Program
 
Mon Oct 22 2018
Plenary Session 2
Chair: Kevin Sivula
09:00 - 09:30
2-K1
Hwang, Yunjeong
Korea Institute of Science and Technology (KIST)
Electrochemical Conversion of CO2 toward Valuable Chemicals for Solar-to-Chemical Conversion Application
Yunjeong Hwang
Korea Institute of Science and Technology (KIST), KR
Authors
Yun Jeong Hwang a, Byoun Koun Min a, Hyeong-Suk Oh a
Affiliations
a, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul, South Korea, Seoul, KR
Abstract

Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Plenary session 1
Chair: Mischa Bonn
09:00 - 09:30
1-K1
Klimov, Victor
Los Alamos National Laboratory, US
Colloidal Quantum Dot Lasing: Historical Perspective and Recent Progress
Victor Klimov
Los Alamos National Laboratory, US

Victor I. Klimov is a Fellow of Los Alamos National Laboratory and the Director of the Center for Advanced Solar Photophysics of the U.S. Department of Energy. He received his M.S. (1978), Ph.D. (1981), and D.Sc. (1993) degrees from Moscow State University. He is a Fellow of both the American Physical Society and the Optical Society of America, and a recipient of the Humboldt Research Award. His research interests include optical spectroscopy of semiconductor and metal nanostructures, carrier relaxation processes, strongly confined multiexcitons, energy and charge transfer, and fundamental aspects of photovoltaics.

Authors
Victor Klimov a
Affiliations
a, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Abstract

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

Dyman S5.1
Chair: Enrique Cánovas
09:30 - 10:00
S5.1-O1
Alimoradi jazi, Maryam
Universiteit Utrecht
Band Occupation and Charge Transport in CdSe Nanocrystal Superlattices
Maryam Alimoradi jazi
Universiteit Utrecht, NL
Authors
Maryam Alimoradi Jazi a, Wiel Evers b, e, Thomas Altantzis c, Christophe Delerue d, Sara Bals c, Laurens Sibbeles e, Arjan Houtepen e, Daniel Vanmaekelberg a
Affiliations
a, Condensed Matter and Interfaces, Debye Institute for nanomaterials science, Utrecht University
b, . Opto-electronic Materials Section, Kavli Institute of Nanoscience, Delft University of Technology
c, Electron Microscopy for Materials Research (EMAT),, University of Antwerp,Groenenborgerlaan 171, 2020 Antwerp
d, IEMN, Department ISEN, 41 boulevard Vauban, F-59046 Lille Cedex, France
e, Department of Chemical Engineering, Optoelectronic Materials, TU Delft, Julianalaan 136, 2628 BL Delft, The Netherlands
Abstract

Colloidal semiconductor nanocrystals have gained interest since their optical and electronic properties can be tuned by varying their shape, size and composition. Recently, 2D square and honeycomb superlattice of lead- and cadmium-chalcogenide quantum dots (QDs) have been prepared. These superstructures are formed by assembling PbSe nanocrystals in a monolayer at the toluene suspension air/interface after which the nanocrystals attach via their four vertical {100} facets [1],[2]. Afterward, cation exchange transforms PbSe into zinc blend CdSe. Theoretical studies show that these 2-D systems have distinct band structures compared to continuous nanosheets, with the appearance of Dirac cones in the case of the honeycomb [3]. Strong electronic coupling via the atomic connections of the QDs in the superstructure may result in a higher mobility compared to the self-assembled lead chalcogenide QDs that are less strongly coupled due to the (in) organic ligands [4].  

In our research, we use electrolyte-gated transistors to study the optoelectronic properties and transport characteristics of 2-D PbSe and CdSe superstructures [5]. The potential of the gate electrode determines the Fermi level with respect to the conduction band (CB) or valence band (VB) of the superstructure. First, to monitor the stability of the superlattice under electron injection we measure the differential capacitance as a function of gate voltage. Second, the conductivity of the network is measured as a function of the Fermi level position. To quantify band occupation into the superlattice, the optical absorption quenching employed. Finally, the mobility of the system is calculated from conductivity and charge density.

We reported the first study of electron transport in a 2-D PbSe system with a square geometry in which band occupation is assured by the electron density of 8 electrons per nanocrystal . The electron mobility between 5 and 18 cm2/Vs is observed for these supersructures [6].­­

In our recent work, we study the electron transport of CdSe superlattices with square and honeycomb geometry. The band occupation is assured by the number of 2 electrons per nanocrystal. The electron mobility of 1 and 10 cm2/Vs is achieved for square and honeycomb geometry respectively.   

1) W.H. Evers et al., Nano Lett., 13, (2013).

2) M.P. Boneschanscher et al., Science, (2014).

3) E. Kalesaki et al., Phys. Rev. B 88, (2013).

4) W.H. Evers et al., Nature Communications 6, (2015).

5) D. Vanmaekelbergh et al., Electrochemica Acta, 53, (2007).

6) M. Alimoradi Jazi et al., Nano Lett., 17, (2017)

10:00 - 10:30
S5.1-O2
Walravens, Willem
Ghent University
Setting Carriers Free – Healing Faulty Interfaces Promotes Delocalization and Transport in Nanocrystal Solids
Willem Walravens
Ghent University, BE
Authors
Willem Walravens a, Filip Geenen b, Eduardo Solano d, Jolien Dendooven b, Athmane Tadjine e, Nayyera Mahmoud a, c, Gunther Roelkens c, Christophe Delerue e, Christophe Detavernier b, Zeger Hens a
Affiliations
a, Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium
b, Conformal Coating of Nanomaterials (CoCooN), Department of Solid-State Sciences, Ghent University, Belgium
c, Ghent University, Photonics Research Group, iGent, Technologiepark Zwijnaarde 15, 9000 Ghent, Belgium
d, NCD beamline, ALBA Synchrotron Light Source, Carrer de la Llum 2-26, 08290 Cerdanyola del Vallès, Spain
e, IEMN, Department ISEN, 41 boulevard Vauban, F-59046 Lille Cedex, France
Abstract

Nanocrystal building blocks can be assembled to make an artificial, nanocrystal solid. The choice of building block and the way they are assembled set up pathways to make new and unique materials with tailored properties. A case in point are superlattices of semiconductor nanocrystals or quantum dots (QDs), which find applications in, e.g., photodetectors, solar cells and field-effect transistors. Quantum dots offer the appealing combination of a tunable band gap, a high absorption coefficients, and a suitability for solution-based processing. QD films are typically produced through, e.g., spincoating, dropcasting or spraycoating. This results in disordered nanocrystal stacks, where poor electronic transport can be caused by excessive surface defects or restricted dot-to-dot hopping. To disentangle such effects, we analyzed the delocalization and transport of charge carriers in 2D superlattices of epitaxially connected QDs. In the case of PbS and PbSe QDs, such superlattices can be formed over several square micrometer. Using elemental analysis and structural analysis by in-situ XRF and GISAXS, respectively, we show that such lattices keep their structural integrity in a wide temperature window, ranging up to 310 ºC and more; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices at temperatures ranging from 75-150 ºC induces a marked redshift of the QD band-edge transition. In fact, the band-edge found after annealing agrees, opposite from state-of-the-art literature, with theoretical predictions on charge carrier delocalization in such epitaxially connected superlattices. In addition, we observe a 1000-fold increases of the charge carrier mobility after mild annealing. While the superstructure remains intact at these temperatures, an XRD rocking curve analysis indicates that annealing markedly decreases the density of grain boundaries. This indicates that the presumably epitaxial connections between QDs in as-synthesized superlattices still form a major source of grain boundaries and defects, to an extend that carrier delocalization over multiple QDs is prevented and dot-to-dot transport remains strongly restricted.

NCFun S3.1
Chair: Tianquan Lian
09:30 - 10:00
S3.1-I1
Lian, Tianquan
Emory University
Exciton Structure and Dynamics in 2D Colloidal Quantum Wells
Tianquan Lian
Emory University, US

Tianquan (Tim) Lian received his PhD degree from University of Pennsylvania (under the supervision of Prof. Robin Hochstrasser) in 1993. After postdoctoral training with Prof. Charles B. Harris in the University of California at Berkeley, Tim Lian joined the faculty of chemistry department at Emory University in 1996. He was promoted to associate professor in 2002, full professor in 2005, Winship distinguished research Professor in 2007, and William Henry Emerson Professor of Chemistry in 2008. Tim Lian is a recipient of the NSF CAREER award and the Alfred P. Sloan fellowship. Tim Lian research interest is focused on ultrafast dynamics in photovoltaic and photocatalytic nanomaterials and at their interfaces.

Authors
Tianquan Lian a
Affiliations
a, Department of Chemistry, Emory University, 1515 Dickey Drive NE, Atlanta, Georgia 30322, USA
Abstract

Cadmium chalcogenide (CdX, X=Se, S, Te) colloidal quantum wells or nanoplatelet (NPLs) have atomically precise thickness of a few CdX layers (1-2 nm) and uniform exciton confinement energy across lateral dimensions of 10s of nanometer and larger. This unique property has led to speculation of coherent delocalization of the exciton center-of-mass over the entire NPL, which would have profound effect on fundamental properties of excitons (including its oscillator, transport mechanism and Auger annihilation) and their applications (such as lasing threshold). In this talk, I will summarize a series of recent studies on fundamental exciton properties in 2D NPLs. We show that at room temperature, exciton transport can be described by 2D diffusion with diffusion constants near the bulk crystal values and hot exciton diffusion completes with exciton relaxation. In CdSe NPLs, the biexciton Auger recombination lifetime does not depend linear on its volume, deviating from the “Universal Volume” scaling law that has been reported for 0D quantum dots. Instead, the Auger lifetime scales linearly with the lateral size because of the size dependent collision frequency, and the Auger lifetime depends sensitively (nonlinearly) on the NPL thickness due to change in the degree of quantum confinement. We suggest Auger lifetime in for other 2D NPLs and 1D nanorods can also be expected to deviate from the volume scaling law because of the different dependences on the quantum confined and non-confined dimensions. We have developed an optical gain model that accounts for the nature of 1D confinement in NPLs and revealed the origin of low gain thresholds in these materials. Finally, we will also discuss conditions under which coherent delocalization of excitons and the Giant Oscillator Strength Transition (GOST) effect can be observed.

10:00 - 10:30
S3.1-I2
Banin, Uri
Hebrew University of Jerusalem
Hybrid Semiconductor-Metal Nanoparticles as Photocatalysts
Uri Banin
Hebrew University of Jerusalem, IL

Professor Uri Banin is the incumbent of the Larisch Memorial Chair at the Institute of Chemistry and the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem (HU). Dr. Banin was the founding director of the Harvey M. Kreuger Family Center for Nanoscience and Nanotechnology (2001-2010) and led the program of the Israel National Nanotechnology Initiative at HU (2007-2010). He served on the University’s Executive Committee and on its board of managers and was a member of the board of Yissum. He served on the scientific advisory board of Nanosys. In 2009 Banin was the scientific founder of Qlight Nanotech, a start-up company based on his inventions, developing the use of nanocrystals in display and lighting applications. Since 2013, Banin is an Associate Editor of the journal Nano Letters. His distinctions include the Rothschild and Fulbright postdoctoral fellowships (1994-1995), the Alon fellowship for young faculty (1997-2000), the Yoram Ben-Porat prize (2000), the Israel Chemical Society young scientist award (2001), the Michael Bruno Memorial Award (2007-2010), and the Tenne Family prize for nanoscale science (2012). He received two European Research Council (ERC) advanced investigator grant, project DCENSY (2010-2015), and project CoupledNC (2017-2022). Banin’s research focuses on nanoscience and nanotechnology of nanocrystals and he authored over 180 scientific publications in this field that have been extensively cited.

Authors
Uri Banin a
Affiliations
a, Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904
Abstract

Hybrid nanoparticles (HNPs) combine disparate materials onto a single nanosystem thus providing a powerful approach for bottom-up design of novel architectures. Beyond the fundamental development in synthesis, the interest in HNPs arises from their combined and often synergetic properties exceeding the functionality of the individual components. These ideas are well demonstrated in hybrid semiconductor-metal nanoparticles, which are the focus of this talk. The synergistic optical and chemical properties of hybrid nanoparticles resulting in light-induced charge separation and charge transfer, allow photocatalytic activity which can promote surface chemistry redox reactions, and open a pathway for converting solar energy to chemical energy stored in a fuel. An additional area of interest is in use of the HNPs for light-induced generation of radicals opening options for light-induced on-demand radicals formation.

We will report on the effects of the surface coating and the co-catalyst metal size on the photocatalytic function of metal tipped semiconductor nanorods as a model hybrid nanoparticle system. Both tested parameters were found to influence the photocatalytic efficiency and charge transfer dynamics. The work combines advances in synthesis of well-controlled hybrid nanoparticles, hydrogen evolution efficiency measurements, steady state and time resolved emission measurements, as well as ultrafast transient absorption measurements to gain a complete view on the effects of these parameters on photocatalysis with metal tipped semiconductor nanorods. A model was devised to capture the essential effects of the size of the metal tip on the photoctalytic efficiency. An additional effect concerns the HNPs functionality under high excitation fluence in the regime of multiexcitons. The understanding of the effects of the hybrid nanosystems properties on the photocatalytic processes contribute to the rational design of hybrid nanostructures in photocatalytic applications. Moreover, use of the HNPs in generation of reactive hydrogen species and its application for controlling enzymatic activity and additional processes will also be highlighted.

PVCon S9.1
Chair: Joaquim Puigdollers
09:30 - 10:00
S9.1-I1
Martorell, Jordi
ICFO - Institut de Ciències Fotòniques
Novel Photonic Approaches to Enhance the Current or Voltage in Organic and Perovskite Solar Cells
Jordi Martorell
ICFO - Institut de Ciències Fotòniques, ES
Authors
Jordi Martorell a
Affiliations
a, ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Av. Carl Friedrich Gauss, num. 3, Castelldefels (Barcelona), 8860, ES
Abstract

A considerable portion of photonics based research to enhance thin film solar cell performance considers the incorporation of plasmonic nano-particles to enhance light absorption and consequently increase the short circuit current. This approach, which has produced some interesting results, suffers from, essentially, two shortcomings: It has a positive effect in only one of the three photovoltaic parameters that determine the power conversion efficiency (PEC), while it may also have a negative impact on the electronic performance of the device. In the work we present, we will discuss several novel photonic approaches, having a minimal impact on electronic aspects of the solar cell not directly linked to light absorption or emission, capable of enhancing either the short circuit current or open circuit voltage for organic and perovskite solar cells.

In one configuration the standard ITO based transparent electrode is replaced by a 1-dimensional multilayer containing, at least, a TiO2 and a Ag layers [1]. This combination forms with the back metal electrode in thin film cells two coupled optical cavities with a resonance degeneracy which can be broken to produce a broadband light trapping. When applied to non-fullerene acceptor based single junction polymer cells we show that PECs close to 14% can be reached.

In a perovskite cell, by naturally transferring the random nano-texturing inherent of the perovskite layer to the back semiconductor/metal interface, where the contrast in the imaginary part of the refractive index is very large, we demonstrate that backscattering reduces light escape leading to an optimal light absorption bringing the PCE from 19.3% to 19.8%. Such path towards an ergodic behavior for maximum light absorption in perovskite cells may lead to the most effective light absorption in such cells [2].

To bring perovskite solar cells towards the Shockley-Queisser limit requires lowering the bandgap while simultaneously increasing the open circuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface non-radiative recombination. We successfully incorporated such large cations in larger than 1 μm perovskite crystals and fabricated cells that exhibited a largely increased fluorescence quantum yield and an open circuit voltage equivalent to 93% of the corresponding radiative limit one.

1 Quan Liu et al., Adv. Energy Mater. 7, Art. No. 1700356 (2017).

2 Hui Zhang et al., ACS Photonics (2018).

10:00 - 10:30
S9.1-O1
Chaturvedi, Neha
King Abdullah University of Science and Technology (KAUST), Division of Physical Sciences and Engineering, Thuwal 23955-6900, Kingdom of Saudi Arabia
High Speed Coating Method for Fabricating Organic Solar Cells with PCE>10%
Neha Chaturvedi
King Abdullah University of Science and Technology (KAUST), Division of Physical Sciences and Engineering, Thuwal 23955-6900, Kingdom of Saudi Arabia
Authors
NEHA CHATURVEDI a, Hanlin Hu a, Nicola gasparini a, Derya Baran a, Aram Amassian a, Iain MuCulloch a
Affiliations
a, King Abdullah University of Science and Technology (KAUST), Division of Physical Sciences and Engineering, Thuwal 23955-6900, Kingdom of Saudi Arabia
Abstract

Organic semiconductors based on conjugated polymers and fullerene acceptors are a large class of materials that have been broadly explored for various applications that allow printed, flexible, stretchable, and large-area electronics like organic field-effect transistors (OFETs) and bulk-heterojunction organic photovoltaics (OPVs). The commercialization of large area photovoltaic devices relies on the capability of coating thick active layers in ambient condition without losing the efficiency. After surpassing the 13% threshold, OPVs becomes a promising approach in the field of thin film photovoltaic technology  but it is limited to thin film only. Till now the performance of scalable printed solar cells is quite low as compared to spin coated devices. In OPVs the photoactive layer, which consists of donor and acceptor materials plays an important role. Initially, the focus of OPV research was limited to fullerene based acceptor PC61BM and PC71BM ([6,6]-phenyl C61-/C71-butyric acid methyl ester). The fullerene based acceptors have some limitations to achieve high efficiency like low absorption coefficient and narrow visible absorption window limiting the light-to-current generation, thus inhibiting the improvement of device performance . From the last few years, non - fullerene acceptors (NFAs) have emerged as a new concept to overcome the limitations associated with fullerene based acceptors.

In this work, we used blend of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2 carboxylate-2-6-diyl)] (PBDTTT-EFT, or more commonly PCE10) : PC71BM (fullerene based) and PBDTTT-EFT: EH-IDTBR (NFA) as an active layer material. We used the wire-bar (WB) coating as well as spin coating (SC) to deposit the active layer of PCE10:PC71BM and PCE10: EH-IDTBR . Thicker active layer (>100 nm) devices based on WB coated shows good performance as compared to devices based on thicker active layer coated by SC. Comparison of the film properties as well as device performance has been carried out when we change the process form lab to scalable (SC to WB) technique. Solar cell (ITO/ZnO/PCE10:PC71BM/MoO3/Ag) based on WB coated PCE10:PC71BM results the PCE of 10.22 % comparable to the device based on SC PCE10:PC71BM with PCE of 10.10%. Devices based on NFAs (ITO/ZnO/PCE10: EH-IDTBR/MoO3/Ag) shows comparatively good performance with PCE of 10.77% for WB coated and PCE of 10.60% for SC devices. Reported WB coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost and high-efficiency OSCs by high-throughput production.

PerFun S7.1
Chair: Matthew C. Beard
09:30 - 10:00
S7.1-I1
Cahen, David
Weizmann Institute of Science
What Remains Special about Halide Perovskites?
David Cahen
Weizmann Institute of Science, 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 of Science & Bar Ilan University
Abstract

Halide Perovskites may be mostly normal (inorganic) semiconductors and, yes, we should be careful to describe them with concepts from organic electronics. HOWEVER, it is remarkable that a material with over-all high quality optoelectronic properties can result from fast, low temperature, solution preparation. Understanding the reason(s) behind this, may help answer the question if this is because Pb is so unique or if we can generalize to find other materials like these. I will consider apparent inconsistencies in what we think we know about their defects, to zoom in on what remains special, and maybe even unique.

work done with Gary Hodes (Weizmann) and many others

Halide Perovskites may be mostly normal (inorganic) semiconductors and, yes, we should be careful to describe them with concepts from organic electronics. HOWEVER, it is remarkable that a material with over-all high quality optoelectronic properties can result from fast, low temperature, solution preparation. Understanding the reason(s) behind this, may help answer the question if this is because Pb is so unique or if we can generalize to find other materials like these. I will consider apparent inconsistencies in what we think we know about their defects, to zoom in on what remains special, and maybe even unique.

10:00 - 10:30
S7.1-I2
Lovrincic, Robert
TU Braunschweig
Compositional Doping of Lead Halide Perovskites
Robert Lovrincic
TU Braunschweig, DE
Robert Lovrincic obtained his PhD in physics from Heidelberg University in 2009. In 2010 he joined the group of Prof. David Cahen at the Weizmann Institute of Science (Israel) as a Minerva and Marie-Curie fellow. He has been a group leader at the Innovationlab in Heidelberg since 2013. His main expertise is the vibrational and structural characterization of electronic materials.
Authors
Robert Lovrincic a, b
Affiliations
a, Institute for High Frequency Technology, TU Braunschweig, Speyerer Str. 4, Heidelberg, 69115, DE
b, InnovationLab, Heidelberg
Abstract

Impressive photovoltaic conversion efficiencies have been achieved within a very short time span with solar cells based on metal halide perovskites. Such a development is especially remarkable given the low temperature preparation methods used for these materials.1 It is generally accepted that defects must be “benign” and thus allow for high-quality devices despite low temperature synthesis. Relatedly, doping in halide perovskites has been achieved by intrinsic defects, however with very limited control over doping levels and spatial profiles.

We will discuss how the soft nature of halide perovskites contributes to the unusual optoelectronic properties.2 For doping via intrinsic defects, we developed a procedure to control and determine the exact stoichiometry of halide perovskite films from infrared spectroscopy, which allows us to quantify the MA content in the films. We present the first perovskite-perovskite homojunction obtained by vacuum deposition of stoichiometrically-tuned methylammonium lead iodide films and devices based thereon.3 We analyzed the resulting thin film junctions by cross-sectional scanning Kelvin probe microscopy, and found a pronounced contact potential difference at the interface between the two differently doped perovskite layers.

 

[1] David Egger, Achintya Bera, David Cahen, Gary Hodes, Thomas Kirchartz, Leeor Kronik, Robert Lovrincic, Andrew Rappe, David Reichman, and Omer Yaffe. What Remains Unexplained about the Properties of Halide Perovskites? Advanced Materials, 30 (20):800691, 2018.

[2] Michael Sendner, Pabitra K. Nayak, David A. Egger, Sebastian Beck, Christian Müller, Bernd Epding, Wolfgang Kowalsky, Leeor Kronik, Henry J. Snaith, Annemarie Pucci, and Robert Lovrincic. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Materials Horizons, 3:613–620, 2016.

[3] Benedikt Dänekamp, Christian Müller, Michael Sendner, Pablo P. Boix, Michele Sessolo, Robert Lovrincic, and Henk Bolink. Perovskite-Perovskite homojunctions via Compositional Doping. The Journal of Physical Chemistry Letters, 9:2770, 2018.

PerMod S8.1
Chair: Alessio Gagliardi
09:30 - 10:00
S8.1-I1
Ruhstaller, Beat
ZHAW Institute of Computational Physics
Electronic, Ionic and Optical Perovskite Solar Cell Modeling and Experimental Validation
Beat Ruhstaller
ZHAW Institute of Computational Physics, CH

Prof. Dr. Beat Ruhstaller is founder of Fluxim and lecturer at the Zurich University of Applied Sciences ZHAW in Winterthur, Switzerland. After a Diploma in Physics from ETH Zürich he obtained his PhD in Physics at the University of California, Santa Cruz (USA), in 2000. He was a postdoc at the IBM Zurich Research Laboratory in the display technology group before joining ZHAW, where he headed the Institute of Computational Physics from 2007 to 2010. In 2006 he founded Fluxim which he has managed as CEO since 2011. Fluxim has successfully brought R&D tool innovations from the lab to the OLED and solar cell market. He has been performing research on both optical, electronic and thermal processes in light-emitting and light-harvesting (organic) semiconductor devices.

Authors
Beat Ruhstaller a, b, Evelyne Knapp a, Martin Neukom a, b, Andreas Schiller a, b, Stéphane Altazin b
Affiliations
a, Zurich Univ. of Appl. Sciences (ZHAW), Inst. of Computational Physics
b, Fluxim Inc.
Abstract

Perovskite cells pose an intriguing modelling challenge as the electrical cell properties are governed by both electronic and ionic charge transport and the optical cell properties need to be carefully optimized when seeking record efficiencies in tandem cell configurations with silicon wafer cells. In this contribution, we give an update on recent advances in both electrical and optical modeling and discuss experimental results.

Negative capacitance and inductive loops in impedance spectroscopy in perovskite solar cells have been described in several recent reports, though their origin remained unclear so far. The negative capacitance and inductive loop may be related to one another as they appear in the same samples but at different applied biases. Similarly, we have demonstrated that ion migration is present even in high-efficiency low-hysteresis perovskite cells [1]. We shed light on the likely physical mechanisms behind these observations and compare devices in the frequency domain at different applied bias by employing a mixed electronic-ionic device model that naturally produces inductive loops and negative capacitance allowing us to study correlations with relevant material parameters.

Moreover we present an optical model implemented in the software SETFOS 4.6 [2] for simulating perovskite/silicon monolithic tandem solar cells that exploit light scattering structures [3]. We validate the model with experimental data of tandem solar cells that either use front- or rear-side textures. The software is used to investigate the potential of different monolithic tandem structures. The p-i-n solar cell architecture is the most promising with respect to achievable photocurrent for both flat and textured wafers. Finally, cesium-formamidinium-based perovskite materials with several bandgaps were synthetized, optically characterized [4] and their potential in tandem devices was quantified by simulations. The most promising tandem has a potential of reaching a power conversion efficiency of 31% [5].

[1] M. Neukom, S. Züfle, E. Knapp, M. Makha, R. Hany, B. Ruhstaller, Solar En. Mat. & Solar Cells, 169 159ff (2017)

[2] T. Lanz, B. Ruhstaller, C. Battaglia, and C. Ballif, J. Appl. Phys. 110, 33111 (2011) and SETFOS 4.6 by Fluxim AG, https://www.fluxim.com, Switzerland

[3] S. Altazin, L. Stepanova, K. Lapagna, P. Losio, J. Werner, B. Niesen, A. Dabirian, M. Morales-Masis, S. de Wolf, C. Ballif, B. Ruhstaller, Proc. 32nd Eur. Photovolt. Sol. Energy Conf. 1276 (2016)

[4] J. Werner et al., ACS Energy Lett. 3, 742–747 (2018)

[5] S. Altazin, L. Stepanova, J. Werner, B. Niesen, C. Ballif, and B. Ruhstaller, Optics Express 26 (10), A579 (2018)

10:00 - 10:30
S8.1-I2
Anta, Juan A.
Universidad Pablo de Olavide
Modeling and Interpretation of Small Perturbation Measurements in Perovskite Solar Cells
Juan A. Anta
Universidad Pablo de Olavide, ES

Juan A. Anta is Full Professor of Physical Chemistry at the University Pablo de Olavide, Seville, Spain. He obtained a BA in Chemistry in the Universidad Complutense of Madrid, Spain and carried out his PhD research at the Physical Chemistry Institut of the National Research Council of Spain. In 1997 and 1998 he was a postdoctoral fellow in the Department of Theoretical Chemistry of the University of Oxford and from mid 1999 to mid 2000 he was research assistant at the Department of Chemistry of the Imperial College, London. His research focuses on solar cell modelling, random-walk methods applied to electron transport in nanostructured devices and disordered semiconductors, and device modeling in Dye-sensitised and perovskite solar cells

Authors
Juan A. Anta a
Affiliations
a, Área de Química Física, Universidad Pablo de Olavide, E-41013 Sevilla, Spain
Abstract

Metal halide perovskites are mixed ionic-electronic conductors extremely efficient for making solar cells, due to its strong absorption in the visible and their relatively slow recombination. Processes like transport, recombination, charge accumulation, hysteresis, etc. occur at very different time scales and determine the photovoltaic performance of the solar cell. Small-perturbation, frequency-modulated optoelectronic techniques such as impedance spectroscopy (IS) or intensity-modulated photocurrent spectroscopies (IMPS/IMVS) are especially suited to detect, deconvolute and quantify all these processes.

 

Nowadays, a full understanding of the typical features observed in the IS and IMPS/IMVS spectra in perovskite solar cells is still missing. Hence, it is not clear yet how properties like the magnitude, locus and the nature of the recombination loss can be identified or quantified from the analysis of the data. Besides, low frequency signals, associated to hysteresis phenomena should also be unambiguously assessed in the spectra. In this talk I discuss the interpretation of the spectra and the use of simple models to rationalize the small-perturbation response of the device and its impact on efficiency-determining dynamic processes.

SolFuel S1.1
Chair: Kevin Sivula
09:30 - 10:00
S1.1-I1
Grimaud, Alexis
College de France
Oxygen Evolution Reaction on the Surface of Transition Metal Oxides – Heterogeneous or Homogeneous catalysis?
Alexis Grimaud
College de France, FR
Authors
Alexis Grimaud a, b
Affiliations
a, Chimie du Solide et de l’Energie, FRE 3677, Collège de France, 75231 Paris Cedex 05, France
b, Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 80039 Amiens Cedex, France
Abstract

The need for better energy storage and conversion devices has never been so urgent in order to enable the rapid deployment of renewable energies and reduce our use of fossil fuels. While batteries spurred the spread of portable electronics, their limited energy density hampers their use for large scale applications. Hydrogen has long been envisioned as a viable energy carrier owing to its very large energy density, nevertheless its electrochemical production by electrolysis, the most efficient carbon-free production route at large scale, greatly suffers from the slow kinetics associated with the oxygen evolution reaction (OER). The key challenges that need to be addressed to improve the OER kinetics are well-spotted and researchers eagerly pushed to better understand the reaction leading to numerous progresses since our early vision of this reaction. Hence, while some works were devoted to finding physical descriptors capable of describing the OER activity following a Sabatier principle, recent developments in the field point towards the complexity of such reaction. Indeed, a substantial body of evidence now points towards the involvement of the bulk chemistry of the most active transition metal oxides in the OER mechanism. Hence, we recently found that bulk oxygen atoms are evolved under OER conditions for cobalt-based perovskites materials,1 eventually triggering a new mechanism into which chemical steps and proton exchange are rate limiting. We could then demonstrate that the origin for the often observed activity-stability relationship for OER electrocatalysts is nested into the existence of a common intermediate which is reactive surface oxygen in the form of oxyl-group.2 Overall, the line becomes blurrier between heterogeneous and homogeneous catalysis when using transition metal oxides as OER catalysts for which complex surface dynamics are at play.3 Hence, efforts must be paid to understanding and stabilizing these intermediates in order to break this relationship and further enhance the stability of OER electrocatalysts. We will therefore discuss in this talk our recent efforts at designing chemical approach to counterbalance the chemical reactivity of the most active transition metal oxides through a combined crystallographic and electrolyte engineering approach.

10:00 - 10:15
S1.1-O1
Kaiser, Bernhard
TU Darmstadt
Nickeloxide Nanoparticles and Thin Films as Catalysts for (Photo)Electrochemical Water Splitting: A Surface Science Study
Bernhard Kaiser
TU Darmstadt, DE

born

Authors
Shasha Tao a, Stephan Wagner a, b, Hannes Radinger a, b, Sven Tengeler a, b, Wolfram Jaegermann a, b, Bernhard Kaiser a, b
Affiliations
a, Institute of Material Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany
b, Graduate School of Energy Science and Engineering, Technische Universität Darmstadt, 64287 Darmstadt, Germany
Abstract

The increasing replacement of fossil fuels by renewable energies from wind and sun requires large energy storage capabilities, because of the only intermittent availability of these sources. Such big capacities can only be provided by the storage in chemical bonds, the simplest being the hydrogen molecule formed by direct electrochemical water splitting. For this purpose, expensive and rare electrocatalysts made from Platinum series metals will have to be replaced by cheaper, more abundant and hazard-free materials. Nickel metal and its oxides are known since many years for their good activity for water splitting.

In our studies Nickeloxide nanoparticles and thin films are prepared by electrochemical deposition as well as by magnetron sputtering. The composition is optimised towards the achievement of an optimum activity for the hydrogen evolution reaction (HER) as well as the oxygen evolution reaction (OER). In order to gain a deeper understanding of the critical parameters for the catalytic activity during the electrochemical reaction, we investigate the chemical composition by XPS and SEM before and after electrochemical testing as well as by in-operando Raman-spectroscopy. The achieved activities are comparable to Platinum for the HER and to RuO2 for the OER. They depend strongly on the chemical composition of the catalyst, which in turn is heavily influenced by the chosen preparation method. Furthermore, we investigate the stability of our catalysts in relation to their activity and composition. The highly active Nickel (oxide/hydroxide) mixture for the HER degrades over time by the complete transformation to the less active pure Nickel-dihydroxide compound. For the OER the pre-treatment of the Nickel compound is of extreme importance in order to form a large amount of the catalytically active NiO(OH) species on the surface during the electrochemical reaction. These nanoparticles show a nearly constant activity for a testing period of 26 hours at a current density of 10 mA/cm2.

10:15 - 10:30
S1.1-O2
Francàs Forcada, Laia
Imperial College London
Spectroelectrochemical Study of the Catalytic Species on the Ni(Fe)OOH and FeOOH Electrocatalysts
Laia Francàs Forcada
Imperial College London, GB
Authors
Laia Francas a, Sacha Corby a, Shababa Selim a, Dongho Lee b, Mesa Camilo a, Robert Godin a, Kyoung Shin-Choi b, James Durrant a
Affiliations
a, Imperial College London, Department of Chemistry, South Kensington Campus, London, SW7 2AZ
b, Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706, USA
Abstract

The study of electrocatalysts for water splitting is pivotal to improving the efficiency of fuel production. This is an attractive field since electrocatalysts can be used: (1) as part of a photoelectrode, for direct sunlight to fuel transformation, or (2) can be used in an electrolyser to transform the electricity generated through a solar panel to produce a fuel. In a complete system it is thought that water oxidation is the limiting process. Therefore, a lot of effort is devoted to finding a better catalyst for this reaction in order to enhance the final performance of the system.

Recently, iron and nickel oxyhydroxides have been appointed as good candidates for the water oxidation reaction in basic conditions. In this context, interest for understanding the origin of their behaviour has grown in recent years. In 2014, Boettcher and co-workers published for the first time that the high activity towards water oxidation of NiOOH was due to Fe incorporation from the electrolyte.[1] Since then, there has been a lot of effort to determine which species are involved in the catalytic reaction, with no consensus reached. Some experiments suggest the presence of Fe(IV) during the catalysis,[2] while on the contrary, other works cannot detect such iron species and suggest that nickel centres are the active sites.[3] Despite this debate, some structural differences have been found due to iron incorporation which leads to an improved performance. However, the mechanistic and kinetic analysis of metal oxide based electrocatalysts, in general, is hampered by the non-ideal nature of these materials. Most of these electrocatalysts present different redox states and a non-planar and dense structure. Thus the interpretation of the traditional electrochemical techniques, such as tafel plots, is more complicated and sometimes not possible.[4]

In this work, we used spectroelectrochemical methods to analyse three different samples: Pure FeOOH, a mixed FeOOHNiOOH sample composed of different layers, and finally Ni(Fe)OOH with spontaneous Fe incorporation. From these measurements, we are able to determine the active species and study the kinetics under catalytic conditions to yield a basic mechanistic picture.

References

[1]          L. Trotochaud, et al. JACS, 2014, 136, 6744-6753.

[2]          J. Y. C. Chen, et al. JACS, 2015, 137, 15090-15093.

[3]          M. Görlin, et at, JACS, 2017, 139, 2070-2082.

[4]          E. Pastor, et al, Nat. Commun., 2017, 8, 14280.

 

10:30 - 11:00
Coffee Break
Dyman S5.2
Chair: Vanessa Wood
11:00 - 11:30
S5.2-O1
Montanarella, Federico
Universiteit Utrecht
Reversible Charge Carrier Trapping Slows Down Förster Energy Transfer in CdSe/CdS Quantum-Dot Solids
Federico Montanarella
Universiteit Utrecht, NL
Authors
Federico Montanarella a, Margherita Biondi a, Stijn Hinterding a, Daniel Vanmaekelbergh a, Freddy Rabouw a
Affiliations
a, Universiteit Utrecht, Princetonplein 1, Utrecht, 3584, NL
Abstract

The dynamics of photoluminescence (PL) from nanocrystal quantum dots (QDs) is significantly affected by reversible trapping of photo-excited charge carriers. This process occurs after up to 50% of the absorption events, depending on the type of QD considered, and can extend the time between photo-excitation and relaxation of the QD by orders of magnitude. Although many opto-electronic applications require QDs assembled into a QD solid, until now reversible trapping has been studied only in (ensembles of) spatially separated QDs. Here, we study the influence of reversible trapping on the excited-state dynamics of CdSe/CdS core/shell QDs when they are assembled into close-packed “supraparticles”. Time- and spectrally resolved PL measurements reveal competition between spontaneous emission, reversible charge carrier trapping, and Förster resonance energy transfer between the QDs. While Förster transfer causes the PL to redshift over the first 20–50 ns after excitation, reversible trapping stops and even reverses this trend at later times. We can model this behavior with a simple kinetic Monte Carlo simulation by considering that charge carrier trapping leaves the QDs in a state with zero oscillator strength from which no energy transfer can occur. Our results highlight that reversible trapping significantly affects the energy and charge carrier dynamics for applications where QDs are assembled into a QD solid.

11:30 - 12:00
S5.2-I1
Boehme, Simon
Vrije Universiteit Amsterdam
Size- and Temperature- Dependent Hot Carrier Cooling in CsPbBr3 Nanocrystals
Simon Boehme
Vrije Universiteit Amsterdam, NL
Authors
Ivan Infante a, Simon Boehme a
Affiliations
a, Department of Chemistry and Pharmaceutical Sciences, Division of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
Abstract

Hot carriers refer to electrons (holes) that are formed from the thermalization of non-equilibrium photoexcited carrier populations following the absorption of above-bandgap photons. These hot carriers (HCs) later equilibrate within few picoseconds with the semiconductor lattice through carrier cooling processes such as carrier phonon scattering, Auger process, etc. The mechanisms and dynamics of HC cooling in semiconductors are of fundamental importance for enhancing device functionalities. Colloidal lead halide perovskite nanocrystals recently emerged as promising candidate materials for many optoelectronic applications. Building efficient and long-lasting devices from perovskite nanocrystals however remains a challenge. In this work, we study the size and temperature dependent cooling process of HC in CsPbBr3 nanocrystals using time-domain density functional theory. We provide detailed insights into the mechanism of cooling by analyzing the effect of the passivating ligands (alkyl ammonium) in this process. We demonstrate that the inorganic lattice plays a much larger role in mediating the thermalization to the band edge.

12:00 - 12:30
S5.2-I2
Beard, Matthew
National Renewable Energy Laboratory,
Controlling the Properties of Colloidal Quantum Dots for Solar Energy Applications
Matthew Beard
National Renewable Energy Laboratory,, US
Authors
Matthew Beard a
Affiliations
a, National Renewable Energy Laboratory, Golden, Colorado, USA
Abstract

Colloidal semiconductor nanocrystals, specifically quantum dots (QDs), are of interest to numerous scientific disciplines due to their highly tunable optical and electronic properties. For many years, various chemical treatments have been developed to fabricate conductive QD arrays for use in solar cells.  While such investigations have lead to increased solar cell performance there is much about how the chemical treatments modify the optical and electrical properties that are not well understood.  We have developed a simple, robust, and scalable solution-phase X-type ligand exchange method for PbS QDs that replaces native surface ligands with functionalized cinnamate ligands, yielding highly tunable, well-defined organic/inorganic hybrid chemical systems. We explore a library of functionalized cinnamic acid molecules to systematically tune PbS QD surface chemistry, and find that thin films of fully ligand exchanged QDs exhibit remarkable band edge shifts: the band edge position of QDs can be tuned over 2.0 eV.  We have also developed simple methods to impurity dope PbS and PbSe QDs with both n and p-type dopants.  We show how the dopants are incorporated and result in doped QDs with well behaved properties.   

NCFun S3.2
Chair: Tianquan Lian
11:00 - 11:30
S3.2-I1
Vanmaekelbergh, Daniel
Universiteit Utrecht
Delayed Exciton Emission in Low-Dimensional CdSe Nanocrystals
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, Freddy Rabouw a, Federico Montanarella a
Affiliations
a, Universiteit Utrecht, Princetonplein 1, Utrecht, 3584, NL
Abstract

Delayed exciton emission has been extensively studied in several types of low dimensional semiconductors but is still not fully understood. The photon energy of delayed emission is identical to that of the spontaneous exciton emission, but it occurs on an extended time scale, from the life time for spontaneous emission (typical 20 ns) to the microsecond and even millisecond regime. In a heuristic way, we might say that the exciton is stored as non-radiative state in the nanocrystal for a variable time without (much) energy loss. The stored energy finally returns to the radiative state and emission can occur.

I will review our work on the characterization of delayed emission of low dimensional CdSe systems (core-shell quantum dots, rods, and platelets) which was performed on the ensemble level as well as on the single-dot level. I will also relate the phenomenon of delayed emission to another extensively studied phenomenon in these systems, namely blinking. In another contribution from our group the interplay between spontaneous emission, delayed emission and energy transfer as observed in QD supraparticles will be presented.

Reduced Auger Recombination in Single CdSe/CdS Nanorods by One-Dimensional Electron Delocalization." Nano Letters 13(10): 4884-4892.

Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots." Nano Letters 15(11): 7718-7725.

Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots." Acs Nano 9(10): 10366-10376.

Temporary Charge Carrier Separation Dominates the Photoluminescence Decay Dynamics of Colloidal CdSe Nanoplatelets." Nano Letters 16(3): 2047-2053.

Composite Supraparticles with Tunable Light Emission." Acs Nano 11(9): 9136-9142.

11:30 - 12:00
S3.2-I2
Beard, Matthew
National Renewable Energy Laboratory,
Multiple Exciton Generation in Semiconductor Nanocrytals
Matthew Beard
National Renewable Energy Laboratory,, US
Authors
Matthew Beard a
Affiliations
a, National Renewable Energy Laboratory, Golden, Colorado, USA
Abstract

Generating multiple excitons by a single high-energy photon is a promising third generation solar energy conversion strategy.  I will discuss recent progress within the Center for Advanced Solar Photophysics (CASP) on both improving the multple exciton generation (MEG) efficiency in heterostructured nanostructures as well as in the using MEG in the production of solar fuels.  We are exploring MEG in PbE|CdE (E = S, Se) Janus-like hetero-nanostructures and find that MEG is enhanced over that of single-component and core/shell nanocrystal architectures. The enhanced MEG arrises due to the asymmetric nature of the hetero-nanostructure that results in an increase in the effective Coulomb interaction that drives MEG and a reduction of the competing hot exciton cooling rate. We find that slowed cooling occurs through effective trapping of hot-holes by a manifold of valence band interfacial states having character of both PbS and CdS. The Janus-like NCs retain their symmetric structure and thus can be easily incorporated as the main absorber layer in functional solid-state solar cell architectures. Finally, based upon our analysis, we provide design rules for the next generation of engineered nanocrystals to further improve the MEG characteristics. In addition, we have developed a PbS QD photoelectrochemical cell that is able to drive a hydrogen evolution reaction with a peak external quantum efficiency (EQE) of over 100%, with the highest EQE at 114±1.3%.  Our results show that the extra carriers produced via MEG can be used to drive a chemical reaction with above unity quantum efficiency thus demonstrating a new direction in exploring high efficiency approaches for solar fuels. I will discuss the potential for using MEG to drive a photochemical reaction as opposed to use in a solar cell. 

 

12:00 - 12:30
S3.2-O1
de Mello Donega, Celso
Universiteit Utrecht
NIR-Emitting CuInS2/ZnS Dot-in-Rod Colloidal Heteronanorods
Celso de Mello Donega
Universiteit Utrecht, NL

Celso de Mello Donega is an Associate Professor in the Chemistry Department of the Faculty of Sciences at Utrecht University in the Netherlands. His expertise is in the field of synthesis and optical spectroscopy of luminescent materials. His research is focused on the chemistry and optoelectronic properties of nanomaterials, with particular emphasis on colloidal nanocrystals and heteronanocrystals.

Authors
Celso de Mello Donega a, Chenghui Xia a, Naomi Winckelmans b, Hans Gerritsen a, Sara Bals b
Affiliations
a, Universiteit Utrecht, Princetonplein 1, Utrecht, 3584, NL
b, 2 EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Abstract

Ternary CuInX2 (X= S, Se) nanocrystals (NCs) have attracted increasing attention as promising alternatives for CdX and PbX NCs due to their low toxicity, large absorption cross-sections across a broad spectral range, and unparalleled photoluminescence tunability, spanning a spectral window that extends from the green to the NIR (~550 to ~1100 nm for X= S). To achieve properties that are inaccessible to single component NCs (such as high PL quantum yields, spatial charge carrier separation, etc.), researchers have been synthesizing colloidal CuInX2-based hetero-NCs (HNCs) (e.g., CuInS2/ZnS concentric core/shell HNCs, CuInSe2/CuInS2 dot-in-rod HNCs). Anisotropic CuInX2-based HNCs are particularly interesting, since they are expected to exhibit novel properties, such as polarized NIR photoluminescence (PL) and spatial charge separation, which are attractive for many applications (e.g., polarized LEDs, photocatalysis and artificial photosynthesis, luminescent solar concentrators, solar cells). Nevertheless, reports on the synthesis of anisotropic CuInX2-based HNCs are scarce. 

In this work, we report a novel two-step pathway that yields CuInS2/ZnS dot core/ rod shell heteronanorods. The wurtzite CuInS2 NCs used as seeds are obtained by cation exchange in template Cu2-xS NCs. The CuInS2 NC seeds are injected together with the S precursor into a hot solution of the Zn precursor and suitable coordinating ligands, which leads to heteroepitaxial growth of ZnS primarily on the cation-rich polar facet of the seeds, as demonstrated by high-angle annular dark-field scanning transmission electron microscopy and electron tomography. The colloidal wurtzite CuInS2/ZnS dot-in-rod heteronanorods have large molar extinction coefficients, and photoluminescence in the NIR (~800 nm) with PLQYs ~20%. Moreover, they exhibit multi-exponential PL decay that is initially rather fast (a few ns), and then slows down to several hundreds of ns, similar to the behavior previously reported for both chalcopyrite and wurtzite isotropic CuInS2/ZnS core/shell HNC, which has been attributed to radiative recombination of a conduction band electron with a hole localized at a Cu ion. The slow radiative recombination dynamics are potentially beneficial for photovoltaic and photocatalytic applications, since long carrier lifetimes are of great importance for effectively extracting charge carriers. 

PVCon S9.2
Chair: Jordi Martorell
11:00 - 11:30
S9.2-I1
Eich, Manfred
Hamburg University of Technology
Nanomaterials for High Temperature Photonics
Manfred Eich
Hamburg University of Technology, DE
Authors
Manfred Eich a, f, P. Dyachenko a, S. Lang a, G. Shang a, Q.Y. Nguyen b, M. Chirumamilla a, K. Knopp a, G. Vaidhyanathan f, S. Molesky e, H. Renner a, A. Yu Petrov a, e, Z. Jacob d, M. Störmer f, T. Krekeler g, M. Ritter g, G. Schneider b
Affiliations
a, Institute of Optical and Electronic Materials, Hamburg University of Technology, Eissendorfer Strasse 38, 21073 Hamburg, Germany
b, Institute of Advanced Ceramics, Hamburg University of Technology, Denickestrasse 15, 21073 Hamburg, Germany
c, University of Alberta, Department of Electrical and Computer Engineering, 9107 - 116 Street, T6G 2V4, Edmonton, Canada
d, Birck Nanotechnology Center, School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47906, USA
e, ITMO University, 49 Kronverkskii Ave., 197101, St. Petersburg, Russia
f, Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502 Geesthacht, Germany
g, Electron Microscopy Unit, Hamburg University of Technology, Eissendorfer Strasse 42, Hamburg 21073, Germany
Abstract

research results will be presented on nanomaterials as selective emitters and for near field radiative transfer for thermophotovoltaics and on tailored photonic glasses as non-iridescent structural colors.

of thermal radiation is a fundamental physical process defined by the dielectric properties of the thermally excited materials. Radiation into far field is described by Planck’s law and is limited by the blackbody emission. In near field, additional thermal energy transfer can be achieved due to evanescent fields, which are orders of magnitude larger than in far field. In the far field, emission, e.g. of long wavelengths below the energy of a semiconductor receiver band gap, can be suppressed in band edge emitters from nanostructured hyperbolic optical metamaterials as well as with resonantly coupled dielectric particle layers on top of plasmonic substrates. We demonstrate selective band edge emitters for thermophotovoltaic devices stable up to 1400°C based on W-HfO2 refractive metamaterials as well as ZrO2 based ceramic particles on tungsten. We further report on ceramic photonic structures as high-temperature compatible structural colors. A careful choice of the interplay between lattice and motif parameters of the photonic glass allows for structural colors with strong color saturation.

References

Shang, G.; Maiwald, L.; Renner, H.; Jalas, D.; Dosta, M.; Heinrich, S.; Petrov, A.; & Eich, M.; Photonic glass for high contrast structural color, Scientific Reports, 8, 7804 (2018)

Dyachenko, P.N.; Molesky, S.; Petrov, A.Y.; Stormer, M.; Krekeler, T.; Lang, S.; Ritter, M.; Jacob, Z.; and Eich, M.; Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions, Nature Communications, vol. 7, no. 11809, pp. 1–8, June 2016

Lang, S.; Sharma, G.; Molesky, S.; Kränzien, P.U.; Jalas, T.; Jacob, Z.; Petrov, A.Y.; and Eich, M.; Dynamic measurement of near-field radiative heat transfer, Scientific Reports, vol. 7, no. 1, p. 13916–13916, October 2017

Leib, E.W.; Pasquarelli, R.M.; do Rosario, J.J.; Dyachenko, P.N.; Doring, S.; Puchert, A.; Petrov, A.Y.; Eich, M.; Schneider, G.A.; Janssen, R.; Weller, H.; and Vossmeyer, T.; Yttria-stabilized zirconia microspheres: novel building blocks for high-temperature photonics, Journal of Materials Chemistry C, vol. 4, no. 1, pp. 62–74, January 2016

 

11:30 - 11:45
Abstract not programmed
11:45 - 12:00
S9.2-O1
Garín Escrivá, Moisés
Silicon Millefeuille: Multiplying Silicon Wafers for Ultrathin Photovoltaics.
Moisés Garín Escrivá
Authors
Moisés Garín a, Chen Jin a, Trifon Trifonov a, Ramón Alcubilla a
Affiliations
a, Universitat Politècnica de Catalunya, Calle Jordi Girona, 31, Barcelona, ES
Abstract

More than 30% of the production cost of commercial solar modules can be attributed to the cost of silicon and wafering. As a result, silicon wafers have reduced their thickness from 350 µm down to 180 µm. However, current sawing technology is reaching its inherent limits due to reduced yield and excessive kerf losses. Here we report on a method that can produce multiple free-standing ultra-thin (<20 µm) silicon layers from one silicon wafer and in a single technological process. This method, that we call “Silicon Millefeuille”, is based on the reorganization of close-packed arrays of pores under a high-temperature annealing in Ar:H ambient. The transformation process takes place in solid phase by surface diffusion, what results in the formation of high-quality monocrystalline thin layers that can later be peeled off into independent ultra-thin substrates. Pores are produced by electrochemical dissolution of n-type silicon in HF solution under back-side illumination. This allows to introduce an in-depth periodic modulation of the pore diameter that defines the number and thickness of the layers produced after the annealing. Furthermore, we show that the precise control of the initial in-depth pore profile has a profound impact on the pore reorganization dynamics, allowing to control the morphology of the thin layers obtained through annealing.

12:00 - 12:30
S9.2-I2
Pazos Outon, Luis
Electrical Engineering and Computer Sciences, University of California, Berkeley
Reaching 28% Efficiency in Thermo-Photovoltaics
Luis Pazos Outon
Electrical Engineering and Computer Sciences, University of California, Berkeley
Authors
Gregg Scranton a, b, Zunaid Omair a, b, Luis Pazos-Outón a, T. Patrick Xiao a, Vidya Ganapati c, Myles Steiner d, Per Peterson e, John Holzrichter f, Eli Yablonovitch a, b
Affiliations
a, Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, California 94720
b, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, Califor- nia 94720, United States
c, Swarthmore College, Swarthmore, Pennsylvania, USA
d, National Renewable Energy Laboratory, Golden, Colorado, USA
e, Department of Nuclear Engineering, University of California at Berkeley
f, Physical Insight Associates, Berkeley, California, USA
Abstract

Photovoltaic (PV) cells are efficient heat engines, converting incident photon energy to electrical energy. In the case of solar PV, the cell receives radiation from the Sun – which can be approximated as a black body at 5500oC – and converts part of the received radiation to electricity. In an ideal solar PV cell, the maximum energy conversion efficiency is ~33.5%, the famous Shockley-Queisser limit. Entropic losses, thermalization, and unused below-bandgap photons are the main limits of solar PV.

If instead of relying on the Sun, a local black body is used as the source of photons, many of the limitations mentioned can be minimized. This idea, known as thermo-photovoltaics (TPV), has been known since 1960. With a local thermal emitter at a suitable temperature these losses can be minimized and high efficiencies are attainable. Previous efforts have attempted to tune the emissivity spectrum of the emitter, minimizing the amount of below‑bandgap radiation reaching the photovoltaic cell.  This approach, however, bring some challenges due to the difficulty of developing a high quality spectral filter that remains stable at high temperatures.

We report on a thermo‑photovoltaic device that relies on the band‑edge of a photovoltaic absorber to spectrally filter the incoming radiation. Photons above the bandgap are converted to electricity, while the unused and unabsorbed photons below the bandgap are reflected by a ~94% reflective rear electrode and recovered by the source of radiation. Our system uses graphite as the blackbody emitter, and In0.55Ga0.45As (bandgap of 0.74eV) as the photovoltaic absorber.  For an emitter temperature of ~1200°C, we report a power conversion efficiency of 28.1%.

PerFun S7.2
Chair: Xing Guichuan
11:00 - 11:15
S7.2-O1
Crothers, Timothy
Department of Physics of University of Oxford
The Role of Photon Reabsorption in Masking Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3Perovskite.
Timothy Crothers
Department of Physics of University of Oxford, GB
Authors
Timothy Crothers a, Rebecca Milot a, Jay Patel a, Elizabeth Parrott a, Johannes Schlipf b, Peter Müller-Buschbaum b, Michael Johnston a, Laura Herz a
Affiliations
a, Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
b, Technische Universität München: Garching, Germany
Abstract

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant.

