchartz
FZ Jülich, DE

He studied electrical engineering in Stuttgart and started working on Si solar cells in 2004 under the guidance of Uwe Rau at the Institute for Physical Electronics (ipe) in Stuttgart. After finishing his undergraduate studies in 2006, he continued working with Uwe Rau first in Stuttgart and later in Juelich on simulations and electroluminescence spectroscopy of solar cells. After finishing his PhD in 2009 and 1.5 years of postdoc work in Juelich, Thomas Kirchartz started a three year fellowship at Imperial College London working on recombination mechanisms in organic solar cells with Jenny Nelson. In 2013, he returned to Germany and accepted a position as head of a new activity on hybrid and organic solar cells in Juelich and simultaneously as Professor for Photovoltaics with Nanostructured Materials in the department of Electrical Engineering and Information Technology at the University Duisburg-Essen. Kirchartz has published >100 isi-listed papers, has co-edited one book on characterization of thin-film solar cells whose second edition was published in 2016 and currently has an h-index of 38.

Authors
Thomas Kirchartz a
Affiliations
a, IEK-5 Photovoltaics, Forschungszentrum Jülich, Wilhelm-Johnen-Straße, Jülich, 52425, DE
Abstract

Both transient and steady state photoluminescence PL have been frequently used to analyze the properties of halide perovskite films1 and recently also layer stacks, i.e. films with interfaces.2-4 Here, we present our current level of understanding of how to analyze the data. In the case of films, long decays in transient PL correlate well with strong steady state PL. The shape of the decays allows us to determine bimolecular and monomolecular recombination coefficients, the former of which is clearly affected by photon recycling.1,5 In the case of films with one interface, we show that high luminescence is still beneficial for high open-circuit voltages in devices and still correlates with long photoluminescence decays.6 We show by simulation how the combination of steady state PL with tr-PL can be used to better understand band alignment at interfaces and how it provides an estimate of the surface recombination velocities. Finally, we discuss the case of layer stacks with two contacts and of full devices. Here, additional effects such as the conductivity and capacitance of contact layers become important. In addition, the size of the laser spot becomes relevant because it affects how much the lateral redistribution of charge carriers via the conductive contact layers affects the result. Finally, we discuss how transient PL and methods like transient photovoltage differ at least quantitatively in the perovskite solar cell. The difference between the two is that transient PL measures the internal voltage, i.e. the quasi-Fermi level splitting, and transient photovoltage measures the external voltage that builds up at the external terminals of the cell. While both decays are affected by the contact layers, the impact is substantially different. The external voltage first has to be build up by charging up the capacitance of the interfacial layers, the internal voltage peaks immediately after the laser pulse and then decays quickly.

   (1)    Staub, F.; Hempel, H.; Hebig, J. C.; Mock, J.; Paetzold, U. W.; Rau, U.; Unold, T.; Kirchartz, T. Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films From Time-Resolved Photoluminescence. Phys. Rev. Applied 2016, 6, 044017.

   (2)    Krogmeier, B.; Staub, F.; Grabowski, D.; Rau, U.; Kirchartz, T. Quantitative Analysis of the Transient Photoluminescence of CH3NH3PbI3/PC61BM Heterojunctions by Numerical Simulations. Sustainable Energy Fuels 2018, 2, 1027-1034.

   (3)    Stolterfoht, M.; Wolff, C. M.; Marquez, J. A.; Zhang, S.; Hages, C. J.; Rothhardt, D.; Albrecht, S.; Burn, P. L.; Meredith, P.; Unold, T. et al. Visualization and Suppression of Interfacial Recombination for High-Efficiency Large-Area Pin Perovskite Solar Cells. Nat. Energy 2018, 3, 847-854.

   (4)    Stolterfoht, M.; Caprioglio, P.; Wolff, C. M.; Marquez, J. A.; Nordmann, J.; Zhang, S.; Rothhardt, D.; Hörmann, U.; Redinger, A.; Kegelmann, L.; Albrecht, S.; Kirchartz, T.; Saliba, M.; Unold, T.; Neher, D. The perovskite/transport layer interfaces dominate non-radiative recombination in efficient perovskite solar cells. Unpublished Work, 2018.

   (5)    Staub, F.; Kirchartz, T.; Bittkau, K.; Rau, U. Manipulating the Net Radiative Recombination Rate in Lead Halide Perovskite Films by Modification of Light Outcoupling. The Journal of Physical Chemistry Letters 2017, 8, 5084-5090.

   (6)    Liu, Z.; Krückemeier, L.; Krogmeier, B.; Klingebiel, B.; Marquez, J. A.; Levcenko, S.; Öz, S.; Mathur, S.; Rau, U.; Unold, T. et al. Open-Circuit Voltages Exceeding 1.26 V in Planar Methylammonium Lead Iodide Perovskite Solar Cells. ACS Energy Lett. 2019, 4, 110-117.

 

PERInt 2.3
Chair: Emilio J. Juarez-Perez
13:30 - 14:00
2.3-O1
Tisdale, William
Massachusetts Institute of Technology
Exciton-Exciton and Exciton-Lattice Interactions in 2D and 0D Perovskites
William Tisdale
Massachusetts Institute of Technology

Will Tisdale joined the Department of Chemical Engineering at MIT in January, 2012, where he holds the rank of Associate Professor and is currently the ARCO Career Development Professor in Energy Studies.  He earned his B.S. in Chemical Engineering from the University of Delaware in 2005, his Ph.D. in Chemical Engineering from the University of Minnesota in 2010, and was a postdoc in the Research Laboratory of Electronics at MIT before joining the faculty in 2012. Will is a recipient of the Presidential Early Career Award for Scientists and Engineers (PECASE), the DOE Early Career Award, the NSF CAREER Award, an Alfred P. Sloan Fellowship, the Camille Dreyfus Teacher-Scholar Award, the AIChE Nanoscale Science & Engineering Forum Young Investigator Award, and MIT’s Everett Moore Baker Award for Excellence in Undergraduate Teaching.

Authors
William Tisdale a
Affiliations
a, Massachusetts Institute Of Technology, 77 Massachusetts Avenue, Room 2-216, Cambridge, 2139
Abstract

When fabricated in a nanostructured form – either as layered 2D quantum wells or colloidal nanocrystals – hybrid perovskite nanomaterials exhibit a combination of interesting properties revealing both quantum mechanical and classical composite effects. In this talk, I will discuss the thermal, vibrational, and excitonic properties of hybrid perovskite nanomaterials as a function of composition, structure, and temperature. In particular, I will discuss ultrafast spectroscopic studies of exciton-lattice interactions in 2D Ruddlesden-Popper perovskites and the unique behavior of biexcitons in colloidal CsPbBr3 nanocrystals. Using a combination of excited state resonant impulsive stimulated Raman scattering (RISRS), low-frequency Raman scattering, density functional theory (DFT), and temperature-dependent photoluminescence, we investigate the effect of organic cation size on exciton-phonon coupling in a series of 2D lead iodide perovskites. We find that inorganic cage motion dominates excited state dynamics in this family, with minimal contribution from the organic cations. In CsPbBr3 nanocrystals, we analyze fluence-dependent transient absorption data to extract the overlapping spectra of exciton and biexciton states while making no assumptions about their spectral lineshapes. From the size-dependence of these spectra, we make conclusions about the nature of exciton-exciton interactions in these colloidal systems and how they differ from conventional II-VI quantum dots.

14:00 - 14:30
2.3-O2
Domanski, Konrad
Fluxim AG
Performance of Perovskite Solar Cells under Real-World Temperature-Illumination Variations in the Lab
Konrad Domanski
Fluxim AG, CH
Authors
Konrad Domanski a, Brian Carlsen b, Anand Agrawalla b, Essa Alharbi b, Michael Graetzel b, Anders Hagfeldt b, Wolfgang Tress b
Affiliations
a, Fluxim AG, Katharina-Sulzer-Platz, 2, Winterthur, CH
b, Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, Lausanne, CH
Abstract

After reaching 20% efficiency, research in perovskite photovoltaics has shifted from a race for efficiency to a race for stability. For efficiency, the standard test conditions (STC) set the rules for the race. However, the term stability is used very broadly and assessed in various ways, meaning different groups are running different races. [1] For the application, however, only energy yields that can be achieved under real-world, long-term operation matter. [2]

In this work [3], we characterize and analyse the performance of 20% efficient perovskite solar cells under simulated ambient conditions based on real temperature and irradiance data of 27 selected days during a year in Sion, Switzerland. Working in a controlled lab environment, a much more reliable and systematic collection of data can be carried out, avoiding parasitic failure mechanisms, and weather conditions of an arbitrary location and time of year can be emulated without physical presence outdoors.

We find that the perovskite solar cell shows a low decrease of efficiency with elevated temperature and low light intensity, maintaining almost optimum values for dominant ambient conditions. Therefore, the resulting year-averaged efficiency (energy produced/total illumination energy) is close to the STC value. The overall energy yield is influenced by reversible degradation (<2% a day), delivering the highest performance in the morning, and ~10% permanent degradation, observable during the year.

Finally, we compare the perovskite with 22% efficient silicon heterojunction devices. Whereas the maximum-power-point voltage of these is differently affected by the weather conditions, the current scales linearly with light intensity for both, which is particularly important when considering 2-terminal perovskite-silicon tandem cells.

14:30 - 14:45
2.3-O3
Wang, Qiong
Young Investigator Group Active Materials and interfaces for stable perovskite solar cells, Helmholtz-Zentrum Berlin für Materialien und Energie
How to Improve the Stability of All Inorganic Perovskite Solar Cells?
Qiong Wang
Young Investigator Group Active Materials and interfaces for stable perovskite solar cells, Helmholtz-Zentrum Berlin für Materialien und Energie, DE
Authors
Qiong Wang a, Antonio Abate a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
Abstract

All inorganic perovskites hold the promise to solve the thermal-stability issue embedded by the organic-inorganic lead halide perovskites, such as methylammonium lead iodide perovskite, and have the ideal optical bandgap for the top cell in a tandem structure with silicon or CIGS cell, thus have become one of the hottest research field in the recent two years. Here, we explored the deposition process, precursors and substrates used for the all inorganic perovskite solar cells. Advanced techniques such as synchrontron-based nano-x-ray fluorescence, GIWAXS, time-resolved/steady state photoluminescence spectroscopy and impedance spectroscopy show that halide segregation is one of the reasons that limit the efficiency and stability of perovskite solar cells. Moreover, we demonstrated that the fast annealing process would effectively solve this issue and give a highly stable perovskite solar cells with an efficiency of 13%. The accelerated ageing measurement show that our devices hold 90% of the initial efficiency after 10,000 hours constant illumination at 1 Sun (AM 1.5G, 100 mW/cm2).

14:45 - 15:00
2.3-O4
Ugur , Esma
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia
Change in Excited State Dynamics of Perovskite Solar Cells after Exposure to Humid Air under Illumination
Esma Ugur
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, SA
Authors
Esma Ugur a, Jafar I. Khan a, Erkki Alarousu a, Martin Ledinsky b, Sandra P. Gonzalez-Lopez a, Ahmed H. Balawi a, Erkan Aydin a, Michele De Bastiani a, Stefaan De Wolf a, Frédéric Laquai a
Affiliations
a, King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, 4700 King Abdullah University of Science and Technology (KAUST), Thuwal, SA
b, Institute of Physics, Academy of Sciences of the Czech Republic., Cukrovarnická, 10, Prague, CZ
Abstract

In a very short time span, the power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has reached 24%, a massive improvement for solution-processed photovoltaic devices. Towards this end, both surface and bulk recombination of photogenerated charge carriers in the perovskite absorber layer is the major efficiency-limiting factors. [1] In this respect, controlling the growth and crystallization of the perovskite thin film is crucial for the performance of the perovskite solar cells as the crystal growth dynamics are very susceptible to the processing conditions [2]. Interestingly, some studies report a beneficial effect from water inclusion during processing, while others claim an adverse effect. Moreover, the effect of humidity and light exposure on the performance of PSCs is still debated. In this study, we fabricated, using a two-step protocol, MAPbI3 perovskite solar cells with SnO2 electron transport layers to study how the device performance and photophysics change upon exposure to humidity and light. Reference devices, not exposed to humidity and light, exhibit 18.7 % PCE with 22.6 mA/cm2 short-circuit current density (Jsc) and 1.14 V open circuit voltage (Voc). After exposing the perovskite absorber layer to 45-55% relative humidity under 1-sun illumination prior to completing the device fabrication, a reduction in Voc to 1.09 V was observed. We study the influence of excess lead iodide (PbI2), which is commonly believed to be passivating the PSCs, on the humidity resistance of MAPbI3 devices. While the Voc value of samples with excess-PbI2 was lowered to 1.10V, we did not observe any change in Jsc. Since Voc losses can be attributed to non-radiative recombination, we performed time-resolved photoluminescence (TR-PL) spectroscopy before and after humid air exposure of perovskite thin films under 1-sun illumination. We will discuss in detail how the exposure to humid air under illumination alters the surface structure of the perovskite samples, affects the device performance, and the photophysical processes.

15:00 - 15:15
2.3-O5
Phung, Nga
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Impact of Alkaline Earth Metal Doping on the Stability of Perovskite Solar Cells
Nga Phung
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Nga Phung a, Hans Köbler a, Diego Di Girolamo b, Thi Tuyen Ngo c, Gabrielle Sousa e Silva a, Ivan Mora-Seró c, Bernd Rech a, Antonio Abate a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, Dept. of Chemistry, University of Rome Sapienza, P.le A. Moro 5, 00185 Rome,Italy
c, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

Halide perovskite solar cells (PSCs) have progressed rapidly to reach an efficiency of more than 24% in laboratory scale to date. However, the long-term stability of PSCs is the main roadblock on the path to industrialization. In this work, we present stability studies on perovskite solar cells utilizing interfacial alkaline earth metal doping.

We are employing a high throughput aging setup to study the long-term stability of the devices. The setup is equipped to measure up to 500 cells simultaneously in maximum power point tracking (MPPT) mode. The combination of full control over temperature, atmosphere and load for such a high number of devices is unique so far and enables significant studies providing reliable data.

One strategy to overcome PSC instability is interfacial doping with alkaline earth metals. We previously demonstrated alkaline earth metal ions (i.e. Mg and Sr) as excellent dopants to improve the device’s performance. The dopants reduce the defect concentrations in the perovskite which in turn significantly improves the open circuit voltage. Nonetheless, when the dopants are intermixed in the precursor solution, this leads to a significant change in the perovskite morphology. To preserve the perovskite morphology, we use dopants as an interlayer in a p-i-n device structure.[1] Following the strategy of interlayer-doping, we present highly stable and reproducible CsMAFA perovskite based[2] solar cells.

15:15 - 15:30
Abstract not programmed
13:30 - 15:30
RadDet 2.3
MapNan 2.3
Chair: Brian Rodriguez
14:00 - 14:30
2.3-I1
Ahmadi, Mahshid
University of Tennessee, Knoxville
Spatially Resolved Carrier Dynamics and Associated Chemical Changes at Hybrid Organic-inorganic Perovskite/Electrode Interfaces
Mahshid Ahmadi
University of Tennessee, Knoxville

Dr. Mahshid Ahmadi received her Ph.D. from Nanyang Technological University, Singapore in 2013. She then worked as a research technology consultant in a start-up solar cell company (HEE) in Dallas, Texas, USA. She is currently working as an assistant professor at Joint Institute for Advanced Materials (JIAM), Department of Materials Science, University of Tennessee, Knoxville. Her research interest includes materials development and electronic device fabrication. Specially, her current research focuses on organic-inorganic halide perovskite photovoltaics and
high energy radiation detectors.

Authors
Mahshid Ahmadi a, Liam Collins b, Kate Higgins a, Matthias Lorenz b, Sergei V. Kalinin b
Affiliations
a, Joint Institute for Advanced Materials, Department of Materials Science and Engineering, University of Tennessee, Knoxville, US
b, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, EE. UU., Oak Ridge, US
Abstract

Beyond the admirable photovoltaic properties, the unique opto-electrical properties of organic-inorganic halide perovskite (OIHP) combined with their relatively low-cost production, have made this class of materials a great candidate in photodetectors1, LEDs2, and radiation sensors3. Key to optimization of these applications is the understanding of the fundamental physical and chemical processes at the electrode interface4, 5, 6. Charge injection at electrode-OIHPs interface may induce interfacial trapped states and recombination regions leading to unfavorable effect on charge collection efficiency,7, 8 induce electrochemical reactions and result in interfacial degradation4, 8. The latter can be pure- metal-OIHP process, or additionally mediated by the presence of environment at the triple-phase boundaries.5, 9

The lack of understanding stems largely from the lack of appropriate tools to capture the electrochemical dynamics on the length scales of the local inhomogeneities and time scales over which the coupled dynamics take place. Here, we implemented Kelvin probe force microscopy (KPFM) for probing charge dynamics at electrode-OIHPs interface on time scales from minutes to seconds (KPFM), milliseconds (time resolved (tr-) KPFM) and microseconds (G-KPFM) to explore the spatial and temporal charge dynamics at MAPbBr3 devices as a model system. KPFM provides a nanoscale profile of device potential as a function of time during application of an external stimuli (light and bias). This allows us to gain insight into the temporal dynamics at the electrode- OIHPs interface activated by electric field. Indeed, the multimodal KPFM is a complementary measurement to the impedance spectroscopy as the impedance provides information on the relaxation process in frequency domain while KPFM provide information on the local relaxation phenomena in time domain. 

Unlike electronic processes, the ionic and electrochemical phenomena can be semi- or non-reversible and highly non-linear. Although evidence of ion migration by KPFM has been reported previously10, 11, standard KPFM is not well-suited to explore time dependent phenomena due to the limited time resolution (~1 sec) of the approach. Therefore, we employ time resolved (tr-) and ultrafast (G-mode) KPFM to directly probe the dynamic spatio-temporal behavior of the in-operandi device. G-KPFM has previously been shown to provide ~10s µs time resolution with 10s nm spatial resolution, making it a suitable approach for detecting and separating field induced fast transport processes12, 13.

            The nature of the electrochemical phenomena at the metal-OHIP interfaces is explored via the in-situ time of flight secondary ion mass spectrometry (ToF – SIMS) measurements. We explore the changes of the ionic concentrations along the surface of the laterally electrode structure under applied bias, identifying the nature of the mobile ionic carriers and their bias responses. The changes in the responses in the presence of illumination are further elucidated.

            The results demonstrate an interplay of several phenomena, including charge injection, recombination and ion migration, leading to an unbalanced charge dynamic in MAPbBr3-Au interface under forward and reverse biases, explaining the origin of the current-voltage hysteresis in these devices. We contrast the bias assisted charge dynamics under both illuminated and dark conditions. Our multimodal time scale KPFM measurements reveal that the charge transport behavior in hybrid perovskites are light, bias and environmental dependent providing a comprehensive picture of overall carrier dynamics and interface properties in MAPbBr3 perovskites with lateral Au electrodes.

14:30 - 14:45
2.3-O1
Axt, Amelie
Max Planck Institute for Polymer Research, Mainz, Germany
Know your full potential: Kelvin probe force microscopy on nanoscale electrical devices and at solid-liquid interfaces
Amelie Axt
Max Planck Institute for Polymer Research, Mainz, Germany, DE
Authors
Amelie Axt a, Ilka Hermes a, Stefan A.L. Weber a
Affiliations
a, Max Planck Institute for Polymer Research, Mainz, Germany, Ackermannweg, 10, Mainz, DE
Abstract

KPFM is widely used to map the nanoscale potential distribution in operating devices, e.g. in thin film transistors, battery materials, solar cells or even in liquids on e.g. membranes and coatings.

Surface potential measurements are crucial for understanding the operation principles of functional nanostructures in electronic devices. Knowing the local surface potential can also contribute to understanding effects like corrosion and membrane aging.

Nevertheless, KPFM is prone to certain imaging artifacts, such as crosstalk from topography or stray electric fields.

We compare different amplitude modulation (AM) and frequency modulation (FM) KPFM methods on a reference structure consisting of a glass-platinum interdigitated electrode array. This structure allows to modify the surface potential externally and minimizes corrosion, while mimicking the sample geometry in device measurements, e.g. on thin film transistors or battery materials.

We found that when operated with a feedback, FM KPFM methods provide more quantitative results that are less affected by the presence of stray electric fields compared to AM KPFM methods[1]. Since a feedback limits the scanning speed, voltage range, is prone to artifacts and is not applicable in liquids, we also tested open loop methods. In open loop operation, FM KPFM methods provide more contrast compared to AM KPFM methods.

 

Figure 1: CPD line profiles of two closed loop KPFM experiments on the same cross section of a mesoscopic perovskite solar cell under short circuit conditions with and without illumination, visualized by the red and blue line, respectively. The cell consisted of a fluorine-doped tin oxide (FTO) electrode, a compact TiO2 electron extraction layer and a mesoscopic TiO2 layer (meso) filled with the perovskite light-absorber methylammonium lead iodide (MAPI). The mesoscopic layer was followed by a compact MAPI capping layer, the hole transport material spiro-OMETAD and a gold electrode. Prior to the measurement, the cross section of the solar cell was polished with a focused ion beam (FIB) to minimize topographic crosstalk. The CPD line profiles on the left were extracted from double side band frequency modulation KPFM (FM sideband) scans in single pass with VAC of 3V [2]. The CPD line profiles on the right were extracted from amplitude modulation KPFM (AM lift mode) scans in lift mode with a tip-sample distance of 10 nm, an oscillation amplitude of ~80 nm and a tip voltage UAC of 1 V. Each line profile is an average of three adjacent scan lines. [1]

Sol2D 2.3
Chair: Xavier MARIE
14:00 - 14:30
2.3-I1
Siebbeles, Laurens
Delft University of Technology (TU Delft), The Netherlands
Nature and Dynamics of Charge Carriers and Excitons in Colloidal 2D Materials
Laurens Siebbeles
Delft University of Technology (TU Delft), The Netherlands, NL

Laurens Siebbeles (1963) is leader of the Opto-Electronic Materials Section and deputy head of the Dept. of Chemical Engineering at the Delft University of Technology in The Netherlands. His research involves studies of the motion of electrons in novel nanostructured materials that have potential applications in e.g. solar cells, light-emitting diodes and nanoelectronics. Materials of interest include organic nanostructured materials, semiconductor quantum dots, nanorods and two-dimensional materials. Studies on charge and exciton dynamics are carried out using ultrafast time-resolved laser techniques and high-energy electron pulses in combination with quantum theoretical modeling.

Authors
Laurens Siebbeles a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
Abstract

We studied the photogeneration, mobility and decay dynamics of charge carriers and excitons in colloidally synthesized or liquid exfoliated 2D materials. Examples include metal chalcogenide nanosheets, superlattices of connected PbSe quantum dots with square or honeycomb geometry, and black phosphorus. The studies were performed using ultrafast pump-probe laser spectroscopy with optical or terahertz conductivity detection.

The composition and nanogeometry of the material were found to have pronounced effects on the relative yield of free mobile charges and neutral excitons. The relative yield of excitons was found to increase with excitation density. This effect could be described on the basis of the Saha equation, which accounts for more charge recombination at higher photoexcitation density. Surprisingly, excitons in CdSe nanosheets are stable even at high densities were they start to exhibit spatial overlap. This counter intuitive result can be understood theoretically from the fact that the Coulomb screening length, and thus the exciton binding energy, remain non-zero even at high density. This is a particular result for 2D materials and does not hold for 3D semiconductors.

The mobility of charge carriers depends strongly on the nanogeometry and material composition. In PbSe honeycomb superlattices mobilities are of the order of 1 cm2/Vs, while in square superlattices and PbS nanosheets values as high as a few hundred cm2/Vs were found. The frequency dependence of the mobility could be described theoretically by the Drude-Smith model, which includes effects of charge scattering on phonons as well as static defects.

14:30 - 14:45
2.3-O1
Horani, Faris
Technion - Israel Institute of Technology
Deciphering the evolution mechanism of 3D β-In2S3 nanoclusters
Faris Horani
Technion - Israel Institute of Technology, IL
Authors
Faris Horani a, Efrat Lifshitz b
Affiliations
a, Technion, Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Haifa, IL
b, Technion, Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Haifa, IL
Abstract

Nanoscale semiconductor materials shaped as urchins and flowers (termed, nanourchins and nanoflowers, respectively) stimulated extensive attention in recent years owing to their branched 3D structure with a large surface-to-volume ratio, thus demonstrating practical applications in catalysis, chemical sensors and various optoelectronic devices. Nanourchins and nanoflowers based on In2S3 semiconductors are of a special interest, due to their low toxicity, activity in the UV-Vis spectral regime, high carrier mobility and unique morphologies. In this study, we explore colloidal growth of β-In2S3 nanourchins and nanoflowers occurring from single-nanodots which over the growth period are self-assembled into branched 3D structures, where shape control is dictated by the synthesis conditions. The current work is an innovative study that describes in detail the growth mechanism that leads to specific structures and surface properties. Advanced electron microscopy methods revealed an elemental distribution across a nanourchin and displayed a dense core with In2S3 composition, surrounded by a shell consisting of In-rich spikes protruding from the core surface. The nanourchins undergo transformation into nanoflowers after altering reaction conditions, involving re-organization of the atoms' positions and formation of a hollow core/shell structure with In2S3/InS composition. Eventually, the nanoflowers decompose into nanoplatelet shapes. The use of specific organic ligands was observed to dictate the formation of the 3D clusters. The proposed formation mechanisms for each nanostructure were corroborated by thermodynamic considerations.

14:45 - 15:00
2.3-O2
Giuffredi, Giorgio
Istituto Italiano di Tecnologia
Nanocrystalline, Mixed-Phase Transition Metal Oxide/Oxy-Chalcogenide Nanostructures for Efficient Hydrogen Evolution Electrocatalysis
Giorgio Giuffredi
Istituto Italiano di Tecnologia, IT
Authors
Giorgio Giuffredi a, b, Alessandro Mezzetti a, Andrea Perego a, Piero Mazzolini a, Greta Tirelli a, Mirko Prato c, Francesco Fumagalli a, Yu-Chuan Lin d, Chenze Liu d, e, Ilia Ivanov d, Alex Belianinov d, Alex Puretzky d, Gerd Duscher d, e, David Geohegan d, Fabio Di Fonzo a
Affiliations
a, Center for Nano Science and Tecnology, Istituto Italiano di Tecnologia, Via Pascoli 70/3, Milano, IT
b, Department of Energy, Politecnico di Milano
c, Materials Characterization Facility, Istituto Italiano di Tecnologia
d, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, EE. UU., Oak Ridge, US
e, University of Tennessee, Knoxville
Abstract

Crystalline Transition Metal Chalcogenides (c-TMDs) gained attention as non-precious Hydrogen Evolution Reaction (HER) electrocatalysts thanks to their abundance, activity and because their morphology strongly influences the HER performance. This relationship has been used as design rule to obtain efficient TMDs catalysts, by either increasing the intrinsic activity of the material through the creation of new HER active sites or by maximizing the effective surface area through morphology and structure control. Amorphous TMDs (a-TMDs) have in principle higher activity thanks to their disordered structure, however the relation between structure and HER performance is unknown and traditional fabrication techniques grant a limited control over the morphology. Moreover, the poor electrical conductivity of TMDs related to their large bandgap hinders the maximum HER rate that can be achieved. These two limitations ultimately lead to unsatisfactory HER performances, when these catalysts are self-supported.

In this work, we study the influence of morphology and composition on HER performance for self-supported nano-crystalline TMDs, focusing on Molybdenum Sulfide (MoSx) and Tungsten Selenide (WSex). We exploit the intrinsic high activity granted by a disordered, metastable structure, leveraging on the HER enhancement granted by the many defective sites of the material, and a precise control over the morphology of the material granted by using Pulsed Laser Deposition (PLD) as synthesis method. Through PLD we fine-tune the morphology of the materials down to the nanoscale, controlling the pore size distribution among the material and the surface area, ranging from compact film to hierarchical nanostructures.

The pristine TMDs exhibit a short-range ordered, quasi-crystalline structure with crystallites embedded in an amorphous matrix and excess amorphous chalcogen. The shared electrochemical activation process, occurring at the beginning of catalysis, transforms the pristine TMDs into the actual HER catalysts, modifying the structure and composition of the material. More in detail, sub-stoichiometric oxide/oxysulfide phases with high electrical conductivity and new under-coordinated sites are formed for MoSx: the incorporation of oxygen in the structure creates a conductive backbone that, we hypothesize, enhances the HER activity of the material and improves the kinetic. For WSex, on the other hand, the different oxide phases that are formed upon electrochemical activation have a different effect: while sub-stoichiometric oxyselenide phase exhibits good electrical conductivity, the high resistivity of stoichiometric tungsten trioxide limits the HER kinetics.

By optimizing the morphology of the nanostructures and exploiting their structure, we reach HER performances among state-of-art for TMDs-based HER catalysts, regardless of their support or crystalline structure. For MoSx, we achieve a Tafel slope of 35 mV⸱dec-1 and a -100 mA⸱cm-2 overpotential (η100) of 169 mV, while for WSex a -10 mA⸱cm-2 overpotential (η10) of 190 mV and a Tafel slope of 65 mV⸱dec-1 are registered. This remarkable performance is obtained by exploiting the short-range ordered structure of the synthesized TMDs and by leveraging on the effect of the oxide layers created during the activation process, whose effect is dependent on the composition of the pristine TMD.

In conclusion, this report shows the importance of morphology and composition on HER performance for TMDs, as reflected by the relation between structure and intrinsic HER catalytic parameters. This relationship represents a promising design rule to obtain materials which, thanks to their performances, may provide a promising alternative to precious catalysts.

15:00 - 15:30
2.3-I2
Zhu, Xiaoyang
Columbia University
Excitons, Phonons, and Electrons in Two-dimensional Semiconductors and Heterojunctions
Xiaoyang Zhu
Columbia University, US

Xiaoyang Zhu is the Howard Family Professor of Nanoscience and a Professor of Chemistry at Columbia University. He received a BS degree from Fudan University in 1984 and a PhD from the University of Texas at Austin in 1989. After postdoctoral research with Gerhard Ertl at the Fritz-Haber-Institute, he joined the faculty at Southern Illinois University as an Assistant Professor in 1993. In 1997, he moved to the University of Minnesota as a tenured Associate Professor, later a Full Professor, and a Merck endowed professor. In 2009, he returned to the University of Texas at Austin as the Vauquelin Regents Professor and served as directors of the DOE Energy Frontier Research Center (EFRC) and the Center for Materials Chemistry. In 2013, he moved to Columbia University. His honors include a Dreyfus New Faculty Award from Dreyfus Foundation, a Cottrell Scholar Award from Research Corporation, a Friedrich Wilhelm Bessel Award from the Humboldt Foundation, a Fellow of the American Physical Society, a Vannevar Bush Faculty Fellow Award from DOD, and an Ahmed Zewail Award from the American Chemical Society. Among his professional activities, he serves on the editorial/advisory boards of Accounts of Chemical Research, Science Advances, Chemical Physics, and Progress in Surface Science, and as a scientific advisor to the Fritz-Haber-Institute of the Max-Planck Society and ShanghaiTech University

Authors
Xiaoyang Zhu a
Affiliations
a, Department of Chemistry, Columbia University, New York, New York 10027, United States
Abstract

Two-dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMDCs), are emerging platforms for exploring a broad range of electronic, optoelectronic, and quantum phenomena. These materials feature strong Coulombic interactions, making them ideal for studying highly correlated phenomena as a function of charge-carrier density. In this talk, I will discuss our recent effort in understanding dynamics at the 2D limit. Using a range of model systems, including TMDC hetero-bilayers, 2D superatomic semiconductors, and 2D magnetic materials, I will discuss how the strong Coulomb interaction is manifested in manybody dynamics, including spin-valley specific scattering, Mott transitions, and electron-phonon coupling. I will address the prospects of quantum fluid phases, such as exciton condensates and 2D superconductivity. Finally, I will discuss the exciting, but somewhat controversial topic of Moire excitons at TMDC heterojunctions, specifically, how fragile the Miore potentials are to sample conditions, such as strain and interfacial contamination. The difficulty in characterizing the detailed sample conditions means that one must exercise caution in attributing spectroscopic findings to Moire excitons.

SolFuel 2.3
Chair: Kevin Sivula
14:00 - 14:30
2.3-I1
Weiss, Emily
Northwestern University
Colloidal Photocatalysis for Multi-Electron Redox Reactions
Emily Weiss
Northwestern University, US
Emily Weiss is an Associate Professor and the Irving M. Klotz Research Professor in the Department of Chemistry at Northwesern University. Emily earned her PhD from Northwestern in 2005, advised by Mark Ratner and Michael Wasielewski. Her graduate work focused on magnetic superexchange interactions of radical ion pairs created by electron transfer within organic donor-acceptor systems. Emily did postdoctoral research at Harvard under George M. Whitesides from 2005-2008 as a Petroleum Research Fund Postdoctoral Energy Fellow, and started her independent career at Northwestern in Fall 2008. Emily’s group studies electronic processes at organic-inorganic interfaces within colloidal and semiconductor and metal nanoparticles. The objectives of this research are to understand the mechanisms of conversion of energy from one class to another (light, heat, chemical potential, electrical potential) at interfaces, to understand the behavior of quantum confined systems far from equilibrium, and to design and synthesize nanostructures that are new combinations of organic and inorganic components.
Authors
Emily Weiss a, Shichen Lian a, Mohamad Kodaimati a, Kevin McClelland a
Affiliations
a, Department of Chemistry, Northwestern University, Evanston, IL 60208, USA.
Abstract

Colloidal quantum dots (QDs) have many advantages of both heterogeneous and homogeneous photocatalysts for multi-electron reactions relevant to solar energy conversion and organic synthesis. This talk focuses on how the surface chemistry of metal chalcogenide QDs can be tuned to promote selectivity for certain reaction pathways, strong particle-molecule interactions, and the formation of colloidally stable assemblies for energy and charge funneling, for reactions such as proton and CO2 reduction, and carbon-carbon coupling. In particular, in situ photoinduced processes at the surfaces of CdS QDs control the products of the 2-electron oxidation of benzyl alcohol to either benzaldehyde or C-C coupled products in water; QD-QD and QD-porphyrin interactions can be tuned to promote the 2-electron reduction of CO2 to CO via an Fe porphyrin catalyst in DMSO and water; and stable colloidal assemblies of donor and acceptor CdSe QDs enable energy transfer-enhanced photocatalytic reduction of H+ to H2 in water.

14:30 - 14:45
2.3-O1
Agosti, Amedeo
University of Bologna, Italy
Towards Solar Factories: Photosynthetic H2 Generation and Organic Transformations for Highly Efficient Solar-to-Chemical Energy Conversion
Amedeo Agosti
University of Bologna, Italy, IT
Authors
Amedeo Agosti a, Yifat Nakibli b, Lilac Amirav b, Giacomo Bergamini a
Affiliations
a, Department of Chemistry ''G. Ciamician'', University of Bologna, Italy, Via Francesco Selmi, 2, Bologna, IT
b, Technion - Israel Institute of Technology, Haifa, IL
Abstract

The framework of solar-to-chemical energy conversion is mapped by an exploding investigation space, aiming at rapid elevation of the technology to commercially relevant performances and processing conditions. Prospective materials and alternative oxidative pathways are revolutionizing water-splitting into decoupled hydrogen and high-value added chemicals production. Yet, pioneering solar refinery systems have been limited to either efficient, but isolated half-reactions or sluggish simultaneous red-ox transformations, hampering the forthcoming adoption of this promising solar-harvesting strategy. Here, we provide the first demonstration of efficient and stable full-cycle redox transformations, synthesizing solar chemicals.

The identification of a successful redox cycle ensued from fluorescent quenching screening, which bridges between optoelectronic material properties and photosynthetic activity. Implementing this approach on hybrid nanorod photocatalysts (CdSe@CdS-Pt), we demonstrate hydrogen production with photon to hydrogen quantum efficiencies of up to ~70%, under visible light and mild conditions, while simultaneously harvesting solar chemical potential for valuable oxidative chemistries. Facile spectrophotometric analyses further show robust photo-chemical and colloidal stability, as well as product selectivity when converting molecules carrying amino- and alcoholgroups, with solar-to-chemical energy conversion efficiencies of up to 4.2%. As such, rigorous spectroscopic assessment and operando characterization yield superior photosynthetic performance, realizing a truly light-triggered catalytic reaction and establishing nanostructured metalchalcogenide semiconductors as state-of-the-art artificial photo-chemical devices.

14:45 - 15:15
2.3-O2
Hetterscheid, Dennis
Leiden University
Very Fast Oxygen Reduction Catalyzed by Cu(tmpa); Towards Hydrogen Peroxide as a Solar Fuel
Dennis Hetterscheid
Leiden University, NL
Authors
Dennis Hetterscheid a
Affiliations
a, Leiden University, Leiden Institute of Chemistry, Leiden, 2300, NL
Abstract

Very fast oxygen reduction catalyzed by Cu(tmpa); Towards hydrogen peroxide as a solar fuel

For an energy transition from a fuel based economy to an economy that is based on solar and wind energy that suffer from intermittency problems it is important to be able to convert electricity into chemical fuels. Besides the production of hydrogen, lately also electrochemical reduction of carbon dioxide and dinitrogen have been of interest to the scientific community. The use of hydrogen peroxide as a fuel, on the other hand, has thus far been overlooked. In part this is due to a lack of good and selective catalysts for the electrochemical synthesis of hydrogen peroxide.

My group has recently shown that hydrogen peroxide can be obtained at the cathode with more than 90% faradaic efficiency using the molecular catalyst Cu(tmpa).[1] Under optimized conditions the catalyst operates with more than a million turnovers per second, and with an onset at a 200 mV overpotential. The catalytic currents are limited by mass transport of buffer and oxygen concentrations.

The key to selectivity lies in that Cu(tmpa) is a single site catalysts that is very limited in how many electrons it can deliver to the substrate simultaneously. In addition ligand exchange reactions, including the dissociation of hydrogen peroxide, are very rapid while Karlin et al. have already established that oxygen binding to Cu(I) occurs through a very rapid reaction.[2]

At the lecture the precise mechanism towards hydrogen peroxide synthesis, bulk electrolysis results and in situ spectroscopy of the H2O2 synthesis will be discussed.

    

References

[1] M. Langerman, D. G. H. Hetterscheid, Angew. Chem., Int. Ed., DOI: 10.1002/anie.201904075

[2] H. C. Fry, D. V. Scaltrito, K. D. Karlin, G. J. Meyer, J. Am. Chem. Soc. 2003, 125, 11866-11871

15:15 - 15:30
2.3-O3
Wenderich, Kasper
University of Twente
Towards (Photo)electrochemical Production of Hydrogen Peroxide by Water Oxidation as a Financial Attractive Alternative to Oxygen Evolution
Kasper Wenderich
University of Twente, NL
Authors
Kasper Wenderich a, Wouter Kwak a, Alexa Grimm b, Mats Wildlock c, Guido Mul a, Bastian Mei a
Affiliations
a, Photocatalytic Synthesis Group, MESA+ Institute of Nanotechnology, University of Twente, P,O. box 217, 7500 AE Enschede, The Netherlands
b, Copernicus Institute of Sustainable Development, Utrecht University, P.O. Box 80125, 3508 TC Utrecht, The Netherlands
c, Nouryon, Bohus, SE 445-80, Sweden
Abstract

Efficient hydrogen (H2) evolution through the (photo)electrochemical splitting of water is essential to drive a green hydrogen economy. At solar-to-hydrogen (STH) efficiencies around 10 %, the average price of H2 produced is slightly higher than $10 000 ton-1 [1]. The current market value of hydrogen from steam methane reforming is only $1 400 ton-1 [1-2], implying that there is little incentive for the development of alternative processes for H2 production. Hence, the economic profitability of hydrogen production through (photo)electrochemical water splitting must be increased. A suitable route might be the simultaneous production of high-value chemicals at the anode.

 

In ‘classic’ electrochemical water splitting, oxygen is formed at the anode [2]:

2 H2O → O+ 4H+ 4e-                                                                                      E0 (O2/H2O) = 1.23 V vs RHE

The net worth of O2, however, is only $35 ton-1. An alternative anodic reaction is the selective partial oxidation of water to hydrogen peroxide (H2O2) [2]:

2 H2O → H2O+ 2H+ 2e-                                                                                E0 (H2O2/H2O) = +1.78 V vs RHE

With a net worth value of $500-1200 ton-1 and a growing H2O2 demand, hydrogen production with concomitant H2O2 has great promise in future (photo)electrochemical applications. Although not many reports have been published on anodic H2O2 production, the feasibility has recently been demonstrated theoretically and experimentally. In particular, BiVO4 has been referred to as a promising (photo)electrocatalyst [3-4]. Furthermore, it has been advocated that using bicarbonate as an electrolyte is essential to achieve high selectivity for anodic H2O2 production.

 

In this work, we present a techno-economic analysis to address whether H2O2 production at the anode as an alternative to oxygen is a feasible method to make (photo)electrochemical H2 production economically more attractive for industry. The minimum hydrogen prices that can theoretically be achieved to reach a break-even-point will be discussed. Furthermore, we performed a sensitivity analysis to demonstrate which parameters need to be optimized to realize these hydrogen prices as quickly as possible. Using the same means, we will elaborate on the techno-economics of simultaneous anodic and cathodic H2O2 production as well. Finally, we confirm experimentally that selective partial oxidation of water to H2O2 is possible. Challenges that need to be overcome will be discussed.

15:30 - 16:00
Coffee Break
16:00 - 17:00
CharDy 2.4
PERInt 2.4
Chair: Emilio J. Juarez-Perez
16:00 - 16:30
2.4-I1
Cahen, David
Weizmann Institute and Bar-Ilan University
Can Halide Perovskites Teach Us New Materials Chemistry and Physics?
David Cahen
Weizmann Institute and Bar-Ilan University, IL

Born in the Netherlands,David Cahen studied chemistry & physics at the Hebrew Univ. of Jerusalem (HUJ), Materials Research and Phys. Chem. at Northwestern Univ, and biophysics of photosynthesis (postdoc) at HUJ and the Weizmann Institute of Science, WIS. After joining the WIS faculty he focused on alternative sustainable energy resources, in particular various types of solar cells. In parallel he researches hybrid molecular/non-molecular systems, focusing on understanding and controlling electronic transport across (bio)molecules. He is a fellow of the AVS and the MRS. He heads WIS' Alternative, sustainable energy research initiative.

Authors
David Cahen a
Affiliations
a, Weizmann Institute and Bar-Ilan University, IL
Abstract

While the fact that (the most popular) Halide Perovskites (HaPs) are only just stable (against decomposition into binaries) may seem to doom them for practical use, actually the opposite is the case. The reason is that this situation and the low activation energy for this process and its reverse (formation from the binaries), leads directly to very low defect densities in their bulk. As energetics at surfaces differ from those in the bulk, the sur- and inter-faces, external and internal, are the loci for most of the defects in HaPs. The above explains their ability to self-repair and many more of their properties.  Naturally there are deeper reasons for such behavior and I will elaborate and explain those in my lecture, as well as possible implications for how we view electronic materials.

16:30 - 16:45
2.4-O1
Futscher, Moritz
AMOLF
The Effect of Manganese Doping on Mobile Ions in Perovskite Light-emitting Diodes
Moritz Futscher
AMOLF, NL
Authors
Moritz Futscher a, Mahesh Gangishetty b, Daniel Congreve b, Bruno Ehrler a
Affiliations
a, Center for Nanophotonics, AMOLF, Science Park 104, The Netherlands
b, Rowland Institute at Harvard, Massachusetts, United States, Edwin H Land Boulevard, 100, Cambridge, US
Abstract

In recent years, halide perovskite light-emitting diodes have attracted a lot of attention. However, most high-performance devices degrade within seconds to minutes. This degradation has been attributed to the migration of mobile ions within the perovskite, impeding commercial applications. Recently it was shown that introducing manganese into perovskite light-emitting diodes enhances their brightness, efficiency, and stability. [1] However, the exact mechanism for these improvements is still under investigation.