We have used optical-pump THz-probe spectroscopy to study the charge-carrier dynamics in a set of dual-source vapour-deposited CH3NH3PbI3 films whose thicknesses vary between 50 and 533 nm [1]. We find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10–10 cm3s–1 is common to all samples irrespective of film thickness [2].

We therefore postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

                                           

References:

1)  Crothers, Timothy W., et al. "Photon reabsorption masks intrinsic bimolecular charge-carrier recombination in CH3NH3PbI3 perovskite." Nano letters 17.9 (2017): 5782-5789.

2)  Davies, Christopher L., et al. "Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process." Nature communications 9.1 (2018): 293.

11:15 - 11:30
S7.2-O2
Jung, Young-Kwang
Yonsei University
Bond Formation and Carrier Confinement at Epitaxial Interface between PbS and CsPbBr3
Young-Kwang Jung
Yonsei University, KR
Authors
Young-Kwang Jung a, Keith T. Butler b, Aron Walsh a, c
Affiliations
a, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 120, KR
b, Lutherford Appleton Laboratory
c, Imperial College London, South Kensington, London,, GB
Abstract

Hybrid halide perovskites are being intensively studied as active layers in photovoltaic cells. They combine cost efficiency due to solution processability and high power conversion efficiency due to electrical transport properties and proper band gap. The chemical stability, which is often affected by quality of surfaces and interfaces, still remains a concern. Meanwhile, lead sulfide (PbS) is rocksalt-structured semiconductor with low band gap, which can be easily made as large single crystals. PbS is also well known for quantum dot material with high radiative efficiency. Recently, quantum-dot-in-perovskite crystals reported as promising optoelectronic material as well. Therefore, understanding the interface between lead sulfide and halide perovskite is important. In this study, we perform first-principles density-functional theory (DFT) calculations to investigate atomic contact properties (e.g. interface geometry) and electronic contact properties (e.g. charge redistribution and band offset). We focus on interface between PbS and CsPbBr3 where epitaxial interface between the both of materials with low lattice strain is possible. Our results predict spontaneous forming of CsPbBr3 - PbS interface is feasible, and consequentially this interface will have a type I band alignment.

11:30 - 11:45
S7.2-O3
Goñi, Alejandro Rodolfo
Pressure-Induced Locking of Methylammonium Cations Versus Amorphization in Hybrid Lead Iodide Perovskites
Alejandro Rodolfo Goñi
Authors
Adrián Francisco-López a, Bethan Charles b, Oliver J. Weber b, María 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, ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain
Abstract

The structural phase behavior of high quality single crystals of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) was revisited by combining Raman scattering and photoluminescence (PL) measurements under high hydrostatic pressure up to ca. 10 GPa. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4 GPa, 2.7 GPa and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in linewidth of the phonon modes of the inorganic cage, similar to the changes observed at the tetragonal-to-orthorhombic phase transition occurring at around 160 K but ambient pressure [1]. This behavior is interpreted as evidence for the locking of the organic cations in the cage voids above 2.7 GPa, due to the reduced volume and symmetry of the unit cell. At the third phase transition, reported here for the first time, the PL is greatly affected, whereas the Raman experiences only subtle changes related to a splitting of some of the peaks. This behavior may indicate a change mostly in the electronic structure with little effect on the crystal structure. Strikingly, no amorphization of the sample was observed up to the highest pressure which reached close to 10 GPa, in frank discrepancy with most of the high-pressure (x-ray) data of the literature [2], which established an onset of 3 GPa for the set in of an amorphous phase in MAPI3.

[1] Leguy, A.M.A. et al., Phys. Chem. Chem. Phys. 2016, 18, 27051–27066.
[2] Postorino, P. & Malavasi, L., J. Phys. Chem. Lett. 2017, 8, 2613–2622 and references therein.

11:45 - 12:00
S7.2-O4
Pydzinska, Katarzyna
Adam Mickiewicz University in Poznań
Transient Absorption of Perovskite Solar Cells. Differences in the Transient Features due to Synthesis Methods and Proper Determination of the Second Order Recombination Rate Constant
Katarzyna Pydzinska
Adam Mickiewicz University in Poznań
Authors
Katarzyna Pydzinska a, Jerzy Karolczak a, Janusz Szeremeta b, Konrad Wojciechowski b, Marcin Ziolek a
Affiliations
a, Faculty of Physics, Adam Mickiewicz University in Poznań, Umultowska 85, 61-614 Poznań, Poland
b, Saule Technologies, Mokotowska 1, Warsaw, 00-640, Warszawa, PL
Abstract

Perovskite solar cells have been extensively developed for few years but still the ultrafast and fast processes occurring in this system are not fully understood. The main tools providing wide view of a charge transport behavior are transient absorption and time-resolved emission spectroscopies. In the femtoseconds to nanoseconds time range two main transient absorption features dominate. The first is the long-wavelength band edge bleach assigned to a state filling and decaying in nanosecond range [1]. The second one is caused by cooling of charges, it takes place in hundreds of femtoseconds and usually has a shape of a band edge bleach first derivative [2].

Comparison of transient absorption signals for differently sensitized methylammonium lead iodide (MAPbI3) solar cells (non-stoichiometric and stoichiometric ratio of PbI2:MAI in precursor solution) will be presented. It was found that modification of the precursor ratio from non-stoichiometric to stoichiometric leads to the drastic changes of transient absorption signal from the typical strong long-wavelength band-edge bleach to the weak derivative-like signal due to the bandgap shift [3].

Transient absorption and emission measurements allows determination of charge injection kinetics for different charge transporting materials and recombination rates within perovskite [2]. Our studies of the hole injection from MAPbI3 to spiro-OMeTAD and its xanthene derivative X60 [4] as well as the recent results of electron injection from MAPbI3 to PCBM, PenPTC and SPPO13 will be presented. Moreover, proper determination of the first and the second order recombination rate constants in perovskite materials will be proposed. In particular, the influence of different transient absorption data treatment (band integral, global analysis based on singular value decomposition and bleach minimum amplitude) on the obtained rate constants will be shown.

 

Acknowledgements

The study was supported by Polish Ministry of Science and Higher Education under project 0019/DIA/2017/46.

 

 

[1]  J. Peng, Y. Chen, K. Zheng, T. Pullerits, Z.Liang, Chem. Soc. Rev., 2017, 46, 5714-5729.

[2] K. Pydzińska, J. Karolczak, I. Kosta, R. Tena-Zaera, A. Todinova, J. Idigoras, J.A. Anta, M. Ziółek, ChemSusChem, 2016, 9, 1547-1659.

[3] K. Pydzińska, J. Karolczak, M. Szafrański, M. Ziółek, RCS Advances, 2018, 8, 6479-6487.

[4] K. Pydzińska,  P. Florczak, G. Nowaczyk, M. Ziółek, Synthetic Metals, 2017, 232, 181-187.

12:00 - 12:15
S7.2-O5
Mingjie, Li
Nanyang Technological University
Slow Cooling and Highly Efficient Extraction of Hot-Carriers in Perovskite Nanocrystals: Towards Hot-Carrier Perovskite Solar Cell
Li Mingjie
Nanyang Technological University
Authors
Mingjie Li a, Tze Chien Sum a
Affiliations
a, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
Abstract

Thermodynamic calculations revealed that single junction solar cell conversion efficiencies can exceed the Shockley-Queisser limits and reach around 66% under 1-sun illumination if the excess energy of hot photogenerated carriers is utilized before they cool down to the lattice temperature (i.e., hot-carrier solar cells).1 Organic–inorganic lead halide perovskite semiconductors have recently emerged as the leading contender in low-cost high-performance solar cells.2,3 The key for the realization of hot-carrier solar cell include the slow hot-carrier cooling and effective extraction of hot-carrier energies which requires fast hot-carrier injection into charge collection layer before hot-carrier cooling down to the lattice temperature. Emulating semiconductor nanoscience, some interesting questions would be if the hot-carrier cooling rate in halide perovskites could be further modulated through confinement effects, and if these hot-carriers can be efficiently extracted. Here, the hot-carrier cooling dynamics and mechanisms in colloidal CH3NH3PbBr3 nanocrystals of different sizes (with mean radius ~2.5–5.6 nm) and their bulk-film counterpart were compared using room-temperature transient absorption spectroscopy. Our results revealed that the weakly quantum confined CH3NH3PbBr3 nanocrystals are very promising hot-carrier absorber materials (~ 2 orders slower hot-carrier cooling times and around 4 times larger hot-carrier temperatures than their bulk-film counterparts). This is attributed to their intrinsic phonon bottleneck and Auger-heating effects at low and high carrier densities, respectively. Importantly, we demonstrate efficient room temperature hot-electrons extraction (up to about 83%) by an energy-selective electron acceptor layer within ~1 ps from surface-treated perovskite nanocrystal very thin films (~30 nm). These new insights would allow the development of extremely thin absorber and concentrator-type hot-carrier perovskite solar cells. 4

 

References:

[1] Ross, R.T. & Nozik, A.J. Efficiency of Hot-Carrier Solar-Energy Converters. J. Appl. Phys. 53, 3813-3818 (1982).

[2] Lee, M.M., Teuscher, J., Miyasaka, T., Murakami, T.N. & Snaith, H.J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 338, 643-647 (2012).

[3] Zhou, H.P. et al. Interface engineering of highly efficient perovskite solar cells. Science 345, 542-546 (2014).

[4] Li, M. et al. Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals. Nat. Commun. 8, 14350 doi: 10.1038/ncomms14350 (2017).

12:15 - 12:30
Abstract not programmed
PerMod S8.2
Chair: Amanda Neukirch
11:00 - 11:30
S8.2-O3
Würfel, Uli
Fraunhofer Institute for Solar Energy Systems ISE
Why Do Perovskite Solar Cells that Have a Reduced Open-Circuit Voltage due to Surface Recombination Not Show Hysteresis?
Uli Würfel
Fraunhofer Institute for Solar Energy Systems ISE, DE
Authors
Uli Würfel a, b, Moritz Unmüssig a, b
Affiliations
a, Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, 79110 Freiburg, Germany
b, Materials Research Center FMF, University of Freiburg
Abstract

The phenomenom of hysteresis in the current-voltage characteristics of perovskite solar cells has been discussed by numerous authors. It is agreed by many that mobile ions or ion vacancies play a significant role in the underlying processes causing this hysteresis. Calao et al. have shown that hysteresis will only occur if ion migration alters the rate of surface recombination. The latter is a loss mechanism which occurs at the interfaces of the perovskite absorber with the adjacent materials, usually electron (ETM) and hole (HTM) transporting material and it depends sensitively on the concentration of electrons and holes in the vicinity of such an interface. As Calao et al. could show there is minimal hysteresis if ETM or HTM layers passivate the corresponding interface to the absorber such that surface recombination becomes insignificant regardless of the distribution of mobile ions or ion vacancies.
We built inverted planar p-i-n perovskite solar cells and observed a high open-circuit voltage (Voc) when NiOx was employed as HTM. In contrast, Voc was significantly reduced if NiOx was replaced by PEDOT:PSS. This reduction of Voc could clearly be attributed to surface recombination occurring at the absorber/HTM interface as confirmed by photoluminescence measurements. Interestingly, the PEDOT:PSS based perovskite solar cells nevertheless did not show any hysteresis. To answer the question why the migrating ions do not cause hysteresis in these kind of devices we performed numerical simulations. We can reproduce the experimental finding when implementing interface traps at the perovskite/PEDOT:PSS interface. These states can lead to a fermi level pinning effect such that the ionic distribution has only very little influence on the concentrations of electrons and holes in the vicinity of that recombination active interface.

[1] P. Calado, A. M. Telford, D. Bryant, X. Li, J. Nelson, B. C. O’Regan, P. R.F. Barnes, Nature Communications 7 (2016), 13831

11:30 - 12:00
S8.2-O1
Koopmans, Marten
Photophysics and OptoElectronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, The Netherlands
A comprehensive numerical study of the implications of s-shaped JV characteristics in perovskite solar cells
Marten Koopmans
Photophysics and OptoElectronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, The Netherlands
Authors
Marten Koopmans a, Jan Anton Koster a
Affiliations
a, Photophysics and OptoElectronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, The Netherlands
Abstract

In perovskite solar cells, achieving consistency in device fabrication is very difficult. Although it easy to measure whether a solar cell performs well, it is not always easy to determine what the problem is in case of a bad solar cell. Many devices that show less than ideal performance show a feature called an ‘s-shape’, where in the JV-characteristic a region exists where the second derivative of the current with respect to voltage is negative. If this s-shape occurs below Voc, this will dramatically decrease the fill factor. Until now some possible causes are known. It has been shown using drift diffusion simulations that in perovskite solar cells, s-shapes can exist under forward bias. Also from organic solar cells it is well known that poor charge transport can lead to s-shapes, but no comprehensive explanation has been shown on these s-shapes in perovskites as of yet.

In this contribution we identify all the processes and parameters that yield s-shapes in the JV-characteristic and can therefore give a complete story on the phenomenon of s-shapes in perovskite solar cells. Furthermore, the occurrence of s-shapes is linked to device characteristics such as transport layer mobility and effects such as charge buildup at certain bias voltages. This is done using very large amounts of simulated solar cells with different parameters, where parameters are chosen such that the whole parameter space of solar cells yielding s-shapes is swept. Each individual solar cell simulation is a drift diffusion simulation including mobile ions where n-i-p or p-i-n structures are assumed. The conclusions from the modeling can be used to pinpoint weak spots of fabricated solar cells and help improve device recipes and fabrication.

12:00 - 12:30
S8.2-O2
Marchal, Nadège
University of Mons (UMONS), Laboratory for Chemistry of Novel Materials, Center for Innovation and Research in Materials and Polymers (CIRMAP), Mons (Belgium)
Effect of Electronically Inert Organic Spacers on the Optoelectronic Properties of 2D Hybrid Perovskites
Nadège Marchal
University of Mons (UMONS), Laboratory for Chemistry of Novel Materials, Center for Innovation and Research in Materials and Polymers (CIRMAP), Mons (Belgium)
Authors
Nadège Marchal a, Claudio Quarti a, David Beljonne a
Affiliations
a, University of Mons (UMONS), Laboratory for Chemistry of Novel Materials, Center for Innovation and Research in Materials and Polymers (CIRMAP), Mons (Belgium)
Abstract

During the last years, three-dimensional hybrid perovskites were in the spotlight because of their very promising properties as semiconductor materials for solar cells, but also for optoelectronic applications in general. Indeed, with a low temperature and precursor solution based synthesis and certified 20% photovoltaic efficiency, 3D hybrid perovskites have been claimed as the new big thing in photovoltaics1. Further studies must be conducted to understand in detail the properties of this material, but especially to overcome one of its main flaws: degradation issues.

Currently, two-dimensional hybrid perovskites are getting attention of the scientific community mainly due to two reasons: the improvement of the stability and the chemical composition versatility, much higher than for the 3D-peovoskites. In fact, in the 3D case, the size of the “cages” created by the octahedral network limits the candidates as organic/inorganic cation, which is not the case of layered 2D hybrid perovskites. With these materials, we are expending the field of possibilities and it can lead to better tunability of the photophysical properties.

We are reporting here (time-dependent) density functional theory calculations [(TD)-DFT] on alkyl-ammonium lead iodide perovskites, and more specifically, the influence of the alkyl chain length of the spacer cation on the electronic structure and optical properties of the material. We predicted significant changes using long or short chains. Indeed, if the inorganic layers fix the electronic properties, the length of the organic cation showed to have an indirect effect. With long alkyl chains, as dodecyl chains, an opening of the electronic band gap occurs, due to the influence of the supramolecular packing on the structure organization of the octahedra network. For the case of long organic spacer dodecylammonium lead iodide perovskites, organic chains adopt a polyethylene-like packing, causing distortions in the inorganic frame and leading to the observed electronic band gap opening. These theoretical results are in agreement with experimental data and demonstrate that organic saturated chains can modify the optoelectronic properties of layered halide perovskite semiconductors2.

1 J. Bisquert, Journal of Physical Chemistry Letters 4(15):2597, 2013

2 C.Quarti, et al., Journal of Physical Chemistry Letters, DOI: 10.1021/acs.jpclett.8b01309

SolFuel S1.2
Chair: Kevin Sivula
11:00 - 11:30
S1.2-O1
Barawi Moran, Mariam
Imdea Energia
Design and Development of a Multilayer Photoelectrode Composed of TiO2 Nanocrystals and a New Nanostructured Conjugate Porous Polymer with Advanced Photoelectrochemical Properties
Mariam Barawi Moran
Imdea Energia
Authors
Mariam Barawi Moran a, Alberto González a, Elena Alfonso a, Alba García a, Carmen López-Calixto a, Marta Liras a, Víctor. A de la Peña O´ Shea a
Affiliations
a, Institute IMDEA Energy
Abstract

Photoelectrochemical water splitting is one of the most interesting alternatives to produce hydrogen in a clean way by solar energy conversion.(1) Despite the huge potential and great advances, new materials need to be developed in order to take this technology to a commercial level. At present, different materials as oxides, oxisulfides and metal chalcogenides are being investigated as photoelectrodes in photoelectrochemical cells. However, achieving high energy conversion efficiencies by using a single material is a very tricky objective. Therefore, hybrid materials are getting a lot of attention lately. (2)

In this work, we present a hybrid material formed by the heterojunction of a novel synthesized organic conductive polymer and TiO2 nanocrystals. The nanostructured conjugated porous polymer is based on dithiothiophene moiety (Nano-CMPDTT) and was synthesized by Sonogashira cross coupling reaction from precursors in mini-emulsion conditions. In order to elucidate the electronic structure and the ability of this material to be used as a photocatalyst, HOMO and LUMO positions were determined by cyclic voltammetry. The energy diagram shows an ideal position of the energy bands in order to use the synthesized polymer as an electron injector to TiO2 in photocatalytic reactions. TiO2 NCs and organic polymer suspensions have been deposited by spin coating in ITO glasses. The formed films have been characterized by X-ray diffraction, SEM, EDX and AFM. Photoelectrochemical measurements were performed in a three electrode cell configuration, using the hybrid material as the working electrode. The hybrid material presents an enhancement in photovoltages and photocurrents values. Electrochemical Impedance Spectroscopy (EIS) was performed to confirm the improved charge transfer observed when illuminating the hybrid material in comparison to the TiO2 nanocrystals alone. In fact, a decrease in the resistance associated with this phenomenon was found. This confirms that the presence of the polymer in the hybrid material increases the absorption of light, charge transfer and reduces electron-hole recombination, making this hybrid a good candidate to be used as a photoelectrode for the hydrogen evolution reaction. In fact, recent results show an improvement in the energy conversion efficiency by using this new hybrid material as electrode compared with regular TiO2.

[1] Z. Chen, H. N. Dinh, E. Miller, Photoelectrochemical Water Splitting (Springer New York, New York, NY, 2013; http://link.springer.com/10.1007/978-1-4614-8298-7), SpringerBriefs in Energy.

[2] M. P. Arciniegas et al., Self-Assembled Dense Colloidal Cu2Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

11:30 - 11:45
S1.2-O2
Corby, Sacha
Imperial College London
Charge Carrier Dynamics in Nanostructured Tungsten Trioxide for Solar Driven Water Oxidation
Sacha Corby
Imperial College London, GB
Authors
Sacha Corby a, Laia Francas a, Shababa Selim a, Michael Sachs a, Andreas Kafizas a, James Durrant a
Affiliations
a, Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ, UK
Abstract

Transition metal oxides are amongst the most widely studied materials for solar water oxidation owing to their earth abundance, good aqueous stability and facile syntheses.1 Understanding the dynamics of the photogenerated charge carriers in these materials is key to highlighting properties that need to be targeted to improve water splitting performance. With a narrower band gap than TiO2, tungsten trioxide (WO3) can absorb a larger proportion of the solar spectrum and has a deeper valence band energy to provide a large thermodynamic driving force for water oxidation.2-3 WO3 is also often reported to have high electrical conductivity, leading to its frequent implementation as an electron transporting layer in various heterojunction photoanodes.2-4 This high conductivity is considered to be a result of a large density of charge carriers, caused by intrinsic oxygen vacancies which can act as n-type dopants.5 However, deviations from stoichiometry have also been suggested to introduce chemical defects that can result in increased trapping of charges which, rather than boost performance, can often introduce additional recombination pathways.6

Using transient diffuse reflectance spectroscopy and transient photocurrent measurements, I will discuss the dynamics of the photogenerated charges in WO3 photoanodes, synthesised by chemical vapour deposition (CVD). The synthesis method generates heavily doped monoclinic WO3-x needles, which are annealed in air at elevated temperature to remove most of the oxygen vacancies. These photoanodes exhibit an early photocurrent onset and a high faradaic efficiency (>85%) for water oxidation. Compared to other transition metal oxide photoanodes, we observed rapid water oxidation (>1 ms) but found the rate of electron extraction is significantly slower (>10 ms). We investigated the effect of oxygen vacancy states on electron transport and found that electron trapping in the needles is significant, proposing a trap-mediated mechanism of electron transport to the back contact. We then used ultrafast transient absorption spectroscopy to examine the bias dependence on bulk recombination processes. Finally, we altered the oxygen vacancy content to determine the overall effect of these defects on charge transport and performance.

 

[1] G. Wang, et al. Energy Environl Sci, 2012, 5, 6180–6187.

[2] Z. Huang, et al. Advanced Materials, 2015, 27 (36), 5309-5327.

[3] X. Liu, et al. PCCP, 2012, 14 (22), 7894-7911.

[4] C.X. Kronawitter, et al. Energy Environl Sci 2011, 4 (10), 3889-3899.

[5] T. Zhu, et al. ChemSusChem, 2014, 7, 2974-2997

[6] M.B. Johansson, et al. J Phys Condens Matter, 2016, 28, 475802.

11:45 - 12:00
S1.2-O3
Huijser, Annemarie
University of Twente
Shedding Light on the Nature of Excited States in a Hydrogen Generating Supramolecular RuPt Catalyst by Ultrafast X-Ray Spectroscopy
Annemarie Huijser
University of Twente, NL
Authors
Annemarie Huijser a, Qing Pan a, David van Duinen a, Mads G. Laursen b, Amal El Nahas c, Pavel Chabera c, Qingyu Kong d, Xiaoyi Zhang d, Kristoffer Haldrup b, Wesley R. Browne e, Grigory Smolentsev f, Jens Uhlig c
Affiliations
a, MESA+ Institute for Nanotechnology, University of Twente
b, Department of Physics, Technical University of Denmark, Fysikvej, 312, Dept. of Physics, Kgs. Lyngby, 2800, DK
c, Lund University, Department of Chemical Physics, Getingevägen 60, Lund, 22241, SE
d, X-ray Sciences Division, Argonne National Laboratory
e, University of Groningen, Stratingh Institute for Chemistry, Molecular Inorganic Chemistry group
f, Paul Scherrer Institute, OLGA/113, Villigen PSI, 5232, CH
Abstract

Organometallic complexes with reactive metal centers are promising candidates for photocatalysis. To improve their performance, insight into the nature of excited states and control thereof is crucial. Ultrafast x-ray spectroscopy is a powerful method to achieve mechanistic insight into processes at catalytic metal moieties and is particularly useful to detect light-induced changes in oxidation state and coordination geometry. It can also be applied to probe the essential charge transfer step to the catalyst in real-time, including the potential involvement of atomic rearrangements.
We recently developed a series of new Ru-metal photocatalysts, of which the RuPt derivative showed a H2 turnover number of 80 after 6 h of irradiation at 470 nm. The photodynamics of RuPt studied by femtosecond transient absorption are highly complex and differ significantly from the RuPd analogue [1], indicating an important role of the catalytic moiety and motivating ultrafast x-ray absorption studies performed at the Advanced Photon Source at Argonne. This work particularly focuses at light-induced redox processes at the Pt moiety and the timescales thereof. Another question to be answered involves the local structure of the catalytic moiety and possible temporal or permanent changes.
The earliest differential absorption spectrum at 30 ps indicates that the Pt moiety is already reduced at this timescale. The signal intensity partially decays on a timescale of ca. 930 ps. The observation of a differential absorption signal at timescales far beyond indicates branching. The spectra at late timescales can be modelled by a hexa-coordinated PtIV (or possibly PtIII) species. This intermediate species may be formed by oxidative addition of iodine, is long-lived (>10 microseconds) and ultimately recovers into the original ground state structure. Hence, activation of the catalyst by a single photon may lead to withdrawal of two electrons. This mechanism presents a new paradigm for H2 generation by supramolecular photocatalysts.

[1] Q. Pan, F. Mecozzi, J.P. Korterik, J.G. Vos, W.R. Browne, A. Huijser, "The Critical Role Played by the Catalytic Moiety in the Early-Time Photodynamics of Hydrogen-Generating Bimetallic Photocatalysts", ChemPhysChem 17, 2654-2659 (2016).   

 

  

12:00 - 12:30
S1.2-O4
Pastor, Ernest
Imperial College London, Department of Chemistry, South Kensington Campus, London, SW7 2AZ
Ultrafast Electron Localisation and Delocalisation in Photoelectrochemical Cells. Towards Control of Excited-State Transport
Ernest Pastor
Imperial College London, Department of Chemistry, South Kensington Campus, London, SW7 2AZ
Authors
Ernest Pastor a, Artem Bakulin a
Affiliations
a, Imperial College London, Department of Chemistry, South Kensington Campus, London, SW7 2AZ
Abstract

Developing energy storage mechanisms is a crucial step to secure a sustainable energy economy. This is particularly important for solar power because there is a mismatch between the periods of peak energy harvesting and peak consumption. Photo-electrocatalytic energy storage has traditionally been dominated by inorganic systems. In particular, transition metal oxides are attractive due to their stability in liquid electrolytes and the remarkable catalytic flexibility of metallic centres. However, inorganic systems often underperform and only poor quantum yields are achieved, even when synthetic care is taken to eliminate defects and impurities.  In the dark, oxides like Fe2O3 and BiVO4 are poor conductors due to the localization of charges on specific atomic sites forming small polarons. Under illumination, transport improves but the mechanisms behind it are unknown. Recent x-ray and terahertz studies have hinted at the existence of light-induced small polarons and their link with structural properties. 1-3 However, these measurements lack device sensitivity and it is difficult to know if such states really influence performance. In this talk, I will present ultrafast optical studies with photocurrent detection of photoelectrochemical devices. These measurements are capable of probing light-initiated changes that control actual device activity.4 I will present data demonstrating that, in oxides such as efficient Fe2O3 thin-films,5 charges intrinsically self-trap within femtoseconds of photo-generation hindering transport. Importantly, I will show how in efficient systems, crossover between molecular-type, localised transport and band-like transport is critical and necessary for activity. This behavior is remarkably similar to that observed in molecular crystals and some of the strategies used to improve molecular semiconductors might apply to metal oxides. I will discuss the material implication of these findings and how it might be possible to achieve synthetic control of charge hopping and delocalisation rates in oxides.

 

References:

1Carneiro L. et al. Nat Mater. 2017, 16, 8, 819 

2Biswas S. et al. Nano Lett. 2018,18,1228

3Butler  K.T. et al. J.Mat.Chem.A, 2016,4,18516

4Bakulin A. et al. J.Phys.Chem. Lett. 2016, 7, 2, 250

5Steier L. et al. ACS Nano, 2015,9,11775

 

12:30 - 14:30
Lunch
Dyman S5.3
Chair: Simon Boehme
14:30 - 15:00
S5.3-O1
Lhuillier, Emmanuel
Sorbonne Universités, UPMC University Paris 06, CNRS-UMR 7588, Institut des Nanosciences de Paris, 4 place Jussieu, 75005 Paris, France
Dynamics in Narrow Band Gap Nanocrystals
Emmanuel Lhuillier
Sorbonne Universités, UPMC University Paris 06, CNRS-UMR 7588, Institut des Nanosciences de Paris, 4 place Jussieu, 75005 Paris, France

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
Emmanuel Lhuillier a, Bertille Martinez a, Clement livache a, nicolas goubet a
Affiliations
a, Sorbonne Universités, UPMC Univ. Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, 4 place Jussieu, 75005 Paris, France
Abstract

Nanocrystals can be used to achieve transition in the mid and far infrared. This is in particular promising for the design of low cost infrared photodetectors. However more need to be understood on the dynamic of carriers in narrow band gap nanocrystals. This question is nevertheless quite challenging since many conventional experimental methods such as time resolved photoluminescence cannot be used because of the low PL efficiency in the infrared nanocrystals and because of difficulties to build infrared optical setup. We have investigated two complementary ways to probe carrier dynamics in narrow band gap colloidal materials which are time resolved photoemission and transient photocurrent. To illustrate these experiments, two systems have been investigated. First I will show that pump probe experiment can be conducted while exciting 2D HgTe nanoplatelets in the near IR and while looking at relaxation with a photoemission probe. This method is very efficient to determine without contact majority and minority carrier lifetime.In the second part of the talk, I will discuss how it is possible to determine band structure parameter such as the Urbach energy from transient photocurrent measurements conducted on HgTe nanocrystals. To do so, we have built a broad bandwidth setup (from ns to ms) and bring evidence for the multi-trapping transport regime.These two methods are extremely complementary to optical time resolved spectroscopy often limited to short dynamics (ns and less)

15:00 - 15:30
S5.3-O2
Prins, Tim
Utrecht University
Doping InP Quantum Dots with Cu+ slows down Hot Electron Cooling
Tim Prins
Utrecht University, NL
Authors
P. Tim Prins a, Pieter Geiregat b, Dirk A. W. Spruijt a, Zeger Hens b, Freddy T. Rabouw a, Celso de Mello Donega a, Daniel A. M. Vanmaekelbergh a
Affiliations
a, Debye Institute for Nanomaterials Science, Utrecht University
b, Physics and Chemistry of Nanostructures Group, Ghent University
Abstract

Nonresonant excitation of colloidal quantum dots (QDs) creates hot carriers that subsequently cool down to the band edges or are trapped in localized states. Carrier cooling and trapping typically happens on timescales from femtoseconds to picoseconds, orders of magnitude faster than the nanosecond to microsecond timescales of radiative recombination. Understanding cooling and trapping is relevant for (hot) carrier extraction in photovoltaics and to increase the luminesence output of QDs used as phosphor.

We investigate carrier cooling and trapping in InP QDs with a ZnSe1-xSx shell. Undoped QDs are compared to Cu+-doped QDs, where the Cu+ ion serves as a designed hole trap. Using pump probe transient absorption spectroscopy with femtosecond time resolution, we are able to monitor the population of electrons in the conduction band.

Our comparative study shows that hot electron cooling is almost an order of magnitude slower in the Cu+-doped QDs than in the undoped QDs. We ascribe this to rapid hole trapping on the Cu+ ion. This confirms the model in which hot electron cooling goes via an Auger-like process, where the hot electron transfers its excess energy to the hole which subsequently relaxes by phonon coupling. In our Cu+-doped QDs the hole is trapped on a Cu+ ion on sub-picosecond timescales, so the Auger cooling pathway is unavailable to the hot electron. Instead it must cool down via another, slower, pathway, most likely by coupling to high-energy vibrations at the surface of the QDs. This must also mean that hole trapping on the Cu+ ion is faster than the Auger cooling timescale of the undoped QDs, which is of the order of 300 fs. Our results provide insight in the behaviour of hot electrons and holes in the short time period after excitation of both Cu+-doped and undoped InP QDs.

15:30 - 16:00
S5.3-O3
Das, Aparajita
Indian Institute of Technology Hyderabad
Poly(3,4-ethylenedioxypyrrole) Counter Electrode and an Electrolyte Additive for Quantum Dot Solar Cells
Aparajita Das
Indian Institute of Technology Hyderabad
Authors
Aparajita Das a, Ankita Kolay a, Deepa Melepurath a
Affiliations
a, Department of Chemistry, Indian Institute of Technology Hyderabad, ODF estate, Yeddumailaram Medak, Telangana, INDIA-502205 Mobile No.(+91) 9705892456., Hyderabad, 502205
Abstract

The operational lifetime and scale-up of quantum dot sensitized solar cells (QDSCs) are largely limited by the hole transport layer, which is usually an aqueous polysulfide solution. The liquid, and alkaline nature of this electrolyte makes it difficult to develop laminated sealed devices. Further, the polysulfide electrolyte composition alters with use (there are observable color and viscosity variations!) thus impacting the cell performance. Besides the photoanode, the counter electrode (CE) also plays a key role in controlling cell response. While a variety of CEs have been attempted in the past: metallic coatings, carbon nanomaterials and conducting polymers, poly(3,4-ethylenedioxypyrrole) or PEDOP has rarely been used in QDSCs.

In view of the above described issues, in this report, QDSCs with a photoanode comprising of N-doped graphene quantum dots (N-GQDs) and cadmium sulfide (CdS)/titania (TiO2) based solar cells with electropolymerized PEDOP@carbon cloth as CE and a polysulfide/SiO2 gel with an electrolyte additive, namely, sodium poly(styrene sulfonate) or NaPSS were developed. The proportion of NaPSS is optimized on the basis of cell performance. A significant improvement in QDSC performance is obtained by incorporating NaPSS in the gel. By using impedance spectroscopy, the role of NaPSS in improving the cell performance is determined. NaPSS increases the recombination resistance for back electron transfer at the photoanode/electrolyte interface, thus increasing the power conversion efficiency to nearly 7% from 6.1% (when no NaPSS is present). NaPSS also imparts an enhanced operational life to the QDSC. Apart from NaPSS, the effectivity of PEDOP as a CE for QDSCs is studied by comparing impedance parameters, electrocatalytic activities and electrical conductivities of PEDOP films with different dopants. Ionically conducting and electrically conducting dopants were attempted. These studies provide a deeper understanding of factors that limit QDSC performances, and help in overcoming them.

 

 

16:00 - 16:30
S5.3-O4
Grimaldi, Gianluca
Delft University of technology
Controlling (hot) electron transfer in quantum dot heterojunction films
Gianluca Grimaldi
Delft University of technology, NL
Authors
Gianluca Grimaldi a, Arjan Houtepen a, R.W. Crisp a, S. Ten Brinck b, F. Zapata b, c, M. van Ouwendorpa a, M. van den Brom a, N. Renaud a, N. Kirkwood a, W. Evers a, d, S. Kinge c, I. Infante b, L.D.A. Siebbeles a
Affiliations
a, Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
b, Department of Theoretical Chemistry, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands
c, Netherlands eScience Center, Science Park 140, 1098 XG Amsterdam, The Netherlands
d, Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
e, Toyota Motor Europe, Materials Research & Development, Hoge Wei 33, B-1930 Zaventem, Belgium
Abstract

Control over the energy alignment of Quantum Dots (QD) heterostructures can be used to unlock new functionalities for QD based optoelectronic devices: from improved carrier separation in type-II heterostructure, to control over hot-carrier transfer in type I heterostructures and lowered Carrier Multiplication threshold in quasi-type-II heterostructures.

We investigated the carrier dynamics in QD heterojunction films composed of PbSe and CdSe QDs. We demonstrate that such films tend to form a type I band alignment in which fast and efficient hot electron transfer from PbSe QDs to CdSe QDs is observed by transient absorption (TA) measurements. ­ The efficiency of the hot electron transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale. Hot-electron solar cells have been proposed as a route towards higher efficiency solar cells, and our observation reveals the possibility to achieve and control hot-electron transfer via energy-structure engineering in QD heterojunctions.

We next attempted to switch the energy alignment between the PbSe QDs and CdSe QDs, using the size-dependence of their energy structure as well as using tailored ligands to shift the energy levels through their surface dipoles. Spectroelectrochemical measurements reveal that we can shift the type I alignment to a type II alignment and TA measurements demonstrate a much-improved efficiency of “cold” electron transfer.

We thus proved that a combination of size-variation and control over surface-passivation allows to span the range between type-I and type-II alignment. One particularly interesting configuration is that of quasi-type-II alignment, where the conduction electron levels are resonant, as this could potentially be used for optimal Carrier Multiplication.

16:30 - 17:00
S5.3-I1
Cánovas, Enrique
IMDEA Nanoscience
Band-Like Charge Transport in a Semiconducting Metal-Organic Framework
Enrique Cánovas
IMDEA Nanoscience, ES

Enrique Cánovas graduated on Applied Physics at Universidad Autónoma de Madrid (2002). After that, he realized a two-years Master of Advanced Studies at Universidad de Valladolid working on the spectroscopic characterization of native and operation-induced defects in high power laser diodes. From 2004 to 2006 he made a second Master of Advanced Studies at Universidad Politécnica de Madrid (Institute of Solar Energy, IES); training focus was on the fabrication, characterization and optimization of solid state solar cells. In 2006 he joined the group of Prof. Martí  and Prof. Luque at IES, where he completed PhD studies on the spectroscopic characterization of novel nanostructures aiming ultra-high-efficiency solar cells. His PhD studies included two placements (covering 9 months in total) at Lawrence Berkeley National Laboratory (USA - with Prof. W. Walukiewicz) and Glasgow University (Scotland - with Prof. Colin Stanley). Between 2010 and 2012 he worked as a postdoc at FOM Institute AMOLF (Amsterdam - The Netherlands, Prof. M. Bonn) on the characterization of carrier dynamics in sensitized solar cell architectures. Between 2012 to 2018 he lead the Nanostructured Photovoltaics Group at Max Planck Institute for Polymer Research (Mainz, Germany). Since April 2018, Enrique Canovas works at IMDEA Nanoscience where he was appointed Assistant Research Proffesor (tenure-track).  His research interests cover all aspects of photovoltaics, nanotechnology and charge carrier dynamics.

Authors
Enrique Canovas a
Affiliations
a, IMDEA Nanoscience, C/faraday, 9, Madrid, 28049, Madrid, ES
Abstract

Metal-organic frameworks (MOFs) are coordination polymers consisting of metal ions connected by organic ligands. Besides the traditional applications in gas storage and separation as well as catalysis, the long-range crystalline order in MOFs and the tunable coupling between their organic and inorganic constituents have recently led to the design and synthesis of (semi-)conducting MOFs, opening the path for their application in opto-electronics. Yet, despite being a critical aspect for the development of MOF based electronics, the true nature of charge transport in MOFs, i.e. whether hopping or band-like transport occurs, has remained unresolved to date.

Here we report a time-resolved high-frequency (terahertz) conductivity study of a newly developed Fe3(THT)2 (THT=2,3,6,7,10,11-hexathioltriphenylene) two-dimensional MOF.  The novel π-d conjugated samples, synthesized through interfacial method at room temperature, are obtained as a large-area, free-standing film with tunable geometry (size and thickness). The Fe3(THT)2 films are conductive (~1 S/cm), porous (specific surface area of 526±5 m2/g) and semiconducting (with a ~250 meV direct bandgap). We demonstrate for the first time band-like charge transport in MOFs. This finding is directly apparent from the Drude-type high-frequency (terahertz) photo-conductivity response obtained in the samples; revealing free-moving, delocalized charge carriers displaying ~200 cm2/Vs mobilities at room temperature; a record mobility for MOFs. The temperature dependence of the mobility reveals that the main scattering mechanism limiting the mobility and hence band-like charge transport in this material is related to impurity scattering, so that material improvements may further increase the mobility.

The demonstration of band-like charge transport in MOFs reveal the potential of (porous) conductive MOFs to be employed as active materials in opto-electronics devices.

NCFun S3.3
Chair: Gordana Dukovic
14:30 - 15:00
S3.3-O1
Post, Christiaan
Universiteit Utrecht
Nano Perforation of InGaAs Quantum wells: a Lithography Route Towards III-V Semiconductors with Honeycomb Nanogeometry
Christiaan Post
Universiteit Utrecht, NL
Authors
L. Christiaan Post a, Tao Xu b, Nathali A. Franchina Vergel b, Yannick Lambert b, Francois Vaurette b, Ludovic Desplanque b, Xavier Wallart b, Bruno Grandidier b, Christophe Delerue b, Daniel A. M. Vanmaekelbergh a
Affiliations
a, Condensed Matter and Interfaces, Debye Institute for nanomaterials science, Utrecht University
b, IEMN, Department ISEN, 41 boulevard Vauban, F-59046 Lille Cedex, France
Abstract

III-V semiconductor quantum wells have obtained a central place in advanced logics and opto-electronics. In more recent research directions heading towards materials with entirely new functions, the effects of a nano scale geometry forming a periodic scattering potential in the lateral directions of the quantum well has been discussed and calculated. In case of a nano scale honeycomb geometry, and entirely new band structure emerges in which the highest valence and lowest conduction bands become Dirac cones at the K-points, while the semiconductor quantum well band gap remains nearly unaltered.

In this presentation we report on the fabrication of a 10 nm thick InGaAs quantum well (QW) on a n-type InP substrate with a honeycomb symmetry structure by creating a triangular anti-lattice inside the QW using high-resolution electron beam lithography. The morphology of the samples are studied using atomic force microscopy (AFM), elementary diffraction spectroscopy (EDS) and cross-section transmission electron microscopy (TEM). The quality of the samples is characterized using scanning electron microscopy (SEM), which is used for an extensive statistical analysis to determine the disorder inside the lattices. The results are supported by theoretical simulations on the bandstructure and density of states (DOS).

15:00 - 15:30
S3.3-O2
Boldt, Klaus
University of Konstanz
Charge Localisation at Strained, Axial Heterojunctions in 1D Semiconductor Nanocrystals
Klaus Boldt
University of Konstanz, DE
Authors
Florian Enders a, Peng Zeng b, Trevor A. Smith b, Jannika Lauth c, d, Laurens D. A. Siebbeles c, Arne Budweg a, Daniele Brida a, Klaus Boldt a
Affiliations
a, University of Konstanz, Universitaetsstr. 10, POB M680, Konstanz, 78457, DE
b, School of Chemistry, The University of Melbourne, Melbourne, Victoria, 3010, AU
c, Department of Chemical Engineering, TU Delft, Julianalaan 136, Delft, 2628 BL, NL
d, Carl von Ossietzky University of Oldenburg, 26129, Oldenburg, Germany, GE
Abstract

Nanocrystals with type-II heterostructures are attractive candidates for applications in which separation of excited charge carriers are exploited, such as photovoltaics [1], upconversion [2], or quantum dot lasing [3]. These properties are in competition with processes involving interfacial trap states, usually associated with the particle surface or strained interfaces. To use semiconductor nanocrystals in technical applications it is of utmost importance to characterise and control these effects.

Here we present a model system made of CdTe-tipped CdSe/CdS seeded nanorods, in which the CdTe particle is separated in space from the CdSe seed while being in electronic contact. The CdTe/CdS interface has type-II character, but charge carrier delocalisation is likely affected by the large lattice mismatch of 10% between CdTe and CdS [4]. We performed ultrafast (femto to picosecond) transient absorption spectroscopy to map carrier dynamics after exciting one or both recombination centres, with seed size and rod length as variable parameters. Conduction band electrons, after exciting the CdTe/CdS interface, are expected to localise in the CdSe seed, which has the lowest energy band edge and acts as a reporter particle. However, while states in the CdS rod get bleached by the electron no signal corresponding to CdSe is detected. We employ electron and hole scavengers to determine the roles of the individual charge carriers, and by this the contribution of Coulomb interactions and interfacial trap states.

This experiment gives fundamental insight into the behaviour of strained nano-heterointerfaces, as well as the effects from size quantisation and mean free path of the carriers. The experimental data is compared to effective mass approximation-based simulations for free carriers. While the basic electronic structure is predicted with sufficient accuracy the comparison highlights the importance to simulate carrier interactions and interfaces on the atomic scale.

[1] Itzhakov, S.; Shen, H.; Buhbut, S.; Lin, H.; Oron, D.; J. Phys. Chem. C 2013, 117 (43), 22203–22210.

[2] Deutsch, Z.; Neeman, L.; Oron, D.; Nat. Nanotech. 2013, 8, 649–653.

[3] Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I. V.; Mcguire, J. A.; Piryatinski, A.; Nature 2007, 447, 441–446.

[4] Jing, L.; Kershaw, S. V.; Kipp, T.; Kalytchuk, S.; Ding, K.; Zeng, J.; Jiao, M.; Sun, X.; Mews, A.; et al.; J. Am. Chem. Soc. 2015, 137, 2073–2084.

15:30 - 16:00
S3.3-O3
Bar-Elli, Omri
Weizmann Institute of Science
Detecting Short Voltage Pulses Using Single Quantum Dots Designed for Action Potential Sensing
Omri Bar-Elli
Weizmann Institute of Science, IL
Authors
Omri Bar-Elli a, Dan Steinitz a, Gaoling Yang a, Ron Tenne a, Anastasia Ludwig b, Yung Kuo c, Antoine Triller c, Shimon Weiss c, d, Dan Oron a
Affiliations
a, Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel
b, LEcole Normale Superieure, Institute of Biologie (IBENS), Paris Sciences et Lettres (PSL), CNRS UMR 8197, Inserm 1024, 46 rue d’Ulm, Paris 75005, France
c, Department of Chemistry and Biochemistry, Department of Physiology, and California NanoSystems Institute, University of California Los Angeles, Los Angeles, California
d, Department of Physics, Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan, 52900, Israel
Abstract

It was recently shown that single colloidal quantum dots (QDs) may exhibit large sensitivities to external electric fields in the form of a spectral shift of the emission wavelength. This opens the possibility of designing QDs’ based local electric field sensors, for example for neuronal membrane action potential (AP) sensing. One of the hurdles on the way to such a goal is fast detection. Typically, slow spectrometers are used to detect the spectral shifts however, AP sensing requires significantly faster detection rates of the order of a millisecond.

The underlying mechanism for the QDs’ response to external electric fields is the quantum confined Stark effect. From symmetry considerations spherical QDs exhibit only a red-shifted emission spectrum in the presence of an electric field. APs consist of a change from negative to positive potential thus, symmetric QDs are of limited value. Asymmetric type II ZnSe/CdS nanorods, on the other hand, exhibit a large linear response, namely both red- and blue-shifts, depending on the orientation of the QD in the electric field. Moreover, the spectral shifts are expected to be correlated with changes in the decay rates.

Here, we demonstrate an experimental setup designed at achieving shot-noise limited sensitivity to emission spectral shifts on time scale suitable for AP sensing. We present experimental results of these phenomena as well as characterize the performance of single QDs as sensors for short millisecond voltage pulses comparable to APs in both duration and amplitude.

16:00 - 16:30
S3.3-I1
Klinke, Christian
Swansea University,
Two-Dimensional Nanostructures: Synthesis and Optoelectronic Transport
Christian Klinke
Swansea University,, GB

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 is supported by an ERC Starting Grant 2012. Since 2013 he is a Heisenberg fellow of the German Funding Agency DFG.

Authors
Christian Klinke a
Affiliations
a, Institute of Physical Chemistry, University of Hamburg
Abstract

Two-dimensional colloidal nanomaterials represent very exciting optoelectronic 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. Such structures combine good lateral conductivity with solution-processability and electronic confinement in height, which allows tuning the effective bandgap of the materials. I will present the syntheses of the materials and the analyses of the optoelectronic transport through these materials in field-effect transistors, solar cells, and spintronic devices. It turns out that the electronic confinement allows for an optimization of their performances.

Two-dimensional colloidal nanomaterials represent very exciting optoelectronic 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. Such structures combine good lateral conductivity with solution-processability and electronic confinement in height, which allows tuning the effective bandgap of the materials. I will present the syntheses of the materials and the analyses of the optoelectronic transport through these materials in field-effect transistors, solar cells, and spintronic devices. It turns out that the electronic confinement allows for an optimization of their performances.

16:30 - 17:00
S3.3-I2
Amirav, Lilac
Technion – Israel Institute of Technology
Sculpting Photocatalysts on the Nano Scale
Lilac Amirav
Technion – Israel Institute of Technology

Amirav is an expert in the use of hybrid nanostructures for renewable energy generation, in particular photocatalytic solar-to-fuel conversion. She has demonstrated success in designing sophisticated heterostructures for the water reduction half reaction. She is particularly interested in photocatalysis on the nano scale and related photophysical and photochemical phenomena. The laboratory’s cutting-edge synthetic effort is combined with development of nontraditional techniques for mechanistic study of charge transfer pathways, and fundamental research on reaction mechanism.

Authors
Lilac Amirav a
Affiliations
a, Schulich Faculty of Chemistry, Technion – Israel Institute of Technology
Abstract

The solar-driven photocatalytic splitting of water into hydrogen and oxygen is a potential source of clean and renewable fuels. However, four decades of global research have proven this multi-step reaction to be highly challenging. The design of effective artificial photo-catalytic systems will depend on our ability to correlate the photocatalyst structure, composition, and morphology with its activity.

I will present our strategies, and most recent results, in taking photocatalyst production to new and unexplored frontiers. I will focus on unique design of innovative nano scale particles, which harness nano phenomena for improved activity, and methodologies for the construction of sophisticated heterostructures. I will share our design rules and accumulated insights, which enabled us to obtain a perfect 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water splitting reduction half reaction. Finally, I will describe our future designs of systems capable of overall water splitting and genuine solar-to-fuel energy conversion.

PVCon S9.3
Chair: Luis Pazos Outon
14:30 - 15:00
S9.3-I1
Ito, Seigo
University of Hyogo
Studies of Tandem Solar Cells and Stability Issue of Perovskite Solar Cells
Seigo Ito
University of Hyogo, JP

Seigo Ito received his Ph.D. from the University of Tokyo (Japan), with a thesis that was the first to discuss Graetzel-type dye-sensitized solar cells in Japan. He worked in the Laboratory of Professor Shozo Yanagida (Osaka University, Japan) for two years, and in the Laboratory of Professor Michael Graetzel, at the Swiss federal Institute of Technology (EPFL) in Lausanne as a postdoctoral scientist for over three years, where his efforts focused on the progress of high-efficiency dye-sensitized solar cells. He is currently professor at University of Hyogo, making new printable cost-effective solar cells.

Authors
Seigo Ito a
Affiliations
a, Department of Materials and Synchrotron Radiation Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo, 671, JP
Abstract

In order to consider the effective structure of silicon solar cells for perovskite/silicon tandem solar cells, the optic and photovoltaic properties of textured and flat silicon surfaces were compared using mechanical-stacking-tandem of 2- and 4- terminal structures by perovskite layers on crystal silicon wafers.  The reflectance of the texture silicon surface in the range of 750-1050 nm could be reduced more than that of the flat silicon surface (from 2.7% to 0.8%), which resulted in increases in average IPCE values (from 83.0% to 88.0%) and current density (from 13.7 mA cm-2 to 14.8 mA cm-2).  Using the texture surface of silicon heterojunction (SHJ) solar cells, the significant conversion efficiency of 21.4% was achieved by 4-terminal device, which was 2.4%-up from that of SHJ solar cells alone.

And, for the progress of perovskite/silicon tandem solar cells, we introduce a totally vacuum-free cost-efficient crystalline silicon solar cells. Solar cells were fabricated based on low-cost techniques including spin coating, spray pyrolysis and screen-printing. A best efficiency of 17.51% was achieved by non-vacuum process with a basic structure of <AI/p+/p-Si/n+/SiO2/TiO2/Ag> CZ-Si p-type solar cells. JSC and VOC of the best cell were measured as 38.1 mAcm-2 and 596.2 mV, respectively with FF of 77.1%. Suns-Voc measurements were carried out and the detrimental effect of the series resistance on the cell performance was revealed. It is concluded that higher efficiencies are achievable by the improvements of the contacts and by utilizing good quality starting wafers.

Finally, for the study of stability test on perovskite solar cells, Carbon-based triple-porous-layer perovskite solar cells without any hole transporting material were selected for investigation to reduce internal degradation issues about thermal stress.  The sealed perovskite solar cells which kept at 100 ˚C performed slow degradation in the power conversion efficiency until 4500 h, but the degradation speed was accelerated after that.  By analyzing the perovskite solar cells aged for 7000 h at 100 ˚C, the results of energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy suggest that, although Pb2+ and I- were sealed inside of the devices, the most of CH3NH3+ diffused out of the sealant UV-curable adhesive at 100 ˚C, which is the reason of the thermal degradation for the sealed perovskite solar cells.

15:00 - 15:30
S9.3-O1
Jooss, Christian
University of Goettingen
Hot Polaron States with Nanosecond Lifetime in Manganite Perovskite Photovoltaic Junctions
Christian Jooss
University of Goettingen, DE
Authors
Christian Jooss a, Dirk Raiser b, Benedikt Ifland a, Mohsen Sotoudeh c, Peter Blöchl c, Michael Seibt e, Tobias Meyer e, Simone Techert d, Birte Kressdorf a, Patrick Peretzki e
Affiliations
a, Institute for Material Physics, University of Goettingen, Friedrich-Hund-Platz 1, D-37077 Goettingen, Germany
b, Max-Planck-Institute for Biophysical Chemistry, Am Faßberg 11, D-37077 Goettingen, Germany
c, Institute for Theoretical Physics, University of Clausthal, Leibnizstrasse 10, D-38678 Clausthal-Zellerfeld, Germany
d, FS-SCS Deutsches Elektronensynchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany
e, IV. Physicak Institute, University of Goettingen, Friedrich-Hund-Platz 1, D-37077 Goettingen, Germ
Abstract

Understanding and controlling the relaxation process of optically excited charge carriers in solids with strong correlations is of great interest in the quest for new strategies to exploit solar energy. Usually, optically excited electrons in a solid thermalize rapidly on a femtosecond to picosecond timescale due to interactions with other electrons and phonons. New mechanisms to slow down thermalization would thus be of great significance for efficient light energy conversion, e.g. in photovoltaic devices. Ultrafast optical pump probe experiments in the manganite Pr0.65Ca0.35MnO3, a photovoltaic and electro-catalytic material with strong polaronic correlations, reveal an ultra-slow recombination dynamics on a nanosecond-time scale [1]. The photo-diffusion of excited electron-hole polaron pairs gives rice to a pronounced photovoltaic effect [1,2] and is studied by electron beam induced current (EBIC) on nanometer length scales [3]. Theoretical considerations suggest that the excited charge carriers are trapped in a hot polaron state. The dependence of the lifetime on charge order implies that strong correlation between the excited polaron and the octahedral dynamics of its environment appears to be substantial for stabilizing the hot polaron [4]. Furthermore, modification of the interfacial atomic and electronic structure of the manganite-titanite junctions gives insights into the processes underlying the transfer of a small polaron across the interface [5].

[1] Evolution of hot polaron states with a nanosecond lifetime in a manganite, D. Raiser, S. Mildner, B. Ifland, M. Sotoudeh, P. Blöchl, S. Techert, C. Jooss, Advanced Energy Materials, 2017, 1602174, DOI: 10.1002/aenm.201602174

[2] Polaron absorption for photovoltaic energy conversion in a manganite-titanate pn-heterojunction, G. Saucke, J. Norpoth, D. Su, Y. Zhu and Ch. Jooss, Phys. Rev. B 85, 165315 (2012).