We recently showed that transient ion drift is a powerful method to quantify activation energy, concentration, and diffusion coefficient of mobile ions in perovskite-based devices. [2] Here we use transient ion drift to study ion migration in perovskite light-emitting diodes with and without manganese addition. We find that the concentration of mobile ions is not influenced by manganese passivation, but that the diffusion coefficient is reduced from 4 x 10-11 cm2s-1 to 2 x 10-11 cm2s-1 and the activation energy is increased from 0.1 eV to 0.3 eV. These changes in the properties of mobile ions serve to rationalize the improved stability in perovskite light-emitting diodes upon magnesium passivation and will lead to a better understanding of ion migration and the influence of passivating agents on that migration and thus on the stability of the devices.

16:00 - 17:00
RadDet 2.4
Sol2D 2.4
Chair: Efrat Lifshitz
16:00 - 16:30
2.4-I1
Even, Jacky
Institut National des Sciences Appliquées, Rennes (FR)
Physical Properties of Pure 2D Layered Hybrid Perovskites: Recent Results
Jacky Even
Institut National des Sciences Appliquées, Rennes (FR), FR

Jacky Even was born in Rennes, France, in 1964. He received the Ph.D. degree from the University of Paris VI, Paris, France, in 1992. He was a Research and Teaching Assistant with the University of Rennes I, Rennes, from 1992 to 1999. He has been a Full Professor of optoelectronics with the Institut National des Sciences Appliquées, Rennes,since 1999. He was the head of the Materials and Nanotechnology from 2006 to 2009, and Director of Education of Insa Rennes from 2010 to 2012. He created the FOTON Laboratory Simulation Group in 1999. His main field of activity is the theoretical study of the electronic, optical, and nonlinear properties of semiconductor QW and QD structures, hybrid perovskite materials, and the simulation of optoelectronic and photovoltaic devices. He is a senior member of Institut Universitaire de France (IUF).

Authors
jacky even a
Affiliations
a, Univ Rennes, INSA Rennes, CNRS, Institut FOTON - UMR6082, F-35000 RENNES
Abstract

Solution-processed organometallic perovskite materials have emerged since 2012 as a promising thin-film photovoltaic technology and since 2015 as promising colloidal nanostructures. The presentation will focus on recent optical spectroscopy and diffraction results on 2D multilayered halide perovskite phases, composed of perovskites multilayers sandwiched between two layers of large organic cations. Such heterostructures initially studied during the 80’s and 90’s, have recently led to the demonstration of improved solar cells photostability under standard illumination as well as humidity resistance. More, sizeable current densities on the order of a few A/cm2 have been injected in such structures for LED applications demonstrating their robustness.

The three sub-classes of monolayered 2D perovskites defined from diffraction analyses in 2017-2018 will be presented, including symmetry analyses of their electronic band structures computed by DFT. Light emission is related for many of these heterostructures to strongly Wannier bound excitons. In this case, intrinsic quantum and dielectric carrier confinements are afforded by the organic inner barriers, lead to a stable Wannier exciton at room temperature. A semi-empirical modelling of the Bethe-Salpeter has been compared to magneto-absorption results on crystalline flakes, including the deviation from 2D Rydberg series and the influence of multilayer thckness. In other situations, the distortions of the perovskite lattice may induce the dissociation of the Wannier excitons at the edges and inside the thin-films or flakes, or the formation of localized excitations, at the origin of below band gap white light emission.

SolFuel 2.4
Chair: Kevin Sivula
16:00 - 16:30
2.4-I1
Tilley, David
University of Zurich
Operando Methods for a Deeper Understanding of Photoelectrochemical Water Splitting Systems
David Tilley
University of Zurich, CH
Authors
David Tilley a
Affiliations
a, Department of Chemistry, University of Zurich, Winterthurerstrasse 190, Zurich, CH-8057
Abstract

Semiconductor-based water splitting and photovoltaic systems are often complex, multilayer systems where each layer and interface is engineered to maximize the efficiency and stability of the device. Due to the multiplicity of interfaces in such devices, identification of the limitations in order to improve the performance is challenging. In this talk, I will discuss operando techniques such as dual working electrode (DWE), photoelectrochemical impedance spectroscopy (PEIS), and transient photocurrent as they are applied to multilayer photocathodes and photoanodes. In water splitting photoelectrodes, the DWE technique can be used to deconvolute the photovoltaic output of the photoabsorber from the electrocatalytic performance of the surface catalyst,[1] and is useful in determining the band alignment in heterojunction systems under operation. I will discuss how impedance spectroscopy can be used to identify the limitations in complex systems such as Au/Cu2O/Ga2O3/TiO2/RuOx photocathode, through analysis of the potential dependent resistances that are detected by PEIS.[2] I will show how a combination of these techniques can be used to gain a clearer picture of the underlying photophysical charge carrier processes that are taking place in these photoelectrodes under practical operational conditions.

16:30 - 16:45
2.4-O1
García Tecedor, Miguel
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
Systematic Analysis of Earth-abundant Based Electrocatalysts for Energy Applications by Spectroscopic Techniques
Miguel García Tecedor
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES
Authors
Miguel García-Tecedor a, Roser Fernández-Climent a, Sixto Giménez a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

Solar water splitting and reduction of atmospheric CO2 are complementary processes to reduce global warming and greenhouse effect by synthesising valuable fuels and chemicals. With the aim of improving the overall efficiency of both processes, finding earth-abundant and cost-effective catalysts is still a key challenge. Ni based oxides constitute one of the most attractive and cost-effective water oxidation electrocatalysts, which represent a real alternative to the scarce RuO2 and IrO2 for the Oxygen Evolution Reaction (OER). On the other hand, Cu based materials are the most studied candidates for driving the CO2 Reduction Reaction (CO2 RR) due to their low cost and their noticeable ability to reduce CO2 to hydrocarbons and alcohols. Understanding the origin of the performance of the employed catalysts for carrying out these reactions is mandatory in order to rationalize the design and synthesis of technologically viable systems, which can be implemented as part of the energy industry.  

The studied electrocatalysts were deposited by different techniques as magnetron sputtering, combustion or wet chemistry methods and complementary structural, optical, electrochemical and spectroscopic techniques were employed to identify the most relevant steps towards water oxidation or CO2 RR, as well as to determine the limiting factors for performance.

16:45 - 17:00
2.4-O2
Abdi, Fatwa
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany
Overcoming Performance Losses in Scaling-up Metal Oxide-based Solar Water Splitting Devices
Fatwa Abdi
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany, DE

Fatwa Abdi is a group leader and the deputy head of the Institute for Solar Fuels, Helmholtz-Zentrum Berlin (HZB). He obtained his undergraduate degree in 2005 from Nanyang Technological University and masters' degree in 2006 from National University of Singapore and Massachusetts Institute of Technology, all in Materials Science and Engineering. After a short stint in the semiconductor industry, he pursued a PhD at TU Delft, the Netherlands, and graduated cum laude in 2013. He was the recipient of Singapore-MIT Alliance fellowship (2005) and Martinus van Marum prize (2014) from the Royal Dutch Society of Sciences and Humanities.

Authors
Ibbi Y. Ahmet a, Yimeng Ma a, Ronald R. Gutierrez b, Roel van de Krol a, Sophia Haussener b, Fatwa F. Abdi a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany, Berlin, DE
b, Laboratory of Renewable Energy Science and Engineering, École Polytechnique Fédérale de Lausanne, Switzerland
Abstract

Progress in the development of metal oxide photoelectrodes in the past 10-15 years has shifted focus towards fabricating practical stand-alone solar water splitting devices. Integrated tandem devices in near-neutral pH electrolytes based on the combination of a BiVO4-based photoanode as a wide-bandgap top absorber and various types of bottom absorbers have been reported, and solar-to-hydrogen (STH) efficiencies approaching 10% have been demonstrated.[1-2] Nevertheless, the majority of these demonstration devices still have active areas of less than 1 cm2. The next step is to move beyond lab-scale experiments and demonstrate large-area devices. Here, we report the scale-up of spray-deposited BiVO4 photoanodes from <1 to 50 cm2, from which we observed a significant performance loss with increased area. An unbiased 50 cm2 PV-PEC water splitting device is demonstrated with an STH efficiency of 2.1%. While this is the highest reported value for a large-area (> 10 cm2) BiVO4-based water splitting device, it is still a factor of 2-3 lower than the efficiency achieved using the corresponding small-area device. By performing a series of control experiments, we found that factors other than the BiVO4 itself, such as substrate conductivity, electrolyte conductivity, and cell geometry, are responsible for a total voltage loss of 600 mV and therefore limit the performance of the large-area photoanode. The different loss mechanisms associated with scaling-up photoelectrodes were further quantified using a finite element analysis model (COMSOL Multiphysics®). Based on these insights, electrochemical engineering strategies to overcome these losses are offered, which would limit the voltage loss for large-area photoelectrodes to less than 50 mV.

17:00 - 19:00
Poster Session
 
Wed Nov 06 2019
Plenary Session 5
Chair: Jacky Even
09:00 - 09:30
5-K1
Mitzi, David
Duke University
Organic-Inorganic Perovskites: Unrivaled Versatility for Semiconductor Design and Fabrication
David Mitzi
Duke University, US

David Mitzi received a B.S.E. in Electrical Engineering from Princeton University in 1985 and a Ph.D. in Applied Physics from Stanford University in 1990. In 1990, he joined the IBM T. J. Watson Research Center and initiated a program examining structure-property relationships, low-cost thin-film deposition techniques and device applications for a variety of electronic materials (e.g., oxides, halides, chalcogenides, organic-inorganic hybrids). Between 2009 and 2014 he managed the Photovoltaic Science and Technology department at IBM, with a focus on developing solution-processed high-performance inorganic semiconductors for thin-film photovoltaic (PV) devices. In July 2015, Dr. Mitzi moved to the Department of Mechanical Engineering and Materials Science at Duke University as a professor. He holds a number of patents and has authored or coauthored more than 180 papers and book chapters.

Authors
David Mitzi a
Affiliations
a, Duke University, PO Box 90281, Durham, 27708, US
Abstract

Although known for more than a century, hybrid (organic-inorganic) halide-based perovskites have received extraordinary attention recently, because of the unique physical properties and chemical diversity of the three-dimensional (3-D) and related lower-dimensional (2-D, 1-D and 0-D) lead-, tin- and germanium-based systems, which make them outstanding candidates for application in photovoltaic and related optoelectronic devices. This talk will emphasize the structural versatility of the hybrid perovskite family [1] and resulting implications for semiconductor design, as well as film/device formation. Examples  include the use of specifically-designed organic cations to stabilize the formation of difficult-to-realize lead-free perovskite semiconductors [2], the incorporation of functional organic cations that directly impact charge carrier dynamics and photophysics [3], and the role that the organic cation plays in tailoring melting properties of hybrid perovskites [4] and how this may connect to developing a solvent-free film formation pathway and associated laminated device framework [5]. Outstanding functionality combined with facile processing, enabled by the chemical devirsity of the hybrid perovskites (in particular, the organic component of the structure), provide two pillars for generating unique physical properties and future application of this family of hybrid semiconductors.

Plenary Session 6
Chair: Erwin Reisner
09:00 - 09:30
6-K1
Zhang, Jenny
University of Cambridge - UK
Semi-artificial Photosynthesis: a Platform for Studying and Wiring Photosynthesis
Jenny Zhang
University of Cambridge - UK, GB

Jenny started her independent research career as a David Phillips Fellow at the University of Cambridge in 2018. Currently she is tailoring photoelectrochemical platforms to rewire photosystem II both in vitro and in vivo for solar fuel and electricity generation. Her work extends into biophotovoltaic and microbial fuel cell studies.

Authors
Jenny Zhang a
Affiliations
a, Department of Chemistry, University of Cambridge - UK, Lensfield Road, Cambridge, GB
Abstract

Semi-artificial photosynthesis is an emergent field that combines the strengths of artificial and biological photosynthesis to rewire pathways for solar-to-charge and chemical conversion that are otherwise inaccessible to either field alone. Here, I present the integration of the water oxidation enzyme, photosystem II (PSII), into tailored high surface-area electrodes, which allows the electrons extracted from the first step of photosynthesis to be harnessed for driving novel endergonic reactions and to probe enzyme functionality.1, 2 For example, PSII has been wired to hydrogenase in different photoelectrochemical configurations to drive light-driven water splitting.3,4 Cyanobacteria and isolated thylakoid fractions has also be wired to electrodes to access the long-lived and unique photo-electrogenic activities of PSII in vivo.5

 

References

[1] M. Kato, J. Zhang, N. Paul, E. Reisner, Chem. Soc. Rev. (2014) 43, 6485–6497

[2] J. Zhang, N. Paul, P. Sokol, E. Romero, R-V. Grondelle, E. Reisner, Nature Chem. Bio.  (2016) 12, 1046–1052.

[3] D. Mersch, C-Y. Lee; J. Zhang, K. Brinkert, J. C. Fontecilla-Camps; A. W. Rutherford, E. Reisner, J. Am. Chem. Soc. (2015) 137, 8541–8549.

[4] K. P. Sokol, W. E. Robinson, J. Warnan, N. Kornienko, M. Nowaczyk, A. Ruff, J. Z. Zhang, E. Reisner (2018) 3, 944-951.

[5] J. Zhang, P. Bombelli, K. Sokol, A. Fantuzzi, A. W. Rutherford, C. Howe, E. Reisner, J. Am. Chem. Soc. (2018) 140, 6-9.

Exciup 1.1
Chair: Bruno Ehrler
09:30 - 10:00
1.1-I1
Friend, Richard
University of Cambridge - UK
New materials for singlet exciton fission to triplet pairs
Richard Friend
University of Cambridge - UK, GB

Richard Friend holds the Cavendish Professorship of Physics at the University of Cambridge. His research encompasses the physics, materials science and engineering of semiconductor devices made with carbon-based semiconductors, particularly polymers. His research advances have shown that carbon-based semiconductors have significant applications in LEDs, solar cells, lasers, and electronics. His current research interests are directed to novel schemes – including ideas inspired by recent insights into Nature’s light harvesting – that seek to improve the performance and cost of solar cells.

Authors
Richard Friend a
Affiliations
a, Cavendish Laboratory, Department of Physics, University of Cambridge, UK, JJ Thomson Avenue, Cambridge, GB
Abstract

Fission of singlet excitons to triplet exciton pairs is well characterised for pentacene and tetracene. Whilst fission to separated triplets in pentacene and TIPS-pentacene is rapid and efficient, for tetracene and TIPS-tetracene there is clear evidence that the singlet exciton first forms a bound triplet-triplet (TT) pair that later separates endothermically to free triplets [1]. The TT state for TIPS tetracene can exist as spin 2 quintet states [2] that show specific TT exchange energies depending on the nearest neighbour tetracene configurations [3]. There is less known about other materials that should show similar energetics for fission. I will report on some new materials systems investigated in Cambridge, including non-covalently coupled pentacene dimers, anda family ofderivatives of indolonaphthyridine thiophene that show clear evidence for singlet fission. I will also explore new approaches for the conversion of triplet excitons to emissive states that can be used for later energy transfer to a conventional solar cell.

10:00 - 10:30
1.1-O1
Daiber, Benjamin
AMOLF
Efficiency Potential and Application of Singlet Fission Enhanced Silicon Solar Cells using Different Energy Transfer
Benjamin Daiber
AMOLF, NL
Authors
Benjamin Daiber a, Koen v.d. Hoven a, Joris Y. Bodin a, Stefan Luxembourg b, Moritz Futscher a, Bruno Ehrler a
Affiliations
a, Center for Nanophotonics, AMOLF, Science Park 104, The Netherlands
b, ECN.TNO
Abstract

Silicon is the dominating solar cell material, therefore add-ons on the silicon solar cell that can improve on the power conversion efficiency are urgently needed. In certain organic materials singlet fission generates two triplet (spin 1) excitons from one singlet (spin 0) exciton. If the triplet excitons are harvested in a silicon solar cell the efficiency could be dramatically increased, as we show computationally. There are different transfer pathways between the organic singlet fission material and silicon. We have simulated the achievable efficiency for each transfer path with realistic assumptions such as a singlet fission quantum efficiency of 1.7 (1.7 e-h pairs per high energy photon), a transmission loss of 5%, and different entropy gains of the Singlet Fission process.

Even with these realistic assumptions, the efficiency of a silicon/singlet fission solar cell can be as high as 34% when combined with the current record silicon solar cell of 27%. We found that dissociating the triplet excitons at the interface leads to a large potential efficiency gain because a triplet energy lower than the silicon bandgap still leads to charge generation, and allows for high current generation. We also find that current singlet fission materials do not absorb light strongly enough, motivating sensitization schemes.  A direct triplet exciton transfer shows lower overall efficiencies because the energy level requirements are more strict, however the solar cell architecture is more elegant since there are no additional contacts needed. Finally, we compare the singlet fission/silicon solar cells to the efficiency potential of perovskite/silicon tandem solar cells. We find that tandem cells are particularly beneficial for a silicon base cell with low efficiency, while a highly efficient silicon solar cells benefits less from the perovskite top cell. In contrast, the efficiency gain from the singlet fission layer is almost constant for all silicon base cells, and for highly efficient silicon cells would clearly outperform a high-efficiency perovskite top cell.

We also tried to realize the direct exciton transfer solar cells we simulated by fabricating a silicon solar cells with a top layer of tetracene. The silicon base cell is back-contacted, so we can HF-etch from one side to have direct access to the silicon <111> sides of pyramidally textured silicon. The photocurrent under a magnetic field can differentiate between photocurrent contributions of singlet and triplet excitons. A newly improved magnetic field dependent photocurrent setup allows us to measure current changes on the order of 0.01% and is a vital tool for a precise attribution of the origin of the photocurrent. We find that after deposition of the tetracene layer we see an injection of singlets or photons into silicon, but after aging the solar cell we see evidence for triplet transfer. The characteristic Merrifield curve (photocurrent as function of applied magnetic field ) inverts, which suggest the injection of triplet excitons from tetracene into silicon. We observe a triplet injection curve for tetracene-silicon solar cells that have been aged for five days in air or six weeks encapsulated in nitrogen atmosphere. We discuss different possible mechanisms for this behavior, a thin layer of silicon dioxide growing between tetracene and silicon and a changing orientation of the tetracene molecules. A better understanding of the energy transfer processes at the interface will be important for increasing the injection efficiency.

OPV 1.1
Chair: Harald W. Ade
09:30 - 10:00
1.1-I1
Nelson, Jenny
Imperial College London
Device-scale and Molecular-scale Modelling of Organic Photovoltaic Devices
Jenny Nelson
Imperial College London, GB

Jenny Nelson is a Professor of Physics at Imperial College London, where she has researched novel varieties of material for use in solar cells since 1989. Her current research is focussed on understanding the properties of molecular semiconductor materials and their application to organic solar cells. This work combines fundamental electrical, spectroscopic and structural studies of molecular electronic materials with numerical modelling and device studies, with the aim of optimising the performance of plastic solar cells. She has published around 200 articles in peer reviewed journals, several book chapters and a book on the physics of solar cells.

Authors
Jenny Nelson a, Mohammed Azzouzi a, Jun Yan a
Affiliations
a, Department of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, UK.
Abstract

In organic heterojunction devices, current generation results from the sequence of photon absorption, charge separation, and charge collection in competition with recombination. To understand and design organic PV devices, we need models of these processes that incoporate both the device architecture and the molecular nature of the materials. Device models work fairly well in describing charge collection and recombination, and resulting curent-voltage curves, but usually with some empirical form for the charge generation efficiency and recombination coefficients. A full description of microscopic processes such as interfacial charge transfer requires molecular scale models. For design purposes, we would like to be able to predict device behaviour from the properties of the molecular components, but it is challenging to combine these aspects in a single model. In this talk we will discuss how time-resolved device models and measurements can be used to probe charge recombination at a device level, while molecular models of charge transfer are able to successfully describe charge dynamics as probed spectroscopically. We will then address the challenges in bringing the two approaches together into a single framework.

10:00 - 10:30
1.1-I2
Kirchartz, Thomas
FZ Jülich
Capacitance-based Characterization of Organic Solar Cells
Thomas Kirchartz
FZ Jülich, DE

He studied electrical engineering in Stuttgart and started working on Si solar cells in 2004 under the guidance of Uwe Rau at the Institute for Physical Electronics (ipe) in Stuttgart. After finishing his undergraduate studies in 2006, he continued working with Uwe Rau first in Stuttgart and later in Juelich on simulations and electroluminescence spectroscopy of solar cells. After finishing his PhD in 2009 and 1.5 years of postdoc work in Juelich, Thomas Kirchartz started a three year fellowship at Imperial College London working on recombination mechanisms in organic solar cells with Jenny Nelson. In 2013, he returned to Germany and accepted a position as head of a new activity on hybrid and organic solar cells in Juelich and simultaneously as Professor for Photovoltaics with Nanostructured Materials in the department of Electrical Engineering and Information Technology at the University Duisburg-Essen. Kirchartz has published >100 isi-listed papers, has co-edited one book on characterization of thin-film solar cells whose second edition was published in 2016 and currently has an h-index of 38.

Authors
Thomas Kirchartz a
Affiliations
a, IEK-5 Photovoltaics, Forschungszentrum Jülich, Wilhelm-Johnen-Straße, Jülich, 52425, DE
Abstract

Methods based on the measurement of impedance or admittance of an organic solar cell are widely used to understand electronic properties such as charge carrier lifetime, mobility, and charge carrier density. Here, I will first briefly review the different methods and then explain the challenges in understanding and interpreting these methods. The first approach is based on measuring the real and imaginary part of the impedance and analyzing the data by fitting equivalent circuit models to the data. This approach usually leads to characteristic time-constants that are affected by recombination but do not directly provide the actual charge carrier lifetimes because of the position dependence of the charge carrier density. A second approach is based on the determination of the charge-carrier density from an integration of the chemical capacitance.1 In this case, the key challenge is to isolate the chemical capacitance of the active layer (that can be used to estimate the carrier concentration in the active layer) from the capacitance of the electrodes. One way of achieving this is to compare high and low frequency capacitances and to assume that the high frequency capacitance is due to the electrode and the additional capacitance at lower frequencies is due to the active layer. This approach has the advantage of providing a charge carrier density over the whole voltage range from reverse bias to forward bias.2 However, the approach is not able to correctly take into account the voltage dependence of the electrode capacitance that is affected by charge redistribution in the active layer itself. A possible solution is to compare the capacitance under illumination vs. the capacitance in the dark and analyze the difference. This gives a rather precise estimate of the charge carrier density at low forward and reverse bias and thereby allows us to determine recombination parameters at these voltages. An obvious disadvantage is however that it doesn’t work at larger forward bias, where charge injection becomes substantial.3 Finally, we explain how the photogenerated excess capacitance at reverse bias can also be used to determine the charge carrier mobility.4

References

   (1)    Brus, V. V.; Proctor, C. M.; Ran, N. A.; Nguyen, T. Q. Capacitance Spectroscopy for Quantifying Recombination Losses in Nonfullerene Small-Molecule Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2016, 6, 1502250.

   (2)    Heiber, M. C.; Okubo, T.; Ko, S. J.; Luginbuhl, B. R.; Ran, N. A.; Wang, M.; Wang, H.; Uddin, M. A.; Woo, H. Y.; Bazan, G. C. et al. Measuring the Competition Between Bimolecular Charge Recombination and Charge Transport in Organic Solar Cells Under Operating Conditions. Energ. Environ. Sci. 2018, 11, 3019-3032.

   (3)    Zonno, I.; Zayani, H.; Grzeslo, M.; Krogmeier, B.; Kirchartz, T. Extracting Recombination Parameters From Impedance Measurements on Organic Solar Cells. Phys. Rev. Applied 2019, 11, 054024.

   (4)    Zonno, I.; Martinez-Otero, A.; Hebig, J. C.; Kirchartz, T. Understanding Mott-Schottky Measurements Under Illumination in Organic Bulk Heterojunction Solar Cells. Phys. Rev. Applied 2017, 7, 034018.

 

PERInt 3.1
Chair: Pablo P. Boix
09:30 - 10:00
3.1-I1
Shen, Qing
The University of Electro-Communications
Phase Stable and Less-Defect Perovskite Quantum Dots: Optical Property, Photoexcited Hot Carrier Dynamics, Charge Transfer and Application to Optoelectronic Devices
Qing Shen
The University of Electro-Communications, JP

Prof. Qing Shen received her Bachelor’s degree in physics from Nanjing University of China in 1987 and earned her Ph.D. degree from the University of Tokyo in 1995. In 1996, she joined the University of Electro-Communications, Japan and became a full professor in 2016. In 1997, she got the Young Scientist Award of the Japan Society of Applied Physics. In 2003, she got the Best Paper Award of the Japan Society of Thermophysical Properties and the Young Scientist Award of the Symposium on Ultrasonic Electronics of Japan. In 2014, she got the Excellent Women Scientist Award of the Japan Society of Applied Physics. She has published nearly 140 peer-reviewed journal papers and book chapters. Her current research interests focus on solution processed nano-materials and nanostructures, semiconductor quantum dot solar cells and perovskite solar cells, and especially the photoexcited carrier dynamics (hot carrier cooling, multiple exciton generation, charge transfer at the interface) in perovskite solar cells, quantum dot and dye sensitized solar cells, organic-inorganic hybrid solar cells.

Authors
Qing Shen a, Feng Liu a, Chao Ding a, Yaohong Zhang a, Taro Toyoda a, Shuzi Hayase a
Affiliations
a, The University of Electro-Communications, Japan
Abstract

  Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability.1 For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus quenching luminescence. Very recently, we have suceeded in synthesis of phase stable and less defect preovksite QDs, including FAPbI3 QDs, CsPbI3 QDs and Sn-Pb alloyed QDs.2-4 We have demonstrated that a high room-temperature photoluminescence quantum yield (PL QY) of up to 100% can be obtained in FAPbI3 and CsPbI3 perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidences the negligible electron or hole trapping pathways in our QDs, which explains such a high quantum efficiency. In addtion, photoexcited hot and cold carrier dynamics as well as charge transfer at the heterojunction of QD/metal oxide were systematically investigated. Solar cells based on these high-quality perovskite QDs exhibit power conversion efficiency of over 12%, showing great promise for practical application. We expect the successful synthesis of the “ideal” perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices in the near future.

10:00 - 10:30
3.1-I2
Saliba, Michael
TU Darmstadt, Optoelectronics
Polyelemental, Multicomponent Perovskite Semiconductor Libraries through Combinatorial Screening
Michael Saliba
TU Darmstadt, Optoelectronics, DE
Authors
Michael Saliba a
Affiliations
a, Adolphe Merkle Institute, University of Fribourg, CH-1700 Fribourg, Switzerland
Abstract

Perovskites have emerged as low-cost, high efficiency photovoltaics with certified efficiencies of 24.0% approaching already established technologies. The perovskites used for solar cells have an ABX3 structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities. This is particularly noteworthy for CsPbX3 and FAPbX3 which are stable at room temperature as a photoinactive “yellow phase” instead of the more desired photoactive “black phase” that is only stable at higher temperatures. Moreover, apart from phase stability, operating perovskite solar cells (PSCs) at elevated temperatures (of 85 °C) is required for passing industrial norms.
Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase” at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures supress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase” of FAPbI3 that can be avoided using the also unstable CsPbI3). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibility and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5)
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The Voc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V, one of the lowest measured on any PV material indicating the almost recombination-free nature of the novel compound. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking. This is a crucial step towards industrialisation of perovskite solar cells.

Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology.
The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6,7)
(1) Jeon et al. Nature (2015)
(2) Lee et al. Advanced Energy Materials (2015)
(3) McMeekin et al. Science (2016)
(4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016)
(5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016).
(6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture. Science (2018)
(7) Saliba. Polyelemental, Multicomponent Perovskite Semiconductor Libraries through Combinatorial Screening. Advanced Energy Materials (2019)

SolCat 1.1
Chair: Ludmilla Steier
09:30 - 10:00
1.1-O1
Janáky, Csaba
University of Szeged
Scaling-up Carbon-dioxide Electroreduction: from Novel Catalysts to Electrolyzer Development
Csaba Janáky
University of Szeged, HU
Authors
Csaba Janáky a, Balázs Endrődi a, Dorottya Hursán a, Egon Kecsenovity a, Richard Jones b
Affiliations
a, University of Szeged, Szeged, HU
b, ThalesNano Inc
Abstract

Electrochemical reduction of CO2 is a promising method for converting a greenhouse gas into value-added products, utilizing renewable energy. Novel catalysts, electrode assemblies, and cell configurations are all necessary to achieve economically appealing performance. In this talk, I am going to talk about two of these aspects, targeted by our laboratory.

First, I am going to present a zero gap electrolyzer cell, which converts gas phase CO2 to products without the need for any liquid catholyte. This is the first report of a CO2 electrolyzer cell, where multiple stacks are connected, thus scaling up the electrolysis process (patent pending, PCT/HU2019/095001). The operation of the cell was validated using both silver nanoparticle and copper nanocube catalysts, and the first was employed for the optimization of the electrolysis conditions. Upon this, CO formation with partial current densities above 250 mA cm2 were achieved routinely, which was further increased to 300 mA cm2 (with ~95 % Faradaic efficiency) by pressurizing the CO2 inlet. Evenly distributing the CO2 gas among the stacks (parallel connection), the operation of the multi-stack cell was identical to the sum of multiple single-stack cells. When passing the CO2 gas through the stacks one after the other (serial gas connection), the CO2 conversion efficiency was increased remarkably. Importantly, the presented electrolyzer simultaneously provides high partial current density, low cell voltage (−3.0 V), high conversion efficiency (up to 40 %), and high selectivity for CO production; while operating at up to 10 bar differential pressure.

In the second part of my presentation I will shed light on the importance of catalyst morphology, using N-doped carbon (N–C) catalysts as a model system.[1]  We found that CO2R activity, selectivity, and stability of N–C electrodes are highly dependent on their porosity. The presence of mesopores was demonstrated to be beneficial in achieving high CO selectivity and current density, with an optimal pore size around 27 nm. Even after convoluting factors other than morphology (e.g., surface chemistry, level of graphitization, surface area), the reasons behind the observed trends are complex. CO2 adsorption properties, wetting characteristics, and geometric effects are jointly responsible for the massive difference in the CO2R performance. All these properties must be taken into consideration when we aim to understand the reduction mechanism on different catalysts and while improving the performance further to a technologically relevant level (as alternatives to precious metal catalysts).

10:00 - 10:30
1.1-I1
Rossmeisl, Jan
University of Copenhagen
Electrocatalysis at the Atomic Scale
Jan Rossmeisl
University of Copenhagen, DK
Jan Rossmeisl is Professor of theoretical chemistry at Department of Chemistry and the Nano-Science center at Copenhagen University. Before joining University of Copenhagen in April 2015, Jan was an Associate Professor and group leader for the theoretical catalysis group at Department of Physics at the Technical University of Denmark Jan holds master’s (2000) and Ph.D (2004) degrees in physics from the Technical University of Denmark. Since 2007 supervisor of more than 20 Ph.D and Post docs. Coauthor of more than 110 publications in peer reviewed journals, co-inventor of 4 patents and co-founder of two startup Companies. Research interests includes: electrocatalysis, energy conversion, atomic scale simulations, rational interface design for catalysis.
Authors
Jan Rossmeisl a
Affiliations
a, Department of Chemistry, Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Abstract

The chemical industry should in the future be based on renewable energy. Therefore, material development for environmentally friendly, electrocatalytic production of valuable chemicals is needed.

Chemicals could be produced using safe, cheap, more environmentally friendly and more abundant reactants than today. The products could be provided on demand at the place where they are needed, reducing expensive and hazardous transport of chemicals. However, stable, efficient and selective catalysts have to be discovered. This requires insight into the surface chemistry at the atomic scale.

The challenge of discovering new catalyst materials is twofold: Firstly, the properties or descriptors of the wanted catalyst have to be identified. Secondly, real materials with the wanted properties should be found.

I will give examples of determining descriptors for different reactions and a method for identify promising catalyst materials based on high entropy alloy, which is a new class of materials with the promise to change the way we discover interesting catalyst materials.

[1] High-entropy alloys as a discovery platform for electrocatalysis, TAA Batchelor, JK Pedersen, SH Winther, IE Castelli, KW Jacobsen, J.Rossmeisl. Joule 3 (3), 834-845, 2019.

[2] Electrochemical CO2 Reduction: A Classification Problem, A Bagger, W Ju, AS Varela, P Strasser, J Rossmeisl, ChemPhysChem 18 (22), 3266-3273, 2017

SolFuel 3.1
Chair: Ian Sharp
09:30 - 10:00
3.1-I1
Mendes, Adélio
LEPABE-FEUP
Towards solarchemistry: direct conversion of sunlight into fuels
Adélio Mendes
LEPABE-FEUP, PT

Professor Adélio Mendes (born 1964) received his PhD degree from the University of Porto in 1993.

Full Professor at the Department of Chemical Engineering of the Faculty of Engineering of the University of Porto. Coordinates a large research team with research interests mainly in dye sensitized solar cells and perovskite solar cells, photoelectrochemical cells including water splitting and solar redox flow batteries, photocatalysis, redox flow batteries, electrochemical membrane reactors (PEMFC, H-SOFC, chemical synthesis), methanol steam reforming, membrane and adsorbent-based gas separations and carbon molecular sieve membranes synthesis and characterization.

Professor Mendes authored or co-authored more than 300 articles in peer-review international journals, filled 23 families of patents and is the author of a textbook; received an Advanced Research Grant from the ERC on dye-sensitized solar cells for building integrated of ca. 2 MEuros and since 2013 he is partner in 4 more EU projects and leads one EU project. Presently he is the leader of a FET Open project, GOTSolar, on perovskite solar cells. He received the Air Products Faculty Excellence 2011 Award (USA) for developments in gas separation and Solvay & Hovione Innovation Challenge 2011 prize, the Prize of Coimbra University of 2016, and the prize of Technology Innovation - 2017 by the University of Porto. Presently, he is the Coordinator of CEner-FEUP, the Competence Center for Energy of the Faculty of Engineering at the University of Porto.

Authors
Paula Dias a, Adelio Mendes a
Affiliations
a, LEPABE - Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias, 4200-465 Porto, Portugal
Abstract

Technologies for harvesting and storage energy from renewable sources need to be scaled up at least six times faster for the world to meet the decarbonization and climate mitigation goals set out in the Paris Agreement, states IRENA in Global Energy Transformation, a Roadmap to 2050. Sunlight is the most abundant and probably as well the most convenient local renewable energy source for producing thermal and electrical energy [1]. Although photovoltaic (PV) electricity is already the cheapest if produced in countries with high solar irradiance, technology gaps still exist for achieving cost-effective scalable deployment combined with storage technologies to provide reliable and dispatchable energy. More recently, the direct conversion of sunlight into storable fuels and feedstock chemicals has been attracting the attention of scientists and entrepreneurs; the name solarchemistry has been coined in this context.

One of the most important technologies which has emerged for converting sunlight into fuels is the photoelectrochemical (PEC) cells which can serve for: a) water splitting; b) charging redox flow cells and; c) CO2 photoelectroreduction. This talk addresses the latest developments in PEC cells for solar water splitting and direct charging redox flow cells. Critical pathways towards economically viability of both technologies in energy markets require: i) the development of stable and efficient earth-abundant photo-absorber materials and electrocatalysts; and ii) the successful optimization of PEC cell architectures suitable for large-scale solar fuels production. These challenges have yet to be overcome and both are still in the research and development stage. Latest developments on hematite photoelectrodes concerning long-term stability [2] and high photovoltage [3] will be discussed along with their larger-scale implementation of these sunlight energy harvesting and storage processes. Photoelectrochemical “CoolPEC” cell, optimized for continuous operation and including improved key features such as: i) simultaneous photoelectrode and cell window; ii) electrolyte feeding manifold, assuring efficient gas bubbles detachment from windows and efficient heat dissipation if concentrated sunlight is used; and iii) narrow, easy-assemble and cost‑efficient embodiment [4], was reported. “CoolPEC” cell was operated over 42 days (1008 h) using a 50 cm2 hematite photoelectrode in a tandem arrangement with silicon heterojunction solar cells, under 1000 W∙m−2 and with constant electrolyte feeding at 45 °C [4]. More recently, a 200 cm2 PEC module (Figure 1), comprising four 50 cm2 PEC cells based on the CoolPEC cell design, was constructed and operated continuously outdoor under concentrated sunlight (up to 14 kW∙m-2), provided by the SoCRatus (DLR. Cologne, Germany), with electrolyte recirculation over four days. When assembled with four multi-photoelectrode windows, the module generated a stable current density of ca. 2.0 mA∙cm-2 at 1.45 V and reached a plateau current of ca. 4.0 mA∙cm‑2 before dark current onset, resulting in a hydrogen production rate of 5.5 × 10‑5 gH2∙h-1·cm-2 (based on the active area). The technology of solar charging redox flow batteries, named solar redox flow cells (SRFC), received recently a boost with the publication of an article reporting the first truly stable aqueous alkaline redox flow battery with a hematite photoanode [5]. SRFC is now a hot research topic with great potential for building integrated since this technology can simultaneously harvest efficiently sunlight to two forms of energy, an electrochemical fuel easily converted in electricity and heat. This technology is expecting to display soon over 10 % of solar to electrochemical energy conversion efficiency and ca. 60 % of heat harvesting for sanitary and thermal comfort uses.

10:00 - 10:15
3.1-O1
Holmes-Gentle, Isaac
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland
Dynamic Process Simulation of a kW Scale Solar Hydrogen Producing System under Concentrated Irradiation
Isaac Holmes-Gentle
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, CH
Authors
Isaac Holmes-Gentle a, Saurabh Tembhurne a, Clemens Suter a, Sophia Haussener a
Affiliations
a, EPFL, Institute of Mechanical Engineering, Station 9, 1015 Lausanne, Switzerland
Abstract

PEC devices can encompass a large spectrum of device types [1], and whilst the theoretical maximum efficiency for each level of integration may be the same for a given conceptual design (i.e. number of junctions, optical and electrical configuration of those junctions) [2], the ease of engineering a feasible device may differ significantly [3]. In this work we investigate the performance of a thermally integrated but light absorption decoupled photo-electrochemical device.

A lab scale prototype (30 W scale) using concentrated light (~470 times concentrated) has previously experimentally demonstrated the advantages of the direct electronic coupling and spatial integration of the electrochemical splitting of water to the photo-voltaic cell, whilst utilising waste heat extracted from the solar irradiance to improve the kinetics of the electrochemical reaction [4]. In this work, the scale up of a similarly integrated device is dynamically simulated in order to gain a greater understanding into the operability and real world performance. The simulated system under study here, is based on the efficient real pilot plant scale (kW scale) demonstration which has been constructed on the EPFL campus. It is composed of a 7 m diameter solar parabolic dish concentrating onto the integrated solar hydrogen device, with the necessary liquid/gas handling periphery systems (e.g. pumping, separation, compression and storage). The hydrogen is produced and stored at 20-30 bar.

The operation, control and optimization of such a novel integrated system is non-trivial, as the integration which permits such high efficiency adds significant complexity to the process engineering. In this study, dynamic process modelling is used to assess the response of the system to daily, seasonal and yearly variations in solar irradiance and ambient temperature. Conversely, the demand for fuel, heat and electricity on campus will also temporally fluctuate and control strategies are found in order to maximise the efficiency or the agreement between production and demand. Time-dependent environmental conditions and realistic component performance data has been employed in order to get meaningful results that can direct the control strategies of the real life concentrated solar to hydrogen pilot plant. Additionally, the capacity to which controlling the flow rate can stabilise both the hydrogen production and performance degradation, as previously reported [5], has been assessed for the scaled system. Finally, the simulated performance of the EPFL scaled co-generation device is compared to similar competing technologies and the future perspective of this type of technology is discussed.

10:15 - 10:30
3.1-O2
Zhang, Qingran
The University of New South Wales
A Fully Reversible Water Electrolyzer Cell made up from FeCoNi (Oxy)hydroxide Atomic Layers
Qingran Zhang
The University of New South Wales, AU

Qingran Zhang holds an M.Eng. degree in environmental engineering from Shanghai University. He is currently a Ph.D. candidate in chemical engineering at the University of New South Wales, working in the Particles and Catalysis Research Group under Prof. Rose Amal. His work focuses on understanding the reaction processes relating to the electrocatalysis and designing the efficient renewable energy conversion systems. 

Authors
Qingran Zhang a, Xunyu Lu a, Rose Amal a
Affiliations
a, The University of New South Wales, The University of New South Wales, Kensington, Sydney, 2052, AU
Abstract

Photovoltaic (PV) cells powered water electrolysis systems provide a facile and viable way to convert the abundant but intermittent solar energy into hydrogen (H2) fuel[1]. The large-scale deployment of such systems requires the development of efficient and cost-effective catalysts for overcoming the sluggish reaction kinetics associated with the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) in water splitting[2]. Further, the integration of water electrolyzer cells with PV cells also calls for the exceptional robustness of the catalyst materials that can effectively withstand the frequent power interruptions caused by cell shutdowns and/or weather changes[3]. To date, these requirements have posed a grand challenge in material development. In this work, atomically thin FeCoNi (oxy)hydroxide nanosheets (FeCoNi-ATNs) were prepared via a facile and scalable one-step bottom-up method. The obtained FeCoNi-ATNs exhibited extremely high mass activities for both OER and HER (1931 A g-1 at 330 mV for OER; 1819 A g-1 at 200 mV for HER) in alkaline solutions, which were among the highest of catalyst materials reported so far. More excitingly, by employing FeCoNi-ATNs as the catalyst material for both anode and cathode, a fully reversible water electrolyzer cell (WEC) was assembled, which exhibited a robust reversibility between two half reactions in water electrolysis under a high current density (100 mA cm-2). The as-fabricated WEC can effectively overcome the stability issues caused by electrode depolarization during frequent power interruptions, an inevitable phenomenon which is commonly brought about by the usage of intermittent renewable energy supplies.

10:30 - 11:00
Coffee Break
Exciup 1.2
Chair: Timothy Schmidt
11:00 - 11:30
1.2-O1
Pandya, Raj
Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK.
Optical Projection and Spatial Separation of Spin Entangled Triplet-Pairs from the S1 (21Ag-) State of Pi-Conjugated Systems
Raj Pandya
Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., GB
Authors
Raj Pandya a, Akshay Rao a
Affiliations
a, Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., J.J. Thomson Avenue, Cambridge, GB
Abstract

The S1 (21Ag-) state is an optically dark state of natural and synthetic pi-conjugated materials that can play a critical role in optoelectronic processes such as, energy harvesting, photoprotection and singlet fission [1,2]. Despite this widespread importance, direct experimental characterisations of the electronic structure of the S1 (21Ag-) wave function have remained scarce and uncertain, although advanced theory predicts it to have a rich multi-excitonic character [3]. Here, studying an archetypal polymer, polydiacetylene, and carotenoids, we experimentally demonstrate that S1 (21Ag-) is a superposition state with strong contributions from spin-entangled pairs of triplet excitons (1(TT)). We further show that optical manipulation of the S1 (21Ag-) wave function using triplet absorption transitions allows selective projection of the 1(TT) component into a manifold of spatially separated triplet-pairs with lifetimes enhanced by up to one order of magnitude and whose yield is strongly dependent on the level of inter-chromophore coupling. Our results provide a unified picture of 21Ag- states in pi-conjugated materials and open new routes to exploit their dynamics in singlet fission, photobiology and for the generation of entangled (spin-1) particles for molecular quantum technologies [4].

 

11:30 - 12:00
1.2-I1
Zhu, Xiaoyang
Columbia University
Understanding and Controlling the Triplet Pair States in Singlet Fission
Xiaoyang Zhu
Columbia University, US

Xiaoyang Zhu is the Howard Family Professor of Nanoscience and a Professor of Chemistry at Columbia University. He received a BS degree from Fudan University in 1984 and a PhD from the University of Texas at Austin in 1989. After postdoctoral research with Gerhard Ertl at the Fritz-Haber-Institute, he joined the faculty at Southern Illinois University as an Assistant Professor in 1993. In 1997, he moved to the University of Minnesota as a tenured Associate Professor, later a Full Professor, and a Merck endowed professor. In 2009, he returned to the University of Texas at Austin as the Vauquelin Regents Professor and served as directors of the DOE Energy Frontier Research Center (EFRC) and the Center for Materials Chemistry. In 2013, he moved to Columbia University. His honors include a Dreyfus New Faculty Award from Dreyfus Foundation, a Cottrell Scholar Award from Research Corporation, a Friedrich Wilhelm Bessel Award from the Humboldt Foundation, a Fellow of the American Physical Society, a Vannevar Bush Faculty Fellow Award from DOD, and an Ahmed Zewail Award from the American Chemical Society. Among his professional activities, he serves on the editorial/advisory boards of Accounts of Chemical Research, Science Advances, Chemical Physics, and Progress in Surface Science, and as a scientific advisor to the Fritz-Haber-Institute of the Max-Planck Society and ShanghaiTech University

Authors
Xiaoyang Zhu a
Affiliations
a, Department of Chemistry, Columbia University, New York, New York 10027, United States
Abstract

Singlet fission refers to the conversion of a singlet state (S1) to two triplet states (2xT1). There have been lingering confusions and debates on the nature of the all-important triplet pair intermediate states. Here we attempt to clarify the confusions from both theoretical and experimental perspectives. We distinguish the triplet pair state which maintains electronic coherence between the two constituent triplets from one which does not. The former is characterized by distinct experimental signatures and its formation may occur via incoherent and/or vibronic coherent mechanisms. We present our recent attempts at tuning the energetics of the triplet pair states and, thus, the energetics of singlet fission; this approach may be used to greatly expand the molecular toolbox for singlet fission. Finally, we discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.

OPV 1.2
Chair: Jenny Nelson
11:00 - 11:30
1.2-O1
azzouzi, mohammed
Department of Chemistry and Centre for Plastic Electronics, Imperial College London
Impact of Marginal Exciton – Charge-transfer State Offset on Charge Generation and Recombination in Polymer: Fullerene Solar Cells
mohammed azzouzi
Department of Chemistry and Centre for Plastic Electronics, Imperial College London, GB
Authors
mohammed azzouzi a, Michelle Vezie a, Jenny Nelson a, Tracey Clarke b, Artem Bakulin c
Affiliations
a, Department of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, UK.
b, Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, GB
c, Department of Chemistry and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London, GB
Abstract

The energetic offset between the initial photoexcited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (Voc). In this work, we use time-resolved spectroscopy and voltage loss measurements to analyse the effect of the exciton-CT state offset on charge transfer, separation and recombination processes in chemically similar blends of a low-bandgap isoindigoid polymer (INDT-S) with fullerenes derivatives of different electron affinity (PCBM and KL). The two blends possess large offsets for hole transfer, but different offsets for electron transfer: a very low offset for the INDT-S:PCBM blend and a higher one for the INDT-S:KL blend. In the case of the lower exciton-CT state offset (INDT-S:PCBM), the photocurrent generation is lower, Voc is higher and non-radiative voltage losses are lower than in INDT-S:KL. By characterising the dynamics of the devices after photoexcitation using both  transient absorption spectroscopy (TAS) and pump push photocurrent (PPPC) , we find that the dynamics of the  INDT-S:PCBM blend shows different excited state dynamics depending on whether the donor or acceptor is photoexcited. Interestingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than in INDT-S:KL upon excitation of the donor. We reconcile these observations using a model for the dependence of Voc on radiative and non-radiative recombination.  We also explain the effect of exciting donor or acceptor on charge transfer and recombination using a simple kinetic model. The results of the model show that hybridisation between the lowest excitonic and CT states can significantly reduce Voc losses whilst still allowing reasonable charge generation efficiency.  

11:30 - 12:00
1.2-O2
Madsen, Morten
University of Southern Denmark
Metal Oxide Interlayers for Scalable Organic Photovoltaic Devices
Morten Madsen
University of Southern Denmark
Authors
Mehrad Ahmadpour a, Andre Luis Fernandes Cauduro b, Jani Lamminaho a, Elodie Destouesse a, Mina Mirsafaie a, Bhushan Ramesh Patil a, William Greenbank a, Brian Julsgaard c, Vida Turkovic a, Peter Balling c, Horst-Günter Rubahn a, Nadine Witkowski d, Andreas Schmid b, Morten Madsen a
Affiliations
a, SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, Sønderborg, DK-6400, Denmark
b, National Center for Electron Microscopy, The Molecular Foundry, Berkeley, California, 94720, US
c, Department of Physics and Astronomy and iNano, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C
d, Sorbonne Universités, UPMC Univ. Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, Place Jussieu, 4, Paris, FR
Abstract

The introduction of non-fullerene acceptors has provided several recent record efficiencies in organic photovoltaic (OPV) cells, reaching now above 16% for single-junction devices. While these developments have provided a strong boost to the OPV field, more efforts have to be devoted to the up-scaling of such high performance OPV devices, which includes scalability of the active layers as well as the employed electrodes and interlayers. In terms of interlayers, metal oxide thin films have been widely used in OPV devices where they act as contact layers selective to either hole or electron transport, and thus support efficient carrier extraction from the cells. Well-known examples are titanium and molybdenum oxides, used in both organic and perovskite solar cells, with new variations appearing as the technologies develop further. In recent work, we have demonstrated that sputtered metal oxides thin films may act as interlayers in organic photovoltaic (OPV) devices, where they can act as both efficient and stable carrier extraction layers.

Here, recent progress made on reactively sputtered metal oxide hole1 and electron2 contact layers is presented. In both systems, a strong correlation between initial material composition and annealing condition to the microstructure of the films is given, leading to a pronounced improvement in their carrier extraction capabilities. Supported by a variety of surface science characterization studies, the importance of the energy band alignment, work function, microstructure, oxygen vacancies, optical and electrical properties and intrinsic stability on their performance as contact layers in OPV devices is discussed. Importantly, a new crystalline MoOx system employed for efficient hole extraction is shown to lead to a significantly prolonged OPV device lifetimes, and a new crystalline TiOx layer is shown to lead to an efficient electron extraction with s-shape free current-voltage characteristics, in striking difference to established TiOx interlayers. In order to meet the requirements on scalable OPV development, the up-scaling of these new metal oxide interlayer systems is discussed, considering recent results on industrially relevant OPV device development3. This includes Sheet-to-Sheet (S2S) and Roll-to-Roll (R2R) processing of OPV devices and modules, using combined solution and vacuum based techniques.

1 M. Ahmadpour, A. L. F. Cauduro, C. Méthivier, B. Kunert, C. Labanti, R. Resel, V Turkovic, H.-G. Rubahn, N. Witkowski, A. K. Schmid and M. Madsen, Crystalline molybdenum oxide layers as efficient and stable hole contacts in organic photovoltaic devices, ACS Appl. Energy Mater. 2, 420 (2019)

2 M Mirsafaei, P B. Jensen, M. Ahmadpour, H. Lakhotiya, J. L. Hansen, B. Julsgaard, H.-G. Rubahn, R. Lazzari, N. Witkowski, P. Balling and M. Madsen, Sputter deposited titanium oxide layers as efficient electron selective contacts in organic photovoltaic devices, under review (2019)

3 E. Destouesse, M. Top, J. Lamminaho, H.-G. Rubahn, J. Fahlteich and M. Madsen,  Slot-die processing and encapsulation of non-fullerene based ITO-free organic solar cells and modules, under review (2019)

12:00 - 12:15
1.2-O3
Paleti, Sri Harish
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia
An Energetic Perspective to Improve the Photostability of Non-Fullerene Acceptor based Organic PhotoVoltaics
Sri Harish Paleti
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, SA
Authors
Sri Harish Kumar Paleti a, Anastasia Markina b, Nicola Gasparini a, Denis Andrienko b, Derya Baran a
Affiliations
a, King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, 4700 King Abdullah University of Science and Technology (KAUST), Thuwal, SA
b, Max Planck Institute for Polymer Research, Mainz, Germany, Ackermannweg, 10, Mainz, DE
Abstract

Organic photovoltaics (OPV) offer unique advantages over conventional photovoltaic technologies, but their shorter operational lifetimes limited their application in the real world. In case of fullerene based devices it’s well established that the photofastness of the donor polymer and dimerization of fullerene are focal reasons for the limited operational lifetime. Recently, the development of non-fullerene acceptors (NFA) have pushed the efficiencies over 17%. However, the photostability of these devices are still are not fully explored. We show that the photobleaching of the NFA limits the operational lifetime in these state-of-the-art devices. Here, we demonstrate that lowering of the excited state energies of NFA’s is a way to improve the photo-stability of the OPVs. To test our hypothesis, high efficiency solar cells were fabricated by blending the benchmark donor polymers, PTB7-Th and PBDB-T with ITIC and its halogenated derivatives (ITIC, ITIC-4F and ITIC-4Cl). Upon constant illumination, the roll-off of short-circuit current density (Jsc) is the main cause of the drop in power conversion efficiencies (PCE) in these devices. We found that the chlorination of the peripheral units (ITIC-4Cl) leads to improved photostability of the devices when compared to the un-substituted NFA (ITIC). This is related to the different photo-bleaching kinetics of the neat acceptor films. Nuclear magnetic resonance (NMR) spectroscopy studies reveal that the photochemical reactions involve the chemical substitution on the peripheral bonds (C-H/F) of the dicyanomethylene-indanone (IC) moiety on respective acceptors. These chemical substitution’s is directly related to the excited state energies of the acceptors rather than the aforementioned bond energies.

PERInt 3.2
Chair: Iván Mora-Seró
11:00 - 11:30
3.2-I1
Palomares, Emilio
Institute of Chemical Research of Catalonia and Institució Catalana de Recerca i Estudis Avançats, ES
Carrier Recombination and Ion Migration: Role of the Contacts.
Emilio Palomares
Institute of Chemical Research of Catalonia and Institució Catalana de Recerca i Estudis Avançats, ES
Authors
Emilio Palomares a, b
Affiliations
a, Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology (BIST), Avinguda dels Països Catalans, 16, Tarragona, ES
b, Institució Catalana de Recerca i Estudis Avançats (ICREA), Spain, Passeig Lluis Companys 23, Barcelona, ES
Abstract

During my lecture I will present our latest results on the characterization of different type of contacts for perovskite solar cells using advanced photo-induced time resolved techniques. Using PICE (Photo-induced charge extraction), TPV (Photo-induced Transient PhotoVoltage) and other techniques, we have been able to distinguish between capacitive electronic charge, carrier recombination, and ion migration. Moreover, we have been able to stablish the existing relationship between the different intrinsic properties of the perovskite material. Moreover, the results allow us to compare different materials, used as hole transport materials (HTM) and electron transport materians ( ETM), and the relationship between their HOMO and LUMO energy levels, the solar cell efficiency and the charge losses due to interfacial charge recombination processes occurring at the device under illumination. These techniques and the measurements carried out are key to understand the device function and improve further the efficiency and stability on perovskite  based solar cells.

11:30 - 12:00
3.2-I2
Srimath Kandada, Ajay Ram
Center for Nano Science and Tecnology, Istituto Italiano di Tecnologia
On the Nature of Exciton-Bath Interactions in Two-Dimensional Lead Halide Perovskites
Ajay Ram Srimath Kandada
Center for Nano Science and Tecnology, Istituto Italiano di Tecnologia, IT
Authors
Ajay Ram Srimath Kandada a, Felix Thouin b, Carlos Silva b
Affiliations
a, Center for Nanoscience and Technology, Istituto Italiano di Tecnologia, Via Pascoli 70/3, Milano 20133, Italy.
b, School of Chemistry and Biochemistry and School of Physics, Georgia Institute of Technology, Atlanta 30338, USA
Abstract

Excitonic interactions in 2D semiconductors garner considerable attention, both due to their relevance in quantum opto-electronics and to the richness of their physics. Quantum-well like derivatives of organicinorganic perovskites are emerging material systems where strongly bound two-dimensional excitons have been observed even at room temperature. Here, we identify and quantify the photophysical factors that govern the multi-body interactions in phenylethylammonium lead iodide, a prototypical 2D perovskite. Employing high-sensitivity coherent non-linear spectroscopy, we measure the homogenous linewidth of multiple excitonic transitions1. By following the evolution of the linewidths with temperature and excitation density, we obtain pertinent insights into the exciton-phonon and exciton-exciton interactions. Based on our recent work2-4, we also argue that polaronic effects are manifested intrinsically in the exciton spectral structure and point to the apparently deterministic role of polaronic effects in excitonic properties. 
 
 
References 
 
1. F. Thouin, D. Cortecchia, A. Petrozza, A. R. Srimath Kandada  and C. Silva, Enhanced screening and spectral diversity in many-body elastic scattering of excitons in two-dimensional hybrid metalhalide perovskites, arXiv:1904.12402[cond-mat.mtrl-sci].  2. A. R. Srimath Kandada and C. Silva, Perspective: Exciton polarons in two-dimensional hybrid metal-halide perovskites, arXiv:1908.03909[cond-mat.mtrl-sci].  3. F. Thouin, A. R. Srimath Kandada, D. Valverde-Chavez, D. Cortecchia, I. Bargigia, A. Petrozza, X. Yang, E. R. Bittner and C. Silva, Electron-phonon couplings inherent in polarons drive exciton dynamics in two-dimensional metal-halide perovskites, Chem. Mater., DOI: 10.1021/acs.chemmater.9b02267. 4. F. Thouin, D. Valverde-Chavez, C. Quarti, D. Cortecchia, I. Bargigia, D. Beljonne, A. Petrozza, C. Silva and A. R. Srimath Kandada, Phonon coherences reveal the polaronic character of the excitons in two-dimensional lead-halide perovskites, Nature Materials 18, 349-356. 
 

SolCat 1.2
Chair: Karen Chan
11:00 - 11:30
1.2-I1
Surendranath, Yogesh
MIT
Mechanistic Insights Into Selective CO2-to-Fuels Catalysis
Yogesh Surendranath
MIT, US
Authors
Yogesh Surendranath a
Affiliations
a, Massachusetts Institute of Technology - USA, Cambridge, US
Abstract

The widespread utilization of renewable energy will require energy dense and cost-effective methods for storage. This challenge could be met by using renewable electricity to drive the reduction of carbon dioxide to energy dense carbonaceous fuels. However, many fuels are accessible over a narrow range in electrochemical potential, requiring a detailed mechanistic understanding of the key factors that control kinetic branching in these reactions. By combining electrochemical kinetic studies and in situ spectroscopies, we have uncovered key factors that control reaction selectivity and have applied this understanding to systematically tune product distributions in CO2-to-fuels catalysis. In particular, we will discuss (1) how electrolytes and solvent compete for adsorption of key intermediates, (2) how proton-coupled electron transfer dynamics provide key branchpoints for CO2 reduction vs H2 production, and (3) how competition between adsorbed H and CO drive C1 vs C2 selectivity. Our latest findings in this areas will be discussed.

11:30 - 11:45
1.2-O1
Mezzavilla, Stefano
Imperial College London
Active Sites for the Electrochemical Reduction of CO2 on Gold Surfaces – a Structure-Sensitivity Study
Stefano Mezzavilla
Imperial College London, GB

I am an independent Imperial College Research Fellow at the department of Materials at Imperial College London. My research interests focus on the design, synthesis and characterization of efficient electrocatalysts for energy conversion processes.

 

 

 

 

Authors
Stefano Mezzavilla a, b, Sebstian Horch b, Ifan Stephens a, Brian Seger b, Ib Chorkendorff b
Affiliations
a, Department of Materials, Imperial College London, United Kingdom, Prince’s Consort Road, South Kensington Campus, London, GB
b, Fysik, Danish Technical University, Fysikvej Bld. 311, Kongens Lyngby, 2800, DK
Abstract

The electrocatalytic reduction of CO2 to CO and syngas, which underpin multi-million tons scale processes such as olefin synthesis, methanol synthesis and Fischer-Tropsch, is a promising strategy to convert CO2 into value-added chemicals and to foster the introduction of renewable electricity in the chemical industry. Gold is the most active electrocatalysts capable to produce CO at low overpotentials and with excellent selectivity [1]. Many strategies, such as nanostructuring [2] and grafting with organic ligands, have been proposed to further enhance its performance. However, the fundamental knowledge of how the atomistic structure of the catalyst surface influences reaction rates and selectivity remains a very important missing fundamental insight.

In this work, we experimentally established – for the first time – that atomic steps and undercoordinated sites control the activity of Au for CO2 reduction [3]. We performed a thorough experimental investigation of gold single crystals having well-defined surface orientations. Low-index single crystals, such as (111), (100) and (110), were compared to a steps-rich (211) surface. The electrochemical reduction of CO2 to CO was found to exhibit a pronounced structure sensitivity: the CO partial current density registered with the most active catalysts (i.e., (110) and (211)) is ca. 20-fold higher than the one measured with Au (100), see Figure 1.

We further established the dominance of steps by selective poisoning experiments: the reaction was found to be largely suppressed if surface defects, such as atomic steps, were selectively blocked with inert (poisoning) Pb atoms.

The findings obtained with these model electrodes provide important targets for the design and synthesis of more efficient nanostructured catalysts. Furthermore, they offer elements to optimize the theoretical description of the electrochemical interface and reaction kinetics, which in turn may strengthen the prediction accuracy of future screening investigations.

 

[1]        Z. P. Jovanov, H. A. Hansen, A. S. Varela, P. Malacrida, A. A. Peterson, J. K. Nørskov, I. E. L. Stephens, I. Chorkendorff, J. Catal. 2016, 343, 215–231.

[2]        W. Zhu, Y.-J. Zhang, H. Zhang, H. Lv, Q. Li, R. Michalsky, A. A. Peterson, S. Sun, J. Am. Chem. Soc. 2014, 136, 16132–16135.

[3]        S. Mezzavilla, S. Horch, I. E. L. Stephens, B. Seger, I. Chorkendorff, Angew. Chem. Int. Ed. 2019, 58,3774–3778

11:45 - 12:00
1.2-O2
Ju, Wen
Technische Universität Berlin
Mechanistic understanding of formaldehyde reduction on metals and M-N-C catalysts
Wen Ju
Technische Universität Berlin, DE
Authors
Wen Ju a, Alexander Bagger b, Frederic Jaouen c, Jan Rossmeisl b, Peter Strasser a
Affiliations
a, Technische Universität Berlin, Straße des 17. Juni, 124, Berlin, DE
b, University of Copenhagen, -, copenhaguen, 0, DK
c, Université de Montpellier, France, Montpellier, FR
Abstract

The direct electrochemical CO2 reduction emerges as an attractive technology for its capability of converting waste CO2 into value-add chemicals and fuels. To achieve a full mechanistic understanding of such electrochemical conversion, vast experimental and computational studies have been implemented, and, aldehydes are found as the reactive intermediates for higher-value hydrocarbons and alcohols. It is worth to notice that, the products spectrum of aldehydes reduction – either to hydrocarbons or alcohols – is contingent on the nature of the catalysts. For instance, aldehydes electrolysis on metallic surface exclusively generates alcohols,[1], [2], [3] whereas the analogous reaction on nonmetallic single-site M–N–C catalysts selectively yields hydrocarbon as the only product.[4], [5] To date, the respective mechanistic routes have not been clearly addressed yet.

In this talk, I present our joint experimental-computational study aiming at understanding the mechanistic details of the hydrocarbons and alcohols formation on typical metal surface and a family of single-site M–N–C catalysts (M = Mn, Fe, Co, Ni). Formaldehyde is used as the single-carbon reactant for this series tests. We monitor the products spectrum of CH2O reduction – methane versus methanol – using chromatograph, and, simulate the atomic reaction steps with density functional theory. It is found that, binding types of the aldehyde, namely, carbon-adsorption or oxygen-adsorption, determines the possibility of its further hydrogenation; while the selectivity, either towards methane or methanol, is controlled by the protonation location. Our finding explains the distinct selectivity- and mechanism- differences of formaldehyde reduction over the metallic and single-site M-N-C catalysts, clarifying one aspect of the complex map of comprehensive electrochemical CO2 reduction.

SolFuel 3.2
Chair: Ian Sharp
11:00 - 11:30
3.2-I1
Deutsch, Todd
National Renewable Energy Laboratory
Photo-Electrochemical Hydrogen Production Systems using III-V Semiconductors: Challenges in Scaling-up from an Electrode to a Device
Todd Deutsch
National Renewable Energy Laboratory, US

Dr. Deutsch has been studying photoelectrochemical (PEC) water splitting since interning in Dr. John A. Turner’s lab at NREL in 1999 and 2000. He performed his graduate studies on III-V semiconductor water-splitting systems under the joint guidance of Dr. Turner and Prof. Carl A. Koval in the Chemistry Department at the University of Colorado Boulder.

Todd officially joined NREL as a postdoctoral scholar in Dr. Turner’s group in August 2006 and became a staff scientist two years later. He works on identifying and characterizing appropriate materials for generating hydrogen fuel from water using sunlight as the only energy input. Recently, his work has focused on inverted metamorphic multijunction III-V semiconductors and corrosion remediation strategies for high-efficiency water-splitting photoelectrodes. Todd has been honored as an Outstanding Mentor by the U.S. Department of Energy, Office of Science nine times in recognition of his work as an advisor to more than 30 students in the Science Undergraduate Laboratory Internship (SULI) program at NREL.

Authors
Todd Deutsch a, James Young a, Walter Klein a, Myles Steiner a
Affiliations
a, Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, US
Abstract

While III-V semiconductors have achieved the highest photo-electrochemical solar-to-hydrogen conversion efficiencies, they are remarkably unstable during operation in a harsh electrolyte. The first part of this talk will focus on the degradation mechanism of inverted metamorphic multijunction (IMM) III-V cells and surface modification strategies aimed at protecting them from photocorrosion. We applied noble metal catalysts, oxide coatings by atomic layer deposition, and MoSin an effort to protect the GaInP2 surface that was in contact with acidic electrolyte. We also grew epitaxial capping layers from III-V alloys that should be more intrinsically stable than GaInP2. The ability of the various modifications to protect the IMM’s surface was evaluated by operating at each electrode at short circuit for extended periods of time.

The second part of this talk will identify the challenges encountered while scaling the IMM III-V absorber areas of from ~0.15 cm2 up to 16 cm2 and incorporating them in a photoreactor capable of generating 3 standard liters of hydrogen in 8 hours under natural sunlight. To successfully scale photo-electrochemical water-splitting technologies from bench to demonstration size requires addressing predictable and unpredictable complications. Despite using Comsol multiphysics to model our photoreactor and identify suitable specifications for a prototype, several practical issues were uncovered during testing that led to multiple iterations of photoreactor design between the initial and final generations. Several bottlenecks that ranged from counter electrode composition and orientation to bubble management needed redress in order to meet our performance targets. Ultimately, the demonstration-scale system was able to generate nearly twice the target volume of hydrogen in an 8-hour outdoor trial.

11:30 - 12:00
3.2-O1
Hannappel, Thomas
Ilmenau University of Technology
Epitaxial Si-based Tandem Device Structures for Efficient Solar Water Splitting
Thomas Hannappel
Ilmenau University of Technology

Thomas Hannappel is W3 full professor (physics) at Ilmenau University of Technology, Germany, department ‘Photovoltaics’, since 2011. Before, he was provisional head of the Institute “Materials for Photovoltaics” at the Helmholtz-Zentrum Berlin and lecturer at the Free University Berlin, where he received his state doctorate in 2005. At Technical University Berlin he obtained his PhD in Physics with studies on ultrafast dynamics of photo-induced charge carrier separation in dye solar cells, he performed at Fritz-Haber-Institute Berlin of the Max-Planck-Society. In 2003/04 he conducted research on silicon/III-V-interfaces at National Renewable Energy Laboratory, Colorado. His current investigations are focused on high-performance solar cells and critical interfaces and he is a key player in the fields solar energy conversion and reactions of critical semiconductor interfaces including silicon/ and germanium/III-V-interfaces, and nano- and quantum-structures.

Authors
Thomas Hannappel a, Supplie Oliver a, Agnieszka Paszuk a, Hans-Joachim Lewerenz b, Matthias M. May c, Lara Eggert a, Andreas Bund a, Wen-Hui Cheng b, Matthias H. Richter b, Frank Dimroth d, Lackner David d, Ohlmann Jens d
Affiliations
a, Ilmenau University of Technology, Institute of Physics, Dep. Fundamentals of Energy Materials, Germany
b, California Institute of Technology, Joint Center for Artificial Photosynthesis, Pasadena, USA
c, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany, Berlin, DE
d, Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany
Abstract

In an efficient solar-driven water-splitting photo-electrochemical (PEC) cell, the individual process steps can be divided into three different steps: (i) generation of appropriate electrochemical potentials by max. light absorption, (ii) non-dissipative transport of charge carriers and (iii) efficient charge separation and high catalytic activity at the solid liquid interface.  Consideration of all energetic and kinetic processes leads to the conclusion that only individual absorber materials are suitable with band gaps, which are too large for an efficient exploitation of the sun light and, therefore, efficient water splitting. Alternatively, tandem layer structures are capable to create a sufficient photo voltage, i.e. a sufficient splitting of the quasi-Fermi levels, and in addition to explore the solar spectrum most efficiently [1]. When using tandem cells, STH efficiencies can be achieved clearly exceeding 20% [2,3]. The difficulty arises to achieve a stable performance and to describe the microscopic processes at the challenging solid-liquid interface. Based on surface chemistry observed in model experiments [4], we firstly applied interfacial processing sequences for the functionalization of highly efficient III-V-semiconductor tandem absorbers [5], secondly studied in situ interfacial chemistry on the atomic scale, and thirdly prepared well-defined surfaces to explore different surface reconstructions on their initial interaction with water and oxygen. Different atomic surface reconstructions of InP and GaP, [100] and [111], prepared by metal-organic vapor epitaxy (MOVPE) and transferred in inert gas to the photo-electrochemical cell were investigated. In order to realize cost-competitive tandem device structures, low-defect III-V semiconductor integration into the mature silicon technology can be accomplished with the involvement of graded layer growth of GaAsxP1-x. For that, we studied graded GaAsP(001) growth in situ with reflection anisotropy spectroscopy (RAS). With increasing As supply, a characteristic spectral fingerprint of the surface reconstruction shifts towards lower photon energies, which is well observable at growth temperature and for a broad range of As concentrations. Within a simplified empirical model, this shift depends approximately linearly on the As content in the GaAsP layer. The As content of individual GaAsP layers can be quantified in situ during growth which is beneficial for process control. It is shown how to control in situ atomic scale preparation and epitaxial growth of efficient device structures for water splitting.

12:00 - 12:15
3.2-O2
Koike, Kayo
RIKEN - Japan
Photoelectrochemical and Electrochemical Properties of GaN Nanowires
Kayo Koike
RIKEN - Japan, JP
Authors
Kayo Koike a, Tevye R Kuykendall b, Shaul Aloni b, Satoshi Wada a, Katsushi Fujii a
Affiliations
a, RIKEN Center for Advanced Photonics
b, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA, Berkeley, US
Abstract

 

In the photosynthesis-II (PS-II), light absorber and water oxidation reaction center are separated. According to this PS-II system, we investigated the separated system of GaN nanowires (NWs) photo-absorber and NiO water oxidation electrocatalyst. The NWs of photo-absorber for our experiments were semi-polar GaN compared to those of GaN single crystal.

 

To characterize the electrochemical properties of semi-polar GaN NWs, a working electrode was prepared from MOVPE grown NW arrays using a vapor-liquid-solid mechanism. The NiO was deposited by Ni(OH)2 dispersed solution. The counter electrode was Pt and the light was a Xe-lamp with an intensity of 500 mW/cm2. The electrolyte was 1.0 M NaOH (pH 13.4). The photocurrent densities were measured for 180 min without bias.

 

We measured the GaN NW arrays using cyclic voltammetry with light illumination to clarify the photoelectrochemical properties. Voc, turn-on voltage, and cathodic current of GaN NWs had completely different results compared to those of GaN single crystal. Especially, cathodic current of GaN NWs was significantly larger than those of GaN single crystal. The resulting which was electrochemical properties likely indicated that the surface absorption species are different between the GaN NWs and single crystal due to the surface orientations.

 

The photocurrent density of GaN NWs without NiO can be observed under the illumination. However, the photocurrent density was observed to decrease rapidly once the reaction started. The surface changed from having a frosted glass appearance before to a mirror-like appearance after the reaction. It means that the GaN NWs dissolved into the electrolyte during the PEC reaction. GaN NWs with NiO showed similar result for that without NiO. The NiO was loaded on GaN NWs thus, it is not good condition to work as catalyst of GaN NWs. GaN NWs with and without NiO were completely dissolved after the 180 min reactions from SEM image. From these results, we see that NiO didn’t work to prevent the anodic corrosion like with GaN single crystals from these results.

 

As shown here, the semi-polar GaN NWs sample shows completely different photoelectrochemical and electrochemical properties compared to GaN single crystal because of their surface crystal orientation. 

12:15 - 12:30
3.2-O3
Cardenas-Morcoso, Drialys
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
“Hybrid Tandem Device Based on Thin-Film Silicon Photovoltaics and Nanostructured Water Oxidation Catalysts for Solar Water Splitting”
Drialys Cardenas-Morcoso
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES
Authors
Drialys Cardenas-Morcoso a, Tsvetelina Merdzhanova b, Vladimir Smirnov b, Friedhelm Finger b, Bernhard Kaiser c, Wolfram Jaegermann c, Sixto Gimenez a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
b, Institut für Energie- und Klimaforschung, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straÿe, 52425 Juelich, Germany
c, FG Oberflächenforschung, Technische Universtität Darmstadt, Otto-Berndt-Straße 3, 64287 Darmstadt, Germany
Abstract

The combination of thin film photo-absorbers with nanostructured metal oxide semiconductor catalysts in a hybrid tandem device stands out as a promising approach for robust and cost-effective ‘bias-free’ conversion of solar energy into chemical energy, stored in solar fuels or added values products. In this context, the integration of a Ni-Fe based electro-catalyst for water oxidation with a triple-junction Si photovoltaic solar cell, is an interesting proposal due to the abundant nature of its constituents and the proven effectivity of each individual component. The water oxidation catalyst (WOC) is composed by the mixed oxide NiFe2O4 -which provides the main actives sites for water oxidation catalysis after electrochemical treatment, and nanostructured α-Fe2O3, acting as co-catalyst. Due to the complexity of Ni-Fe catalyst, a rigorous analysis of the electrode surface is needed in order to reveal the role of each component during water splitting operation. Further, with the WOC coupling to a Si-based triple-junction photovoltaic cell (a-Si:H/a-Si:H/μc-Si:H), 7.7 % of Solar-To-Hydrogen conversion efficiency for the tested photovoltaic-electrochemical cell was achieved. These studies sustain that integrated photovoltaic-electrochemical configurations constitute an attractive and viable alternative to efficient and low-cost solar energy conversion when using Earth-abundant materials.

12:00 - 13:30
Lunch
12:30 - 14:00
Lunch
Exciup 1.3
Chair: Artem Bakulin
14:00 - 14:15
1.3-O1
Jones, David
University of Melbourne
Non-traditional Singlet Fission Materials
David Jones
University of Melbourne, AU
Authors
David Jones a
Affiliations
a, School of Chemistry, Bio21 Institute, University of Melbourne, , Parkville, VIC 3010, Australia.
Abstract

Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excited state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells raises the maximum theoretical efficiency of a solar cell from the Schockly-Queisser Limit of 33% to around 45% by effectively harvesting the energy from high energy photons. SF has been reported and extensively studied in crystalline acenes, and more recently acene dimers to better understand the fundamental photophysics and materials requirements for SF. Incorporation of these SF materials in to functional solar cells, although demonstrating modest efficiency enhancements, has had limited success. In our efforts to produce higher efficiency printed organic solar cells we had the desire to incorporate solution processible SF materials in printed organic solar cells, however most of the reported SF materials are highly crystalline and either do not promote SF in the solid state or controlling crystallisation is difficult. We aim to remove the local order constraint in high efficiency solid-state SF materials by, i) designing intra-molecular SF materials, and ii) using secondary self-association to pre-organise chromophores. Multiple exciton generation (MEG) through singlet fission (SF) is a spin allowed process whereby a singlet excited state is split into two triplet excitons. Inclusion of MEG chromophores into solar cells raises the maximum theoretical efficiency of a solar cell from the Schockly-Queisser Limit of 33% to around 45% by effectively harvesting the energy from high energy photons, with a potential 35% reduction in delivered cost per Watt of power. SF has been reported and extensively studied in crystalline acenes, and more recently acene dimers to better understand the fundamental photophysics and materials requirements for SF. Incorporation of these SF materials into functional solar cells, although demonstrating modest efficiency enhancements, has had limited success. In our efforts to produce higher efficiency printed organic solar cells we had the desire to incorporate solution processible SF materials in printed organic solar cells, however most of the reported SF materials are highly crystalline and either do not promote SF in the solid state or controlling crystallisation is difficult. We aim to remove the local order constraint on high efficiency solid-state SF materials by, i) designing intra-molecular SF materials, and ii) using secondary self-association to pre-organise chromophores.

We have recently published our proof of principle sduies on new SF systems.[3] Here we report our recent studies of new solid-state singlet fission materials using liquid crystallinity to promote self-assembly and to pre-organise triplet host chromophores. Using design criteria outlined by Busby et al. [1], suggesting an Acceptor-Donor-Acceptor (A-D-A) structure may support intra-molecular SF. However, we have used the core Donor in intra-molecular SF materials to promote self association, removing a requirement for local order in the triplet host chromophores. We have used strong pi-pi interactions of substituted hexabenzocoronene (HBC) donors to promote strong self-assembly, coupled with thienyl-substituted diketopyrrolopyrrole (TDPP) as the triplet host. In addition, we needed to design our intra-molecular SF host, with i) a singlet energy around 2.0 eV if TDPP was to be our triplet host, ii) strong self-association through the core, and iii) solution processability. Thin films of the discotic liquid crystalline FHBC(TDPP)2 material form hexagonally packed columns and has a singlet energy level of 2.00 eV [2]. SF studies on FHBC(TDPP)2 demonstrate a triplet yield of 150% in amorphous thin films, increasing to 170% in thermally annealed films.

14:15 - 14:30
1.3-O2
M. Gholizadeh, Elham
The University of New South Wales
Oxygen-Enhanced Upconversion of near Infrared Light from Below the Silicon Band Gap
Elham M. Gholizadeh
The University of New South Wales, AU
Authors
Elham M. Gholizadeh a, Timothy Schmidt a
Affiliations
a, PhD student
Abstract

Solar cells have deficiencies such as disability in absorbing all the sunlight’s wavelengths. Upconversion is one of the processes which helps solar cells to overcome this problem. However, people still need to improve Upconversion yield to make it applicable in solar cells. Two kinds of molecules are used in Upconversion: Sensitizer and emitter molecules. During this process two low energy photons convert to higher energy one.


Upconversion (UC) systems still need to overcome some challenges. For instance, Oxygen is one of the big limitations for these systems. It can kill triplet before they transfer to the emitter molecule. Moreover, Sensitizer’s absorption needs to improve, and it is also desired to bring Upconversion below silicon band gap.
Previously, we have introduced Violanthrone as the molecule which would be able to prevent this bottle neck [2]. In fact, this molecule’s triplet state is below singlet oxygen. So not only oxygen would not be a problem any more, but also can help the process.

The aim of present going experiments is to use Violanthron-79 as the emitter in the mixture of PbS QDs as the sensitizer. The idea of this project is to make a system with a wide range of absorption which is not oxygen sensitive. Ultimately, we can even make the system better and use different types of QDs to bring UC below the Si band gap.

In this system, the oxygen will not be destructive. Moreover, it would behave as a transistor for triplets. So transferring triplets from QDs to Violanthrone would be faster and more efficient. The other point is about fundamental quantum dots ligands which are oleic acid with a long 18 Carbons chain. According to Nienhaus et.al. [3], we also believe these long ligands will be a big barrier for triplet transferring from QDs to Violanthrone. So ligand exchange has been designed. In terms of Ligand exchange the basic oleic acid (OA) ligands will partially change by Tips tetracene carboxylic acid (Tips-TC). So, it is expected that triplets transfer from QDs to the Tips-TC and then transfer to the Violanthone molecule. Then, Triplet-Triplet annihilation (TTA) happens between two Violanthrone molecule and finally the higher energy photon will emit from singlet state of Violanthrone. It is expected oxygen facilitates triplet energy transfer from Tips-TC to Violanthrone since Violanthrone has lower triplet sate than Oxygen.

We are already using QDs which absorb 846 nm and emit 960 nm as the sensitizer. Ultimately, QDs with the absorption below silicon band gap will be applied.
 

14:30 - 15:00
1.3-I1
Grozema, Ferdinand
Delft University of Technology, The Netherlands
Triplet Dynamics in Perylenediimides
Ferdinand Grozema
Delft University of Technology, The Netherlands, NL
Authors
Ferdinand Grozema a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
Abstract

Perylene diimides (PDIs) are exceptionally stable dyes that have several characteristics that make them attractive for application in organic photovoltaics. These include a strong absorption in the visible and excellent electron accepting properties. In addition, energetic characteristics of the lowest singlet and triplet level are close to ideal for singlet fission. A particular attractive aspect of PDIs is their synthetic versability. Introduction of sidechains allows a very wide tuning of the crystal packing and hence the electronic properties. In this contribution the effect of crystal packing in PDI materials on a variety of electronic propeeties will be discussed. This includes singlet fission, upconversion and triplet diffusion. The results show a strong correlation between crystal packing and singlet fission efficiency. The dynamics of singlet fission is studied by a combination of ultrafast spectroscopy and theoretical approaches. The subsequent triplet diffusion is studied by time-resolved microwave conductivity. The results show that PDIs have great promise for different processes in organic photovoltaics.

15:00 - 15:30
1.3-I2
Campos, Luis
Department of Chemistry, Columbia University, New York, New York 10027, United States
Materials Design for Third Generation Solar Cells
Luis Campos
Department of Chemistry, Columbia University, New York, New York 10027, United States

Luis M. Campos is an Associate Professor in the Department of Chemistry at Columbia University. He was born on this planet, just like you. Luis grew up in Guadalajara, Mexico, and moved at the age of eleven to Los Angeles, California. He received a B.Sc. in Chemistry from CSU Dominguez Hills in 2001, and a Ph.D. from the Department of Chemistry & Biochemistry at UCLA in 2006 working under the supervision of M. A. Garcia-Garibay and K. N. Houk. At UCLA, he was awarded the NSF Predoctoral Fellowship, Paul & Daisy Soros Fellowship, and the Saul & Silvia Winstein Award for his graduate research in solid-state photochemistry. Switching to materials chemistry, he went to UCSB as a UC President's Postdoctoral Fellow to work under the supervision of C. J. Hawker at the Materials Research Laboratory. At Columbia, his group’s research interests lie in physical macromolecular chemistry. To date, he has co-authored over 100 articles and 13 patents; and he has received various awards, including the ACS Arthur C. Cope Scholar Award, ONR Young Investigator Award,NSF CAREER Award, 3M Non-Tenured Faculty Award, I-APS Young Faculty Award, the Journal of Physical Organic ChemistryAward for Early Excellence, and the Polymers Young Investigator Award. In addition to these research accolades, Luis has been recognized for his pedagogical contributions by the Cottrell Scholar Award, Columbia University Presidential Teaching Award, and the Camille Dreyfus Teacher-Scholar Award.

Authors
Luis Campos a
Affiliations
a, Department of Chemistry, Columbia University, New York, New York 10027, United States
Abstract

Organic materials offer a rich palate to be decorated with functional units in order to tune various properties. For example, the ability to generate multiple excitons from a single photon (singlet fission in molecular materials) has the potential to significantly enhance the photocurrent in single-junction solar cells, and thus raise the power conversion efficiency from the theoretical limit of 33% to 44%. However, there is a paucity of materials that undergo efficient singlet fission. Our group is interested in designing building blocks that are capable of generating triplet pairs in modular small molecules and polymers. In this vein, the reverse process of singlet fission – triplet fusion – provides the ability to upconvert low energy unabsorbed infrared photons into visible energy that can be used to improve the light absorption in solar cells. Here, we discuss our efforts into developing triplet fusion materials and the new potential applications. This talk will provide an overview on our approach to the design, synthesis, and evaluation of the materials for singlet fission and triplet fusion.

PERInt 3.3
Chair: Qing Shen
13:30 - 13:45
3.3-O1
Hossain, Ihteaz Muhaimeen
Institute of Microstructure Technology, Karlsruhe Institute of Technology
Perovskite Tandem Photovoltaics: Employing 2D/3D Perovskite Heterostructure for Perovskite Top Solar Cell with Engineered Bandgap
Ihteaz Muhaimeen Hossain
Institute of Microstructure Technology, Karlsruhe Institute of Technology, DE
Authors
Ihteaz M. Hossain a, b, Saba Gharibzadeh a, b, Paul Fassl a, b, Bahram A. Nejand a, b, Raphael Schmager a, Tobias Abzieher b, Jonas A. Schwenzer b, Uli Lemmer a, b, Bryce S. Richards a, b, Ulrich W. Paetzold a, b
Affiliations
a, Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
b, Karlsruhe Institute of Technology (KIT), Light Technology Institute (LTI), Engesserstrasse 13, 76131 Karlsruhe, Germany
Abstract

The material class of mixed-halide organic-inorganic hybrid perovskites exhibits a tunable bandgap (EG) from 1.5 - 3.1 eV, simply by adjusting the ratio of the halide precursors. This characteristic makes these materials excellent candidates for low-cost multi-junction photovoltaics. In particular, wide-bandgap perovskites (WBP) with an EG ~ 1.7 eV are attractive top-cell materials to improve the power conversion efficiency (PCE) of single-junction silicon or CIGS solar cells in a two-terminal or four-terminal (4T) tandem configuration.

Recently, we developed a 2D/3D perovskite heterostructure by spin-coating n-butylammonium bromide (BABr) on top of a 3D double-cation WBP (EG ~ 1.72 eV) absorber (FA0.83Cs0.17Pb(I0.6Br0.4)3), resulting in a record open-circuit voltage of up to 1.31 V and a stable output power of up to 19.4% [1]. In this contribution, we show that double-cation perovskite solar cells employing various ratios of halide precursors (FA0.83Cs0.17Pb(IxBr1-x)3; 0.45 < x < 0.75) with 2D/3D heterostructure yield enhanced open-circuit voltages and PCEs over a wide range of bandgaps (1.63 – 1.83 eV) compared to reference devices without the heterostructure. The stable performance of devices and high reproducibility of the passivation approach is verified. We will also show our latest developments of the 4T tandem solar cells using such semitransparent perovskite solar cells with engineered bandgap, leading to PCEs > 26%. Possible strategies to further increase the tandem solar cell performance towards a PCE > 30% will be discussed.