[3]  Low energy scanning transmission electron beam induced current for nanoscale characterization of p-n junctions, Patrick Peretzki, B. Ifland, C. Jooss, M.Seibt, Phys. Status Solidi RRL 11,1600358 (2017)

 

[4] Contribution of Jahn-Teller and charge transfer excitations to the photovoltaic effect of manganite/titanite heterojunctions, B. Ifland, J. Hoffmann, B. Kressdorf, V. Roddatis, M. Seibt and Ch. Jooss, New Journal of Physics, 19 (2017) 063046

[5 ] Current–voltage characteristics of manganite–titanite perovskite junctions, B. Ifland, P. Peretzki, B. Kressdorf, Ph. Saring, A. Kelling, M. Seibt and Ch. Jooss, Beilstein Journal of Nanotechnology, 2015, 6, 1467–1484

15:30 - 16:00
S9.3-O2
Tavakoli, Nasim
AMOLF
Combining 1D and 2D waveguiding properties for ultrathin tandem solar cells
Nasim Tavakoli
AMOLF, NL
Authors
Nasim Tavakoli a, Esther Alarcon Llado a
Affiliations
a, Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
Abstract

As an effective way to surpass the Shockley-Queisser efficiency limit multijunction solar cells have been designed and developed for many years now. Silicon, in particular, is very appealing as the low bandgap material in a tandem design since we already have a mature understanding of its optical and electronic properties and its fabrication technology is widely available. For the high bandgap, III-V materials are shown to be promising in both light management and carrier management. However, fabricating monolithic III-V on Si multijunction is still challenged by various limiting factors such as lattice mismatching, high fabrication cost, and the size/weight of the tandem designs which makes them unsuitable for many applications. One way to tackle these issue is epitaxial growth of vertically standing III-V nanowires on Si ultrathin substrate. This design has many advantages: Not only the wires can overcome the mismatching problem thanks to their intrinsic strain relaxation properties, they also create a natural anti-reflection coating.

In this work, we study light-matter interactions in GaAs-based nanowire arrays on ultrathin silicon film with the dual goal of obtaining large absorption in the array and improving light trapping in the bottom thin film cell. By performing FDTD simulations we show that the coupling of the incident light to the HE11 waveguiding mode of the wires not only boosts the absorption in the wires themselves, but also efficiently transfers the non-absorbed light to Si bottom cell. Moreover, the grating properties of the array is capable of changing the momentum of the transmitted light. In this way, light is trapped in the Si thin film. As a result, we induce higher absorption for the wavelengths close to the bandgap of silicon where the absorption coefficient is very low.

To conclude, by optimizing the geometry of both each wire and the grating we are able to firstly couple the light into waveguiding modes of each wire and later couple the transmitted/scattered light into waveguiding modes of the ultrathin silicon layer underneath. By combining these 1D and 2D waveguiding properties a high efficiency ultrathin and flexible tandem cell is designed.

 

16:00 - 16:30
S9.3-O3
Suárez, Isaac
University of Valencia
Wearable Amplifier-Photodetector System Based on PMMA/Perovskite Waveguides Integrated on a Wearable Nanocellulose Substrate
Isaac Suárez
University of Valencia
Authors
Isaac Suárez a
Affiliations
a, University of Valencia
Abstract

In the last years semiconductor organometallic halide (CH3NH3PbX3, X=Cl, Br, I) perovskites (MHP) have emerged as an outstanding material to develop a new generation of photonics and electronics devices [1]. MHPs present excellent conductivity, light detection and emission properties, which have been successfully exploited in the implementation of broad range of optoelectronic devices. Examples include solar cells with efficiencies higher than 22%, broad band photodetectors, efficient optical sources, or optical amplifiers with low thresholds. Nevertheless, in spite of this important progress, most of this works use a rigid substrate to fabricate the device, while the integration of MHPs in flexible substrate is still at its very beginning. These new kinds of substrates, however, represent an important trend in optoelectronics, not only due to their wide range of applications, but also to the possibility to implement wearable devices directly in contact with the skin or clothes. In this work, MHP materials are successfully incorporated on a nanocellulose (NC) substrate with the intention to to construct wearable devices with new functionalities based on the light emission/detection properties of MHPs. In particular, a bilayer Poly(methyl methacrylate) /MHP deposited on NC  resulted in a suitable waveguide to demonstrate amplification of the spontaneous emission (ASE) with a threshold as low as 3-4 nJ [2-3].  Moreover, when a photodetector system is integrated within the waveguide, the device provides a photocurrent useful to monitor the light/ASE propagated/generated along the structure [4]. This approximation paves the road of new wearable systems with a broad range of applications.

[1] I. Suárez: Active photonic devices based on colloidal semiconductor nanocrystals and organometallic halide perovskites, Eur. Phys. J. Appl. Phys., vol. 75, 30001, 2016.

[2] T.T. Ngo, et al.: Enhancement of the Performance of Perovskite Solar Cells, LEDs, and Optical Amplifiers by Anti‐Solvent Additive Deposition," Adv. Mater., vol.  29, pp.  1604056, Dec. 2016

[3] I. Suárez et al.: Polymer/perovskite amplifying waveguides for active hybrid silicon photonics. Adv. Mater., vol. 27, 6157-6162, 2015.

[4] I. Suárez et al.: Integrated Optical Amplifier-Photodetector on a Wearable Nanocellulose Substrate. Adv. Opt. Matter., online available.

16:30 - 17:00
S9.3-O4
Murcia, Sebastian
Integration of Adapted Thin-film Photovoltaics into Solar Vanadium Redox Flow Batteries for Energy Storage
Sebastian Murcia
Authors
Sebastián Murcia-López a, Nina Carretero a, Cristina Flox a, Félix Urbain a, Joan R. Morante a, b, Teresa Andreu a
Affiliations
a, Catalonia Institute for Energy Research, IREC. Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain.
b, University of Barcelona, C. Martí i Franqués, 1, 08028 Barcelona, Spain
Abstract

The electrical conversion and storage of solar energy is a crucial world target in the long-term scenario. Therefore, the use and integration of photovoltaic (PV) technologies into different electrochemical processes for obtaining solar fuels have been strongly developed in the last years. Following this approach, the integration of PV and other energy storage systems such as batteries is a logical approach that pursues simplification of capture and storage of the solar energy through direct conversion into (electro)chemical energy. These so-called solar batteries offer the advantage of carrying out in a single device, a process normally done in several steps in two independent units, which result expensive, bulky and inefficient. Among several other configurations, the application of this concept to redox flow batteries has attracted attention considering their advantages, including decoupling of energy and power and large-scale development.

Despite the obvious interest in these systems, the direct influence of the redox potential (i.e., selected redox pair) into the operation point of the PV, constitutes an important challenge in the design of such devices. Therefore, some studies have focused on metal oxides and/or on PV tandem configurations, for instance CdS/DSSC, being applied to organic redox pairs and/or to vanadium redox flow batteries (VRFB) reaching limited state of charge (SoC).

In this work, we report the adaptation and integration of thin film PV to VRFB in a single device. Copper Indium Gallium Selenide (CIGS) modules have been adapted by the interconnection of CIGS commercial cells deposited on flexible metallic substrates. This way, three and four-cell modules leading to different open circuit voltages (OCV) and i-V performances were integrated in VRFB with modified carbon felt (CF) electrodes. Two kinds of batteries reaching different cell voltages were evaluated.

A very close dependence between the VRFB cell potential and the photocurrent of the photovoltaic system has been observed, ultimately influencing the overall capacity of the battery. In particular, there is a clear difference between using 3 or 4 CIGS cells, due to the different operation points. Therefore, it is obvious that adapting of the PV is necessary before its integration. Ultimately, the four-cell module has provided enough photovoltage for carrying out the unbiased photo-assisted charge of the battery up to SoC 100%, with acceptable charge time and overall round-trip energy conversion efficiency of around 4%.

PerFun S7.3
Chair: David Cahen
14:30 - 15:00
S7.3-I1
Beard, Matthew
National Renewable Energy Laboratory,
Time-Resolved Optical Studies of Perovskite Polycrystalline Films, Single Crystals and their Surfaces
Matthew Beard
National Renewable Energy Laboratory,, US
Authors
Matthew Beard a
Affiliations
a, National Renewable Energy Laboratory, Golden, Colorado, USA
Abstract

We have used time-resolved transient spectroscopies to better understand both bulk carrier dynamical processes as well as surface carrier dynamics. We employed transient reflection spectroscopy to measure the surface carrier dynamics in methylammonium lead iodide perovskite single crystals and polycrystalline films. We find that the surface recombination velocity (SRV) in polycrystalline films is nearly an order of magnitude smaller than that in single crystals, likely due to unintended surface passivation of the films during synthesis. In spite of the low SRV, surface recombination limits the total carrier lifetime in polycrystalline thin films, meaning that recombination inside grains and at grain boundaries is less important than top and bottom surface recombination. The suppressed SRV in the polycrystalline films appears to be related to an excess of methylammonium compared to the single crystals surfaces, determined by X-ray photoelectron spectroscopy analysis.  We studied the charge carrier dynamics in 2D Ruddlesden-Popper perovskites PEA2PbI4·(MAPbI3)n−1(PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4) single crystals. We have also studied carrier dynamics in Perovskite QD samples and will discuss recent results. 

15:00 - 15:30
S7.3-I2
Guichuan, Xing
University of Macau
Light Emission from Metal-Halide Perovskites
Xing Guichuan
University of Macau, CN

Dr. Guichuan Xing received his bachelor Degree from Fudan University (China) in 2003 and PhD in physics from National University of Singapore (Singapore) in 2011. From 2009 to 2016, he worked as a research fellow in Prof. Tze Chien Sum group at Nanyang Technological University. Dr. Xing joined the Institute of Applied Physics and Materials Engineering (IAPME), University of Macau in 2016 as an assistant Professor. His research interest includes developing and applying ultrafast nonlinear spectroscopic techniques to probing, understanding and controlling the fundamental charge and energy carrier generation, transport and relaxation processes in novel optoelectronic systems for energy conversion/storage and light emission applications.

Authors
Guichuan Xing a
Affiliations
a, Institute of Applied Physics and Materials Engineering, University of Macau, Macau SAR, China 999078
Abstract

Recently, the low-temperature solution-processed metal-halide perovskites have demonstrated great potential in light harvesting applications. The light to electricity power conversion efficiency up to 23.25% has been achieved. The primary advantages of these lead based perovskites for solar cells are the large photon absorption coefficient, long-range balanced charge carrier diffusion lengths, low trap density, high charge carrier mobility, fast and efficient photo-generated exciton dissociation and slow electron-hole bimolecular recombination. Additional to these properties, the tunable optical direct bandgap over a wide range also makes these perovskites good candidates for using in light emission applications (Lasing & Electroluminescence). However, the relatively slow electron-hole bimolecular recombination in the traditional three-dimensional perovskites that drives their outstanding photovoltaic performance is a fundamental limitation for the light emission. Here, we show that the perovskite light emission efficiency could be greatly enhanced by tailoring the bimolecular-type recombination in three-dimensional crystals to excitonic-type recombination in low-dimensional crystals.

References

[1] Guichuan Xing, Bo Wu, Xiangyang Wu, Mingjie Li, Bin Du, Qi Wei, Jia Guo, Edwin K. L. Yeow, Tze Chien Sum, Wei Huang, Nature Communications, 8, 14558 (2017).

[2] Guichuan Xing, Mulmudi Hemant Kumar, Wee Kiang Chong, Xinfeng Liu, Yao Cai, Hong Ding, Mark Asta, Michael Grätzel, Subodh Mhaisalkar, Nripan Mathews, Tze Chien Sum, Advanced Materials, 28, 8181-8196 (2016).

[3] Guichuan Xing, Nripan Mathews, Swee Sien Lim, Natalia Yantara, Xinfeng Liu, Dharani Sabba, Michael Grätzel, Subodh Mhaisalkar, Tze Chien Sum, Nature Materials, 13, 476-480 (2014).

[4] Guichuan Xing, Nripan Mathews, Shuangyong Sun, Swee Sien Lim, Yeng Ming Lam, Michael Grätzel, Subodh Mhaisalkar, Tze Chien Sum, Science, 342, 344-347 (2013).

[5] Fei Yan, Jun Xing, Guichuan Xing, Lina Quan, Swee Tiam Tan, Jiaxin Zhao, Rui Su, Lulu Zhang, Shi Chen, Yawen Zhao, Alfred Huan, Edward H Sargent, Qihua Xiong, Hilmi Volkan Demir, Nano Lett., 18, 3157-3164 (2018)

 

Acknowledgements: Financial support from the Macau Science and Technology Development Fund (FDCT-116/2016/A3 and FDCT-091/2017/A2), Research Grant (SRG2016-00087-FST and MYRG2018-00148-IAPME) from the University of Macau, the Natural Science Foundation of China (91733302, 61605073 and 2015CB932200) is gratefully acknowledged.

15:30 - 16:00
S7.3-O1
Pedesseau, Laurent
INSA, FOTON, UMR CNRS 6082
Black Phases of CsPbI3: Structural and Theoretical Studies
Laurent Pedesseau
INSA, FOTON, UMR CNRS 6082, FR

He obtained his MSc in condensed matter from the University of Montpellier in 2001. In 2004, he received his PhD in physics from the University of Toulouse for atomistic empirical simulations applied to Civil Engineering materials. In 2013, he was appointed as assistant professor at FOTON laboratory (INSA Rennes) to work on III–V semiconductor nanostructures for silicon photonics, hybrid-perovskites for photovoltaics, and optoelectronic device simulations for optical telecommunication.

Authors
Laurent Pedesseau a, Arthur Marronnier b, Guido Roma c, Soline Boyer-Richard a, Jean-Marc Jancu a, Yvan Bonnassieux b, Claudine Katan d, Constantinos Stoumpos e, Mercouri Kanatzidis e, Jacky Even a
Affiliations
a, Fonctions Optiques pour les Technologies de l’Information (FOTON), Institut National des Sciences Appliquées (INSA) de Rennes, CNRS, UMR 6082, 35708 Rennes, France
b, LPICM, CNRS, Ecole Polytechnique, Université Paris-Saclay, F-91128 Palaiseau, France
c, Service de Physique de l’Etat Condensé CEA, CNRS, Universite Paris Saclay CEA Saclay , l’orme des merisiers bat 772, 91191 Gif sur Yvette Cedex FRANCE
d, Univ Rennes, ENSCR, INSA Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR6226, F-35000 RENNES
e, Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208, USA
Abstract

In less than 10 years, hybrid organic-inorganic perovskites have emerged as a new generation of absorber materials for high-efficiency and low-cost solar cells [1], [2]. More recently, fully inorganic perovskite quantum dots (QD) also led to promising efficiencies [3], [4] and then become a serious alternative to hybrid organic-inorganic perovskites. Currently, the record efficiency for QD solar cells is obtained with CsPbI3. High resolution in-situ synchrotron XRD measurements have been performed on CsPbI3 as a function of the temperature and revealed a highly anisotropic variation of the lattice parameters. Moreover, CsPbI3 can be temporarily maintained in a perovskite-like structure down to room temperature, stabilizing a metastable perovskite polytype (black-phase) crucial for photovoltaic applications. Structural, vibrational and electronic properties of the three experimentally observed black phases are further scrutinized using theoretical approaches [5], [6]. A symmetry-based tight-binding model, calibrated with self-consistent GW calculations including spin-orbit coupling, affords further insight into their electronic properties. A Rashba effect is thus predicted for both cubic and tetragonal phases when using the symmetry breaking structures obtained through frozen phonon calculations.

The ab initio simulations have been performed on HPC resources of CINES under the allocation 2017-[x2017096724] made by GENCI (Grand Equipement National de Calcul Intensif).

[1] A. Kojima, et al., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J.

Am. Chem. Soc. 2009, 131, 6050−6051.

[2] Best research-cell efficiencies; https://www.nrel.gov/pv/assets/images/efficiency-chart.png (accessed Nov 7, 2017).

[3] H. Bian et al., Graded Bandgap CsPbI2+xBr1-x Perovskite Solar Cells with a Stabilized Efficiency of 14.4%, Joule (2018), https://doi.org/10.1016/j.joule.2018.04.012

[4] E. M. Sanehira, et al., Enhanced Mobility CsPbI3 Quantum Dot Arrays for Record-efficiency, High-voltage Photovoltaic Cells. Sci. Adv. 2017, 3, eaao4204.

[5] A. Marronnier et al., Structural Instabilities Related to Highly Anharmonic Phonons in Halide Perovskites. . J. Phys. Chem. Lett. 2017, 8, 2659−2665

[6] A. Marronnier et al., Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells. ACS Nano 2018, 12, 3477−3486

16:00 - 16:30
S7.3-I3
Ruhman, Sanford
Hebrew University of Jerusalem
Gauging Photorefractive Effects on Transient Absorption in Lead Iodide Perovskite Thin Films by Comparison to Nanocrystals.
Sanford Ruhman
Hebrew University of Jerusalem, IL

Sanford Ruhman is a full professor of Chemistry at the Hebrew University. His work concentrates on applications of femtosecond spectroscopy in condensed phases. As a pioneer in the field of femtosecond photochemistry his group was the first to report conservation of coherence from reactants to dissociation products in solutions, and to utilize impulsive Raman probing of photoproducts. His current interests include fundamental ultrafast excitonics in nanocrystals and photovoltaic materials, ultrafast photobiology, and applications of impulsive vibrational spectroscopy to probe light induced dynamics in liquids and solids. Over the years he has served as the Director of the Farkas Minerva center for light induced processes at the Hebrew University, and as the head of the Institute of Chemistry there.

Authors
Tufan Ghosh a, Sigalit Aharon a, Lioz Etgar a, Sanford Ruhman a
Affiliations
a, Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel
Abstract

Due to their sizable refractive index, reflectivity of visible light off the lead halide perovskite - air interface exceeds 15%. This has prompted a number of investigations into the prominence of photo-reflective contributions to pump-probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI3) perovskite. The rationale is to compare pump-probe measurements samples where the relative contributions of changes in the real and imaginary components of the refractive index are very different. The absorption cross section of a nanocrystal of volume v in terms of its complex refractive index and that of its (non-absorbing) surroundings n0 is

 

is a local field factor which must be included since it depends on the same refractive indices. For a polycrystalline film or a crystal, reflectivity at the interface with a non-absorbing dielectric whose refractive index is n0, is given by the Fresnel relation {2}:

 

The first derivatives of sabs or of R (both of which reduce transmission) with respect to n or k quantify their sensitivity to variations in either constant. By analyzing how complex refractive index changes impact the two experiments we can show that reflectivity changes due to variations in n would not only differ in amplitude but even in sign in both experiments. The results presented in the figure below demonstrate virtually identical TA data for both samples proving that changes in absorption and not reflection dominate transient transmission measurements in thin films of these materials. None of the characteristic spectral signatures reported in such experiments is exclusively due to, or even strongly affected by changes in sample reflectivity. This finding is upheld by another experiment where a methyl ammonium lead iodide perovskite film was formed on high index flint glass, and probed after pump irradiation from either face of the sample. We conclude that interpretations of ultrafast pump-probe experiments on thin perovskite films in terms of photo-induced changes in absorption alone are qualitatively sound, requiring relatively minor adjustments to factor in photo-reflective effects.

16:30 - 17:00
S7.3-O2
Kepenekian, Mikael
CNRS
Making and breaking the exciton in layered halide hybrid perovskites
Mikael Kepenekian
CNRS, FR
Authors
Mikael Kepenekian a, Boubacar Traore a, Jean-Christophe Blancon b, Hsinhan Tsai b, Wanyi Nie b, Konstantinos Stoumpos c, Laurent Pedesseau d, Claudine Katan a, Sergei Tretiak b, Mercouri Kanatzidis c, Jacky Even d, Aditya Mohite b
Affiliations
a, Univ Rennes, ENSCR, INSA Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR6226, F-35000 RENNES
b, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
c, Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
d, Univ Rennes, INSA Rennes, CNRS, Institut FOTON - UMR6082, F-35000 RENNES
Abstract

Layered halide hybrid organic−inorganic perovskites [1] have been the subject of intense investigation before the rise of three-dimensional (3D) halide perovskites and their impressive performance in solar cells. Recently, layered perovskites have also been proposed as attractive alternatives for photostable solar cells [2] and revisited for light-emitting devices. Interestingly, these performances can be traced back to extremely efficient internal exciton dissociation through edge states identified on thin films and single crystals [3].

Layered perovskites present fascinating features with inherent quantum and dielectric confinements imposed by the organic layers sandwiching the inorganic core, and computational approaches have successfully help rationalized their properties (excitonic, Rashba effects, etc.) [4-6]. Here, we propose a joint spectroscopic and computational investigation to unravel the origin of the recently identified layer-edge states in layered Ruddlesden-Popper phases with inorganic layers containing n = 1 to 4 octahedra. We show that for n > 2, the system presents a localized surface state within the band gap.

Based on our conclusion, we propose an elastic model providing design principles for future layered perovskites with optimized properties for photovoltaics or light emission.

 

References

[1] L. Pedesseau et al., ACS Nano (2016), 10, 9776.

[2] H. Tsai et al.,Nature (2016), 536, 312.

[3] J.-C. Blancon et al., Science (2017), 355, 1288.

[4] M. Kepenekian et al., ACS Nano (2015), 12, 11557.

[5] D. Sapori, M. Kepenekian, L. Pedesseau, C. Katan, J. Even, Nanoscale (2016), 8, 6369.

[6] M. D. Smith et al., Chem. Sci. (2017), 8, 1960.

PerMod S8.3
Chair: Edoardo Mosconi
14:30 - 14:45
S8.3-O4
Bou, Agustín
Universitat Jaume I
Inductive Loop in the Impedance Response of Perovskite Solar Cells Explained by Surface Polarization Model
Agustín Bou
Universitat Jaume I, ES
Authors
Elnaz Ghahremanirad a, b, Agustín Bou a, Saeed Olyaee b, Juan Bisquert a
Affiliations
a, Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló, Spain
b, Nano-photonics and Optoelectronics Research Laboratory (NORLab), Shahid Rajaee Teacher Training University, St. Shabanloo, Ave. Lavizan, Tehran
Abstract

The analysis of perovskite solar cells by impedance spectroscopy has provided a rich variety of behaviors that demand adequate interpretation. Two main features have been reported: First, different impedance spectral arcs vary in combination; second, inductive loops and negative capacitance characteristics appear as an intrinsic property of the current configuration of perovskite solar cells. Here we adopt a previously developed surface polarization model based on the assumption of large electric and ionic charge accumulation at the external contact interface. Just from the equations of the model, the impedance spectroscopy response is calculated and reproduces and explains the mentioned general features. The inductance element in the equivalent circuit is the result of the delay of the surface voltage and depends on the kinetic relaxation time. The model is therefore able to quantitatively describe exotic features of the perovskite solar cell and provides insight into the operation mechanisms of the device.

14:45 - 15:00
S8.3-O5
Seidu, Azimatu
Aalto University, Department of Applied Physics
Database-driven study for hybrid perovskite coating materials
Azimatu Seidu
Aalto University, Department of Applied Physics
Authors
Azimatu Seidu a, Lauri Himanen a, Jingrui Li a, Patrick Rinke a
Affiliations
a, Department of Applied Physics, Aalto University, P.O.Box 11100, FI-00076 Aalto, Finland
Abstract

Recently, hybrid perovskite solar cells (HPSCs) have achieved conversion efficiencies > 22 % and
have revived the search for clean, affordable and efficient energy. However, the practical realization
of this hope is pending due to problems related to the stability of HPSCs in moisture, heat,
ultraviolet light and oxygen rich environments [1]. Thus, the search is underway for protective coating materials to protect HPSCs. A proper coating should have a wide band gap in order to serve as a good window
material, exhibit minimum lattice mismatch at the coating-perovskite interface and most
importantly, be resistance to environmental conditions. In this study, we consider a series of ABX3 perovskites, with  A = MA/Cs, B = Sn/Pb and X = Cl/Br/I. To select possible protective-coating candidates, we
filtered the large Aflow [2] database to collect materials with wide band gap (≥ 3 eV) and further
sorted them based on their solubility in water, toxicity and abundance. To avoid large lattice
mismatch, we only considered rectangular surfaces of coating materials and limited the mismatch to be
within -5 and 5 %. For instance, for MAPI3, we found the following promising coating candidates, NiO, PbTiO3, BaZrO3 , ZnO and GaN, whose lattice mismatch are -0.35, -0.10, 0.37, -0.04 and 0.64%, respectively.
In addition to protecting the stability of HPSCs, our collected coating materials have the potential to serve as efficient transport layers in the HPSC architecture. Our search will not only improve the stability of HPSCs but also serve as a starting point in the search of novel device materials for emergent HPSC technologies.
Keywords
hybrid, perovskites, resistance, lattice mismatch

Reference
[1] M. A. Green, A. Ho-Baille, and H. J Snaith, Nature Photon. 8, 506(2014).
[2] R. H. Taylor, F. Rose, C. Tohler, O. Levy, K. Yang, M. B. Nardelli, and S. Curtarolo,
Computational Materials Science 93, 178 (2014).

15:00 - 15:30
S8.3-I1
Neukirch, Amanda
Geometry Distortion and Small Polaron Binding Energy Changes with Ionic Replacement in Halide Perovskites
Amanda Neukirch
Authors
Amanda Neukirch a, Jacky Even b, Claudine Katan c, Sergei Tretiak a
Affiliations
a, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
b, INSA FOTON 6082 CNRS Rennes
c, CNRS, Institut des sciences chimiques de Rennes
Abstract

Solution-processed halide perovskites have demonstrated remarkable performances in optoelectronic devices and applications. Despite the extraordinary progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices remain open. Here we report the results from an in-depth computational study of small polaron formation utilizing information from electronic structure, charge density, and reorganization energy calculations on isolated structures. Local lattice symmetry, electronic structure, and electron phonon coupling are interrelated in polaron formation in hybrid halide perovskites. To illustrate these aspects, first principles calculations are performed on CsPbI3, CsSnI3, CsPbBr3, MAPbI3, FAPbI3, MAPbBr3, FAPbBr3, MASnI3, and FASnBr3. This study will focus on how ionic exchange changes the geometry and polaron binding energy in the material. It is found that in all cases that hole polaron formation is associated with smaller binding energies and lattice contraction, while electron polaron formation exhibits larger polaron binding energies, lattice expansion, and Jahn Teller like distortions.

15:30 - 16:00
S8.3-O1
Bisquert, Juan
Institute of Advanced Materials (INAM), Universitat Jaume I
Theory of Frequency Perturbation Techniques for Perovskite Solar Cells
Juan Bisquert
Institute of Advanced Materials (INAM), Universitat Jaume I, 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, Agustín Bou a, Sandheep Ravishankar a
Affiliations
a, Institute of Advanced Materials (INAM), UniversitatJaume I, 12006 Castelló, Spain
Abstract

The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. The application of frequency techniques is a major tool for the analysis of perovskite solar cells. Here we describe the general theory and methods of small perturbation frequency modulated techniques. These methods connect a broad range of experimental tools that interrogate the system in specific steady-state conditions.1 the first method we discuss is the simulation of the dynamic response of the perovskite/contact interface. Here we find that it is necessary to establish the main polarization conditions, and how they are coupled to recombination and charge transfer, eventually leading to a typical behaviour of negative capacitance. Secondly we investigate the physical meaning of light modulated techniques as IMPS and IMVS when applied to charge collection in perovskite solar cells. Third we introduce a light-to-light impedance that is able to provide information on phenomena of photon diffusion as in photon recycling regime.2

(1) Bertoluzzi, L.; Bisquert, J. Investigating the Consistency of Models for Water Splitting Systems by Light and Voltage Modulated Techniques, J. Phys. Chem. Lett. 2017, 8, 172-180.

(2) Ansari-Rad, M.; Bisquert, J. Theory of Light-Modulated Emission Spectroscopy, J. Phys. Chem. Lett. 2017, 8, 3673-3677.

 

16:00 - 16:30
S8.3-O2
Wolf, Matthew
University of Bath
Meso-Scale Modelling of Charge Transport in Halide Perovskites
Matthew Wolf
University of Bath, GB
Authors
Matthew Wolf a, Lewis Irvine a, Ian Thompson a, Alison Walker a
Affiliations
a, Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
Abstract

The fundamental nature of charge carrier transport (band-like or polaronic), and the influence thereupon of various scattering mechanisms and defect distributions are of central importance to the operation of semi-conductor based devices. While there have been numerous investigations aiming to understand these effects in hybrid halide perovskites, there remains much to be understood [1,2]. The structural and compositional complexity of perovskite based solar cells renders it extremely difficult to disentangle these effects, and theoretical simulations can provide valuable insights and predictions. So far modelling has focused on atomistic [3] and continuum [4] length scales, but a model bridging these scales, while taking into account all of the aspects described above, is lacking.

Here, we will describe a “device Monte Carlo” meso-scale model, based on well established semi-classical transport theory, which takes into account the band structure of the material, phonon and defect scattering, and electrostatic fields arising from inhomogeneities in defect and carrier concentrations, using parameters derived from experiment and ab initio calculations. We will present the results of the application of this model to charge carrier transport in hybrid halide perovskites, with a particular emphasis on current–voltage characteristics and the experimentally observed effects of changing defect distributions under illuminatation [5].

References

[1] T. Brenner et al., Nat. Rev. Mater. 1 (2016) 15007

[2] L. M. Herz, ACS Energy Lett. 2 (2017) p1539

[3] C. Motta and S. Sanvito, J. Phys. Chem. C 122 (2018) p1361

[4] S. E. J. O’ Kane et al., J. Mater. C 5 (2017) p452

[5] G. Y. Kim et al., Nat. Mater. 17 (2018) p445

16:30 - 17:00
S8.3-O3
Wood, Vanessa
ETH Zurich
Vibrations and Electron-Phonon Coupling in Lead Halide Perovskite Nanocrystals
Vanessa Wood
ETH Zurich, CH

Vanessa Wood is a professor in the Department of Information Technology and Electrical Engineering at ETH Zurich, where she heads the Laboratory for Nanoelectronics. Before joining ETH in 2011, she was a postdoctoral associate in the laboratory of Professor Yet-Ming Chiang and Professor Craig Carter in the Department of Materials Science and Engineering at MIT, performing research on novel lithium-ion battery systems. She received her MSc and PhD from the Department of Electrical Engineering and Computer Science at MIT. Her graduate work was done in the group of Professor Vladimir Bulović and focused on the development of optoelectronic devices containing colloidally synthesized quantum dots.

Authors
Vanessa Wood a
Affiliations
a, ETH Zurich, Department of Information Technology and Electrical Engineering
Abstract

Knowledge of the vibrational structure of a semiconductor is essential for explaining its optical and electronic properties and enabling optimized materials selection for optoelectronic devices. However, experimental measurement of the vibrational density of states of nanomaterials is particularly challenging. In this contribtion, I will describe recent work my group has carried out, investigating electron-phonon interactions through a variety of computational and experimental techniques. In particular, we have performed ab-initio molecular dynamics simulations on CsPbBr3 perovskite nanocrystals in order to gain insight in the electronic and vibrational structure of these materials. We haved studied the influence of the large surface to volume ratio in nanocrystals versus bulk crystals, and shown that our computational results match the first experimental measurements of the vibrational denisty of states of CsPbBr3 perovskite nanocrystals obtained using inelastic x-ray scattering (IXS).  We also show how the power spectrum of the electron and hole wavefunction dephasing can be used to investigate which phonons couple strongly to electrons and holes and mediate non-radiative transitions. 

SolFuel S1.3
Chair: Nestor Guijarro Carratala
14:30 - 15:00
S1.3-I1
Artero, Vincent
CEA Grenoble
Molecular-Based H2-Evolving Photocathodes
Vincent Artero
CEA Grenoble, FR

Vincent Artero was born in 1973. He is a graduate of the Ecole Normale Supérieure (Ulm; D/S 93) and of the University Pierre et Marie Curie (Paris 6). He received the Ph.D. degree in 2000 under the supervision of Prof. A. Proust. His doctoral work dealt with organometallic derivatives of polyoxometalates. After a postdoctoral stay at the University of Aachen (Aix la Chapelle) with Prof. U. Kölle, he joined in 2001 the group of Prof. M. Fontecave in Grenoble with a junior scientist position in the Life  Science Division of CEA. Since 2016, he is Research Director at CEA and leads the SolHyCat group. His current research interests are in bio-inspired chemistry including catalysis related to hydrogen energy and artificial photosynthesis.

Vincent Artero received the "Grand Prix Mergier-Bourdeix de l'Académie des Sciences" in 2011 and has been  granted with a Consolidator Grant from the European Research Council (ERC, photocatH2ode project 2012-2017). He's a member of the Young academy of Europe (YAE). He currently acts as Chair of the Scientific Advisory Board of the ARCANE Excellence Laboratory Network  (LABEX) for bio-driven chemistry in Grenoble and as co-head of the French network (CNRS-Groupement de recherche) on Solar Fuels. Since 2016, Vincent Artero is associate editor of the Royal Society of Chemistry journal "Sustainable Energy and Fuels". From January 2018 onward, he actsas associate editor of the Royal Society of Chemistry  flagship journal "Chemical Science"

Authors
Vincent ARTERO a, b, c, d
Affiliations
a, Universite Grenoble Alpes
b, CNRS, Université de Rennes 1, Campus de Beaulieu, Rennes, 35000, FR
c, CEA Grenoble, Avenue des Martyrs, 17, Grenoble, FR
d, Laboratoire de Chimie et Biologie des Métaux
Abstract

Mimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a primary target for artificial photosynthesis,1 which requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. We will report on our contribution to the development of various series of catalysts for H2 evolution,2-4 including the reinvestigation of amorphous molybdenum sulfide5 and to the establishment of methodologies towards the rational benchmarking of their catalytic activity. Besides, we will also describe our effort towards the combination of such catalysts with various photoactive motifs for the preparation of photoelectrode materials6-10 that can be implemented into photoelectrochemical (PEC) cells for water splitting.

 

References

1.         N. Queyriaux, N. Kaeffer, A. Morozan, M. Chavarot-Kerlidou and V. Artero, J. Photochem. Photobiol. C, 2015, 25, 90-105.

2.         D. Brazzolotto, M. Gennari, N. Queyriaux, T. R. Simmons, J. Pécaut, S. Demeshko, F. Meyer, M. Orio, V. Artero and C. Duboc, Nat. Chem., 2016, 8, 1054-1060.

3.         N. Kaeffer, M. Chavarot-Kerlidou and V. Artero, Acc. Chem. Res., 2015, 48, 1286–1295.

4.         T. N. Huan, R. T. Jane, A. Benayad, L. Guetaz, P. D. Tran and V. Artero, Energy Environ. Sci., 2016, 9, 940-947.

5.         P. D. Tran, T. V. Tran, M. Orio, S. Torelli, Q. D. Truong, K. Nayuki, Y. Sasaki, S. Y. Chiam, R. Yi, I. Honma, J. Barber and V. Artero, Nat. Mater., 2016, 15, 640-646.

6.         J. Massin, M. Bräutigam, N. Kaeffer, N. Queyriaux, M. J. Field, F. H. Schacher, J. Popp, M. Chavarot-Kerlidou, B. Dietzek and V. Artero, Interface Focus, 2015, 5, 20140083.

7.         N. Kaeffer, J. Massin, C. Lebrun, O. Renault, M. Chavarot-Kerlidou and V. Artero, J. Am. Chem. Soc., 2016, 138, 12308−12311.

8.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, C. Laberty-Robert, S. Campidelli, R. de Bettignies, V. Artero, S. Palacin and B. Jousselme, Energy Environ. Sci., 2013, 6, 2706-2713.

9.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, R. Cornut, V. Artero and B. Jousselme, ACS Appl. Mater. Interfaces, 2015, 7, 16395–16403.

10.       A. Morozan, T. Bourgeteau, D. Tondelier, B. Geffroy, B. Jousselme and V. Artero, Nanotechnology, 2016, 27, 355401.

15:00 - 15:30
S1.3-O1
Shalom, Menny
Department of Chemistry, Ben Gurion University of the Negev, Beer Sheba, Israel
Graphitic Carbon Nitride Layers as Light-Harvesting Semiconductors for Photoelectrochemical Cells
Menny Shalom
Department of Chemistry, Ben Gurion University of the Negev, Beer Sheba, Israel
Authors
Menny Shalom a
Affiliations
a, Department of Chemistry, Ben Gurion University of the Negev, Beer Sheba, Israel
Abstract

One of the most promising future sources of alternative energy involves water-splitting photoelectrochemical cells (PECs) – a technology that could potentially convert sunlight and water directly to a clean, environmentally-friendly, and cheap hydrogen fuel. Practical PEC-mediated hydrogen production requires robust and highly efficient semiconductors, which should possess good light-harvesting properties, a suitable energy band position, stability in harsh condition, and a low price. Despite great progress in this field, new semiconductors that entail such stringent requirements are still sought after. Over the past few years, graphitic carbon nitride (g-CN) has attracted widespread attention due to its outstanding electronic properties, which have been exploited in various applications – including in photo- and electro-catalysis, heterogeneous catalysis, CO2 reduction, water splitting, light-emitting diodes, and PV cells. gCN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, to date, only a few reports regarded the utilization of g-CN in PECs, due to the difficulty in acquiring a homogenous g-CN layer on a conductive substrate and to our lack of basic understanding of the intrinsic layer properties of g-CN. In this talk I will introduce new approaches to grow g-CN layers with altered properties on conductive substrates for photoelectrochemical application. The growth mechanism as well as their chemical, photophysical, electronic and charge transfer properties will be discussed.

15:30 - 15:45
S1.3-O2
Sachs, Michael
Imperial College London, Department of Chemistry, South Kensington Campus, London, SW7 2AZ
Understanding Hydrogen Evolution Activity of Linear Organic Photocatalysts
Michael Sachs
Imperial College London, Department of Chemistry, South Kensington Campus, London, SW7 2AZ
Authors
Michael Sachs a, Reiner Sebastian Sprick b, Drew Pearce c, Sam J. Hillman c, Adriano Monti d, Anne A. Y. Guilbert c, Nick J. Brownbill b, Stoichko Dimitrov a, Frédéric Blanc b, Martijn A. Zwijnenburg d, Jenny Nelson c, James R. Durrant a, Andrew I. Cooper b
Affiliations
a, Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ, UK
b, Department of Chemistry, University of Liverpool, Crown Street, Liverpool, Liverpool, GB
c, Department of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, UK., South Kensington, London SW7 2AZ, UK, London, GB
d, Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, GB
Abstract

While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic photocatalysts are currently gaining substantial momentum - particularly due to their much higher synthetic flexibility. For instance, their optical band gap can be tuned continuously throughout large parts of the solar spectrum by copolymerising suitable monomers in defined ratios.1 This tunability has sparked intense research interest in organic photocatalysts,2,3 however, the fundamental understanding of photoinduced processes in these systems has stayed behind the rapid development of new materials. Some parallels can be drawn to organic photovoltaics where comparable materials are used, but especially the aqueous environment makes polymer photocatalysts distinct from other applications. To understand what dictates their performance and how structurally similar polymers can exhibit very different degrees of hydrogen evolution activity,4 photophysical processes in these materials require further investigation.

The combined study presented here is the first in-depth investigation of hydrogen evolution activity of linear conjugated polymers and combines materials development with spectroscopic characterisation and computational modelling. We investigate a series of polymers with strikingly different hydrogen evolution activity, including some of the highest performing photocatalysts reported to date in this class of materials. A comparison to structurally related polymers with significantly lower activity allows us to identify the key determinants of hydrogen evolution activity in this series. To this end, we use transient absorption spectroscopy to monitor photogenerated reaction intermediates on time sales of femtoseconds to seconds after light absorption and correlate the type and yield of observed intermediates with the hydrogen evolution activity of the respective polymer. Computational simulations and calculations build on this transient data and extend the observations to the role of the solvent environment in the photoinduced reaction sequence. The presented results can provide design strategies for new materials and thus have implications for the development of more efficient organic photocatalysts.

 

References

1. Sprick, R. S. et al. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 137, 3265–3270 (2015).

2. Zhang, G., Lan, Z.-A. & Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chemie Int. Ed. 55, 15712–15727 (2016).

3. Vyas, V. S., Lau, V. W. & Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Production. Chem. Mater. 28, 5191–5204 (2016).

4. Sprick, R. S. et al. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts. Angew. Chemie Int. Ed. 55, 1792–1796 (2016).

 

 

15:45 - 16:15
S1.3-O3
Karadas, Ferdi
Ferdi Karadas
An Earth Abundant Dye-Sensitized Photoanode for Water Oxidation
Ferdi Karadas
Ferdi Karadas
Authors
Ferdi Karadas a, T. Gamze Ulusoy Ghobadi b
Affiliations
a, Bilkent University, Ankara, Turkey
b, UNAM, Bilkent University, Ankara, Turker
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) either connected to the chromophore (a dyad assembly) or directly to the semiconductor. In this assembly, chromophore produces an electron-hole pair upon excitation with visible light followed by the injection of electrons to the semiconductor and then to the cathode compartment while holes are transferred to the catalytic site to activate the catalyst for water oxidation.

Despite the recent promising studies, achieving high stability for both the catalyst and the chromophore under DS-PEC operating conditions, extending the light absorption to visible and near IR region, and developing entirely earth abundant assemblies still remain as key challenges. 

Our recent research efforts have recently been devoted to developing dye-sensitized photoanodes involving cyanide-based assemblies. For this purpose, pentacyanoferrate building block was used not only as donor system for the construction of a new donor-acceptor chromophore but also as a cyanide precursor to produce a heterogeneous cobalt-iron Prussian blue electrocatalyst film. The photoelectrochemical performance and superior stability of the Prussian blue film coupled with TiO2 will be presented in detail.

16:15 - 16:30
S1.3-O4
Trompoukis, Christos
Department of Information Technology, Ghent University
Porosity as an Ionic Shortcut: Porous Multi-Junction Thin-Film Silicon Solar Cells for Scalable Solar Water Splitting
Christos Trompoukis
Department of Information Technology, Ghent University, BE

Christos received in 2008 his B.Sc in Physics from the University of Athens in Greece and, in 2010 his M.Sc in Physics from KTH Royal Institute of Technology in Stockholm, Sweden. During his Ph.D. he was part of imec's solar cell group where he was working on photonic nanostructures for advanced light trapping in thin silicon solar cells. In 2015 he received his Ph.D. degree in Engineering Science from KU Leuven and started investigating solar hydrogen fuel cells as a postdoctoral researcher between KU Leuven's surface chemistry and catalysis group and UGent's Photonics group. 

Authors
Christos Trompoukis a, Jan-Willem Schüttauf c, Tom Bosserez b, Ji-Yu Feng d, Aimi Abass e, Jan Rongé b, Roel Baets a, Johan Martens b
Affiliations
a, Ghent University, Photonics Research Group, iGent, Technologiepark Zwijnaarde 15, 9000 Ghent, Belgium
b, KU Leuven, Centre for Surface Chemistry and Catalysis, Celestijnenlaan 200F, 3001 Leuven, Belgium
c, Swiss Center for Electronics and Microtechnology (CSEM), PV Center, Rue Jaquet-Droz 1, 2000 Neuchâtel, Switzerland
d, Ghent University, Department of Solid State Sciences, Krijgslaan 281/S1, 9000 Gent, Belgium
e, Karlsruhe Institute of Technology (KIT), Institute of Nanotechnology, 76021 Karlsruhe, Germany
Abstract

Integrated wireless monolithic solar water splitting devices, i.e. monoliths submerged in the electrolyte, is a promising approach for low-cost photoelectrochemical solar water splitting [1]. However, such a device design poses a significant limitation: the ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This turns out to be a bottleneck for reaching high efficiencies and maintain optimum performance when it comes to up-scaling.

In this work, we present a novel approach to tackle the aforementioned limitation. Our device design is based on the concept of porosity as an ionic shortcut, implemented in silicon-based monoliths for low-cost and scalable solar water splitting [2, 3]. Simulation and experimental results towards enabling the proof of concept device consisting of porous multi-junction thin-film silicon solar cells on perforated substrates are presented. Based on simulations, we highlight how porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. As a result, the overpotentials to drive the water splitting reaction can be reduced by more than 400 mV. Experimentally, the impact of porosity (square array of holes with a period of 100 μm and a diameter of 20 μm) on single-junction and multi-junction amorphous and microcrystalline thin-film silicon solar cells is explained. A minimal decrease in VOC is seen, with porous triple-junction thin-film silicon solar cells reaching a value of 1.98 V. Additionally, we discuss the implementation of surface coatings by atomic layer deposition to i) alleviate the material degradation that occurs during silicon etching (electrical passivation) and ii) enable a chemically stable operation (protection against material corrosion). Finally, to demonstrate the beneficial effect of porosity on the hydrogen production we focus on two systems: i) a well-known simplified system based on platinum nanoparticle decorated porous silicon photocathodes (Von ≈ 475 mV) immersed in a sulfite scavenger (cell potential ΔΕ0 = 170 mV) in absence of electrical bias and ii) the envisage device of porous multi-junction silicon solar cell monoliths in unbiased solar water splitting conditions. Overall, keeping a device oriented point of view, we discuss the results and challenges in our approach as well as some design guidelines.

References:

[1] S. Y. Reece et al., Science 334, 645-648 (2011).

[2] T. Bosserez et al., J. Phys. Chem. C 120 (38), 21242 – 21247 (2016)

[3] C. Trompoukis et al., Sol. Energy Mater. Sol. Cells 182, 196–203 (2018).

16:30 - 16:45
S1.3-O5
de la Peña O'Shea, Víctor A.
IMDEA Energía
Solar Fuels Productions by Artificial Photosynthesis: From Inorganic Semiconductors to Hybrid Multifunctional Materials
Víctor A. de la Peña O'Shea
IMDEA Energía
Authors
Alba García Sanchez a, Patricia Reñones a, Carmen García a, Elena Alfonos a, Laura Collado a, Raul Perez Ruiz a, Mariam Barawi a, Igancio Villar a, Marta Liras a, Fernando Fresno a, Víctor Antonio de la Peña O'Shea a
Affiliations
a, 1 Photoactivated Proceses Unit IMDEA Energy Institute
Abstract

Photocatalytic conversion of CO2 and H2O is an interesting route to produce fuels and chemicals [1]; this process is also known as Artificial Photosynthesis (AP). In last years, extensive efforts have been made to develop efficient catalytic systems capable of harvesting light absorption and reducing CO2 especially when using water as the electron donor. Several modification pathways of inorganic semiconductors have been performed to improve the reaction efficiencies, including tailoring of the band structure, doping with metals and non-metallic elements and deposition of metal nanoparticles, etc [1-2].

Herein, we report different strategies and modifications of photocatalysts to increase process performance. Among them, an interesting approach to improve charge separation in photocatalytic systems is the use of heterojunctions. In this line, the combination of different semiconductors with noble metal nanoparticles or organic semiconducting polymers leads to a separation of the photogenerated charge carriers and thus to increasing their life time, facilitating charge transfer to adsorbed molecules.

The main products, using bare TiO2, were CO and H2, with low concentrations of CH4. The deposition of surface plasmon nanoparticles (SP-NPs) leads to changes in the selectivity to higher electron-demanding products, such as CH4. TAS measurements confirm that this behaviour is due to the electron scavenging ability of SP-NPs.

The H2 evolution rates of organo-inorganic hybrid materials, is increased with the polymer content reaching the optimum with IEP-1@T-10 which improves the activity of TiO2 by a factor of 40. These hybrid materials also show a dramatic reactivity improvement in CO2 photoreduction. Hybrid materials also show a great change in the selectivity, enhancing the relative production of methane vs. carbon monoxide, and largely promoting selectivity towards hydrogen. Meanwhile, bare TiO2 gives rise to the formation of CO, a small amount of CH4 and H2 and traces of CH3OH and higher hydrocarbons (C2 mainly). To explain this behaviour a combination of in-situ NAP-XPS, FTIR, TAS spectroscopies and theoretical tools has been used, showing a more efficient light absorption and charge transfer in the hybrid photocatalyst compared with bare materials.

References

[1] V. A. de la Peña O’Shea, D. P. Serrano, J. M. Coronado, “Current challenges of CO2 photocatalytic reduction over semiconductors using sunlight”, in Molecules to Materials—Pathway to Artificial Photosynthesis, Ed. E. Rozhkova, K. Ariga (Eds.), Springer, London, 2015.

[2] L. Collado, A. Reynal, J.M. Coronado, D.P. Serrano, J.R. Durrant, V.A. de la Peña O'Shea, Appl. Catal. B: Environ. 178, 177, 2015

 
Tue Oct 23 2018
Dyman S5.4
Chair: Stefano Sanvito
09:00 - 09:30
S5.4-O1
Annadata, Harshini
Department of Chemistry, National University of Singapore
Tuning the Tunnelling Decay Coefficient Using Single Polarizable Atoms via Energy Level Engineering of the Frontier Orbitals
Harshini Annadata
Department of Chemistry, National University of Singapore, SG

Ms. Harshini V. Annadata is a 5th year PhD student in the Nijhuis group and belongs to the Department of Chemistry, NUS.  She has experience with computational chemistry techniques like VASP, SIESTA and Gaussian09. She also has experience in soft x-ray spectroscopies including PES, NEXAFS and CHC.

Authors
Xiaoping Chen a, Harshini Annadata a, David Egger b, Christian Nijhuis a, c, d
Affiliations
a, Department of Chemistry, National University of Singapore, 21 Lower Kent Ridge Road, Singapore, 119077, SG
b, Institute of Theoretical Physics, University of Regensburg, Universitätsstraße 31, 93040 Regensburg, Germany
c, Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
d, NUSNNI-Nanocore, National University of Singapore, Singapore 117411, Singapore *Correspondence to: chmnca@nus.edu.sg
Abstract

Understanding the charge transport rates across molecules and molecule-electrode interfaces  is important in many areas of research including chemistry, biology, and nanoscience.1-2 A crucial parameter is the tunnelling decay coefficient (β, in Å-1 or nC-1) which determines how quickly the current across the junction decreases as a function of the length of the molecule. Usually, the value of β can be changed by changing the chemical structure of the molecular backbone,3-4 but β also depends on the type of the binding with the electrodes for conjugated systems.3 We have reported that SAMs of S(CH2)nX where X = H, F, Cl, Br, or I, have increasingly high currents with increasing polarizability of X.5 Here we report a new approach to tune β by simply changing X. In this system, eutectic gallium-indium alloy (EGaIn) was used as the top electrode, a monolayer of S(CH2)nX was self-assembled on a Ag surface which also served as the bottom electrode. We found that as the polarizability of X increases from X = F to I, β decreased from 0.97 ± 0.04 nC-1 to 0.34 ± 0.01 nC-1 and the dielectric constant εr increased from 2.5 ± 0.6 to 8.9 ± 1.6, respectively. DFT calculations show that the electrostatic potential profile of the SAM depends on X. More specifically, we found that the HOMO-1 is the dominant conduction orbital that is highly effected by X resulting in the tunnelling barrier height and thus the decay coefficient. In other words, the value of β can be controlled by using one polarizable atom without the need to change the molecular backbone.

References:

(1)           Stubbe, J. et al. Chem. Rev. 2003, 103, 2167.

(2)           Heitzer, H. M. et al. Acs Nano 2014, 8, 12587.

(3)           Kim, B. et al. Am. Chem. Soc. 2011, 133, 19864.

(4)           Xie, Z. et al. Acs Nano 2015, 9, 8022.

(5)           Wang, D. et al. Adv. Mater. 2015, 27, 6689.

09:30 - 10:00
S5.4-O2
Vicent-Luna, José Manuel
Department of Physical, Chemical and Natural Systems, University Pablo de Olavide
Molecular Dynamics Analysis of Charge Transport in Ionic Liquid Electrolytes containing added salt with Mono, Di, and Trivalent Metal Cations
José Manuel Vicent-Luna
Department of Physical, Chemical and Natural Systems, University Pablo de Olavide, ES
Authors
José Manuel Vicent Luna a, Azaceta Eneko b, Said Hamad a, José Manuel Ortiz Roldán a, Ramón Tena Zaera b, Sofía Calero a, Juan Antonio Anta a
Affiliations
a, Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide, 41013 Sevilla, Spain
b, CIDETEC, Parque Tecnológico de San Sebastián, Paseo Miramón, 196, Donostia–San Sebastián 20014 (Spain)
Abstract

Among many other applications, room temperature ionic liquids (ILs) are used as electrolytes for storage and energy conversion devices. In this context, rechargeable batteries are extended and useful devices to store energy. Since their discovery in 1970s, Li-ion batteries have become popular energy storage solutions and nowadays represent a promising alternative to conventional devices. The increasing requirements and power of these batteries, and disadvantages such as degradation or high flammability, make essential the search of alternative materials. Thanks to the high abundancy of Na as a raw material, Na-ion batteries have attracted intense attention as potential candidates for the replacement of Li-ion batteries. Other alternatives to Li-ion batteries based on multivalent metal cations are emerging in recent years. The ability of these metal species to transfer more than one electron can be useful to obtain faster charge rates. In this work, we investigate at microscopic level the structural and dynamical properties of 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide [C4PYR]+[Tf2N]- IL-based electrolytes for metal-ion batteries. We carried out molecular dynamics simulations of electrolytes mainly composed of [C4PYR]+[Tf2N]- IL with the addition of Mn+-[Tf2N]- metal salt (M = Li+, Na+, Ni2+, Zn2+, Co2+, Cd2+, and Al3+, n = 1, 2, and 3) dissolved in the IL. The addition of low salt concentration lowers the charge transport and conductivity of the electrolytes. This effect is due to the strong interaction of the metal cations with the [Tf2N]- anions, which allows for molecular aggregation between them. We analyze how the conformation of the [Tf2N]- anions surrounding the metal cations determine the charge transport properties of the electrolyte. We found two main conformations based on the size and charge of the metal cation: monodentate and bidentate (number of oxygen atoms of the anion pointing to the metal atoms). The microscopic local structure of the Mn+-[Tf2N]- aggregates influences the microscopic charge transport as well as the macroscopic conductivity of the total electrolyte.

Acknowledgements. The research leading to these results has received funding from the Andalucía Region (FQM-1851).

10:00 - 10:30
S5.4-O3
Colchero, Jaime
Slow Charge Relaxation in Highly Disordered Electronic Systems studied by Electrostatic Scanning Force Microscopy
Jaime Colchero
Authors
Jaime Colchero a, Miguel Ortuño a, Maria Fernandez M.F. Orihuela a, Andres Somoza a, Emin Istif b, Sandra Víctor-Roman b, Thierry Grenet c, Julien Delahaye c, Ana Maria Benito b, Wolfgang Maser b, Elisa E. Palacios-Lidón a
Affiliations
a, Universidad de Murcia, (Campus Espinardo) Universidad de Murcia, Murcia, ES
b, Instituto de Carboquimica ICB-CSIC, Miguel Luesma Castan 4, Zaragoza, 50018, ES
c, Institut Néel - CNRS, Grenoble
Abstract

While charge transport in highly ordered system is generally well understood, correct modelling of conductivity in highly disordered system quite often presents an important theoretical but also experimental challenge. Slow conductance relaxation has been studied in many disordered insulators using field effect measurements. After a quench at low temperatures, a change in the gate voltage is accompanied by a sudden increase in the conductivity, slowly decreases with a roughly logarithmic dependence on time. Memory effects and aging are often seen in the same type of experiments. These glassy behaviour has been interpreted in different ways, but there is a growing tendency to explain them in terms of electron glasses, i.e., systems with states localized by the disorder and long-range Coulomb interactions between carriers.

The use of local probe techniques, such as scanning Kelvin probe microscopy (SKPM), presents two advantages as compared with the conductance measurements performed so far: i) it allows a study of the phenomena at the nanometer scale, and ii) samples with larger resistances can be measured. In the present work Dynamic AFM is used to characterize the electronic properties of two quite different nanoscale highly disordered and low conductivity systems. On the one hand highly resistive granular metal grains[1], and on the other Graphene Oxide islands[2].

We apply Electrostatic and Kelvin Force Microscopy to these samples. In addition, their time evolution is studied using “movies” where topography and surface potential are acquired simultaneously. Our AFM studies suggest evidence of the formation of an electron glass on the materials studied. This evidence includes the presence of domains on the surface potential, uncorrelated with topograph. The fluctuations of the surface potential are compatible with variations of the Coulomb energy of a single charge over the distance between domains. At the same time, the fact that the fluctuations are larger than kT and that time correlations are dominated by a broad distribution of characteristic times can be naturally explained within the electron glass model. When the conducting polymers are excited with light the surface potential relaxes logarithmically with time, as usually observed in electron glasses.


 

[1] M. Ortuño, E. Escasaín, E. López-Elvira, A. Somoza, J. Colchero and E. Palacios-Lidón. Scientific Reports, Scientific Reports 6, article #21647 (2016).

[2]M.F. Orihuela, A.M. Somoza, J. Colchero, M. Ortuño, E. Palacions-Lidón, Physical Review B 95(20) 205427 (2017).

NCFun S3.4
Chair: Mischa Bonn
09:00 - 09:30
S3.4-I1
Oron, Dan
Weizmann Institute of Science
Luminescence Upconversion in Designer Semiconductor Nanocrystals
Dan Oron
Weizmann Institute of Science, IL
Authors
Dan Oron a
Affiliations
a, Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel
Abstract

One of the potential pathways for exceeding the Shockley-Queisser efficiency limit of photovoltaic cells is via incoherent upconversion of low-energy photons. An ‘ideal’ upconversion system would have high efficiency, a broad response and a low saturation intensity, all encapsulated in an easily deposited system. Past colloidal upconversion nanocrystals were based on rare-earth doped oxides or on organic triple-triplet annihilation polymers. Several years ago we introduced double quantum dots as an alternative upconversion mechanism. In this system a low bandgap dot is coupled to a high band gap dot through a tunneling barrier. Interband absorption in the low bandgap dot is followed by intraband absorption which enables to transfer the excitation to the higher bandgap dot, eventually leading to emission of a higher energy photon. The formation of the latter system and recent advances in the design of double quantum dots will be discussed. We also introduce a new hybrid organic-inorganic particle where upconversion is mediated by ligand induced mid-gap states. Importantly, this latter system relies on direct excitation of charge transfer state and can therefore be potentially extended deeper into the infrared. The state of the art of all these designs will be discussed, as well as the potential for low cost upconversion add-ons to present day solar cells.

09:30 - 10:00
S3.4-I2
Cánovas, Enrique
IMDEA Nanoscience
Hot Electron Transfer Efficiency at Quantum Dot-Oxide Interfaces
Enrique Cánovas
IMDEA Nanoscience, ES

Enrique Cánovas graduated on Applied Physics at Universidad Autónoma de Madrid (2002). After that, he realized a two-years Master of Advanced Studies at Universidad de Valladolid working on the spectroscopic characterization of native and operation-induced defects in high power laser diodes. From 2004 to 2006 he made a second Master of Advanced Studies at Universidad Politécnica de Madrid (Institute of Solar Energy, IES); training focus was on the fabrication, characterization and optimization of solid state solar cells. In 2006 he joined the group of Prof. Martí  and Prof. Luque at IES, where he completed PhD studies on the spectroscopic characterization of novel nanostructures aiming ultra-high-efficiency solar cells. His PhD studies included two placements (covering 9 months in total) at Lawrence Berkeley National Laboratory (USA - with Prof. W. Walukiewicz) and Glasgow University (Scotland - with Prof. Colin Stanley). Between 2010 and 2012 he worked as a postdoc at FOM Institute AMOLF (Amsterdam - The Netherlands, Prof. M. Bonn) on the characterization of carrier dynamics in sensitized solar cell architectures. Between 2012 to 2018 he lead the Nanostructured Photovoltaics Group at Max Planck Institute for Polymer Research (Mainz, Germany). Since April 2018, Enrique Canovas works at IMDEA Nanoscience where he was appointed Assistant Research Proffesor (tenure-track).  His research interests cover all aspects of photovoltaics, nanotechnology and charge carrier dynamics.

Authors
Enrique Canovas a, b, Hai Wang b, Mischa Bonn b
Affiliations
a, IMDEA Nanoscience, C/faraday, 9, Madrid, 28049, Madrid, ES
b, Max Planck Institut for Polymer Research, Ackermannweg 10, Mainz, 55128, DE
Abstract

The maximum efficiency reachable by a photovoltaic device based on a single absorber is thermodynamically limited to ~33%; the Shockley−Queisser (SQ) limit. To a large extent, this efficiency limit is determined by waste heat in the absorber; waste heat which is generated by the thermalization of “hot” charge carriers generated in the material following the absorption of above-bandgap high-energy photons. Several approaches have been proposed in order to bypass thermal losses in solar cell devices, among them, hot carrier solar cells (HCSCs) are distinctly the most promising concept when considering photo-conversion efficiency limits (reaching ~74%). However, practical implementation of operational HCSCs prototypes remains a big challenge; this is in part due to the difficulties on finding/engineering systems where hot carriers are efficiently collected at room temperature.

In this communication, by employing time resolved THz spectroscopy (TRTS), we demonstrate highly-efficient room-temperature hot electron transfer (HET) at QD/mesoporous oxide interfaces. The emergence of HET is directly apparent from photon-energy dependent TRTS measurements. When the samples are irradiated with photon energies matching the QD bandgap, the ET dynamics are monophasic and defined by a time constant of ~10ps. When the samples are irradiated with above QD bandgap photon energies, the ET dynamics become bi-phasic with characteristic time constants of ~10ps and <1ps respectively (representing cold and hot ET components respectively). For even higher photon energies (~3eV photons onto ~1eV bandgap PbS QDs) the ET dynamics become again monophasic with sub-ps time constants (≤0.1ps, limited by the TRST setup resolution). In this case, the HET collection efficiency for photo-generated carries reaches unity quantum yield. Finally, from temperature dependent analysis of interfacial QD-oxide dynamics, we resolve that HET rates (and hence efficiency) are substantially enhaced as the temperature of the system is reduced. This observation is fully consistent with the “a pirori” expectation that HET efficiency is determined by kinetic competition between QD-to-oxide HET rate and hot electron cooling rate within the QD.

Our results reveal the effect and interplay of key parameters governing hot electron transfer at QD-oxide interfaces. The foreseen potential and constraints for the analyzed systems for the realization of hot carrier solar cells prototypes will be briefly discussed.

10:00 - 10:30
S3.4-I3
Delerue, Christophe
IEMN - UMR 8520
Theory of Localized Surface Plasmon Resonance in Doped Semiconductor Nanocrystals
Christophe Delerue
IEMN - UMR 8520, FR
Authors
Christophe Delerue a
Affiliations
a, IEMN, UMR-CNRS 8520, Villeneuve d'Ascq, France
Abstract

Nanocrystals of heavily-doped semiconductors have recently emerged as very promising materials for plasmonics. In contrast to nanocrystals of noble metals, their Localized Surface Plasmon Resonance (LSPR) can be easily tuned in energy by controlling the carrier concentration through doping. In addition, due to the low concentration of carriers compared to metals, the LSPR can be extended to infrared and near-infrared ranges. Recent experimental studies have demonstrated the existence of LSPR in doped nanocrystals of Si and different types of oxides (ZnO, SnO2, In2O3). However, the physics of the LSPR in these NCs is not totally understood. In this talk, I will review recent theoretical studies performed to clarify a certain number of issues. The evolution with doping concentration of the optical processes from single-electron transitions to collective excitations will be described. The conditions required for the emergence of plasmonic modes will be discussed. The results of atomistic calculations will be compared with those of more classical approaches. The intrinsic mechanisms at the origin of plasmon damping in doped ZnO nanocrystals will be analyzed. In this case, the theoretical simulations show that the intrinsic line width of the LSPR can be below 80 meV, in agreement with recent experiments [1]. These results confirm that doped ZnO nanocrystals are very promising for the development of IR plasmonics.

[1] Delerue C. “Minimum Line Width of Surface Plasmon Resonance in Doped ZnO Nanocrystals”. Nano Letters 17 (12), 7599-7605 (2017).

PVCon S9.4
Chair: Alejandro Perez-Rodriguez
09:00 - 09:30
S9.4-I1
Brammertz, Guy
imec-department imomec
Wide Band Gap Kesterite Absorbers for Tandem or Semi-transparent Solar Cells
Guy Brammertz
imec-department imomec, BE

Guy Brammertz graduated in 1999 from the University of Liège (Belgium) in Applied Physics. In 2003 he obtained his Ph.D. from the University of Twente (The Netherlands) defending a thesis about his work on superconducting Josephson junction photon detectors carried out for the European Space Agency. He then joined imec in 2004, where he first was involved in the LogicDram program aiming at the fabrication of Ge and III-V 35 nm gate length MOS transistors for CMOS applications. His work focused on electrical and optical characterization as well as passivation of electrical defects at Ge and III-V/oxide interfaces. In 2011 he joined the imec photovoltaic program, where he is now working on the fabrication and characterization of thin film solar cells based on Cu(In,Ga)(S,Se)2 (CIGS), Cu2ZnSn(S,Se)4 (CZTS) and Cu2ZnGe(S,Se)4 (CZGS) absorbers.

Authors
Guy Brammertz a, b, Leo Choubrac e, Thierry Kohl a, b, Jessica deWild a, b, Marc Meuris a, b, Jef Poortmans b, c, d, Bart Vermang b, c
Affiliations
a, Imec division IMOMEC (partner in Solliance & EnergyVille), Wetenschapspark 1, 3590 Diepenbeek, Belgium.
b, Institute for Material Research (IMO), Hasselt University (partner in Solliance & EnergyVille), Agoralaan gebouw H, Diepenbeek, 3590, Belgium.
c, imec (partner in Solliance & EnergyVille), Kapeldreef 75, Leuven, 3001, Belgium.
d, Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium.
e, Institut des matériaux Jean rouxel, Université de Nantes, France
Abstract

In the present talk we discuss the opportunities and challenges for Kesterite based high band gap absorber layers to act as functional layers in tandem or semi transparent thin film solar cells. Materials based on the kesterite crystal structure have already shown their good properties as thin film solar cell absorbers. Depending on the elements in the crystal, a high band gap value can be achieved, which could present an opportunity for these type of materials to be used as a top cell in tandem devices or as an active layer in a semi-transparent thin film solar cell. After a general introduction we present the properties of Cu2ZnGe(S,Se)4 based solar cell devices, which have a band gap in the 1.4 to 2 eV range depending on the S content in the layer. The crystallization process is analyzed and the solar cell properties of such devices are presented in detail. Solar cell conversion efficiencies in excess of 8 % could be achieved with this material system but some challenges remain. We will compare these results to other high band gap chalcogenide thin film absorber materials.

09:30 - 10:00
S9.4-O1
Alcobé, Xavier
CZTSe Solar Cells Developed On Alternative Substrates for Advanced Applications
Xavier Alcobé
Authors
Ignacio Becerril-Romero a, Simón Lopez-Marino a, Marcel Placidi a, Moisés Espíndola-Rodríguez a, Florian Oliva a, Victor Izquierdo-Roca a, Yudania Sánchez a, Xavier Alcobe b, Edgardo Saucedo a, Paul Pistor a, c
Affiliations
a, Institut de Recerca en Energia de Catalunya (IREC), Barcelona, Spain
b, Centres Científics i Tecnològics (CCiTUB) de la Universitat de Barcelona, C/ Lluis Solé i Sabaris 1-3, 08028 Barcelona, Spain
c, Martin-Luther-University Halle-Wittenberg, Halle (Saale), Germany
Abstract

Thin-film technologies represent one step forward in the range of applicability of photovoltaics (PV) since they can be manufactured onto virtually any surface or material giving them an enormous versatility compared to Si-based solar cells. Flexible and light-weight substrates like thin metallic or polymeric foils are particularly attractive for thin-film PV since they allow powering portable electronics, wearables and IoT devices adapting to any shape and without adding extra weight. These characteristics also make them interesting for applications in the space and transportation industry. In addition, flexible substrates are expected to reduce PV fabrication costs through roll-to-roll high-throughput industrial processes. Another interesting niche of application that can benefit from the versatility of thin-film PV is building-integrated photovoltaics (BIPV). Solar devices can be directly deposited on materials commonly used in construction such as ceramics or glazing so that solar devices are made integral parts of buildings reducing manufacturing and installation costs as well as avoiding the need of extra land allocation for power generation which are often regarded as two critical factors that may restrict the massive deployment of photovoltaics. Among thin-film technologies, Cu2ZnSn(S1-xSex)4 compounds, also known as Kesterites, stand out by the earth-abundancy and non-toxicity of its constituent elements that makes them compatible with a future mass deployment of PV. Thus, this work explores the implementation of a Cu2ZnSnSe4 (CZTSe) sequential fabrication process based on the selenization of sputtered metallic stack precursors onto different substrates: polyimide (PI), stainless steel (SS) and ceramic tiles. However, the increased applicability range of these substrates has a dark side since they lack several favorable properties of soda-lime glass (SLG): favourable thermomechanical properties and beneficial alkali (Na and K) composition that diffuse into the absorber during its synthesis and are fundamental for high efficiency kesterite devices. This way, SS and ceramic possess rough surfaces that complicate the deposition of thin films and detrimental impurities in their composition that can hinder the performance of the devices. As for PI it has a low thermal robustness that limits the synthesis temperature below 500ºC. In addition, none of them contain alkalis thus requiring the development of extrinsic doping procedures. In this work, we present different strategies to overcome the specific problems of each of the substrates and demonstrate their suitability as substrates for CZTSe solar cells.

10:00 - 10:30
S9.4-I2
Würfel, Uli
Fraunhofer Institute for Solar Energy Systems ISE
Factors Determining the Electrode Selectivity in Solar Cells and their Impact on Device Performance
Uli Würfel
Fraunhofer Institute for Solar Energy Systems ISE, DE
Authors
Uli Würfel a
Affiliations
a, Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, 79110 Freiburg, Germany
Abstract

In a solar cell, an ideally selective electron (hole) contact would only exchange electrons (holes) with the conduction (valence) band of the absorber material. This would ensure that no losses occur due to surface recombination
and the open-cicuit voltage would be determined solely by generation and recombination in the bulk. We had shown that selectivity depends on the conductivity of the minority carriers in the vicinity of the contacts [1].
Introducing this general concept allows also to show that a solar cell does not require an electric field in the dark und thus that Voc can be much larger than the built-in voltage. After that different approaches will be discussed how to achieve a high degree of contact selectivity in solar cells such as the mobility junction, the pn junction, the heterojunction and more recent concepts such as thin layers with strong dipole moment altering the effective work function of the electrode [2,3]. Results from intensity and temperature-dependent measurements in combination with numerical simulations will be discussed in detail. Finally, examples from our own labs shall demonstrate which phenomena can arise from the selectivcity of the contacts in the case of silicon, organic and perovskite solar cells [4,5].

[1] U. Würfel, A. Cuevas, P. Würfel, IEEE J. Photovoltaics 5 (2015), 461
[2] U. Würfel et al., Adv. Energy Mater. 6 (2016), 1600594
[3] C. Reichel, U. Würfel et al., J. Appl. Phys. 123 (2018), 024505
[4] J. Reinhardt, M. Grein, C. Bühler, M. Schubert and U. Würfel, Adv. Energy Mater. 4 (2014), 1400081
[5] A. Spies, T. Sarkar, M. List and U. Würfel Adv. Energy Mater. 7 (2017), 1601750

PerFun S7.4
Chair: Robert Lovrincic
09:00 - 09:30
Abstract not programmed
09:30 - 10:00
S7.4-I1
Olthof, Selina
Universität zu Köln
Unravelling the Electronic Structure of Hybrid Perovskites and their Interfaces
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 hybrid organic - inorganic perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaic; but other fields, like light emitting diodes, show great potential as well. In such devices, the function and performance depend crucially on the proper alignment of the energy level landscape throughout the device, i.e. allowing for efficient charge transport across the various interfaces. Here, an advantage of these novel semiconductors is that the electronic structure and band gap energy can be readily tuned by changing the compositions of the perovskite.

In this talk, I will discuss recent findings regarding the variations in electronic structure of hybrid perovskites, covering all lead and tin based halide systems using a combined DFT and UV-/ inverse/ x-ray photoelectron spectroscopy study. Furthermore, with these surface sensitive techniques, the energetic alignment at interfaces between different layers can be probed in-situ by performing a stepwise film preparation. Looking at various bottom contacts I will show that chemical interactions, band bending, and interface dipole formation play an important role.

10:00 - 10:30
S7.4-O1
Pazos Outon, Luis
Electrical Engineering and Computer Sciences, University of California, Berkeley
Fundamental Efficiency Limit of Lead Iodide Perovskite Solar Cells
Luis Pazos Outon
Electrical Engineering and Computer Sciences, University of California, Berkeley
Authors
Luis Pazos-Outon a, T. Patrick Xiao a, Eli Yablonovitch a
Affiliations
a, Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, California 94720
Abstract

Lead halide materials have seen a recent surge of interest from the photovoltaics community following the observation of surprisingly high photovoltaic performance, with optoelectronic properties similar to GaAs. This begs the question: What is the limit for the efficiency of these materials? It has been known that under 1-sun illumination the efficiency limit of crystalline silicon is ∼29%, despite the Shockley–Queisser (SQ) limit for its bandgap being ∼33%: the discrepancy is due to strong Auger recombination. In this article, we show that methyl ammonium lead iodide (MAPbI3) likewise has a larger than expected Auger coefficient. Auger nonradiative recombination decreases the theoretical external luminescence efficiency to ∼95% at open-circuit conditions. The Auger penalty is much reduced at the operating point where the carrier density is less, producing an oddly high fill factor of ∼90.4%. This compensates the Auger penalty and leads to a power conversion efficiency of 30.5%, close to ideal for the MAPbI3 bandgap.

PerMod S8.4
Chair: Beat Ruhstaller
09:00 - 09:30
S8.4-I1
Richardson, Giles
University of Southampton
How Transport Layer Properties affect Perovskite Solar Cell Performance: Insights from a Coupled Charge Transport/ion Migration Model
Giles Richardson
University of Southampton, GB
Authors
Giles Richardson a
Affiliations
a, University of Southampton, Southampton, GB
Abstract

The effects of transport layers on perovskite solar cell performance, in particular anomalous hysteresis, are investigated. A model for coupled ion vacancy motion and charge transport is formulated and solved in a three-layer planar perovskite solar cell. Its results are used to demonstrate that the replacement of standard transport layer materials (spiro-OMeTAD and TiO2) by materials with lower permittivity and/or doping leads to a shift in the scan rates at which hysteresis is most pronounced to rates higher than those commonly used in experiment. These results provide a cogent explanation for why organic electron transport layers can yield seemingly “hysteresis-free” devices but which nevertheless exhibit hysteresis at low temperature. In these devices the decrease in ion vacancy mobility with temperature compensates for the increase in hysteresis rate with use of low permittivity/doping organic transport layers. Simulations are used to classify features of the current-voltage curves that distinguish between cells in which charge carrier recombination occurs predominantly at the transport layer interfaces and those where it occurs predominantly within the perovskite. These characteristics are supplemented by videos showing how the electric potential, electronic and ionic charge profiles evolve across a planar perovskite solar cell during a current-voltage scan. Design protocols to mitigate the possible effects of high ion vacancy distributions on cell degradation are discussed. Finally, features of the steady-state potential profile for a device held near the maximum power point are used to suggest ways in which interfacial recombination can be reduced, and performance enhanced, via tuning transport layer properties. 

 

09:30 - 10:00
S8.4-I2
Gagliardi, Alessio
Technische Universitaet Muenchen
Simulation of Perovskite Solar Cells: the Role of Internal Interfaces
Alessio Gagliardi
Technische Universitaet Muenchen, DE
Authors
Alessio Gagliardi a
Affiliations
a, Technische Universität München, Karlstraße 45, München, 80333, DE
Abstract

Perovskite solar cells have gathered a large interest in the last years as a very compelling and promising photovoltaic technology thanks to many interesting properties such as a wide spectrum of deposition techniques, a simple integration with both organic and inorganic materials and, most important of all, a high light power conversion efficiency.

Perovskite materials have also challenged the scientific community due to the many different physical processes that concur to set the optical and electrical properties: from ferroelectricity [1], to ion migration [2], defects and different recombination processes [3]. An important aspect of perovskite films is the presence of interfaces, both due to grain boundaries as well as due to interfaces between the perovskite layer and the charge selective contacts. Although many progresses have been obtained in the quality of the film, still grain boundaries within the perovskite film in fabricated devices are present. These interfaces can play a major role in setting device performances and hysteresis effects.

The effect of these grain boundaries and interfaces have been investigated by many groups, we refer here to just one reference [4], but the effect to free charges and ion migration is still under debate.

In the present work we theoretically investigate the effect of ion migration with the presence of grain boundaries. The analysis is performed using two different models: a drift-diffusion model to study the role of the mesoporous electron selective contact [5] and kinetic Monte Carlo [6] for the effect of grains and grain boundaries.

References

[1] A. Pecchia et al., Nano Lett., 16, 988 (2016)

[2] J. M. Azpiroz et al., Energy & Environmental Science, 8, 2118-2127 (2015)

[3] L. M. Herz, Annual Rev. Phys. Chemistry, 67, 65-89 (2016)

[4] B. Roose et al., Nano Energy, 39, 24-29 (2017)

[5] A. Gagliardi and A. Abate, ACS Energy Lett., 3, 163 (2018)

[6] T. Albes, A. Gagliardi, Physical Chemistry Chemical Physics, 19 (31), 20974-20983 (2017)

 

10:00 - 10:30
S8.4-O1
Meggiolaro, Daniele
Defects in Lead Halide Perovskites: a Computational Perspective
Daniele Meggiolaro
Authors
Daniele Meggiolaro a, b, Edoardo Mosconi b, Filippo De Angelis a, b
Affiliations
a, D3-Computation, Istituto Italiano di Tecnologia, Via Morego 30, Genova, IT
b, Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di ScienzeTecnologie Molecolari (ISTM-CNR), Via Elce di Sotto 8, 06123, Perugia, Italy.
Abstract

Lead halide perovskites are promising materials for new generation photovoltaics, exceeding 22% of efficiency in solar cells devices.[1] The presence of native defects, however, can strongly affect their efficiency due to charge trapping processes which can limit the lifetime of the photogenerated charge carriers. In this work a state of the art Density Functional Theory (DFT) study of native defects in MAPbI3 and MAPbBr3 is presented, aimed to unveil the nature of deep charge traps in these materials and the associated defects chemistry. The technical aspects of the computational modelling of defects are illustrated, with particular emphasis on the role of theory in the accurate evaluation of defects properties. The role of spin-orbit and self interactions corrections are discussed, as well as the performance of different corrections schemes in the supercell approach.[2] Good practices and open issues in the technical modelling of defects in these materials are discussed. Thus, a global picture of the defects chemistry in these perovskites is provided by the analysis of the associated formation energies in different conditions of growth and of the thermodynamic ionization levels. Our discussion shows that the defects chemistry of these materials is intrinsically dominated by halide chemistry, that is at the heart of their high defects tolerance.[3]     

References

[1] Wehrenfennig et al. Adv. Mater. 2014, 26, 1584-1589.

[2] Komsa et al. Phys. Rev. B 2012, 86, 045112.

[3] Meggiolaro et al. Energy Environ. Sci. 2018, 11, 702-713.

SolFuel S1.4
Chair: Vincent Artero
09:00 - 09:30
S1.4-I1
Cowan, Alexander
University of Liverpool
Sum-Frequency and Surface Sensitive Spectroscopy of Electrode and Photoelectrode Surfaces
Alexander Cowan
University of Liverpool, GB
Authors
Alexander Cowan a
Affiliations
a, Department of Chemistry, University of Liverpool, Crown Street, Liverpool, Liverpool, GB
Abstract

There is a need to rationalise the catalytic mechanisms occurring during solar fuels production if the design rules for improved materials are to be identified. With both direct solar to fuels materials (e.g. photoelectrodes) and indirect systems (e.g. PV driven electrolysis) there is a common challenge – how to characterise short-lived intermediates that are present only at the electrode surface in the presence of a high concentration of bulk solvent/reagent.

IR-Vis Sum-Frequency Generation (SFG) spectroscopy specifically probes molecules at interfaces offering a sensitive probe of only the catalytically relevant species at the electrode surface. SFG has been widely used to study model electrochemical systems but rarely to explore the mechanisms of carbon dioxide reduction and water oxidation. Here I will describe both early experiments to examine photoelectrode surfaces and our recent reports on the mechanisms of carbon dioxide reduction by group 6 and 7 transition metal electrocatalysts which allow for the identification of new catalytic intermediates and the first in-situ observation of the “protonation-first” pathway by [Mn(bpy)(CO)3Br]. We will also discuss how the potential and light dependence of the non-resonant response recorded during the SFG experiment can provide important insights into the changing nature of the double layer structure during the study of solar fuels materials.

09:30 - 10:00
S1.4-O1
Guijarro Carratala, Nestor
École Polytechnique Fédérale de Lausanne EPFL
Operando Potential-Sensing at the Semiconductor-Liquid Junctions: Tracking the Surface Energetics and Interfacial Kinetics during Photoelectrosynthetic Reactions
Nestor Guijarro Carratala
École Polytechnique Fédérale de Lausanne EPFL, CH
Authors
Nestor Guijarro a, Yongpeng Liu a, Florian Le Formal a, Liang Yao a, Kevin Sivula a
Affiliations
a, Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, Ecole Polytechnique Fédérale de Lausanne, Station 6, CH-1015, Switzerland
Abstract

Storing the energy of sunlight into feedstock chemicals or energy-rich compounds, such hydrogen, appears as an enticing option to broaden the utilization of solar energy far beyond classical photovoltaics. Among different emerging technologies, photoelectrochemical (PEC) water splitting stands out with the promise of competitive solar-to-hydrogen conversion efficiencies (predicted to be above 20 %) and a conveniently simple design.1 These devices, comprising two photoactive electrodes wired-stacked together, to leverage their complementary light absorption, and directly immersed in an aqueous electrolyte, generate under illumination a voltage greater than 1.23 V, driving the photoelectrosynthetic reactions of H2 and O2 separately at the different electrode-electrolyte interfaces. Unfortunately, current conversion efficiencies are far below the expectations. In recent years, there has been an encouraging progress on the refinement of the bulk properties of the semiconducting electrodes (viz. nanostructuring, doping, band gap engineering, etc.) as well as on the design of more active electrocatalyst, both contributing to an improved performance. However, another key component of these devices, the semiconductor-liquid junction (SCLJ), where the complex reactions occur and therefore, whose electrochemical and catalytic properties (namely, surface potential, overpotential, kinetics of charge transfer and recombination) control the conversion efficiency, remains hazy posing a major bottleneck for enhancing the conversion efficiency. A better understanding on the electrocatalytic properties of the SCLJ could not only nail down the processes limiting the performance of these devices but also provide specific routes to patch them.

Over the last few years, a wide variety of in-operando electrochemical based techniques have burst in the field of electrochemical-based solar fuel production offering new insights on the nature of intermediate species, the functioning of electrocatalysts, the carrier dynamics within the electrodes, etc.2 Here, we introduce a new technique where a network-type electrical-contact at the electrode-electrolyte interface afford direct probing of the surface carrier dynamics when combined with a transient photocurrent/photovoltage technique. This technique applied to model semiconducting materials incorporating state-of-the-art electrocatalyst demonstrates to provide unprecedented access to steady-state (interfacial energetics) and transient (interfacial kinetics) electrochemical information of the SCLJ in-operando. We believe that these new tools will help to forge a better understanding on the SCLJ and to establish the designing principles for a next generation of more efficient electrodes.

 

[1] M. S. Prévot, K. Sivula. J. Phys. Chem. C, 2013, 117, 17879-17893

[2] W. A. Smith, I. D. Sharp, N. C. Strandwitz, J. Bisquert. Energy Environ. Sci. 2015, 8, 2851-2862

10:00 - 10:15
S1.4-O2
Starr, David
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
Identification and Light-Induced Suppression of Surface States on BiVO4 Photanodes
David Starr
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, DE
Authors
David E. Starr a, Marco Favaro a, Fatwa F. Abdi a, Marlene Lamers a, Michael Kanis a, Hendrik Bluhm b, c, Ethan Crumlin c, Roel van de Krol a
Affiliations
a, Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
b, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
c, Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
Abstract

Due to their potential long term stability, ease of synthesis, and low production cost, semiconducting metal oxide materials have received much attention for use as photoanode materials in photoelectrochemical water splitting devices. To date, most research has focused on binary semiconducting oxide materials. Since no binary oxide material has currently met all the criteria listed above, researchers have expanded the materials search database to include more complex multinary oxides. Among the multinary oxides investigated, bismuth vanadate, BiVO4, is the highest performing material. However, charge carrier recombination at the BiVO4/electrolyte interface remains a limitation. Reactions at the BiVO4/electrolyte interface may give rise to surface states that can act as relay sites for charge injection into the electrolyte, but also 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 X-ray photoelectron spectroscopy (AP-XPS) to gain a molecular-level understanding of the BiVO4/aqueous electrolyte interface. With soft X-rays water adsorption from the gas phase at pressures up to a few Torr can be studied providing information about the early stages of solid/electrolyte interface formation. The tender X-ray form (AP-HAXPES) can be used to directly interrogate a solid surface under a bulk-like electrolyte film that is tens of nanometers thick. Our AP-XPS measurements on the BiVO4(010) single crystal surface indicate that the surface is significantly hydroxylated by ~0.5 Torr. Surface hydroxylation is accompanied by reduced vanadium at the surface which leads to occupied states above the valence band maximum. These states may act as recombination centers on BiVO4-based photoanodes. Using AP-HAXPES we have studied the open-circuit behaviour of the thin-film BiVO4/potassium phosphate electrolyte interface when illuminated with a solar simulator. Upon illumination we observe spectral changes consistent with the formation of a thin bismuth phosphate layer and significant restructuring of the electrolyte near the interface. Bismuth phosphate formation under illumination may quench surface states that have been observed in capacitance versus voltage scans. Surface state suppression by bismuth phosphate formation is a also potential explanation for the increase in performance of BiVO4 photoanodes that have undergone light soaking. In general, these results provide fundamental information about the complex chemical behaviour of semiconductor/electrolyte interfaces in water splitting devices.

10:15 - 10:30
S1.4-O3
Bozheyev, Farabi
National Laboratory Astana, 53 Kabanbay Batyr St., 010000 Astana, Kazakhstan
Passivation of the Surface of Transition Metal Dichalcogenides with Catalysts for Solar Hydrogen Evolution
Farabi Bozheyev
National Laboratory Astana, 53 Kabanbay Batyr St., 010000 Astana, Kazakhstan

Farabi Bozheyev received his master degree in 2011 and his Ph.D in 2015 on specialization of "Condensed Matter Physics" from the National Research Tomsk Polytechnic University (TPU). He worked at TPU as an engineer researcher from 2010 to 2015 on metal sulfide nanopowders (MoS2, WS2, ZnS, FeS) produced by a self-propagating high-temperature synthesis for solar energy and tribological applications. He joined the magnetron sputtering group as a guest scientist in 2013-2014 and further as a postdoc in 2016-2017 at Helmholtz-Zentrum für Materialien und Energie (HZB), where he investigated Mo3S13 nanoclusters, WS2 and WSe2 thin films for solar hydrogen evolution. Currently, he is a senior researcher at the National Laboratory Astana working on the modification of nanoparticles and thin films of transition metal dichalcogenides (TMDs) by ion and electron beams/pulses. His scientific interests are 2D materials, nanoparticles and thin films of TMDs, reactive magnetron sputtering, atomic layer deposition, solar and hydrogen energy.

Authors
Farabi Bozheyev a, Fanxing Xi b, Dennis Friedrich b, Marat Kaikanov a, Alexander Tikhonov c, Sebastian Fiechter b, Klaus Ellmer d
Affiliations
a, National Laboratory Astana, 53 Kabanbay Batyr St., 010000 Astana, Kazakhstan
b, Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
c, School of Science and Technology, Nazarbayev University, 53 Kabanbay Batyr St., 010000 Astana, Kazakhstan
d, Optotransmitter-Umweltschutz-Technologie e.V. (O.U.T.), Köpenicker Str. 325b, 12555 Berlin, Germany
Abstract

The development of efficient photoelectrochemical (PEC) cells for solar hydrogen production is necessary for a prospective renewable energy supply of mankind. Semiconducting films in PEC cells are used as absorber layers in a triple system of electrolyte-catalyst-semiconductor for generating hydrogen which can be stored and afterwards reburned to water in gas turbines, fuel cells or for the production of chemicals [1]. Due to the low efficiency and poor stability of the absorber layers in the electrolyte their performance must be considerably improved.

We were successful in the preparation of highly (001)-textured polycrystalline WSe2+x thin films with a Pd-promoter on TiN:O back contact substrates [2]. In this process, in a first step X-ray amorphous Se-rich WSex films have been deposited at room temperature by a reactive magnetron sputtering onto a thin metal-promoter Pd film which were afterwards annealed in an H2S(e)/Ar atmosphere. During the crystallization process a liquid promoter PdSex forms and migrates to the surface of the growing WSe2 film. It acts as a solvent agent and accumulates at the non van-der-Waals-planes finally initiating the lateral growth of WSe2 platelets and their coalescence. A maximum hole mobility of 70 cm2 V-1s-1 was reached for our polycrystalline films, which is close to the value for a single crystalline WSe2 (100 cm2 V-1s-1).

Recently, we improved the photoelectrochemical performance using Pt [3], Rh and ammonium thiomolybdate (ATM: (NH4)2Mo3S13) layers on top of the WSe2 photocathode for hydrogen evolution reaction in an acidic solution. The surface states of WS2 and WSe2 crystallites can be passivated by photodeposition of the metal catalysts such as Pt, Rh and Ag. We show that the ATM can be used instead of expensive conventional metal catalysts on the WSe2 photocathode. In addition, the ATM layer forms a heterojunction with the WSe2 film, which improves the charge carrier separation, and allows lowering the prize for preparation of catalyst layers on top of different types of semiconductors.

 

1. Fichtner, M. J. Alloys Comp. 509S, S529-S534 (2012).

2. Bozheyev, F., Friedrich, D., Nie, M., Rengachari, M. & Ellmer, K. Phys. Stat. Sol. A 211, 2013-2019 (2014).

3. Bozheyev, F., Harbauer, Zahn, C., Friedrich D., & Ellmer, K. Sci. Rep. 7, 16003 (2017).

10:30 - 11:00
Coffee Break
Dyman S5.5
Chair: Leeor Kronik
11:00 - 11:30
S5.5-I1
Sanvito, Stefano
Trinity College Dublin
Different Flavours of Constrained Density Functional Theory for Charge Dynamics in Organic Materials
Stefano Sanvito
Trinity College Dublin, IE
Authors
Stefano Sanvito a
Affiliations
a, School of Physics and CRANN, Trinity College Dublin
Abstract

Constructing a materials-specific theory of charge dynamics in organic single materials is a complex prob- lem, where the computation of accurate structural and vibrational properties needs to be coupled to ways of determining the charge mobility characteristics. In particular one needs an accurate method for describing excitations, which is also scalable to reasonably large systems. Here I will discussed how different flavours of constrained density functional theory (CDFT) can achieve such goal.

Firstly I will consider the most conventional form of CDFT, which allows one to calculate the energy of systems with displaced electron densities (e.g. in a charge transfer process). Such scheme can be used to extract a number of quantities important for charge dynamics. Here I will make examples of the calculation of 1) the charge transfer energies of molecules on surfaces, so to derive accurate level alignments [1,2]; 2) the quasi-particle gap renormalisation in molecular crystals [3]; 3) the reorganisation energy of molecules in the gas phase and on surfaces [4].

Then I will move to show a recently implemented scheme, which uses CDFT to compute elementary excitations in molecules [5]. This method, which we have named excitonic DFT (XDFT), calculates the M-particle excited state of an N-electron system, by optimizing a constraining potential to confine N−M electrons within the ground-state Kohn-Sham valence subspace. The efficacy of XDFT will be demonstrated by calculating the lowest single-particle singlet and triplet excitation energies of the well-known Thiel molecular test set, with results which are in excellent agreement with time-dependent density functional theory (TDDFT).

[1]A.M.Souza, I.Rungger, C.D.Pemmaraju, U.Schwingenschloegl and S.Sanvito, Constrained-DFTmethod for accurate energy-level alignment of metal/molecule interfaces, Phys. Rev. B 88, 165112 (2013).

[2]  Subhayan Roychoudhury, Carlo Motta and Stefano Sanvito, Charge transfer energies of benzene physisorbed on a graphene sheet from constrained density functional theory, Phys. Rev. B 93, 045130 (2016).

[3] A. Droghetti, I. Rungger, C.D. Pemmaraju and S. Sanvito, Fundamental gap of molecular crystals via constrained Density Functional Theory, Phys. Rev. B 93, 195208 (2016).

[4] Subhayan Roychoudhury, David D. O’Regan and Stefano Sanvito, Wannier-function-based constrained DFT with nonorthogonality-correcting Pulay forces in application to the reorganization effects in graphene- adsorbed pentacene, Phys. Rev. B 97, 205120 (2018).

[5] Subhayan Roychoudhury, Stefano Sanvito and David D. ORegan, XDFT: an efficient first-principles method for neutral excitations in molecules, arXiv:1803.01421 (2018).

11:30 - 12:00
S5.5-I2
Barth, Johannes
Technical University of Munich
Vibrational Excitations & Conformational Switching in Single-Molecule Junctions
Johannes Barth
Technical University of Munich, DE
Authors
Johannes V. Barth a, Hai Bi a, Carlos-Andres Palma a, Yuxian Gong a, Peter Hasch a, Mark Elbing b, Marcel Mayor b, c, Joachim Reichert a
Affiliations
a, Physics Department, Technical University of Munich, D-85748 Garching, Germany
b, Institute of Nanotechnology, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany
c, Department of Chemistry, University of Basel, CH-4056 Basel, Switzerland.
Abstract

The development of robust, chemically-sensitive techniques is crucial for the advancement of single-molecule electronics. Studies in single-molecule junctions largely rely on indirect electrical characterization to statistically evaluate the chemistry and quality of the established circuits. One fundamental challenge is the direct, quantitative determination of charge-vibrational coupling for well-defined single-molecule junctions. The ability to record molecular charge-vibrational coupling for individual species grants access to the determination of maximal charge transport efficiencies for specific molecular configurations and currents. Here we explore the charge-vibrational coupling for current-carrying tethered molecules by combined vibrational and metal-molecule-metal junction current-voltage spectroscopy. By inspecting the steady-state vibrational distribution during charge transport in a bis-phenyl-ethynyl-anthracene derivative by Raman scattering, we deduce a coupling constant of ≈0.35 vibrational excitations per charge carrier. Furthermore we follow the conformational response of a two-state molecular switch. Specifically, we remove the ground state polarizability and symmetry of a known p-terphenyl-4,4´´-dithiol (TPD) molecule by employing the 2,2´,5´,2´´-tetramethylated (TM-TPD) derivate. Whereas the highly sterically hindered, non-planar TM-TPD, lacking π-conjugation, in its pristine conformation does not exhibit a Raman signature, a marked on/off modulation of the single-molecule Raman signal exceeding a factor of 100 is achieved via redox state control by means of the applied voltage.

Support by the Deutsche Forschungsgemeinschaft (DFG) via SPP 1234 (Grant RE2592) & Munich Centre for Advanced Photonics (MAP), the European Research Council via Advanced Grant MolArt (n° 247299) and Chinese Scholarhip Council (H.B., Y.G.) is gratefully acknowledged.

Key refs.: JACS 140 (2018) 4835 | Nature Comm. 7 (2016) 10700 | Nature Nanotechn. 7 (2012), 673

12:00 - 12:30
S5.5-I3
Cahen, David
Weizmann Institute of Science
Proteins as Optoelectronic Materials…
David Cahen
Weizmann Institute of Science, 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 of Science & Bar Ilan University
Abstract

Electron transport (ETp), i.e., electronic conduction, across protein monolayers in a solid state–like configuration is surprisingly efficient, comparable, length-normalized, to completely conjugated molecules. This is amazing as apparently nature does not use this capability, except for electron transfer, ET, within and some times between redox proteins, a process that is coupled to ionic transport.

Nature regulates ET via redox chemistry, while for ETp a redox process is not a necessary condition. This allows studying ETp via non-redox proteins, such as rhodopsins and albumins; remarkably, ETp is quite efficient also across these proteins.

If contact to the external world does not limit ETp, i.e., intra-protein transport dominates, there seems to be no barrier for transport. As ETp is temperature-independent, it may well be coherent…. This may be important also for electron transfer, ET, which involves injection and extraction of electrons, the analogue of which for ETp is the coupling to the electrodes. Then, efficient ETp via non-redox proteins suggests why there are redox centres in ET proteins, ito protect it from high reducing power.

I will discuss experimental data that illustrate our main results as well as some more recent ones on multi-heme proteins and on protein multilayers, which raise even more questions.1,2

* work with Mordechai Sheves & Israel Pecht, Weizmann Inst.

References

C. Bostick et al.  Rep. Prog.Phys., 81 (2018) 026601, “Protein bioelectronics:a review of what we do and do not know” doi.org/10.1088/1361-6633/aa85f2 ,

N. Amdursky et al., Adv. Mater. 42, (2014) 7142 “Electronic Transport via Proteins” 10.1002/adma.201402304

NCFun S3.5
Chair: Mischa Bonn
11:00 - 11:30
S3.5-I1
Schaller, Richard
Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
Ultrafast Structural Studies of Semiconductor Nanocrystals: Transient Disordering and Recrystallization
Richard Schaller
Department of Chemistry, Northwestern University, Evanston, IL 60208, USA

Since 2010, Richard D. Schaller has held a joint appointment as both a research scientist in the Center for Nanoscale Materials at Argonne National Lab and as an assistant professor in the Department of Chemistry at Northwestern University. Schaller’s research focuses on spectroscopy and physical chemistry of semiconductor nanomaterials From 2002 to 2010, Schaller was a Reines Distinguished Postdoctoral Fellow and then a permanent technical staff member at Los Alamos National Lab with Dr. Victor Klimov. Schaller obtained his PhD in physical chemistry from UC Berkeley in 2002 with Prof. Richard Saykally in nonlinear optics and near-field optics. In 2012, he was selected by the National Academy of Sciences as a Kavli Fellow participant.

Authors
Richard Schaller a, b
Affiliations
a, Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
b, Argonne National Laboratory, Center for Nanoscale Materials, 9700 South Cass Avenue Bldg 440, Lemont, Illinois 60439, US
Abstract

Colloidally prepared, quantum-confined, semiconductor nanocrystals offer tunable energy gaps, strong photoluminescence, and, in some cases, optical gain and lasing [1]. We report ultrafast optical pump, X-ray diffraction probe experiments performed at Argonne National Lab’s Advanced Photon Source with CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump intensity. Shifts of diffraction peaks relate lattice heating and peak amplitude reduction conveyed transient lattice disordering (or melting). For smaller NCs, melting was observed upon absorption of as few as ∼15 electron–hole pair excitations per NC on average (0.89 excitations/nm3 for a 1.5 nm radius) with a similar electron-hole pair density inducing disordering for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. These findings suggest a need to take into account nanomaterial physical stability and transient electronic structure for high intensity excitation applications such as lasing and solid-state lighting.
[1] Klimov et al. Science, 290, 314 (2000).; Kazes et al. Adv. Mater. 14, 317 (2002).
[2] Kirschner et al. Nano Lett. 17, 5314 (2017).

11:30 - 12:00
S3.5-I2
Ruhman, Sanford
Hebrew University of Jerusalem
Spectator Exciton Unveils Spin Blockades in the Cooling of Hot Multi-Excitons
Sanford Ruhman
Hebrew University of Jerusalem, IL

Sanford Ruhman is a full professor of Chemistry at the Hebrew University. His work concentrates on applications of femtosecond spectroscopy in condensed phases. As a pioneer in the field of femtosecond photochemistry his group was the first to report conservation of coherence from reactants to dissociation products in solutions, and to utilize impulsive Raman probing of photoproducts. His current interests include fundamental ultrafast excitonics in nanocrystals and photovoltaic materials, ultrafast photobiology, and applications of impulsive vibrational spectroscopy to probe light induced dynamics in liquids and solids. Over the years he has served as the Director of the Farkas Minerva center for light induced processes at the Hebrew University, and as the head of the Institute of Chemistry there.

Authors
Sanford Ruhman a, Efrat Lifshitz a, Roi Baer a, Tufan Ghosh a
Affiliations
a, Hebrew University of Jerusalem, Institute of Chemistry, Casali Center for Applied Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904, IL
Abstract

Decades of investigation show that inter-band photoexcitation of quantum dots is followed by rapid relaxation of hot carriers to the quantized band edge states within one or two picoseconds. Due to the large oscillator strength and low degeneracy of the band edge exciton transition, evolution in its intensity and spectrum have played a pivotal part in probing quantum dot exciton cooling. These changes start as a bi-exciton spectral shift while carriers are hot, changing to a bleach due to state filling once the exciton relaxes. Accordingly, kinetics of the BE bleach buildup has served to characterize the final stages of carrier cooling, and its amplitude per cold exciton the degeneracy of underlying electronic states.

Hot multi-excitons (MX) add a new relaxation process to this scenario. Auger recombination (AR) reduces an N exciton state to N-1 plus heat, initially. Again, investigation of AR dynamics is based on the amplitude and decay kinetics of the BE bleach. Interpretation of such data was based on the following assumptions: 1) that ultrafast cooling of hot excitons leads directly to occupation of the lowest electron and hole states (in accordance with the lattice temperature and the state degeneracy), and 2) that aside from mild spectral shifts induced in the remaining band edge transitions, after carrier cooling is over the BE bleach increases linearly with N until state filling is complete.

To test these assumptions, three pulse pump-probe experiments were conducted in our lab, measuring fs transient transmission (TT) of PbSe nanocrystals in the presence and absence of single cold spectator excitons. Results show that the bleach introduced by a second hot exciton falls significantly from that introduced by the spectator. Later we will describe a recent extension to CdSe which shows that adding an additional hot exciton to the cold spectator reduces the band edge bleach merely by a half! We conclude that the source of incomplete bleaching by the second exciton are hitherto unrecognized random spin orientation conflicts between the two conduction electrons in the crystallites. The presence of this effect both in lead salts and in CdSe NCs demonstrates its generality. This new discovery imposes new restrictions on the utility of the BE exciton transition as a universal “exciton counter” in experiments with all kinds of semiconductor NCs
 

12:00 - 12:30
S3.5-I3
Chergui, Majed
Ecole Polytechnique Fédérale de Lausanne, Suisse
Charge Carrier and Phonon Dynamics in Transition Metal Oxide and in Lead-Halide Perovskite Nanoparticles
Majed Chergui
Ecole Polytechnique Fédérale de Lausanne, Suisse

Majed Chergui is Professor of Physics and Chemistry at the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland. He received his Bachelor’s degree in Physics and Mathematics from Chelsea College (University of London), then his Master’s degree and in 1981, his Ph.D. in Molecular Physics from the Université Paris-Sud (Orsay). Thereafter, he spent six years at the Free University of Berlin (Germany), before moving to become in 1993 full professor of Physics at the Université de Lausanne, then to the EPFL in 2003.

He is best known for developing new ultrafast spectroscopic techniques and methods, which he applied to some of the most important problems in molecular spectroscopy and dynamics. In particular, he pioneered ultrafast X-ray spectroscopy and demonstrated its power for observing chemical transformations in molecules, solutions and nanoparticles, with femtosecond temporal and sub-Ångstrom spatial resolution. This work opened a new field of research which has influenced many international groups, especially at X-ray Free electron laser centers. Parallel to these achievements, he developed new ultrafast spectroscopic tools in the deep-ultraviolet (deep-UV), and in particular, he pioneered 2-dimensional deep-UV spectroscopy, with which he addressed electron transfer in proteins and charge carrier dynamics in transition metal oxide nanoparticles and solids.

With these various tools, he solved several fundamental questions regarding photoinduced phenomena in coordination chemistry complexes, in protein dynamics and in semiconductors, such as metal oxides. Among some of the highlights of his work are the description of the spin dynamics in metal complexes, the identification of solvation changes around photoexcited solutes, the unravelling of electron transfer processes concurrent with FRET in biological systems.

Chergui is the founding editor-in-chief of “Structural Dynamics” (AIP Publishing). He was awarded the Kuwait Prize for Physics (2009), the Humboldt Research Award (2010), the 2015 Earle K. Plyler Prize for Molecular Spectroscopy & Dynamics of the American Physical Society and the 2015 Edward Stern Award of the International X-ray Absorption Society.

Authors
Majed CHERGUI a
Affiliations
a, Ecole Polytechnique Fédérale de Lausanne, Suisse
Abstract

Using a combination of steady-state and ultrafast deep-ultraviolet (UV) we have identified the nature of the transitions at the optical gap of the much studied anatase TiO2 nanoparticles. The first excitation is a strongly bound 2-dimensional exciton in the 3D lattice of the material.[1] We also find that coherent acoustic phonons confined in the nanoparticles selectively modulate the oscillator strength of the 2D exciton, and theory shows that the deformation potential is at the origin of the coherent phonon wavepackets.[2] Further studies of the charge electron and hole trapping in anatase TiO2[3] and in ZnO[4] using ultrafast X-ray spectroscopy will be presented.

Finally, time resolved X-ray studies of CsPbBr3 perovskite NPs reveal the nature of the electron-hole recombination across the band gap, with the electrons being delocalized in the conduction band, while holes form small polarons in the valence band.[5]

 

[1] Strongly bound excitons in anatase TiO2 single crystals and nanoparticles
E. Baldini, L. Chiodo, S. Moser, J. Levallois, E. Pomarico, G. Auböck, A. Magrez, L. Forro, M. Grioni, A. Rubio and M. Chergui. Nature Communications 8 (2017) 13

[2] Phonon-Driven Selective Modulation of Exciton Oscillator Strengths in Anatase TiO2 Nanoparticles

E. Baldini, T. Palmieri, A. Dominguez, P. Ruello, A. Rubio and M. Chergui. Nanoletters (under review)

[3] Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2

F. G. Santomauro, A. Lübcke, J. Rittmann, E. Baldini, A. Ferrer, M. Silatani, P. Zimmermann, S. Grübel, J. A. Johnson, S. O. Mariager, P. Beaud, D. Grolimund, C. Borca, G. Ingold, S.L. Johnson, M. Chergui

Scientific Reports 5 (2015) 14834-1-6

[4] Revealing hole trapping in ZnO nanoparticles by time-resolved X-ray spectroscopy

Thomas J. Penfold, Jakub Szlachetko, Fabio G. Santomauro, Alexander Britz, Wojciech Gawelda, Gilles Doumy, Anne Marie March, Stephen H. Southworth, Jochen Rittmann, Rafael Abela, Majed Chergui and Christopher J. Milne. Nature Communications 9 (2018) 478

[5] Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

Fabio G. Santomauro, Jakob Grilj, Lars Mewes, Georgian Nedelcu, Sergii Yakunin, Thomas Rossi, Gloria Capano, André Al Haddad, James Budarz, Dominik Kinschel, Dario S. Ferreira, Giacomo Rossi, Mario Gutierrez Tovar, Daniel Grolimund, Valerie Samson, Maarten Nachtegaal, Grigory Smolentsev, Maksym V. Kovalenko, and Majed Chergui. Structural Dynamics 4 (2017) 044002

PVCon S9.5
Chair: Guy Brammertz
11:00 - 11:30
S9.5-I1
Placidi, Marcel
Catalonia Institute for Energy Research (IREC)
TCMs for Next Generation Thin Film Photovoltaics
Marcel Placidi
Catalonia Institute for Energy Research (IREC), ES
Authors
Marcel Placidi a
Affiliations
a, Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Sant Adrià del Besòs, 08930 Barcelona
Abstract

Several emerging photovoltaic (PV) applications are currently requiring the development and implementation of transparent conductive materials (TCMs) beyond their traditional use as front electrodes in solar cells, and not only letting the light reaching the junction. Next generation PV devices with higher functionalities as transparent/semitransparent devices for glass-based building integration elements and windows require the integration of TCMs at several levels of the device architecture, with functions as charge transport or even light absorber layers. This is also required for advanced device architectures aiming towards improving the device efficiency as bifacial cells and tandem/multijunction concepts. The TCMs family includes thus electrodes, selective contacts, and even absorber layers presenting a certain degree of transparency (defined by a wide bandgap).