13:45 - 14:00
3.3-O2
Lemercier, Thibault
LEPMI/Université Savoie Mont-Blanc - CEA Liten/DTS/SMPV/LMPO
Compatible Integration of ITO in NIP and PIN Perovskite Solar Cells for Semi-transparent Devices Using Same ETL and HTL
Thibault Lemercier
LEPMI/Université Savoie Mont-Blanc - CEA Liten/DTS/SMPV/LMPO
Authors
Thibault Lemercier a, b, Lara Perrin a, Emilie Planès a, Solenn Berson b, Lionel Flandin a
Affiliations
a, LEPMI / Université Savoie Mont Blanc
b, CEA Liten/DTS/SMPV/LMPO, 50 Avenue du Lac Léman, Le Bourget du Lac, 73375, FR
Abstract

The application of perovskite materials in the photovoltaic field has already led to a fast rise in conversion efficiency, currently above 24%. Thanks to their unique properties [1], these materials are also good candidates for tandem cells if used as the high bandgap subcell [2]. This is the long term goal of this work.

Here, we present an accepted optimized mono-cation perovskite formulation [3]: CH3NH3PbI3{Cl} (chlorine doped MAPbI3) for a development in both NIP and PIN-type structures, using the same materials for n-type and p-type layers: respectively, tin oxide (SnO2) and poly-triarylamine (PTAA). The perovskite layers deposited on either SnO2 and PTAA have been investigated with several characterization tools: UV-visible absorption, photoluminescence (PL), X-ray diffraction and photo-current (J-V) measurements in case of devices. Despite an un-similar PL behavior, we have found rather close properties for the two perovskites grown on either SnO2 and PTAA.

The integration of transparent electrode consisting in ITO deposited by sputtering at room temperature has also been scouted, as a proof of concept for semi-transparent perovskite solar cells for both NIP and PIN-type structures. Their respective J-V measurements have been performed and compared each other depending on the direction of light illumination (glass or metal side). We have notably noticed different current densities, which have been accurately correlated to optical losses calculations in reflection and absorption. Even if this developed PIN-type structure remains less efficient than the NIP one, obtained results highlight the fact that the PIN-type structure seems optically more favorable for a monolithic tandem application.

14:00 - 14:30
3.3-O3
Hutter, Eline
Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
Manipulating Halide Segregation in Mixed-Halide Perovskites with Pressure
Eline Hutter
Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
Authors
Eline Hutter a, Loreta Muscarella a, Lucie McGovern a, Bruno Ehrler a
Affiliations
a, Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
Abstract

Mixing iodide and bromide in MAPb(I1-xBrx)perovskites is an effective strategy to obtain any desired bandgap intermediate to MAPbI3 (1.6 eV) and MAPbBr3 (2.3 eV). However, a major drawback of these mixed-halide perovskites is that under illumination (for x > 0.2), the halides segregate into iodide-rich and bromide-rich domains.[1] This makes the bandgap of mixed-halide perovskites unstable, which is detrimental for solar cell performance. That is, the iodide-rich lower bandgap domains act as a recombination centre for charges, which prevents them from being collected.

In this study, we investigate to what extent this halide segregation can be manipulated under pressure. Using pressure-dependent photoluminescence, we find that the pressure-dependent emission energy of the iodide-rich phase is positive (i.e. dEg/dP > 0). Notably, this is in contrast with pure MAPbI3 and MAPbBr3, where the bandgap shows a negative pressure dependence (i.e. dEg/dP < 0). Interestingly, this observation suggests that the composition of the iodide-rich phase depends on the pressure, indicating that the threshold for halide segregation shifts to higher x-values for increased pressure. In addition, we use pressure-dependent transient absorption spectroscopy to investigate the energetic landscape of the halide segregated perovskites and the rate at which charges funnel from the higher bandgap to the lower bandgap domains. Most importantly, our results suggest that contracting the unit cell of mixed-halide perovskites improves their stability against halide segregation for low x-values, paving the way toward rational design of stable perovskites with tunable bandgaps.

SolCat 1.3
Chair: Matthew Mayer
13:30 - 13:45
1.3-O1
Sebastian Pascual, Paula
University of Copenhagen
Surface Sensitivity and Electrolyte Effects on Cu Single-crystalline Electrodes for CO Electroreduction
Paula Sebastian Pascual
University of Copenhagen, DK
Authors
Paula Sebastian Pascual a, Alexander Bagger a, Jan Rossmeisl a, Maria Escudero-Escribano a
Affiliations
a, Department of Chemistry, Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Abstract

Cu is the only monometallic catalyst capable to electroreduce CO2 beyond CO and produce a large variety of fuels and valuable chemicals (hydrocarbons and alcohols). However, the reaction presents low selectivity and mainly competes with the hydrogen evolution reaction, thus affecting the efficiency of the overall process. Fundamental work on well-ordered electrified interfaces, i.e. involving Cu single crystalline electrodes (Cu(hkl)), is pivotal to understand the CO2RR mechanism because both specific active site and electrolyte controls the selectivity of the reaction. [1][2]

In the present communication, we have systematically investigated Cu(111) and Cu(100) single crystalline electrodes in contact with phosphate buffer solutions and in a wide range of pH (from pH 1.5 to 12.5). Phosphate is known to adsorb specifically on Cu electrodes. [3][4] Here, we show that the anion adsorption is strongly dependent on the geometry of the active site, as well as on the bulk solution pH. In parallel, we have saturated the different buffer solutions with CO (key intermediate of the CO2RR on Cu), aiming to compare how the interfacial properties of Cu(hkl) would be affected or modified under reaction conditions. Furthermore, we attempt to assess a few fundamental aspects that highly influence the CO electroreduction, such as pH effect, specific anion adsorption and/or the stability of the surface under reductive conditions. Figure 1 shows the voltammetric response of Cu(100) (Fig. 1A) and Cu(111) (Fig. 1B)  in contact with a 0.1M phosphate buffer neutral solution  and recorded at 50mV/s. The quasi-reversible peak recorded on both surfaces (red solid line) corresponds to the phosphate displacement by CO, and its potential position is sensitive to the active site. The enlargement of the cathodic scan towards the CO reduction (black dashed line) modifies this voltammetric feature, showing the potential dependence of the interfacial processes on Cu(hkl). 

13:45 - 14:00
1.3-O2
Liu, Kai
Delft University of Technology, The Netherlands
How Local Reaction and Process Conditions Influence CO2 Reduction to Multicarbon Products on Copper Gas-Diffusion electrodes
Kai Liu
Delft University of Technology, The Netherlands, NL
Authors
Kai Liu a, Nathan Nesbitt a, Thomas Burdyny a, Wilson Smith a
Affiliations
a, Delft University of Technology, The Netherlands, Julianalaan, 136, Delft, NL
Abstract

The electrochemical reduction of carbon dioxide (CO2) may provide a future sustainable and profitable way to utilize the most abundant greenhouse gas to produce high-value chemicals and fuels. Recently, the application of gas-diffusion electrodes (GDE) for CO2 reduction reaction (CO2RR) has rapidly increased and can boost the maximum reaction rate of CO2RR by over ten-fold, surmounting the mass transport limitations and concentration polarizations, which was previously a result of low CO2 availability at the surface of the catalyst in H-cells.

Understanding electrochemical CO2 reduction and catalytic activity in gas-diffusion systems is complicated by the large number of interconnected factors including the catalytic material and composition, active surface area, mass transport of reagents and products and the choice of electrolyte. By using controlled copper nanostructures we show that despite differences in CO2 reduction activity, selectivity and working electrode potential on copper catalysts at low current densities, experimental performance is nearly identical in 1 M KOH, KHCO3 and KCl at current densities of 300 mA/cm2. Experimentally, we demonstrate ethylene selectivities between 50-55% in all three electrolytes at local-pH-corrected working potentials of -0.9 to  1.0 V vs a reversible hydrogen electrode. By modeling both gas-phase and liquid phase transport of reagents and products in a gas-diffusion electrode, we find that the local reaction environment for all electrolytes converges, explaining the relative trends in performance at both low and high current densities. Further, using experimental selectivity and gas-channel concentrations as an input for the upgraded model, we determine that the local CO concentration at the catalyst’s surface approaches that of CO2 due to accumulating concentration of CO in the gas-channel and transport limitations through the gas-diffusion layer. The insights gained from these experimental and modeling results are then used experimentally to show that manipulating local dissolved CO2 and CO concentrations by varying CO2 flow rates can almost linearly change the Faradaic efficiency of oxygenate production at 200 mA/cm2. More importantly, these findings suggest that newly-considered operational performance metrics, such as CO2 single-pass conversion efficiencies, will come to play an extremely important role in C2/C3 product selectivities on copper-based catalysts by substantially changing the ratio of CO2/CO at a catalyst’s surface.

14:00 - 14:30
1.3-O3
Seger, Brian
Technical University of Denmark (DTU)
Analyzing the Complete Carbon Balance in High Current Density Electrochemical CO2 Reduction Reactors
Brian Seger
Technical University of Denmark (DTU), DK
Authors
Brian Seger a, Gaston Larrazabal a, Ming Ma a, Ib Chorkendorff a, Kasper Therkildsen b
Affiliations
a, Technical University of Denmark (DTU), Frederiksborgvej 399, Roskilde, 0, DK
b, Siemens Corporate Technologies, Günther-Scharowsky-Str.1, Erlangen, 91058, DE
Abstract

This talk will focus on understanding the complete carbon balance on electrochemical CO2 reduction reactors operating at industrial relevant current densities (100-500 mA/cm2).  Both a gas diffusion approach with a liquid catholyte as well as a zero gap membrane electrode assembly (MEA) approach will be discussed. 

In the MEA approach, a Ag membrane was used as a CO2 reduction cathode and a greater than 95% faradaic efficiency to CO is obtained at 200 mA/cm2 (~ 3.1 V).  Insertion of a reference electrode allowed us to roughly attribute losses to either cathode, anode, or the membrane.  Gas chromatography of the anode, in addition to the cathode, shows both the faradaic efficiency of oxygen evolution as well as the CO2 crossover across an anion exchange membrane and how this varies as a function of current.  The production of formate is analyzed as well as its crossover to the anode. 

For the gas diffusion electrode approach, sputtered Cu was used as a cathodic catalyst.  As expected, high faradaic efficiencies to ethylene and ethanol were seen as well as other multi-carbon products.  CO2 reduction products diffusing through to the anode were analyzed as well as oxygen and CO2. CO2 crossover and oxygen evolution faradaic efficiencies as a function of operating current and other reactor parameters will be discussed.  This talk will also demonstrated the role of varying electrolyte pH and how this effects performance from an overall carbon balance standpoint.

14:30 - 15:00
1.3-I1
Roldan Cuenya, Beatriz
Fritz Haber Institute of the Max Planck Society
Size, Shape, Composition and Electrolyte Effect in CO2 electroreduction
Beatriz Roldan Cuenya
Fritz Haber Institute of the Max Planck Society, DE

Since 2017 Beatriz Roldan Cuenya has been a Director at the Fritz Haber Institute of the Max Planck Society in Berlin (Germany). There she heads the Department of Interface Science. She moved from the Ruhr-University Bochum (Germany), where she became a professor of Physics in 2013. Prior to that, Beatriz Roldan Cuenya was a professor of Physics at University of Central Florida (USA).

She carried out her postdoctoral research in the Department of Chemical Engineering at the University of California Santa Barbara (2001-2003). Prof. Roldan obtained her PhD in Physics from the University of Duisburg-Essen (Germany) summa cum laude in 2001. She completed her M.S./B.S. in Physics with a minor in Materials Science at the University of Oviedo, Spain in 1998. During her academic career Prof. Roldan received an Early CAREER Award from the US National Science Foundation (2005) and the international Peter Mark Memorial award from the American Vacuum Society (2009). She is the author of more than 110 peer-reviewed publications and 3 book chapters and has given over 100 invited talks. She presently serves as Associate Editor of ACS Catalysis, in the editorial advisory board of the Surface Science journal and in the Advisory Committee of the Office of Basic Energy Sciences of the US Department of Energy.

Prof. Roldan’s research program explores the novel physical and chemical properties of size and shape-selected nanostructured materials, with emphasis on advancing the field of nanocatalysis through in situ and operando characterization of catalysts at work.

Authors
Beatriz Roldan Cuenya a
Affiliations
a, Department of Interface Science, Fritz-Haber-Institute of the Max Planck Society, 14195 Berlin Germany
Abstract

The utilization of fossil fuels as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO2. The electrochemical conversion of CO2 (CO2RR) to chemicals and fuels driven by electricity derived from renewable energy has been recently recognized as a promising strategy towards sustainable energy.

In this talk I will provide examples of recent advances in the development of highly active nanostructured surfaces and nanoparticle (NP) electrocatalysts (Cu, Ag, Zn, and Cu-M with M = Co, Zn, Ag, Pd, Au) and how their structure (crystal orientation, atomic arrangement, size, shape, defects), oxidation state and composition influence their selectivity in CO2RR. Additionally, the determining role of the electrolyte in the re-structuring of the electrode surface and its activity and selectivity will be illustrated by adding cations and anions (Cs+, Li+, Na+, K+, I-, Br-, Cl-) to aqueous electrolytes.

The importance of in situ and operando characterization methods (e.g. EC-AFM, Liquid-TEM, XAS, NAP-XPS) to gain in depth understanding on the evolution of the structure and surface composition of the CO2RR catalysts under working conditions will be also demonstrated. These results are expected to open up new routes for the reutilization of CO2 through its direct selective conversion into higher value products.

15:00 - 15:30
1.3-I2
Kenis, Paul
University of Illinois at Urbana-Champaign
Co-Electrolysis for Efficient Electroreduction of CO2 to Intermediates Fuels or Chemicals
Paul Kenis
University of Illinois at Urbana-Champaign, US
Authors
Paul Kenis a, b
Affiliations
a, University of Illinois at Urbana-Champaign, South Mathews Avenue, 600, Urbana, US
b, International Institute for Carbon-Neutral Energy Research(I2CNER), Kyushu University
Abstract

Using CO2 as a feedstock for the production of formic acid, CO, ethylene, and ethanol, all key building blocks for the synthesis of fuels and chemicals, is one of several approaches being explored to help reduce anthropogenic CO2 emissions, while also reducing society’s dependence on diminishing fossil fuel reserves [1].  A range of active electrocatalysts for the selective reduction of CO2 have been reported. For CO production selectivity easily exceeds 95%, and current densities approaching 500 mA/cm2 can be achieved, while overall energy efficiencies are 50-60% [2,3].  Also, ever more selective catalysts for ethylene / ethanol production are being developed.  Recently we reported an electrodeposited copper-silver alloy catalyst able to produce ethylene and ethanol at a combined selectivity exceeding 80% (3:1 ethylene to ethanol) at a rate of 170 mA/cm2 [4].  Techno-economic analyses of these processes indicate that the availability and cost of renewable energy is the most important factor in determining economic feasibility [5].

After a brief summary of state-of-the-art electrocatalysts for the reduction of CO2 to CO and to ethylene / ethanol this presentation will focus first on a number of factors, such as electrolyte composition and pH, that help to optimize the electrocatalytic reduction of CO2 to CO and C2 hydrocarbons.  The second and main part of the presentation will explore anode chemistries to help improve the economics of CO2 conversion at scale.  An analysis of Gibbs free energies indicates that about 90% of the total energy required for CO2 electrolysis is consumed at the anode when the oxygen evolution reaction (OER) is taking place there.  In other words, 90% of the energy is used to produce oxygen for which no large market exists.  Organic chemicals often can be oxidized at significantly lower potentials than the OER.  We identified glycerol as an easily oxidizable and abundantly available option, being a large volume by-product of industrial biodiesel and soap production [6].  Using a 2M glycerol solution as the anolyte lowers the overall cell potential by approximately 0.8 V, regardless of the CO2 electroreduction chemistry on the cathode (CO, formic acid, or ethylene/ethanol formation).  The 0.8 V lower cell potential translates to a 45-53% reduction in overall energy requirement and improves the energetic efficiency by about 15%.  The presentation will conclude with a summary of the improvements in techno-economic feasibility and in life-cycle analysis upon co-electrolysis of CO2 (cathode) and glycerol (anode).

OPV 1.3
Chair: Martin Pfannmöller
14:00 - 14:30
1.3-I1
Ade, Harald W.
North Carolina State University
Phase Behavior, Miscibility, and Stability of Non-Fullerene Organic Solar Cells
Harald W. Ade
North Carolina State University
Authors
Harald Ade a
Affiliations
a, North Carolina State University, 911 Partners Way, EB1 Room 2009, Raleigh, 27606
Abstract

The organic solar cell (OSC) field has been revolutionized by the development and use of novel non-fullerene small molecular acceptors with efficiencies now reaching >16% in several systems. Many reports of OSC blends focus primarily on the device performance aspect of the solar cell and the device stability and mechanical durability of non-fullerene OSCs have received less attention. Developing devices with both high performance and long-term stability remains a challenge, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and by desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OSC commercialization. We will discuss the current understanding of the phase behavior of OSC mixtures and the relation of phase behavior to performance, processing needs (e.g., kinetic quench), and morphological stability via meta-stability or vitrification. A large range of miscibility (from hyper-miscibility to strong hypo-miscibility) is observed in a number of systems, including a temperature dependence that can be a complex mixture of upper- and lower critical solution temperature behavior for both the binodal and the liquidus. The measurements and concepts presented should help to create molecular structure-function relationships that would allow some predictive guidance on how desired phase behavior and vitrification properties can be targeted by specific chemical design, and how simple measurements such as differential calorimetry can be used to screen properties that impact stability.

14:30 - 14:45
1.3-O1
Nelson, Jenny
Imperial College London
Relating Microstructure Behaviour to Charge Transfer States Properties and Energy Losses in Organic Bulk Heterojunction Solar Cells
Jenny Nelson
Imperial College London, GB

Jenny Nelson is a Professor of Physics at Imperial College London, where she has researched novel varieties of material for use in solar cells since 1989. Her current research is focussed on understanding the properties of molecular semiconductor materials and their application to organic solar cells. This work combines fundamental electrical, spectroscopic and structural studies of molecular electronic materials with numerical modelling and device studies, with the aim of optimising the performance of plastic solar cells. She has published around 200 articles in peer reviewed journals, several book chapters and a book on the physics of solar cells.

Authors
Jun Yan a, Elham Rezasoltani a, Mohammed Azzouzi a, Flurin D. Eisner a, Anne A. Y. Guilbert a, Jenny Nelson a
Affiliations
a, Department of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, UK.
Abstract

 

The microstructure of donor: acceptor blend photoactive layers in organic solar cells directly influences the properties of interfacial charge-transfer (CT) states, and thereby influences the minimum value of the energy loss that can be achieved in the photovoltaic device. However, relatively few studies are able to relate blend phase behaviour directly to device energy losses. In this work, we study the correlation between the phase behaviour of two poly-3-hexylthiophene (P3HT): acceptor blends made using different non-fullerene acceptors, namely IDTBR and IDFBR, and the energy losses of the blend devices. Whilst the two acceptors are chemically very similar, small differences in molecular structure result in large differences in the degree of crystallisation and mixing in the blend films. In particular, P3HT:IDTBR blends shows a higher degree of crystallinity and more phase separated domains, whereas P3HT:IDFBR blends show lower crystallinity and more mixing of polymer and acceptor. The different blends exhibit different trends in terms of energy loss as a function of blend composition, indicating disparate mechanisms on energy losses. To understand this, I will quantify the radiative and nonradiative component of energy losses and apply a model of interfacial charge recombination [1] and models of charge transport in order to relate the measured energy losses to the transport and recombination dynamics in the different blends and finally to the blend microstructure.  I will bring the observations together to discuss the relationship between phase behaviour and recombination losses using the P3HT:acceptor systems as a model.

14:45 - 15:00
1.3-O2
Wilken, Sebastian
Linköping University, Sweden
Watching Space Charge Build up in an Organic Solar Cell
Sebastian Wilken
Linköping University, Sweden, SE
Authors
Sebastian Wilken a, b, c, Oskar J. Sandberg d, Dorothea Scheunemann a, b, c, Ronald Österbacka b
Affiliations
a, Linköping University, Sweden, SE-581 83, Linköping, SE
b, Åbo Akademi University, Finland, Porthaninkatu, 3, Turku, FI
c, Institute of Physics, Carl von Ossietzky University of Oldenburg, 26111 Oldenburg, Germany
d, Department of Physics, Swansea University, Singleton Park, Swansea, SA2 8PP, Wales, United Kingdom
Abstract

The emergence of non-fullerene acceptors pushed organic photovoltaics (OPVs) to new heights, with efficiencies over 16% now being reported for single-junction devices [1]. However, there are still very few systems that maintain their performance at technically relevant thicknesses of 300 nm and more. One reason why efficient thick-film OPVs are so challenging is because of undesirable space-charge effects. There are basically two reasons for the accumulation of space charge in OPVs: unintentional doping [2,3] and imbalanced transport of electrons and holes [4]. In both cases, the electric field in the active layer is redistributed such that the photocurrent becomes space-charge limited, i.e., it shows a square-root dependence on the voltage. But then how can we distinguish whether the device limitations are caused by doping or imbalanced charge transport?

Here, we show that this can be done by determining the width of the space-charge region (w) as a function of the photogeneration rate (G). First, we show analytically that only imbalanced charge transport leads to a characteristic G-1/4 dependence of w. We then present a simple and robust method how the build-up of space charge with increasing generation can be monitored in a real operating device. The method is based on a light-intensity dependent measurement of the external quantum efficiency (EQE). Using a numerical reconstruction approach [5], we are able to attribute changes in the height and the spectral shape of the EQE with light intensity to variations of the space-charge region width. We demonstrate our approach for 300 nm thick OPV devices with a 10 times higher electron than hole mobility. We show that the photocurrent is completely governed by the mobility mismatch, while doping, but also other effects reducing the photocurrent as a function of voltage, such as a field-dependent generation, can be ruled out.

15:00 - 15:15
1.3-O3
Alkarsifi, Riva
Aix-Marseille University, Centre Interdisciplinaire de Nanosciences de Marseille CINaM, UMR CNRS 7325, Marseille, France
Highly efficient doped Nickel Oxide Nanocrystal based inks for Solution-Processed Hole Extraction Layers in Polymer Solar Cells
Riva Alkarsifi
Aix-Marseille University, Centre Interdisciplinaire de Nanosciences de Marseille CINaM, UMR CNRS 7325, Marseille, France, FR
Authors
Riva Alkarsifi a, Yatzil Avalos a, Pavlo Perkhun a, Mats Fahlman b, Christine Videlot-Ackermann a, Olivier Margeat a, Jörg Ackermann a
Affiliations
a, CINaM - UMR 7325 CNRS - Aix Marseille Université Campus de Luminy – Case 913 13288 MARSEILLE Cedex 09
b, 2Department of Physics Chemistry and Biology Linkoping University 58183 Linkoping, Sweden
Abstract

Polymer solar cells are nowadays amongst the most promising photovoltaic approaches for next generation PV applications as efficiencies have been increased over the last years over 16 % by the raising of new non-fullerene acceptors (NFA). [1] For their industrialization there are still remaining challenges related to large scale fully solution processing of high efficiency solar cells as record efficiency are obtained only at very small areas using hole extraction layers based on evaporated MoOx. Solution-processable Hole Extraction ( SHEL) materials compatible with the new NFA materials are not yet studied and are thus one of the major challenges today. Commercially existing solution for SHELs are based on PEDOT:PSS and alternatives are needed as PEDOT:PSS is not compatible with many NFAs as will be shown in this work.  Semiconducting metal oxide are promising hole extraction layers in photovoltaic devices because of their potentially superior electronic properties and stability compared to the organic materials. Among these, MoOx, WOx, GO and NiOx are well studied for hole extraction but suffer from high resistivity, which limits their application to only very thin layers. However, creating metal vacancies and doping with some elements can improve their conductivity. [2] Several chemical and physical methods were reported for the preparation of these oxides but to meet the requirements for large-scale, low cost and roll-to-roll production, the solution processable methods are more desirable.

Here we present a highly efficient SHEL based NiOx nanoparticles using different dopants (Li, Cu, and Sn) dispersed in isopropanol. The initial low work function of theses doped materials was further increased by doping the nanocrystal solutions with an organic acceptor, namely F4-TCNQ. [3] Theses hybrid NiOx SHELs allow replacing PEDOT:PSS with comparable device performance in normal device structures in fullerene based solar cells. High efficiency polymer solar cells based on NFAs reaching efficiency over 12% using evaporated MoOx as hole extraction layer are used to study the impact of the NiOx doping on the performance depending on the device structure, stability, air and thick layer processing of these novel SHEL inks compared to PEDOT:PSS.

 

15:15 - 15:30
1.3-O4
Avalos Quiroz, Yatzil Alejandra
CINaM, CNRS, Aix Marseille University, Marseille, France
Correlation of detailed photodegradation study of ITIC derivative acceptors in polymer blends and its impact on the stability in polymer solar cells.
Yatzil Alejandra Avalos Quiroz
CINaM, CNRS, Aix Marseille University, Marseille, France
Authors
Yatzil Avalos a, Agnès Rivaton b, Carmen M. Ruiz c, David Duché c, Jean-Jacques Simon c, Pavlo Perkhun a, Olivier Margeat a, Christine Videlot-Ackermann a, Lydia Cabau d, Olivier Bardagot d, Uyxing Vongsaysy e, Mélanie Bertrand e, Renaud Demadrille d, Jörg Ackermann a
Affiliations
a, CINaM - UMR 7325 CNRS - Aix Marseille Université Campus de Luminy – Case 913 13288 MARSEILLE Cedex 09
b, Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, F-63000 Clermont−Ferrand, France
c, Aix-Marseille Univ., Univ. Toulon, UMR CNRS 7334, Institut Matériaux Microélectronique Nanoscience de Provence (IM2NP), Marseille, France
d, Univ. Grenoble Alpes, CEA, CNRS, IRIG, SyMMES, F-38000 Grenoble
e, ARMOR, Organic Photovoltaics Division, Armor Sustainable Energies (ASE), La Chevrolière, France
Abstract

Intense research in the field of novel non-fullerene acceptors (NFAs) has allowed to increase the power conversion efficiency in organic photovoltaics over 17 %, making organic solar cells nowadays one of the most promising approaches for new generation PV [1]. Therefore, the long-term stability of these new photovoltaic materials, especially the identification of potential photodegradation processes, must be addressed now in depth to prepare the way towards commercialization.

Amongst NFAs, there are two successful molecule classes based on linear A–D–A architectures [2], which are derivatives of IDTBR on the one side, and ITIC and its various derivatives (ITIC-Th, ITIC-4F) on the other side. Recently, the photostability of two IDTBR derivatives has been studied, and it has been shown that crystallinity arises from specific chemical structure design is essential for high photostability [3].

Here we present a detailed study of photochemical and thermal stability of photoactive layers composed of ITIC derivatives NFAs blended with PBDB-T (PCE12), PTB7-Th (PCE10) and the new halogenated derivative of PCE12, PBDB-T-2F (PCE13) [4]. Recently, Brabec and coll. [5] have studied the stability of polymer solar cells using  ITIC derivatives (ITIC, ITIC-4F, ITIC-M, ITIC-DM, ITIC-Th). However, there is no analysis relating potential photodegradation processes to the device performance. The stability of high efficiency solar cells using PCE13 as donor have not yet been studied deeply. It is known that the resistance to degradation mainly depends on the chemical structure of the active layer components, the crystallinity nature of the materials and species generated in the excited state.

 In this work, we study the stability of ITIC derivates en details in thin layers as well as in polymer solar cells.. Especially, we focus on their sensitivity to singlet oxygen 1O2, a very reactive transient species which can dramatically affect the stability of the molecule [6], is addressed. Furthermore, we discuss the role of molecular structure and conformation on the acceptor stability under photochemical and thermal stress in presence (extrinsic stability) and in absence (intrinsic stability) of oxygen, in order to understand the degradation mechanisms in the core of the molecule itself. The degradation kinetics of the ITIC derivates in polymer blends are compared to pure acceptor in order to study potential stabilization effects in the bulk heterojunction. The obtained degradation kinetics of the different ITIC derivates is then compared to the degradation of the corresponding solar cells. The device stability of high efficiency solar cells of over 12% using PCE-10, PCE-12 and PCE-13 as donor polymers are compared to solar cells using IDTBR as acceptor.

PERFuDe 1.3
Chair: David Mitzi
15:00 - 15:30
1.3-I1
Even, Jacky
Institut National des Sciences Appliquées, Rennes (FR)
About the Usefulness of Symmetry and Empirical Approaches for the Theoretical Study of Bulk Halide Perovskites and Halide Perovskite Nanostructures
Jacky Even
Institut National des Sciences Appliquées, Rennes (FR), FR

Jacky Even was born in Rennes, France, in 1964. He received the Ph.D. degree from the University of Paris VI, Paris, France, in 1992. He was a Research and Teaching Assistant with the University of Rennes I, Rennes, from 1992 to 1999. He has been a Full Professor of optoelectronics with the Institut National des Sciences Appliquées, Rennes,since 1999. He was the head of the Materials and Nanotechnology from 2006 to 2009, and Director of Education of Insa Rennes from 2010 to 2012. He created the FOTON Laboratory Simulation Group in 1999. His main field of activity is the theoretical study of the electronic, optical, and nonlinear properties of semiconductor QW and QD structures, hybrid perovskite materials, and the simulation of optoelectronic and photovoltaic devices. He is a senior member of Institut Universitaire de France (IUF).

Authors
jacky even a
Affiliations
a, Univ Rennes, INSA Rennes, CNRS, Institut FOTON - UMR6082, F-35000 RENNES
Abstract

3D halide perovskites have emerged as a new class of semiconductors. Layered halide perovskites and colloidal perovskite nanostructures are also very attractive semiconductor nanostructures for applications. These materials are intensively studied nowadays at the microscopic level using available density functional theory computer codes. However, the quantitative and predictive descriptions of some basic properties such as phonons, electron-phonon coupling, structural phase transitions, excitons, multiphoton absorption and nanocrystal electronic fine structures, carrier relaxation, high injection regime … are still lacking. The presentation will review some recent studies where empirical or semi-empirical theoretical approaches have provided both new methodologies and concepts, as well as numerical results which are out of reach with DFT codes and available computational resources. Some of these classical semiconductor approaches are based on simplified and symmetry-based representations of the halide perovskite lattice allowing progressively accounting for various advanced topics. Examples will be given for 3D materials, layered perovskites and nanocrystals.

SolFuel 3.3
Chair: David Tilley
14:00 - 14:30
3.3-I1
Sivula, Kevin
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland
Materials for Robust, Inexpensive and High Performance Photoelectrochemical Fuel Production
Kevin Sivula
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, CH

Kevin Sivula obtained a PhD in chemical engineering from UC Berkeley in 2007. In 2011, after leading a research group in the Laboratory of Photonics and Interfaces at EPFL, he was appointed tenure track assistant professor. He now heads the Laboratory for Molecular Engineering of Optoelectronic Nanomaterials (http://limno.epfl.ch) at EPFL.

Authors
Kevin Sivula a
Affiliations
a, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Lausanne, CH
Abstract

The development of robust and inexpensive semiconducting materials that operate at high efficiency are needed to make the direct solar-to-fuel energy conversion by photoelectrochemical (PEC) cells economically viable. In this presentation the strategy of PEC solar fuel production is introduced and our laboratory’s progress in the development new light absorbing materials and co-catalysts will be discussed along with the application toward overall solar water splitting tandem cells for H2 production. Specifically, this talk will highlight recent results with the ternary oxides (CuFeO2 and ZnFe2O4) 2D transition metal dichalcogenides, and organic (π-conjugated) semiconductors as solution-processed photoelectrodes. With respect to ternary oxides, in our recent work [1,2] we demonstrate state-of-the-art photocurrent with optimized nanostructuring and addressing interfacial recombination by the electrochemical characterization of the surface states and attached co-catalysts. In addition, we report an advance in the performance of solution processed two-dimensional (2-D) WSe2 for high-efficiency solar water reduction by gaining insight into charge transport and recombination by varying the 2D flake size [3] and passivating defect sites [4]. Finally, with respect to π-conjugated organic semiconductors, in our recent work [5] we demonstrate a π-conjugated organic semiconductor for the sustained direct solar water oxidation reaction. Aspects of catalysis and charge-carrier separation/transport are discussed.

14:30 - 15:00
3.3-I2
Ludwig, Alfred
Ruhr-University Bochum
Discovery of Solar Water Splitting Materials in Multinary Metal Oxide Systems by Combinatorial Synthesis and High-Throughput Characterization of Thin-Film Materials Libraries
Alfred Ludwig
Ruhr-University Bochum, DE
Authors
Alfred Ludwig a, Swati Kumari a, Helge Stein a, Mona Nowak a, Chinmay Khare a, Kirill Sliozberg b, Ramona Gutkowski b, Joao Junqueira b, Wolfgang Schuhmann b
Affiliations
a, Chair for Materials Discovery and Interfaces, Institute for Materials, Ruhr University Bochum, D-44801 Bochum
b, Analytical Chemistry–Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum, D-44801 Bochum, Germany
Abstract

Semiconducting metal oxide thin films are candidates for photoelectrochemical (PEC) solar water-splitting applications due to their abundance, light absorption properties and stability in aqueous media. To identify materials with optimized properties, thin-film materials libraries, exhibiting combined thickness and compositional gradients, were synthesized by combinatorial reactive co-sputtering from elemental targets on platinized 100 mm diameter wafers in several complex multinary oxide systems: Fe-W-Ti-O, Fe-Cr-Al-O, Cu-Si-Ti-O, V-X-O, and Bi-V-X-O. The libraries can be heat-treated in oxygen-containing atmosphere for further crystallization and oxidation. High-throughput characterization of the samples contained in these libraries (342 measurement areas on each library) comprises EDX and XPS for composition, XRD for crystal structure, SEM and AFM for surface morphology, the use of high-throughput test stands for electrical and optical properties (color, transmission), and an optical scanning droplet cells for the elucidation of photoelectrochemical properties including potential-dependent photocurrent, IPCE values, photocurrent spectra, Tauc plots etc.. The analysis of the obtained data enables to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density in functional phase diagrams. Several promising compositions were identified and further investigated. Finally, we demonstrate the combinatorial glancing angle sputter deposition (GLAD) approach for the fabrication of thin film materials libraries consisting of columnar nanostructures.

15:00 - 15:15
3.3-O1
Andrei, Virgil
University of Cambridge - UK
Scalable Photoelectrochemical Perovskite-BiVO4 Tandem Devices for Solar Fuel Synthesis
Virgil Andrei
University of Cambridge - UK, GB
Authors
Virgil Andrei a, Haijiao Lu a, Sebastian D. Pike a, Robert L. Z. Hoye b, Bertrand Reuillard a, Shahab Ahmad c, Dominic S. Wright a, Michael De Volder c, Richard Friend b, Erwin Reisner a
Affiliations
a, Department of Chemistry, University of Cambridge - UK, Lensfield Road, Cambridge, GB
b, Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., J.J. Thomson Avenue, Cambridge, GB
c, NanoManufacturing Group, Department of Engineering, University of Cambridge, Cambridge, United Kingdom.
Abstract

As the energetic demands of our society keep rising, any emerging technology faces the challenge of matching the commercial benefits presented by fossil fuels in terms of energy storage and mobility. Since a vast part of the modern energetic infrastructure already relies on fuels, their production in a sustainable manner stands out as an obvious solution. Therefore, the light-driven conversion of small molecules such as water and CO2 into so-called solar fuels (e.g. H2, CO) represents an attractive alternative for simultaneous energy harvesting and storage.[1,2] While great progress has been made in the development of light absorbers maximizing the solar spectrum coverage, their integration with catalysts into photoelectrochemical (PEC) devices for fuel production still poses challenges. Beside the performance and stability of common PEC prototypes, their scalability and choice of catalyst are also major factors which must be considered for commercial applications.

In this work, we address those issues by developing approaches to synthesize and characterize up-scaled PEC devices, which are able to produce fuels autonomously in the absence of external bias. To overcome the overpotential losses of electroreduction, we introduce state-of-the-art photocathodes obtained by protecting triple cation mixed halide perovskite solar cells with a Field’s metal encapsulant. Their PEC performance is benchmarked using a platinum nanoparticle catalyst for proton reduction, reaching photocurrents as high as -14 mA cm-2 at 0 V against the reversible hydrogen electrode (RHE) under 1 Sun irradiation.[3,4,5] By combining the perovskite photocathodes with robust BiVO4 photoanodes, tandem PEC devices can be obtained which sustain unassisted water splitting at a solar-to-hydrogen conversion of approximately 0.3%. The PEC devices present remarkable stabilities of up to 20 h under operation in an aqueous medium,[4,5] revealing key general insights for the encapsulation of perovskite optoelectronic devices.

To investigate the scalability of our “artificial leaves”, we prepare devices of sizes up to 10 cm2 which reveal a similar performance to their 0.25 cm2 counterparts. The PEC tandems are characterized in a versatile 3D-printed PEC cell, which can accommodate a wide array of samples due to its modular design.[5] The potential for further device up-scaling is revealed by fabricating 25 and 300 cm2 doped BiVO4 panels from bismuth (transition metal) polyoxovanadate single-source precursors.[6] The overall findings are applicable to a wide range of photoelectrochemical systems employing various photoabsorbers.[7] Looking beyond water splitting, we will also discuss our recent progress on the development of PEC devices that can couple the more challenging CO2 reduction to water oxidation, with the ultimate goal of contributing towards a circular carbon economy via photoelectrocatalysis.

15:15 - 15:30
3.3-O2
Ahmet, Ibbi
Helmholtz-Zentrum für Materialien und Energie GmbH, Hahn-Meitner-Platz 1 14109 Berlin
Planar and Nanostructured n-Si/Metal-Oxide/WO3/BiVO4 Monolithic Tandem Devices for Unassisted Solar Water Splitting
Ibbi Ahmet
Helmholtz-Zentrum für Materialien und Energie GmbH, Hahn-Meitner-Platz 1 14109 Berlin

Post Doc working for Pof. Roel van de Krol at the Institute for Solar Fuels within the Helmholtz-Zentrum Berlin (HZB). I have experience in air sensitive chemistry, characterization of main group complexes, (photo)electrochemistry, and thin film deposition/characterisation. Developed novel single molecular precursors/ solutions for the controlled/selective phase pure deposition of structured metal oxide/chalcogenide semiconductor materials, in order to explore their potential applications in a range of energy harvesting technologies.

Experienced in the up-scalling of solar water splitting devices and seek to uncover and resolve the current engineering challenges. 

I have personal interest in chemical deposition processes and seek to explore the effects of surface and gas phase nucleation/crystallisation on the controlled growth of planar and nanostructured materials. 

Other projects focus on:

-The fabrication, investigation and optimision of photoelectrodes consisiting of nanostructured heterjunctions, where we aim to study the role of interfaces and buffer layers on photovoltage and surface recombination, in order to achieve overal solar driven water splitting. 

-The role of bulk and surface impurities, dopants and intrinsic defects on the photovoltage and charge carrier dynamics of a given material.

-Exploration of polymorphism and metastable phases. 

-Formation of the gradient doped materials to enhance charge seperation. 

 

 

Authors
Ibbi Ahmet a, Sean Berglund a, Abdelkrim Chemseddine a, Peter Bogdanoff a, Raphael Präg a, Roel van de Krol a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany, Berlin, DE
Abstract

A fully integrated monolithic device for solar water splitting consisting of two photo-absorber layers of different ideal band gaps has many benefits.1-4 We have investigated a series of tandem n-n heterojunctions of n-Si and BiVO4, which provide a photo-voltage sufficient for unassisted solar water splitting. We used a scalable deposition method for producing WO3 nano-rods via aerosol assisted chemical vapor deposition (AA-CVD), which serves as an electron-conducting scaffold for the subsequent formation of WO3/BiVO4 core shell nanostructures.5,6 Here we present a series of planar and nanostructured core-shell devices composed of n-Si/SiO2/TiO2 or SnO2(interface)/WO3(scaffold)/BiVO4(photo-absorber) heterojunctions. Solid-state electrical measurement have been used to probe the junction type and barrier height of the junction formed at the Si/metal oxide interfaces. Between n-Si/SiO2 and WO3 or BiVO4 we observe the formation of interfacial defects and unfavorable band alignment, which prevents charge transfer, diminishes the n-Si photovoltage, and induces recombination. This could be mitigated with the deposition of a thin TiO2 or SnO2 film onto the n-Si/SiOx interface, which act as effective passivation layers. Film thicknesses between 50 - 100 nm are optimum and result in the lowest onset potentials. Whilst the planar structured  devices showed better onset potentials (lower than -0.2 VRHE) and thus higher n-Si photo-voltages, the nano-structured core shell devices exhibited larger photocurrents for water splitting due to light scattering. We find that surface etching of n-Si with HF to remove the intrinsic SiO2 can be detrimental to the effective n-Si photovoltage, whereas surface cleaning with NH4OH enhances the performance compared to untreated n-Si/SiO2 wafers. Optimized stand-alone tandem photoanodes consisting of Pt/Cu/In:Ga/n-Si/SiO2/TiO2(100 nm)/WO3(core-shell)/BiVO4/Fe(Ni)OOH achieved a stable photocurrent density of 0.25 mAcm-2 in 1.0 M KBi pH 9.3 buffer solution for up to 2 hours under simulated AM 1.5 G illumination without an applied bias. Differential electrochemical mass spectroscopy (DEMS) shows a faradaic efficiency of ~98% for hydrogen production. These results illustrate the importance of suitable interfacial layers in planar and nanostructured tandem devices.

15:30 - 16:00
Coffee Break
Exciup 1.4
Chair: Artem Bakulin
16:00 - 16:30
1.4-O1
Zaykov, Alexandr
Institute of Organic Chemistry and Biochemistry of the CAS
Singlet Fission: Chromophores for Exciton Downconversion
Alexandr Zaykov
Institute of Organic Chemistry and Biochemistry of the CAS, CZ
Authors
Alexandr Zaykov a, Josef Michl b, Zdeněk Havlas a, Eric Buchanan b, Milena Jovanović b
Affiliations
a, Institute of Organic Chemistry and Biochemistry of the CAS, Flemingovo náměstí, 2, Praha, CZ
b, Department of Chemistry and Biochemistry, University of Colorado, Boulder 80309, US
Abstract
Singlet fission is a photophysical phenomenon in which a singlet exciton shares energy with a ground state molecule forming two triplet excitons. This phenomenon was unveiled more than half a century ago but the pursuit of it was resurrected only recently due to the fact that it might lead to an efficient way of exciton downconversion. The harnessing of it may lead to a jump in efficiency of solar cells breaking the Shockley-Queisser limit.[1],[2] Because the phenomenon includes the interaction of at least two molecules, the aim of our work is to provide simple methodology of obtaining optimal mutual disposition of such chromophore molecules. This is done through a scan of the whole 6-D space of rotations and translations of two chromophore molecules. We show this on a model molecule of ethylene but also on molecules of practical interest. In all these cases, qualitative guidelines may be observed that steer away from simple comparison of coupling elements expanding it with the discussion of effects of dimer interaction (Davydov splitting) affecting the viability of the examined structure. The results were obtained using an in-house developed program package called "Simple". This package is built upon a theory that revolves around Fermi’s golden rule with various simplifications that have been added to achieve even sub-millisecond calculation times per dimer structure. Following the calculation of SF coupling elements, semi-classical Marcus approximation is employed to explore the kinetics of SF. The other mode program Simple could be used in is as a quick analysis tool of an already synthesized and characterized crystal structures.
OPV 1.4
Chair: Uli Würfel
16:00 - 16:15
1.4-O1
Engmann, Vida
SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark
Biomimetic Additive-Assisted Stabilization of Organic Solar Cells
Vida Engmann
SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark
Authors
Vida Turkovic a, Michela Prete a, Mikkel Bregnhøj b, Liana Inasaridze c, Dmytro Volyniuk d, Filipp A. Obrezkov e, Juozas V. Grazulevicius d, Sebastian Engmann f, g, Horst-Günter Rubahn a, Pavel A. Troshin c, e, Peter Remsen Ogilby b, Morten Madsen a
Affiliations
a, SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, Sønderborg, DK-6400, Denmark
b, Aarhus University
c, Institute of Problems of Chemical Physics of Russian Academy of Sciences
d, Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu rd. 19, LT-50254, Kaunas, Lithuania.
e, Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, Nobel st. 3, Moscow, RU
f, National Institute of Standards and Technology, Gaithersburg, MD, USA
g, Theiss Research, La Jolla, CA, USA
Abstract

Organic solar cells have recently reached power conversion efficiencies of over 16%, highlighting the stability as their last remaining weak point. Their organic nature makes them strongly influenced by stresses such as oxygen, light, heat and humidity, which can be commonly found in their working environment. Incorporation of stabilizing additives (antioxidants, radical scavengers [1], hydroperoxide decomposers [2], UV absorbers [3]) in active layers of organic solar cells is an attractive approach for inhibiting degradation as it is both inexpensive and easily upscalable, and it does not introduce further complexity into the device architecture. Here we present our recent results on long-term stability improvement by biomimetic singlet oxygen quenching compounds. Microscopic and spectroscopic characterization was used to monitor chemical degradation over time, which is discussed in terms of concentrations of radicals and singlet oxygen over the course of degradation. The reported results and methods indicate a desirable route for mitigating degradation in organic solar cells.

PERFuDe 1.4
Chair: David Mitzi
16:00 - 16:30
1.4-I1
Stoumpos, Constantinos
University of Crete
Structure-Property Relations Two-Dimensional Halide Perovskites
Constantinos Stoumpos
University of Crete, GR
Authors
Constantinos Stoumpos a
Affiliations
a, Department of Materials Science and Technology, University of Crete, 71003 Heraklion, Crete, Greece
Abstract

Halide perovskites continue to impress with the remarkable semiconducting characteristics and the breadth of their potential applications. Celebrating a decade from the original implementation of CH3NH3PbI3 in solar cells, halide perovskites can now consistently produce competitive power-conversion-efficiencies (PCE > 20%) presenting a strong candidacy for real-life technological applications. A class of perovskites that has gained enormous momentum in last five years, however, is that of the two-dimensional (2D) halide perovskites. Originally thought of as a mere curiosity among its congeners, the key feature of increasing the perovskite stability promoted 2D perovskites as popular materials among the perovskite photovoltaics community. The added benefit of controlling the perovskite growth orientation on the films, thus partially compensating the loss of one “semiconducting dimension”, made them even more attractive for consideration in devices. Yet, it is precisely that “missing dimension” that makes 2D perovskites remarkable, since they provide a direct macroscopic avenue to study the properties of the quantum regime.

Akin to their 3D analogues, 2D perovskites possess a direct band gap electronic structure, while providing access to a much wider selection of organic cations- plain or bearing functional groups- able to intercalate between the inorganic perovskite sheets. 2D halide perovskites, having a general formula, (RNH3)An-1MnX3n+1, (R- = Aryl or alkyl group); (A+ = Cs, CH3NH3, HC(NH2)2); (M2+ = Ge, Sn, Pb); (X- = Cl, Br, I), provide an enormous chemical toolbox that allows fine-tuning of the optical, electrical and dielectric properties of the materials. Unsurprisingly, the key properties of these low-dimensional semiconductors, which form continuous homologous series, are still dictated by the structural configurations and conformations of the metal-halide layers and can be controlled by employing simple synthetic chemistry. In this presentation, the key structural aspects of the 2D perovskites will be surveyed, illustrating the 2D perovskite formability and structural chemistry and how those can be advantageously exploited to obtain promising materials that can be utilized in optoelectronic applications.

16:30 - 16:45
1.4-O1
Grozema, Ferdinand
Delft University of Technology, The Netherlands
Towards Two-dimensional Hybrid Perovskites with Functional Organic Components
Ferdinand Grozema
Delft University of Technology, The Netherlands, NL
Authors
Ferdinand Grozema a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
Abstract

Two-dimensional halide perovskites are analogues of 3D perovskites that are of strong interest for photovoltaics. These 2D-analogues have properties that are markedly different than those of the corresponding 3D perovskites, for instance a considerably larger band gap and much stronger interactions between electrons and hole in the inorganic part of the material, i.e. a much large exciton binding energy. We have shown in a recent study that also the exciton binding energy can be tuned over a large range by varying this thickness.

Until now, most of the large organic cations used in 2D or quasi-2D perovskite materials only act as a spacer, defining the dimensionality of the system. Their HOMO-LUMO gap is generally very large and the electronic properties of the resulting materials are fully determined by the properties of the inorganic layers.

In this work, we aim to introduce additional functionality in the organic part. An example of such functionality is the introduction of electron donors or acceptors resulting in enhanced charge separation. We have explored the introduction of functional organic chromophores theoretically by density functional theory calculations and show that it is possible to introduce conjugated molecules that have a significant effect on the electronic structure. Strong electron acceptors or donors lead to conduction band or valence band edges that are localized on the organic part of the materials. This could lead to enhanced charge separation.

In a subsequent step we have synthesized 2D perovskites with functional organic chromophore and show, by time-resolved spectroscopy, that this indeed leads to charge separation where either the electron or the hole remains on the inorganic layers. Using microwave conductivity we show that this indeed leads to a strongly enhanced photoconductivity. This works shows that it is possible to introduce specific functionality in the organic spacer layer in order to tune 2D perovskite for specific application

16:45 - 17:00
1.4-O2
Milic, Jovana
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland
Supramolecular Engineering of Layered Hybrid Perovskite Materials for Stable Perovskite Solar Cells
Jovana Milic
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, CH

Dr. Jovana V. Milić obtained her PhD in the Department of Chemistry and Applied Biosciences at ETH Zurich in July 2017, with the pioneering work on photo-redox switchable molecular grippers in the research group of Prof. François Diederich. Her research interests encompass (supra)molecular engineering of molecular machines and bioinspired organic materials with the aim of developing functional nanotechnologies. Since October 2017, she works as a scientist with Prof. Michael Graetzel in the Laboratory for Photonics and Interfaces at EPFL on the development of innovative photovoltaic materials implementing the concepts of supramolecular engineering, with the current focus on dye-sensitized and hybrid perovskite solar cells. For more information, refer to Jovana’s LinkedIn profile (linkedin.com/in/jovanavmilic), ORCID 0000-0002-9965-3460, and Twitter (@jovana_v_milic).

Authors
Jovana Milic a, Dominik Kubicki a, b, Lyndon Emsley b, Michael Graetzel a
Affiliations
a, Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, Station 6, CH-1015 Lausanne, Lausanne, CH
b, Laboratory of Magnetic Resonance, EPFL, Switzerland
Abstract

While hybrid perovskite solar cells attract considerable attention due to their remarkable solar-to-electric power conversion efficiencies,[1] their limited stability and molecular-level engineering remain challenging.[1–4] In contrast to three-dimensional perovskites, their layered analogs show promising stabilities.[1,5-6] In order to benefit from their enhanced stabilities without compromising the performance, we demonstrate a supramolecular strategy in the design of organic spacers based on fine-tuning noncovalent interactions complemented by their structural adaptability.[4-6] These systems are devised to interact with the perovskite surface in a manner uniquely assessed by solid-state NMR spectroscopy.[3-4,6] As a result, we obtain solar cells with superior properties based on formamidinium lead iodide compositions with efficiencies surpassing those of the state-of-the-art formamidinium-based layered perovskites, accompanied by enhanced stabilities.[5-6] This has been investigated using a combination of techniques to unravel the design principles and exemplify the role of supramolecular engineering in advancing perovskite solar cell research.

SolCat 1.4
Chair: Beatriz Roldan Cuenya
16:00 - 16:15
1.4-O3
Rasul, Shahid
Northumbria University
Recycling CO2 to Produce Renewable Fuels
Shahid Rasul
Northumbria University, GB
Authors
Shahid Rasul a, b, Eileen Yu b
Affiliations
a, Northumbria University, 302 Whynne Jones Building, Newcastle Upon Tyne, GB
b, Newcastle University, Newcastle University, Bedson Building, Newcastle upon Tyne, 0, GB
Abstract

Energy storage is one of the major challenges regarding renewable energy systems due to their intermittent nature. One of the practical solutions is to store renewable energy in chemical bonds (high energy density fuels) as nature does in photosynthesis process. Nature recycles CO2 in presence of sunlight and water to renewable fuels. Renewable fuels have two extraordinary advantages; 1) Renewable fuels provide efficient energy storage option for “surplus renewable energy” into chemical bonds of high energy density fuels and 2) Renewable fuels utilize existing energy supply infrastructure without any further requirements. However, electrochemical production of renewable fuels from waste CO2 and H2O to energy dense chemicals is one of the major challenges due to absence of suitable electrocatalysts.  A suitable electro-catalyst should exhibit high catalytic activity (i.e. high current density) at low overpotential (energy efficiency) towards a single energy rich product (selectivity), be inactive for competitor reactions such as hydrogen evolution (HER) and have good stability under reaction conditions. Herein we report, we report a novel strategy based on improved material design (Cu, Sn, In, Sb, Pb)[1-3] and enhanced mass transfer for eCO2R to affordable and scalable CO production. Our results suggest that our designed electrocatalysts suppress the reduction of H+ and simultaneously promote the conversion of CO2, which is highly desired in the electrochemical reduction of CO2 in aqueous medium.

16:15 - 16:30
1.4-O4
Giuffredi, Giorgio
Istituto Italiano di Tecnologia
Hierarchical, Quasi-1D CuOx-derived Nanostructured Copper Catalysts for CO2 Reduction
Giorgio Giuffredi
Istituto Italiano di Tecnologia, IT
Authors
Giorgio Giuffredi a, b, Federica Arena a, b, Hilmar Guzman c, Cesare Cosentino d, Simelys Hernandez c, Fabio Di Fonzo a
Affiliations
a, Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, 20133 Milano, IT
b, Department of Energy, Politecnico di Milano
c, Department of Applied Science and Technology, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
d, Ronzoni Institute for Chemical and Biochemical Research
Abstract

Copper is one of the most promising CO2 reduction electrocatalysts, because it allows to reduce CO2 to complex hydrocarbons with good efficiency, thus allowing to store renewable energies-generated electricity in the form of chemical energy of carbon-based fuels. Bulk polycrystalline copper, however, has unsatisfactory activity and requires high overpotentials for the reduction reaction. Oxide-derived copper (OD-Cu) catalyst are one of the most efficient way to increase the intrinsic activity of Cu: this class of materials have higher selectivity for complex hydrocarbons and longer stability. Moreover, their activity and selectivity are strongly affected by the morphology, which controls the mass transport of the reactants to the active sites and influences the local pH.

In this contribution, we study the influence of morphological and structural features of OD-Cu hierarchical nanostructured electrocatalysts on their CO2 reduction activity and selectivity. We synthesize a nano-crystalline copper oxide (CuOx) material, which exhibits a high availability of defective, under-coordinated surface sites granting a good reduction activity, exploiting Pulsed Laser Deposition (PLD) as synthesis method. We leverage on the advantages of this technique, which allows to finely tune the morphology and porosity features of the synthesized material, to control the aspect ratio, the nanostructuring and the pore distribution of the CuOx nanostructures. With this approach, it is possible to evaluate the effect of the nanoscale morphological features on the overall catalytic activity of the OD-Cu material. Characterization of the CuOx nanostructures reveals an amorphous matrix with embedded crystallites of CuO and Cu2O, differently from other reported oxide-derived Cu catalysts. Electrochemical characterization of the CuOx nanostructures shows a clear trend between synthesis parameters and electrochemical surface area.

CO2 reduction performance of the oxide-derived Cu nanostructures is assessed through chronoamperometric measurements in aqueous electrolyte, showing for the best-performing catalyst remarkable FE at low applied overpotentials. A FE of 40% towards CH4 is calculated by means of gas-chromatography, while through 1H-NMR spectroscopy a total FE for liquid products in the 45-50% range is registered at -0.4 VRHE. Moreover, the OD-Cu nanostructures shows good stability for at least one hour of prolonged electrochemical operation.

This contribution shows how the combined effect of a OD-Cu material with a highly-defective, metastable structure and a fine control over the catalyst morphology can represent an efficient way to enhance the activity of a Cu-derived CO2 reduction catalyst.

16:30 - 16:45
1.4-O1
Varandili, Seyedehbehnaz
Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Interfacial Synergy in Cu/metal oxide Nanocrystalline Heterodimers for Enhanced CO2 Electroreduction
Seyedehbehnaz Varandili
Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Authors
Seyedehbehnaz Varandili a
Affiliations
a, Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Abstract

Synergistic effects at metal/metal oxide interfaces have been associated with highly active and selective catalytic motifs. So far, such interactions have been rarely explored to enhance the selectivity in the electrochemical CO2 reduction reaction. Herein, we discuss our recent work on Cu/CeO2-x heterodimers (HDs) which highlights the beneficial effects of such interface towards promoting the electrochemical CO2 reduction reaction (CO2RR) [1]. We have developed a colloidal seeded-growth synthesis to connect the two highly mismatched domains (Cu and CeO2-x) and to tune the extension of the interface by varying the size of the Cu domain. The Cu/CeO2-x HDs exhibit state-of-the-art selectivity towards CO2RR (up to ~80%) against the competitive hydrogen evolution reaction (HER) and high faradaic efficiency for methane (up to ~54%) at -1.2 VRHE, which is ~5 times higher than that obtained when the Cu and CeO2-x nanocrystals are physically mixed. Operando X-Ray absorption spectroscopy along with other ex-situ spectroscopies evidences the partial reduction from Ce4+ to Ce3+ in the HDs during CO2RR. The DFT study of the active site motif in reducing condition proposed that lowest free energy pathway utilizes O-vacancy site with intermediates binding to both Cu and Ce atoms, a configuration which allows to break the CHO*/CO* scaling relation. Within this concept, other metal oxides are under investigation to demonstrate the influence of support in the catalytic properties of Cu-metal oxide interface.

16:45 - 17:00
1.4-O2
Iyengar, Pranit
Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Facet Dependent Reactivity of Copper Nanocrystals for Electrochemical CO2 Reduction to Valuable Products
Pranit Iyengar
Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Authors
Pranit Iyengar a, Gian Luca De Gregorio a, Raffaella Buonsanti a
Affiliations
a, Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Abstract

Copper is the best candidate as metallic electrocatalyst to target commercially valuable hydrocarbons in the electrochemical COreduction reaction (CO2RR). Here we present our work on translating its structure-dependent selectivity to proof of concept devices (H-type and gas-fed cells) using colloidally synthesized Cu nanocrystals in the shape of octahedra (111 terminated) and cubes (100 terminated).[1-2] In addition to shape-dependence we show size dependence. Among the cubic nanocrystals (Cucub NCs), the 44 nm ones showed the highest selectivity towards ethylene, reaching up to 44%.[1] Octahedral nanocrystals (Cuoh NCs) were synthesized with size between 75–310 nm and the smallest one showed the best faradaic efficiency up to 55% for methane.[2] We explain both results through a dual facet mechanisms involving the (100)/(110) interface for the Cucub and (111)/(110) interface for the Cuoh.

Finally, we discuss our very recent results concerning the catalytic performance of the two NCs at industrially relevant conditions (i.e. at -100 to -300 mA/cmin a gas-fed flow electrolyzer). Here, we find that the facet-based control on product selectivity remains consistent even in such conditions.

SolFuel 3.4
Chair: David Tilley
16:00 - 16:15
3.4-O1
Son, Min-Kyu
Kyushu University
Investigation on Characterization of Sputtered Lanthanum Iron Oxide Film for Durable Photoelectrochemical Water Splitting
Min-Kyu Son
Kyushu University
Authors
Min-Kyu Son a, Tatsumi Ishihara a
Affiliations
a, International Institute for Carbon-Neutral Energy Research(I2CNER), Kyushu University
Abstract

 Photoelectrochemical(PEC) water splitting has received considerable attention as a promising way for producing eco-friendly and sustainable hydrogen since it was observed by Fujishima and Honda in 1972 [1]. In the PEC water splitting, p-type semiconductors have been widely used to implement photocathodes for generating hydrogen because minority carriers in the p-type semiconductor easily move to the water interface, enabling to split water to the hydrogen. Lanthanum iron oxide (LaFeO3) is one of attractive p-type semiconductors for PEC water splitting [2,3]. It is visible light responsive with a band gap of 2.1~2.4 eV, allowing to utilize the sufficient sunlight. In addition, it has favorable energy band positions for PEC water splitting, especially, with the conduction band lying more negative of hydrogen evolution reaction potential. Furthermore, it is durable in the aqueous solution in contrast with copper oxide, which is a typical p-type semiconductor in the PEC water splitting. Hence, it is beneficial for the sustainable hydrogen production via the PEC water splitting. In this work, we fabricate the LaFeO3 photocathode by the plasma sputtering deposition, which is simple, low cost and able to yield the uniform stoichiometric film. The film is sputtered with a deposition ratio of 0.048 nm/s using the specific conditions with a base pressure below 1.0 x 10-3 Pa, a working pressure of 5 Pa and a RF power of 70 W. The post annealing process is sequentially carried out in the air to make crystalline LaFeO3 film. The as-deposited LaFeO3 film is amorphous, but it converts into the crystalline LaFeO3 by annealing process at the temperature above 550ºC. As a result, the crystalline LaFeO3 photocathodes show the improved PEC performance, while the amorphous LaFeO3 photocathodes show negligible PEC performance in the strong alkaline solution. The optimization and characterization of sputtered LaFeO3 photocathodes are also investigated. It paves the way for the development of durable PEC water splitting system for a hydrogen fuel based economy.

16:15 - 16:30
3.4-O2
Gottesman, Ronen
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
High-quality Stable CuBi2O4 Photoelectrodes by Combining Pulsed Laser Deposition and Rapid Thermal Processing
Ronen Gottesman
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE

Born in Canada, Ronen Gottesman received his B.Sc. in Biophysics from the Department of Physics at Bar-Ilan University (BIU), Israel. He later conducted his Ph.D. at BIU in Physical Chemistry on the study of fundamental working mechanisms in photovoltaic systems which are based on nanoporous electrodes and hybrid perovskite absorbers. Currently, as a postdoc in the Institute for Solar Fuels at Helmholtz-Zentrum Berlin (HZB) his research topic is new complex metal oxides and oxynitrides photoabsorbers for solar fuels production, and the development of syntheses methods based on PLD+RTP and combinatorial approaches.

Authors
Ronen Gottesman a, Angang Song a, Roel van de Krol a, Abdelkrim Chemseddine a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
Abstract

A new approach for fabricating high-quality ternary photoelectrodes such as CuBi2O4 will be presented. Pulsed laser deposition (PLD)[1] is used to deposit the binary oxides Bi2O3 and CuO on FTO substrates, and rapid thermal processing (RTP)[2] is used to achieve an efficient solid-state reaction between the two oxide films. This study shows that when Bi2O3 is deposited first, phase-pure films of CuBi2O4 are obtained. A comparative study with conventional furnace annealing (FA) reveals the importance of radiative annealing in the processing of complex metal oxides photoelectrodes. Furthermore, the much shorter annealing times allow the use of FTO substrates at temperatures up to 650 °C, also resulting in a low thermal budget (the product of process temperature and processing time at an elevated temperature). The formation mechanism of the CuBi2O4 was studied with a variety of structural, chemical, and optical characterization techniques. The photoelectrochemical properties of the RTP and FA processed photoelectrodes were investigated and compared to other CuBi2O4 electrodes made by other techniques such as spray-pyrolysis[3] or drop-casting.[4] The RTP processed photoelectrodes exhibit improved properties and unprecedented photoelectrochemical stability without the addition of protection layers. Finally, photoelectrochemical H2 production of the RTP processed photoelectrodes is confirmed.

16:30 - 16:45
3.4-O3
Wilson, Anna
Imperial College London
Investigating the Enhanced Performance of WO3 Photoanodes from the Addition of Pd Co-catalysts
Anna Wilson
Imperial College London, GB
Authors
Anna Wilson a, Sacha Corby a, Laia Francás a, James Durrant a, Andreas Kafizas a
Affiliations
a, Department of Chemistry, Imperial College London, South Kensington Campus London, London, GB
Abstract

Photoelectrochemical (PEC) water splitting enables the utilisation of sunlight to obtain a renewable hydrogen source. Semiconducting metal oxides are promising materials for PEC photoanodes, and in particular WO3-based materials due to their low-cost, natural abundance and stability in acidic media. In this work, substoichiometric WO3 nanoneedles are synthesised in a single step via aerosol-assisted chemical vapour deposition (AA-CVD) as an up-scalable method. Pd nanoparticles are deposited as an additional step, also via AA-CVD. Physical characterisation of the materials using techniques including X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Raman spectroscopy, confirm their nanoneedle morphology, monoclinic crystalline phase and detailed chemical composition. The photocurrent and oxygen generation capabilities of the different sample compositions are studied, with their charge carrier dynamics analysed via transient techniques (transient absorption spectroscopy and transient photocurrent measurements) to aid understanding of the underlying performance enhancement mechanisms. It is clear from our study that Pd deposition dramatically enhances the photoanode performance, with regards to both photocurrent generation and faradaic water oxidation capabilities. Annealing the photoanodes, to optimise the oxygen vacancy concentration in WO3 and oxidise Pdto Pd2+ species, is crucial for achieving these improvements. The PdOx co-catalyst is found to increase the rate of charge extraction, seen in transient photocurrent measurements, and reduce side reactions at the photoanode surface.  

16:45 - 17:00
3.4-O4
Irani, Rowshanak
Helmholtz-Zentrum für Materialien und Energie GmbH, Hahn-Meitner-Platz 1 14109 Berlin
Interface Energetics and Photoelectrochemistry of MnOx-modified Ta-O-N Photoanodes
Rowshanak Irani
Helmholtz-Zentrum für Materialien und Energie GmbH, Hahn-Meitner-Platz 1 14109 Berlin
Authors
Rowshanak Irani a, Paul Plate a, Peter Bogdanoff a, Fatwa Firdaus Abdi a, Roel van de Krol a, Karsten Harbauer a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
Abstract

Sustainable approaches in utilizing solar energy to produce chemical fuels would provide a way to meet the world increasing energy demand without the negative environmental effects of burning fossil fuels. One particularly interesting method is solar water splitting, where energy from sunlight is used to produce hydrogen and oxygen from water. In this process, an aqueous-stable semiconducting photoelectrode is required in order to sufficiently absorb visible light and efficiently oxidize/reduce water on its surface. Since water oxidation is typically the limiting reaction, many semiconductor photoelectrodes need to be decorated with additional water oxidation co-catalysts, such as CoPi, FeOOH, NiFeOx, and MnOx, in order to improve the performance [1-5]. However, the mechanisms at the semiconductor/co-catalyst interface and how they are influenced by the interface energetics are not yet fully understood. Here, we use a well-defined MnOx-based co-catalysts deposited by atomic layer deposition (ALD) and semiconducting Ta-O-N thin films as a model system in order to investigate the process. We successfully prepared different phases (e.g. Ta2O5, TaOxNy, Ta3N5) with varying valence band maximum positions by systematically controlling the partial pressure of NH3, H2 and H2O during post-annealing of Ta thin films [6]. The valence band position of the MnOx co-catalyst was also shifted by introducing Ni as dopant (Ni:MnOx). Photoelectrochemical studies with and without hole scavenger reveal that MnOx and Ni:MnOx enhances the photocurrent of Ta-O-N films by a factor of ~4 without affecting the charge injection efficiency. Therefore, higher charge separation efficiency should play the crucial role in the photocurrent increase. Open circuit potential (OCP) and in-line X-ray photoelectron spectroscopy (XPS) data suggested that this is attributed to the formation of a band bending at the Ta-O-N/MnOx and Ta-O-N/Ni:MnOx interface. This is further supported by analyzing Ta-O-N films with varying thickness of the co-catalysts; thicker co-catalysts result in increasing band bending, which correlates with the photocurrent trend as well as the effective carrier diffusion length measured by time-resolved microwave conductivity (TRMC). Finally, the influence of the band offsets between the different photoanode in the Ta-O-N systems and the co-catalysts is discussed.

 
Thu Nov 07 2019
Exciup 2.1
Chair: Luis Campos
09:00 - 09:30
2.1-O1
Seki, Kazuhiko
AIST
Diffusion-limited Geminate Delayed Fluorescence by Singlet Fission and Triplet Fusion
Kazuhiko Seki
AIST, JP
Authors
Kazuhiko Seki a, Tomoaki Yago b, Ryuzi Katoh c
Affiliations
a, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b, Saitama University, 255 Shimo-Okubo, Sakura, Saitama, 338, JP
c, Nihon University, College of Engineering, 1 Nakagawara, Tokusada, Tamura, Koriyama, JP
Abstract

Singlet fission (SF) has been extensively studied through the use of magnetic field effects. We studied delayed fluorescence originating from diffusion-mediated reversible geminate fusion of triplets. We extended the Johnson−Merrifield model to study magnetic field effect on delayed fluorescence originating from diffusion-mediated reversible geminate fusion of triplets. We have shown that a wide range of SF materials exhibit power-law asymptotic decay. The asymptotic power law dependence appears after the transit time. The transit time to the asymptotic power law is inversely proportional to the value of the mutual diffusion constant of triplet pairs. The temperature dependence of the asymptotic emission decay supports a mechanism involving delayed fluorescence mediated by diffusion. Moreover, we show that the decay lines at different field strengths will not cross each other when fission occurs effectively. In the opposite limit when fusion occurs effectively the two decay lines will cross each other.  We also study the effect of crystal anisotropy on the delayed fluorescence originating from geminate fusion of triplets.

09:30 - 10:00
2.1-I1
Bakulin, Artem
Imperial College London
Carrier-Carrier vs Carrier-Phonon Interactions in Lead-halide Perovskite Materials: Role of Carrier Density, Nanoconfinement, and Surface Ligands
Artem Bakulin
Imperial College London, GB
Authors
Artem Bakulin a
Affiliations
a, Department of Chemistry and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London, GB
Abstract

The major efficiency limit in conventional solar cells is imposed by the rapid relaxation of above-bandgap “hot” carriers via electron-phonon coupling. To battle this limitation a number of non-trival photophysical phenonmena are investigated including singlet fission, carrier multiplication and hot carrier extraction. Lead-halide perovskites (LHPs) currently hold the efficiency record for solution-processable solar cells, and previous observations of slow hot-carrier cooling in these materials have piqued a deeper interest into their application in disruptive next-generation photovoltaics. However a coherent picture of carrier-carrier and carrier-phonon interactions in these systems is still under development.

Here we implement an ultrafast “pump-push-probe” technique to study the sub-ps cooling in LHP materials and use the observed cooling dynamics as probes for carrier-carrier and carrier-phonon interactions. We demonstrate that cooling in the all-inorganic CsPbBr3 is slower than its hybrid counterparts (e.g. FAPbBr3) in the high carrier density regime, owing to the relative abundance of optical phonon modes associated with the organic cation. The specific rates of cooling and their dependence on the hot and cold carrier densities are used to extract polaron sizes and coupling parameters for different perovskite materials. We scrutinise the thermal equilibration between cold and hot states in the single- and multiple-exciton-per-NC cases, and remark on the effect of surface ligands on the cooling dynamics.

10:00 - 10:30
2.1-O2
Ferro, Silvia
AMOLF
Harnessing Singlet Fission for Perovskite Photovoltaic Applications
Silvia Ferro
AMOLF, NL
Authors
Silvia Ferro a, Bruno Ehrler a
Affiliations
a, Institute AMOLF
Abstract

  

Current photovoltaic devices feature significant thermalisation losses reducing their efficiency. Within this context, singlet fission represents a powerful strategy to make a better use of the high-energy part of the solar spectrum, hence enhancing single junction solar cell performance. More in details, singlet fission is a carrier multiplication process in organic semiconductors where one photo-excited singlet exciton state is converted into two spin-triplet excitons, each carrying about half the energy of the originally excited singlet state. As two carriers are produced for each photon absorbed, photovoltaic devices based on singlet fission materials – typically, conjugated organic molecules - represent a great promise for a consistent increase of the efficiency of solar cells.
In parallel, metal-halide perovskites have recently attracted wide attention as the absorber material for solar cells.  
Our work is aimed at investigating electron transfer dynamics between singlet fission materials (e.g. tetracene, pentacene, rubrene) and suitable perovskites. For efficient singlet fission solar devices, the bandgap of the absorbing perovskite material needs to be well-matched with the triplet state energy of the conjugated organic molecule - around half the bandgap of the singlet fission material itself. For these reasons, low bandgap perovskites are employed in our investigation. We study photoluminescence and exciton dynamics to investigate the charge and energy transfer across the organic/perovskite interface. Computational methods based on first-principles charge and excited state dynamics  across the heterojunction are also employed  to provide insights for the development of high-performance hybrid solar devices.

 

 

 

 

  

NCFun 1.1
Chair: Ivan Infante
09:00 - 09:30
1.1-I1
Cossairt, Brandi
University of Washington
Understanding and Directing the Structure and Properties of Indium Phosphide Nanocrystals through Chemistry
Brandi Cossairt
University of Washington, US

Education and Professional Positions 2012-Present: Assistant Professor University of Washington Department of Chemistry 2010-2012: NIH NRSA Postdoctoral Fellow Columbia University 2010: PhD Inorganic Chemistry Massachusetts Institute of Technology 2006: BS Chemistry California Institute of Technology Awards 2015: Sloan Research Fellowship 2015: 3M Non-Tenured Faculty Award 2015: Seattle Association for Women in Science Award for Early Career Achievement 2014: University of Washington Innovation Award 2010: Ruth L. Kirschstein National Research Service Award Postdoctoral Fellowship, National Institutes of Health 2010: Alan Davison Ph.D. Thesis Prize, Massachusetts Institute of Technology 2009: Young Investigator Award, Division of Inorganic Chemistry, American Chemical Society

Authors
Brandi Cossairt a
Affiliations
a, University of Washington, Department of Chemistry, Seattle, WA 98195-1700
Abstract

Indium phosphide is the leading Cd-free quantum dot material for application in photoluminescence down-conversion display and lighting technologies. Our ability to control the structure and interfacial chemistry of InP nanocrystals underpins our ability to elicit desirable function from these materials. Our lab has shown that InP deviates in its properties from conventional II-VI materials in many respects, requiring innovation in synthesis and interfacial design. In particular, structurally deviant, kinetically persistent, and atomically-precise cluster intermediates have proven critical to unravelling the non-classical mechanisms by which InP may grow and have been instrumental in furthering our understanding of the very nature of quantum confined nanostructures. In this talk I will highlight our recent work on understanding and directing the structure and properties of these materials using chemistry, moving towards improved quantum efficiency, color purity, and stability. In addition, efforts at chemically directed mechanisms to achieve novel (hetero)structure types will be presented.

09:30 - 10:00
1.1-I2
Hens, Zeger
Ghent University
Properties of the Bright Exciton in InP Quantum Dots
Zeger Hens
Ghent University, BE

Prof. Z. Hens received his PhD in applied physics from Ghent University in 2000, worked as a postdoctoral fellow at Utrecht University and was appointed professor at the Ghent University department of inorganic and physical chemistry in 2002. His research concerns the synthesis, processing and characterization of colloidal nanocrystals.

Authors
Zeger Hens a
Affiliations
a, Department of Chemistry, Ghent University, Krijgslaan 281 - S3, 9000 Gent, Belgium
Abstract

In recent years, quantum dots made of III-V semiconductors have attracted increasing interest as a non-restricted alternative to the more prevailing CdSe-based QDs. In this presentation, we discuss the properties of the bright exciton in InP QDs using a combination of ensemble and single dot studies.

Using cryogenic photoluminescence microscopy, we first show that the emission features of single InP QDs exhibit a transition from single exciton to trion and to biexciton emission with increasing excitation power, not unlike CdSe-based QDs. Following this confirmation that emission involves exciton, we analyze the bright exciton emission in more detail using fluorescence line narrowing spectroscopy in magnetic fields and four-wave mixing spectroscopy. Most importantly, we find that the bright exciton line splits into 3 different sub-levels with increasing magnetic field strength, whereas the exciton dephasing rate increases strongly with increasing temperatures, even at temperatures of 4.2 K.

While a 3-fold degenerate bright exciton is expected for spherically symmetrical nanocrystals of zinc blende semiconductors, it was found that a slight shape anisotropy sufficed to fully split this isotropic exciton in the case of CdSe QDs. We argue that the particular light to heavy hole mass ratio in InP QDs may render the bright exciton in such QDs insensitive to shape anisotropy, which makes that the bright exciton in InP QDs is effectively isotropic. Not only does this interpretation account for the fluorescence line narrowing spectroscopy, it also explains why thermalization across different exciton levels still determines the bright exciton dephasing at cryogenic temperatures.

The observation of nearly isotropic excitons in InP QDs is of fundamental and practical interest, making such QDs ideal model system to study the size-dependence of exchange interaction or providing single photon emitters with orientation-independent properties.

10:00 - 10:30
1.1-I3
Jeong, Sohee
Sungkyunkwan University, Republic of Korea
III-V Colloidal Nanocrystals: Control over the Covalent Surfaces
Sohee Jeong
Sungkyunkwan University, Republic of Korea, KR
Authors
Sohee Jeong a
Affiliations
a, Sungkyunkwan University, Republic of Korea, 2066 Seobu-ro, Jangan-gu, Suwon, KR
Abstract

Wet chemistry synthesis and size/shape control of colloidal III-V (InP, InAs) semiconductor nanocrystals has been challenging, compared to well-established synthetic solution chemistry for II-VI (CdSe, CdS) and IV-VI (PbS, PbSe) semiconductor nanocrystals. The difficulty stems largely from the limited choices of precursors suitable for solution chemistry of the III-V materials. Approaches on introducing clusters as a single source precursors has been attempted to control the kinetics of crystallization. At more fundamental level, however, the difficulty lies in the inherent covalency of the III-V materials, which does not allow an easy solution chemistry to control their surfaces. Therefore, the key is how to control the surface chemistry of rather covalent III-V materials.

Based on our previous experience on the atomic control of surface chemistry for IV-VI rock-salt materials, a novel surface chemistry of tetrahedrally-coordinated III-V colloidal quantum dots will be presented. Further control over the specific facet based on understanding of surface energy allow morphology evolution from tetrapod to tetrahedral shape.

OPV 2.1
Chair: Thomas Kirchartz
09:00 - 09:15
2.1-O1
Eisner, Flurin
Department of Chemistry and Centre for Plastic Electronics, Imperial College London
Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells
Flurin Eisner
Department of Chemistry and Centre for Plastic Electronics, Imperial College London, GB
Authors
Flurin Eisner a, Mohammed Azzouzi a, Zhuping Fei b, Martin Heeney b, Jenny Nelson a
Affiliations
a, Department of Physics, Imperial College London, UK
b, Department of Chemistry and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London, GB
Abstract

The primary block to achieving higher efficiencies in bulk-heterojunction organic solar cells (BHJ-OSCs) is energy losses through non-radiative pathways due to the decay of a charge-transfer state (CTS) to the ground state via energy transfer to vibrational modes. It has previously been suggested that the open circuit voltage (Voc) in OSCs is largely determined by the energy of the donor-acceptor CT state[1], and thus a large number of recent studies have focussed on increasing the energy of CT states by minimizing the energy offset between the donor and acceptor. This relationship can be rationalized by understanding that a higher overlap of the vibrational modes of the CT and ground states increases the rate of non-radiative recombination. However, more recent studies have found that increasing the CT state energy does not always result in a reduction in the non-radiative voltage losses, which suggests that other properties of the CT state to ground state transition appear to affect the trend[2].

Here, we systematically investigate the effect that the CT state properties have on the voltage losses of BHJ-OSCs by using a series of increasingly fluorinated PBDB-T donors, in conjunction with a variety of different fullerene and non-fullerene acceptors[3]. Firstly, we show that by downshifting both the HOMO and LUMO through fluorination of the donor, the energy of the donor-acceptor CTS can be effectively moved closer to the first excited state. Secondly, by performing a detailed voltage loss analysis of the various blends we find that the non-radiative voltage losses are not reduced systematically for higher CT state energies; instead, for systems where the CT state and first excited state energies are very close in energy the non-radiative voltage losses are reduced significantly and can reach as low as 0.22V while maintaining a high PCE over 9%.  We suggest that this reduced non-radiative voltage loss is due to the hybridization of the CT state with the first excited state, as has been previously suggested in other systems[4]. Using a model to quantify the non-radiative voltage loss, we simulate the latter behaviour through an increase in both the CT state energy and the oscillator strength of the CT state to ground state transition. We propose that the increase of the oscillator strength may be due to the effect of hybridization that allows the CT state to borrow oscillator strength from the first excited state through the intensity borrowing mechanism. From the results of our model we show that in order to increase non-radiative voltage losses further, material combinations with strong electronic coupling between CT and first excited states as well as high excited state to ground state oscillator strengths should be used. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

09:15 - 09:30
2.1-O2
Köntges, Wolfgang
University of Heidelberg
Optimal Interfacial Composition and Crystallinity of Non-Fullerene Acceptor Blends for Organic Photovoltaics
Wolfgang Köntges
University of Heidelberg, DE
Authors
Wolfgang Köntges a, Pavlo Perkhun b, Rasmus R. Schröder a, c, Elena Barulina b, Olivier Margeat b, Christine Videlot-Ackermann b, Jörg Ackermann b, Martin Pfannmöller a
Affiliations
a, Centre for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
b, CINaM, CNRS, Aix Marseille University, Marseille, France
c, Cryo Electron Microscopy, BioQuant, Heidelberg University Hospital, Heidelberg, Germany
Abstract

The photovoltaic properties of an organic solar cell (OSC) crucially depend on the morphology of the active layer. An optimal bulk morphology for a high efficiency OSC can be achieved by using solvent additives combined with thermal annealing during device fabrication [1, 2]. Understanding the appearance of an optimal morphology and the mechanisms occurring during the fabrication process is therefore indispensable for further development of materials and OSCs. While the size of pure material domains should be within the range of the exciton diffusion length, the exciton dissociation decisively depends on the composition and extent of the donor-acceptor interface. The crystallinity of the different materials and the relative crystal orientation have a strong influence on the charge transport mechanisms. These three morphological parameters appear to have the most impact on the photovoltaic properties of an OSC.

We demonstrate the combination of conventional transmission electron microscopy (TEM) and analytical TEM (ATEM) allowing the simultaneous visualization of all three morphological parameters in non-fullerene acceptor (NFA) based blends. The interest in NFAs such as ITIC does not only arise from the increase in efficiency and stability of NFA based OSCs in the last years [3]. Furthermore, in some NFA blends highly efficient OSCs can be fabricated with negligible driving force for charge separation [4]. Our results reveal in detail the morphological parameters that contribute to an optimal PBDB-T:ITIC blend agreeing with atomistic simulations performed on this system [5, 6]. More significantly, our data implies that the presence of the polymer PBDB-T in this system forces the formation of particular ITIC crystals during thermal annealing. This process is even more enhanced by using solvent additives such as 1,8-diiodooctane (DIO) or chloronaphthalene (CN). These results explain the increase in efficiency for an PBDB-T:ITIC based OSC in relation to fabrication parameters.

An example of correlative conventional TEM and ATEM imaging is shown in Figure 1 for a 30 nm thin non-annealed PBDB-T:ITIC blend processed with DIO. For visualizing the material distribution of NFA based blends low-energy-loss ATEM and machine learning were applied (cf., details in [7, 8] for fullerene based blends). For PBDB-T and ITIC only minor spectral differences are found in the electron energy loss spectra (see Figure 1 a). However, appropriate data processing yields inverse material contrast due to minor spectral fingerprints (Figure 1 b and c). A material distribution map is computed after applying machine learning to a whole series of inelastic images. Additionally, crystalline areas visualized by conventional TEM on the same sample position can be correctly assigned to material phases. The spatial correlation of material composition with crystallinity yields a full view of the nanoscale morphology enabling correct interpretation of molecular arrangement over the whole sample area (Figure 1 d). An exact measurement of the lamellar spacing in both the acceptor (marked in red) and the donor (marked in blue) regions shows different crystal configurations typical for both materials (Figure 1 e and f).

We will further present the morphology of PBDB-T:ITIC layers processed with DIO and CN combined with thermal annealing. In comparison to the morphology shown in Figure 1 the annealed layers show different morphological parameters in agreement with atomistic simulations [6]. Our results imply the drastic influence of the molecular structure of both materials on the morphology formation during the fabrication process. This may help in designing new polymers and small molecules not only matching in their energetic properties, but also their molecular structure.

09:30 - 10:00
2.1-I1
Pfannmöller, Martin
Centre for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
Understanding the Photophysical Processes within Organic Photovoltaic Blends by Functional Imaging in an Analytical Electron Microscope
Martin Pfannmöller
Centre for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
Authors
Martin Pfannmöller a
Affiliations
a, Centre for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
Abstract

One of the most crucial parameters determining photophysical processes and performance of organic solar cells is their electronic structure. This is indirectly related to intrinsic materials properties, e.g. energy levels or crystallization behavior, of the donor and acceptor constituents. Importantly, the electronic structure is directly related to the type and distribution of phases within a donor:acceptor blend, which forms a cell’s photoactive layer. Parameters to consider here are large vs small phase domains, crystalline vs amorphous etc. These indirect and direct properties lead to a highly complex parameter space of a functional morphology. For instance, an acceptor material that tends to form crystallites in pure form might be “forced” into amorphous domains by the thermodynamic or kinetic behavior when combined with a specific donor material. Another aspect is that at domain interfaces or within an intermixed phase, local entropic effects or charge transfer can lead to subtle changes in the electronic structure [1].

Since the electronic processes within a specific morphology are strongly interdependent, we call the components that determine this morphology functional phases. Typically, the size of the functional phases is in the range of only a few to several ten nanometers, which creates great demands on a characterization technique. There are many valuable tools from spectroscopy and microscopy available that are used to decipher a subset of morphological parameters. Here, we focus on the capabilities of analytical transmission electron microscopy (ATEM). It is well known that TEM provides high resolution. However, we show that only by adding analytical, or spectroscopic, modes in a spatially resolved manner, the various functional phases can be identified [2]. The reason is that the electronic structure of a functional phase is reflected in a fingerprint-like signal within low-energy electron loss spectra (EEL spectra). Subtle signal differences are recovered by machine learning algorithms. An example of a computed phase distribution based on spatially resolved EEL spectra (spectrum image = stack of images at different energy losses) is given in Figure 1a for the prominent model system P3HT:PC60BM. Figure 1b shows a small part (i.e. one image at a specific energy loss) of the spectrum image with reliable contrast – based on material properties - between the phases. This type of functional imaging provides insights into the two-dimensional (2D) phase distribution. Information is based on projection data through the three-dimensional (3D) sample object. Therefore we extent the method by spectral tomography to map the electronic structures in 3D (see Figure 1c) [3]. It shows that phase assignments from 2D imaging are valid.

It will be discussed how ATEM is used to identify the functional phases in fullerene and non-fullerene acceptor blends. Non-fullerene acceptors in conjunction with polymers form a particularly challenging system for visualization since their electronic structures are exceedingly similar. However, successful mapping of the phases is highly rewarding since a detailed nanoscale visualization is lacking so far and the most recent record efficiency exceeds 16% in a single junction cell [4]. In addition, novel concepts of spectral imaging with analytical electron microscopy will be introduced. These methods promise enhanced energy resolution or assignment of functional signals to the underlying electronic blend structure based on secondary or back-scattered electron spectra.