The transparent conductive oxides (TCOs) are the most common TCMs encountered in literature, thanks to their high electrical conductivity and light transmittance, and are often used as high-performance n-type conductors in a plethora of transparent devices (LEDs, gas sensors, solar cell, etc). Less is found regarding p-type conductors, even if recently Cu-based materials (oxides and chalcogenides) have gained interest in the field, especially thinking in their integration as interlayer electrodes in tandem devices. Another example of TCMs for PV relies in the selective contacts, generally based on metallic oxides/chalcogenides, exploiting kinetics at the interfaces with the absorber, establishing different conductivities for electrons and holes in different regions of the device, thus creating effective separation and selective transport. Finally, wide bandgap absorbers, also respond to the criteria of (semi-) transparent conductive material, thus completing the TCMs family.

After making a brief review of the most common uses of TCMs for PV applications, summarizing their main potential and challenges, a special focus will be put on their integration with existing thin film technologies, in particular with kesterite. The main results of the optimization of the replacement of the Mo back contact (commonly used in the kesterite technology) by TCOs will be presented. In particular the required functionalization (involving TCMs as interlayers improving the valence band alignment) of the TCOs for several anionic compositions of the kesterite absorbers (i.e. bandgaps) will be presented, such as the first results of integration in a tandem structure.

11:30 - 11:45
Abstract not programmed
11:45 - 12:00
S9.5-O2
BAILO BOBI, EDUARD
Francisco Albero - FAE
Printing Based Processes for Low Cost CRM Free Sustainable Technologies on Ceramic Ecofriendly Substrates for BIPV Applications.
EDUARD BAILO BOBI
Francisco Albero - FAE, ES
Authors
EDUARD BAILO BOBI a, b, c, BEATRIZ MEDINA-RODRIGUEZ a, MONICA COLINA b, MIREIA BLANES a, c, MARCEL PLACIDI b, FRANCISCO RAMOS a, EDGARDO SAUCEDO b, ALBERT CIRERA c, ALEJANDRO PEREZ b, c
Affiliations
a, Francisco Albero - FAE, Rafael Barradas 1, Hospitalet de Llobregat, Barcelona, 08908, ES
b, Catalonia Institute for Energy Research, IREC. Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain.
c, IN2UB, Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, C. Martí i Franquès 1, 08028 Barcelona, Spain
Abstract

The manufacturing cost of thin film solar cells and modules can be significantly reduced through the replacement of vacuum deposition steps by solution-based chemical routes. In the case of kesterite PV technologies, use of solution-based processes for the preparation of the device absorbers has demonstrated the possibility to achieve device efficiencies similar to those obtained with vacuum-based processes that are used for the industrial implementation of chalcogenide technologies. Kesterites are Cu2Zn,Sn(S,Se)4 compounds that are receiving an increasing interest for the development of PV devices free of critical raw materials (CRM), which are strongly relevant for a sustainable mass-deployment of the proposed applications. Kesterite based technologies benefit also from a high level of technological compatibility with technologies that are already at industrial production scale, as CIGS. In this case, relevant challenges to achieve a full exploitation of the cost reduction potential of these technologies are related to: i) the use of processes scalable to industrial production scale; and ii) avoiding the use of highly toxic or hazardous reagents as hydrazine. This gives a strong interest to printing based processes that are compatible with very high throughput processes. Between them, ink-jet printing processes have an additional advantage related to the possibility for the definition of spatially resolved processes, which provides with a higher degree of device design flexibility.

Nowadays, Building integrated photovoltaics (BIPV) has acquired a great interest within the possible applications for thin film photovoltaic devices. It is for this reason that it is vitally important to develop technologies and processes compatible with their integration on substrates alternative to glass, as ceramic based architectural substrates.

This work is focused on the analysis of the manufacturing cost reduction of thin film solar cells and modules by replacing vacuum process steps involved on the precursor preparation by in-air screen printing based processes on ceramics substrates. Solution-based in conjunction of inkjet technology were used for the preparation of the PV active layers on molybdenum coated vitro-ceramic substrates made from recovered material in order to demostrate the viability of these techlogogies for the preparation of more ecofriendly and sustainable opto-electronic devices for BIPV applicationa. With this in mind, promising results were obtanained on cell device prototypes getting around 75% of efficientcy on ceramic substrate comparing with the average value on soda-lime reference substrate, and their scalability for the developmnet of medium size module prototypes has been investigated.

With this approach, the aim is add value to non-vacuum technologies for their future transition for the industrial production of sustainable cost efficient BIPV elements and systems.

12:00 - 12:15
S9.5-O1
Jawhari, Tariq
Thin Film Sb2(S,Se)3 Based Solar Cell: Emergent Technology Compatible with Ubiquitous Applications
Tariq Jawhari
Authors
Pedro Vidal a, Victor Izquierdo-Roca a, Markus Neuschitzer a, Edgardo Saucedo a, Lorenzo Calvo-Barrio b, c, Tariq Jawhari b, Alejandro Perez-Rodriguez a, c
Affiliations
a, Catalonia Institute for Energy Research, IREC. Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain.
b, Centres Científics i Tecnològics (CCiTUB) de la Universitat de Barcelona, C/ Lluis Solé i Sabaris 1-3, 08028 Barcelona, Spain
c, IN2UB, Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, C. Martí i Franquès 1, 08028 Barcelona, Spain
Abstract

Sb2(S,Se)3, is a relevant semiconductor free of critical raw materials that is receiving an increasing interest for photovoltaic (PV) applications, demonstrating solar cells in superstrate configuration with a record efficiency of 6.5%. The semiconductor is characterised by a 1D crystalline formation which in principle favours formation of benign grain boundaries and anisotropic conduction properties. Additionally Sb2Se3 has a high degree of flexibility in terms of substrate type, due to its relatively low synthesis temperature (300-400 ºC). This allows the use of different substrates as polymers, steels, ceramics and TCO/glass. The compound has also the possibility to tune the band gap between 1.1 and 1.9 eV, which opens very interesting perspectives for wide band gap solar cells suitable for energy harvesting in indoor applications, semi-transparent devices or high efficiency tandem devices. This high versatility make this compound very promising for ubiquitous applications based on the integration of PV devices in products and systems requiring light weight, mechanical flexibility and/or optical transparency. 

This work reports a systematic study of Sb2Se3 layers and substrate configuration solar cells that were fabricated using a reactive annealing treatment of Sb evaporated precursors. The study analyses the dependence of the physico-chemical properties of the layers and the optoelectronic characteristics of the devices on the pressure and temperature used during the Se reactive annealing process, as well as on the characteristics of the substrate layers, having used different kinds of substrates including both transparent (FTO) and non-transparent (Mo, Al, Au, Ag, W) back contact materials. The results obtained have allowed us to achieve  reproducible functional devices using near atmospheric pressure at 320ºC on Mo coated soda lime glass substrate with a promising efficiency of 5.6% close to the 6.5% certified world record.

This study involved a deep characterization of the fundamental properties of the synthesised layers that that have been correlated with the optoelectronic characterization of the devices. The results allow us to observe the formation of continuous layers with large and homogeneous crystals, reporting for the first time a weak PL close to 1.3eV in agreement with the band gap value obtained by IQE. Finally, the systematic vibrational characterization of the layers performed under resonant and non- resonant Raman conditions –including measurements on reference single crystal Sb2Se3 – has allowed the identification of 15 Raman peaks of the compound.

PerFun S7.5
Chair: Robert Lovrincic
11:00 - 11:30
S7.5-O1
Bisquert, Juan
Institute of Advanced Materials (INAM), Universitat Jaume I
Ionic Transport, Defects and Electrooptical Response of Perovskite Solar Cells
Juan Bisquert
Institute of Advanced Materials (INAM), Universitat Jaume I, 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, Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castellón de la Plana, Castellón, España, Castellón de la Plana, ES
Abstract

The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. This class of semiconductors presents remarkable bulk electronic and optical properties, but the contacts to the device are a key aspect of the operation and show important dynamic interactions. We describe the results of analysis of kinetic phenomena using frequency modulated 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.1 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 shows the identification of the impedance of ionic diffusion by measuring single crystal samples.2 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 deffects of the structure.3 We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent. We describe important insinghts to the measurement of EQE in frequency modulated conditions, which shows that the quantum efficiency can be variable at very low frequencies.4

(1)         Lopez-Varo, P.; Jiménez-Tejada, J. A.; García-Rosell, M.; Ravishankar, S.; Garcia-Belmonte, G.; Bisquert, J.; Almora, O. Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment, Adv. Energy Mater. 2018, 1702772.

(2)         Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A. Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals, ACS Energy Lett. 2018.

(3)         Li, C.; Guerrero, A.; Zhong, Y.; Gräser, A.; Luna, C. A. M.; Köhler, J.; Bisquert, J.; Hildner, R.; Hüttner, S. Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites, Small 2017, 1701711.

(4)         Ravishankar, S.; Aranda, C.; Boix, P. P.; Anta, J. A.; Bisquert, J.; Garcia-Belmonte, G. Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells, J. Phys. Chem. Lett. 2018, 3099-3104.

 

11:30 - 12:00
S7.5-O2
Pockett, Adam
SPECIFIC, Swansea University
Ion Migration in Triple Mesoporous 2D/3D Perovskite Solar Cells
Adam Pockett
SPECIFIC, Swansea University
Authors
Adam Pockett a, Jenny Baker a, Trystan Watson 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 wide spread 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.
The current state-of-the-art devices use a mixed cation perovskite, consisting of methylammonium and 5-aminovaleric acid (5-AVA). The AVA containing perovskite has been shown to give greater stability and performance – linked to 2D/3D structuring of the perovskite as well as interfacial modifications at the TiO2 surface. The cells undergo a slow light soaking effect during which time the JV performance of the device is vastly improved. They also show improvement when exposed to a high relative humidity.
A striking feature observed using TPV measurements is the presence of a negative photovoltage transient, comparable to that observed in our previous work on planar TiO2 devices at low temperature. This behaviour suggests the presence of high rates of interfacial recombination at the TiO2 surface. In carbon based cells the phenomena is observed at room temperature and is very slow to disappear under continued illumination. For the planar devices the negative transient was shown to diminish over time as ions in the perovskite redistributed, leading to a reduction in the recombination rate. We show that in the carbon devices the exceptionally slow dynamic behaviour observed at room temperature has a similar origin linked to the effects of ion migration – activation energy calculated to be 0.4 eV (in the range of many literature values for iodide migration). However, it takes place at a much slower rate due to the 2D AVA based perovskite hindering iodide ion migration – attempt frequency reduced by several orders of magnitude compared to pure MAPI devices. We show that the inhibited ion migration is the dominant affect rather than the AVA having a direct impact at the TiO2 interface by adsorption via the carboxylic acid group. This inhibition of iodide migration is also linked to the increased stability demonstrated for these devices.

12:00 - 12:30
S7.5-O3
Guerrero Castillejo, Antonio
Universidad Jaume I
Ionic Diffusion Quantification in Lead Halide Perovskites
Antonio Guerrero Castillejo
Universidad Jaume I, ES
Over the last few years Antonio Guerrero has been involved in the development of new semiconductors materials, their characterization and understanding their physical properties. After completing a PhD on the development of new organometallic catalysts for the production of polymers he took a job at Cambridge Display Technology. During four years Antonio developed some of the state of the art materials used in polymer Light Emitting Diodes (P-LEDs). Currently, Antonio is interested in gaining understanding on the operational principles of organic solar cells. At the group of Photovoltaic and Optoelectronic devices in Castell�n (Spain) he is developing some Impedance Spectroscopy modeling tools to understand the electrical response of OPVs.
Authors
Antonio Guerrero a
Affiliations
a, Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castellón de la Plana, Castellón, España, Castellón de la Plana, ES
Abstract

Lead halide perovskites have long been known to be ionic and electronic semiconductors.1, 2 Recently, lead halide perovskite have revolutionized the photovoltaics field with impressive efficiencies reaching ~23 %. Unlike most photoactive materials, ionic conductivity plays a key role in perovskites during photovoltaic device operation. In this presentation it is described how to characterize the ionic properties of lead halide perovskites by advanced electrical and optical techniques.3 Approaches to minimize the electronic contribution to the measured current are used so the ionic current can be probed. For example, Impedance Spectroscopy (IS)  reveals the characteristic signature of ionic diffusion in monocrystalline devices which do not contain HTL. This is the Warburg element and transmission line equivalent circuit in MAPbBr3 and ion accumulation at the MAPbBr3/Au interface typical for non-reactive contacts. Alternatively, measurements of confocal photoluminescence (PL) in interdigitated electrodes as a function of the applied bias reveals that the ionic movement dramatically modifies the PL emission properties that also allows the calculation of the diffusion coefficient of the material.4

 

 

References

1.         Kuku, T. A.; Salau, A. M., Electrical conductivity of CuSnI3, CuPbI3 and KPbI3. Solid State Ionics 1987, 25, 1-7.

2.         Kuku, T. A., Ionic transport and galvanic cell discharge characteristics of CuPbI3 thin films. Thin Solid Films 1998, 325, 246-250.

3.         Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A., Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals. ACS Energy Lett. 2018, 1477-1481.

4.         Li, C.; Guerrero, A.; Zhong, Y.; Gräser, A.; Luna, C. A. M.; Köhler, J.; Bisquert, J.; Hildner, R.; Huettner, S., Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites. Small 2017, 13, 1701711.

PerMod S8.5
Chair: Juan A. Anta
11:00 - 11:30
S8.5-O1
Le corre, vincent
University of Groningen
Transport Layers Limit the Efficiency of Perovskite Solar Cells: an Experimental and Theoretical Study.
vincent Le corre
University of Groningen, NL
Authors
Vincent Le Corre a, Lorena Perdigón Toro b, Markus Feuerstein b, Martin Stolterfoht b, Dieter Neher b, L. Jan Anton Koster a
Affiliations
a, Photophysics and OptoElectronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, The Netherlands
b, Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24-25, Potsdam, 14476
Abstract

Perovskite solar cells (PSCs) are the current rockstar of photovoltaic research attracting more and more attention. With efficiency now reaching up to 23% PSCs are on the way of catching up with classical inorganic solar cells. However, PSCs have not reached their full potential yet. In fact, their efficiency is limited, on the one hand, by non-radiative recombination, mainly via trap states located either at the grain boundaries or at the interface between the perovskite and the transport layers. On the other hand, it is limited by losses due to the poor transport properties of the commonly used transport layers.  Indeed, state-of-the-art transport layers (e.g. TiO2, PCBM and Spiro-OMeTAD…) suffer from rather low mobilities, typically within 10-4 – 10-2 cm2 V-1 s-1, when compared to the high mobilities, 1 – 10 cm2 V-1 s-1, measured for perovskite using field-effect transistors or space-charge-limited-current measurement.

In this work, the effect of the mobility, thickness and doping density of the transport layers was investigated by means of a combined experimental and modeling analysis. For the experiment, two sets of devices made of a triple-cation perovskite were studied, including n-i-p and  p-i-n structures demonstrating efficiencies of up to 20%. For the two structures, the thickness and doping density of one of the transport layers were varied in order to understand their effect on the performance and especially on the FF. In addition, we performed a transient extraction experiment to look at the influence of the transport layers properties on the rate of extraction. The experimental results were then reproduced using drift-diffusion simulations to explain how and by how much every single parameter influences the extraction and the performance. A new and simple formula was also introduced to easily calculate the amount of doping necessary to counterbalance the low mobility of the transport layer.

In conclusion, this work presents a comprehensive analysis of the effects of the different properties of a transport layer on the efficiency of PSCs. We also present general guidelines on how to optimize a transport layer to avoid losses.

11:30 - 12:00
S8.5-O2
Garcia-Rosell Molina, Manuel
Universidad de Granada
Influence of Ion Migration on the performance of the Selective Contact Heterojunctions in Perovskite Solar Cells
Manuel Garcia-Rosell Molina
Universidad de Granada, ES
Authors
Manuel García-Rosell a, Agustín Bou b, Juan A. Jiménez-Tejada a, Juan Bisquert b, Pilar Lopez-Varo a
Affiliations
a, Departamento de Electrónica y Tecnología de Computadores, CITIC-UGR, Universidad de Granada, 18071 Granada, Spain
b, Institute of Advanced Materials (INAM), University Jaume I, Avenida de Vicent Sos Baynat, s/n, 12006 Castelló de la Plana, Castellón (Spain)
Abstract

Experimental and theoretical studies show that the presence of mobile ions in perovskite solar cells (PSCs) modifies the electronic operation of the device [1-3]. The effects of the ion migration on the PSC performance can be enhanced or attenuated with the selective contacts (charge-transport-layer /perovskite heterojunctions) [2]. Thus, the knowledge of the mechanisms that take place at the selective contacts is crucial for the optimization of PSCs.

Anomalous high values of the low-frequency capacitance at open-circuit (OC) and short-circuit (SC) indicate a high accumulation of charge at the heterojunctions, which could hinder the extraction of charge and increase hysteresis in current-voltage curves [2]. This accumulation of charge can be affected by the presence of ionic species. Our goal is to quantify this accumulation of charge as a function of the different physical mechanisms that take place along its bulk and heterojunctions [2]. 

To investigate this issue, we developed a simulation model based on the drift-diffusion equations with specific boundary conditions at the heterojunctions [1-2]. The effect of ion migration on the charge and energy profile distributions along the PSC in OC and SC conditions was analyzed. We conclude that the accumulation of charge at the interfaces is strongly affected by the specific contact materials, and critically depends on a compromise among the presence of ions, the values of the carrier mobility, and the interfacial and bulk recombination parameters.

1. López-Varo, P. et al. ACS Energy Letters 1450-1453(2017)

2. García-Rosell, M. et al. J. Phys. Chem. C. (2018)

3. Reenen, S. et al. J. Phys. Chem. Lett. 6, 3808-3814(2015)

12:00 - 12:30
S8.5-O3
Gallardo, Juan Jesús
University of Cádiz
CsSnBr3, a Lead-Free Perovskite with Photocatalytic Activity
Juan Jesús Gallardo
University of Cádiz, ES
Authors
Juan Jesus Gallardo a, Javier Navas a, Fran Reyes-Perez a, Teresa Aguilar a, Rodrigo Alcántara a, Concha Fernández-Lorenzo a
Affiliations
a, University of Cádiz, Campus Universitario Rio San Pedro, Puerto Real, ES
Abstract

In this work, we have synthesized and characterized a totally inorganic, lead-free and non-soluble in water perovskite with catalytic properties, demonstrated by the photodegradation of an organic dye as crystal violet used, for example, in textile industries. From characterization, XRD revealed the presence of cubic (Pm-3m) and tetragonal (I4/mcm) CsSnBr3 perovskite. The formation of the perovskite is supported by the Goldschmidt’s tolerance factor and octahedral factor calculations. From XPS, we have observed the presence of Sn2+, Sn4+ and the formation of non-stoichiometric tin oxide on the surface. UV-Vis spectroscopy showed high absorption of light in the visible range of the electromagnetic spectrum, with an optical band gap of 1.74 eV. Adsorption and photocatalysis tests have been performed under photovoltaic standard conditions (1 sun, AM1.5G solar spectrum and 25 ºC). The evolution of the system, followed by UV-Vis spectroscopy, showed a photodegradation of the dye in presence of the perovskite. A maximum of 73.1% of photodegradation of the crystal violet has been reached so the synthesized perovskite is, a priori, a suitable and eco-friendly material for removing dyes and other contaminants from the environment. A possible mechanism for the photocatalytic process has been proposed.

SolFuel S1.5
Chair: Alexis Grimaud
11:00 - 11:30
S1.5-I1
Reisner, Erwin
University of Cambridge
Solar-Driven Fuel Synthesis with Hybrid Semiconductor Systems
Erwin Reisner
University of Cambridge, 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
Abstract

The synthesis of solar fuels and chemicals through artificial photosynthesis does not only require the coupling of solar light absorption and charge separation, but also the direct pairing with 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 semiconductor hybrid materials to perform efficient full redox cycle solar fuel catalysis with inexpensive components, and our first steps in extending this approach for sustainable biomass photoreforming and fine chemical synthesis.

 

Representative recent references

(1) “Solar Hydrogen Generation from Lignocellulose”

Kuehnel, Reisner, Angew. Chem. Int. Ed., 2018, 57, 3290.

(1) “Photocatalytic CO2 Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals”

Kuehnel, Orchard, Dalle, Reisner, J. Am. Chem. Soc., 2017, 139, 7217.

(2) “Solar-driven reforming of lignocellulose to H2 with a CdS/CdOx photocatalyst”

Wakerley, Kuehnel, Orchard, Ly, Rosser, Reisner, Nature Energy, 2017, 2, 17021.

(3) “Enhancing Light Absorption and Charge Transfer in Carbon Dots through Graphitization and Core N-doping”

Martindale, Hutton, Caputo, Prantl, Godin, Durrant, Reisner, Angew. Chem. Int. Ed., 2017, 56, 6459.

(4) “Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes”

Hutton, Reuillard, Martindale, Caputo, Lockwood, Butt, Reisner, J. Am. Chem. Soc., 2016, 138, 16722.

(5) “Solar-driven Reduction of Protons Coupled to Alcohol Oxidation with a Carbon Nitride-Catalyst System”

Kasap, Caputo, Martindale, Godin, Lau, Lotsch, Durrant, Reisner, J. Am. Chem. Soc., 2016, 138, 9183.

(6) “Clean Donor Oxidation Enhances H2 Evolution Activity of a Carbon Dot-Catalyst Photosystem”

Martindale, Joliat, Bachmann, Alberto, Reisner, Angew. Chem. Int. Ed., 2016, 55, 9402.

(7) “Electrocatalytic and Solar-driven CO2 Reduction with a Mn Catalyst Immobilized on Mesoporous TiO2

Rosser, Windle, Reisner, Angew. Chem. Int. Ed., 2016, 55, 7388.

11:30 - 11:45
S1.5-O1
Aldakov, Dmitry
CEA-Grenoble
Efficient Hydrogen Photoproduction in Water with Hybrid Systems Composed of Quantum Dots and Molecular Catalysts
Dmitry Aldakov
CEA-Grenoble
Authors
Martina Sandroni a, b, Jérôme Fortage b, Marie-Noëlle Collomb b, Peter Reiss a, Dmitry Aldakov a
Affiliations
a, UMR5819 SyMMES CEA-CNRS-UGA CEA-Grenoble INAC/SyMMES 17 rue des Martyrs 38054 GRENOBLE CEDEX 9 FRANCE
b, DCM, Univ. Grenoble Alpes, CNRS, 38000 Grenoble, France
Abstract

Semiconductor nanocrystals (quantum dots) emerged in the last years as an appealing alternative to molecular photosensitizers, owing to their superior stability, intense light absorption and bright luminescence. In particular, non-cadmium quantum dots seem to be a particularly interesting choice as they display good optoelectronic properties while containing no toxic elements with respect to CdSe and CdTe.[1] In the field of artificial photosynthesis, very efficient ”hybrid” photocatalytic systems for hydrogen production in water were obtained by associating Cd-based quantum dots as photosensitizers with molecular H2-evolving catalysts in presence of a sacrificial reductant. [2,3]

In this communication, we describe new hybrid systems associating environmentally friendly (Cd-free) quantum dots with molecular catalysts based on earth-abundant metals, in order to perform photocatalytic H2 production in purely aqueous environment.

Core-shell CuInS2/ZnS nanocrystals capped with glutathione were synthesized in the aqueous phase, and their structural and optical properties were fully characterized. The nanocrystals exhibit a broad absorption throughout the visible range with orange luminescence in aqueous solution. For the photocatalysis process the colloidal solution was mixed with a cobalt macrocyclic catalyst and a sacrificial reductant, and the H2 production under irradiation was quantified by gas chromatography. This hybrid system exhibited remarkable activity for hydrogen production under visible light irradiation at pH 5.0 with up to 7700 and 1010 turnover numbers versus catalyst and QDs, respectively. The CIS/ZnS nanoparticles were also compared to widely studied CdSe nanocrystals, using the same catalyst, and the former give remarkably better performances in terms of TON. [4]

 

1. M. Sandroni et al. ACS Energy Lett., 2017, 2, 1076–1088.

2. C. Gimbert-Suriñach et al., J. Am. Chem. Soc. 2014, 136, 7655-7661.

3. Z.J. Han et al., Science 2012, 338, 1321-1324

4. M. Sandroni et al. Energy Environ. Sci., 2018, DOI: 10.1039/c8ee00120k.

11:45 - 12:00
S1.5-O2
Alfonso González, Elena
Institute IMDEA Energy
Characterization of Cu2-xTe Nanocrystals for Photoelectrochemical Cells
Elena Alfonso González
Institute IMDEA Energy

Elena Alfonso González graduated in Chemistry from the Complutense University of Madrid in 2014. Her Bachelor thesis was “Search of high-temperature superconductors based on M-1212 structure with Ru in the charge reservoir”. She got a Master of Science degree in Advanced Spectroscopy in Chemistry from Lille and Leipzig Universities in 2016, with a Master thesis entitled “Z-scheme based photocatalytic water splitting by modification of TiO2 and Fe2O3 semiconductors with Pt and RuOx promotors”.

In the professional field, she did an internship in IMDEA Energy Institute in 2015 about the characterization by DRIFTS in situ of catalysts based on TiO2 for the photorredution of CO2. She also did an internship in the CSIC Institute of Ceramics and Glass in 2013 concerning the synthesis and characterisation of thermoelectric materials.

Since February 2017 she is a predoctoral researcher in the Photoactivated Processes Unit in IMDEA Energy Institute.

Authors
Elena Alfonso González a, Mengjiao Wang b, Mariam Barawi a, Luca De Trizio b, Liberato Manna b, Víctor A. de la Peña O'Shea a
Affiliations
a, IMDEA Energy Institute
b, Instituto Italiano di Tecnologia
Abstract

Artificial photosynthesis is a very promising method to turn solar energy into fuels.1 In a photoelectrochemical (PEC) cell, the generation of H2 by water splitting takes place on two different electrodes, which facilitates the separation of the products by the addition of a proton exchange membrane.  PEC cells have led to some of the highest solar-to-hydrogen efficiencies achieved to date.2 The setup of a successful PEC cell requires the preparation of efficient and stable photoelectrodes.

Subestequiometric copper telluride, Cu2-xTe, is a very interesting material that, as far as we know, has not been used in a PEC cell yet. It has already been used in photovoltaic cells due to its small band gap of around 1.5 eV and its high conductivity.3 In this work, we used Cu2-xTe nanocrystals synthesized by a colloidal method4 to prepare thin films and use them as photoelectrodes. After their organic capping was successfully removed by a thermal treatment in Ar, two different crystallographic phases were obtained depending on the heating temperature. Then, the photoelectrochemical and optical properties of both phases were measured. Electrochemical impedance spectroscopy was used to build the Mott-Schottky diagrams and determine the flat band potential, charrier density and conduction type, which is p-type for both phases. This conductivity type was ratified by photopotential measurements. Both electrochemical impedance spectroscopy and UV-Vis-nIR spectroscopy allowed us to build the bands energy diagram, which confirms the ability of this material to reduce water to hydrogen. This means that Cu2-xTe thin films are good candidates to be used as photocathodes, which was also confirmed by the measurement of their photocurrents. As they show some stability problems, two strategies were proved to solve this: the addition of an amorphous TiO2 layer and the change of the electrolyte pH. Moreover, H2 generation by water splitting in a two-electrode PEC cell was observed and quantified.

References

1.           Sivula, K. & van de Krol, R. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 1, 1–16 (2016).

2.           Young, J. L. et al. Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures. Nat. Energy 17028, 1–8 (2017).

3.           Rungtaweechai, N. & Tubtimtae, A. Cu2-xTe/MnTe co-sensitized near-infrared absorbing liquid-junction solar cells. Mater. Lett. 158, 70–74 (2015).

4.           Arciniegas, M. P. et al. Self-Assembled Dense Colloidal Cu2Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

12:00 - 12:30
S1.5-O3
Gimenez Julia, Sixto
Institute of Advanced Materials (INAM), Universitat Jaume I
Photocatalytic and Photoelectrochemical Degradation of Organic Pollutants with All-Inorganic Metal Halide Perovskite Quantum Dots
Sixto Gimenez Julia
Institute of Advanced Materials (INAM), Universitat Jaume I, ES

Sixto Gim�nez (1973, M. Sc. Physics 1996, Ph. D. Physics 2002) is researcher at Universitat Jaume I de Castell� (Spain). His professional career has been focused on the study of particulated materials. 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 (2003-2006) where he focused on the development of non-destructive and in-situ characterization techniques of the sintering behavior of metallic porous materials. In 2006-2007, he was responsible for a new research line on nanostructured particulated materials for magnetic applications at CEIT (Spain). 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 devices and biosensors based on nanoscaled materials, particularly studying the optoelectronic and electrochemical responses of the devices by electrical impedance spectroscopy. He has co-authored more than 30 papers in international journals and has been awarded with a Ramon y Cajal fellowship for 2008-2012.

Authors
Sixto Gimenez Julia a, Drialys Cardenas Moscoso a, Andrés F. Gualdrón-Reyes a, b, c, Ana Beatriz Ferreira-Vitoreti a, d, e, Miguel García-Tecedor a, Seon Joon Yoon a, Mauricio Solis de la Fuente e, Iván Mora-Seró a
Affiliations
a, Institute of Advanced Materials (INAM), Universitat Jaume I, 12071 Castelló, Spain
b, Centro de Investigaciones en Catálisis (CICAT), Universidad Industrial de Santander, Sede UIS Guatiguará, Piedecuesta, Santander, Colombia. C.P. 681011
c, Centro de Investigación Científica y Tecnológica en Materiales y Nanociencias (CMN), Universidad Industrial de Santander, Piedecuesta, Santander, Colombia. C.P. 681011
d, Department of Natural Science, Federal University of São João del-Rei, 36301-160 São João del-Rei, Brazil
e, CAPES Foundation, Ministry of Education of Brazil, Brasília, 70040-020, Brazil
f, Lawrence Berkeley National Laboratory, Energy Technologies Area, 1 Cyclotron Road, Berkeley, California 94720, United States of America
Abstract

All-Inorganic Halide Perovskite Quantum Dots (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different disciplines like photovoltaics, lasing and emission.[1] In the present talk, we will discuss about the application of these materials for solar-driven environmental remediation. To that aim, it is essential to elucidate the energy levels of CsPbxSn1-xX3 QDs by different techniques and to identify candidate target molecules, which can be oxidized with photogenerated holes in these QDs. As a proof of concept, we demonstrate the efficient photocatalytic and photoelectrochemical oxidation of 2-Mercaptobenzothiazol with CsPbBr3 QDs.

12:30 - 14:30
Lunch
Dyman S5.6
Chair: Thomas Kirchartz
14:30 - 15:00
S5.6-O1
Hernangomez Perez, Daniel
Quantum Transport Properties of Pi-Conjugated Linear Molecular Junctions
Daniel Hernangomez Perez
Authors
Daniel Hernangomez-Perez a, Suman Gunasekaran b, Iryna Davydenko c, Seth Marder c, Ferdinand Evers a, Latha Venkataraman b, d
Affiliations
a, Institute of Theoretical Physics, University of Regensburg, D-93050 Regensburg, Germany
b, Department of Chemistry, Columbia University, New York, New York 10027, United States
c, School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
d, Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
Abstract

We report recent experimental and theoretical results for molecular junctions based on \pi conjugated wires. These wires represent a class of linear molecules whose transport properties can be understood in the framework of the topological Su-Schrieffer-Heeger model for polyacetylene. We present an in-depth theoretical analysis based on tight-binding and ab-initio simulations of their coherent transport properties and show that, under certain conditions and depending on the chain parity (even /odd) and length, the conductance at the Fermi level can depend very weakly or even increase with the wire length. For short odd chains, we also provide experimental evidence of the role of the external environment in their charge transport properties: we study conductance trends in single-molecular junctions of polymethine dyes and prove that the trends can also be altered by the choice of the embedding solvent. Overall, our results suggest a way of enabling efficient electron transport at the nanoscale with one-dimensional wires.Iryna Davydenko

15:00 - 15:30
S5.6-O2
Hofmann, Felix
Exciton Gating and Triplet Deshelving in Single Dye Molecules Excited by Perovskite Nanocrystal FRET Antennae
Felix Hofmann
Authors
Felix J. Hofmann a, Maryna I. Bodnarchuk b, c, Loredana Protesescu b, c, Maksym V. Kovalenko b, c, John M. Lupton a, Jan Vogelsang d
Affiliations
a, Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany
b, ETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland
c, Empa – Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129, CH-8600 Dübendorf, Switzerland
d, Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5 – 13, 81377 München, Germany
Abstract

The extraordinary absorption cross-section and high photoluminescence (PL) quantum yield of perovskite nanocrystals make this type of material attractive to a variety of applications in optoelectronics. For the same reasons, nanocrystals are also ideally suited to function as nanoantennae to excite nearby single dye molecules by fluorescence resonant energy transfer (FRET). Here, we demonstrate that FAPbBr3 perovskite nanocrystals, of cuboidal shape and approximately 10 nm in size, are capable of selectively exciting single Cyanin 3 molecules at a concentration a hundredfold higher than standard single-molecule concentrations. This FRET antenna mechanism increases the effective brightness of the single dye molecules one hundredfold. Photon statistics and emission polarization measurements provide evidence for the FRET process by revealing photon-antibunching with unprecedented fidelity and highly polarized emission stemming from single dye molecules. Remarkably, the quality of single-photon emission improves two-fold compared to emission collected directly from the nanocrystals since the higher excited states of the dye molecule act as effective filters to multiexcitons. The same process gives rise to efficient deshelving of the molecular triplet state by reversed intersystem crossing, translating into a reduction of the PL saturation of the dye, thereby increasing the maximum achievable PL intensity of the dye by a factor of five.

15:30 - 16:00
S5.6-O3
Olthof, Selina
Universität zu Köln
Metal Oxide Layers in Perovskite Solar Cells: a Double-Edged Sword
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, Kai Brinkmann b, Ting Hu b, Klaus Meerholz a, Thoams Riedl b
Affiliations
a, Institute of Physical Chemistry, University of Cologne, Germany
b, Institute of Electronic Devices, University of Wuppertal, Germany
Abstract

Perovskite based solar cells have shown impressive progress in recent years. Now that high efficiencies are in reach, more and more effort is put into understanding the underlying device physics and to improve the devices intrinsic and extrinsic stability.

Touching on both of these subjects, I want to first discuss recent findings regarding the implementation of metal oxide layers into perovskite solar cell device stacks. These can be used to form impenetrable barrier layers which prevent the ingress of humidity as well the egress of perovskite decomposition products. Using this strategy, the overall decomposition of perovskite can be significantly suppressed, leading to outstanding solar cell device stability.

On the other hand, we find for a variety of systems that directly interfacing the perovskite to metal oxide layers can trigger a complex variety of reactions, which significantly alter the composition of the perovskite at the interface and lead to the presence of degradation products.  These thin interlayers play an important role for film formation and charge extraction and will therefore influence the overall device performance.

16:00 - 16:30
S5.6-I1
Herz, Laura
University of Oxford
Effect of Nanoscale Phenomena on Charge Conduction and Recombination Mechanisms in Hybrid Perovskites
Laura Herz
University of Oxford, GB

Laura Herz is a Professor of Physics at the University of Oxford. She received her PhD in Physics from the University of Cambridge in 2002 and was a Research Fellow at St John's College Cambridge from 2001 - 2003 after which she moved to Oxford. Her research interests lie in the area of organic and organic/inorganic hybrid semiconductors including aspects such as self-assembly, nano-scale effects, energy-transfer and light-harvesting for solar energy conversion.

Authors
Laura Herz a
Affiliations
a, Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
Abstract

Photovoltaic devices based on hybrid metal halide perovskites are rapidly improving in power conversion efficiency. While these materials are generally viewed as three-dimensional, the effects of nano-scale interfaces within the bulk films are still poorly understood. Such interfaces may appear between the perovskite layers and the charge extraction layers, or within the perovskite layers, at grain boundaries, of when passivating or hydrophobic interlayers are included.

We show here that photon reabsorption in lead iodide perovskite layers is strongly influenced by layer thickness and interfaces formed with charge-extraction layers[1]. Such photon re-absorption is found to reduce the apparent bi-molecular charge-carrier recombination rate constant with increasing film thickness, while the intrinsic value can be fully explained as the inverse process of absorption[2].

In addition, we discuss the role of nanoscale interfaces[3,4], energetic disorder[5], and passivating interlayers[6] on the dynamics of charge-carriers in various metal halide perovskites. Lowering of the perovskite dimensionality is shown to have effects similar to those known for classic inorganic semiconductors, such as enhancing bimolecular and Auger recombination and reducing trap-mediated recombination through surface passivation. Such extrinsic effects will also reduce the charge-carrier mobility below the maximum attainable value near 100cm2/(Vs) for MAPbI3[7].

[1] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, and L. M. Herz,Nano Lett. 17, 5782 (2017).

[2] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, and L. M. Herz, Nature Communications 9, 293 (2018).

[3] D. P. McMeekin, Z. Wang, W. Rehman, F. Pulvirenti, J. B. Patel, N. K. Noel, S. R. Marder, M. B. Johnston, L. M. Herz, and H. J. Snaith, Adv. Mater., 29 (2017), p. 1607039.

[4] R. L. Milot, R. J. Sutton, G. E. Eperon, A. A. Haghighirad, J. M. Hardigree, L. Miranda, H. J. Snaith, M. B. Johnston, and L. M. Herz, Nano Letters, 16 (2016), pp. 7001-7007.

[5] A. D. Wright, R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, and L. M. Herz, Adv. Func. Mater., 27 (2017), p. 1700860.

[6] Z. Wang, Q. Lin, F. P. Chmiel, N. Sakai, L. M. Herz, and H. J. Snaith, Nature Energy, 2 (2017), p. 17135.

[7] L. M. Herz, ACS Energy Lett., 2 (2017), pp. 1539-1548.

16:30 - 17:00
S5.6-I2
Kronik, Leeor
Weizmann Institute of Science
Structural Dynamics in Lead-Halide Perovskites from First-Principles
Leeor Kronik
Weizmann Institute of Science, IL
Authors
Leeor Kronik a
Affiliations
a, Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel.
Abstract

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

NCFun S3.6
Chair: Enrique Cánovas
14:30 - 15:00
Abstract not programmed
15:00 - 15:30
S3.6-I1
Dukovic, Gordana
University of Colorado Boulder, US
Excited State Processes in Semiconductor Nanocrystals and their relationships with Light-Driven Multi-Electron Catalysis
Gordana Dukovic
University of Colorado Boulder, US
Authors
Gordana Dukovic a
Affiliations
a, University of Colorado Boulder, US
Abstract

This presentation will focus on the excited state behavior of semiconductor nanocrystals as light absorbers that, coupled with redox catalysts, drive light-driven multi-electron transfer reactions. Reactions of interest include hydrogen generation, carbon dioxide reduction, nitrogen fixation, and water oxidation. We have demonstrated that nanocrystal excited state behavior, charge transfer dynamics, and surface chemistry play a governing role in the overall photochemistry of nanocrystal-catalyst complexes. This presentation will describe our most recent findings about how the reactions of interest can be driven and controlled through manipulation of nanocrystal excited state dynamics. In particular, the presentation will focus on: (i) measurement (by transient absorption spectroscopy), kinetic modeling, and control of electron transfer kinetics for injection of photoexcited electrons into redox enzymes; and (ii) elucidation of the motion of photoexcited holes on nanocrystal surfaces, using a combination of transient absorption measurements, modeling, and theory, and the implications of this motion on oxidation photochemistry.

SolFuel S1.6
Chair: Alexander Cowan
14:30 - 15:00
S1.6-I1
Surendranath, Yogesh
MIT
Bridging Molecular and Heterogeneous Electrocatalysis For Solar Fuels Production
Yogesh Surendranath
MIT, US
Authors
Yogesh Surendranath a
Affiliations
a, Massachusetts Institute of Technology, Cambridge, MA 0213
Abstract

The efficient production of solar fuels requires catalysts capable of accelerating complex multi-electron reactions at electrified interfaces. These reactions can be carried out at the metallic surface sites of heterogeneous electrocatalysts or via redox mediation at molecular electrocatalysts. Molecular catalysts yield readily to synthetic alteration of their redox properties and secondary coordination sphere, permitting systematic tuning of their activity and selectivity. Similar control is difficult to achieve with heterogeneous electrocatalysts because they typically exhibit a distribution of active site geometries and local electronic structures, which are recalcitrant to molecular-level synthetic modification. However, metallic heterogeneous electrocatalysts benefit from a continuum of electronic states which distribute the redox burden of a multi-electron transformation, enabling more efficient catalysis. We have developed a simple synthetic strategy for conjugating well-defined molecular catalyst active sites with the extended states of graphitic solids. Electrochemical and spectroscopic data indicate that these graphite-conjugated catalysts do not behave like their molecular analogues, but rather as metallic active sites with molecular definition, providing a unique bridge between the traditionally disparate fields of molecular and heterogeneous electrocatalysis. Our efforts to deploy these new hybrid materials for solar fuels production will be discussed.

15:00 - 15:30
S1.6-O1
Guerrero Castillejo, Antonio
Universidad Jaume I
Synthesis of Dimensionally Stable and Porous Electrodes for Photoelectrochemical Applications
Antonio Guerrero Castillejo
Universidad Jaume I, ES
Over the last few years Antonio Guerrero has been involved in the development of new semiconductors materials, their characterization and understanding their physical properties. After completing a PhD on the development of new organometallic catalysts for the production of polymers he took a job at Cambridge Display Technology. During four years Antonio developed some of the state of the art materials used in polymer Light Emitting Diodes (P-LEDs). Currently, Antonio is interested in gaining understanding on the operational principles of organic solar cells. At the group of Photovoltaic and Optoelectronic devices in Castell�n (Spain) he is developing some Impedance Spectroscopy modeling tools to understand the electrical response of OPVs.
Authors
Antonio Guerrero a
Affiliations
a, Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castellón de la Plana, Castellón, España, Castellón de la Plana, ES
Abstract

Dimensionally stable electrodes that are able to sustain the harsh chemical environment involved during electrochemical reactions is a requirement for materials to reach industrial applications. Dimensionally stable anodes (DSA) have been obtained previously for the electrochemical generation of chlorine gas and for water splitting applications.1 So far the most successful approach has been the generation of a thin layer of a stable electrocatalytic and conductive coating deposited on a base metal, usually Ti. Unfortunately, this approach renders materials with a small contact surface area between catalyst and electrolyte which could limit the overall activity of the reaction. The current presentation will focus on how DSA can be prepared from powder metallurgy processes. The composition can be controlled to modify the porosity of the electrode. This type of electrodes can be used in combination with photovoltaic materials by using different photoelectrochemical configurations for generation of solar fuels.2-4

References

1.         Mousty, C.; Fóti, G.; Comninellis, C.; Reid, V., Electrochemical behaviour of DSA type electrodes prepared by induction heating. Electrochim. Acta 1999, 45, 451-456.

2.         Guerrero, A.; Bisquert, J., Perovskite semiconductors for photoelectrochemical water splitting applications. Current Opinion in Electrochemistry 2017, 2, 144-147.

3.         Guerrero, A.; Haro, M.; Bellani, S.; Antognazza, M. R.; Meda, L.; Gimenez, S.; Bisquert, J., Organic photoelectrochemical cells with quantitative photocarrier conversion. Energy Environ. Sci. 2014, 7, 3666-3673.

4.         Haro, M.; Solis, C.; Blas‐Ferrando Vicente, M.; Margeat, O.; Dhkil Sadok, B.; Videlot‐Ackermann, C.; Ackermann, J.; Di Fonzo, F.; Guerrero, A.; Gimenez, S., Direct Hydrogen Evolution from Saline Water Reduction at Neutral pH using Organic Photocathodes. ChemSusChem 2016, 9, 3062-3066.

15:30 - 15:45
S1.6-O3
Suter, Silvan
École Politechnique Fédérale Lausanne
Linking Morphology and Multi-Physical Transport in Porous Copper Electrodes
Silvan Suter
École Politechnique Fédérale Lausanne
Authors
Silvan Suter a, Sophia Haussener a
Affiliations
a, Laboratory of Renewable Energy Science and Engineering, École Polytechnique Fédérale de Lausanne, Switzerland
Abstract

The morphology of copper electrodes significantly affects the performance of electrochemical CO2 reduction devices. Porous morphologies are important to increase the active reaction sites and to tune the selectivity towards desired products, but it can also limit the mass transport through the pore space. A better understanding of morphology-induced performance limitations or enhancements in porous electrodes is needed.

We used a coupled experimental-numerical approach to quantitatively characterize morphologically-complex porous electrodes (micrometer thick macroporous films with mesoporous structural details). We utilized a 3D-microscopy method, FIB-SEM tomography [1] with a high resolution of 4x4x4 nm3, to obtain a grey value array representing the photoelectrode morphology. The digital structure was segmented based on trainable machine-learning algorithms to subsequently quantify performance-related morphological parameters.

We applied this method to a hierarchically structured indium−tin oxide (ITO) electrode, covered by electrodeposited copper. The ITO scaffold has a macroporous inverse opal (IO) architecture and a mesoporous skeleton to increase the effective surface area [2]. Various samples with IO film thickness of 10 – 30 μm and pore diameters of 0.5 – 2 μm were scanned.

The digitalized electrode morphologies were then used in direct pore-level simulations to understand the mass transport within the macro- and mesopores. The diffusive transport of reactants and products in the electrolyte were investigated with a finite volume solver on a meshed representation of the exact geometries. Local current densities at the solid-liquid interface and pH distributions in the pore space were determined for the different film thickness and pore diameters.

The FIB-SEM tomography, with its nanometer-scale resolution, and the advanced pore-level simulations allowed for direct linking of the multi-physical transport to the morphological parameters of the porous ITO/Cu films. This study lays the ground for the optimization of the CO2 reduction efficiency by tuning the morphological parameters on digitally modified electrode representations (e.g. modified pore diameters, pore distribution and film thickness).

 

References

[1]         M. Cantoni and L. Holzer, “Advances in 3D focused ion beam tomography,” MRS Bull., vol. 39, no. 4, pp. 354–360, 2014.

[2]         D. Mersch et al., “Wiring of Photosystem II to Hydrogenase for Photoelectrochemical Water Splitting,” 2015.

15:45 - 16:15
S1.6-O2
CARRETERO, NINA M
Catalonia Institute for Energy Research – IREC, Sant Adrià del Besòs, Barcelona 08930, Spain
Bi-Metallic Sn-Pd Catalysts for CO2 Electroreduction to Formic Acid: Modulation of Overpotential and Stability
NINA M CARRETERO
Catalonia Institute for Energy Research – IREC, Sant Adrià del Besòs, Barcelona 08930, Spain
Authors
Nina M. Carretero a, Maria D. Hernández-Alonso b, Joan Ramon Morante a, c, Teresa Andreu a
Affiliations
a, Catalonia Institute for Energy Research, IREC. Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain.
b, Repsol Technology Center, Carretera de Extremadura A-5, km 18, 28935 Móstoles, Madrid, ES
c, University of Barcelona, C. Martí i Franqués, 1, 08028 Barcelona, Spain
Abstract

 

The electrocatalytic reduction of CO2 has attracted high attention recently, not only to reduce the carbon footprint, but for the possibility to produce value-added chemicals, such as carbon monoxide, formic acid, methanol or more complex organic molecules like ethanol or ethylene, under ambient temperature and pressure conditions. However, the real application of these electrochemical techniques are still a challenge, and efforts have to be made in decrease the overpotential, improve the product selectivity and stability of the electrocatalyst. Among all the CO2 reduction products that can be obtained, we have focused our research in formic acid, as it has been receiving great industrial attention for its several applications in fuel cells, textile or chemical industries.

We have studied two different metals and the combination of both as electrocatalysts for CO2 reduction to formic acid: tin (Sn)1 and palladium (Pd)2. Electrodeposited Sn has shown a potential around -0,8 VRHE at 10 mA/cm2 and a faradaic efficiency to formate around 70% using KHCO3 as electrolyte. On the other hand, under the same experimental conditions, the electrode containing Pd nanoparticles supported on carbon black shows a potential around -0.25 VRHE at 10 mV/cm2 and more than 80% of faradaic efficiency. The cathode potential obtained with Pd electrodes in comparison with Sn is 500 mV lower however, the catalyst working at such high current density is rapidly poisoned with CO (subproduct of the CO2 reduction) and the faradaic efficiency to formic is decreased dramatically. In this work, we present how the combination of both catalysts can be used to modulate the overpotential of the CO2 electroreduction reaction, the selectivity and the electrode stability under continuous operation. Also, a novel technique of co-electrodeposition of both metals is achieved, showing an alternative method to produce easily tunable electrodes, which facilitates and enhance the possible coupling to a photo-absorbing element in a complete photoelectrochemical cell.

References:

1. J. Mater. Chem. A,2016, 4, 13582–13588

2. ACS Energy Lett. 2016, 1, 764−770

16:15 - 16:30
S1.6-O4
magnan, helene
SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif-sur-Yvette, France
Unraveling the Role of Ferroelectricity in Solar Water Splitting Improvement
helene magnan
SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif-sur-Yvette, France, FR
Authors
magnan helene a, stanescu dana a, brehin julien a, barbier antoine a
Affiliations
a, Service de Physique de l’Etat Condensé CEA, CNRS, Universite Paris Saclay CEA Saclay , l’orme des merisiers bat 772, 91191 Gif sur Yvette Cedex FRANCE
Abstract

The transformation of solar energy into chemical energy stored in the form of 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 the strong electron-hole recombination may explain their low efficiency. It has been recently proposed to use the spontaneous electric field of a ferroelectric compound to separate photogenerated charges in photoanode. In our laboratory we have been developing model oxide thin films for solar water splitting prepared by oxygen assisted molecular beam epitaxy for several years, in order to understand the relevant parameters to improve photoanodes performance. In this context, in order to unravel the role of ferroelectricity we have been investigating epitaxial TiO2 films and TiO2/BaTiO3 heterojonctions deposited on Pt(001) where BaTiO3 is the prototypical ferroelectric material. We have studied the growth, the electronic structure, the ferroelectric properties and the photoelectrochemical performance (photocurrent and impedance spectroscopy) as a function of the position of the ferroelectric layer in the film (above, in the middle and below) and of the orientation of ferroelectric polarization. Thereby, we show that the TiO2 films adopt a TiO2-B (001) structure for thickness lower than 3 nm and an anatase (001) structure for higher thicknesses, while the BaTiO3 films adopt a tetragonal (001) structure with a natural electrical polarization perpendicular to the surface. We show that the performance of the TiO2 photoanode can be improved by a polarized layer of BaTiO3, the best improvement is when the ferroelectric film is below the TiO2 film and when the electrical polarization is downward polarized. We explain this finding by the presence of an internal electric field which favors the separation of photo-generated charges, and explain how the electronic structure is modified by the electrical polarization in each case.

16:30 - 16:45
S1.6-O5
Urbain, Félix
Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Sant Adrià del Besòs, 08930 Barcelona
Scalable Production of Syngas from Solar CO2 Recycling
Félix Urbain
Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Sant Adrià del Besòs, 08930 Barcelona

Dr.-Ing. Félix Urbain studied Materials Science and Engineering at the RWTH Aachen University (Germany), where he received his M.Sc. degree in 2012. He pursued his PhD at the Institute of Photovoltaics (IEK-5) in the Jülich Research Centre. His attention was focused on hydrogen production via H2O splitting. In 2016, he was awarded with the degree Dr.-Ing. summa cum laude (top 3% of RWTH Aachen University), sustaining the world record in solar-to-hydrogen conversion efficiency of 9.5% for thin film silicon based systems. Currently, he works as a Post-Doc at IREC in Spain, and his activity is dedicated to photoelectrochemical CO2 recycling. He is author of 22 papers on ISI international journals, 24 conference contributions (4 invited talks), and one patent.

Authors
Félix Urbain a, Nina M. Carretero a, Teresa Andreu a, b, María Dolores Hernández Alonso c, Joan Ramón Morante a, d
Affiliations
a, Institut de Recerca en Energia de Catalunya (IREC), Barcelona, Spain
b, Universitat Politècnica de Catalunya, Calle Jordi Girona, 31, Barcelona, ES
c, Repsol Technology Center, Carretera de Extremadura A-5, km 18, 28935 Móstoles, Madrid, ES
d, Universitat de Barcelona, Unitat de Biofísica, Facultat de Medicina, C/ Casanova 143, Barcelona, 08036, ES
Abstract

In this contribution, we report on strategies for high yield syngas production from solar CO2 recycling. To make CO2 re-use available at industry level, versatile and cost-effective processes at large-scale are in dire need. Therefore, we focused our study on efficient processes, which are scalable to large areas and compatible with state-of-the-art photovoltaics and electrocatalysts. Efficient processes include low overpotentials for the CO2 reduction reaction (CO2RR) and the oxygen evolution reaction (OER), respectively, along with excellent selectivity for the desired products (in particular for the case of CO2RR). We demonstrate that the coating of three-dimensional metallic foam electrodes with adapted nanosized catalysts resulted in high yield CO2 conversion to syngas. Furthermore, we provide evidence that photovoltaic structures based on cheap and mature silicon technology can be adapted to provide the required photovoltage to break the CO2 molecule into the targeted product.

In particular, we have investigated the application of silicon heterojunction photovoltaic cells as photoanodes, which requires meeting challenges, such as increasing the photovoltage without impairing the photovoltaic efficiency; protection of the solar cell by robust coatings to increase the stability in aqueous electrolytes; and the decoration with catalysts ensuring an efficient OER. We show that photovoltages up to 2.5 V with photocurrent densities up to 7.5 mA/cm2 can be reached by connecting four HIT cells in series. Furthermore, for the rear contact of the HIT cells we explored the applicability of metallic foams (e.g. Ni and Cu) loaded with metallic particles as OER catalyst. We demonstrate OER overpotentials below 300 mV (for 10 mA/cm2).