10:00 - 10:30
2.1-O3
Palacios Gomez, David
Durham University
Impact of Morphology in Cascade Ternary Organic Photovoltaic Devices
David Palacios Gomez
Durham University, GB

 

My name is David A. Palacios G. and I am currently studying a PhD in engineering at Durham University in the Next Generation Materials and Microsystems Group, where I work in the organic photovoltaics field. I finished my undergrad studies in industrial engineering (2009) at the Universidad Del Valle de Mexico (UVM). Afterwards, I studied a specialty degree in sustainable development (2011) and a master in administration (2013) at the Universidad of Sonora (UNISON). In addition, prior to my PhD, I studied a master in energy management and renewable sources (2016) at the Monterrey Institute of Technology and Higher Education (ITESM). I have also taken supplementary courses in electronics, mechanics and solar energy as part of my professional development. Furthermore, since I concluded my undergrad studies, I worked 4 years as a Process Engineer for Rober Bosch GmbH in an electronics factory where I was responsible for implementing and improving manufacturing processes. Following my job in Bosch, I worked 4 years at FEMSA group in Mexico as a Project Engineer Coordinator where I was in charge of developing and implementing projects mainly in the operations area.
Presently, I am working with organic solar cells, particularly investigating fundamental processes in energy cascade ternary blends, regarding the impact between morphology, recombination mechanisms and performance. 

Authors
David Palacios-Gomez a, Ali Huerta-Flores a, Christopher Pearson a, Faisal Alanazi b, Budhika Mendis b, Christopher Groves a
Affiliations
a, Durham University, School of Engineering, South Road, Durham, 0, GB
b, Durham University Physics department
Abstract

 

In this contribution, we examine the use of cascaded energy heterostructure to control loss mechanisms in organic photovoltaic devices (OPVs). It has been proposed that introducing a third component between the donor and acceptor with carefully selected HOMO and LUMO can encourage the spatial separation of electron-hole pairs, and this reduce recombination [1]. Here, we specifically focus on the impact of ternary blend morphology on the effectiveness of the cascade on reducing recombination in a pair of OPV blend systems.

 

 We examine both PTB7:ICBA:PC71BM and P3HT:ICBA:PC71BM ternary blends, and compare these devices against PTB7:PC71BM and P3HT:PC71BM binary controls with and without 3% of 1,8-diiodooctane (DIO). The choice of these of materials sets give rise to a cascaded heterojunction structure, shown in Fig. 1. In the series of devices fabricated, the fraction of ICBA was varied from 0% (i.e. binary) to 30% to build a picture of how the ternary blend operates. We show that PTB7-based ternaries with 25% concentration of ICBA and DIO have a higher PCE due to an increase in external quantum efficiency (EQE) from 23% in the reference blend to 48% in the ternary blend at 600 nm respectively, notwithstanding the poor absorption of ICBA.  However, we only find this when DIO is added. The morphology of these blends were investigated by TEM and AFM. We find that, like PTB7:PC71BM binary OPVs, that blends with DIO had significantly reduced fullerene aggregation compared to those without (Fig. 2 a and b). In contrast to PTB7, the P3HT-based ternaries showed reduced EQE as ICBA was added. TEM imaging showed that P3HT-based ternaries did not show as significant a degree of fullerene aggregation as the PTB7-based devices without DIO. We attribute this to the difference in the molecular morphology of PTB7- and P3HT-based blends, where the former has been shown to exhibit crystallites of a few molecules [2], while the latter produces intercalated mixed phases [3] (Fig. 2 c, d). Therefore, we hypothesize that ternary blends can improve OPV efficiency, provided that the smallest length scale of phase separation is in the order of a few nm. Thus, we show in this paper two important findings; that this technique can work and ternary OPVs can outperform binary blends, but that in order to do this the morphology of the ternary blend must be optimized by careful control of morphology.

 

 

[1]        Groves, C. (2013). Energy & Environmental Science 6(5): 1546-1551.

 

[2]        Collins, B. (2013). Adv. Energy Mater. (3)1: 65-74.

 

[3]        Miller, N.C. (2012). Nano Lett. 12(3): 1566-1570.

PERFuDe 2.1
Chair: Constantinos Stoumpos
09:00 - 09:30
2.1-I2
Barnes, Piers
Imperial College London
The Physics of Perovskite Devices and Interfaces
Piers Barnes
Imperial College London, GB
Piers studied for his first degree in Physics at the University of Bristol, graduating in 1998. For his PhD (2002) he investigated the physics of polycrystalline ice at the British Antarctic Survey in Cambridge. After two Antarctic field seasons (2000/2001 and 2002/2003) drilling and measuring the world�s oldest ice core record recovered from Dome C he moved to the University of New South Wales in Australia. In 2004 he made a change in research direction to develop photoelectrodes for solar water splitting after winning a post-doctoral fellowship with the CSIRO. In 2006 he was awarded an Ig Nobel prize for �calculating the number of photographs that must be taken to (almost) ensure that nobody in a group photo will have their eyes closed�. Piers returned to the UK (2007) to work as a Research Associate at Imperial College London on interpreting loss mechanisms in dye sensitised solar cells where developed new approaches to simulating and characterising behaviour in this class of devices. In October 2011 he began an EPSRC fellowship at Imperial to measure model and exploit molecular wiring in hybrid optoelectronic devices.
Authors
Piers Barnes a
Affiliations
a, Imperial College London, South Kensington, London,, GB
Abstract

The presence of mobile ionic charge in metal halide perovskite semiconductors requires the introduction of additional charge carrier variables to both time-dependent and steady-state descriptions of their devices. We have developed Driftfusion1: flexible, open-source, drift-diffusion simulation software capable of describing the evolution of electron, hole and ion concentrations, and electrostatic potential, in one-dimension across any number of different semiconductor layers. The simulations allow the underlying factors controlling the behaviour of optoelectronic devices containing mobile ions such as perovskite solar cells to be understood and predicted2-4. Based on insights gained from these simulations we have developed an intuitive circuit model of perovskite device behaviour. The circuit model treats interfaces in the device by coupling the electrostatic potential of mobile ionic charge to the rate of charge transfer across interfaces using bipolar transistor elements to describe the process. The model allows impedance spectra of mixed conducting devices to be fit using physically meaningful parameters, and explains the large apparent capacitive and inductive behaviours often observed at low frequencies, as well as giving a more general description of the large perturbation behaviour of the devices.5

 

1            Calado, P., Gelmetti, I., Azzouzi, M., Hilton, B. & Barnes, P. R. F.     (https://github.com/barnesgroupICL/Driftfusion, 2018).

2            Calado, P. et al. Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis. Nat Commun 7, 13831 (2016).

3            Calado, P. et al. Identifying Dominant Recombination Mechanisms in Perovskite Solar Cells by Measuring the Transient Ideality Factor. Physical Review Applied 11, 044005 (2019).

4            Calado, P. & Barnes, P. R. F. Is it possible for a perovskite p-n homojunction to persist in the presence of mobile ionic charge? arXiv:1905.11892 (2019).

5            Moia, D. et al. Ionic-to-electronic current amplification in hybrid perovskite solar cells: ionically gated transistor-interface circuit model explains hysteresis and impedance of mixed conducting devices. Energy Environ. Sci. 12, 1296-1308 (2019).

09:30 - 09:45
2.1-O1
Unold, Thomas
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Photoluminescence Quantum Efficiency, Carrier Lifetime and Quasi-Fermi Level Splitting in Highly-efficient Perovskite Solar Cells
Thomas Unold
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Thomas Unold a, Martin Stolterfoht b, Christian Wolff b, Pietro Caprioglio b, Jose Marquez-Prieto a, Sergej Levcenco a, Dieter Neher b, Thomas Kirchartz c
Affiliations
a, Department Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Hahn-Meitner-Platz, 1, Berlin, DE
b, Universität Potsdam - Physik weicher Materie, Karl-Liebknecht Straße 24-25, Potsdam-Golm, 14476, DE
c, Forschungszentrum Juelich GmbH, 52425 Juelich
Abstract

Although hybrid perovskite solar cells currently feature record efficiencies well beyond 20%, the best open-circuit voltages (VOC) still fall short of what is expected from the radiative efficiency limit. The VOC is determined by carrier recombination processes in the solar cell including bulk, interface and/or contact recombination.[1] Under ideal conditions the open-circuit voltage approaches the internal quasi-Fermi level splitting (QFLS), which may be estimated by measuring the external photoluminescence quantum yield (PLQY) and the absorption properties [2]. On the other hand the PLQY and quasi-Fermi level splitting can also be estimated through time-resolved photoluminescence (TRPL), if the radiative recombination constant and absorption are taken into account [3]. An examination of the literature shows that the reported PLQY values and lifetimes for measured open-circuit voltages varies sometimes by orders of magnitude which is difficult to reconcile from a theoretical perspective. We present QFLS and Voc values derived from PLQY and TRPL for different halide perovskite materials and show that careful consideration of the above points leads to a consistent picture. Possible sources of error in the analysis will be discussed and guidelines how to cross-check data will be presented.

09:45 - 10:00
2.1-O2
Fassl, Paul
Karlsruhe Institute of Technology (KIT)
Modelling Self-Absorption Induced Red-Shift of the Photoluminescence of Perovskite Thin Films to Estimate the Internal Photoluminescence Quantum Efficiency and Escape Probability
Paul Fassl
Karlsruhe Institute of Technology (KIT), DE
Authors
Paul Fassl a, b, Vincent Lami c, d, Raphael Schmager a, b, David Becker-Koch c, d, Isabel Allegro a, b, Uli Lemmer a, b, Yana Vaynzof c, d, Bryce S. Richards a, b, Ulrich W. Paetzold a, b
Affiliations
a, Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
b, Light Technology Institute, Karlsruhe Institute of Technology, Engesserstrasse 13, 76131 Karlsruhe, Germany
c, Centre for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
d, Kirchhoff-Institute for Physics, Ruprecht-Karls-University Heidelberg, Im Neuenheimer Feld, 227, Heidelberg, DE
Abstract

The high refractive index of perovskite semiconductors at their emission wavelength (n ~ 2.2-2.6) results in a narrow emission escape cone and a large amount of photoluminescence (PL) to be trapped in perovskite thin films. Due to the strong band edge absorption and very small luminescent stokes shift, this leads to considerable photon reabsorption and reemission – an effect known as photon recycling (PR) [1] – and a concurrent red-shift of propagating PL [2]. As recently highlighted, the self-absorption induced redistribution of charge carriers and outcoupling of propagated PL (e.g. due to scattering) can lead to a significant misinterpretation of measurements of the intrinsic bimolecular recombination and charge carrier diffusion coefficients as well as of asymetric PL spectral shapes [3, 4]. For perovskite films with low non-radiative recombination losses, PR can be of great advantage as it allows higher charge carrier densities at steady-state and, thus, increased device open-circuit voltage [5]. However, most current models still start from the simplified assumption that all initially trapped PL will be reabsorbed at some point without the possibility of being outcoupled beforehand, likely overestimating the effect of PR [6].

In this study, we show that in external photoluminescence quantum efficiency (PLQE) measurements employing a typical integrating sphere setup, the PL spectra of polycrystalline perovskite films with layer thicknesses typically employed for thin film solar cells (~150–400 nm), exhibit a notable red-shift and broadening. Such asymetric spectra are often ignored or misinterpreted in the literature, and to our knowledge there is only one recent study that attempts to model them for thin films under the assumption of self-absorption alone [4]. We developed an all-optical model that considers both PR and the outcoupling of initially trapped propagated PL at the crystal surface and grain boundaries due to scattering, which can reproduce several spectral shapes with varying extents of red-shifts. We confirm our assumptions by various control experiments and benchmark the model for perovskite thin films with different grain size, thickness and composition. Our model allows to estimate the escape probability as well as the initially generated internal PL intensity and thus to calculate the internal PLQE. Our approach serves as a new guideline for estimating these important parameters directly from measurements of the external PLQE and a PL spectrum using an integrating sphere setup.

10:00 - 10:30
2.1-I1
Catchpole, Kylie
Australian National University
Understanding Interfaces and Transport Layers in Perovskite Solar Cells
Kylie Catchpole
Australian National University, AU

Kylie Catchpole is Professor in the Research School of Engineering at the Australian National University.  She has over 100 scientific publications, with a focus on using new materials and nanotechnology to improve solar cells.  She completed her PhD at ANU and was a postdoctoral fellow at the University of New South Wales and the FOM Institute for Atomic and Molecular Physics in Amsterdam before returning to ANU in 2008.  In 2013 she was awarded a Future Fellowship from the Australian Research Council and in 2015 she was awarded the John Booker Medal for Engineering Science from the Australian Academy of Science. 

Authors
Kylie Catchpole a
Affiliations
a, The Australian National University, Canberra ACT 0200, Australia, AU
Abstract

Interfaces and transport layers are crucial to high performance perovskite solar cells.  However, so far the mechanisms behind the performance of particular materials and material combinations have not been well understood.  In this talk we discuss the effect of transport, recombination and capacitance in interpreting the behaviour of several transport layers.  In particular, we focus on TiO2 and MoOx. 

For the TiO2/p+Si interface, we discuss the role of interface states and band alignment in achieving an Ohmic contact.  This behaviour has previously been viewed as hopping within the TiO2.  We have also see evidence in favour of band alignment shifts and against hopping conduction within thin layers of MoOx, demonstrating the need to perform several types of measurements supported by modelling to assign a mechanism. 

Band offsets at interfaces can have several other effects on cell performance. We have seen that band alignment shifts together with improved conduction are able to improve performance of indium doped TiO2.  We have also shown that band offsets can also affect hysteresis behaviour of perovskite solar cells, including leading to inverted hysteresis. 

Finally, it is also important to consider the transport layers in assessing capacitance.  For example,  TiO2 can become depleted during capacitance measurements, which is important to take account of when interpreting the results.

For all of these types of measurements, experimental evidence is often indirect, and there for a combination of several types of measurements together with modelling of the mechanisms is required for a full understanding of the behaviour.

SolCat 2.1
Chair: Matthew Mayer
09:00 - 09:30
2.1-I1
Reisner, Erwin
University of Cambridge - UK
Solar-driven Utilization of CO2 with Molecularly-Engineered Semiconductor Hybrid Systems
Erwin Reisner
University of Cambridge - UK, GB

Erwin Reisner received his education and professional training at the University of Vienna (with Prof Bernhard K. Keppler), the Massachusetts Institute of Technology (with Prof Stephen J. Lippard) and the University of Oxford (with Prof Fraser A. Armstrong) before starting his independent career as a University Lecturer at Cambridge and Fellow of St. John’s College in 2010. He holds an EPSRC Career Acceleration Fellowship and heads the Christian Doppler Laboratory for Sustainable SynGas Chemistry. His group develops artificial photosynthesis by combining chemical biology, synthetic chemistry and materials chemistry.

Authors
Erwin Reisner a
Affiliations
a, University of Cambridge - UK, The Old Schools, Trinity Ln, Cambridge CB2 1TN, UK, Cambridge, GB
Abstract

The synthesis of solar fuels and chemicals through artificial photosynthesis allows the direct pairing of light absorption to drive chemical redox processes. This approach is a one-step and versatile alternative to the more indirect coupling of a photovoltaic cell with electrolysis and enables potentially the synthesis of a wide range of fuels and feedstock chemicals. A common drawback in most artificial photosynthetic systems and organic photocatalysis is their reliance on expensive materials and device architectures, which challenges the development of ultimately scalable devices. Another limitation in many approaches is their inefficiency and reliance on sacrificial redox reagents, which may be system damaging and often prevent truly energy-storing chemistry to proceed. This presentation will give an overview about our recent progress in developing molecular-semiconductor hybrid systems for CO2 reduction with a focus on synthetic 3d transition metal complexes and biological catalysts. We will explore strategies for catalyst immobilization on semiconductor particles and photoelectrodes and their coupling to water oxidation for the assembly of proof-of-concept CO2 conversion devices.

Representative recent references on metal complex-semiconductor hybrid systems:

   Dalle, Warnan et al., Chem. Rev., 2019, 119, 2752–2875.

   Leung, Vigil, Warnan et al., Angew. Chem. Int. Ed., 2019, 58, 7697–7701.

   Leung, Warnan et al., Nature Catal., 2019, 2, 354–365.

    

Representative recent references on enzyme-semiconductor hybrid systems:

   Kornienko et al., Nature Nanotechn., 2018, 13, 890–899.

   Miller et al., Angew. Chem. Int. Ed., 2019, 58, 4601–4605.

   Sokol, Robinson et al., J. Am. Chem. Soc., 2018, 140, 16418–16422.

09:30 - 09:45
2.1-O1
Shankar, Ravi
Imperial College London
Porous Boron Oxynitride for Combined CO2 Capture and Photoreduction
Ravi Shankar
Imperial College London, GB
Authors
Ravi Shankar a, Michael Sachs b, Laia Francàs b, Daphné Lubert-Perquel c, Gwilherm Kerherve d, Anna Regoutz d, Camille Petit a
Affiliations
a, Barrer Centre, Department of Chemical Engineering, Imperial College London United Kingdom, South Kensington Campus, Exhibition Road, London, GB
b, Department of Chemistry, Imperial College London, South Kensington Campus London, London, GB
c, London Centre for Nanotechnology and Department of Materials, Imperial College London, United Kingdom, South Kensington Campus, Prince’s Consort Road, London, GB
d, Department of Materials, Imperial College London, United Kingdom, Prince’s Consort Road, South Kensington Campus, London, GB
Abstract

Designing robust, metal-free photocatalysts that can efficiently facilitate the conversion of sunlight into chemical energy remains an ongoing challenge in the field of nanomaterials. Porous, amorphous materials are typically not employed for photocatalytic purposes as the abundance of defects can lead to low charge mobility and favour bulk electron-hole recombination. However, a disordered nature in the material can lead to porosity, which in turn promotes both interfacial catalyst-reactant interactions and fast charge transfer to the reactants.

Here, we demonstrate that moving from hexagonal boron nitride (h-BN), a well-known crystalline insulator, to porous boron oxynitride (BNO), we create a semiconductor, which is able to photoreduce CO2 in a gas/solid phase, under both UV-vis and pure visible light, in ambient conditions, without the need for cocatalysts. The materials were synthesized using a bottom-up approach and characterized using a range of analytical techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, FT-IR spectroscopy, N2 sorption, UV-Vis diffuse reflectance spectroscopy, electron paramagnetic resonance analysis, and valence band X-ray photoelectron spectroscopy. The materials were then tested for CO2 reduction and their performances were mapped against the chemical, structural and optical properties of the material.

The material is able to selectively evolve CO and maintains its photocatalytic stability over several catalytic cycles. The performance of this material, which is yet to be optimized, is on par with that of TiO2, the benchmark in the field. Through this study, we provide insight into the role of chemical and structural features of porous BN on CO2 photoreduction. Owing to the chemical and structural tunability of porous BN, these findings highlight the potential of porous BN-based structures for heterogeneous photocatalysis and solar fuels synthesis. These findings could have key implications in designing and tailoring a new class of robust, metal-free photocatalysts to facilitate challenging photochemical reactions.

09:45 - 10:00
2.1-O2
May, Matthias
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Beyond Solar Fuels: Photoelectrochemical Approaches to Negative Emissions
Matthias May
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE

Matthias May studied physics in Stuttgart, Grenoble, and Berlin, with a focus on condensed matter and computational physics. In his diploma thesis (2010), he investigated charge-density wave phase transitions using photoelectron spectroscopy. For his PhD studies at Humboldt-Universität zu Berlin and Helmholtz-Zentrum Berlin on III-V semiconductors for solar water splitting, he won a scholarship of Studienstiftung des deutschen Volkes. He received his PhD end of 2014 and worked in his first postdoctoral position on high-efficiency water splitting. From 2016 to 2018, he was postdoctoral fellow at the Chemistry Department of the University of Cambridge, funded by the German Academy of Sciences Leopoldina, modelling optical properties of solid-liquid interfaces. His main scientific interests lie in the area of highly correlated electron systems and semiconductor-interfaces, both from an experimental and modelling perspective.

Authors
Matthias May a, Kira Rehfeld b
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Germany, Berlin, DE
b, Ruprecht-Karls-Universität Heidelberg, Institute of Environmental Physics, 69120 Heidelberg, Germany
Abstract

Negative emissions are an integral part of most scenarios for limiting global warming to 2 °C or below. However, the feasibility of the various technological approaches to remove dilute CO2 from the atmosphere for permanent storage is still unclear.[1,2] While natural photosynthesis is scalable and features established products for long-term storage, its low conversion efficiency translates to vast footprints in terms of the required area.[1,3] Here, photoelectrochemical approaches could reduce the conflict with food production as they provide significantly higher efficiencies, require less water, and do not depend on arable land.[3] We give an overview of the approaches discussed in the literature, from negative-emissions solar hydrogen production[2] to photoelectrochemical CO2 reduction. Furthermore, we introduce a definition of solar-to-carbon efficiency, which is better suited to rank approaches for negative emissions. We show that the most efficient products in terms of CO2 removal efficiency deviate from those considered for solar fuels, which therefore require new absorber-catalyst combinations. Finally, we discuss feasibility, land footprint, and costs of these photoelectrochemical carbon sinks for the -10 Gt/year target in 2050.

10:00 - 10:30
2.1-O3
Mas Marzá, Elena
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
Photoelectrosynthesis of Imines
Elena Mas Marzá
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES
Authors
Elena Mas-Marzá a, Ramón Arcas-Martínez a, Laxman Gouda a, Francisco Fabregat-Santiago a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

Photoelectrocatalysis is an attractive alternative to conventional electrochemical procedures mainly because this procedure allows decreasing the applied potential due to energy assistance from sunlight. The vast majority of the applications in photolectrocatalysis have mainly focused on the study of the oxidation of H2O and the reduction of CO2,[1-3] leaving aside its exploitation for the synthesis of more complex organic molecules with high added value. Considering the high potentials, both reducing and oxidizing, that can be obtained from photoelectrochemical processes, these are suitable candidates for designing synthetic routes that allow the preparation of organic products that involve redox transformations in the starting reagents. In this scenario, there are very few examples described in the literature to date where photoelectrodes are used to obtain added value organic species. Recently, it has been described the use of BiVO4 and WO3 photoelectrodes for the oxidation of 5-hydroxymethylfurfural, [4] benzyl alcohols,[5-6] furan, [7] tetralines[5], cyclohexane,[8] and glycerol;[9] and Fe2O3 photoanodes for the photoelectrocatalytic amination of arenes.[10] In all these cases, the desired product is generated in one of the (photo)electrodes, whereas in the other one the oxidation (or reduction) of a sacrificial agent takes place.

We propose to make a step further in this field, so that both processes, oxidation and reduction, in the photoelectrocatalytic route lead to the synthesis of chemicals of added value without the need of using agents of sacrifice or charge donors. Thus, all the chemicals used are involved in the formation of the final product. In this work we present a route to produce imines, a product of interest as building block in chemical industry, from the photoelectrocatalytic oxidation of alcohols to aldehydes combined with the reduction of nitroarenes into amines. Yields higher than 60 % have been found for the conversion of alcohol into the corresponding aldehyde, after 13 hours reaction, using BiVO4 as photoelectrode. The contribution of the different process, i.e. alcohol UV oxidation in the presecen of O2, photocatalytic and photoelectrocatalytic oxidation, to the overall photoelectrochemical reaction are evaluated. The production of side products of interest as H2O2 has also been identified.

10:30 - 11:00
Coffee Break
Exciup 2.2
Chair: Ferdinand Grozema
11:00 - 11:30
2.2-O1
Lissau, Jonas Sandby
University of Southern Denmark
Routes towards Improved Solar Energy Conversion in Organic and Hybrid Solar Cells via Photon Upconversion
Jonas Sandby Lissau
University of Southern Denmark
Authors
Jonas Sandby Lissau a, Malika Khelfallah a, Morten Madsen a
Affiliations
a, SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, Sønderborg, DK-6400, Denmark
Abstract

Upconversion of low-energy photons transmitted by traditional single-threshold solar cells is a promising approach to overcome their theoretical efficiency limit. Due to their relatively high-energy absorption threshold dye-sensitized and organic solar cells have a particular high loss of low-energy photons and consequently a high potential for improvement by photon upconversion.

We have investigated photon upconversion via triplet fusion on dye-sensitized nanostructured metal oxides [1], which has been applied in the first examples of intermediate band dye-sensitised solar cells [2]. More recently we applied photon upconversion via triplet fusion in organic molecules tailored to improve the efficiency of organic solar cells. This approach can be synthetically tuned to match the spectral requirements of the solar cell technology. In addition, molecular photon upconversion benefits from spin-allowed broadband absorbing transitions, which facilitates reasonable upconversion efficiency under solar flux [1].

Specifically, we investigate in this work solid-state systems based on palladium(II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine as a triplet sensitizer and rubrene as a triplet-triplet annihilator. This upconverter was first demonstrated by Singh-Rachford and Castellano in solution and solid polymer films [3]. After light absorption (735 nm) and intersystem crossing in the triplet sensitizer, the resulting triplet state is transferred to rubrene. Following triplet energy migration, two rubrene triplet states can annihilate (triplet fusion) to produce a high-energy singlet state, which decays by emission of an (upconverted) high-energy photon (573 nm). New results focusing on the integration of this system in organic solar cell devices will be discussed.

To further boost the photon upconversion efficiency, metal nanostructures tailored for plasmon resonance at the absorption band of the upconverter are integrated, using new guidelines for nano-particle design optimization demonstrated in lanthanide based photon upconverters (Madsen, S. P. et al., J. Phys. D: Appl. Phys. 2019, submitted). Combined, this work thus addresses new routes for integration of molecular upconverter systems in organic solar cell devices.

11:30 - 12:00
2.2-I1
Schmidt, Timothy
UNSW Australia
Photochemical upconversion and photovoltaics
Timothy Schmidt
UNSW Australia, AU
Authors
Timothy Schmidt a
Affiliations
a, ARC Centre of Excellence in Exciton Science, School of Chemistry, UNSW Sydney, Australia
Abstract

Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. While systems have been developed for upconverting light from the biological tissue window near 800 nm, they remain susceptible to quenching by oxygen. Here we demonstrate an upconversion composition using semiconductor nanocrystal sensitizers that employs molecular triplet states below the singlet oxygen energy.[1] We show that, contrary to the usual expectation, the admission of oxygen enhances the intensity of upconverted light and significantly speeds up the photochemical processes involved. Further, we demonstrate photochemical upconversion from below the silicon band gap in the presence of oxygen.
These results establish a new strategy for circumventing the problem of oxygen in photochemical upconversion and lay the foundation for an expansion of this process into new applications. In particular, we report progress towards a bifacial silicon solar cell which utilizes photochemical upconversion.

NCFun 1.2
Chair: Zeger Hens
11:00 - 11:30
1.2-I1
Neale, Nathan
National Renewable Energy Laboratory
Surface Chemistry Effects on Quantum Confinement in Group IV Nanocrystals
Nathan Neale
National Renewable Energy Laboratory, US

Nate Neale received his B.A. degree in chemistry from Middlebury College in 1998, where he studied radical substitution reactions at activated arenes and the binding mode of cisplatin, a common commercial anti-cancer drug, to a model DNA fragment. His scientific training continued as a graduate student under Prof. T. Don Tilley at the University of California, Berkeley, investigating the mechanism by which a transition-metal catalyst facilitates the polymerization of stannanes to polystannanes, a class of inorganic polymers with unique optical and electronic properties. As a postdoctoral researcher at NREL, he worked on controlling the synthesis and surface chemistry of TiO2 nanostructures for dye-sensitized solar cells in the laboratories of Dr. Arthur J. Frank. After a brief stint at the University of Colorado, Boulder, during which time he worked in collaboration with Dr. Frank, Dr. Arthur J. Nozik, and Prof. David Jonas on photoelectrodes for photoelectrochemical water splitting, he returned to NREL as a staff scientist in 2008. His current research interests are focused on tailoring the chemical structure and photophysics of nanostructured inorganic semiconductors and catalysts for photovoltaics, solar fuels, batteries, and related energy conversion and storage concepts.

Authors
Nathan Neale a, Michael Carroll a, Rens Limpens a, Lance Wheeler a, Gregory Pach a
Affiliations
a, Chemical and Nanoscale Sciences Center, National Renewable Energy Laboratory
Abstract

We have been exploring the surface functionalization of group IV (Si, Ge) and III–V (InxGa1-xP, etc.) nanocrystals (NCs) to understand how surface chemistry influences the fundamental processes (charge generation, separation, and recombination) as well as inter-NC charge transfer. These studies are all enabled by nonthermal plasma synthesis that provides clean, highly reactive NC surfaces. Subsequent surface chemistry manipulation yields NCs largely free from competing surface defect states that are thus amenable to detailed spectroscopic studies.

For example, spectroscopic interrogation of plasma-synthesized Si NCs have provided insight into their electron-phonon interactions, quasi-direct optical transitions, and exciton formation dynamics. Doped Si NCs are easily accessible using this technique, which has allowed us to probe the degree of interaction between free carriers and photogenerated electron-hole pairs.[1,2] In addition, we have demonstrated cationic ligand exchange reactions on plasma-synthesized Ge NCs that enables effective electronic coupling and thus inter-particle charge transport in Ge NC films.[3] Finally, recent work has shown that we can use the plasma method to control the morphology of InxGa1-xP NCs from hollow to solid, demonstrating the viability of this method in accessing difficult-to-synthesize semiconductor nanostructures.[4]

In this presentation, we will leverage our deep understanding of the surface chemistry to reveal a new way to modulate the emission properties in Si and Ge NCs as detailed in our published[5] and ongoing work. The optical properties of Si and Ge NCs are a subject of intense study and continued debate. The photoluminescence (PL) in particular is known to depend strongly on the surface chemistry, with electron-hole recombination pathways derived from the semiconductor band-edge, surface-state defects, or combined NC-conjugated ligand hybrid states. We will describe the effect of different saturated surface functional groups—alkyls, amides, alkoxides, and alkylthiolates—on the emission properties in nonthermal plasma-synthesized Si and Ge NCs. For example, we find a systematic and size-dependent high-energy (blue) shift in the PL spectrum of Si NCs with amide and alkoxy functionalization relative to alkyl. Converse, alkylthiolate ligands result in a low-energy (red) shift in Si NCs. These results suggest that the atom bound to the Si NC surface strongly interacts with the Si NC electronic wave function and modulates the Si NC quantum confinement, revealing a potentially broadly applicable correlation between the emission energy in quantum-confined structures and the ligand binding group.

11:30 - 12:00
1.2-I2
Kulik, Heather
Massachusetts Institute of Technology (MIT)
Electronic Structure Origins of Surface-Dependent Growth in III–V Quantum Dots
Heather Kulik
Massachusetts Institute of Technology (MIT), US
Authors
Heather Kulik a
Affiliations
a, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, US
Abstract

Indium phosphide quantum dots (QDs) have emerged as a promising candidate to replace more toxic II–VI CdSe QDs, but production of high-quality III–V InP QDs with targeted properties requires a better understanding of their growth. I will describe our work in developing a first-principles-derived model that unifies InP QD formation from isolated precursor and early stage cluster reactions to 1.3 nm magic sized clusters and rationalize experimentally observed properties of full sized >3 nm QDs. Our first-principles study on realistic QD models reveals large surface-dependent reactivity for all elementary growth process steps, including In-ligand bond cleavage and P precursor addition. I will describe the correlation of thermodynamic trends to kinetic properties at all stages of growth. This analysis suggests the presence of labile and stable spots on cluster and QD surfaces. Correlation of electronic or geometric properties to energetics identifies surprising sources for these variations: short In···In separation on the surface produces the most reactive sites, at odds with conventional understanding of strain (i.e., separation) in bulk metallic surfaces increasing reactivity and models for ionic II–VI QD growth. We rationalize these differences by the covalent, directional nature of bonding in III–V QDs and explained by descriptors derived directly from the In–O bond density. The unique constraints of carboxylate and P precursor bonding to In atoms rationalize why all sizes of InP clusters and QDs are In-rich but become less so as QDs mature. Time permitting, I will discuss how these observations could be used to suggest alternate growth recipes that take into account strong surface-dependence of kinetics as well as the shapes of both In and P precursors for better control of kinetics and surface morphology in III–V QDs.

12:00 - 12:30
1.2-O1
Schimpf, Alina
University of California San Diego
Synthesis of Monodisperse and Size-Tunable Colloidal Copper Phosphide Nanocrystals by Redox Disproportionation of Aminophosphine
Alina Schimpf
University of California San Diego, US
Authors
Alina Schimpf a, Alexander Rachkov a
Affiliations
a, University of California San Diego, Gilman Drive, 9500, San Diego, US
Abstract

Copper chalcogenide nanomaterials have attracted significant research interest in due to their remarkable compositional and structural versatility, enabling them to support large densities of delocalized charge-carriers. Synthetic advancements in colloidal preparations of copper chalcogenides have allowed tunability of material properties such as copper vacancies, crystallographic phase, monodispersity, size, morphology, and hierarchical assembly. Deliberate targeting of structure and composition in copper chalcogenide nanomaterials is essential because it imparts control over their resulting optoelectronic and plasmonic properties that have use in a range of applications from sensing to therapeutics. Nanoscale copper phosphide remains relatively unexplored compared to its  chalcogenide analogs due to a low degree of synthetic sophistication afforded by typical colloidal metal phosphide precursors, such as tri-n-octylphosphine and tris(trimethylsilyl)phosphine. Recent studies of colloidal indium phosphide nanocrystals have utilized an alternative aminophosphine precursor to direct syntheses that allow for a better mechanistic understanding. This design paradigm is extended in this work to Cu3−xP nanocrystals with the purpose of accessing tunability of their material and optical properties. A new high-quality, one-pot, heat-up strategy for making colloidally stable, plasmonic Cu3−xP nanoplatelets is presented. Size-tunability of the Cu3−xP nanoplatelets with maintenance of high monodispersity is achieved by careful tuning of heating profile and reagent composition. The use of molecular redox agents to modulate the localized surface plasmon resonance of nanoscale Cu3−xP is also explored.

OPV 2.2
Chair: Morten Madsen
11:00 - 11:30
2.2-I1
Yao, Huifeng
Institute of Chemistry, Chinese Academy of Sciences (ICCAS)
Optimization of Active Layers in Highly Efficient Organic Solar Cells
Huifeng Yao
Institute of Chemistry, Chinese Academy of Sciences (ICCAS)
Authors
Huifeng Yao a
Affiliations
a, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Abstract

Abstract

Recent progress of organic solar cells (OPVs) is dominated by the development of non-fullerene acceptors (NFAs). The power conversion efficiencies (PCE) of single-junction OSCs have been boosted to over 16%, showing its great potential in practical applications.[1, 2] In our works, we focus on optimizing the active layers of OPV cells to get highly efficient devices. In this talk, I will present some examples relating to how we design NFAs. Furthermore, I will introduce our method in optimizing the blend morphology of the active layer. In detail, I will talk about our recent work about the chlorinated NFAs.[3] Based on this acceptor, we achieved high PCEs of over 16%. Detailed studies imply that the reduced non-radiative loss contributes greatly to the improvement of the device performance. In addition, I will also discuss our morphology optimization method using volatilizable solid additives.[4]

References

J. Yuan, Y. Zhang, L. Zhou, G. Zhang, H.-L. Yip, T.-K. Lau, X. Lu, C. Zhu, H. Peng, P. A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y. Li, Y. Zou, Joule 2019, 3, 1140.

B. Fan, D. Zhang, M. Li, W. Zhong, Z. Zeng, L. Ying, F. Huang, Y. Cao, Sci. China Chem. 2019.

Y. Cui, H. Yao, J. Zhang, T. Zhang, Y. Wang, L. Hong, K. Xian, B. Xu, S. Zhang, J. Peng, Z. Wei, F. Gao, J. Hou, Nat. Commun. 2019, 10, 2515.

R. Yu, H. Yao, L. Hong, Y. Qin, J. Zhu, Y. Cui, S. Li, J. Hou, Nat. Commun. 2018, 9, 4645.

11:30 - 12:00
2.2-O1
Perkhun, Pavlo
Aix-Marseille University, Centre Interdisciplinaire de Nanosciences de Marseille CINaM, UMR CNRS 7325, Marseille, France
Reducing Performance Losses in High Efficiency Digital Printed Polymer Solar Cells Using Non-fullerene Acceptors
Pavlo Perkhun
Aix-Marseille University, Centre Interdisciplinaire de Nanosciences de Marseille CINaM, UMR CNRS 7325, Marseille, France, FR
Authors
Pavlo Perkhun a, Elena Barulina a, Sadok Ben Dkhil b, Pascal Pierron b, Wolfgang Köntges c, Martin Pfannmöller c, Antonio Guerrero d, Christine Videlot-Ackermann a, Olivier Margeat a, Jean-Jacques Simon e, Jörg Ackermann a
Affiliations
a, Aix Marseille Univ, CNRS UMR 7325, CINaM, Marseille, France.
b, Dracula Technologies, 4 rue Georges Auric, 26000 Valence, France
c, Centre for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
d, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
e, Aix Marseille Univ, CNRS UMR 7334, IM2NP, Marseille, France
Abstract

During the last two years, the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) could be increased beyond 14% for single and 17.3% for tandem heterojunctions due to the discovery of novel non-fullerene acceptor (NFA) [1, 2]. In the field of NFAs, we have recently studied ternary blend approaches to increase open circuit voltage [3] as well the determination of charge transfer state energies from luminescence spectra in solar cells [4]. As now the performance of NFA based PSCs makes it highly promising as photovoltaic technology for a large number of niche markets, it is important to study aspects such as processing of PSCs with industrial relevant materials and printing techniques. Ink-jet or digital printing is one of the promising techniques. Indeed it allows to generate PSCs controlled in shape and size for niche markets demanding personalized engineering of solar cells. Recently ink jet printing was used to print PSC with high efficiencies [5] still ~5 % and tunability in shape [6]. Improvements in the efficiency can be expected by the usage of novel NFAs family of the ITIC based on indacenodithieno-[3, 2-b]-thiophene, (IDT), usually end-capped with 2-(3-oxo-2, 3-dihydroinden-1-ylidene)-malononitrile [7] in combination with more efficient printable interfacial layers and electrodes. We recently demonstrated that specific formulations of PEDOT:PSS allow to print highly efficient PSC using fullerene based acceptor with efficiency over 6% revealing the high potential of PEDOT:PSS for ink jet printing.

In order to go beyond this efficiency and to benefit from the unique properties of the new generation of non fullerene acceptors, we studied systems consisting of ITIC and ITIC-4F acceptors and PBDB-T and PBDB-T-SF donors towards stable fully solution processed high efficiency OSCs. First we optimized interfacial layers and ink formulation in O-xylene as a “green” solvent for fully solution processing of PCSs based on NFAs resulting in the PCEs >12% for the PBDB-T-SF/ITIC-4F system. We show that replacement of MoOx as HTL by PEDOT:PSS in an inverted structures introduce only performance losses from 9.4% for PBDB-T:ITIC to 7.7% making fully printed high efficiency PSCs possible. Finally, we present cells with printed active layers based on ITIC-4F processed in air with PCEs >10% and evaluate their stability under storage conditions and under illumination compared to devices that were processed by spin coating under Argon. In order to meet industrial requirements we printed the cells with thick layers of photoactive material (~300 nm) and compare their performance to the thin one (~100 nm). The nanoscale morphology and quality of the ink-jet printed active layers are characterized by analytical TEM and AFM techniques. The impact of printed layers on charge transport and recombination in the solar cells is characterized by impedance spectroscopy and is compared with the cells made by spin-coating.

12:00 - 12:30
2.2-O2
Nieto Diaz, Balder Adad
Durham University
Organic Photovoltaic Blends Diluted with Inert Polymers for Enhanced Lifetime: Impact of Blend Microstructure and Processing Additives
Balder Adad Nieto Diaz
Durham University, GB
Authors
Balder Adad Nieto Diaz a, Christopher Pearson a, Christopher Groves a
Affiliations
a, Durham University, School of Engineering, South Road, Durham, 0, GB
Abstract

Organic photovoltaics (OPVs) are a promising renewable energy technology due to their scalable manufacture and recent substantial gains in power conversion efficiency to 15.6% [1].  Commercialisation of OPVs will require similar step-changes in the lifetime to be made.  OPV layers are susceptible to physical or chemical degradation with different processes occurring at different aging times [2], which in turn are accelerated by water [3] and oxygen [4].  Recent work by Al-Busaidi et al [5] has shown the addition of an inert polymer, poly (methyl methacrylate) (PMMA) to a donor-acceptor blend, yielding Ternary OPVs, increases lifetime due to the PMMA scavenging water in the active later.  Here we show the impact of the Ternary blend morphology on both the lifetime and performance for two donor-acceptor systems: well-studied P3HT:PC61BM and higher-performance PTB7:PC71BM.

 

Ternary P3HT:PC61BM:PMMA blends were shown to have better initial performance, and lifetimes more than double than P3HT:PC61BM controls, depending upon the molecular weight of the PMMA. This in turn is shown to be due to a significant reduction in the rate at which Jsc degrades.  AFM studies showed that islands comprising PMMA form and increase in size with increasing weight percent (wt%) and molecular weight (MW) of PMMA as shown in Figure 1, thus we can relate the distribution of PMMA to its lifetime-extending functionality.  

 

Processing additives such as DIO are a common tool to further control morphology, however, it was found that using DIO in P3HT:PC61BM:PMMA and PTB7:PC71BM:PMMA OPVs blends led to worse initial performance and lifetime than the relevant binary control. This is important since PTB7:PC71BM OPVs are commonly processed with DIO to limit fullerene aggregation. The use of processing additives has been shown to have an impact on the chemical stability of PTB7 [6] and by adding PMMA this degradation appeared to be hastened. However, we show that processing the PTB7:PC71BM:PMMA blends without DIO recovers the beneficial effects upon lifetime shown in the P3HT based devices.  Therefore, we have demonstrated that dilution of a donor-acceptor OPV blend with PMMA can increase lifetime, but that the normal range of morphology optimising tools may be limited.

 

[1]  Yuan et al., Joule 3, 1–12, 2019 [2]  W. Ma, et al., Adv. Funct. Mater., vol. 15, no. 10, pp. 1617–1622, 2005. [3]  C.H. Peters, et al., Adv. Energy Mater., 1491–494, 2011. [4]  A. Tournebize, et al., Chem. Mater. 25, 4522–4528, 2013. [5]  Al-Busaidi, Z., et al., Sol. Ener. Mater. and Sol. Cells 160: 101-106, 2017. [6]  Kettle, J., et al., Organic Electronics 39, 222-228, 2016.

PERFuDe 2.2
Chair: Piers Barnes
11:00 - 11:30
2.2-I1
Knapp, Evelyne
ZHAW
Consistent Device Model of a Perovskite Solar Cell for Multiple Experiments
Evelyne Knapp
ZHAW, CH

Dr. Evelyne Knapp is a research associate at the Institute of Computational Physics at the Zurich University of Applied Sciences in Winterthur, Switzerland. She holds a Diploma and Ph.D. degree in Computational Science and Engineering from ETH Zurich.

Authors
Evelyne Knapp a, Andreas Schiller a, b, Martin T. Neukom a, b, Simon Züfle a, b, Beat Ruhstaller a, b
Affiliations
a, Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), 8401 Winterthur (Switzerland)
b, Fluxim AG, Katharina-Sulzer-Platz, 2, Winterthur, CH
Abstract

Drift-diffusion models enhanced with ionic transport have successfully been used to describe hysteresis in current-voltage curves, extraordinarily high low-frequency capacitance under illumination and other particularities of perovskite solar cells. Nevertheless, previous studies focus on a single experiment and its model description.