 

The CO2RR was performed by using large-scale metallic foam electrodes as highly conductive catalyst scaffolds. In this context, we developed a deposition process, which enables tunable coating of Cu and Ni foams, respectively, with highly active nanosized metal catalysts, such as Zn or Ag, for selective syngas production. The performance of the as-produced electrodes has been evaluated in terms of product selectivity, Faradaic efficiency, overpotentials, and stability. The developed electrodes exhibit overpotentials below 400 mV for CO2RR. Furthermore, stable and tunable H2:CO ratios between 5 and 1 along with high CO Faradaic efficiencies of up to 96% and CO current densities of 40 mA/cm2 were measured. Finally, we demonstrate a bias-free operation of the complete  device (2-electrode configuration) providing a photocurrent density of 5.0 mA/cm2 measured under 100 mW/cm2 illumination. This corresponds to a solar-to-syngas conversion efficiency of 4.3%.

joint session S7/S8
Chair not set
14:30 - 15:00
S7/S8-I1
De Angelis, Filippo
ISTM-CNR Perugia, IT
Defect photophysics of metal-halide pero
Filippo De Angelis
ISTM-CNR Perugia, IT

Filippo De Angelis is senior research scientist and a deputy director at the CNR Institute of Molecular Sciences and Technology, in Perugia, Italy. He is the founder and leader of the Computational Laboratory for Hybrid/Organic Photovoltaics. He earned a BS in Chemistry in 1996 and a PhD in Theoretical Inorganic Chemistry in 1999, both from the University of Perugia. He is an expert in the development and application of quantum mechanical methods to the study of hybrid/organic photovoltaics and materials for energy applications. He is Fellow of the European Academy of Sciences. He has published >270 papers with > 17000 citations.

Authors
Filippo De Angelis a, b
Affiliations
a, Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Perugia, Italy
b, CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
Abstract

The relatively weak bond of metal-halide perovskites  (MHPs) gives rise to an inherently soft crystal lattice which is naturally prone to disorder, [1] associated to formation of defects. Defects introducing levels in the material’s band-gap may act as traps and recombination centers for photogenerated charge carriers, limiting the device performance and possibly impacting the device temporal stability. Defects may also introduce ionic mobility channels in MHPs. Ion migration is boosted by the presence of vacancies and interstitial defects, acting as shuttles for ion hopping.[2] If the migrating defects are also charge traps, as it occurs for iodine defects in MAPbI3, one has migrating traps which can respond to the action of an electric field [3] and to the presence of photogenerated carriers.[4, 5] Some of the traps may also undergo photochemical reactions, such as the reported release of molecular iodine under light irradiation[6, 7]. Defects may also lay behind the reported material transformation under light exposure, followed by very slow relaxation to initial conditions.[8,9]

Theoretical and computational modeling is a complementary tool for rationalizing experimental results, on the one hand, and to direct experiments and device fabrication towards innovative concepts, on the other hand.  Several computational studies have already been carried out on native defects in MHPs, employing Density Functional Theory. The complex interplay of electronic structure and dynamical features of MHPs, however, poses challenging problems to the accuracy and reproducibility of these calculations.[10] Here we present what we believe are the “best practices” in defect modeling of metal-halide perovskites with selected examples of applications related to the effect of electric fields and charge carriers on the structural and electronic properties of perovskites relevant to stability and solar cell operation.

References:

[1] Conings, B. et al. Adv. Energy Mater. 2015, 5, 1500477.

[2] Mosconi, E.; De Angelis, F. ACS Energy Lett. 2016, 1, 182-188.

[3] Chen, B. et al. Nat. Mater., 2018, in press.

[4] Birkhold, S.T.  et al. ACS Energy Lett. 2018, 3, 1279−1286

[5] Meggiolaro, D. et al. Energy Environ. Sci. 2018, 11, 702-713.

[6] Meggiolaro, D. et al. ACS Energy Lett., 2018, 3, 447–451.

[7] Kim, G.Y. et al. Nat Mater 2018, 17, 445-449.

[8] Gottesman, R. et al.  J. Phys. Chem. Lett. 2014, 5, 2662-2669.

[9] Tsai, H. et al. Science 2018, 360, 67-70.

[10] Meggiolaro, D.; De Angelis, F. ACS Energy Lett. 2018, 2206-2222.

15:00 - 15:30
S7/S8-I2
Yamashita, Koichi
The University of Tokyo
Charge Separation and Charge Carrier Trapping of Lead Iodide Perovskites
Koichi Yamashita
The University of Tokyo, JP
Koichi Yamashita obtained his PhD from Kyoto University in 1982 supervised by Prof. Kenichi Fukui. He was postdoctoral fellow with Prof. William H. Miller at the University of California, Berkeley for 1982-84. He moved to Okazaki in 1984 to join the group of Prof. Keiji Morokuma as Research Associate at Division of Theoretical Study of Institute of Molecular Science. In 1991 he became Senior Researcher at Institute of Fundamental Chemistry directed by Prof. Kenichi Fukui. In 1994 he moved to Tokyo to join the group of Prof. Kimihiko Hirao as Associate Professor in Department of Applied Chemistry at University of Tokyo. He has been Full Professor at the University of Tokyo since 1997.
Authors
Koichi Yamashita a
Affiliations
a, Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo
Abstract

The high performance of recently emerged lead halide perovskite-based photovoltaic

devices has been attributed to remarkable carrier properties in this kind of material:

long carrier diffusion length, long carrier lifetime, and low electron-hole recombination

rate. However, the mechanism of the charge separation is still not fully understood. In my talk, it will be demonstrated that the charge separation is induced by the structural fluctuation of the inorganic lattice using first-principles molecular dynamics simulations [1]. On the other hand, charge carrier trapping at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells. In practice, it is one of the main limiting factors for carrier lifetime.  Surface defects responsible for carrier trapping are clarified by comprehensive first-principles investigations and it is proposed that PbI2-rich condition is preferred to MAI-rich one, while intermediate condition has possibility to be the best choice [2].

References

[1] H. Uratani and K. Yamashita, J. Phys. Chem. C, 121, 26648−26654 (2017).

[2] H. Uratani and K. Yamashita, J. Phys. Chem. Lett., 8, 742−746 (2017).

15:30 - 16:00
S7/S8-I3
Mosconi, Edoardo
CNR-ISTM Perugia
First Principles Modeling of Mixed 2D/3D Organohalide Perovskites
Edoardo Mosconi
CNR-ISTM Perugia, IT
Authors
Edoardo Mosconi a, García Espejo García Espejo b, Filippo De Angelis a
Affiliations
a, Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di ScienzeTecnologie Molecolari (ISTM-CNR), Via Elce di Sotto 8, 06123, Perugia, Italy.
b, University of Cordoba, Department of Physical Chemistry and Applied Thermodynamics, Córdoba, Spain
Abstract

While 3D perovskites are the materials currently leading the field for photovoltaics, 2D hybrid organic/inorganic layered materials have a much broader versatility in accommodating a vast variety of organic molecules and are providing a long-term devices stability. In particular, we obtained a one-year stable perovskite device by engineering an ultra-stable 2D/3D perovskite junction with a PCE of 14.6% in standard mesoporous solar cells.[1] Hybrid organic-inorganic multidimensional perovskites, also known as Ruddlesden-Popper perovskites, are composed of 3D domains separated by large organic cations. The mixing of the two mainly studied types of perovskites supposes the combination of the good properties from each one. Due to the 3D domains, Ruddlesden-Popper perovskites can absorb radiation in a wide range of the electromagnetic spectrum. Moreover, the environmental stability issue characteristic of 3D perovskites is solved thanks to the good stability provided by the 2D domains, in which the larger amount of organic phase acts as barrier against water and moisture penetration.[1] The main problem of this material arises from its photoexcitation and the consequent generation of the electron-hole pairs. Holes and electrons keep confined in the inorganic layers due to the electric isolation of the organic cations in 2D domains, i.e.: while their diffusion along the 3D domains is excellent, it is quite poor across the 2D ones. In order to find a solution, we will explore the possibility of improving the conductivity of charge carriers across 2D domains by the insertion of specifically designed conducting organic cations. The chemical structure of these cations should be suitable for the purpose, for instance by embedding aromatic rings or conjugated multiple bonds and allowing the selective transport of a single carrier type. The desired cation properties will be finely computed in order to realize a Ruddlesden-Popper perovskite with high and selective vertical conduction, see Fig. 1.

[1] Grancini, G.; Roldán-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M., et al. Nat Commun, 8 (2017) 15684.

16:00 - 16:30
S7/S8-I4
Katan, Claudine
Institut des Sciences Chimiques de Rennes, CNRS
Halide Perovskites: Recent Advances in Optoelectronic Properties from Atomic Scale Modelling
Claudine Katan
Institut des Sciences Chimiques de Rennes, CNRS

Claudine Katan (born Hoerner) received her Ph.D. in physics (nonlinear optics) from the University of Strasbourg (ULP), France in 1992. She subsequently served as a lecturer in physics at the University of Rennes (UR1), France, before being appointed as a CNRS Research Investigator in the Physics Department at Rennes in 1993. Until 2003, her research interests concerned the properties of molecular charge-transfer crystals and the topology of electron densities mainly through approaches based on density functional theory (e.g. the CP-PAW code by P. E. Blöchl, IBM-Zurich). She then joined the Chemistry Department at Rennes and turned her research interests toward the structural, electronic and linear/nonlinear optical properties of molecular and supramolecular chromophores using various theoretical approaches—from modeling to state-of-the-art electronic structure calculations (e.g. CEO methodology by S. Tretiak, LANL) . Since the end of 2010, her research has also been devoted to 3D and 2D crystalline materials of the family of halide perovskites based on solid-state physics concepts. Overall, her theoretical work is closely related to the experimental research developed in-house and through international collaboratorations.

Authors
Claudine Katan a, Boubacar Traore a, Mikaël Képénékian a, Laurent Pedesseau b, Jean-Christophe Blancon c, Wanyi Nie c, Hsinhan Tsai c, d, Sergei Tretiak c, Constantinos Stoumpos e, Mercouri Kanatzidis e, f, Aditya Mohite c, g, Jacky Even b
Affiliations
a, Univ Rennes, ENSCR, INSA Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR6226, F-35000 RENNES
b, Univ Rennes, INSA Rennes, CNRS, Institut FOTON - UMR6082, F-35000 RENNES
c, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
d, Department of Materials Science and Nanoengineering, Rice University, Houston, Texas 77005, USA
e, Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA
f, Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
g, Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, USA
Abstract

Both all inorganic and hybrid halide perovskites have undeniably remarkable characteristics for next-generation photovoltaics, which deserve to be better understood. There are many different perovskite structures that are currently widely explored as absorber materials among which 3D AMX3 and 2D A2 A’n-1 Mn X3n+1 frameworks, where A, A’ are cations, M is a metal, X is a halide. Here, through a couple of recent examples including newly discovered halide perovskite phases [1], we will discuss their optoelectronic properties based on first-principles calculations and semi-empirical modelling. Impact of interfaces [2], structural fluctuations [3], quantum and dielectric confinements [4] on charge carriers and excitons will be inspected. Particular attention will be paid on excitonic effects comparing the results of model calculations with low temperature optical spectroscopy and 60-Tesla magneto-absorption [5]. Theoretical inspection of low energy states associated with electronic states localized on the edges of the perovskite layers [6] will also be shown to provide guidance for the design of new synthetic targets [7] taking advantage of experimentally determined elastic constants [8].

[1] C. M. M. Soe et al. JACS, 139, 16297, 2017; L. Mao et al. JACS, 140, 3775, 2018; X. Li et al. submitted.

[2] W. Nie et al. Adv. Mater. 30, 1703879, 2018.

[3] M. A. Carignano et al. J. Phys. Chem. C, 121, 20729, 2017; A. Marronnier et al. ACS Nano, 12, 3477, 2018; L. Zhou et al. ACS Energy Lett., 3, 787–793, 2018; C. Katan et al. Nature Materials, 17, 377, 2018.

[4] B. Traore et al. ACS Nano, 12, 3321, 2018.

[5] J.-C. Blancon et al. Nature Com. in press (arXiv:1710.07653).

[6] J.-C. Blancon et al. Science, 355, 1288, 2017.

[7] M. Kepenekian et al. arXiv:1801.00704.

[8] A. C. Ferreira et al. arXiv:1801.08701.

PerMod S8.6
Chair: Claudio Quarti
16:30 - 17:00
S8.6-I1
Rothlisberger, Ursula
École Polytechnique Fédérale de Lausanne EPFL
Modelling Nucleation and Growth of Lead Halide Perovskites
Ursula Rothlisberger
École Polytechnique Fédérale de Lausanne EPFL, CH

Ursula Rothlisberger was born in Switzerland and obtained her diploma in Physical Chemistry from the University of Bern. She earned her Ph.D. degree at the IBM Zurich Research Laboratory in R�schlikon. From 1992�1995, she worked as a postdoctoral fellow, first at the University of Pennsylvania in Philadelphia (USA) and then at the Max-Planck-Institute for Solid State Physics in Stuttgart, Germany. In 1996, she moved as a Profile 2 Fellow of the National Science Foundation to the ETH in Zurich. One year later, she became Assistant Professor of Computer-Aided Inorganic Chemistry at the ETH Zurich, and in 2002 she accepted a call for a position as Associate Professor at the �cole Polytechnique F�d�rale de Lausanne (EPFL). Since 2009, she has been working as a full Professor in Computational Chemistry and Biochemistry at the EPFL. In 2001, she received the Ruzicka Prize, and in 2005, the World Association of Theoretically Oriented Chemists (WATOC) awarded her the Dirac Medal for "the outstanding computational chemist in the world under the age of 40". Ursula Rothlisberger is an expert in the field of density functional based mixed quantum mechanical/molecular mechanical molecular dynamics simulations in the ground and electronically excited states. She has published more than 200 original publications in peer-reviewed journals and various review articles in specialized journals and as book chapters.

Authors
Ursula Roethlisberger a, Paramvir Ahlawat a, Michele Parrinello b
Affiliations
a, Swiss Federal Institute of Technology, EPFL, ISIC, LCBC, CH-1015, Lausanne, Switzerland
b, ETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland
Abstract

Sample preparation of perovskite materials has a crucial impact on their optoelectronic properties and has indeed been a dominant factor for the rapid advances in photoconversion efficiencies. However, very little is currently known about the microcopic details that determine the nucleation and crystal growth proces. Such a knowledge could be a starting point to enable control over the crystallization process and a rational optimization of preparation conditions.

In principle, molecular dynmaics simulations can provide atomistic insight into complex phenomena but direct simulations of the nucleation process are highly challenging due to the large activation barriers that are involved and the high-dimensionality of the available phase space. Here, we present enhanced sampling molecular dynamics simulations based on well-tempered metadynamics simulations about the nucleation process of lead halide perovskites from solution. Choosing appropriate collective varaibles, it has been possible for the first time to monitor the nucleation and growth of such a multicomponent system (containing, lead ions, halide anions, monovalent cations and solvent molecules). These simulations demonstrate  the influence that different solvents play in this process and reveal a pivotal role of the monovalent cations.

17:30 - 19:00
Poster Session
 
Wed Oct 24 2018
Plenary Session 4
Chair: David Egger
09:00 - 09:30
4-K1
Wood, Vanessa
ETH Zurich
Charge Transport in Nanocrystal Solids
Vanessa Wood
ETH Zurich, CH

Vanessa Wood is a professor in the Department of Information Technology and Electrical Engineering at ETH Zurich, where she heads the Laboratory for Nanoelectronics. Before joining ETH in 2011, she was a postdoctoral associate in the laboratory of Professor Yet-Ming Chiang and Professor Craig Carter in the Department of Materials Science and Engineering at MIT, performing research on novel lithium-ion battery systems. She received her MSc and PhD from the Department of Electrical Engineering and Computer Science at MIT. Her graduate work was done in the group of Professor Vladimir Bulović and focused on the development of optoelectronic devices containing colloidally synthesized quantum dots.

Authors
Vanessa Wood a
Affiliations
a, Laboratory for Interfacial Chemistry, ETH Zurich, Vladimir-Prelog-Weg 2, Zurich, 8093, CH
Abstract

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

Plenary session 3
Chair: Shannon Boettcher
09:00 - 09:30
3-K1
Toma, Francesca
Lawrence Berkeley National Laboratory
A New Portrait of Functional Complex Interfaces for Solar Fuel Devices
Francesca Toma
Lawrence Berkeley National Laboratory, US
Authors
Francesca Maria Toma a, b
Affiliations
a, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, Califor- nia 94720, United States
b, JCAP, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Mail Stop 976, Berkeley, CA 94720, US
Abstract

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

Dyman S5.7
Chair: David Cahen
09:30 - 10:00
S5.7-O1
Lovrincic, Robert
TU Braunschweig
Optical Phonons in Halide Perovskites and Implications for Charge Transport
Robert Lovrincic
TU Braunschweig, DE
Robert Lovrincic obtained his PhD in physics from Heidelberg University in 2009. In 2010 he joined the group of Prof. David Cahen at the Weizmann Institute of Science (Israel) as a Minerva and Marie-Curie fellow. He has been a group leader at the Innovationlab in Heidelberg since 2013. His main expertise is the vibrational and structural characterization of electronic materials.
Authors
Christian Gehrmann a, David Egger a, Robert Lovrincic b, c
Affiliations
a, Institute of Theoretical Physics, University of Regensburg, D-93050 Regensburg, Germany
b, Institute for High Frequency Technology, TU Braunschweig, Speyerer Str. 4, Heidelberg, 69115, DE
c, InnovationLab, Heidelberg
Abstract

Metal-halide perovskites are promising materials for opto-electronic applications. Their mechanical and electronic properties are directly connected to the nature of their lattice vibrations. Whereas the mid infrared (IR) range contains mainly information on the internal vibrations of the methylammonium cation, the lead-halide lattice vibrations are located in the far IR.

We will report far-IR spectroscopy measurements of lead halide perovskite thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra.1 We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites. We determine the frequencies of both the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, and upper limits for the phonon-scattering limited charge carrier mobilities. Furthermore, we compare our experimental results to molecular dymanics simulations of lead halide perovskites. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

(1) Sendner, M.; Nayak, P. K.; Egger, D. A.; Beck, S.; Müller, C.; Epding, B.; Kowalsky, W.; Kronik, L.; Snaith, H. J.; Pucci, A.; Lovrincic, R. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Mater Horiz 2016, 3 (6), 613–620.

10:00 - 10:30
S5.7-I1
Kirchartz, Thomas
FZ Jülich
Long Lifetimes and Small Phonon Energies in Metal-Halide Perovskite 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, IEK5-Photovoltaics, Forschungszentrum Jülich, 52425 Jülich, Germany
Abstract

Solar cells based on metal-halide perovskite absorber layers have resulted in outstanding photovoltaic devices with long non-radiative lifetimes as a crucial feature enabling high efficiencies. Long non-radiative lifetimes occur if the transfer of the energy of the electron-hole pair into vibrational energy is slow, due to, e.g., a low density of defects, weak electron phonon coupling or the release of a large number of phonons needed for a single transition. Here, we discuss the implications of the known material properties of metal-halide perovskites (such as permittivities, phonon energies and effective masses) and combine those with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI3 lead to a strong dependence of recombination rates on trap position, which can be readily deduced from the underlying physical effects determining non-radiative transitions. Here, we show that this is important for the non-radiative recombination dynamics of metal-halide perovskites, as it implies that these systems are rather insensitive to defects that are not at midgap energy. This can lead to long lifetimes, which indicates that the low phonon energies are likely an important factor for the high performance of optoelectronic devices with metal halide perovskites. 

NCPhot S4.1
Chair: Daniel Vanmaekelbergh
09:30 - 10:00
S4.1-O1
Urbonas, Darius
IBM Research - Zurich
Tunable Nanoscale defect Cavities for Exciton-Polariton Condensates at Room Temperature
Darius Urbonas
IBM Research - Zurich, CH
Authors
Darius Urbonas a, Thilo Stöferle a, Fabio Scafirimuto a, Rainer F. Mahrt a
Affiliations
a, IBM Research−Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
Abstract

We use tunable nanoscale defect cavities to create zero-dimensional exciton-polaritons [1]. Data on the strong coupling as well as mode engineering is shown as a function of cavity detuning. At ambient conditions, we observe non-equilibrium exciton-polariton condensation [2] with strong lateral confinement on the wavelength scale [3]. Threshold and line narrowing are analyzed as a function of excitation density. Both, in real and momentum space we observe the distinct signature of strong transversal confinement in the condensation. First order coherence properties are investigated by means of a Michelson interferometer.

As a building block towards extended lattices, we realize two coupled cavities by focused ion beam milling and thermal scanning probe lithography. These photonic molecules are investigated by means of atomic force microscopy and optical measurements, to compare both fabrication methods. Furthermore, we investigate different ways to improve the optical properties of the active material e.g., photo-degradation and inhomogeneity.

These are the initial steps towards studying quantum fluids in extended, arbitrary potential landscapes at ambient conditions.

References

[1] D. Urbonas et al., ACS Photonics 3 (9), 1542–1545 (2016).

[2] J. D. Plumhof et al., Nat. Mater. 13, 247 (2014).

[3] F. Scafirimuto et al., ACS Photonics 5 (1), 85–89 (2018).

10:00 - 10:30
S4.1-O2
Chandrasekaran, Vigneshwaran
Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium
Spectral Dynamics of Linearly Polarized Bright Exciton in InP/ZnSe Colloidal Quantum Dots
Vigneshwaran Chandrasekaran
Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium
Authors
Vigneshwaran Chandrasekaran a, Lorenzo Scarpelli b, Francesco Masia b, Dorian Dupont a, Mickäel D. Tessier a, Pieter Geiregat a, Iwan Moreels a, Wolgang Langbein b, Zeger Hens a
Affiliations
a, Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium
b, School of Physics and Astronomy, Cardiff University, CF24 3AA Cardiff, United Kingdom
Abstract

Spectroscopy of single colloidal quantum dots (QDs), especially at cryogenic temperature, helps to understand the inherent properties of nanocrystals that are often hidden in ensemble level studies. This applies in particular to InP-based QDs, which attract increasing interest as Cd-free alternative nanocrystals yet were hardly investigated at the single QD level. Here, we discuss the photoluminescence properties of single InP/ZnSe QDs, both at room temperature and at cryogenic temperature. While ensemble level measurements feature a luminescent linewidth of around 50 nm, we find that emission spectrum of single InP/ZnSe QDs can have a linewidth as narrow as 14 nm (50 meV). Hence, the relatively broad emission line that characterizes ensembles of InP-based QDs is by no means an intrinsic material property. In addition, we found that InP/ZnSe QDs combine a nearly blinking free emission with a high purity single photon emission (g2(0)<0.03), also well beyond the saturation intensity.[1] Cryogenic single QD spectra, on the other hand, consist of zero-phonon lines that can be as narrow as 40 µeV. Polarization resolved spectra point to a linearly polarized spectral doublet from the bright exciton. At lower excitation intensities, jitter was negligible and spectra could be integrated for tens of seconds without erasing the doublet splitting of 1.2 meV. At higher excitation intensities, jitter becomes more severe and switching between emission from the exciton-doublet and the trion-singlet can be observed. This indicates that spectral jitter finds its origin in changes of the local electric field caused by the temporal trapping of one charge carrier. In summary, we find conclude that single InP QDs have emission characteristics similar to the extensively studied CdSe-based QDs. Moreover, the narrow emission lines, limited jitter and fluorescence intermittency of single InP/ZnSe QDs holds great promise to further explore these materials as a solution-processable single-photon emitter and improve the ensemble level characteristics of these materials.


Reference:
[1] Chandrasekaran, V.; Tessier, M. D.; Dupont, D.; Geiregat, P.; Hens, Z.; Brainis, E. Nano Letters 2017, 17 (10), 6104–6109.

PerFun S7.7
Chair: Koichi Yamashita
09:30 - 10:00
S7.7-I1
Deschler, Felix
University of Cambridge
Investigation of Electronic and Structural Dynamics in Metal-Halide Perovskites with Ultrafast Spectroscopy
Felix Deschler
University of Cambridge, GB
Authors
Felix Deschler a
Affiliations
a, University of Cambridge, UK
Abstract

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

The properties and dynamics of the perovskites’ electronic states are controlled by the crystal structure and symmetry. Strong spin-orbit coupling is predicted to introduce Rashba-type state splitting in the electronic band structure. Together 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 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 more sustainable lead-free variants. I will discuss how the crystal structure affects the properties of electronic states, and how we can use these modifications to create novel optoelectronic devices.

10:00 - 10:30
S7.7-I2
Bakulin, Artem
Imperial College London
Organic Cation in Hybrid Perovskite Materials and Interfaces
Artem Bakulin
Imperial College London, GB
Authors
Artem Bakulin a
Affiliations
a, Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ, UK
Abstract

Three-dimensional lead-halide perovskites combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine a number of bulk-sensitive and surface-selective spectroscopic techniques to eluscidate the structure and dynamics of organic cations.

We use ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic cation orientation in a films of pure and mixed tri-halide perovskite materials. For pure MAPbX3 (X=I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. Much slower dynamics, up to partial immobilisation of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3 and MAPb(BrxI1-x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell.

We also applied surface sensitive vibrtational sum-frequency generation spectroscopy (VSFG) to address the orientation of cations at the perovrkite active layer interfaces with different electron- and hole-extracting materials.We found that at perovskite spiro interface cations are patially immobilised that can be and evidence of high interfacial charge density. 

The observed dynamics and structural information are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronics.

SolFuel S1.7
Chair: Shannon Boettcher
09:30 - 10:00
S1.7-I1
Galan-Mascaros, Jose Ramon
Institute of Chemical Research of Catalonia (ICIQ)
A-LEAF: A European-Wide Approach Towards Solar Fuels
Jose Ramon Galan-Mascaros
Institute of Chemical Research of Catalonia (ICIQ), ES
Authors
Jose Ramon Galan-Mascaros a, b
Affiliations
a, Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology (BIST), Avinguda Paisos Catalans 16, Tarragona 43007, Catalonia, Spain
b, ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain
Abstract

During this presentation we will introduce the A-LEAF project: "An artificial leaf: a photo-electro-catalytic cell from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels" (FET-PROACT-732840). This project, funded by the European Union, is a collaborative effort by thirteen partners from eight European countries, coordinated by the Institute of Chemical Research of Catalonia (ICIQ), in Spain.

Our A-LEAF initiative aims to design and develop a fully functional scheme to transform sunligh, water and carbon dioxide into useful, sustainable and environmentally neutral fuels and/or fine chemicals. We have gathered a truly multidisciplinary consortium to cover all scientific expertise needed to overcome the major challenges: physicists to achieve sunlight capture; surface scientists to use this energy to oxidize water, extracting protons and electrons; electrochemists to use these equivalents to reduce carbon dioxide into useful stock; and system engineers to implement and fine-tune all components into a viable and cost-effective process. Because, in addition to the general scientific challenge, we have committed to use, exclusively, scalable processes, earth abundant non-critical raw materials and inexpensive platforms. Only with these added values, we may dream of bringing artificial photosynthesis, and solar fuels, to the future Energy pull, where societal impact is the ultimate target.

Our  thirteen partner institutions: ICIQ, ETH Zürich, Universitet Leiden, IMDEA Nanociencia, Technische Universität Wien, Universitat Jaume I, Imperial College, Technische Universität Darmstadt, Forschungzentrum Jülich, Université de Montpellier, INSTM and COVESTRO; started the collaborative work in Januay 2017, and the major results and current state of the project (successful achievements and unexpected problems) will be highlighted and discussed.

10:00 - 10:30
S1.7-O1
Abdi, Fatwa
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels
Overcoming the Limitations of BiVO4 Photoanodes through Anion and Cation Substitution
Fatwa Abdi
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels

Fatwa Abdi is a staff scientist and a working group leader at 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
Marlene Lamers a, Marco Favaro a, David Starr a, Ibbi Ahmet a, Wenjie Li b, Lydia Wong b, Roel van de Krol a, Fatwa Abdi a
Affiliations
a, Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
b, School of Materials Science and Engineering Nanyang Technological University, Singapore
Abstract

Efforts in the solar water splitting field for the past decade have established BiVO4 as the highest performing metal oxide photoanode. Within this period, the AM1.5 photocurrents of BiVO4 have been increased from hundreds of mA/cm2 to ~7 mA/cm2.[1] However, high photocurrents (> 5 mA/cm2) are only achieved with nanostructuring, which poses additional complexities (e.g., light scattering) for the design and scalability of a tandem device for water splitting. Non-nanostructured BiVO4 is limited by its poor carrier transport properties. In addition, with reported photocurrents already within 90% of the theoretical maximum, the solar-to-hydrogen (STH) efficiency of BiVO4 is limited to < 9% due to its relatively wide bandgap of 2.4-2.5 eV.

Here, we demonstrate that these limiting factors can be alleviated by controlled anion and cation substitution in BiVO4. To overcome the bandgap limitation, we incorporated sulfur into BiVO4 by post-annealing in sulfur-rich atmosphere. As a result, the bandgap is reduced by up to ~0.3 eV, which increases the theoretical maximum STH efficiency to ~12%. We confirmed that sulfur substitutes oxygen in the lattice of BiVO4 by a series of structural and chemical characterization (e.g., XRD, Raman, XPS). Moreover, hard X-ray photoelectron spectroscopy (HAXPES) reveals that the bandgap decrease is a result of an upward shift of the valence band maximum. Simultaneously, time-resolved microwave conductivity (TRMC) measurements reveal an improvement of charge carrier transport by the incorporation of sulfur; the mobility increases by a factor of ~5. Wavelength-dependent TRMC measurements confirm the photoactivity of the sulfur-incorporated BiVO4 up to 560 nm, well beyond the bandgap of typical BiVO4 films. In addition, we successfully introduced calcium into BiVO4 thin films;[2] calcium substitutes bismuth and acts as an acceptor-type dopant. Although it seems counterintuitive to introduce an acceptor dopant into n-type BiVO4, this would allow the fabrication of p-type, and eventually p-n homojunctions based on BiVO4. HAXPES measurements reveal that calcium out-diffuses towards the surface of the film, thereby creating a spontaneous p-n homojunction within the film. As a result of the internal electric field, the carrier separation efficiency was enhanced by a factor of ~2. Overall, this work underlines the importance of controlled ionic substitution in complex metal oxides, which may not only improve the performance but also enable new device architectures.

[1] Abdi and Berglund, J. Phys. D. Appl. Phys. (2017)

[2] Abdi et al. ChemPlusChem (2018)

10:30 - 11:00
Coffee Break
Dyman S5.8
Chair: Freddy Rabouw
11:00 - 11:30
S5.8-I1
Bakulin, Artem
Imperial College London
Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites
Artem Bakulin
Imperial College London, GB
Authors
Artem Bakulin a
Affiliations
a, Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ, UK
Abstract

In conventional solar cells (SCs), above-bandgap “hot” carriers (HCs) rapidly lose their excess energy to vibrations in the semiconductor lattice via electron-phonon coupling. This thermalisation to the band edge forms the main part of the Shockley-Queisser limiting efficiency. Semiconductors with reduced carrier cooling rates are desirable to exceed this limit via a hot-carrier architecture, where the hot carriers are extracted before they are fully cooled. Lead-halide perovskites are solution processible solar cell materials, which exhibit exceptional power conversion efficiencies and a tunable band gap. Recent measurements indicate slow cooling at high carrier densities in these material systems. Here we use ultrafast infrared intraband spectroscopy to directly compare the dynamics of carrier cooling in a range of five commonly studied lead-halide perovskites: FAPbI3, FAPbBr3, MAPbI3, MAPbBr3 and CsPbBr3. We measure this cooling as occuring within ~100-900 fs, depending on both the carrier density, nhot (slower cooling at higher nhot) and choice of cation (with the slowest cooling in the all-inorganic Cs-based system). These observations support the existence of a “hot-phonon bottleneck” and assert the role of lattice vibrations towards HC cooling.

11:30 - 12:00
S5.8-O1
Deschler, Felix
University of Cambridge
Origin of High Photoluminescence in Mixed-Cation Perovskites: Photodoping from energetic disorder
Felix Deschler
University of Cambridge, GB
Authors
Felix Deschler a, Sascha Feldmann a, Stuart MacPherson a, Satyaprasad Senanayak a, Jasmine Rivett a, Mojtaba Abdi Jalebi a, Guangjun Nan b, David Beljonne b, Sam Stranks a, Michael Salia c
Affiliations
a, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom, JJ Thomson Avenue, 9, Cambridge, GB
b, University of Mons (UMONS), Laboratory for Chemistry of Novel Materials, Center for Innovation and Research in Materials and Polymers (CIRMAP), Mons (Belgium)
c, Adolphe Merkle Institute, University of Fribourg, CH-1700 Fribourg, Switzerland
Abstract

Metal-halide perovskites show exceptional optoelectronic properties for next generation photovoltaics and light-emitting diodes. Recently, monovalent cation substitution has been reported to generate luminescence very efficiently, yet the underlying photo-physics remain to be understood.

Here, we study the origin of this increased brightness by combining transient absorption and photoluminescence (PL) to track charge carrier dynamics in thin films. Unexpectedly, we find that the recombination regime changes from the previously-reported second to first order regime dynamically within tens of nanoseconds after excitation, in line with fluence-dependent PLQE measurements. In temperature-dependent PL we find a redshift of the luminescence with decreasing temperature, directly mapping localized shallow traps. Supported by DFT calculations and transistor measurements we propose that energetic disorder in the distribution of electronic states leads to spatial accumulation of charges, creating n- and p-type regions. Our results indicate that strong luminescence can be achieved in mixed-cation perovskites even at low carrier densities and thereby provides a roadmap for highly efficient LEDs.

NCPhot S4.2
Chair: Marcus Scheele
11:00 - 11:30
S4.2-O1
Lifshitz, Efrat
Technion
Spin Properties in II-VI and Perovskites Colloidal Quantum Dots
Efrat Lifshitz
Technion, IL
Authors
Efrat Lifshitz a, Joanna Dehnel a, Alyssa Kostadinov a, Yahel Barak a
Affiliations
a, Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute, Solid State Institute, Technion, Haifa 32000
Abstract

Colloidal semiconductor quantum dots (CQDs) have been at the forefront of scientific research for more than two decades, based on their size tunable properties. Although implementation of CQDs in opto-electronic devices already occurs, various fundamental issues with a direct impact on technology are left as open questions. Recent years showed an interest in the investigation of magneto-optical properties of various CQDs with substantial importance for opto-electronic and spin-based devices.

Here we include the study of two different CQD platforms: (1) Synthesis and magneto-optical characterization of spectrally stable pure and diluted magnetic semiconductor CQDs from the II-VI semiconductor family (e.g., CdTe/CdSe and Mn@CdTe/CdSe); (2) Magneto-optical characterization of perovskite CQDs of the type APbBr3 (A - methylamonium or Cs+). Both systems show intriguing spin properties of special scientific and technological interests. The uniqueness of the spin properties and their novelty will be the focus during the talk.

CdTe/CdSe colloidal quantum dots with special composition, including soft boundary (alloying) at the core/shell interface or a giant core or a shell, possess quasi type-II configuration and show blinking-free behavior. The Mn+2 doping induces internal spin interactions between photo-generated species and the dopant spins, leading to giant magnetization or to an internal energy transfer into the dopant orbitals, and consequence emission from host-dopant hybrid- or from dopant atomistic-states. The current study developed a method to position the Mn ions selectively either at the core or at the shell. The magneto-optical measurements, including the use of optically detected magnetic resonance, exhibited resonance transitions related to the coupling of the Mn electron and the nuclear spins with the individual photo-generated carriers. The work was done in collaboration with the laboratory of Prof. Volkan Hilmi Demir from Bilkent and University and NTU.

The perovskites are minerals that have been studied extensively in the past. They are the focus of new interest in recent years, due to their exceptional performance in photovoltaic cells. Perovskites semiconductors possess high absorption coefficients as well as long-range transport properties. Currently, they are also prepared in the form of CQDs with very interesting properties including ferroelectricity, magnetism and exciton effects. The magneto-optical measurements of excitons in CsPbBr3 as individuals were investigated by monitoring the micro-photoluminescence spectra in the presence of an external magnetic field, while monitoring either the circular or linear polarization components.

11:30 - 12:00
S4.2-I1
Stöferle, Thilo
IBM Research - Zurich
Superfluorescence from Lead Halide Perovskite Quantum Dot Superlattices
Thilo Stöferle
IBM Research - Zurich, CH

Dr. Thilo Stöferle has been a permanent Research Staff Member at the IBM Research – Zurich Laboratory since August 2007. His current research interests are quantum simulation and quantum fluids, Bose-Einstein condensates with exciton-polaritons, integrated high Q/V cavities, nanophotonic lasers and switches. Another focus is on hybrid nanocomposite quantum materials for strong-light matter interaction and opto-electronic applications.

Authors
Gabriele Rainò a, b, c, Michael Becker c, d, Maryna Bodnarchuk b, Rainer Mahrt c, Maksym Kovalenko a, b, Thilo Stöferle c
Affiliations
a, Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zurich, 8093 Zurich, Switzerland
b, Laboratory of Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland
c, IBM Research – Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
d, 4Optical Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland
Abstract

Fully inorganic cesium lead halide (CsPbX3, where X = Cl, Br, I) perovskite-type nanocrystals are colloidal quantum dots with bright, narrowband emission that is tunable by composition and size over a wide spectral range with room-temperature photoluminescence quantum yield of up to 90%[1]. A pecularity of this material is the almost blinking-free emission at low temperature[2] that originates from a triplet state with exceptionally high oscillator strength[3].

We use densely packed arrays of up to several millions of these nanocrystals, known as superlattices, produced by means of solvent-drying-induced spontaneous assembly[4]. Such ensemble of emitters behaves dramatically different from its individual constituents due to coherent interaction enabled by the strong light-matter interaction and the excellent monodispersity of the quantum dots. The collective coupling gives rise to an intriguing many-body quantum phenomenon, resulting in short, intense bursts of light: so-called superfluorescence. We observe a comprehensive set of key signatures like dynamically red-shifted emission with more than twenty-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham–Chiao ringing behaviour at high excitation density.

This is the first demonstration of optical collective behaviour and extended coherent states with nanocrystals, opening up new opportunities for high-brightness and multi-photon quantum light sources.

 

References:
[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)
[2] Rainò et al., ACS Nano 10, 2485–2490 (2016)
[3] Becker et al., Nature, 553, 187-193 (2018)
[4] Rainò et al., arXiv 1804.01873 (2018)

 

12:00 - 12:30
S4.2-I2
Langbein, Wolfgang
Cardiff University, School of Physics and Astronomy
Coherent Exciton Dynamics in Colloidal Quantum Dots
Wolfgang Langbein
Cardiff University, School of Physics and Astronomy, GB

Wolfgang Langbein (ResearcherID B-1271-2010) was born in Würzburg, Germany, in 1968. He received his Diplom in physics from the University of Kaiserslautern in 1992, and his PhD degree in physics from the University of Karlsruhe in 1995. From 1995 to 1998, he was assistant research professor at the Mikroelektronik Centret, Denmark. From 1998 to 2004, he was with the University of Dortmund, where received his Habilitation in 2003. In 2004 he was appointed senior lecturer in the School of Physics, Cardiff University, promoted to Reader in 2006 and to Personal Chair in 2007. His current research interests are (i) characterization and ultrafast spectroscopy of semiconductor nanostructures, microcavities, and quantum-dot optical amplifiers. (ii) application of optical spectroscopy and imaging to life-science, including the techniques of coherent Raman scattering microscopy and label-free optical biosensors using microcavities or plasmonics.

Authors
Wolfgang Langbein a
Affiliations
a, Cardiff University, School of Physics and Astronomy, The Parade, Cardiff, GB
Abstract

Coherent quantum dynamics of excitons in semiconductor quantum dots (QDs) are of key interest, besides fundamental physics, for many applications ranging from quantum computing to advanced photonic devices. With the advances in colloidal synthesis, high-quality semiconductor nanocrystals can be fabricated at lower cost, and more flexibility in size, shapes and compositions. Despite its importance, measuring the exciton dephasing time in colloidal nanocrystals is technically challenging. Using three-beam transient resonant four-wave mixing technique in heterodyne detection not affected by spectral diffusion, we have measured the temperature-dependent ground-state exciton dephasing dynamics in CdSe/ZnS wurtzite QDs, CdSe/CdS spherical zincblende and rod-shape wurtzite QDs with variable core diameter and shell thickness / rod length, CdSe nanoplatelets, PbS QDs, InP/ZnSe QDs, and perovskite (CsPbBr2Cl) QDs.

In these structures, the importance of phonon-assisted transitions, and the zero-phonon line (ZPL) dephasing by phonon-mediated spin-relaxation and radiative decay at low temperature are vastly varying. In PbS QDs, the peculiar band structure allows coupling with phonons at the zone edge (X-point), resulting in dominating phonon assisted transitions [1] even at low temperatures with sub-picosecond dephasing and a ZPL weight of less than 7%. In CdSe QDs [2,3] and dots in rods of similar size, the ZPL weight instead is above 50% since only zone-center phonons can couple, and the ZPL dephasing is limited by spin-relaxation into dark states on a 10-1000ps time scale, faster than the radiative lifetime of about 10ns. In InP QDs, a non-toxic alternative to CdSe, a similar behaviour is observed. In CdSe nano-platelets, the quasi two-dimensional confinement leads to quantum-well type behaviour, with a large exciton coherence area, such that the exciton dephasing is dominated by a fast radiative decay in the 1ps range [4]. In perovskite CsPbBr2Cl QDs, the exciton ground state is bright, and a radiatively limited dephasing in the 10-100ps range is observed at low temperatures.  

[1] F. Masia et. al., Phys. Rev. B 83, 201309(R) (2011) DOI:10.1103/PhysRevB.83.201309
[2] F. Masia et al. Phys. Rev. Lett. 108, 087401 (2012) DOI:10.1103/PhysRevLett.108.087401
[3] N. Accanto et al., ACS Nano 6, 5227-5233 (2012) DOI:10.1021/nn300992a
[4] A. Naeem et al., Phys. Rev. B 91, 121302(R) (2015)  DOI 10.1103/PhysRevB.91.121302

PerFun S7.8
Chair: Koichi Yamashita
11:00 - 11:30
Abstract not programmed
11:30 - 12:00
S7.8-O1
Röhm, Holger
LTI, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany
Ferroelectric domains in methylammonium lead iodide perovskite solar cells
Holger Röhm
LTI, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany
Authors
Holger Röhm a, Tobias Leonhard a, b, Michael J. Hoffmann b, c, Alexander Colsmann a, b
Affiliations
a, Light Technology Institute, Karlsruhe Institute of Technology, Karlsruhe, 76131, 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

Within five years, methylammonium lead iodide (MAPbI3) solar cells have quickly reached remarkable power conversion efficiencies rivaling those of established technologies. However, arguably, the toxic and water-soluble lead compound may be an obstacle on their way to market. The quest for alternative, non-toxic photo harvesters is partly hampered by a lack of fundamental understanding of the crystal grain’s characteristics and energy conversion mechanisms. As part of this process, the scientific community controversially discusses the importance of ferroic properties for the exceptional performance of MAPbI3 light-harvesting layers, including claims of non-ferroelectricity, anti-ferroelectricity, ferroelectricity and ferroelasticity. Simulations have predicted ferroelectricity in MAPbI3 with alternating polarized domains ruling the charge carrier transport [1]. Understanding the crystallographic cause and the effects of the crystal’s ferroelectricity would therefore provide helpful guidance for the quest to find non-toxic MAPbI3 replacements.

We explore the ferroic properties of methylammonium lead iodide perovskite solar cells by piezoresponse force microscopy (PFM) [2][3]. In vertical and horizontal PFM imaging, we find 90 nm wide ferroelectric domains of alternating polarization. High-resolution photo-conductive atomic force micrographs under illumination also show alternating charge carrier extraction patterns which we attribute to the local vertical polarization components within the ferroelectric domains.

We apply these techniques to investigate formation of polarized domains during thermal treatment and study their influence on the performance of perovskite solar cells. Annealing steps, commonly only viewed as a means of crystal growth and precursor conversion, prove to directly influence the formation, shape and polarization direction of ferroelectric domains in perovskite thin films.

 

References:

[1] D. Rossi, A. Pecchia, M. Auf der Maur, T. Leonhard, H. Röhm, M. J. Hoffmann, A. Colsmann, A. D. Carlo, Nano En. (2018).

[2] H. Röhm, T. Leonhard, M.J. Hoffmann, A. Colsmann, Energy Environ. Sci. (2017).

[3] T. Leonhard, A. Schulz, H. Röhm, S. Wagner, F. Altermann, W. Rheinheimer, M.J. Hoffmann, A. Colsmann, submitted.

12:00 - 12:30
S7.8-O2
Chirvony, Vladimir
Instituto de Ciencia de los Materiales, Universidad de Valencia, P.O. Box 22085, 46071 Valencia, Spain
Long-Lived Luminescence and Slow Carrier Diffusion in Metal Halide Perovskites as a result of Multiple Trapping and De-Trapping by Shallow Non-Quenching Traps
Vladimir Chirvony
Instituto de Ciencia de los Materiales, Universidad de Valencia, P.O. Box 22085, 46071 Valencia, Spain
Authors
Vladimir Chirvony a, b, Juan Martínez-Pastor a, Henk Bolink b
Affiliations
a, Instituto de Ciencia de los Materiales, Universidad de Valencia, c/Catedrático J. Beltrán, 2, Paterna 46980, Spain
b, Instituto de Ciencia Molecular, Universidad de Valencia, c/Catedrático J. Beltrán, 2, Paterna 46980, Spain
Abstract

Slow carrier recombination in metal halide perovskites (MHP) is considered the origin of the observed large charge carrier diffusion length. However, the reasons for this slow charge carrier recombination remains unclear. In this report, on the basis of novel experimental data and the results of modeling, we propose that the observed long luminescence lifetimes in MHP are due to the extremely fast capture of carriers by shallow non-quenching traps, followed by their much slower release back to the conduction band. Multiple (tens to thousands of times) repetition of this loss-free process can explain the observed slow luminescence decay after pulsed photoexcitation (the so-called "delayed luminescence") [1, 2]. In the case of polycrystalline layers and in single crystals of MHP, the result of this multiple capture of carriers by shallow non-quenching traps are rather low values of the diffusion coefficient found for this type of materials [2].

REFERENCES

[1] Chirvony, V. S.; González-Carrero, S.; Suárez, I.; Galian, R. E.; Sessolo, M.; Bolink, H. J.; Martínez-Pastor, J. P.; Pérez-Prieto, J. Delayed luminescence in lead halide perovskite nanocrystals. J. Phys. Chem. C 2017, 121, 13381-13390.

[2] Chirvony, V. S.; Martínez-Pastor, J. P. Trap-limited dynamics of excited carriers and delayed luminescence in metal halide perovskites. J. Phys. Chem. Lett. 2018, accepted.

 

 

 

SolFuel S1.8
Chair: Francesca Toma
11:00 - 11:30
S1.8-O1
Osterloh, Frank
University of California
Electronic Structure Basis for Enhanced Overall Water Splitting Photocatalysis of Doped Strontium Titanate in Direct Sunlight
Frank Osterloh
University of California

Frank Osterloh is an inorganic chemist working on nanoparticle devices for artificial photosynthesis. He received M.A. and Ph.D. degrees in chemistry in 1994 and 1997 from the Carl von Ossietzky University in Oldenburg, Germany, while working with late Prof. Siegfried Pohl. After completing postdoctoral training with Prof. Richard H. Holm at Harvard University, he joined the faculty at the Chemistry Department at the University of California, Davis in 2000. Frank has authored 90 scientific papers, incl. four review articles and several book chapters. His research contributions were recognized with the ACS Inorganic Nanoscience award (2010) and the Richard A. Glenn Award of the ACS (2014, Division of Fuel Chemistry). In 2016, he was named Fellow of the Royal Society of Chemistry. He currently serves as associate editor for the ‘Journal of Materials Chemistry A’, published by the RSC, and on the editorial advisory board of ChemNanoMat (Wiley-VCH).

Authors
Frank Osterloh a, Zeqiong Zhao a, Renato Gonçalves b, Muhammad Huda c, Sajib Barman c, Emma Willard a, Russell Perry a, Edaan Byle a
Affiliations
a, University of California at Davis
b, University of São Paulo (IFSC/USP)
c, University of Texas, Arlington
Abstract

Recently, Ham et al. reported that aluminum incorporation into SrTiO3 microparticles followed by modification with Rh2-yCryO3 produces a superior overall water splitting catalyst with 30% apparent quantum efficiency at 350 nm. Based on transient IR spectroscopy, the improved activity was attributed to the removal of Ti3+ recombination sites. Here we use results from photoelectron spectroscopy (XPS) and density functional theory to show that Al3+ incorporation into the perovskite lattice not only reduces the number of Ti3+ deep recombination sites and but also promotes the formation of oxygen vacancies in the material. The oxygen vacancies form a mini-band 0.2 eV above the valence band whose energy position strongly depends on the spatial separation between Al3+ sites and oxygen vacancies. The vacancy band is believed to aid photochemical charge separation in this metal oxide. Particle suspensions of the material inside of a baggie reactor promote overall water splitting under direct sunlight illumination with 0.1% solar to hydrogen efficiency.

References

Ham, Y., T. Hisatomi, Y. Goto, Y. Moriya, Y. Sakata, A. Yamakata, J. Kubota, and K. Domen, Flux-Mediated Doping of SrTiO3 Photocatalysts for Efficient Overall Water Splitting. Journal of Materials Chemistry A, 2016. 4(8): p. 3027-3033. http://dx.doi.org/10.1039/C5TA04843E

11:30 - 11:45
S1.8-O2
Jiang, Chang-Ming
Walter Schottky Institut and Physics Department, Technische Universität München, 85748 Garching, Germany
Electronic Structure of CuFeO2 Photocathode Studied by Resonant Inelastic X-ray Scattering
Chang-Ming Jiang
Walter Schottky Institut and Physics Department, Technische Universität München, 85748 Garching, Germany
Authors
Chang-Ming Jiang a, Ian Sharp b, Jason Cooper a
Affiliations
a, Joint Center for Artificial Photosynthesis and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
b, Walter Schottky Institut and Physics Department, Technische Universität München, 85748 Garching, Germany
Abstract

Cu(I)-based delafossites (CuMO2) have garnered significant attention for their small optical bandgaps, strong light absorption characteristics, and native p-type conductivities, which make them suitable photocathode candidates for solar fuel production. While resolving the inherently complex electronic structures of these ternary oxides is often challenging, doing so is imperative for understanding optical excitations and carrier transport mechanisms, as well as for furthering material design and discovery. Combining density functional theory (DFT) calculations and X-ray spectroscopy techniques, this research gives a detailed portrait of the band structure of rhombohedral 3R-CuFeO2, prepared by reactive co-sputtering. In particular, element-specific contributions from 3d orbitals of Fe and Cu, as well as 2p orbitals of O, in the valence and conduction bands are revealed by resonant inelastic X-ray scattering (RIXS) measurement. By combining this knowledge of electronic structure, with measurements of optical properties, carrier relaxation dynamics, and photoelectrochemical performance characteristics, this research provides insights into current limitations of this novel photocathode material and informs strategies to overcome them.

11:45 - 12:00
S1.8-O3
Andreu, Teresa
Catalonia Institute for Energy Research (IREC)
Surface states in BiVO4 photoanodes for water oxidation: tuning the electron trapping process
Teresa Andreu
Catalonia Institute for Energy Research (IREC), ES
Authors
Teresa Andreu a, Sebastian Murcia-López a, Qin Shi b
Affiliations
a, Department of Advanced Materials for Energy, Catalonia Institute for Energy Research (IREC), Catalonia, Spain
b, College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, PR China
Abstract

The nanostructured W-doped BiVO4 photoanodes were prepared by electrospinning. The role of surface states (SS) during water oxidation for the as-prepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 VRHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (NSS) with respect to the charge donor density (Nd), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest Nd and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V5+/V4+ in semiconductor bulk to form water oxidation intermediates by electron trapping process. Otherwise, the irreversible surface reductive reaction of VO2+ to VO2+ by electron trapping process raises the surface recombination. W doping does have an effect on the surface properties of BiVO4 electrode. It can tune the electron trapping process to obtain high concentration of i-SS and less surface recombination. This work gives a further understanding for the enhancement of PEC performance caused by W doping in the field of charge transfer at the semiconductor/electrolyte interface.

12:00 - 12:15
S1.8-O4
Kölbach, Moritz
Helmholtz Center Berlin for Materials and Energy
α-SnWO4: A New Promising Photoanode Material for Solar Water Splitting
Moritz Kölbach
Helmholtz Center Berlin for Materials and Energy, DE
Authors
Moritz Kölbach a, Inês Jordão Pereira b, Karsten Harbauer a, Sean Berglund a, Paul Plate a, Dennis Friedrich a, Roel van de Krol a, Fatwa F. Abdi a
Affiliations
a, Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
b, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Ed C8, Lisboa, PT
Abstract

One of the critical challenges for efficient solar water splitting is the identification of a stable photoelectrode material with a bandgap of 1.6 - 1.9 eV that can be used as a top absorber in an efficient D4 tandem device. Recently, n-type α-SnWO4 was identified as a potential photoanode candidate due to the combination of its ideal bandgap (~1.9 eV) and a very negative photocurrent onset potential (~0 V vs. RHE) [1-3]. However, up to now, α-SnWO4 photoelectrodes have shown very low photoconversion efficiencies. The reason for this is not fully understood, and many of the essential material parameters are still elusive.

In this study, we identify the major limiting parameters of pulsed laser-deposited α-SnWO4 photoanodes: (I) the oxidation of Sn2+ to Sn4+ at the surface creating a hole blocking layer and pushing the photoconversion efficiency to almost zero, and (II) the relatively poor charge carrier dynamics resulting in a mismatch between the charge carrier diffusion length and the light penetration depth. To address these challenges, we explored several strategies such as the deposition of a hole-conducting NiOx protection layer resulting in a new benchmark sulfite oxidation photocurrent of 0.75 mA cm-2 at 1.23 V vs. RHE. We furthermore show that a high-temperature treatment enhances the charge carrier transport by improving the film crystallinity, resulting in a charge carrier diffusion length comparable to state-of-the-art BiVO4 photoanodes. Finally, the remaining challenges for oxygen evolution using our α-SnWO4 films will be discussed.

Our findings provide important insights into the limitations and key properties of α-SnWO4 photoelectrodes and will help to further improve the performance of this promising photoanode material.