We present and discuss a variety of steady state, transient and frequency domain experiments on vacuum-deposited methyl ammonium lead iodide perovskite solar cells that are successfully reproduced by a 1D mixed ionic-electronic drift-diffusion model. Remarkably, only one single parameter set was used to simulate the nine experiments.

The comprehensive description paves the way for a quantitative understanding of perovskite solar cell devices. In the simulation, we reduce the cell to a 3-layer device that consists of the transport layers and perovskite. We discuss the impact of certain model ingredients such as ionic charges and trap states on the set of experiments, and show the limitations of the current model.

Further, we look at the upscaling of perovskite solar cells and other applications.

11:30 - 11:45
2.2-O1
Ahlawat, Paramvir
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland
Molecular Dynamics Simulations of Nucleation of Lead Halide Perovskites
Paramvir Ahlawat
Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, CH

I am a PhD student at EPFL. I am working on atomistic simulations of nucleation and crystal growth of lead halide perovskites.

Authors
Paramvir Ahlawat a, Michele Parrinello b, c, Ursula Rothlisberger a
Affiliations
a, Laboratory of Computational Chemistry and Biochemistry, Dept. of Chemistry, Ecole Polytechnique Fédérale de Lausanne, 4107, Lausanne, 1015, CH
b, Department of Chemistry and Applied Biosciences, ETH Zürich, Wolfgang-Pauli-Strasse 10, Zurich 8049, Switzerland
c, Università della Svizzera italiana (USI), Lugano
Abstract

Control over morphology is essential to manufacture high efficiency and high stability perovskite solar cells (PSCs). Nucleation regulate the morphological evolution. Therefore, it is utterly important to get the atomic level details into the nucleation process of these materials. For this, experimental techniques are mostly limited by temporal and spatial resolution. In our alternate approach, we perform molecular dynamics (MD) simulations of nucleation of lead halide perovskites[1]. However, nucleation is a rare event and MD simulation suffer from unaffordable time scales. Multi-component nature of our system adds an extra level of difficulty. To overcome this, we employ enhanced sampling technique of metadynamics which explore the free energy surface based on collective variables (CVs). In this study, we design experimentally measured quantities as CVs and succesfully observe the formation of perovskite crystals in our simulations[1]. To shed light, we depict the molecular details of different stages involved in formation of perovskite crystals and find that monovalent cations initiate and control the nucleation. Our simulations also reveal the atomic level structural details of intermediate phases[2] during crystallization. We argue the role of these intermediates in stability and efficiency of PSCs and show the effects of temperatures on free energy profile of these structures. Based on our insights, we demonstrate new experiments to better control the morphology via: spin coating and homogeneous nucleation. In the end, we discuss our compelling efforts and insights on simulating nucleation of highly complex mixed-cations systems on TiO2[3].

References:

[1] Ahlawat, P., Piaggi, P., Graetzel, M., Parrinello, M., & Rothlisberger, U. (2018). Atomistic Mechanism of the Nucleation of Methylammonium Lead Iodide Perovskite from Solution. arXiv preprint arXiv:1810.00759v2

[2] P. Ahlawat at el., in preparation

[3] P. Ahlawat at el. in preparation

11:45 - 12:00
2.2-O2
Sytnyk, Mykhailo
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Epitaxial Metal Halide Perovskites by InkJet Printing
Mykhailo Sytnyk
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), DE
Authors
Mykhailo Sytnyk a, AmirAbbas YousefiAmin a, Tim Freund a, Wolfgang Heiss a, Christina Harreiss b, Erdmann Spiecker b, Valentine V. Volobuev c, d, Jędrzej Korczak c, Тоmasz Story c, Gunther Springholz e, Annemarie Pfnür f, Klaus Götz f, Tobias Unruh f, Kamalpreet Singh g, Oleksandr Voznyy g, Ole Lytken h
Affiliations
a, Friedrich Alexander University Erlangen-Nuremberg, Erlangen, DE
b, Institute of Micro- and Nanostructure Research (IMN), Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 6, 91058 Erlangen, Germany
c, International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL-02668 Warsaw, Poland
d, National Technical University "KhPI", Kyrpychova Str. 2, 61002 Kharkiv, Ukraine
e, Institut für Halbleiterphysik, Johannes Kepler Universität at Linz, 4040 Linz, Austria
f, Friedrich Alexander University Erlangen-Nuremberg, Erlangen, DE
g, University of Toronto, King's College Road, 10, Toronto, CA
h, Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany
Abstract

Epitaxial thin film growth is the best method to obtain high-quality crystalline materials for (opto-) electronic applications. Metal halide perovskite semiconductors show high performance in polycrystalline films used for solar cells and even better electronic parameters in single crystals. The epitaxy of single crystalline layers is still in its infancy. While deposition of perovskite in a vacuum on various substrates resulted in crystallites on appropriate substrates, obtaining continuous single crystalline films has still to be shown. Here we attempt this goal by inkjet printing from precursor solutions, which in general is a very cheap and versatile technology for deposition of materials on predefined locations. This development is an advancement to the recent demonstration of epitaxy by spin casting [1], in which the formation of continuous perovskite films was discussed, but not fully demonstrated.
We found that the key towards a successful growth of epitaxial structures, which can be eventually merged to a continuous crystalline film, include (i) the appropriate choice of crystalline substrate, (ii) the surface preparation and activation, (iii) the formation of an appropriate metal halide buffer layers, (iv) the choice of humidity in the deposition ambient, (v) the substrate temperature, and the (iv) amount of deposited material. Our experimental findings are accompanied by density-function theoretical calculations of interface free-energies, predicting the advantages of applying a metal halide interlayer for epitaxial growth of perovskites upon lead chalcogenide crystalline substrates. In particular, we obtained most promising results with PbS and PbTe single crystalline substrates with [100] and [111] surface orientation, providing small lattice mismatches with the deposited methylammonium lead halide perovskite epitaxial structures. While the importance of the substrate pretreatments is confirmed by X-ray photoemission spectroscopy, the coherence between the lattices of the epitaxial perovskite structures is evidenced by X-ray pole figures and by high-resolution transmission electron microscopy. The obtained epitaxial structures include planar islands with either hexagonal or cubic shapes, whose edges are oriented according to the crystal orientation of the substrates. Increasing the amount of deposited material by increasing the ejection frequency of the inkjet printer allowed merging the individual epitaxial islands, to form quasi-continuous films, with some voids, as evidenced by electron- and optical microscopy. Most interestingly, even though the lead-chalcogenides used as substrates exhibit smaller band-gap energies as the epitaxial perovskite structures on top of them, the perovskite microstructures provide luminescence with bright intensity, making them promising for the development of future electronic devices. 
 

12:00 - 12:15
2.2-O3
Mathies, Florian
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Opportunities of Inkjet-printed Organic Metal Halide Perovskite Solar Cells
Florian Mathies
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Florian Mathies a, Hampus Näsström a, Oleksandra Shargaieva a, Gopinath Paramasivam a, Eva Unger a, b
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, Department of Chemical Physics and NanoLund, Lund University, Sweden, Lund, SE
Abstract

Organic metal halide perovskites (OMHP) have seen a dramatic increase in photovoltaics over the past decade. Intensive research on OMHP materials has demonstrated outstanding optical and electronic properties and has resulted in a record power conversion efficiency (PCE) of more than 24%. Besides, OMHPs have a high fault tolerance, regardless of the various manufacturing techniques from vacuum processing to coating and printing of these layers. These properties make OMHPs highly for photovoltaics, but also for lighting and sensor applications.

The main promises for the use of inkjet printing for perovskite solar cells are low production costs, high throughput and a high degree of design and substrate flexibility. Original publications show the potential of ink-jet printed perovskites for harvesting and lighting [1, 2]. Understanding the formation of OMHPs during the deposition and post-treatment process is essential to obtain high quality perovskite layers. By controlling the ink formulation, as well as the post-processing parameters, we are able to control the film roughness and thickness, thus changing the crystallization dynamics. The optimization of the OMHP absorbent layer by a multi-pass printing approach and a mild vacuum annealing step leads to an increase in grain size, photon absorption, and PCE. Solar cells in n-i-p architecture obtain a PCE of 15% on a sub-cm² cell area size.

A future application of printed OMHP solar cells may be building integrated photovoltaics. The production of colored OMHP solar cells benefits from the additive and substrate independent pressure approach [3]. The independently printed perovskite solar cell and the luminescent dye layer thus give more freedom in the production design. The results show that a high color perception of the OMHP solar cell using light absorbing materials of different colors only leads to a small reduction in efficiency.

12:15 - 12:30
2.2-O4
Brinkmann, Kai
University of Wuppertal, Germany
Intrinsic ALD Barriers Enable Processing on Top of Perovskite Solar Cells from Environmentally Friendly Solvents
Kai Brinkmann
University of Wuppertal, Germany, DE
Authors
Kai Oliver Brinkmann a, Tobias Gahlmann a, Junjie He a, b, Christian Tückmantel a, Manuel Theisen a, Tim Becker a, Johannes Bahr a, Cedric Kreusel a, Jun Song b, Junle Qu b, Thomas Riedl a
Affiliations
a, University of Wuppertal, Germany, Rainer-Gruenter-Straße, 21, Wuppertal, DE
b, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education Guangdong Province, College of Optoelectronic Engineering Shenzhen University, China
Abstract

Hybrid halide perovskites are an extremely promising material platform for next generation photovoltaics. To exploit the full potential of the technology and to reduce production costs, vacuum‑free roll-to-roll manufacturing with environmentally benign chemicals would be desired. In the vast majority of reports on perovskite solar cells, the top electrode is typically thermally evaporated in vacuum. On the other hand, various solvents used in the deposition of the top electrode by spray coating, doctor blading or printing may critically damage the perovskite below.[1,2] for example, e.g. silver nanowires (Ag-NW) processed from water or carbon processed as a paste from ethoxyethylacetate (high boiling, incombustible) destroy the perovskite at an instant.

Here we will show that the implementation of an electrically conductive and physically dense (impermeable) barrier grown by ALD is able to efficiently block the ingress of corrosive solvents into the perovskite stack. As an example, ALD grown SnOx which is transparent and conductive, can be utilized as electron transport layer in the device stack and can concomitantly function as an internal barrier.[3,4] Based on this concept, we are able to present the first perovskite solar cells with liquid processed semitransparent Ag-NW electrode from an aqueous dispersion. The resulting semi-transparent cells show power conversion efficiencies exceeding 15%. A detailed study of the interface of SnOx/Ag-NW by kelvin probe and photoemission spectroscopy identifies an energy barrier that can be eliminated by UV-illumination or by the addition of a dedicated interlayer with high carrier density (n > 1018). In a similar sense we will present the first proof of concept devices with a doctor bladed Carbon top electrode in p-i-n architecture. Hence the implementation of impermeable ALD SnOx layer can be expected to be the enabler for a plethora of subsequent processes without being limited by the sensitivity of the perovskite.

SolCat 2.2
Chair: Matthew Mayer
11:00 - 11:30
2.2-I1
Ager, Joel
Lawrence Berkeley National Laboratory
Cascade Catalysis Controls Selectivity in Electrochemical Carbon Dioxide Reduction
Joel Ager
Lawrence Berkeley National Laboratory, US

Joel W. Ager III is a Staff Scientist in the Materials Sciences Division of Lawrence Berkeley National Laboratory and an Adjunct Full Professor in the Materials Science and Engineering Department, UC Berkeley. He is a Principal Investigator in the Electronic Materials Program and in the Joint Center for Artificial Photosynthesis (JCAP) at LBNL and in the Berkeley Educational Alliance for Research in Singapore (BEARS) where he serves as Co-Lead PI of the eCO2EP project with Cambridge University. He graduated from Harvard College in 1982 with an A.B in Chemistry and from the University of Colorado in 1986 with a PhD in Chemical Physics.  After a post-doctoral fellowship at the University of Heidelberg, he joined Lawrence Berkeley National Laboratory in 1989. His research interests include the fundamental electronic and transport properties of semiconducting materials, discovery of new photoelectrochemical and electrochemical catalysts for solar to chemical energy conversion, and the development of new types of transparent conductors. Professor Ager is a frequent invited speaker at international conferences and has published over 300 papers in refereed journals.  His work is highly cited, with over 30,000 citations and an h-index of 85 (Google Scholar). 

Authors
Joel Ager a
Affiliations
a, University of California at Berkeley and Lawrence Berkeley National Laboratory
Abstract

Inspired by nature’s use of multiple enzymes to produce multi-carbon products from CO2 and sunlight [1], it is attractive to consider analogous cascade catalysis schemes in electrochemical CO2 reduction to achieve higher selectivity than what is possible now with conventional metal electrocatalysts [2]. I will discuss the opportunities and challenges of such approaches, with an emphasis on the management of the transport of intermediates between the active sites and the matching of the reaction rates [3]. 

A two-catalyst electrochemical cascade will be employed, with a CO intermediate being produced by Au or Ag undergoing further conversion to C2/C3 products on Cu. Both simulations and experiment show that sequential/tandem cascade catalysis can be affected on the micron scale, with diffusional transport of the intermediate CO in the liquid phase. This is possible due to the far higher density of catalytic sites (and corresponding molar fluxes) on the surface of a metal electrocatalyst, compared to enzymatic systems. Specifically, for microfabricated bimetallic systems (Au/Ag and Cu), CO transport and further conversion is possible for length scales up to fractions of the diffusion layer thickness (~100 mm) [4]. Simulations show that the local CO concentration can greatly exceed the solubility limit. Increasing the relative areal coverage of Ag or Au increases the CO concentration at the Cu surface, which leads to increases selectivity to oxygenates (e.g. ethanol, acetate, acetaldehyde, propanol), consistent with trends reported previously for CO reduction performed at elevated pressure [5].

Convective transport of intermediate CO is also possible. Using of independently controllable Ag and Cu cathodes in a laminar flow cell allows for both efficient conversion of the CO intermediate and tuning of the oxygenate yield [6]. Finally, realization of such a cascade approach in a “nanocoral” Ag-Cu bimetallic electrocatalyst has enabled the demonstration of solar-driven conversion of CO2 to hydrocarbon and oxygenates with an overall efficiency of over 5%, ~10x that of natural photosynthesis [7]. Recent work on integrating bimetallic cascade electrocatalysts directly on solar-driven photocathodes will also be described [8].

 

1. Shi, J.; Jiang, Y.; Jiang, Z.; Wang, X.; Wang, X.; Zhang, S.; Han, P.; Yang, C. Chem. Soc. Rev. 2015, 44, 5981–6000.

2. Yang, K. D.; Lee, C. W.; Jin, K.; Im, S. W.; Nam, K. T. J. Phys. Chem. Lett. 2017, 8, 538–545.

3. Wheeldon, I.; Minteer, S. D.; Banta, S.; Barton, S. C.; Atanassov, P.; Sigman, M. Nat. Chem. 2016, 8, 299–309.

4. Lum, Y.; Ager, J. W. Energy Environ. Sci. 2018, 10, 2935-2944.

5. Li, C. W.; Ciston, J.; Kanan, M. W. Nature 2014, 508 (7497), 504–507

6. Gurudayal; Perone, D.; Malani, S.; Lum, Y.; Haussener, S.; Ager, J. W. ACS Appl. Energy Mater. 2019, acsaem.9b00791.

7. Gurudayal; Bullock, J.; Srankó, D. F.; Towle, C. M.; Lum, Y.; Hettick, M.; Scott, M. C.; Javey, A.; Ager, J. W. Energy Environ. Sci. 2017, 10, 2222–2230.

8. Gurudayal; Beeman, J. W.; Bullock, J.; Wang, H.; Eichhorn, J.; Towle, C.; Javey, A.; Toma, F. M.; Mathews, N.; Ager, J. W. Energy Environ. Sci. 2019, 12 (3), 1068–1077.

11:30 - 12:00
2.2-O1
Fischer, Anna
University of Freiburg, Germany
SnIn@InSnOx core@shell Nanoparticles as Electrocatalysts for CO2 Electroreduction to Formate
Anna Fischer
University of Freiburg, Germany, DE

Since Aug. 2014:

Professor for “Inorganic Functional Materials” and head of the NANOMATERIAL group at the IAAC of the Ludwigs-Universität-Freiburg

2009 – 2014:

Group Leader within the framework of UniCat (DFG Exzellenz Cluster), Technische Universität Berlin, Institut für Chemie

Research on "Nanostructured electrodes for (bio)-electrocatalysis“

2008 – 2009:

Post-Doc at the MPIKG, Department of Biomaterials, Golm, Germany

2005 – 2008:

Dissertation at the Max-Planck-Institute of Colloids and Interfaces (MPIKG), Golm, Germany

“Synthesis of nanostructured metal nitrides through reactive hard-templating“

2000 – 2005:

Education in chemistry, Paris, France

Authors
Laura C Pardo-Perez a, d, Detre Teschner b, c, Elena Willinger b, Anna Fischer d, e, f
Affiliations
a, Institute of Chemistry, Technical University Berlin, Berlin, Germany
b, Department of Inorganic Chemistry, Fritz-Haber-Institute der Max-Planck-Gesellschaft, Berlin, Germany
c, Department of Heterogeneous Reactions, Max-Planck-Institute for Chemical Energy Conversion, Mülheim an der Ruhr, Germany
d, Institute for Inorganic and Analytical Chemistry, Inorganic Functional Materials Lab, University of Freiburg, Germany
e, Freiburg Materials Research Center (FMF), University of Freiburg, Stefan-Meier-Str. 21, Freiburg, 79104, DE
f, Freiburg Center for Interactive Materials and Bioinspired Technologies (FIT), University of Freiburg, 79110 Freiburg, Germany.
Abstract

The electrochemical reduction of CO2 (CO2RR) has gained increasing attention in the last years for the production of sustainable fuels. To overcome the large overpotentials and poor product selectivity associated to that process, highly efficient and selective electrocatalysts are required. Recently, Sn/SnOx and In/InOx composites have been reported active for CO2RR with high selectivity towards formate formation.

In the present work, we report on the activity and selectivity for CO2RR of “SnIn@InSnOx” core@shell nanoparticles. As oxide derived catalysts, these complex nanostructures are formed in-situ by the reduction of tin-rich indium tin oxide (ITOTR) thin films as bimetallic precatalyst precursor during CO2RR.

The in-situ formed core@shell particles were found to catalyze the CO2RR with a high mass activity of 546 A.gIn+Sn -1 and a high formate faradaic efficiency of 80%; performance, which outperforms other Sn and In nanopßarticle based CO2RR electrocatalysts reported so far.

In addition, ex-situ XPS analysis revealed that the presence of oxidized Sn and In species at the particles surface favors the formation of formate, revealing the importance of the oxide shell in the CO2 reduction mechanism.

Finally, preconditioning, applied potential and gas atmosphere all influenced the particle size and restructuring dynamics during extended electrolysis of these complex structures, and hence their activity and selectivity, but in all cases the “SnIn@InSnOx” core@shell structure was preserved throughout the different electrolysis conditions essayed.

12:00 - 13:30
Lunch
12:30 - 14:00
Lunch
Exciup 2.3
Chair: Ferdinand Grozema
13:30 - 14:00
2.3-O1
Nienhaus, Lea
Florida State University
NIR-to-visible Upconversion Sensitized by Bulk Lead Halide Perovskites
Lea Nienhaus
Florida State University, US
Authors
Sarah Wieghold a, Alexander Bieber a, Zachary VanOrman a, Lea Nienhaus a
Affiliations
a, Florida State University, 95 Chieftan Way, Tallahssee, 32312, US
Abstract

The sub-bandgap onset of rubrene-based organic light emitting diodes serves as an indicator of triplet exciton sensitization by carrier injection. Hence, materials which have a proper band alignment to allow for direct charge injection into the triplet state can enable a new path in sensitizing excitonic upconversion. In particular, bulk lead halide perovskite (LHPs) thin films have recently emerged as efficient sensitizers for near-infrared-to-visible upconversion. The upconversion process is based on triplet-triplet annihilation (TTA) in the annihilator rubrene. Conservative estimates result in upconversion efficiencies upwards of 3%. [1]

Understanding the upconversion mechanism is crucial for the advancement of upconversion devices. Our observations indicate that non-radiative trap filling in the LHP film and charge transfer to rubrene are competing pathways. As a result, we obtain lower intensity thresholds Ith denoting efficient upconversion using thicker perovskite films. Results indicate that the power-dependence of the upconverted emission is non-linear even above the Ith value, due to the bimolecular nature of the triplet sensitization mechanism. However, a trade-off can be observed: despite low Ith values for thicker LHP films, with increasing film thickness parasitic reabsorption of the singlets created by TTA also increases, which diminishes the visible light output of the device.  [2]

Two other unusual effects have been observed in these bilayer devices: i) two rise times in the upconverted photoluminescence dynamics, and ii) a reversible photobleach of the upconverted emission. Both effects can be traced back to the existing triplet population level and the resulting population-dependent diffusion length, indicating that further optimization of the device is still needed for real-world applications. [3]

14:00 - 14:15
2.3-O2
Gray, Victor
Physical Chemistry, Department of Chemistry, Ångström Laboratory, Uppsala University,Sweden
Energetic Dependence of Triplet Energy Transfer to PbS Quantum Dots for Singlet-Fission Based Photo-multiplication
Victor Gray
Physical Chemistry, Department of Chemistry, Ångström Laboratory, Uppsala University,Sweden, SE
Authors
Victor Gray a, b, Jesse Allardice b, Simon Dowland b, Zhilong Zhang b, James Xiao b, Neil Greenham b, Akshay Rao b
Affiliations
a, Department of Chemistry, Ångström Laboratory, Uppsala University, Lägerhyddsvägen, 1, Uppsala, SE
b, Cavendish Laboratory, Department of Physics, University of Cambridge, UK, JJ Thomson Avenue, Cambridge, GB
Abstract

Singlet-fission (SF) is a carrier multiplication process in organic materials where a photo-excited singlet state decays into two triplet excitons, each with roughly half the excitation energy. Integrated properly with a photovoltaic (PV) device the singlet-fission material can generate two charge carrier pairs per absorbed photon, leading to a significant increase in device performance. However, major challenges remain in how to integrate the SF material efficiently. A promising solution is to re-emit the exciton energy from the two triplet excitons as two low energy photons that can be re-absorbed by the PV device. This scheme allows for the decoupling and separate optimization of the PV device and SF-photomultiplier material. Unfortunately, triplet excitons are inherently dark. By transferring triplet excitons into emissive PbS quantum dots we are able to convert these dark states into photons. Here I will present our latest work on PbS-TIPS-Tetracene hybrid materials that show a 120% exciton multiplication with corresponding increas in the PbS emission when exciting the SF material. I will focus on the triplet energy transfer from the SF material TIPS-Tetracene to PbS quantum dots, and how it can be optimized through surface engineering with various ligands and quantum dot size. 

14:15 - 14:30
2.3-O3
Maiti, Sourav
Delft University of Technology (TU Delft), The Netherlands
Dynamics of Singlet Fission in Tetracene and Triplet Transfer to Silicon
Sourav Maiti
Delft University of Technology (TU Delft), The Netherlands, NL
Authors
Sourav Maiti a, Silvia Ferro b, Benjamin Daiber b, Alyssa van den Boom c, Sidharam Pujari c, Han Zuilhof c, Bruno Ehrler b, Sachin Kinge d, Laurens D. A. Siebbeles a
Affiliations
a, Delft University of Technology, The Netherlands, Julianalaan, 136, Delft, NL
b, Center for Nanophotonics, AMOLF, Science Park 104, The Netherlands
c, Laboratory of Organic Chemistry, Wageningen University, The Netherlands, Stippeneng, 4, Wageningen, NL
d, Materials Research & Development, Toyota Motor Europe, Hoge Wei 33, Belgium
Abstract

The photovoltaic efficiency of Si solar cells can be enhanced beyond the Shockley-Queisser limit by adding an organic singlet fission layer. The high energy photons are absorbed in the organic layer to create a singlet exciton, which splits into two triplet excitons via singlet fission. The challenge is to harvest the triplets with Si. The triplets should diffuse to the interface where both charge and energy transfer to Si are possible.

We study the efficiency of singlet fission, triplet diffusion, charge and energy transfer to Si by a combination of time-resolved spectroscopic techniques with optical and microwave/terahertz conductivity measurements. We are using tetracene as a singlet fission layer because the triplet energy (~1.25eV) is larger than the Si band-gap (~1.1eV), therefore energy transfer from the triplets to Si is energetically favorable. The singlet decay and triplet formation is monitored through transient absorption spectroscopy as both the singlet and triplet have distinct characteristics in the transient spectra. The initial findings suggest that the singlet fission occurs efficiently. However, the triplet transfer process to Si will significantly depend on the nature of Si surface and orientation of tetracene on Si. Therefore, particular attention is paid to the nature of the Si surface and coupling of functionalized tetracene to the Si surface.  Results of the effects of the abovementioned parameters on the efficiency of energy/charge transfer from the singlet fission layer to Si will be presented.

14:30 - 14:45
2.3-O4
MacQueen, Rowan
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Thin film halide perovskite as a triplet fusion sensitizer: present status and open questions
Rowan MacQueen
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Frederik Eistrup a, Klaus Schwarzburg a, Sergiu Levcenco a, Dennis Friedrich a, Thomas Unold a, Klaus Lips a, Eva Unger a, b, Rowan MacQueen a
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, Department of Chemistry & NanoLund, Lund University, Sweden
Abstract

Sensitized triplet fusion upconversion occurs via the collision of triplet-excited annihilator molecules [1]. The optically dark triplet states are populated by triplet sensitizers, which usually consist of molecule-like absorbers such as semiconductor nanocrystals, organometallic complexes, or thermally active delayed fluorescence molecules. In a new approach to triplet sensitization, intended to remove the usual excitonic character of the triplet sensitizer, electron-hole pairs optically excited in a thin film of lead halide perovskite semiconductor are used to drive triplet formation and subsequent triplet fusion upconversion in an adjacent small molecule layer. The approach is reminiscent of an organic light-emitting diode, with key differences arising from the simple bilayer nature of the device. Recent results show that this is a viable approach to implementing triplet fusion upconversion, although the efficiency of the system is thus far modest [2]. In this work, we present our current understanding of the thin film perovskite triplet sensitization mechanism, and discuss the potentials and pitfalls for this method of photon upconversion. We also examine the broader implications of the process as an example of radiationless energy transfer at a hybrid semiconductor interface, and discuss how this may shed light on the nature of surface states in thin film lead halide perovskites.

14:45 - 15:15
2.3-I1
Castellano, Felix
North Carolina State University
Triplet Migration Across Quantum Dot-Molecular Interfaces
Felix Castellano
North Carolina State University

Felix (Phil) Castellano earned a B.A. in Chemistry from Clark University in 1991 and a Ph.D. in Chemistry from Johns Hopkins University in 1996. Following an NIH Postdoctoral Fellowship at the University of Maryland, School of Medicine, he accepted a position as Assistant Professor at Bowling Green State University in 1998. He was promoted to Associate Professor in 2004, to Professor in 2006, and was appointed Director of the Center for Photochemical Sciences in 2011. In 2013, he moved his research program to North Carolina State University where he is currently the Goodnight Innovation Distinguished Chair. He was appointed as a Fellow of the Royal Society of Chemistry (FRSC) in 2015. His current research focuses on metal-organic chromophore photophysics and energy transfer, photochemical upconversion phenomena, solar fuels photocatalysis, energy transduction at semiconductor/molecular interfaces, photoredox catalysis, and excited state electron transfer processes.     

Authors
Felix Castellano a
Affiliations
a, North Carolina State University, 911 Partners Way, EB1 Room 2009, Raleigh, 27606
Abstract

The generation and transfer of triplet excitons across semiconductor nanomaterial-molecular interfaces will play an important role in emerging photonic and optoelectronic technologies and understanding the rules that govern such phenomena is essential.[1] The ability to cooperatively merge the photophysical properties of semiconductor quantum dots, with those of well-understood molecular chromophores is therefore paramount. CdSe semiconductor nanocrystals, selectively excited by green light, engage in interfacial Dexter-like triplet-triplet energy transfer with surface-anchored polyaromatic carboxylic acid acceptors, thereby extending its excited state lifetime by 5 orders-of-magnitude.[2] Net triplet energy transfer also occurs from surface anchored molecular acceptors to freely diffusing molecular solutes, further extending the triplet exciton lifetime while sensitizing singlet oxygen in aerated solution. The successful translation of triplet excitons from semiconductor nanoparticles to bulk solution implies a general paradigm that such materials are effective surrogates for molecular triplets. Several examples of newly conceived donor-acceptor quantum dot-molecule constructs will be presented.   

Inspired by the notion that semiconductor nanocrystals present molecular-like photophysical and photochemical properties, 1-pyrenecarboxylic acid (PCA)-functionalized CdSe quantum dots are shown to undergo thermally activated delayed photoluminescence.[3] This phenomenon results from a near quantitative triplet-triplet energy transfer from the nanocrystals to PCA, producing a molecular triplet-state ‘reservoir’ that thermally repopulates the photoluminescent state of CdSe through endothermic reverse triplet-triplet energy transfer. The resultant photoluminescence properties are systematically and predictably tuned through variation of the quantum dot–molecule energy gap, temperature, and the triplet-excited-state lifetime of the molecular adsorbate. The concepts developed here appear to be generally applicable to semiconductor nanocrystals interfaced with molecular chromophores enabling potential applications of their combined excited states.

SolCat 2.3
Chair: Joel Ager
13:30 - 14:00
2.3-I1
Strasser, Peter
Technical University of Berlin
Mechanistic Studies of the Electrochemical CO2 Reduction on Single Site, Metallic and Hybrid Electrocatalysts
Peter Strasser
Technical University of Berlin, DE

Peter Strasser is the chaired professor of �Electrochemistry for energy conversion and storage� at the Chemical Engineering Division of the Department of Chemistry at the Technical University of Berlin. Prior to his appointment, he was Professor at the Department of Chemical and Biomolecular Engineering at the University of Houston. Before moving to Houston, Prof. Strasser served as Senior Member of staff at Symyx Technologies, Inc., Santa Clara, USA. In 1999, Prof. Strasser earned his doctoral degree in Physical Chemistry and Electrochemistry from the �Fritz-Haber-Institute� of the Max-Planck-Society, Berlin, Germany, under the direction of the 2007 Chemistry Nobel Laureate, Professor Gerhard Ertl. In the same year, he was awarded the �Otto-Hahn Research Medal� by the Max-Planck Society. In 1996, Dr. Strasser was visiting scientist with Sony Central Research, Yokohama, Japan. He studied chemistry at Stanford University, the University of Tuebingen, and the University of Pisa, Italy. Professor Strasser is interested in the fundamental Materials Science and Catalysis of electrified liquid solid interfaces, in particular for renewable energy conversion, energy storage, production of fuels and chemicals.

Authors
Peter Strasser a
Affiliations
a, Dept. of Chemistry, Technical University Berlin, Strasse des 17. Juni 124, TC 03, 10623 Berlin, Germany
Abstract

    

The direct electrochemical CO2 reduction reaction on solid surfaces offers intriguing fundamental scientific as well as practical technical challenges and opportunities. Controlling the selectivity is key to turn this process into a practical process. To achieve this, more fundamental mechanistic work and understanding is needed.

In this talk, I will highlight some of our recent advances in the understanding of the chemical mechanism of the direct electrochemical reduction of CO2 into value-added fuels and chemicals on metallic surfaces, on non-metallic single-site electrocatalysts and on metallic/non-metallic tandem schemes. DFT based computational mechanistic predictions are tested by experiments and plausible reaction pathways and intermediates and their binding is discussed. Metallic catalysts comprise Cu and Cu-based alloys, non-metallic single site catalysts include high surface area solid carbons with atomically dispersed Metal-nitrogen moieties.

 

 

  

 

14:00 - 14:30
2.3-I2
Chan, Karen
Technical University of Denmark (DTU)
The Effect of the Electrolyte on Electrochemical CO2 Reduction
Karen Chan
Technical University of Denmark (DTU), DK
Authors
Karen Chan a
Affiliations
a, Technical University of Denmark (DTU), Frederiksborgvej 399, Roskilde, 0, DK
Abstract

Electrochemical CO2 reduction has been shown to be extremely sensitive to the composition of the electrolyte. In this talk I will discuss our theoretical investigations of 1) the effect of pH and 2) the effect of cation identity for CO2 reduction on Au, Ag, and Cu electrodes. Critical to both effects is the interfacial field- stabilization of relevant polar intermediates, which result in large shifts in activity with pH and ion identity. Based on ab initio simulations of reaction energetics and a modified Poisson Boltzmann approach to the ion distribution, we formulate corresponding kinetic models which are evaluated against experimental activity data.  We discuss the implications of our findings for catalyst design and electrolyte engineering.

 

 

 

                                                                                 

 

 

 

 

                       

14:30 - 14:45
2.3-O1
Ly, Khoa
Fakultät für Chemie und Lebensmittelchemie, Technische Universität Dresden
Operando Vibrational Spectroelectrochemistry for Studying CO2 Reduction Catalysis Promoted by Molecularly-defined Electrocatalysts
Khoa Ly
Fakultät für Chemie und Lebensmittelchemie, Technische Universität Dresden, DE
Authors
Khoa Hoang Ly a
Affiliations
a, Fakultät für Chemie und Lebensmittelchemie, Technische Universität Dresden, Dresden, DE
Abstract

Catalytic CO2 conversion to chemical feedstocks and carbon-based fuels powered by renewable energy sources constitutes a promising approach towards achieving a closed carbon cycle. Advancing this technology in the future relies strongly on optimization of the present catalytic systems to drive the conversion reaction with minimal energy losses and precise selectivity. In this respect, molecular catalytic centers as well as electrocatalytically-active 2D framework materials heterogenized onto suitable conductive support materials have emerged as interesting catalytic systems with prospective application potentials. The rational improvement such systems is based on the availability of in-depth information on the catalytic reaction mechanism that would allow for precise fine-tuning of the catalytic properties via chemical variations. To this end, particularly vibrational spectroelectrochemistry has established as a valuable tool to reveal and monitor the complex reactions. In contrast to other spectroscopic methods, this technique can be readily employed under in situ (aqueous) conditions and derives directly data on the catalytic steps at a molecular level.

This presentation discusses the versatility of (surface-enhanced) vibrational spectroelectrochemistry to study CO2 reduction catalysis specifically promoted by highly-active heterogenized molecular transition metal complexes and phthalocyanine-based 2D framework materials. The examples from the latest research include the application of operando IR absorption and confocal (resonance) Raman spectroscopy coupled to electrochemical control revealing crucial information on the reaction mechanism of the heterogenized catalysts with relevant implications on their afforded reactivity and selectivity.[1,2] The presentation concludes with an assessment of future prospects and novel experimental designs to increase the wealth of information that operando vibrational spectro-electrochemistry can derive.

14:45 - 15:15
2.3-O3
Velasco Vélez, Juan Jesús
Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck- Gesellschaft, Faradayweg 4-6, Berlin, 14195, Germany
In situ X-ray Spectroscopy Investigation of the Cathodic Electroreduction of CO2 into Valuable Chemical Feedstocks onto Copper Based Catalysts
Juan Jesús Velasco Vélez
Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck- Gesellschaft, Faradayweg 4-6, Berlin, 14195, Germany
Authors
Juan J. Velasco Vélez a, Cheng-Hao Chuang b, Dunfeng Gao a, Qingjun Zhu a, Travis Jones a, Emilia Carbonio a, Peter Strasser c, Beatriz Roldán-Cuenya a, Robert Schlögl a, Axel Knop-Gericke a
Affiliations
a, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, Berlin, 14195, DE
b, Department of Physics, Tamkang University, No.151, Yingzhuan Rd., Tamsui Dist., New Taipei City, 25137, TW
c, TU Berlin, Str. des 17. Juni 135, ER 1-1, Berlin, 10623, DE
Abstract

Among technologies to minimize the impact of CO2 gas emission, the electrocatalytic route of energy conversion becomes a key issue because the electricity produced by renewable sources of energy, like solar and wind, can be used to convert CO2 into valuable chemical feedstocks. Over the last decades several materials able to reduce electrochemically CO2 in aqueous solution to produce hydrocarbons have been identified not efficient and stable for practical use. In this direction, copper is unique due to its ability to electro-reduce CO2 to hydrocarbons and alcohols in aqueous electrolytes, as was probed by Hori et al1. Nevertheless, the selective electroreduction of CO2 into fuels is challenging due to the multiple complex proton-coupled electron transfer steps that must occur2. This complex network makes the cathodic CO2 reduction reaction (CO2RR) behave with relative low current density and high overpotential as well as electrode deactivation over time. Moreover, the lack of information on the electronic structure of Cu during both the fabrication process and under the catalytic reaction makes it difficult to design more efficient and stable electrocatalysts. By tracking the electronic structure of the Cu catalysts, using in situ X-ray spectroscopies, we were able to tune and precisely set the initial Cu redox state, such as Cu0, Cu+ and Cu2+, by controlled applied potential protocols3. Also, we traced the variations and modifications in the electronic structure (oxidation state) of the Cu catalysts during applied potential scans or steps and, in particular, under catalytic CO2RR conditions. These experiments (combined with calculations) yielded unambiguous information of the catalyst redox processes governing the CO2RR, as well as the nature of the active sites. In addition, the active/inactive and stable/unstable oxidation states depending on the applied potential and electrolyte were revealed, as shown in the figure. Here, we will report on the in situ preparation of catalysts and on the in situ monitoring of their electronic structure modification during preparation and electrocatalytic reaction for the next systems:

i) Electrodeposited Cu with accurately controlled oxidation state

ii) Thermal copper oxides with accuratelly controlled oxidation state

iii) CuNi and CuZn alloys

15:15 - 15:30
2.3-O2
Wagner, Andreas
Department of Chemistry, University of Cambridge - UK
Host-guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as Hybrid System in CO2 Reduction
Andreas Wagner
Department of Chemistry, University of Cambridge - UK, GB
Authors
Andreas Wagner a, Khoa Ly a, g, Nina Heidary a, h, István Szabó b, Tamás Földes b, Khaleel Assaf c, Steven Barrow d, i, Kamil Sokołowski d, e, Nikolay Kornienko a, h, Moritz Kuehnel j, a, Edina Rosta b, Ingo Zebger f, Werner Nau c, Oren Scherman d, Erwin Reisner a
Affiliations
a, Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of Chemistry, University of Cambridge, Lensfield Road, GB
b, Department of Chemistry, King's College London, United Kingdom, Trinity Street, 7, London, GB
c, Department of Life Sciences and Chemistry, Jacobs University Bremen, Germany, Campus Ring, 1, Bremen, DE
d, Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge - UK, Lensfield Road, GB
e, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
f, Technische Universität Berlin, Straße des 17. Juni, 124, Berlin, DE
g, Fakultät für Chemie und Lebensmittelchemie, Technische Universität Dresden, Dresden, DE
h, Department of Chemistry, Université de Montréal, Roger-Gaudry Building, Montreal, Quebec, H3C 3J7, Canada
i, School of Applied Chemistry and Environmental Science, RMIT University, Melbourne, 3000, Victoria, Australia
j, Department of Chemistry, Swansea University, College of Science, Singleton Park, Swansea SA2 8PP, U.K.
Abstract

A major challenge in electrocatalysis is the rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways. In this work, we present a model system introducing the concept of surface-bound host-guest chemistry in CO2 electrocatalysis. The functionalization of pristine gold (Au) with cucurbit[6]uril (CB[6]) nanocavities was studied as a hybrid organic-inorganic model system that utilizes host-guest chemistry to influence heterogeneous electrocatalytic reactions.

The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations support capture and reduction of CO2 inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO3 at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO2 concentration close to the gold interface. Electrocatalytic CO2 reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO2 reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in CO current density, but almost invariant H2 production.

Based on the obtained results, design criteria of molecular cavities that allow to overcome current limitations in heterogeneous CO2 electrocatalysis are discussed. We believe that the methodology and molecular insights in the presented work will provide fruitful basis for future design concepts of molecularly engineered catalytic environments through interfacial host-guest chemistry.

NCFun19 1.3
Chair: Jonathan Owen
14:30 - 15:00
1.3-I1
Houtepen, Arjan
Delft University of Technology, The Netherlands
Electrochemical Control over Semiconductor Nanomaterials: Doping and Surface Reduction
Arjan Houtepen
Delft University of Technology, The Netherlands, NL

Arjan Houtepen obtained his PhD Cum Laude under supervision of prof. Vanmaekelbergh at Utrecht University and subsequently became tenure track assistant professor in Delft. In 2009/2010 he was a visiting scientist in the group of prof. Feldmann in Munich. At present he is associate professor in the optoelectronic materials section at Delft University.

Authors
Arjan Houtepen a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
Abstract

Understanding and controlling the electrochemical properties of semiconductor nanomaterials is of great importance. These electrochemical properties include non-radiative recombination (i.e. electron and hole trapping are redox reactions), the stability of nanomaterials (i.e. many decomposition pathways are redox reactions) and the (im)possibility of electronic doping.

We use electrochemistry to control the Fermi-level in films of colloidal nanomaterials. This can result in filling of electron/hole traps in the bandgap and/or in n or p doping by the introduction of electrons (holes) into the conduction (valence) band. I will present how this can be used to probe electron/hole traps in the band gap of CdSe/CdS/ZnS QDs and nanoplatelets via spectroelectrochemistry. I will discuss how certain traps are actually formed by the introduction of charges, and how this may be avoided by proper control of the surface.[1]

Electrochemical control over the density of electrons or holes can be of great importance for the rational design of semiconductor devices such as LEDs, solar cells and lasers. For instance, it is known that the lasing threshold in semiconductor lasers is reduced by doping. I will discuss the systematic reduction of the optical gain threshold in CdSe/CdS/ZnS QD films, studied by a combination of electrochemical doping and ultrafast transient absorption spectroscopy.

Finally, I will discuss recent attempts to stabilize the charge density after electrochemical doping, so that electrochemical control over de doping density may be used to create functional devices.

15:00 - 15:15
1.3-O1
Gudjonsdottir, Solrun
Delft University of Technology (TU Delft), The Netherlands
On the Stability of Permanent Electrochemical Doping of Quantum Dot, Fullerene and Conductive Polymer Films in Frozen Electrolytes for Use in Semiconductor Devices
Solrun Gudjonsdottir
Delft University of Technology (TU Delft), The Netherlands, NL
Authors
Solrun Gudjonsdottir a, Ward van der Stam b, Christel Koopman a, Bob Kwakkenbos a, Arjan Houtepen a
Affiliations
a, Delft University of Technology (TU Delft), The Netherlands, Van der Maasweg, 9, Delft, NL
b, Utrecht University, Princetonplein, 1, Utrecht, NL
Abstract

Semiconductor films that allow facile ion transport can be electronically doped via electrochemistry, where the amount of injected charge can be controlled by the potential applied. To apply electrochemical doping to the design of semiconductor devices the injected charge has to be stabilized to avoid unintentional relaxation back to the intrinsic state. Until now, complete doping stability has only been gained at cryogenic temperatures. At this temperature, the electrolyte solvent is frozen and both external dopants and impurities are immobilized. In addition, electrochemical side reactions of the semiconductor material itself are slowed down. However, freezing of the electrolyte does not need to occur at cryogenic temperatures.