 

[1] Ziani et al., APL Materials, 3 (2015) 096101

[2] Zhu et al., ACS Appl. Mater. Interfaces, 9 (2017) 1459-1470

[3] Pyper et al., Chin. Chem. Lett., 26 (2015) 474-478

12:15 - 12:30
S1.8-O5
Tang, PengYi
Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC & BIST
Bottom-up Engineering of Hematite Nanowire Heterostructures for Photoelectrochemical Water Splitting
PengYi Tang
Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC & BIST, ES
Authors
PengYi Tang a, b, HaiBing Xie a, LiJuan Han c, Carles Ros b, Marti Biset Peiro b, José Ramón Galán-Mascarós c, d, Teresa Andreu b, Joan Ramon Morante b, Jordi Arbiol a, d
Affiliations
a, Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain
b, Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Sant Adrià del Besòs, Barcelona 08930, Catalonia, Spain
c, Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology (BIST), Avinguda Paisos Catalans 16, Tarragona 43007, Catalonia, Spain
d, ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain
Abstract

The development of technologies for H2 production or CO2 reduction strongly relies on an abundant supply of protons and electrons liberated by water oxidation. [1-2] Therefore, photoelectrochemical (PEC) water oxidation is an important anodic half-cell process in the development of a sustainable artificial solar fuel system. In the PEC devices design, coupling water oxidation catalysts with active photoanode materials has become the most promising methodology, since the attachment/integration of the catalyst on the semiconductor light absorbers could kinetically facilitate interfacial charge transfer reactions.

In this contribution, we have fabricated ITO/Fe2O3/Fe2TiO5/FeNiOOH multi-layers nanowire heterostructures via combination of sputtering, hydrothermal, ALD, photo-electrodepositon methods for photoelectrochemical (PEC) oxygen evolution application. Structural, spectroscopic and electrochemical investigations disclose that the origin of the superior catalytic performance is owing to the interfacial coupling effect of ITO underlayer (Sn doping and conductivity promoter), ultrathin Fe2TiO5 coating (Ti doping, energetics and surface state density modulation) and FeNiOOH eletrocatalyst (varying surface state energy level). [2]

Meanwhile, an alternative earth-abundant CoFe prussian blue analogues (CoFe PBA) is incorporated in Fe2O3/Fe2TiO5 core-shell type II heterojunction nanowires as photoanodes for PEC water oxidation. The observed photocurrent is improved from 0.12 mA cm-2 to 1.25 mA cm-2 at 1.23 V vs. RHE under illumination by involvement of ultrathin Fe2TiO5 layer and CoFe PBA WOCs coating. Further investigation of the PEC mechanisms via photoelectrochemical impedance spectroscopy unveils that the enhanced PEC performance is attributed to the enhanced charge transfer efficiency owing to the tuned energy level and density of surface state. [3-4]

References

[1] Félix Urbain, Pengyi Tang, Nina M. Carretero, Teresa Andreu, Luís G. Gerling, Cristóbal Voz, Jordi Arbiol, Joan R. Morante, Energy & Environmental Science, 10, 2256-2266 (2017).

[2] Pengyi Tang, HaiBing Xie, Carles Ros, LiJuan Han, Martí Biset-Peiró, Yongmin He, Wesley Kramer, Alejandro Perez-Rodriguez, Edgardo Saucedo, Jose Galan-Mascaros, Teresa Andreu, Joan R. Morante, Jordi Arbiol, Energy & Environmental Science, 10, 2124-2136 (2017).

[3] Lijuan Han, Pengyi Tang, Alvaro Reyes-Carmona, Barbara Rodriguez-Garcia, Mabel Torrens, Joan Ramon Morante, Jordi Arbiol, Jose Ramon Galan-Mascaros, Journal of the American Chemical Society, 138, 16037-16045 (2016).

[4] PengYi Tang, LiJuan Han, Paul Paciok, Marti Biset Peiro, Hong-Chu Du, Xian-Kui Wei, Lei Jin, Hai-Bing Xie, Qin Shi, Teresa Andreu, Joan Ramon Morante, Mónica Lira-Cantú, Marc Heggen, Rafal E. Dunin-Borkowski, José Ramón Galán-Mascarós, Jordi Arbiol, to be submitted.

12:30 - 14:30
Lunch
NCPhot S4.3
Chair: Efrat Lifshitz
14:30 - 15:00
S4.3-I1
Delerue, Christophe
IEMN - UMR 8520
Theory of Optical Properties of 2D Semiconductor Nanocrystal Lattices
Christophe Delerue
IEMN - UMR 8520, FR
Authors
Athmane Tadjine a, Delerue Christophe a
Affiliations
a, IEMN, UMR-CNRS 8520, Villeneuve d'Ascq, France
Abstract

Recent progress in colloidal chemistry have led to the synthesis of new types of two-dimensional (2D) lattices of PbX (X=Se, S, Te) nanocrystals [1,2]. In these lattices, each nanocrystal is epitaxially connected to its neighbors, the 2D materials are single crystalline in absence of disorder. Remarkably, square and honeycomb lattices can be synthesized using the same initial nanocrystals, allowing to investigate the role of the lattice geometry on the electronic properties. In addition, cation exchange processes can be used to transform PbX lattices into CdX ones. Theoretical studies accompanying these experiments have already demonstrated that these 2D lattices are characterized by very interesting band structures, including in some cases Dirac and non-trivial flat bands. In the present talk, theoretical works on the optical properties of 2D lattices will be presented. The effects of the epitaxial bonds between neighbor nanocrystals on the optical properties will be discussed. The theoretical calculations will be compared to recent experimental studies [3]. The differences between IV-VI (PbX) and II-VI (CdX) materials, between square and honeycomb lattices, will be reviewed.

[1] W. H. Evers, B. Goris, S. Bals, M. Casavola, J. de Graaf, R. van Roij, M. Dijkstra, and D. Vanmaekelbergh, Nano Lett. 13, 2317 (2013).

[2] M. P. Boneschanscher, W. H. Evers, J. J. Geuchies, T. Altantzis, B. Goris, F. T. Rabouw, S. A. P. van Rossum, H. S. J. van der Zant, L. D. A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A. V. Petukhov, S. Bals, and D. Vanmaekelbergh, Science 344, 1377-1380 (2014).

[3] M. Alimoradi Jazi et al., Nano Lett. 17, 9 5238-5243 (2017).

 

15:00 - 15:30
S4.3-O1
Budniak, Adam
Technion – Israel Institute of Technology
Investigation of CrPS4 in Bulk and Few-Layers Form
Adam Budniak
Technion – Israel Institute of Technology
Authors
Adam K. Budniak a, Niall A. Killilea b, Amir Abbas Yousefi Amin b, Szymon J. Zelewski c, Jan Kopaczek c, Esty Ritov a, Yaron Amouyal a, Wolfgang Heiss b, Robert Kudrawiec c, Efrat Lifshitz a
Affiliations
a, Technion – Israel Institute of Technology
b, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Martenstrasse 7, Erlangen, DE
c, Wroclaw University of Technology, Wyb. Wyspianskiego 27, Wroclaw, 50, PL
Abstract

Ever since the exfoliation of graphite into an atomically thin monolayer, known today as graphene [1], two dimensional (2D) materials have been of central interest for a variety of electronic applications. 2D materials belong to a large family of anisotropically active compounds which have strong, covalent bonds within a layer while in between layers there are only weak van der Waals interactions that can be overcome, obtaining molecularly thin sheets. Such a reduction of dimensionality has a profound impact on properties, known to vary strongly with respect to the number of atomic layers.

As graphene applications in electronics has thus far been hindered by its non-existent band gap, layered semiconductors are studied as potential candidates for future devices. Many Transition Metal Dichalcogenides (TMDs) including MoS2, MoSe2, WS2, WSe2 have been thoroughly investigated, however in order to meet rising demands new families of 2D semiconductors are studied. One family of such is the transition metal thiophosphates, denoted MPSx, for x=3 or 4; for example bulk crystals of CrPS4 - chromium thiophosphate – which has been examined in the past for applications in lithium batteries. Nowadays, this compound has once again gained scientific interest due to its optical anisotropic properties and the possibility to obtain and study its few- and monolayer systems [2].

In this work, bulk crystals of CrPS4 were obtained by vapor transport synthesis (furnace method), followed by structure and composition confirmation via different techniques, for example Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), Powder X-Ray Diffraction (PXRD) and Raman spectroscopy. Optical properties, such as band gap and optical transitions were investigated by Solid State UV-VIS Spectroscopy, PhotoAcoustic Spectroscopy (PAS) [3] and Modulated Spectroscopy (MS) [3]. Later, bulk crystals of chromium thiophosphate (CrPS4) were exfoliated in liquid to obtain few layers systems and photoconductivity measurements were used to ascertain photoactive properties, both of re-stacked films and bulk crystals.

 

Acknowledgments:
This work was supported by the European Comission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCA-ITN-642656)
This work was performed within the grant of the National Science Centre Poland (OPUS 11 no. 2016/21/B/ST3/00482).
S.J.Z. also acknowledges the support within the ETIUDA 5 grant from National Science Center Poland (no. 2017/24/T/ST3/00257).

References:
[1] Novoselov et.al., Science, vol 306, no. 5696 (2004)
[2] Lee et.al., ACS Nano vol. 11, no. 11 (2017)
[3] Zelewski & Kudrawiec, Scientific Reports, vol. 7 no. 15365 (2017)

Sol2D S6.1
Chair: Christian Klinke
14:30 - 15:00
S6.1-I1
Norris, David
ETH Zurich
The Growth Mechanism of Semiconductor Nanoplatelets
David Norris
ETH Zurich, CH

David J. Norris is currently the Director of the Optical Materials Engineering Laboratory and Professor of Materials Engineering at ETH Zurich. He received his B.S. and Ph.D. degrees in Chemistry from the University of Chicago (1990) and MIT (1995), respectively. After an NSF postdoctoral fellowship at the University of California, San Diego, he joined the NEC Research Institute in Princeton in 1997 where he led a photonics research group. He then became an Associate Professor (2001-2006) and Professor (2006-2010) of Chemical Engineering and Materials Science at the University of Minnesota. In 2010, he moved to his current position at ETH Zurich. Prof. Norris is a Fellow of the American Physical Society and the American Association for the Advancement of Science. He received the Golden Owl award at ETH in 2012 for excellence in teaching. He was awarded an Advanced Grant from the European Research Council (2014-2019). In 2015, he was the recipient of the Max R�ssler Prize.

Authors
David Norris a
Affiliations
a, Optical Materials Engineering Laboratory, ETH Zürich, Swiss
Abstract

Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. These materials were discovered when liquid-phase chemical syntheses of spherical nanocrystals were modified. However, despite intense interest in their properties, the mechanism behind their highly anisotropic shape and precise atomic-scale thickness remained unclear, and even counter-intuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead halide perovskites). Here we describe an intrinsic instability in growth kinetics that can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface, and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.

15:00 - 15:30
S6.1-I2
Lifshitz, Efrat
Technion
Magneto-Optical Properties of Two-Dimensional Semiconductors: Transition Metal Phosphorous Trichalcogenides, Indium Chacogenides and Magnetically Doped Colloidal Nanoplatelets
Efrat Lifshitz
Technion, IL
Authors
Efrat Lifshitz a
Affiliations
a, aculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute and Grand Technion Energy Program, Technion – Israel Institute of Technology, Haifa, 32000, Israel
Abstract

Two-dimensional (2D) semiconductors are finding a renewed interest in recent years, due to their combination of physical properties, e.g., mobility, fluorescence and spin selectivity, with a potential implementation in new and emerging opto-electronic and spin-electronic devices. The current work discusses various magneto-optical phenomena, found in two different 2D systems: The transition metal phosphorous trichalcogenides, indium chalcogenides and magnetically doped colloidal nanoplatelets. The magneto-optical properties are investigated by following optical polarization in the presence of an external magnetic field, as various temperatures, as well as implementing the use of optically detected magnetic resonance spectroscopy.

The transition metal phorphorous trichalcogenides resemble the most common layered dichalcogenides, but one-third of the metals are replaced with a P-P pair; hence, the chemical formula is written as M2/3(P-P)1/3X2 or M2P2X6. The dilution of metal site by non-magnetic atoms, endows a column like arrangement of the remaining metals, leading to a special magnetic properties, from a full antiferromagnetic Neel, through antiferromagnetic zigzag to ferromagnetic character. The various arrangements can be tuned by variation of the metal cations (among the first row of transitions metal atoms). The work focuses on the influence of the created magnetism on the magneto-optical properties of the M2P2X6 semiconductors. In addition, the talk will report about the synthesis and characterization of In2S3 layered compounds, exploring the various possible structural phases and their magnetically doped derivatives.

Transition metal dopant embedded in colloidal semiconductor nanoplatelets (NPLs) exhibit special magnetic properties, resemble the bulk diluted magnetic semiconductors. However, the NPLs confined thickness induces an extremely intense spin-exchange interaction between the resident photo-excited carriers and the guest magnetic spins. Such an interaction leads to a giant magnetization and g-factor, and consequently endows the materials with special magneto-optical properties. The work emphases the investigation of the spin-exchange interaction while varying the magnetic dopants, by following variation in the magneto-optical properties, when detecting either an ensemble of NPLs or a focus on a single platelet. **

** Magnetically doped NPLs project was carried out in collaboration with Prof. Volkan Hilmi Demir and his groups from Nanyang Technological University – NTU Singapore 639798, Singapore and from Bilkent University, Ankara 06800, Turkey.

15:30 - 16:00
S6.1-I3
Achtstein, Alexander
TU Berlin
Tuning the Photonic Properties of Colloidal Quantum Wells
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
Alexander Achtstein a, Riccardo Scott a, Jan Heckmann a, Anatol Prudnikau b, Artsiom Antanovich b, Nina Owschimikow a, Nicolai Grosse a, Mikhail Artemyev b, Ulrike Woggon a, Juan Climente c
Affiliations
a, Technical University of Berlin, Straße des 17. Juni, Berlin, DE
b, Institue for Physico-Chemical Problems, Belorussian State University, Minsk
c, Universitat Jaume I, Av/ Sos Baynat s/n, Castelló de la Plana, ES
Abstract

We introduce the electronic and exciton dynamic properties of these nanoplatelets and their hetero structures and demonstrate for example that ligand induced strain in these colloidal quantum wells with finite size results in a strong alteration of the transition energies. We show that intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy efficient display technology and propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic and directed photoluminescence.  Our analysis of the two-dimensional k-space distribution of the nanoplatelet absorption and emission reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters for photonic applications. We also demonstrate by 2D k-space spectroscopy that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets, thus promoting them as a new class of directional two-photon absorbers with extremely large cross sections.

References:                                                                                                                     1. Scott and Achtstein et al., Nature Nanotechnology 12, 1155 (2017)                        2. Heckmann and Achtstein et al., Nano Letters 17, 6321, (2017)                                3. Antanovich and Achtstein et al., Nanoscale, (2017); DOI: 10.1039/c7nr05065h

16:00 - 16:30
S6.1-I4
Demir, Hilmi Volkan
Nanyang Technological University & Bilkent University
Colloidal Photonics of Atomically Flat 2D Nanocrystals
Hilmi Volkan Demir
Nanyang Technological University & Bilkent University
Authors
Hilmi Volkan Demir a, b
Affiliations
a, Nanyang Technological University (NTU), Singapore
b, Bilkent University, Turkey
Abstract

 

Solution-processed semiconductor nanocrystals have attracted great interest in photonics including high-purity color conversion and enrichment in quality lighting and display backlighting. These nanocrystals span different types and heterostructures of semiconductors in the forms of colloidal quantum dots and rods to a more recently emerging class of colloidal quantum wells. Here we will talk about colloidal photonics using the family of quasi-2D, tightly-confined, atomically flat nanocrystals. Here we will show that custom-design 2D heteronanoplatelets uniquely offer record high optical gain coefficients and ultra-low threshold stimulated emission. In addition, we will show that controlled stacking of these nanoplatelets provides us with the ability to further tune and master their excitonic properties. Also, we will discuss doping of these nanoplatelets with Cu for high-flux solar concentration properties and with Mn for precise wavefunction-engineered magnetic properties. Given their most recent accelerating progress, these solution-processed quantum materials hold great promise to challenge their epitaxial counterparts in semiconductor optoelectronics in the near future.

  

16:30 - 17:00
S6.1-I5
Sun, Liangfeng
Bowling Green State University
Synthesis and Optical Spectroscopy of Colloidal PbS Nanosheets
Liangfeng Sun
Bowling Green State University, US
Authors
Liangfeng Sun a, b, Zhoufeng Jiang a, b, Antara Debnath Antu a, Shashini Premathilka a, b, Yiteng Tang a, b, Ghadendra Bhandari a, Kamal Subedi a, Matthew Leopold a, Nick Reilly a, Simeen Khan a, Douglas Dimick a, Cody Stombaugh a, Angelic Mandell a, Yufan He b, c, Peter Lu b, c, Jianjun Hu d, Andrey Voevodin d, Ajit Roy d, Paul Roland e, Randy Ellingson e, Joey Leffler a, Alexey Zayak a
Affiliations
a, Department of Physics and Astronomy, Bowling Green State University, Bowling Green, Ohio 43403, United States
b, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United States
c, Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, United States
d, Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, United States
e, Department of Physics and Astronomy, Wright Center for Photovoltaics Innovation and Commercialization, School of Solar and Advanced Renewable Energy, University of Toledo, Toledo, Ohio 43606, United States
Abstract

Colloidal PbS nanosheets represent an important infrared 2D nanomaterial. They have tunable energy gaps and much higher charge carrier mobility than quantum-dot films, which makes them ideal for optoelectronic and electronic applications including photovoltaic devices and field-effect transistors.

The growth of PbS nanosheets is confirmed by electron microscopy and photoluminescence spectroscopy. Their thickness can be tuned by changing the reaction temperature during the synthesis. Recent research also demonstrates that the lateral size of the nanosheets can be systematically tuned between 20 nm to a few hundred nanometers. Core/shell PbS/CdS nanosheets can be synthesized using a cation exchange method. This method can protect the PbS core as well as further tuning the core thickness. Surface passivation of the nanosheets using organic molecules is also proven to be very effective to suppress the surface defects and improve the optical properties.

The energy gap of the PbS nanosheets can be tuned by changing their thickness. The thickness dependent energy gap is a unique feature of the exciton under one-dimensional confinement. In contrast to quantum dots, the confinement energy can be achieved in a typical nanosheet is smaller than a quantum dot. However, the maximum energy gap of the nanosheet can still reach 1 eV which is about more than twice of the energy gap of the bulk PbS.

The optical absorption of typical PbS nanosheets shows a step-like spectrum. The step edge is nearly coincident with the photoluminescence peak, indicating a negligible Stokes shift. The absorption spectrum of the nanosheets of 20 nm in lateral size shows an excitonic peak similar to quantum dots, which is possibly due to the additional lateral confinement.

The absolute photoluminescence quantum yield of the PbS nanosheets can be measured accurately by using an integrating-sphere technique. Well-passivated PbS nanosheets typically show 20% to 30% photoluminescence quantum yield. The maximum photoluminescence quantum yield has reached 60%, which is about twice of the quantum yield from PbS quantum dots of the same energy gap.

The time-resolved photoluminescence of the PbS nanosheets shows a distinct fast decay followed by a slow decay. The fast decay accounts for more than 90% of the total luminescence intensity. The exciton radiative lifetime derived from the time-resolved photoluminescence and the quantum yield is much shorter than the PbS quantum dots. It is an indication of giant oscillation strength transition which is a consequence of large exciton coherence volume in a nanosheet.

WatSpl S2.1
Chair: Christina Scheu
14:30 - 15:00
S2.1-I1
Fischer, Anna
University Freiburg
Improving BiVO4 thin Film Photoanodes for Light-Induced Water Oxidation
Anna Fischer
University Freiburg, 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
Martin Rohloff a, b, c, Björn Anke c, Spark Zhang d, Christina Scheu d, Martin Lerch c, Anna Fischer a, b
Affiliations
a, University Freiburg, IAAC, Germany
b, FIT, University Freiburg, Germany
c, TU Berlin, Inorganic Chemistry, Germany
d, Max-Planck-Institut für Eisenforschung, Germany
Abstract

The n-type semiconductor bismuth vanadate (BiVO4) is a promising material as photoanode for light induced water oxidation. Its absorption in the visible range (band gap energy of 2.4 eV), its suitable band edge positions compared to the OER half reaction, its stability against photo-corrosion as well as its low cost make BiVO4 one of the most interesting ternary oxide materials for light-induced oxygen evolution from water.1 One major drawback for BiVO4 is its poor electronic conductivity, which can however be overcome by applying three strategies: i) adjustment of thin film properties (especially thickness), ii) n-type doping by cation and more recently anion substitution and iii) heterojunction design (type II). Within the present talk, I will give an overview of the improvements we achieved for novel sol-gel based BiVO4 thin film photoanodes following each of these strategies allowing us to go all the way from low to high performance BiVO4 photoanodes.

First we developed a new, sol-gel-based synthesis involving simple dip-coating and calcination allowing the easy and reproducible fabrication of porous BiVO4 and Mo‑doped BiVO4 thin film photoanodes.2 The obtained thin films crystallize in the monoclinic scheelite structure in micrometre large, two-dimensional, single-crystalline porous domains with wall features in the range of the hole diffusion length of BiVO4. Optimization of the electron transport properties resulting in higher PEC performance was realized by cation and anion doping using Molybdenum and Fluorine, respectively.3 Finally, our new synthesis approach could be easily applied for the fabrication of BiVO4/WO3 type II heterojunctions following a simple layer-by-layer deposition. It is shown that precise control of the layer morphology and the overlapping interface between the layers is an indispensable prerequisite for high performance WO3/BiVO4 heterojunction photoanodes.

This work was funded by the DFG SPP1613 program.

1 Z.-F. Huang, L. Pan, J.-J. Zou, X. Zhang, L. Wang, Nanoscale 2014, 6, 14044.

2 M. Rohloff, B. Anke, S. Zhang, U. Gernert, C. Scheu, M. Lerch, A. Fischer, Sustainable Energy Fuels 2017, 1, 1830.

3 B. Anke, M. Rohloff, M. G. Willinger, W. Hetaba, A. Fischer, M. Lerch, Solid State Sci. 2017, 63, 1.

15:00 - 15:15
S2.1-O1
Granone, Luis Ignacio
Leibniz University Hannover, Institute for technical chemistry
Effect of the Degree of Inversion on the Photocatalytic Activity of Spinel ZnFe2O4
Luis Ignacio Granone
Leibniz University Hannover, Institute for technical chemistry
Authors
Luis I. Granone a, b, Ralf Dillert a, b, Detlef W. Bahnemann a, b, c
Affiliations
a, Institute of Technical Chemistry, Gottfried Wilhelm Leibniz University Hannover, Callinstrasse 3, D-30167 Hannover, Germany
b, Laboratory of Nano- and Quantum-Engineering (LNQE), Wilhelm Leibniz University Hannover, Schneiderberg 39, D-30167 Hannover, Germany
c, Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State University, Ulyanovskaya str. 1, 198504 Peterhof, Saint-Petersburg, Russia
Abstract

Recently, due to promising results in the fields of photocatalysis and photoelectrocatalysis, the attention has been focused on ferrites as new visible light-active materials [1,2]. Properties such as narrow band gap energies (≈ 2 eV), high stability, abundance, and low cost make ferrites promising for solar energy production and solar remediation. Zinc ferrite (ZnFe2O4) is one of the most widely studied compounds belonging to the spinel ferrite family. However, various and even contradictory results regarding the photocatalytic activity of ZnFe2O4 have been reported in the scientific literature [3]. ZnFe2O4 crystallize in a phase-centered cubic spinel structure with Fe3+ and Zn2+ ions occupying tetrahedral or octahedral sites[1]. When the Fe3+ and Zn2+ ions are arranged in octahedral and tetrahedral sites, respectively, the ferrite exhibits a so-called normal spinel structure (T[Zn]O[Fe2]O4). However, when all the Zn2+ ions at the tetrahedral sites are exchanged by Fe3+ ions from octahedral sites, the compound adopts a so-called inverse spinel structure (T[Fe]O[ZnFe]O4). The degree of inversion, x, defined as the fraction of Zn2+ ions occupying octahedral sites, can consequently adopt values from 0 (normal structure) to 1 (inverse structure) according to T[Zn1-xFex]O[ZnxFe2-x]O4 with 0 ≤ x ≤ 1. The degree of inversion closely depends on the synthetic route.

For the first time, the effect of the degree of inversion on the physicochemical properties affecting the photocatalytic behavior of ZnFe2O4 has been investigated. Interestingly, as the degree of inversion increases, the conductivity of the material increases exponentially and an influence on the photocatalytic activity is observed. Thus, the degree of inversion plays a fundamental role and is a parameter of utmost importance to be investigated in order to get a meaningful approach regarding the photocatalytic performance of spinel ferrites.

 

[1] R. Dillert, D. H. Taffa, M. Wark, T. Bredow and D. W. Bahnemann, APL Materials, 2015, 3, 104001.

[2] D. H. Taffa, R. Dillert, A. C. Ulpe, K. C. Bauerfeind, T. Bredow, D. W. Bahnemann and M. Wark, Journal of Photonics for Energy, 2016, 7, 012009.

[3] A. Arimi, L. Megatif, L. I. Granone, R. Dillert and D. W. Bahnemann, Journal of Photochemistry and Photobiology A: Chemistry, 2018, DOI: 10.1016/j.jphotochem.2018.03.014.

15:15 - 15:30
S2.1-O2
hajiyani, hamidreza
Universität Duisburg-Essen
Origin of Enhanced Efficiency of Tin-doped Ultrathin Hematite Photoanodes for Water-Splitting
hamidreza hajiyani
Universität Duisburg-Essen, DE
Authors
Hamidreza Hajiyani a, Alexander G. Hufnagel b, Siyuan Zhang c, Thomas Bein b, Dina Fattakhova-Rohlfing d, e, Christina Scheu c, Rossitza Pentcheva a
Affiliations
a, Department of Physics, Theoretical Physics and Center of Nanointegration (CENIDE), Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
b, Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstraße 5-13 (E), 81377 Munich, Germany
c, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany
d, Institut für Energie- und Klimaforschung, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straÿe, 52425 Juelich, Germany
e, Universität Duisburg-Essen, Germany
Abstract

Using a combination of experimental and theoretical methods we explore the beneficial effect of Sn(IV) doping in ultrathin hematite photoanodes for water oxidation. A series of hematite photoanodes with tailored Sn-doping profiles were prepared by alternating atomic layer deposition. Using data from spectrophotometry and intensity-modulated photocurrent spectroscopy we deconvoluted the overall efficiency and obtained the individual process efficiencies for light harvesting, charge separation and charge transfer. Photoanodes with Sn-doping both on the surface and in the subsurface region show the best performance with enhanced charge separation and charge transfer efficiency. Density functional theory calculations with a Hubbard U parameter were performed to investigate the causes of the efficiency improvement considering both Fe2O3 (0001), as well as Fe2O3 (11-26) surface orientation, as identified from micrographs at atomic resolution. The energetics of surface intermediates during the oxygen evolution reaction reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe2O (11-26) orientation shows a significantly lower overpotential, one of the lowest reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation). Moreover, the subsurface Sn-doping introduces a band bending that helps to improve the charge separation efficiency.

 

We acknowledge funding by SPP1613 and computational time at MagntUDE.

[1] A. G. Hufnagel, H. Hajiyani, S. Zhang, T. Li, O. Kasian, B. Gault, B. Breitbach, T. Bein, D. Fattakhova-Rohlfing, C. Scheu and R. Pentcheva, Adv. Funct. Mater. (accepted).

15:30 - 15:45
S2.1-O3
Eichberger, Rainer
Helmholtz Center Berlin for Materials and Energy
The Transport Pathways of Charge Carriers in CuWO4 for Photocatalysis
Rainer Eichberger
Helmholtz Center Berlin for Materials and Energy, DE
Authors
Sönke Müller a, James Hirst a, Hannes Hempel b, Daniel Peeters c, Alexander Sadlo c, Oliver Mendoza d, Dariusz Mitoraj d, Dennis Friedrich a, Anjana Devi c, Radim Beranek d, Rainer Eichberger a
Affiliations
a, Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
b, Department Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
c, Inorganic Materials Chemistry, Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
d, Institute of Electrochemistry, Ulm University, Albert-Einstein-Allee 47, 89069 Ulm, Germany
Abstract

We report on the carrier transport properties of CVD-grown CuWO4 films for solar water splitting with varying copper-to-tungsten stoichiometry within the borders of the binary metal oxides CuO and WO3. Time-resolved terahertz (TRTC) and microwave conductivity (TRMC) measurements addresses the bulk dynamics in different time windows from sub-ps to ms. In addition, photo-induced absorption (PIA) in a photo-electrochemical cell is applied to study the temporal behavior of excited carriers in the vicinity of the semiconductor/electrolyte interface under varying bias. A charge carrier mobility of ∼6× 103 cm2 V1 s1 and a diffusion length of and 30 nm is determined for a CuWO4 absorber film deposited with a copper-to-tungsten ratio (Cu/W) of 1.1 which also provides the best photocurrents in assembled photo-electrochemical cells. This value is comparable to undoped BiVO4 where poor carrier transport is governed by small polaron formation leading to slow mobility values. We compare the experimental results with measurements performed on dip-coated CuWO4 samples and other metal oxides such as BiVO4 and Cu2O. Our findings establish new insights into the advantages and limits of CuWO4-based photoanodes that can be possibly used in a tandem configuration on top of a highly absorbing semiconductor with optimal electronic properties.

 

[1] D. Peeters,  O. Mendoza Reyes,  L. Mai,  A. Sadlo,  S. Cwik,  D. Rogal, lH.-W. Becker, H. M.  Schütz, J. Hirst, S. Müller, D. Friedrich, D. Mitoraj, M. Nagli, M. Caspary Toroker, R.Eichberger, R. Beranek, A. Devi, J. Mater. Chem. A, adv. article (2018)

[1] M. Ziwritsch, S. Müller, H. Hempel, T. Unold, F. F. Abdi, R. v. d. Krol, D. Friedrich, R. Eichberger, ACS Energy Lett., 1 , 888 (2016)

[2] F. F. Abdi, T. J. Savenije, M. May, B. Dam, R. van de Krol, J. Phys. Chem. Lett. 4, 2752 (2013)

[4] J. Ravensbergen, F. F. Abdi, J. H. van Santen, R. N. Frese, B. Dam, R. van de Krol, J. T. M. Kennis, J. Phys. Chem. C, 118, 27793 (2014)

 

15:45 - 16:00
Discussion
16:00 - 16:30
S2.1-I2
Smirnov, Vladimir
Forschungszentrum Jülich
Multijunction Si Solar Cells for Integrated Photo-Electrochemical Devices
Vladimir Smirnov
Forschungszentrum Jülich, DE
Authors
Vladimir Smirnov a, Katharina Welter a
Affiliations
a, IEK5-Photovoltaics, Forschungszentrum Jülich, 52425 Jülich, Germany
Abstract

The application of multijunction solar cells in photoelectrochemical (PV-EC) devices for hydrogen production is addressed. Integrated PV-EC devices are composed of a ‘traditional’ photovoltaic (PV) cell combined with an electrochemical (EC) cell, presenting a promising approach to produce hydrogen and other fuels. The requirements for PV-EC devices and strategies for the solar cell development will be discussed. The results on the photovoltaic development of multijunction silicon based cells will be presented, focusing on a wide range of photovoltages and photocurrents in various systems, including the adoption of the cells to function on either cathode or anode sides of the system. A prototype integrated PV-EC system based on silicon multijunction solar cells can yield solar-to-hydrogen efficiencies (STH) of 9.5%.

The paths of device upscaling beyond laboratory size and the influence of varied illumination conditions close to obtained outdoor will also be discussed. This includes the effects of spectral quality, intensity and incident angle on the performance of both photovoltaic cells and PV-EC devices.

16:30 - 16:45
S2.1-O4
Melder, Jens
University of Freiburg, Freiburg Materials Research Center (FMF)
Electrochemical Water Oxidation by MnOx/CFP – pH Dependence of the Catalytic Activity
Jens Melder
University of Freiburg, Freiburg Materials Research Center (FMF), DE
Authors
Jens Melder a, Stefan Mebs b, Philipp Heizmann a, Holger Dau b, Philipp Kurz a
Affiliations
a, Institut für Anorganische und Analytische Chemie und Freiburger Materialforschungszentrum (FMF), Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg
b, Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
Abstract

The efficient catalysis of the four-electron oxidation of water to molecular oxygen (oxygen evolution reaction, OER) is a central challenge for the development of devices for the conversion of electrical energy, ideally from renewable sources, into storable chemical energy (e.g. by electrolysis or “artificial leaves”). Up to now, a plethora of earth-abundant, non-toxic catalysts (e.g. Ni/Fe, Co, Mn oxides)1 for OER is known. Most of these possess high activities and stabilities under strongly alkaline pH conditions. In contrast, the only convincing materials for OER in acidic media are based on the scarce elements Ru and Ir, while especially Ni and Co based electrodes suffer from corrosion and/or show little activity.2

Inspired by the oxygen evolving complex (OEC) of Photosystem II, the biological catalyst for this reaction, several manganese oxide (MnOx) polymorphs have been tested as heterogeneous water oxidation (electro-)catalysts. Here we found that amorphous layered or tunnelled manganese oxides show good activities and stabilities also for strong acidic reaction media.3 Recently, we developed a route to directly prepare coatings of amorphous MnOx on free-standing carbon based electrode supports (e.g. carbon-fiber-paper, CFP) by a simple, scalable redox deposition approach.4

This presentation will deal with a study where the electrocatalytic performance of such MnOx/CFP-anodes was tested under different electrolyte conditions. A special focus was laid on the influence of the pH on activity and stability. Additionally, the MnOx/CFP-electrodes were characterized before and after electrolysis by means of XRD, SEM/EDX, vibrational- and X-ray absorption spectroscopy. Details of the preparation, characterization, electrocatalytic performance and corrosion stability will be discussed together with possible reasons for the different behavior of the electrocatalyst at different pHs. We will report on an efficient catalyst for OER at acidic and near-neutral solution, a pH range highly desirable for (photo-)electrochemical devices, such as polymer electrolyte membrane (PEM) electrolysers or artificial leaves.

References

1 N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337–365.

2 L. C. Seitz, C. F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H. Y. Hwang, J. K. Nørskov and T. F. Jaramillo, Science, 2016, 353, 1011–1014.

3 C. E. Frey and P. Kurz, Chem. A Eur. J., 2015, 21, 14958–14968.

4 J. Melder, W. L. Kwong, D. Shevela, J. Messinger and P. Kurz, ChemSusChem, 2017, 4491–4502.

16:45 - 17:00
S2.1-O5
Ludwig, Alfred
Ruhr-University Bochum
Combinatorial Fabrication and High-Throughput Characterization of Thin Film Metal Oxide Libraries for Solar water Splitting
Alfred Ludwig
Ruhr-University Bochum, DE
Authors
Alfred Ludwig a, Mona Nowak a, Swati Kumari a, Helge S. Stein a, Ramona Gutkowski b, Joao Junqueira b, Wolfgang Schuhmann b
Affiliations
a, Institute for Materials, Ruhr-Universität Bochum, D-44801 Bochum, Germany
b, Analytical Chemistry–Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum, D-44801 Bochum, Germany
Abstract

Semiconducting metal oxide thin films are promising 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. High-throughput measurements of compositional, structural and functional data on the materials libraries were performed by automated thickness measurements, energy-dispersive x-ray analysis (EDX), X-ray diffraction (XRD) and PEC analyses using an optical scanning droplet cell in 342 measurement areas on each of the materials libraries. Furthermore, the microstructure of selected thin films was characterized by electron and atomic force microscopy. The analysis of the obtained data enables to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density. Several promising compositions were identified using the combinatorial approach. Furthermore, we demonstrate the combinatorial glancing angle sputter deposition (GLAD) approach for the fabrication of thin film materials libraries consisting of columnar nanostructures.

 
Thu Oct 25 2018
NCPhot S4.4
Chair: Zeger Hens
09:00 - 09:30
Abstract not programmed
09:30 - 10:00
S4.4-O1
Becker, Michael
IBM Research – Zurich
Bright Triplet Emission from Lead Halide Perovskite Nanocrystals
Michael Becker
IBM Research – Zurich, CH
Authors
Michael A. Becker a, b, Roman Vaxenburg c, Georgian Nedelcu d, e, Loredana Protesescu d, e, Andrew Shabaev c, Peter Sercel f, Michael J. Mehl g, John G. Michopoulos h, Samuel G. Lambrakos h, Noam Bernstein h, John L. Lyons h, Maryna I. Bodnarchuk d, e, Rainer F. Mahrt a, Thilo Stoeferle a, Maksym V. Kovalenko d, e, David J. Norris b, Gabriele Raino a, d, e, Alexander L. Efros h
Affiliations
a, IBM Research – Zurich Research Laboratory, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
b, Optical Materials Engineering Laboratory, ETH Zürich, Leonhardstrasse 21, 8092 Zurich, Switzerland
c, George Mason University, Fairfax VA, 22030, USA
d, Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, 8093 Zürich, Switzerland
e, Laboratory for Thin Films and Photovoltaics, Empa − Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
f, T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
g, U.S. Naval Acadamy, Annapolis, MD 21402, USA
h, Naval Research Laboratory, Washington DC, 20375, USA
Abstract

The emission of fully inorganic cesium lead halide (CsPbX3, where X = I,Br,Cl) perovskite-type nanocrystals is tunable over a wide energy range with ultrahigh photoluminescence quantum yields of up to 90%[1] and exhibits narrow emission lines. Due to their facile solution processability and their potential for high-efficiency photovoltaics and light sources they have gained enormous interest.

Experiments on single perovskite quantum dots reveal a unique energetic level structure with a lowest bright triplet state[2], thus enabling photon emission rates ~20 and ~1000 times higher compared to any other conventional semiconductor nanocrystals at room and cryogenic temperatures, respectively. We investigate the nature of this exceptionally fast photon emission by temperature dependent quantum yield measurements. Furthermore we discriminate it from composition dependent “A-type” blinking behaviour in intensity-decay time correlation measurements and demonstrate stable, narrowband emission, with suppressed blinking and small spectral diffusion[3] for single CsPbBr2Cl nanocrystals. By means of polarization dependent high resolution spectroscopy, the complex nature of the exciton fine structure splitting and charged exciton emission has been characterized.

Based on these measurements, supported by effective-mass models and group theory calculations, we conclude that the triplet exciton state is responsible for the extraordinary photon emission properties of lead halide perovskites. Our results can assist to identify other semiconductors that exhibit bright triplet excitons, with potential implications for improved optoelectronic devices.

 

 

References:

[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)

[2] Becker et al., Nature, 553, 187-193 (2018)

[3] Rainò et al., ACS Nano 10, 2485–2490 (2016)

10:00 - 10:30
S4.4-O2
Tomar, Renu
Ghent university
A Quantitative Study of Optical Gain Mechanisms in Quasi-2D Solution Processable Materials
Renu Tomar
Ghent university, BE
Authors
RENU TOMAR a, ADITYA KULKARNI b, KAI CHEN c, SHALINI SINGH a, LAURENS SIEBBELES b, JUSTIN HODGKISS c, PIETER GEIREGAT a, ZEGER HENS a
Affiliations
a, Ghent university, Krijgslaan 281 - S3, Gent, BE
b, Opto-electronic materials section, Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology
c, School of Chemical and Physical Sciences,Victoria University of Wellington, New Zealand
Abstract

Two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications due to their pronounced excitonic features leading to unique properties in terms of light emission. However, only a few studies focus on the use of these materials for light amplification or net optical gain development and the ensuing high carrier density photo-physics. The beneficial nature of the strong excitonic effects on optical gain remain hence unquantified and , despite the large binding energies, it remains unclear what the involvement of is at the concomitant high carrier densities. Here, we use colloidal 2D CdSe nanoplatelets as a model system and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several distinct and carrier density-dependent optical gain regimes exist for these materials. At low density, optical gain is found to originate from excitonic molecules delivering large material gains up to 20.000 cm-1, yet with an Auger limited lifetime of few hundred picoseconds. At increasing pair density, we observe a surprising transition to a combined regime of blue-shifted and disruptively large optical gain, combined with the typical exciton mediated gain. We show that this peculiar situation originates from a carrier cooling bottleneck at high density. Surprisingly, the insulating (multi-)exciton gas is found to co-exist with the conductive phase in a density regime nearly one order of magnitude beyond the expected Mott transition.  The ensuing exciton ground state absorption even counter-acts the development of net optical gain in certain spectral regions. Our results shed a new light on the disruptive photo-physics of high binding energy excitons in strongly excited 2D materials and pave the way for the development of more efficient broadband optical gain media and/or high density excitonic devices such as polariton lasers.

Plenary Session 5
Chair: Daniel Vanmaekelbergh
09:00 - 09:30
5-K1
Kovalenko, Maksym
ETH Zurich
Colloidal Nanocrystals of APbX3 Perovskites [A=Cs+, CH(NH2)2+, X=Cl-, Br-, I-]: Surface Chemistry, Self-Assembly and Potential Applications
Maksym Kovalenko
ETH Zurich, CH

Maksym Kovalenko has been a tenure-track Assistant Professor of Inorganic Chemistry at ETH Zurich since July 2011 and Associate professor from January 2017. His group is also partially hosted by EMPA (Swiss Federal Laboratories for Materials Science and Technology) to support his highly interdisciplinary research program. He completed graduate studies at Johannes Kepler University Linz (Austria, 2004-2007, with Prof. Wolfgang Heiss), followed by postdoctoral training at the University of Chicago (USA, 2008-2011, with Prof. Dmitri Talapin). His present scientific focus is on the development of new synthesis methods for inorganic nanomaterials, their surface chemistry engineering, and assembly into macroscopically large solids. His ultimate, practical goal is to provide novel inorganic materials for rechargeable Li-ion batteries, photovoltaics, and optoelectronics. He is the recipient of an ERC Starting Grant 2012, Ruzicka Preis 2013 and Werner Prize 2016

Authors
Maksym Kovalenko a
Affiliations
a, Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zurich, 8093 Zurich, Switzerland
b, Laboratory for Thin Films and Photovoltaics, Empa − Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
Abstract

Colloidal lead halide perovskite nanocrystals (APbX3, NCs, A=Cs+, FA+, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs1-xFAxPbI3 and FAPbI3 reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as CsxFA1–xPb(Br1–yIy)3 NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, CsxFA1–xPb(Br1–yIy)3 NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

 

 

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

M. V. Kovalenko et al. Science 2017, 358, 745-750

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

I. Lignos et al. ACS Nano 2018, DOI: 10.1021/acsnano.8b01122

F. Krieg et al. ACS Energy Letters 2018, 3, 641–646.

Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

SPMEn S10.1
Chair: Sascha Sadewasser
09:00 - 09:15
Abstract not programmed
09:15 - 09:45
S10.1-I1
Weber, Stefan
Max Planck Institut for Polymer Research
Watching Ions Move: Scanning Probe Microscopy on Perovskite Solar Cells
Stefan Weber
Max Planck Institut for Polymer Research, DE

Stefan Weber (born 1981) studied physics at the University of Konstanz. For his PhD thesis, he joined the Max Planck Institute for Polymer Research in 2007, where he studied organic electronic materials with atomic force microscopy in an international German-Korean research-training group. He then went to University College Dublin, where he studied high-resolution force microscopy at liquid-solid interfaces. Since 2012 he has been group leader at the MPI-P and, since 2014, a junior professor in the physics department of the Johannes Gutenberg University Mainz. Since his doctoral thesis, he has been working on the application and further development of force microscopy methods. He aims at understanding basic mechanisms in nanostructures as found e.g. in solar cell materials.

Authors
Stefan A.L. Weber a, b, Ilka Hermes a, Anders Hagfeldt c, Michael Graetzel d, Wolfgang Tress c, Rüdiger Berger b
Affiliations
a, Max Planck Institute For Polymer Research, Max Planck Institute for Polymer Research Ackermannweg 10, Mainz, DE
b, Institute of Physics, Johannes Gutenberg University Mainz
c, Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne
d, Laboratory of Photonics and Interfaces (LPI), Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
Abstract

Perovskite solar cells have electrified the solar cell research community with astonishing performance and surprising material properties. Very efficient (>20 %) devices with perovskite layers of low defect density can be prepared by cheap and simple solution based processes at moderate temperatures (<150°C). For commercializing this technology, a stable and reliable operation has to be ensured. In perovskite solar cells, however, the output power is strongly influenced by the history of the device in terms of bias voltage (causing hysteresis) or illumination (known as light soaking effect). The underlying process is assumed to be the slow migration of ionic charges within the perovskite layer.

In my presentation I will demonstrate how scanning probe microscopy can help understanding these processes. Using Kelvin probe force microscopy, we were able to follow the vertical charge distribution in the active perovskite layer of an operating device. We observed the formation of a localized interfacial charge at the anode interface, which screened most of the electric field in the cell. The formation of this charge happened within 10 ms after applying a forward voltage to the device. After switching off the forward voltage, however, these interfacial charges were stable for over 500 ms and created a reverse electric field in the cell. This reverse electric field directly explains higher photocurrents during reverse bias scans by electric field-assisted charge carrier extraction. We thereby show that instead of the slow migration of mobile ions, the formation and the release of interfacial charges is the dominating factor for current-voltage hysteresis.

09:45 - 10:00
Abstract not programmed
10:00 - 10:30
S10.1-I2
LECLERE, Philippe
University of Mons
Light on Organic and Hybrid Photovoltaic Devices : The Key Role of Scanning Probe Microscopy
Philippe LECLERE
University of Mons, BE

Philippe Leclère received a PhD in Physics from the University of Liège (Belgium) in 1994. He joined the group of Jean-Luc Brédas at the University of Mons in 1995 as a research fellow. From 2000 to 2004, he worked as research associate and served as research coordinator at the Materia Nova Research Center. During this period, he spent 4 months (in 1999) in the group of Jean-Pierre Aimé at the University of Bordeaux (France) and one year (2003) in the group of E.W. (Bert) Meijer at the Eindhoven University of Technology (TU/e) in the Netherlands. In October 2004, he became Research Associate of the Belgian National Fund for Scientific Research (FRS - FNRS) in the group of Roberto Lazzaroni at the University of Mons. In October 2014, he became Senior Research Associate of the FRS - FNRS. Since 2003, he is still visiting scientist at the Institute of Complex Molecular Systems at TU/e. His research interests mostly deal with the characterization by means of scanning probe microscopy techniques of the morphology and the nanoscale mechanical, electrical properties of organic and hybrid supramolecular (nano)structures, build by self-assembly of functional (macro)molecules. He is (co)author of over 160 chapter books and papers in international peer-reviewed journals. Hirsch Factor : 38

Authors
Philippe LECLERE a
Affiliations
a, University of Mons (UMONS), Laboratory for Chemistry of Novel Materials, Center for Innovation and Research in Materials and Polymers (CIRMAP), Mons (Belgium)
Abstract

Recent research and progress in organic photovoltaic (OPV) repeatedly insist on the importance of the molecular organization of the compounds forming the active bulk-heterojunction (BHJ) blends. The morphology of the blend has been to tremendously affect both the charge transfer at the donor-acceptor interface and the carrier transport to the electrodes. And still, for each material combination, much remains to be understood to fully assess its specific and ultimate morphology. For this purpose, high resolution characterization methods are of primary interest to locally depict tthe photovoltaic process. Conductive Atomic Force Microscopy (C-AFM) and Kelvin Probe Force Microscopy (KPFM) have already proven to be of significant help to yield nanoscale two-dimensional mapping of electrical properties. C-AFM and related PeakForce TUNA emerged as powerful technique to electrically evidence phase separation in blends. An additional key feature lies in local I-V curve providing useful information about the charge transport mechanisms within the materials forming the blends. Quantitative measurements leading to local determination of hole mobility have already been reported. It appears that upon illumination the technique has shown to be sensitive to photocurrent. With photoconductive-AFM (pc-AFM), a dedicated external calibrated module has been recently introduced allowing full quantitative mapping of photovoltaic mechanisms.

 

In this lecture, we will carefully analyze, as a model sample, BHJ made of poly(3-hexylthiophene) (P3HT) and fullerene derivative (PCBM). While photocurrent is determined at 0 V DC bias, additional parameters including the open-voltage, the fill factor and the resistances can be obtained spanning the DC bias between the probe and the sample back electrode.(KPFM is also used in this work to delineate phase separation and potential variations at interfaces. Upon illumination, photovoltage can also be evidenced. With the additional external calibrated illumination module, mapping of photovoltage in BHJ blends can be obtained, opening the doors of local characterization of charge transfer at donor-acceptor interfaces, where crucial processes are occurring in photovoltaic devices.

 

In fine, we will also address the photovoltaic properties of  other systems such hybrid TiO2 nanopillars : conjugated polymers.

  

Sol2D S6.2
Chair: Sandrine Ithurria
09:30 - 10:00
S6.2-O1
Mews, Alf
University of Hamburg
Synthesis and Electrical Properties of Photoactive Two Dimensional SnS Nanosheets
Alf Mews
University of Hamburg, DE
Authors
Alf Mews a, Monika Kobylinski a, Charllotte Ruhmlieb a, Vi Pham a, Jan Siebels a, Andreas Kolditz a
Affiliations
a, Institute of Physical Chemistry, University of Hamburg
Abstract

Tin(II)sulfide nanosheets with lateral sizes in the range of several micrometers and thicknesses of only tens of nanometers are synthesized by injection of S-oleylamine into a hot solution of oleylamine, oleic acid, HMDS and tin chloride. We will present structural data to follow the nanosheets growth and show how additional octadecene can influence the lateral growth direction to form either rectangular or hexagonal nanosheets.

In the second part we focus on the electrical properties of individual hexagonal SnS-nanosheets. Here we present data from combined Kelvin Probe Force Microscopy (KPFM) with simultaneous Scanning Photocurrent Measurements (SPCM) to attribute the electro-optical properties to mutual interaction between locally generated charge carriers and electric potentials. For example the occurrence of zero-bias photocurrent can directly be attributed to changes of the band bending due to the optically generated charge carriers.

10:00 - 10:30
S6.2-I1
Buhro, William
Washington University
Facile Surface Exchanges in 2D CdSe Nanocrystals
William Buhro
Washington University, US

Professor William E. Buhro earned an A.B. in Chemistry in 1980 at Hope College (Holland, Michigan) and a Ph.D. in Chemistry in 1985 at the University of California, Los Angeles. His dissertation research focused on organometallic chemistry. He was then awarded the first Chester Davis Research Fellowship at Indiana University, where he was a postdoctoral fellow from 1985-1987. In 1987 he joined the Department of Chemistry at Washington University as an assistant professor. Buhro twice received the Washington University Council of Arts and Sciences Faculty Award for Teaching (1990, 1996), the Emerson Electric Co. Excellence in Teaching Award (1996), and was named a National Science Foundation Presidential Young Investigator (1991-1996). In 2010 Buhro received the St. Louis Award from the ACS St. Louis Section, and was named a Fellow of the American Chemical Society. He is currently the George E. Pake Professor in Arts & Sciences, Chair of the Department of Chemistry, and an editor of the ACS journal Chemistry of Materials. His research interests in nanoscience include the synthesis of nanocrystalline materials, especially pseudo-1D and 2D colloidal semiconductor nanocrystals, the spectroscopic properties of quantum nanostructures, and mechanisms of nanocrystal growth.

Authors
William Buhro a
Affiliations
a, Department of Chemistry, Washington University, St. Louis, MO 63119, USA
Abstract

The surface ligation of semiconductor nanocrystals determines their optical and transport properties, and purposeful control of surface chemistry is under active investigation.  Colloidal 2D CdSe nanoribbons or quantum belts provide an ideal system for the study of rapid, reversible, and complete exchange of surface ligation at room temperature.  L-type primary-amine ligation is readily replaced by Z-type Lewis acid ligation of type MX2 (M = Zn, Cd; X = carboxylate or halide).  Re-exposure to primary amines removes the Z-type ligands and restores L-type ligation.  Primary-amine ligation is readily converted to bound-ion-pair X-type ligation upon exposure of the quantum belts to acids HX (X = halide, carboxylate, or nitrate).  Salts of type R4NX or NaX (X = carboxylate or halide) are unable to displace L-type ligation, but readily substitute for Z-type ligation, forming bound-ion-pair X-type ligation.  This bound-ion-pair X-type ligation is readily exchanged to L-type ligation upon exposure to primary amines.  The surface exchanges are characterized by a variety of means, and the absorption and emission spectra of the quantum belts are particularly sensitive to surface ligation.  Such surface exchanges promise to optimize the properties of the 2D nanocrystals.

WatSpl S2.2
Chair: Laurence Peter
09:00 - 09:30
S2.2-I1
Scheu, Christina
Max-Planck-Institut für Eisenforschung Düsseldorf
Nb3O7OH Nanoarrays for Photocatalytic Water Splitting: Defects, Dopants, and Stability of co-Catalysts
Christina Scheu
Max-Planck-Institut für Eisenforschung Düsseldorf, DE

Prof. Christina Scheu has a diploma degree in physics and did her doctorate at the Max-Planck-Institute for Metals Research in Stuttgart (Germany) in the field of material science. She spent two years as a Minerva Fellow at the Technion - Israel Institute of Technology – in Haifa, Israel. 2008 she was appointed as a full professor at the Ludwig-Maximilian-University (Munich, Germany). Since April 2014 she holds a joint position as a full professor at the RWTH Aachen, and as an independent group leader at the Max-Planck-Institut für Eisenforschung GmbH (MPIE) in Düsseldorf Germany. Her expertise is the structural and chemical analysis of functional materials with ex-situ and in-situ transmission electron microscopy and electron energy loss spectroscopy and correlation to optical, electronic and electrochemical properties. The investigated materials range from (photo)catalyst for hydrogen production to electrodes and membranes for polymer based fuel cells.

Authors
Sophia Betzler a, Thomas Gänsler b, Katharina Hengge b, Anna Frank b, Siyuan Zhang b, Christina Scheu b
Affiliations
a, Ludwig-Maximilians-Universität (LMU) München
b, Max-Planck-Institut für Eisenforschung Düsseldorf, Max-Planck-Straße, 1, Düsseldorf, DE
Abstract

Novel semiconducting nanostructured oxides have gained interest in photocatalytic water splitting where they can act as electrode material. One candidate is the n-type semiconductor Nb3O7(OH), which can be fabricated as 3D nanoarray using a hydrothermal synthesis approach [1]. The 3D nanoarray consists of nanowires arranged perpendicular to each other. The growth defects within the Nb3O7(OH) nanostructure such as stacking faults are the key parameters which determine the functionality as will be discussed in the talk. The nanostructures have been studied in-depth using advanced transmission electron microscopy including electron energy loss spectroscopy to determine the oxidation state of the individual atoms as well as to analyze the band gap on the nanometer scale. In addition, electron tomography and focused ion beam slicing have been used to obtain the 3D morphology of the Nb3O7(OH) array after various growth stages. The functional properties of the Nb3O7(OH) arrays can be improved by the incorporation of Ti within the orthorhombic crystal structure which leads to a higher hydrogen production rate in light driven water splitting experiments [2]. For these measurements, a Pt co-catalyst is deposited on the nanowire array. In order to understand the stability and degradation behavior of the co-catalyst we plan to perform identical location transmission electron microscopy measurements similar as we have done for a Pt/Ru electrocatalyst [3]. Such measurements enable tracking of individual nanoparticles and allow to determine the dominating degradation mechanisms such as catalyst dissolution or Oswald ripening down to the atomic scale. 