Here we investigate the possibility of stabilizing electrochemically doped semiconductor films at room temperature using a large variety of electrolyte solvents with melting points above room temperature (RT). We show electrochemical doping for three different QD materials (ZnO, CdSe and CdSe/CdS QDs), two fullerenes (C60 and PCBM) and two conductive polymers (P3DT and P3HT). By using room temperature freezing, we show that the doping stability can be increased immensely, in some cases from few seconds to over an hour. By using PEG as the electrolyte solvent, ZnO QD samples are still degenerately doped after few days (compared to around half an hour in common electrolyte solvents). However, even at a low rate, injected electrons do leave the QDs. On the other hand by using succinonitrile at reduced temperatures (-75 °C), complete doping stability has been achieved.

At last by combining the results from many different solvents, we have found a solvent which is completely frozen at room temperature. That is, injected charges can’t be removed from a ZnO QD film, even if an external bias is applied over the film. These results highlight the potential of using solidified electrolytes to stabilize injected charges, which is a promising step toward making semiconductor devices based on electrochemically doped semiconductor films.

15:15 - 15:30
1.3-O2
Thiel, Felix
Institute of Physical Chemistry, University of Hamburg
Cation Exchange Reactions in Nanorods: Vacancy-Mediated Diffusion in Cu-deficient Cu(2-x)S Nanorods during the Formation of a Ternary System
Felix Thiel
Institute of Physical Chemistry, University of Hamburg, DE
Authors
Felix Thiel a, b, Cristina Palencia Ramirez a, b, Horst Weller a, b
Affiliations
a, Institute of Physical Chemistry, University of Hamburg, Martin-Luther-King-Platz, 6, Hamburg, DE
b, The Hamburg Centre for Ultrafast Imaging
Abstract

Copper indium dichalcogenide, specifically CuInS2 (CIS), presents as a valuable material as a light absorber in quantum dot and thin film photovoltaics, due to its high absorption coefficient, and tunable optical properties from the visible spectrum to the near-infrared. CIS, with a band gap of 1.45 eV, could be a suitable alternative to more toxic, heavy metal based materials, such as CdSe.

Well established direct synthetic routes exist to produce small spherically shaped CIS NCs. However, achieving different morphologies with high control is still a challenge. Therefore, cation exchange reactions have proven to be an excellent alternative. This synthetic strategy is based on using one nanostructure as a template to generate the final structure with the desired composition via exchange of ions. This is usually achieved by partially or fully exchanging lattice cations, while retaining the initial anionic sublattice of the NCs.  

We synthesized large copper sulfide nanorods in batch with aspect ratios of approximately 2:1, with reproducible and narrow size distributions. Copper sulfide exists in various crystal phases, ranging from stoichiometric low chalcocite Cu2S to various Cu-deficient phases, e.g. djurleite Cu1.94S and roxbyite Cu1.78S. Cu-deficient copper sulfide phases exhibiting vacancies are a very interesting starting material, since they provide pathways for cation diffusion.

Our synthesized copper sulfide nanorods exhibit the Cu-deficient phase of djurleite Cu1.94S with sizes of approximately 40 nm by 80 nm. CE reactions were carried out under different reaction conditions, i.e. by varying the temperature, and observing the reaction progress at different times. The reaction rate does not change, even with a large surplus of In3+ guest cations in the reaction solution, pointing towards zero order reaction kinetics. The progress of powder diffraction patterns not only exhibits the emergence of reflections, but in addition shifting of reflections. This points towards an alloying process, constantly changing lattice parameters, opposed to two distinct crystal phases in a core/shell mechanism, which would present in two sets of patterns changing in intensities, without much reflection shifting.

OPV 2.3
Chair: Jörg Ackermann
14:00 - 14:30
2.3-O1
Khan, Jafar Iqbal
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia
Non-geminate Recombination Limits Fill Factor in Polymer:ITIC Bulk Heterojunction Solar Cells
Jafar Iqbal Khan
King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, SA
Authors
Jafar Khan a, Yuliar Firdaus a, Federico Cruciani a, Shengjian Liu a, Denis Andrienko b, Thomas Anthopoulos a, Pierre Beaujuge a, Frederic Laquai a
Affiliations
a, King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, 4700 King Abdullah University of Science and Technology (KAUST), Thuwal, SA
b, Max Planck Institute for Polymer Research, Mainz, Germany, Ackermannweg, 10, Mainz, DE
Abstract

 

Non-fullerene acceptors have emerged as promising fullerene replacements in organic photovoltaics and recently power conversion efficiencies have surpassed 16%. Despite the rapid progress, fundamental understanding of the photo-physical processes is still lacking and identifying the loss mechanisms in devices is important for future material design and device optimization. Here, we investigate the impact of polymer main chain and side chain substitution in BDT-thiophene copolymers on the performance of three bulk heterojunction solar cells that use ITIC as nonfullerene acceptor: PBDT[2HT]:ITIC (2.5%), PBDT(T)[2H]T:ITIC (4.2%) and PBDT(T)[2F]T:ITIC (9.8%). We observe more geminate recombination in the alkoxy-substituted PBDT[2HT]:ITIC blend in addition to nongeminate recombination of free charges, limiting both the short circuit current and fill factor. The alkylthiophene-substituted PBDT(T)[2HT]:ITIC blend exhibits less geminate recombination but significant nongeminate recombination, limiting the fill factor to about 40%, while backbone fluorination in PBDT(T)[2F]T:ITIC leads to fast and efficient charge separation and significantly reduced non-geminate recombination, resulting in fill factors in excess of 60%. Time-delayed collection field measurements showed that charge generation in PBDT(T)[2F]T:ITIC is field-independent, while a weak field dependence is observed for the PBDT[2H]T:ITIC system. Our findings provide important structure-property relations for the design of novel polymer:NFA systems.

  

14:30 - 14:45
2.3-O2
Kammerer, Jochen
University of Heidelberg
Morphology of NFA Organic Photovoltaic Blends by Automated Segmentation of Spatially Resolved Electron Spectra
Jochen Kammerer
University of Heidelberg, DE
Authors
Jochen Kammerer a, b, Rasum R. Schröder a, c, Pavlo Perkhun d, Olivier Margeat d, Wolfgang Köntges a, Christine Videlot-Ackermann d, Jörg Ackermann d, Irene Irene Wacker a, c, Martin Pfannmöller a
Affiliations
a, Centre for Advanced Materials (CAM), Heidelberg University, Heidelberg, Germany
b, Karlsruhe Institute of Technology (KIT), Light Technology Institute (LTI), Engesserstrasse 13, 76131 Karlsruhe, Germany
c, Cryo Electron Microscopy, BioQuant, Heidelberg University Hospital, Heidelberg, Germany
d, Aix-Marseille University, Centre Interdisciplinaire de Nanosciences de Marseille CINaM, UMR CNRS 7325, Marseille, France.
Abstract

The record for the conversion efficiencies of singe junction organic photovoltaic cells has recently leapt from about 14 % [1] to 16 % [2] within one year. This huge increase is not only owed to optimizing the acceptor molecules in terms of electronic structures but also an improved blend morphology.  Its optimization is typically a result of educated guesses combined with trial and error instead of direct investigation of the cell’s morphology. The reason is that donor polymers and acceptor molecules show similar chemical composition and properties. Thus, standard microscopy methods fail to generate contrast between the different materials [3]. Most studies rely on atomic force microscopy, deducing the blend morphology from the surface profile of the sample.   

We introduce a novel technique to visualize the morphology of fullerene (FA) and non-fullerene acceptor (NFA) bulk heterojunctions at the nanoscale. The method is based on spatially resolved secondary (SE) and backscattered electron (BSE) spectra, which directly relate to material properties [4]. Unsupervised machine learning reveals similarities within the datasets and allows to assign them to the different phases. Exemplary results for FA and NFA blends based on SE and BSE spectra are displayed in fig. 1. An interfacial mixed phase can be identified in both cases separating donor and acceptor domains.

The spectra are acquired with the aberration corrected prototype ultra-low voltage scanning electron microscope (ULVSEM) Zeiss DELTA® [5]. This microscope can retain high resolution down to beam electron energies ≥ 20 eV. This is radically lower compared to energies of ≥1 keV and 30 – 300 keV in conventional SEM and transmission electron microscopy, respectively. Several benefits for the investigation of organic electronics result from ultra-low landing energies: (i) New contrast mechanisms arise; (ii) the interaction volume of electron probe and sample is drastically reduced. Thus, signal mixing across interfaces is diminished and the signal originates directly from the surface. (iii) We observe decrease of beam damage, reducing the alteration of the sample during investigation. We expect that the opportunity to gain insight into local electronic properties from nano-resolved electron spectra will lead to a full understanding of interfacial effects and charge separation processes in the near future.

14:45 - 15:15
2.3-I1
Maes, Wouter
Hasselt University, IMO, Diepenbeek (Belgium)
Understanding Batch-to-Batch Variations of Push-Pull Type Conjugated Polymers for Organic Photovoltaics
Wouter Maes
Hasselt University, IMO, Diepenbeek (Belgium)

Wouter Maes got his PhD in Chemistry with Professor Wim Dehaen at the Katholieke Universiteit (KU) Leuven (Belgium) in 2005. After post-doctoral stays at the KU Leuven (postdoc of the Research Foundation – Flanders, FWO; with Professor Wim Dehaen), the Université Pierre et Marie Curie, Paris (with Professor Eric Rose) and Oxford University (with Professor Harry Anderson), he became Assistant Professor at Hasselt University in 2009, where he was promoted to Associate Professor in 2014 and Professor (Hoogleraar) in 2018. His research activities deal with the design and synthesis of organic semiconducting materials (with an emphasis on conjugated polymers) and their application in organic electronic devices (organic solar cells, photodetectors, transistors, light-emitting diodes) and advanced healthcare, pursuing rational structure-property relations (see https://www.uhasselt.be/DSOS). These activities are generally combined with more in-depth material and device physics studies within the framework of the Institute for Materials Research (IMO-IMOMEC) of Hasselt University.

Authors
Wouter Maes a, Omar Beckers a, Koen Vandewal a, Pieter Verstappen a
Affiliations
a, UHasselt – Hasselt University, Institute for Materials Research (IMO-IMOMEC), Agoralaan – Building D, 3590 Diepenbeek, Belgium
Abstract

Donor-acceptor or push-pull type conjugated polymers have become a dominating class of active materials in the field of organic electronics. Their adjustable light-harvesting, charge transfer and charge transport characteristics have been beneficially applied in organic photovoltaics, photodetectors and thin-film transistors. The conventional synthetic approach towards these push-pull polymers is based on Suzuki or (mostly) Stille cross-coupling of complementary functionalized heterocyclic precursors. In the ideal world, this should give rise to a perfect alternation of the employed building blocks throughout the polymer backbone and this alternation of electron rich (donor/push) and electron deficient (acceptor/pull) moieties leads to a substantial decrease of the bandgap. In recent years, however, it has become increasingly clear that the ‘real’ structure of the resulting alternating copolymers is often quite different from the projected one. Structural imperfections can for instance result from homocoupling of two identical building blocks. Furthermore, the end groups of these donor-acceptor copolymers are often also not those expected or targeted. In this contribution, recent results from our group will be presented, providing insights on the impact of homocoupling ‘defects’ on the device characteristics of organic solar cells. Additionally, different types of end groups were identified via MALDI-TOF mass spectrometry.

15:15 - 15:30
2.3-O3
Prel, Alexis
Laboratoire ICube, Université de Strasbourg, CNRS, France.
A Nanomorphology Taxonomy for Organic Solar Cells Modeling
Alexis Prel
Laboratoire ICube, Université de Strasbourg, CNRS, France., FR
Authors
Alexis Prel a, Abir Rezgui a, Anne-Sophie Cordan a, Yann Leroy a
Affiliations
a, Laboratoire ICube, Université de Strasbourg, CNRS, UMR 7357, 23 rue du Loess, 67037 Strasbourg, France
Abstract

Nanoscale morphology is an essential feature of bulk-heterojunction organic solar cells. In the past, disordered geometries have helped to push efficiencies beyond what early bilayer strategies could achieve, by providing a compromise between interface processes and transport along percolation pathways. [1]  It is therefore desirable to acquire a fundamental understanding of the relationship between the heterojunction's nanoscale morphology and the transport of exciton or free carriers in these devices.

In this context, I use numerical simulations to investigate the influence on transport of various morphologic traits such as the tortuosity of the conduction pathways, the presence of dead-ends or the fraction of disconnected domains. In this contribution, I show how much can be learned from applying drift-diffusion models to a few representative test cases. I argue that these can form a basis for a better understanding of structure-property relationships in organic solar cells, as depicted in the attached figure. For instance, in open-circuit conditions, the surface-to-volume ratio of the conduction pathways is a sufficient geometrical descriptor, whereas a quantitative definition of tortuosity is needed to describe short-circuit conditions. In the future, a strong knowledge of these basic cases could provide the missing link between extensive morphology information (e.g. as obtained by electron microscopy or X-ray diffraction) and interpretation in terms of transport and device performances.

This is embedded in a parameter extraction procedure to quantitatively interpret characterization data, and infer morphologic information. To this end, I use a Bayesian framework [2] to aggregate information from multiple techniques. This makes the inference more robust [3], and takes the algorithm presented in reference [4] one step further. I demonstrate with simple examples why this should be done routinely to avoid dramatic interpretation errors.

Possible applications include the use of non-destructive techniques on fully grown devices (e.g. I-V curve, TPC, TPV, CELIV, …) as a way to compare active layer depositions or identify degradation mechanisms.

PERFuDe 2.3
Chair: Jovana Milic
14:00 - 14:30
2.3-I1
Bernardi, Marco
Caltech, USA
Advances in Computing Charge Transport in Perovskite Materials from First Principles
Marco Bernardi
Caltech, USA
Authors
Marco Bernardi a
Affiliations
a, Department of Applied Physics, California Institute of Technology,, Pasadena, California 91125
Abstract

Perovskite materials typically exhibit soft phonon modes associated with phase transitions and electron-phonon interactions strong enough to lead to the formation of polarons. The presence of polarons and soft modes makes first-principles calculations of charge transport in perovskite materials highly challenging. This talk will discuss new approaches for treating the electron-phonon coupling due to soft phonon modes in perovskites, as well as a cumulant diagram-resummation approach for rigorously computing the carrier mobility in the large polaron regime. We apply these approaches to cubic SrTiO3 perovskite as a paradigmatic case, analyzing in detail soft mode and beyond-quasiparticle polaron contributions to charge transport. These advances lead to the first accurate ab initio prediction of the temperature dependence [1] and absolute value [2] of the mobility in SrTiO3, providing long-sought microscopic details about strong electron-phonon coupling and charge transport in perovskite materials.  We discuss application of these concepts to halide perovskites, for which we will show preliminary results if time permits.

[1] J.-J. Zhou, O. Hellman, M. Bernardi, "Electron-Phonon Scattering in the Presence of Soft Modes and Electron Mobility in SrTiO3 Perovskite from First Principles."
Physical Review Letters 121, 226603 (2018)
[2] J.-J. Zhou, M. Bernardi, "Unveiling the Origin of Charge Transport in SrTiO3 Beyond the Quasiparticle Regime." Preprint: arxiv 1905.03414

14:30 - 14:45
2.3-O1
Delport, Géraud
Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK.
Understanding the Influence of the Microscopic Structure of 2D and 3D Perovskites Materials on the Local Diffusion of Carriers.
Géraud Delport
Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., GB
Authors
Géraud Delport a, Camillle Stavrakas a, Edward Barnard b, Miguel Anaya a, Samuel D. Stranks a
Affiliations
a, Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., J.J. Thomson Avenue, Cambridge, GB
b, Molecular Foundry, Lawrence Berkeley National Laboratory, California 94720, USA, US
Abstract

Hybrid Organic perovskites (HOP) are remarkable materials for light emission and photovoltaics applications. Yet, the interplay between their structural and the optoelectronic properties are not fully understood. In particular, the ability of charges to diffuse through the HOP structure is one of the most important properties for applications.  Different techniques have been employed to evaluate the long range mobility/diffusion process in HOP materials. Among them, time resolved photoluminescence (TR-PL) microscopy is one of the most versatile[i]. It allows to investigate the diffusion with high spatial resolution (nanometre to micrometre range).  

In this work, we study the diffusion of carriers in 2D and 3D perovskites single crystals. By successive measurements, we build a statistics of diffusion coefficients, ranging between 0.05 to 2 cm2.s-2, in agreement with previous studies[ii]. We highlight the influence of the local structure and traps on the diffusion coefficient and the diffusion lengths.  To understand the influence of traps in details, we study the changes in diffusion behaviour upon different excitation densities or operating conditions (gas,temperature…). Finally, we perform time resolved two-photon (2P) photoluminescence tomography[iii], to probe the differences between the diffusive  behaviour at the surface and in the bulk of the crystals. This work provides a better understanding about the chemical and physical factors that still limit the diffusion of carriers in HOP materials. This study will provide insight to further improve HOP materials and devices performances.


 

14:45 - 15:00
2.3-O3
Koliogiorgos, Athanasios
Faculty of Electrical Engineering, Czech Technical University in Prague, Prague, Czech Republic
Electronic and Optical Properties of ABX3 (A = Cs, CH3NH3/B = Ge, Pb, Sn, Ca, Sr/X = Cl, Br, I) Perovskite Quantum Dots
Athanasios Koliogiorgos
Faculty of Electrical Engineering, Czech Technical University in Prague, Prague, Czech Republic
Authors
Athanasios Koliogiorgos a, Christos Garoufalis b, Iosif Galanakis b, Sotirios Baskoutas b
Affiliations
a, Faculty of Electrical Engineering, Czech Technical University in Prague, Prague, Czech Republic
b, Department of Materials Science, University of Patras, Greece, GR
Abstract

Perovskite quantum dots (QDs) constitute a novel and rapidly developing
field of nanotechnology with promising potential for optoelectronic applications. However,
few perovskite materials for QDs and other nanostructures have been theoretically explored.
In this study, we present a wide spectrum of different hybrid halide perovskite cuboid-like
QDs with the general formula of ABX3 with varying sizes well below the Bohr exciton radius, with a focus on lead-free compounds. Density functional theory (DFT) and time-dependent DFT calculations were employed to
determine their structural, electronic, and optical properties. Our calculations include both
stoichiometric and nonstoichiometric QDs, and our results reveal several materials with high
optical absorption and application-suitable electronic and optical gaps. A computational attempt to explore the size gap between ultrasmall quantum dots and bulk material is also included. Our study highlights
the potential as well as the challenges and issues regarding nanostructured halide perovskite
materials, laying the background for future theoretical and experimental work.

15:00 - 15:15
2.3-O4
Breternitz, Joachim
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Crystallography of Hybrid Halide Perovskites: Fundamental Reasoning of Ferroelectricity in MAPbI3
Joachim Breternitz
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Joachim Breternitz a, Frederike Lehmann a, b, Sarah Barnett c, Hariott Nowell c, Susan Schorr a, d
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, Universität Potsdam, Institute of Chemistry, 14469 Potsdam, Germany.
c, Diamond Light Source, Didcot OX11 0DE, UK.
d, Freie Universität Berlin, Arnimallee 14, Berlin, DE
Abstract

Hybrid halide perovskites have clearly made a spectacular appearance in the field of solar absorber materials. [1] With cell efficiencies steeply rising, the fundamental understanding of these materials is not always commensurate. A thorough understanding of the structural features of perovskites is most important, as it is closely linked to the electronic structure of the respective materials.

One prominent example are the reports of piezoelectric and ferroelectric effects in MAPbI3 at room temperature, [2] which are contradictory to the generally accepted crystal structure in the space group I4/mcm. While the latter is centrosymmetric, a polar space group, i.e. the breaking of inversion symmetry in the structure, would be a necessary prerequisite for ferroelectricity. A convenient explanation for this apparent discrepancy would be a symmetry breaking due to the non-symmetric molecular cation orientation and dynamics, as has been suggested in the past. [3] Since the molecule is heavily disordered, [4] a non-linear effect would probably disappear on a macroscopic scale if it is only caused by this.

We will open the discussion with the question how to define perovskites [5] and which structural features are crucial for the categorisations. Further, we will set out to categorize the different structures of halide perovskites in the form of a Bärnighausen tree and will further elucidate the importance of understanding group-subgroup relationships between the different crystal structures observed in halide perovskites. With this fundamental crystallographic basis, we set out to elucidate the question of ferroelectricity in MAPbI3 at ambient conditions.

For this, we present a detailed crystallographic investigation of the possible structural origin of ferroelectricity in MAPbI3. In short, the breaking of inversion symmetry is a consequence of maximizing hydrogen bonding between the molecular cation CH3NH3+ and the surrounding I- anions. This highlights how the organic cation plays a major role on the structural properties of this material. We will elucidate how the disordered molecular cation influences its surrounding and finally deliver a conclusive explanation for ferroelectricity from a crystallographic point-of-view.

15:15 - 15:30
2.3-O2
Benin, Bogdan
Laboratory of Inorganic Chemistry, Department of Chemistry & Applied Biosciences, ETH Zurich
Low-dimensional Tin-halides: Properties and Novel Applications
Bogdan Benin
Laboratory of Inorganic Chemistry, Department of Chemistry & Applied Biosciences, ETH Zurich, CH
Authors
Bogdan Benin a, b, Sergii Yakunin a, b, Dmitry Dirin a, b, Maksym Kovalenko a, b
Affiliations
a, Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zurich, 8093 Zurich, Switzerland
b, Empa, Swiss Federal Laboratories for Materials Science and Technology, Nanoscale Materials Science, 8600 Dübendorf, Switzerland
Abstract

Three-dimensional (3D) lead-halide perovskites continue to receive tremendous amounts of attention owing to their unique, and apparently, defect-tolerant photophysical and charge-transport properties. The discovery and subsequent investigation of these characteristics has spurred the innovative use of these materials in numerous applications such as solar cells, photodetectors, and hard-radiation detectors. While these fields continue to benefit from the use of lead-halide perovskites, the search for lead-free alternatives has so far achieved limited success. Newly discovered, ternary, metal-halide based materials seldom adhere to a cubic, 3D perovskite structure, but rather tend to adopt lower-dimensional structures with reduced connectivity between polyhedra and altered characteristics. In the case of zero-dimensional (0D), fully disconnected, materials, a unique set of properties are observed.

 

One recent entry to the growing list of luminescent 0D materials is the fully-inorganic, perovskite-derived, Cs4SnBr6, which exhibits room-temperature, broad-band photoluminescence (PL) centered at 540 nm with a quantum yield (QY) of 15±5% (Fig. 1a). Additionally, a compositional series Cs4-xAxSn(Br1-yIy)6 (A = Rb, K; x ≤ 1, y ≤ 1) can be prepared with PL that is tunable from 500-620 nm and a compositionally tunable Stokes shifts (Fig. 1b).[1] Furthermore, these materials and other low-dimensional tin-halides such as (C4N2H14I)4SnI6 and [C(NH2)3]2SnBr4 all share an additional property – highly temperature-sensitive PL lifetimes (Fig. 1c).[2] Not only are these lifetimes invariant on excitation power density, encapsulation, oxidation, spectral position, or other potential defects, they are highly reproducible with a variation of ca. 40 picoseconds over the course of 50 consecutive measurements. With these highly reproducible and thermally ultra-sensitive lifetimes, cutting-edge remote optical thermography can be achieved with a thermometric precision of up to 13 mK (Fig. 1d).

15:30 - 16:00
Coffee Break
NCFun19 1.4
Chair: Heather Kulik
16:00 - 16:15
1.4-O1
Sugathan, Anumol
Indian Institute of Science
Spectroscopic Insights into the Electronic Structure of Copper Iron Sulfide Nanocrystals
Anumol Sugathan
Indian Institute of Science, IN
Authors
Anumol s a, Biswajit Bhattacharyya a, V. V. R. Kishore a, Abhinav Kumar a, Guru Pratheep Rajasekar a, D. D. Sarma a, Anshu Pandey a
Affiliations
a, Solid State and Structural Chemistry Unit, Indian Institute of Science (IN)
Abstract

CuFeS2 quantum dots (QDs) have emerged as promising alternative to conventional lead and cadmium based QDs in recent years, courtesy of their narrow band gap and environmentally benign nature [1]. However, the full potential of this material remains unrealized because of the lingering doubts regarding their true nature. For instance, despite being a semiconductor, CuFeS2 QDs bear a striking resemblance to gold nanoparticles. The similarities extend all the way from the purple color of the QD solution and the metallic golden luster of the QD films to even the optical absorption spectrum of the QD solution.  In particular, there is a bump at ~500 nm in the absorbance spectra of the QDs which look very similar to the localized surface plasmon resonance (LSPR) in gold nanoparticles. The literature reports on this feature, however, are conflicting and this band has been variously assigned to a LSPR as well as to an intermediate band by different researchers [2, 3]. A material which can support a LSPR could be useful for charge transport, while a material with an intermediate band could find direct uses in light harvesting. Given the vastly different implications of either of the two interpretations, it is quite necessary to understand the real nature of the 500 nm feature; i.e. whether it’s a LSPR or an intermediate band. Here, I will briefly describe the various techniques, namely, optical, ultrafast and electrical measurements undertaken by us to understand the true nature of the 500 nm feature and hence the material [4]. 

16:15 - 16:30
1.4-O2
Palencia Ramírez, Cristina
University of Hamburg
Formation Dynamics of Nanocrystals: In-situ Observation of the Growth of CdSe NCs Via Magic-sized Clusters Intermediates.
Cristina Palencia Ramírez
University of Hamburg, DE
Authors
Cristina Palencia Ramírez a, b, Robert Seher a, b, Jan Krohn a, Felix Thiel a, b, Felix Lehmkühler b, c, Horst Weller a, b
Affiliations
a, Institute of Physical Chemistry. University of Hamburg
b, The Hamburg Centre for Ultrafast Imaging
c, Deutsches Elektronen-Synchrotron (DESY)
Abstract

The pioneer synthesis of nanocrystals (NCs) published in 1993 opened up new routes to prepare highly monodisperse quantum dots. This, together with the outstanding electronical, optical and surface properties observed in different kinds of such semiconductor nanocrystals, has triggered an increased scientific interest on these nanomaterials. As a result, well-defined methods to synthesize and characterize NCs have been developed, and very precise control on the synthesis allows obtaining a variety of crystal structures, shapes and sizes. Despite this, usually these advances are the result of trial-mistake approaches and this is due to the lack of knowledge in the events concurring during the formation of such nanocrystals. The key point to investigate the formation dynamics of nanocrystals lies on the possibility to in-situ characterize nucleation and growth events, which take place from the millisecond to second time window.

To this aim, we have designed and assembled a state-of-the-art continuous-flow device to perform the synthesis of CdSe NCs at very different reaction conditions. The addition of optical flow cells and X-ray transparent flow cells enables in-situ characterization by means of optical spectroscopy and SAXS/WAXS experiments (synchrotron radiation facilities). With this reactor we have studied the growth dynamics of CdSe nanocrystals. Our results show that CdSe magic sized clusters are formed always as intermediates in the formation of CdSe nanocrystals. Whether they can be observed or not in solution depends mainly on the reaction temperature, which is closely related to the monomer concentration and the nucleation rate. A series of experiments utilizing worked-up CdSe clusters demonstrates our proposed new growth mechanism, according to which, CdSe nanocrystals are formed subsequently to burst dissolution of small CdSe magic sized clusters and further growth into larger clusters and nanocrystals. This new growth mechanism suggests that the classical nucleation theory, as we know it, is no longer suitable to explain formation processes in nanocrystals.

16:30 - 17:00
1.4-I1
Voznyy, Oleksandr
University of Toronto
Ab Initio Studies of Surface Chemistry and Exciton Fine Structure in Semiconductor Nanocrystals
Oleksandr Voznyy
University of Toronto, CA

Alex earned his Ph.D. in physics of semiconductors from Chernivtsi National University, Ukraine for his work on electronic properties of nitride semiconductor alloys.

In 2004 he joined the Quantum Semiconductors and Bionanophotonics lab at University of Sherbrooke as a postdoc, working on theoretical modeling of laser-assisted quantum well intermixing and self-assembly processes of organic monolayers on metal and semiconductor surfaces for applications in bio-sensing.

In 2008 he moved to Quantum Theory Group at National Research Council of Canada in Ottawa, where he worked on many-body problems in epitaxial and colloidal semiconductor and graphene quantum dots; in particular, simulations of multi-exciton generation, Auger processes and optical properties of nanocrystals used in hybrid polymer-semiconductor solar cells.

Alex joined Ted Sargent’s Nanomaterials for Energy Group in 2011 and worked on characterization and modeling of the semiconductor nanocrystal surfaces and developing the synthesis methods for nanomaterials with improved optical and transport properties for photovoltaics.

In 2018, Alex joined the Department of Physical and Environmental Sciences at the University of Toronto, Scarborough as an Assistant Professor in Clean Energy. His topics of interest are materials for energy storage and novel materials discovery using high-throughput experiments and machine learning.

Authors
Oleksandr Voznyy a
Affiliations
a, University of Toronto, King's College Road, 10, Toronto, CA
Abstract

Colloidal semiconductor nanocrystals are a highly attractive class of materials for coherent light emission, with implications for lasing, light-emitting diodes, and quantum computing. Fine-tuning their properties for the above applications requires exact understanding of their surface chemistry in order to achieve near-unity quantum yields, and exciton fine structure, in particular, spacing and polarization of their triplet and singlet states. 

Dynamic formation of defects in CdSe nanocrystals in response to n-doping has been widely explored. However, true nanocrystals are p-doped due to exposure to air ambient. Here we explore what defects could form under such conditions, discuss their implications for lasing, and suggest avenues to suppressing them.

Another class of nanocrystals, CsPbBr3, have seen a lot of controversy regarding their exciton fine structure. To aid in the resolution of this debate, we performed investigation of the fine structure of the triplet emission properties in these materials. Using the wave functions generated via DFT calculations including spin-orbit coupling for cubic, orthorhombic and tetragonal caesium lead halide perovskite nanocrystals of ~3 nm in diameter, we further augmented them with Coulomb coupling between the exciton configurations, to resolve the absorption and emission fine structure in a configuration interaction method.

 

16:00 - 17:00
OPV 2.4
PERFuDe 2.4
Chair: Kylie Catchpole
16:00 - 16:30
2.4-I1
Mora-Seró, Iván
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain
Characterization of Transport and Recombination in Perovskite Solar Cells by Impedance Spectroscopy
Iván Mora-Seró
Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, ES

Iván Mora-Seró (1974, M. Sc. Physics 1997, Ph. D. Physics 2004) is researcher at Universitat Jaume I de Castelló (Spain). His research during the Ph.D. at Universitat de València (Spain) was centered in the crystal growth of semiconductors II-VI with narrow gap. On February 2002 he joined the University Jaume I. From this date until nowadays his research work has been developed in: electronic transport in nanostructured devices, photovoltaics, photocatalysis, making both experimental and theoretical work. Currently he is associate professor at University Jaume I and he is Principal Researcher (Research Division F4) of the Institute of Advanced Materials (INAM). Recent research activity was focused on new concepts for photovoltaic conversion and light emission based on nanoscaled devices and semiconductor materials following two mean lines: quantum dot solar cells with especial attention to sensitized devices and lead halide perovskite solar cells and LEDs, been this last line probably the current hottest topic in the development of new solar cells.

Authors
Iván Mora-Seró a
Affiliations
a, Universitat Jaume I, Institute of Advanced Materials (INAM) - Spain, Avinguda de Vicent Sos Baynat, Castelló de la Plana, ES
Abstract

Recombination is a key parameter to stablish the performance of a Solar Cell and stablish their limitations for a further optimization process. On the other hand, charge transport processes are usually not considered in the limitation of perovskite solar cell performance as 3D halide perovskites presents good transport properties. However, selective contact or the use of bulky cations in 2D/3D perovskite can also introduce a transport limitation in the cell performance. These parameters can be easily determined in other kind of solar cells as in dye sensitized solar cells (DSSCs) by impedance spectroscopy, but in the case of Perovskite Solar Cells (PSCs) this determination have been elusive. Impedance spectroscopy is a non-destructive characterization technique that can help in the understanding of these devices. Impedance spectroscopy is a characterization method in the frequency domain that allows to decouple physical processes with different characteristic times at the working conditions i.e. under illumination and applied bias. In this talk we show as solar cells using mesoporous TiO2scaffold can behave as DSSCs when the amount of deposited perovskite is low enough. As the amount of perovskite in the cell increases the behavior resembles the observed for PSCs. The analysis by impedance spectroscopy of these samples have allow as to connect both kind of devices and determine that transport and recombination are coupled in PSCs. Despite this limitation we show different examples in which impedance spectroscopy provide an excellent guide to determine the limiting processes in different kind of PSCs, providing consequently important clues to experimentalist to optimize its performance.

16:30 - 16:45
2.4-O1
Canil, Laura
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
Work Function Tuning through Self-Assembling Monolayers of Fluorinated Molecules
Laura Canil
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, DE
Authors
Laura Canil a, Antonio Abate a, b
Affiliations
a, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, DE
b, University of Naples Federico II, Corso Umberto I 40, Naples, IT
Abstract

With a power conversion efficiency over 24%, perovskite solar cells (PSCs) are considered a rising star in the solar energy. Nowadays a lot of research is focused on the improvement of the device performance through the employment of new materials or architectures [1,2]. With regard to this, the importance of interfaces in such a system is well known and in particular a crucial role is played by the energy level alignment of the different layers [3].

A good match between the electronic bands is required in order to obtain high performance PSCs structures, to this purpose we functionalize the interface between the perovskite and the charge selective contacts within the device. We make use of specific molecule-to-substrate interactions to self-assembly perfluorinated small molecules on the perovskite surface. This results in the formation of an interfacial dipole and leads to a shift in the perovskite work function, which allows us to tune the perovskite energy levels and therefore be flexible in the choice of the transport layers. Our results show that with this technique even materials with a non optimal energy level alignment can lead to device performances comparable with the state of art.

Moreover, the formation of a nanometer thick perfluorinated layer results in hydrophobicity of the perovskite surface, which enhances stability by preventing the ingress of water from the atmosphere.

Notably, such a functionalization can be done through scalable solution processing methods, which are compatible with fast output production including roll-to-roll and inject printing. We investigate the impact of the functionalization on material and device by characterizing the change in the energetics of the system and correlating them with the PSCs performance.

The interface functionalization with perfluorinated molecules is an effective new approach to improve energy levels alignment and enhance PSCs performance. This technique can be used as a universal tool to tune the work function and control its shift, which leads to more flexibility in the choice of materials and structure.

16:45 - 17:00
Abstract not programmed
SolCat 2.4
Chair: Víctor A. de la Peña O'Shea
16:00 - 16:30
2.4-O1
Hod, Idan
Ben-Gurion University of the Negev, Israel
Metal-Organic Frameworks as a Heterogeneous Platform for (Photo)-Electrocatalytic CO2 Reduction
Idan Hod
Ben-Gurion University of the Negev, Israel, IL
Authors
Idan Hod a, Ran Shimoni a, Itamar Liberman a, Raya Ifraemov a, Wenhui He a, Chanderpratap Singh a
Affiliations
a, Ben-Gurion University of the Negev, Israel, Beer-Sheva, IL
Abstract

In a world that is running out of natural resources, there is a growing need to design and develop sustainable and green energy resources. In that respect, photo-electrocatalytically driven reactions for the production of alternative fuels (such as CO2 reduction) hold the potential to provide a route for future carbon neutral energy economy. Nevertheless, the slow kinetics of those catalytic reactions demands the development of efficient catalysts in order to drive it at lower overpotentials. Indeed, a variety of molecular catalysts based on metal complexes are capable of electrochemically reducing CO2. Yet, despite the significant progress in this field, practical realization of molecular catalysts will have to involve a simple and robust way to assemble high concentration of these catalysts in an ordered, reactant-accessible fashion onto a conductive electrode. 

In this talk, I will present our recent proof-of-principle study on electrocatalytic CO2 reduction activity of MOFs incorporating molecular catalysts such as Fe-tetraphenylporphyrin and Mn(bpy)(CO)3Br. Our group utilizes Metal-Organic Frameworks (MOFs) as a platform for heterogenizing molecular electrocatalysts. Their unique properties (porosity and flexible chemical functionality), enables us to use MOFs for integrating all the different functional elements needed for efficient catalysts: 1) immobilization of molecular catalysts, 2) electron transport elements, 3) mass transport channels, and 4) modulation of catalyst secondary environment. Thus, in essence, MOFs could possess all of the functional ingredients of a catalytic enzyme.  

16:30 - 16:45
2.4-O2
Guntern, Yannick
Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Nanocrystal/Metal-Organic Framework Hybrids as Electrocatalytic Platform for CO2 Conversion
Yannick Guntern
Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Authors
Yannick T. Guntern a, James R. Pankhurst a, Raffaella Buonsanti a
Affiliations
a, Laboratory of Nanochemistry for Energy, EPFL, Switzerland
Abstract

The tunable chemistry linked to the organic/inorganic components in colloidal nanocrystals (NCs) and metal-organic frameworks (MOFs) offers a rich playground to advance the fundamental understanding of materials design for various applications. Here, we present an approach which combines these two classes of materials by synthesizing NC/MOF hybrids wherein Ag NCs are in intimate contact with Al-PMOF ([Al2(OH)2(TCPP)], TCPP = tetrakis(4-carboxyphenyl)porphyrin), to form Ag@Al-PMOF. The hybrid thin films are synthesized by combining colloidal chemistry, atomic layer deposition (ALD) and solvothermal chemical conversion, thereby preserving electrical contact of the NCs with a conductive substrate. This key feature has allowed us to explore Ag@Al-PMOF as electrocatalysts for the CO2 reduction reaction (CO2RR) and how the synergistic interactions between the two components improve the catalytic performance. We show that the pristine interface between the NCs and the MOFs accounts for electronic changes in the Ag, which suppress the hydrogen evolution reaction (HER) and promote the CO2RR. We also demonstrate a minor contribution of mass transfer effects imposed by the porous MOF layer under the chosen testing conditions. Furthermore, we find an increased morphological stability of the Ag NCs when combined with the Al-PMOF. The synthesis method is general and applicable to other metal NCs, thus revealing a new way to think about rationally tailored electrocatalytic materials to steer selectivity and improve stability.

16:45 - 17:00
2.4-O3
Arena, Federica
Center for NanoScience and Technology, Italian Institute of Technology, Via Pascoli 70/3, 20133 Milano, Italy
Bioelectrochemical TiN|FDH Catalyst for CO2 Reduction to HCOOH
Federica Arena
Center for NanoScience and Technology, Italian Institute of Technology, Via Pascoli 70/3, 20133 Milano, Italy
Authors
Federica Arena a, b, Giorgio Giuffredi a, b, Stefano Donini a, Emilio Parisini a, Fabio Di Fonzo a
Affiliations
a, National Center for Nanoscience and Technology
b, Department of Energy, Politecnico di Milano
Abstract

 

Among the many strategies proposed to convert generated CO2 into value-added chemicals and energy carriers involves its electrochemical conversion using bioelectrochemical systems (BES)[1]. The advantage of this new technology lies in the possibility of exploiting the enzyme properties catalysing the reduction reaction with a high selectivity and specificity toward products with a low overpotential applied.

 

In this work, we present a novel BES, where the NAD-dependent enzyme Formate Dehydrogenase from Thiobacillus sp. KNK65MA [2] is expressed by heterologous production in E. coli BL21 (DE3) and deposited on a nanostructured mesoporous support of Titanium Nitride (TiN) realized by Pulsed Laser Deposition. Thanks to this method, we can realize a nanostructured support with high surface area and tree-like morphology. By optimizing the synthesis parameters, it is possible to obtain a nanostructured, hierarchical support that maximizes the available surface area for catalyst absorption and enhances the bio-interface between enzyme and inorganic electrode. We quantify the amount of immobilized enzymatic catalyst on the nanostructure through standard enzymatic assays, demonstrating that the nanostructuration of the TiN support increases the surface area available for enzyme immobilization, achieving a maximum enzyme adsorption of 59 µg cm-2 for the BES. The TiN support is firstly characterized electrochemically, to verify its mechanical and chemical stability in the electrolyte solution. Subsequently, the enzymatic electrosynthesis of formic acid from CO2 is investigated at different applied potentials, showing a productivity for formic acid that ranges from 1.5 to 3.7 mmol mg-1enzyme h-1 according to the applied overpotentials. Finally, post-catalysis characterization of the hybrid system shows that the amorphous nanostructure does not undergo any important modifications in its morphology and composition, thus demonstrating the mechanical stability of the TiN scaffold.

 

This performance, which is unparalleled in previous studies involving enzymes of the FDH family immobilized on inorganic supports, stems from a combination of are the best for inorganic support-immobilized enzymes of the FDH family, achieved thanks to the high reducing activity of TsFDH and the high contact area offered by the nanostructured TiN support, and demonstrates the feasibility and as well as the potential for a biotechnological device in terms offeaturing product specificity and stability.

  

17:00 - 19:00
Poster Session
 
Fri Nov 08 2019
Plenary Session 7
Chair: Paulina Plochocka
09:00 - 09:30
7-K1
Zhu, Xiaoyang
Columbia University
Ferroelectric Polarons in Lead Halide Perovskites
Xiaoyang Zhu
Columbia University, US

Xiaoyang Zhu is the Howard Family Professor of Nanoscience and a Professor of Chemistry at Columbia University. He received a BS degree from Fudan University in 1984 and a PhD from the University of Texas at Austin in 1989. After postdoctoral research with Gerhard Ertl at the Fritz-Haber-Institute, he joined the faculty at Southern Illinois University as an Assistant Professor in 1993. In 1997, he moved to the University of Minnesota as a tenured Associate Professor, later a Full Professor, and a Merck endowed professor. In 2009, he returned to the University of Texas at Austin as the Vauquelin Regents Professor and served as directors of the DOE Energy Frontier Research Center (EFRC) and the Center for Materials Chemistry. In 2013, he moved to Columbia University. His honors include a Dreyfus New Faculty Award from Dreyfus Foundation, a Cottrell Scholar Award from Research Corporation, a Friedrich Wilhelm Bessel Award from the Humboldt Foundation, a Fellow of the American Physical Society, a Vannevar Bush Faculty Fellow Award from DOD, and an Ahmed Zewail Award from the American Chemical Society. Among his professional activities, he serves on the editorial/advisory boards of Accounts of Chemical Research, Science Advances, Chemical Physics, and Progress in Surface Science, and as a scientific advisor to the Fritz-Haber-Institute of the Max-Planck Society and ShanghaiTech University

Authors
Xiaoyang Zhu a
Affiliations
a, Department of Chemistry, Columbia University, New York, New York 10027, United States
Abstract

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids.  Here we explain the essential physics in this class of materials based on their dielectric functions and dynamic symmetry breaking on nano scales. We show that the dielectric function in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole, resulting a quasiparticle which we call a ferroelectric large polaron, a concept similar to solvation in chemistry. Compared to a conventional large polaron, the collective nature of polarization in a ferroelectric large polaron may give rise to order(s)-of-magnitude larger reduction in the Coulomb potential. Using two-dimensional optical Kerr effect spectroscopy, we directly probe the energetics and local phonon responses of ferroelectric polarons. The ferroelectric polaron may explain the defect tolerance and low recombination rates of charge carriers in lead halide perovskites and the slow cooling of hot carriers, as well as providing a design principle of the “perfect” semiconductor for optoelectronics.

Plenary Session 8
Chair: Ivan Infante
09:00 - 09:30
8-K1
Talapin, Dmitri
University of Chicago
Self-organization of Electrostatically and Sterically Stabilized Colloidal Nanocrystals: The Roles of Topology, Image Charges and Non-classical Nucleation
Dmitri Talapin
University of Chicago, US
Dmitri Talapin is a Professor of Chemistry at University of Chicago. His research interests revolve around inorganic nanomaterials, spanning from synthetic methodology to device fabrication, with the desire of turning colloidal nanostructures into competitiv