[1] S. B. Betzler, A. Wisnet, B. Breitbach, C. Mitterbauer, J. Weickert, L. Schmidt-Mende, and C. Scheu, J. Mater. Chem. A, 2014, 2, 12005.

[2] S. B. Betzler, F. Podjaski, K. Bader, M. Beetz, K. Hengge, A. Wisnet, M. Handloser, A. Hartschuh, B. V. Lotsch, C. Scheu, Chemistry of Materials, 2016, 28, 7666.

[3] K. Hengge, T. Gänsler, E. Pizzutilo, C. Heinzl, M. Beetz, K. J. J. Mayrhofer, C. Scheu, International Journal of Hydrogen Energy 2017, 42 (40), 25359.

[4] The author would like to thank the colleagues and co-workers who contributed to this work and the German Research Foundation (DFG) for financial support.

09:30 - 09:45
S2.2-O3
Ronge, Emanuel
University of Goettingen
In-Situ Transmission Electron Microscopy Analysis of a Calcium-Birnessite Water-Oxidation Catalyst
Emanuel Ronge
University of Goettingen, DE
Authors
Emanuel Ronge a, Vladimir Roddatis a, Jonas Ohms b, Philipp Kurz b, Christian Jooss a
Affiliations
a, University of Goettingen, Friedrich-Hund-Platz 1, Goettingen, DE
b, Albert-Ludwigs University of Freiburg
Abstract

Finding electro-catalysts for driving the oxygen evolution reaction (OER) at the minimum overpotential remains the bottle neck in water splitting. Birnessite is a promising earth-abundant electrode material, since its layered calcium manganese oxide structure with intercalated crystal water possibly allows bulk OER activity [1,2].

We study a nanocrystalline type of highly active electrode, prepared by dropping ink solutions on top of a substrate. The structure of the highly porous electrodes with a grain size down to a few nanometres are analysed with High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED). To gain more information about the mechanism of water oxidation we investigated the interaction of the electrode with the electrolyte. Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-Ray Spectroscopy (EDX) indicate an intercalation of the electrolyte cation phosphorous while calcium was depleted from the electrode. The exchange mainly happens at the surface and at grain boundaries.

To further explore the active centres of water oxidation with Birnessite we took steps towards in-situ TEM studies in water vapour. The Environmental TEM (ETEM) offers to investigate Birnessite active states in electric potentials close to operando conditions. Based on our studies we present conclusions on the involved OER mechanism and design strategies for highly active and stable electro-catalysts.

 

[1] S. Y. Lee, D. González-Flores, J. Ohms, T. Trost, H. Dau, I. Zaharieva and P. Kurz, ChemSusChem, 2014, 7, 3442-3451

[2] C. E. Frey and P. Kurz, Dalton Trans. , 2014, 43, 4370-4379

09:45 - 10:00
S2.2-O4
Petersen, Thorben
Carl von Ossietzky University Oldenburg
Quantum Chemical Investigation of Water Splitting on ideal TiO2-Anatase(101)
Thorben Petersen
Carl von Ossietzky University Oldenburg, DE
Authors
Thorben Petersen a, Thorsten Klüner a
Affiliations
a, Carl von Ossietzky University Oldenburg, Carl von Ossietzky Starß2 9-11, Oldenburg, 26129, DE
Abstract

Titania-based photocatalysts represent a promising class of materials to  split water in its elementary components due to their high abundancy and stability [1]. Albeit their low solar visible light exploitation, their crystal  structure and electronic properties are well-known and facilitate the  investigation of the several fundamental aspects involved in photocatalytic water splitting. A detailed understanding of these key steps will subsequently allow for the design of an appropriate photocatalyst.
Most recently, the anatase modification of titanium dioxide (a-TiO2) emerged as a widely applied material since it is the majority phase of TiO2-nanoparticles and shows enhanced photocatalytic activity [1,2]. The intermediates during the water oxidation pathway on the a-TiO2(101) surface were already identified on GGA-PBE level of theory [3]. The key step was determined to consist of the first proton removal induced through a photogenerated hole (H2O + h+ → OH + H+ ). The influence of the photogenerated hole on the dissociation process was addressed controversially by theoretical studies: whereas combined PBE/HSE06 studies indicate that hole-trapping only occurs after dissociation through the OH-anionic species [4], recent work employing the B3LYP functional gives evidence for a concerted proton/hole transfer [5]. In addition, as observed by experimental TPD and TOF methods, OH radicals are found to be ejected from a well-defined a-TiO2(101) surface after irradiation [6].
In order to get a more sophisticated insight into this yet indetermined reaction, we use two different theoretical approaches in this contribution: We firstly identify the active sites of the H2O/a-TiO2 system through periodic slab calculations using hybrid DFT functionals (PBE0/HSE06). Afterwards, these data will provide the basis for an embedded cluster approach allowing for accurate post-HF methods. As a result, we will present potential energy surfaces of a single water molecule on a-TiO2(101).

[1] F. De Angelis, C. Di Valentin, S. Fantacci et al., Chem. Rev. 114 (2014) 9708.
[2] A. Barnard, P. Zapol, L. Curtiss, Surf. Sci. 582 (2005), 173.
[3] Y.-F. Li, Z.-P. Liu, L. Lui, W. Gao, J. Am. Chem. Soc. 132 (2010) 13008.
[4] W.-N. Zhao, Z.-P. Liu, Chem. Sci. 5 (2014) 2256.
[5] C. Di Valentin, J. Phys.: Condens. Matter. 28 (2016) 074002.
[6] Z. Geng, X. Chen, W. Yang et al., J. Phys. Chem. C 120 (2016) 26807.

10:00 - 10:15
S2.2-O1
Fischer, Thomas
University of Cologne
Oxide Bilayers as High Efficiency Water Oxidation Catalysts through Electronically Coupled Phase Boundaries
Thomas Fischer
University of Cologne
Authors
Sanjay Mathur a, Lasse Jürgensen a, Yakup Gönüllü a, Jennifer Leduc a, Thomas Fischer a
Affiliations
a, Department of Chemistry, Inorganic Chemistry, University of Cologne, Greinstr. 6, Cologne 50939, DE
Abstract

New semiconductor metal oxides capable of driving water-splitting reactions by solar irradiation alone are required for sustainable hydrogen production. Whereas most metal oxides only marginally deliver the photochemical energy to split water molecules, uranium oxides are efficient photoelectrocatalysts due to their absorption properties (Eg ~ 2.0 - 2.6 eV) and easy valence switching among uranium centers that additionally augment the photocatalytic efficiency. Although considered a scarce resource, the abundance of uranium compounds in the environment is manifested in the huge quantities of stored UF6 gas, produced as waste streams in the nuclear fuel enrichment process. Here we demonstrate that thin films of depleted uranium oxide (U3O8) and their bilayers with hematite (a-Fe2O3) are high activity water oxidation catalysts due to electronically coupled phase boundaries. The electronic structure of uranium oxides showed an optimal band edge alignment in U3O8//Fe2O3 bilayers (DFT calculations) resulting in improved charge-transfer at the heterojunction as supported by TAS and XAS measurements. The enhanced photocurrent density of the heterostructures with respect to well-known hematite offers unexplored potential of uranium oxide in artificial photosynthesis.

10:15 - 10:30
S2.2-O2
Lukic, Sasa
University of Duisburg - Essen
Generation of Zinc-Gallium-Oxynitride Nanoparticles from CVS Powders for Photocatalytic Water Splitting
Sasa Lukic
University of Duisburg - Essen, DE
Authors
Sasa Lukic a, Jasper Menze b, Martin Muhler b, Markus Winterer a
Affiliations
a, Nanoparticle Process Technology (NPPT), Institute for Combustion and Gas Dynamics, University of Duisburg-Essen, 47057 Duisburg, Germany
b, Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44780 Bochum, Germany
Abstract

The development of semiconductors that split water photocatalytically under visible-light irradiation is a path to the efficient conversion of solar energy. Various oxides with d0 and d10 electron configuration possess such photocatalytic activity, but suffer from poor oxygen and hydrogen evolution or work only in the ultraviolet regime [1]. Domen et. al. developed gallium-oxynitride (Ga1-xZnx)(N1-xOx) as such a material by nitriding mixture of Ga2O3 and ZnO in a solid solution. It is capable of absorbing visible light efficiently with a bandgap of 2,6 eV [2,3].

In the first step we are producing nanoparticles by Chemical Vapor Synthesis. By adjusting process parameters such as temperature, pressure and precursor evaporation rate, we vary the characteristics of the materials, such as: surface area, crystallinity and particle size [1]. Exploiting the advantages of the synthesis from the gas phase we are able to obtain pure phase or mixture of ZnO and β-Ga2O3, as well as the complex spinel phase ZnGa2O4. In second step these powders are nitrided thermally or with assistance of a microwave plasma to obtain the desired oxynitrides. It is known that photocatalytic activity of such a material strongly depends on crystallinity and its composition. Varying the nitridation time from 0.17 h to 10 h zinc and oxygen concentration in zinc-gallium oxyntrides decreases, which is associated with a change in band-gap energy.

Incorporation of nitrogen and elimination of oxygen and zinc during nitridation process should take place at the surface of β-Ga2O3 and ZnO nanoparticles, parallel with diffusion of the constituent ions to form the stoichiometric solid solution. X-Ray diffraction analyzed by Rietveld refinement reveals crystal phases, cell parameters, as well as atomic composition. These results are supported by High-Resolution Scanning-Electron Microscopy in combination with Energy Dispersive Spectroscopy. The specific surface area of the samples is analyzed by using low temperature nitrogen adsorption. The bandgap is determined by using Ultraviolet-visible Spectroscopy.

[1] S. Lukic et al., ChemSusChem, 10, (2017), 4190-4197.

[2] K. Maeda et al., J. Am. Chem. Soc. 127, (2005), 8286-8287.

[3] K. Maeda et al., J. Phys. Chem. B 109, (2005), 20504-20510.

10:30 - 11:00
Coffee Break
NCPhot S4.5
Chair: David Cheyns
11:00 - 11:30
S4.5-O1
Rabouw, Freddy
Universiteit Utrecht
Probing Electromagnetic Modes at Optical Frequencies with Eu3+-Doped Nanocrystals
Freddy Rabouw
Universiteit Utrecht, NL
Authors
Freddy Rabouw a, Raphael Brechbühler a, Patrik Rohner a, Boris le Feber a, Tim Prins a, Dimos Poulikakos a, David Norris a
Affiliations
a, ETH Zürich, Vladimir Prelog Weg 2, 8093 Zürich, CH
Abstract

Controlling the flow of light at length scales below the diffraction limit is the quest of nanophotonics. The experimental mapping of electromagnetic modes could provide fundamental insights into nanophotonic systems and facilitate their rational design.

We propose NaYF4 nanocrystals doped with Eu3+ ions as nanoscopic probes for electromagnetic modes at optical frequencies. Eu3+ ions feature several electronic transitions throughout the visible spectrum with electric- or magnetic-dipole character. This enables the nanoprobes to sense both the electric and magnetic components of optical modes at the probes’ location. We verify our concept by mapping the photonic modes close to a metallic mirror. Further, we adapt the method to study surface plasmon polaritons (SPPs), electromagnetic modes confined to metal–dielectric interfaces. By placing the nanoprobes locally at varying distances from a plasmonic reflector, we study plasmonic modes with a resolution well beyond the diffraction limit of light. Our results highlight how a well-designed plasmonic environment can be utilized to control the emission directionality of SPP sources and to selectively enhance electric-dipole-forbidden optical transitions of quantum emitters.

11:30 - 12:00
S4.5-O2
Benin, Bogdan
Laboratory of Inorganic Chemistry, Department of Chemistry & Applied Biosciences, ETH Zurich
Highly Emissive Self-Trapped Excitons in Fully Inorganic Zero-Dimensional Tin Halides
Bogdan Benin
Laboratory of Inorganic Chemistry, Department of Chemistry & Applied Biosciences, ETH Zurich, CH
Authors
Bogdan Benin 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, Laboratory of Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland
Abstract

The formation of bound excitons via the spatial localization of charge carriers has long been a goal for luminescent semiconductors. Often, this has been accomplished through the formation of nanocrystals either through top-down or bottom-up methods. Alternatively, zero dimensional (0D) materials structurally impose carrier localization and result in the formation of highly localized Frenkel excitons. Recent works on perovskite-derived, hybrid organic-inorganic, 0D Sn(II) materials have demonstrated that high quantum yield emission from self-trapped excitons is possible when octahedra are isolated. As a new entry to the family of luminescent 0D materials, the fully-inorganic, perovskite-derived Cs4SnBr6 exhibits broad-band photoluminescence centred at 540 nm with a quantum yield of 15±5% at room temperature.[1] A compositional series, following the general formula Cs4-xAxSn(Br1-yIy)6 (A = Rb, K; x ≤ 1,y ≤ 1), can be synthesized by solid-state methods. Furthermore, the emission of these materials ranges from 500 nm – 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands. Finally, utilizing density functional theory calculations, the self-trapped exciton was ascribed to pseudo-Jahn-Teller distorted octahedra.

[1] Benin, B.M.,*; Dirin, D.N.*; Morad, V.; Woerle, M.; Yakunin, S.; Raino, G.; Nazarenko, O.; Fischer, M.; Infante, I.; Kovalenko, M.V. submitted.

12:00 - 12:30
S4.5-O3
Christodoulou, Sotirios
ICFO - The Institute of Photonic Sciences
High Performance Optoelectronic Devices Based on Bright Perovskite Nanocrystals Synthesized at Room Temperature
Sotirios Christodoulou
ICFO - The Institute of Photonic Sciences, ES
Authors
Sotirios Christodoulou a, Francesco Di Stasio a, Santanu Pradhan a, Inigo Ramiro a, Yu Bi a, Aleksandros Stavrinadis a, Gerasimos Konstantatos a, b
Affiliations
a, ICFO – The Institute of Photonic Sciences, Mediterranean Technology Park, Av. Carl Friedrich Gauss 3, 08860 Castelldefels (Barcelona)
b, ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain
Abstract

The excellent optoelectronic properties of perovskite nanocrystals (NCs) such as enhanced photoluminescence quantum yield (PLQY) and tunable emission wavelength has stimulated a widespread investigation of this class of semiconductors.[1] Very recently, it has been demonstrated that CsPbBr3 NCs can reach near-unity PLQY in solution. Yet, retaining the PLQY in film is not trivial; since the NCs are not as well passivated as in solution and close packing can lead to energy-transfer to trap-states and increased self-absorption.

Here, a room temperature synthesis of perovskite NCs displaying near-unity PLQY in solid state films is presented.[2]  Spin-coated films of the obtained CsPbBr3 NCs show PLQY values approaching unity (>95%), thanks to the combination of a novel synthesis at room temperature, and a post synthetic treatment. The as-obtained NCs show PLQY = 80% in spin-coated films. Further enhancement of the PL efficiency is obtained via addition of PbBr2. Following the synthesis, the obtained NCs were employed in optoelectronic devices. Efficient solar cells based on mixed-halide (CsPbBrI2) NCs obtained via anion exchange reactions under ambient conditions were fabricated.[3]. Solar cell devices operating in the wavelength range 350−660 nm were fabricated in air with two different deposition methods: single step (SP) and layer-by-layer (LbL). The solar cells display a photoconversion efficiency of 5.3%, independently of the active-layer fabrication method, and open circuit voltage (Voc) up to 1.31 V, among the highest reported for perovskite-based solar cells with bandgap below 2 eV, clearly demonstrating the potential of this material.

The high potential of the material is further tested in light-emitting diodes (LEDs) employing an inverted structure comprising of ZnO nanoparticles as an electron-transport layer and a conjugated polymer hole-transport layer. The LEDs demonstrate an external-quantum-efficiency of 6.04%, with luminance of 12998 Cd/m2 and low efficiency droop (around 10%). Importantly, such high efficiency was achieved by substituting Cesium with Formamidinium in line with our synthetic procedure. These results show the versatility of our synthetic protocol while the material quality is pointed out by the high performance of the optoelectronic devices. 

 

References

[1]       L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh, M. V Kovalenko, Nano Lett. 2015, 15, 3692.

[2]       F. Di Stasio, S. Christodoulou, N. Huo, G. Konstantatos, Chem. Mater. 2017, 29, 7663.

[3]       S. Christodoulou, F. Di Stasio, S. Pradhan, A. Stavrinadis, G. Konstantatos, J. Phys. Chem. C 2018, 122, 7621-7626.

SPMEn S10.2
Chair: Rüdiger Berger
11:00 - 11:30
S10.2-I1
GREVIN, BENJAMIN
CNRS
Imaging the Photocarrier Dynamics in Organic, Hybrid and Two-Dimensional Photovoltaic Materials by Time-Resolved Kelvin Probe Force Microscopy
BENJAMIN GREVIN
CNRS, FR

Benjamin Grévin is a graduate of the Institut National Polytechnique de Grenoble (INPG) and of the former University Joseph Fourier Grenoble I (University Grenoble Alpes, UGA). He received the Ph.D. degree in 1998 under the supervision of Dr. Y. Berthier. His doctoral work dealt with NMR investigations of high Tc superconductors and related cuprates. After a postdoctoral stay at the Condensed Matter Research Department of Geneva University in the group of Prof. Ø. Fisher, he joined in 2000 the UMR5819 joint research center (CEA-CNRS-UGA). He was awarded the bronze medal of CNRS in 2005 and obtained the accreditation to direct research (Habilitation à diriger les recherches, HdR) in 2006. His current research projects as CNRS Research Director deal with the development of advanced scanning probe microscopy techniques (nc-AFM/KPFM, time-resolved surface photo-voltage imaging), for local investigations of the opto-electronic properties of model organic (donor-acceptor BHJ and molecular self-assemblies), hybrid perovskites and 2D TMDC materials.

Authors
Benjamin GREVIN a
Affiliations
a, UMR5819 SyMMES CEA-CNRS-UGA CEA-Grenoble INAC/SyMMES 17 rue des Martyrs 38054 GRENOBLE CEDEX 9 FRANCE
Abstract

The development of third generation photovoltaics relies on the use of materials displaying a heterogeneous morphology at the mesoscopic or nanoscopic scale. A universal problem consists in identifying the sources of carrier losses (due to chemical, structural and interfacial defects) by recombination of photo-generated charge carriers. In nano-phase segregated organic bulk heterojunction (BHJs) thin films, a number of questions remain open concerning the impact of the donor-acceptor phases and interfaces morphology on the photo-carrier dynamics. It is also crucial to assess the impact of grain boundaries, chemical impurities and other local defects on the photo-carrier recombination in polycrystalline films of hybrid organic-inorganic perovskites.

In this communication, we will present the state of the art and ongoing developments in local measurements of the photo-carrier dynamics in organic and hybrid solar cell materials by time-resolved Kelvin probe force microscopy. After introducing the basic concepts of time-resolved surface photo-voltage (trSPV) imaging by KPFM under frequency-modulated illumination[1-3] ( FMI-KPFM), we will discuss several key issues and technical hints. What is the achievable lateral and temporal resolution? How shall we take into account photo-induced capacitive changes[4] in the analysis of FMI-KPFM data? We will also introduce a new experimental methodology combining FMI-KPFM with surface potential transients imaging, and we will explain what its benefits are for a proper data analysis and for simultaneous investigations of “fast” and “slow” SPV dynamics.

All items will be discussed in the light of experimental results obtained on BHJs, hybrid perovskites thin films and single crystals. In the last case, we will moreover show that the surface photovoltage and crystal photostriction can be simultaneously investigated by implementing a specific experimental protocol. Last, we will explain how the comparison with model photovoltaic type-II interfaces based on 2D transition metal dichalcogenides heterojunctions[5] shall help us in understanding the more complex case of BHJs.  

References

[1] M. Takihara, T. Takahashi, T. Ujihara, Appl. Phys. Lett., 2008, 93, 021902 (3pp).

[2] G. Shao, M. Glaz, M. Fei, H. Ju, D. Ginger, ACS Nano, 2014, 8, 10799-10807.

[3] P. A. Fernández Garrillo, Ł. Borowik, F. Caffy, R. Demadrille, B. Grévin, ACS Appl. Mater. Interfaces, 2016, 8, 31460–31468.

[4] Z. Schumacher, Y. Miyahara, A. Spielhofer, P. Grutter, Phys. Rev. Appl. 2016, 5, 044018 (6pp).

[5] Y. Almadori, D. Moerman, J. Llacer Martinez, Ph. Leclère, B. Grévin, accepted for publication in BJNANO

[6] Y. Almadori, N. Bendiab, B. Grévin, ACS Appl. Mater. Interfaces, 2018, 10, 1363-1373.

11:30 - 11:45
S10.2-O1
Heger, Jan-Frederik
Hochschule Esslingen
AFM Based 3D-Modeling of a Fuel Cell Electrode
Jan-Frederik Heger
Hochschule Esslingen, DE
Authors
Jan-Frederik Heger a, Matthias Simolka a, Arnulf Latz b, c, Volker Schmidt d, Renate Hiesgen a
Affiliations
a, University of Applied Sciences Esslingen, Kanalstrasse 33, 73728 Esslingen, Germany
b, German Aerospace Center, Institute of Engineering Thermodynamics, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany
c, Helmholtz Institute Ulm, Helmholtzstr. 11, 89081 Ulm, Germany
d, Ulm University, Helmholtzstr. 18, 89069 Ulm, Germany
Abstract

To understand the characteristics of fuel cell electrodes, a compound of platinum nanoparticle-covered mesoporous carbon und ionomer, they have to be examined at a nanometer scale. Their structure, porosity, and the properties of the inner surfaces at this scale determine their behavior. In particular, the lateral variation of surface energy and ionomer coverage has a major influence on water balance and transport resistance. To visualize this structure in two dimensional cuts SEM is being used. Combined with material-sensitive AFM measurements physical surface properties can be derived and modeled. In this presentation an approach to model the inner structure of a fuel cell electrode and the properties of inner surfaces is presented. The electrode is examined with material-sensitive AFM. Derived from deformation, adhesion and DMT-modulus mappings, an automated separation into the different phases is put into effect. The so constructed segmented image is rendered into 3D by its height information.

By ion milling further cuts of the same material are being done in layers of few micrometers. The AFM analysis of these series can be used to generate a larger 3D model of the electrode. A method to combine the series of layers to a 3D-model is suggested.

11:45 - 12:00
S10.2-O2
Toma, Francesca
Lawrence Berkeley National Laboratory
Nanoscale Imaging of Charge Carrier Transport in Water Splitting Photoanodes
Francesca Toma
Lawrence Berkeley National Laboratory, US
Authors
Johanna Eichhorn a, Francesca Maria Toma a
Affiliations
a, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, Califor- nia 94720, United States
Abstract

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Herein, we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity dependent current-voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space-charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. In addition, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on this material. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding local functional characteristics of water splitting photoanodes and their effects on the macroscopic performance, and for devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments. 

12:00 - 12:15
S10.2-O3
Nicoara, Nicoleta
International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal
Impact of Alkali post Deposition Treatments on the Grain Boundary Properties in Cu(In,Ga)Se2
Nicoleta Nicoara
International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal
Authors
Nicoleta Nicoara a, Roby Manaligod a, Philip Jackson b, Dimitrios Hariskos b, Wolfram Witte b, Sascha Sadewasser a
Affiliations
a, International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal
b, Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), 70563 Stuttgart, Germany
Abstract

A renewed interest in the role of grain boundaries in chalcopyrite-based thin-films solar cells is triggered by the highly efficient energy conversion obtained through the application of alkali post deposition treatments (PDT) [1,2]. Recent compositional studies indicate the presence of alkali metals at the Cu(In,Ga)Se2 (CIGSe) absorber surface and also in the bulk, localized especially at the grain boundaries (GB) [3-5]. Assessing the impact of GB properties on the efficiency requires a throughout insight into the nanoscale structural, chemical, and electronic properties of CIGSe. In this contribution, Kelvin probe force microscopy (KPFM) is used to study the local electronic properties at GBs by spatially resolved imaging of the surface potential. From a statistical analysis we obtain the alkali-dependent (K, Rb, and Cs) potential variation across the GBs and compare the results with those obtained for samples subjected to an 8 minutes chemical bath deposition (CBD) of Zn(O,S). Different types of GBs are mainly found: majority neutral (the potential difference between GB and grain interior at the sides is negligible) and GBs with positive/upward potential increase. In average, however, a lower barrier potential and more homogenous potential variation is found for RbF. The early stage deposition of solution-grown Zn(O,S) buffer slightly reduce (in average) the potential difference at GBs in both cases.

[1] P. Jackson et al., Phys. Status Solidi RRL 9, 28 (2015); P. Jackson et al., Phys. Status Solidi RRL 10, 583 (2016).

[2] Press Release: Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 22.9%; http://www.solar-frontier.com/eng/news/2017/1220_press.html.

[3] P.-P. Choi, O. Cojocaru-Miredin, R. Wuerz, and D. Raabe, J. Appl. Phys. 110, 124513 (2011).

[4] D. Abou-Ras, B. Schaffer, M. Schaffer, S. S. Schmidt, R. Caballero, and T. Unold, Phys. Rev. Lett. 108, 075502 (2012).

[5] O. Cojocaru-Miredin, P.-P. Choi, D. Abou-Ras, S. Schmidt, R. Caballero, and D. Raabe, IEEE J. Photovoltaics 1, 207 (2011).

12:15 - 12:30
S10.2-O4
Berger, Rüdiger
MPIP
Local Current Imaging through TiO2 Thin Films
Rüdiger Berger
MPIP
Authors
Rüdiger Berger a, Alexander Klasen a, Stefan Weber a, b, Ilka Hermes a, Wolfgang Tremel b, Ewald Johannes a, Amelie Axt a, Phillip Baumli a
Affiliations
a, MPI for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
b, Johannes Gutenberg University, 55128 Mainz, Germany
Abstract

We applied peak force based conductive scanning force microscopy (cSFM) to investigate local conductance differences in TiO2 anatase thin films. We found that the current perpendicular to the interface increased by two orders of magnitude after a UV-ozone (UVO) treatment. This increase in current is attributed to a reduction of oxygen vacancies at the surface after UVO-treatment. Cleaning, i.e. removal of hydrocarbons plays only a minor role.

Thin films of titanium dioxide (TiO2) are applied in electronics, e.g. thin film transistors, anode materials for Lithium ion batteries, photoanodes for water oxidation and as hole blocking layer in perovskite solar cells (PSC). TiO2 anatase films can have a lower conductance compared to electron blocking layers like P3HT, PCPDTBT and spiro-OMEOTAD [1]. Thus the conductivity of TiO2 films is crucial for the electron charge transport and it can limit the solar cell power conversion efficiency (PCE). Therefore, an improved conductance perpendicular trough this layer is highly desirable [2]. Various cleaning procedures were reported for anatase TiO2 thin films which result in an increase in PCE [3-7]. However, the cleaning mechanism and the local conductivity were not investigated in detail on a nanometer scale. We could show that the PCE of planar PSC can be increased by 2% to a maximum of 15.4% by a controlled UV-ozone (UVO) treatment. Finally, peak force based cSFM is stable for over 3 million singe force distance curves. This stability allows to compare cSFM current images quantitatively for differently treated samples.

References:

[1] T. Leijtens, et al., Adv. Mater. 2013, 25, 3227–3233.

[2] C. Liu, et al., ACS Appl. Mater. Interfaces 2015, 7, 1153–1159.

[3] L. Cojocaru, et al., Chem. Lett. 2015, 44, 674–676.

[4] F. Zhang, et al., Chem. Mater. 2016, 28, 802–812.

[5] X. Ma, et al., ChemPhysChem 2017, 1–9.

[6] W. Ke, et al., Nat. Commun. 2015, 6, 6700.

[7] I. S. Kim, et al., ACS Appl. Mater. Interfaces 2016, 8, 24310–24314.

Sol2D S6.3
Chair: Christophe Delerue
11:00 - 11:30
S6.3-I1
Bester, Gabriel
University of Hamburg
Atomistic Theory of Excitonic Effects in 2D Materials
Gabriel Bester
University of Hamburg, DE
Authors
Gabriel Bester a
Affiliations
a, Universität Hamburg
Abstract

One important feature of two-dimensional semiconductors (e.g., transition metal
dichalcogenide monolayers or phosphorene) is the strong Coulomb and Exchange interactions between photo-generated charge carriers resulting from the unusual carrier screening and its environmental sensitivity.   Particular interesting is the formation of bound states involving multiple carriers, such as positively and negatively charged trions (3 particles),  biexcitons and more. The low screening situation leads to quasiparticles that can remain stable even near-room temperature. I will address the corresponding situation describe some of the atomistic theoretical approaches available. En emphasis will be placed on the effects of the environment on the quasiparticle binding energies (exciton binding energies and the shift of exciton transitions relative to trions and biexcitons). Empirical theoretical approaches will be contrasted to more ab-initio descriptions, which still suffer from the high computational demand. Finally, the results will be compared to —the rather well established— situation in semiconductor quantum dots where screening effects tend to be rather well described by bulk screening.

11:30 - 12:00
S6.3-O1
Singh, Shalini
Ghent university
The Surface Chemistry of Colloidal II-VI Two-Dimensional Nanoplatelets
Shalini Singh
Ghent university, BE
Authors
Shalini Singh a, Renu Tomar a, Stephanie ten Brinck b, Jonathan De Roo a, Pieter Geiregat a, José C. Martins c, Ivan Infante b, Zeger Hens a
Affiliations
a, Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium
b, Department of Theoretical Chemistry, Vrije Universiteit, 1081 HV Amsterdam, Netherlands
c, NMR and Structural Analysis Unit, Ghent University, 9000 Gent, Belgium
Abstract

The nanocrystal-ligand interface of colloidal semiconductors has attracted significant research interest owing to their dominant role in tuning and tailoring the physio-chemical properties of such functional nanomaterials. For metal chalcogenide quantum dots (QDs), a comprehensive picture has emerged over the past decade on surface passivation and photoluminescence relationship. The surface of QDs is metal rich passivated by X-type ligands. Displacing them from the surface results in a marked drop of PLQY, which could be reversed by readsorption of ligands. DFT studies indicate that this is linked to the formation of under-coordinated surface Se which behave as traps for electrons or holes. QDs are nanometre-size crystallites with crystal facets of few square nanometres. This differs markedly from 2-D nanocrystals which are few monolayers thick with atomically flat top and bottom surfaces measuring hundreds of square nanometre e.g. CdSe nanoplatelets. A pronounced interplay between the ligand capping and optical properties of nanoplatelets has been reported. But, research on CdSe nanoplatelets is often hampered by their limited colloidal stability and unwanted stacking in bundles, two aspects that are linked to nanoplatelet-ligand interface. This combination of promising properties and cumbersome processing calls for an in-depth study of their surface chemistry. Here, the question as to whether concepts developed for multifaceted QDs can be transferred to nanoplatelets, which are terminated solely by atomically flat interfaces, stands out.

In this work, detailed study of the surface-chemistry of CdSe nanoplatelets is reported. We first introduce an improved synthesis strategy for nanoplatelets that yields colloidally stable and aggregation free nanoplatelets suspensions. Despite large surface area, these core-only nanoplatelets have a PLQY as high as 55%. Elemental analysis show nanoplatelets are Cd rich.1H NMR analysis show that Cd excess comes with a surface termination by X-type carboxylate ligands, a binding motif similar to QDs. Addition of an L-type ligand displaces cadmium-carboxylate complexes from surface. The displacement isotherm unravelled that the surface features adsorption sites with different binding energies for CdX2. We further emphasize the validity of a multiple-site model by DFT calculations, which yield a variation in binding sites from edges to the centre of the facet. Moreover, we analysed different types of mid-gap trap states and site dependent surface reconstruction the nanoplatelet undergoes with displacement of the ligands. Finally, this surface-model for CdSe nanoplatelets is most likely not restricted to CdSe only and should be considered when analysing the surface reactions of another 2-D nanoplatelet system.

12:00 - 12:30
S6.3-I2
Rodina, Anna
Ioffe Physical Technical Institute
Spin-Dependent Optical Properties of CdSe Nanoplatelets
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 our recent results on the optical and magneto-optical studies of charged and neutral excitons in ensemble of CdSe-based colloidal nanoplatets (NPLs). The influence of the temperature and external magnetic field on the time-resolved dynamics of photoluminescence (PL) allows us to distinguish between the recombination originating from neutral and changed excitons. We compare the dependence of the fine structure energy splitting of the neutral exciton on the NPLs thickness determined by different experimental methods with the theoretical calculations. We show that at low temperatures the PL in CdSe bare NPLs is determined by the radiative recombination of the lowest dark (spin-forbidden) exciton. We propose an effective mechanism of the dark exciton activation via the exchange interaction with surface dangling bonds. We discuss the mechanisms of the dangling bond spins alignment in external magnetic field and its effect on the exciton spin polarization [3].

The sign of the circular polarization of the PL induced in magnetic field allows us to distinguish between the recombination originating from negative or positive trions. The theoretical analysis of the magnetic field dependences of the degree of circular polarization (DCP) and of the Spin Flip Raman Scattering (SFRS) energy shift allows us to determine the effective g-factor controlling the Zeeman splitting of the spin sublevels of resident electrons, excitons and holes in the negative trion in external magnetic field. The maximum value of the DCP and the SFRS intensity allow us to extract an information about the preferable orientation of the NPLs in the ensemble.

This work was supported in part by the Russian Foundation for Basic Research (Grant No. 17-02-01063 and Grant No. 15-52-12015).

[1] E. V. Shornikova, L. Biadala, D. R. Yakovlev, V. F. Sapega, Y. G. Kusrayev, A. A. Mitioglu, M. V. Ballottin, P. C. M. Christianen, V. V. Belykh, M. V. Kochiev, N. N. Sibeldin, A. A. Golovatenko, A. V. Rodina, N. A. Gippius, A. Kuntzmann, Ye Jiang, M. Nasilowski, B. Dubertret, M.Bayer, Nanoscale 10, 646 (2018).

[2] E. V. Shornikova, L. Biadala, D. R. Yakovlev, D. H. Feng, V. F. Sapega, N. Flipo, A. A. Golovatenko, M. A. Semina, A. V. Rodina, A. A. Mitioglu, M. V. Ballottin, P. C. M. Christianen, Y. G. Kusrayev, M. Nasilowski, B. Dubertret, M. Bayer, Nano Letters 18, 373 (2018).

[3] A.V Rodina, A.A. Golovatenko, E.V. Shornikova,  D.R. Yakovlev, Al.L. Efros, Journal of Electronic Materials 4, 2018

WatSpl S2.3
Chair: Bruce Parkinson
11:00 - 11:30
S2.3-I1
Marschall, Roland
Justus Liebig University Giessen
Nanostructured Spinel Ferrite Materials for Photoelectrochemical Water Splitting
Roland Marschall
Justus Liebig University Giessen, 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. Since 07/2013, he is Emmy-Noether Young Investigator at the Justus-Liebig-University Giessen. His current research interests are heterogeneous photocatalysis, especially photocatalytic water splitting using semiconductor mixed oxides, and synthesis of oxidic mesostructured materials for energy applications. In 2014, he received the ADUC habilitation award for best German young research group leader.

Authors
Kristin Kirchberg a, Roland Marschall a
Affiliations
a, Justus Liebig University Giessen, Heinrich Buff Ring 58, Giessen, DE
Abstract

We have developed a straightforward microwave synthesis protocol using acetylacetonate and acetate precursors to produce nanocrystals of the earth-abundant cubic spinel ferrites MgFe2O4 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 or seed-mediated growth method. 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 thin films were fabricated by sol-gel synthesis, using a polymer-templating approach previously reported by Haetge et al.[3]. By means of the amphiphilic diblockcopolymer poly(isobutylene)-block-poly(ethylene oxide) (PIB50-b-PEO45), ordered mesopores are obtained after dip-coating by evaporation-induced self-assembly [4] 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 ZnFe2O4 with a crystallite size of 15 nm of. 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.

 

References

[1] C. Suchomski, B. Breitung, R. Witte, M. Knapp, S. Bauer, T. Baumbach, C. Reitz, T. Brezesinski, Beilstein J. Nanotechnol. 2016, 7, 1350

[2] K. Kirchberg, A. Becker, A. Bloesser, T. Weller, J. Timm, C. Suchomski, R. Marschall, J. Phys. Chem. C 121 (2017) 27126−27138

[2] J. Haetge, C. Suchomski, T. Brezesinski, Inorg. Chem. 2010, 49, 11619.

[3] C. J. Brinker et al., Adv. Mater. 1999, 11, 579.

11:30 - 11:45
S2.3-O1
Ahn, Hyo-Jin
A Strategy to Decrease the High Onset Potential of Hematite Photoanodes by Gradient Doping and Decoration with Zn-Co Layered Double Hydroxide
Hyo-Jin Ahn
Authors
Hyo-Jin Ahn a, b, Anandarup Goswami b, Francesca Riboni a, Stepan Kment b, Alberto Naldoni b, Radek Zboril b, Patrik Schmuki a
Affiliations
a, University of Erlangen-Nürnberg, IMEET, Martensstraße 7, Erlangen, 91058, DE
b, Regional Centre of Advanced Technologies and Materials
Abstract

Over the past years, α-Fe2O3 (hematite) has been considered as a promising photoanode material in photoelectrochemical (PEC) water splitting. In spite of significant success in obtaining relatively high PEC performance, the main drawbacks hindering practical application at hematite are related to an intrinsically poor charge transport and an inferior kinetics of the oxygen evolution reaction on the photoelectrode surface. In this presentation, we will discuss a strategy for reducing onset potential of hematite photoanode by coupling of gradient Sn4+ ion doping and Zn-Co LDH serving as a highly active OER catalyst. The gradient Sn doping in hematite influences the space charge layer (SCL), which is critical for charge separation and thus for an enhanced photoelectrochemical water splitting performance (PEC). The OER catalyst, i.e. Zn-Co LDH nano-sheets have been synthesized by a simple microwave treatment. The synergistic effect of Zn-Co LDH decoration and gradient Sn4+ doping results in decreasing the onset potential of more than 300 mV, from 0.86 VRHE to 0.54 VRHE, and in increasing the photocurrent density from 0.60 mA/cm2 to 2.00 mA/cm2 at 1.50 VRHE. Our approach demonstrates strategies to overcome onset potential limitations as well as poor OER properties of hematite and leads to a remarkably improved PEC water splitting performance.

11:45 - 12:00
S2.3-O2
Nong, Hong Nhan
Technische Universität Berlin
Operando Studies of Hole-Doped IrNiOx core-shell electrocatalysts for Water Oxidation in acidic Environment
Hong Nhan Nong
Technische Universität Berlin
Authors
Hong Nhan Nong a, d, Tobias Reier a, Paul Paciok b, Detre Teschner c, d, Marc Heggen b, Valeri Petkov e, Robert Schlögl c, d, Travis Jones d, Peter Strasser a
Affiliations
a, Dept. of Chemistry, Technical University Berlin, Strasse des 17. Juni 124, TC 03, 10623 Berlin, Germany
b, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52428 Jülich, Germany
c, Dept. of Inorganic Chemistry, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
d, Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany
e, Dept. of Physics, Central Michigan University, Mt. Pleasant, MI 48859, USA.
Abstract

The electro-oxidation of water to oxygen (oxygen evolution reaction, OER) in (photo-)electrolytic devices yields the electrons and protons required to form molecular fuels1,2: This is why it is expected to play a major role in the development of future (photo-)electrochemical energy conversion and storage technologies. However, the slow rate of water oxidation remains a key challenge that requires fundamental understanding and the design of more active and stable OER electrocatalysts3,4.

To further this development, we probe the local geometric ligand environment and the electronic metal states of O-coordinated Ir centers in Ni-leached IrNi@IrOx metal-oxide core-shell nanoparticles5 — one of the most active OER electrocatalysts known to date — under catalytic oxygen evolution condition using operando spectroscopic techniques, resonant high-energy XRD and differential atomic pair correlation analysis, with support of density functional theory (DFT) calculations. Ni-leaching generates lattice vacancies, which in turn produce uniquely shortened Ir-O metal-ligand bonds and an unusually large number of d-band holes in the Ir oxide shell under OER. DFT calculations show this increase in formal Ir oxidation state drives the formation of O 2p holes on the oxygen ligands in direct proximity to lattice vacancies, resulting in the highly covalent and uniquely short Ir-O bonds seen experimentally. We argue the electrophilic character of these ligands renders them susceptible to nucleophilic acid-base-type O-O bond formation at reduced kinetic barriers, resulting in strongly enhanced OER reactivities. Together, our findings advance both our fundamental understanding of the exceptional reactivity of bimetallic core-shell Ir catalysts and provide a roadmap to tailoring of other “hole-doped” core-shell catalysts for water oxidation at a molecular level.

References

Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. Chemcatchem 2010, 2, 724.

Olah, G. A.; Goeppert, A.; Prakash, G. K. S. J. Org. Chem. 2009, 74, 487.

Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. Science 2011, 334, 1383.

Reier, T.; Nong, H. N.; Teschner, D.; Schlögl, R.; Strasser, P. 2017, 7, 1601275.

Nong, H. N.; Gan, L.; Willinger, E.; Teschner, D.; Strasser, P. Chem. Sci. 2014, 5, 2955.

12:00 - 12:15
S2.3-O3
Toimil-Molares, Maria Eugenia
GSI Helmholtzzentrum
Photoelectrochemical Performance of Arrays of Cu2O/TiO2 and Au/Cu2O/TiO2 Nanowires Fabricated by Electrodeposition
Maria Eugenia Toimil-Molares
GSI Helmholtzzentrum, DE
Authors
Maria Eugenia Toimil-Molares a, Florent Yang a, Dimitri Korjakin a, Christina Trautmann a, b, Christopher Schröck a, b
Affiliations
a, GSI Helmholtz Centre for Heavy Ion Research, Planckstrasse 1, Darmstadt, 64291, DE
b, Institute of Material Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany
Abstract

For semiconductor nanowire structures the ratio of the minority charge carrier diffusion length over the light absorption depth is significantly reduced compared to bulk materials. In addition, nanostructured semiconductors exhibit larger surface-to-volume ratio. The dimensions of the nanostructures and their geometrical arrangement influence relevant processes such as light absorption, as well as charge separation and transport. Ion-track nanotechnology combined with electrodeposition and atomic layer deposition, enables the controlled synthesis of 3D architectures of semiconductor nanostructures with tailored composition, size and density.

Among the various materials studied as photocathodes for solar hydrogen production, Cu2O is a promising candidate with a predicted solar-to-hydrogen conversion efficiency of ~18%. Moreover, Cu2O is cheap, earth-abundant, non-toxic, and is also scalable and compatible with low-cost fabrication processes. Currently, the main challenge for Cu2O-based photocathodes is their chemical instability in aqueous solution, which can be improved by the use of suitable passivation coatings.

Here, we present the synthesis and characterization of two types of semiconductor nanowire-based photocathodes: (i) highly textured single-crystalline p-type Cu2O nanowire arrays prepared by electrodeposition in etched ion-track polymer membranes, (ii) Au/Cu2O core-shell nanowire arrays prepared by electrodeposition of Cu2O on arrays of free-standing single-crystalline Au nanowires. Polymer membranes with controlled nanochannel density (typically 108 - 1010cm-2) and channel diameter (~20 - 250 nm) are fabricated by swift heavy ion irradiation and selective chemical etching. Subsequently, either Cu2O or Au nanowires with lengths up to 10 µm are synthesized by electrodeposition in the etched nanochannels. Optimized deposition conditions yield single-crystalline nanowires of both materials. After electrodeposition, the polymer membranes are dissolved in an organic solvent. The from the template released Cu2O nanowire arrays are directly coated with a thin and conformal TiOpassivation film by atomic layer deposition (ALD). The Au nanowire arrays, in turn, are first coated with a Cu2O layer by electrodeposition and then with a TiOfilm by ALD. For optimization, the thickness of both layers is systematically varied during the synthesis.

The photoelectrochemical performance of both types of nanowire-based photoelectrodes is studied as a function of parameters such as nanowire length, diameter, and density. In particular, the influence of the core-shell geometry, and the resulting minimization of the diffusion length for both minority and majority charge carriers is discussed.

12:15 - 12:30
S2.3-O4
Bubeck, Cora
University of Stuttgart
Perovskite-type Oxynitrides LaTaO2N and LaTaON2 – Synthetic Strategies
Cora Bubeck
University of Stuttgart
Authors
Cora Bubeck a, Marc Widenmeyer a, Gunther Richter b, Mauro Coduri c, Eduardo Salas Colera c, Songhak Yoon a, Frank Osterloh d, Anke Weidenkaff a
Affiliations
a, University of Stuttgart, Institute for Material Science, Heisenbergstraße 3, 70569 Stuttgart, Germany
b, Central Scientific Facility Thin Film Laboratory, Max Planck Institute for Intelligent Systems, Heisenbergstraße 3, 70569 Stuttgart, Germany
c, European Synchrotron Radiation Facility (ESRF), 71 Avenue des Martyrs, 38000 Grenoble, France
d, Department of Chemistry, University of California, One Shields Avenue, Davis, CA, 95616, USA
Abstract

LaTaON21,2 is a well-known perovskite-type material with defined composition, oxidation state, and physical properties, which make it favorable for solar water splitting.3 This Ta5+-containing oxynitride can be synthesized, e.g. from LaTaO4. The crucial synthetic step for perovskite-type oxynitrides is the ammonolysis, typically a thermal treatment in flowing NH3. Several parameters such as temperature and heating ramp, reaction time, and gas flow rate have to be carefully adjusted.4,5 Besides, the anionic composition can be altered via the adjustment of the precursor reactivity. A thorough tuning of all above mentioned parameters allowed us a controlled variation of the anionic composition from LaTaON2 to LaTaO2N. This consequently leads to a reduction of Ta5+ to Ta4+. A great influence of this oxidation state change on the physical properties, particularly the light absorption, the charge separation, and the surface redox activity, is expected.6

References:

(1)         Marchand, R.; Pors, F.; Laurent, Y. Ann. Chim. Fr. 1991, 16, 553–560.

(2)         Marchand, R.; Antoine, P.; Laurent, Y. J. Solid State Chem. 1993, 107, 34–38.

(3)         Liu, M.; You, W.; Lei, Z.; Takata, T.; Domen, K.; Li, C. Chinese J. Catal. 2006, 27 (7), 556–558.

(4)         Ebbinghaus, S. G.; Abicht, H. P.; Dronskowski, R.; Müller, T.; Reller, A.; Weidenkaff, A. Prog. Solid State Chem. 2009, 37 (2-3), 173–205.

(5)         Widenmeyer, M.; Peng, C.; Baki, A.; Xie, W.; Niewa, R.; Weidenkaff, A. Solid Sate Sci. 2016, 54, 7–16.

(6)         Bubeck, C.; Widenmeyer, M.; Richter, G.; Coduri, M.; Salas Colera, E.; Yoon, S.; Osterloh, F.; Weidenkaff, A. in preparation.

12:30 - 14:30
Lunch
14:30 - 16:00
Internal Project Meeting
NCPhot S4.6
Chair: Maksym Kovalenko
14:30 - 15:00
S4.6-O1
Gupta, Shuchi
The Institute of Photonic Sciences-ICFO
Colloidal Quantum Dots-Graphene based Broadband Photodetector
Shuchi Gupta
The Institute of Photonic Sciences-ICFO, ES
Authors
Shuchi Gupta a, Stijn Goossens a, Gabriele Navickaite a, Carles Monasterio a, Juan José Piqueras a, Raúl Pérez a, Gregory Burwell d, Ivan Nikitskiy a, Tania Lasanta a, Teresa Galán a, Eric Puma a, Alba Centeno b, Amaia Pesquera b, Amaia Zurutuza b, Frank Koppens a, c, Gerasimos Konstantatos a, c
Affiliations
a, The institute of photonic sciences, The Barcelona institute of science and technology, Av. Carl Friedrich Gauss 3, Castelldefels (Barcelona), Spain
b, Graphenea SA, Tolosa Hiribidea 76, Donostia - San Sebastian, Spain
c, ICREA – Institució Catalana de Recerça i Estudis Avançats, Lluis Companys 23, 08010 Barcelona, Spain
d, Centre for NanoHealth, Swansea University
Abstract

Semiconductor colloidal quantum dots (QDs) offer a realm of opportunities especially, in terms of tuning the band gap, manipulating the trap states in a very precise manner, controlling their electronic doping character and their ease of manufacturing and processing in devices. Moreover, when combined with 2D materials, the resultant structures can be used for a diverse range of applications offering photodetectors with exceptionally high responsivity and sensitivity.1 During my presentation, I will talk about quantum dot-graphene hybrid photodetector devices for food inspection, spectrometers, imaging purposes, monitoring vital health parameters and night vision leveraging the unique opportunity of detecting photons from the UV up to the short-wave infrared in a single material platform.  In this photodetector, we make use of the large absorption cross section of QDs and high carrier mobility of graphene to demonstrate our complementary metal-oxide–semiconductors (CMOS) compatible infrared image sensors.2 Unlike commercial detectors, the hybrid detector can be used simultaneously in UV, visible and infrared light conditions at room temperature with measured detectivity upto 1012 Jones. Such detectors exhibit response times of 0.1–1 ms that make them suitable for video frame rate as Stijn Goossenswell as spectrometry applications.

 

[1] G. Konstantatos, et al., Nature Nanotechnol., 7 (June 2012)

[2] Goossens et al., Nat. Phot. 11 (June 2017)

 

15:00 - 15:30
S4.6-O2
Scheele, Marcus
University of Tuebingen
Coupled Organic-Inorganic Nanostructures for Optical Switches
Marcus Scheele
University of Tuebingen, DE
Authors
Marcus Scheele a
Affiliations
a, University of Tuebingen, Auf der Morgenstelle 18, Tuebingen, 72076, DE
Abstract

Optical switches are key components for data processing on the basis of “silicon photonics”, in which they perform the crucial conversion of a photonic information from an optical fiber into an electric information for a silicon-based processing unit. The status of the switch is controlled by an external light source, emitting at a wavelength suitable to be absorbed by the conductive channel to photo-induce additional charge carriers and modulate the current output of the switch in close analogy to a classic transistor. This presentation details how hybrid superlattices of semiconducting nanocrystals and organic pi-systems with long-range order are applied as active layers in functional optical switches. The particular novelty for optical switching is an activated absorption mechanism, in which stimulation with one optical signal sensitizes the material towards an amplified recognition of a second optical stimulus. Several examples with different material combinations are presented and the importance of exciton formation as well as charge transfer across the inorganic-organic interface is discussed.

15:30 - 16:00
S4.6-O3
Pradhan, Santanu
ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology
Highly Efficient PbS Colloidal Quantum Dot Based Infrared Light Emitting Diodes through Suprananocrystalline Matrix Engineering
Santanu Pradhan
ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, ES
Authors
Santanu Pradhan a, Francesco DiStasio a, Yu Bi a, Shuchi Gupta a, Sotirios Christodoulou a, Alexandros Stavrinadis a, Gerasimos Konstantatos a, b
Affiliations
a, 1ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
b, ICREA—Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spain
Abstract

The rapid growth of colloidal quantum dot (CQD) opto-electronics establishes it as one of the most promising new generation technology related to photo-detection, photovoltaic (PV) and light emitting diodes (LEDs) [1]. Although CQD based LEDs enjoy tremendous success in the visible range [2], not so can be claimed about their infrared counterpart, which has a number of applications, including night vision, remote sensing, spectroscopy and biological imaging. The main reason for the poor performance in the infrared is the low photoluminescence quantum yield (PLQY) of ligand exchanged CQD solids. Several techniques like synthesis of core-shell structures [3], chemical passivation with perovskite matrix [4] etc. improve the efficiency but still they are far from their potential. We report here a passivation technique based on suprananocrystalline matrix engineering which leads to record external quantum efficiency (EQE).

Mixed ligand treatment on short-wave infrared (SWIR) based PbS quantum dot solar cell leads to record high PV performance [5]. Yet, the ligand treated SWIR QD solids showed a mere 2% PLQY due to a large amount of non-radiative recombination. The matrix presented here, passivates the non-radiative recombination channels and improve the PLQY over 60%. Careful optimization of the matrix and device architecture leads to record peak EQE of ~7.9% and peak power conversion efficiency of ~9.3% with emission at 1400 nm. Furthermore, PV devices based on this passivation technique showed record high open circuit voltage (0.69 V corresponding to 0.92 eV QD bandgap) confirming the effectiveness of the passivation.

References:

[1] Kagan, C. R., Lifshitz, E., Sargent, E. H., & Talapin, D. V. Buliding devices from colloidal quantum dots. Science 353, aac5523 (2016).

[2] Shirasaki, Y., Supran, G. J., Bawendi, M. G. & Bulović, V. Emergence of colloidal quantum-dot light-emitting technologies. Nat. Photonics 7, 13–23 (2013).

[3] Supran, G. J. et al. High-performance shortwave-infrared light-emitting devices using core–shell (PbS–CdS) colloidal quantum dots. Adv. Mater. 27, 1437–1442 (2015).

[4] Gong, X. et al. Highly efficient quantum dot near-infrared light-emitting diodes. Nat. Photonics 10, 253–257 (2016).

[5] Bi, Y. et al. Infrared solution‐processed quantum dot solar cells reaching external quantum efficiency of 80% at 1.35 µm and JSC in excess of 34 mA cm-2. Adv. Mater. 30, 1704928 (2018).

16:00 - 16:30
S4.6-I2
Lhuillier, Emmanuel
Sorbonne Universités, UPMC University Paris 06, CNRS-UMR 7588, Institut des Nanosciences de Paris, 4 place Jussieu, 75005 Paris, France
Intraband transition in narrow band gap nanocrystals
Emmanuel Lhuillier
Sorbonne Universités, UPMC University Paris 06, CNRS-UMR 7588, Institut des Nanosciences de Paris, 4 place Jussieu, 75005 Paris, France

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
Emmanuel lhuillier a, Adrien Robin a, Clement Livache a, Bertille Martinez a, Nicolas goubet a
Affiliations
a, Sorbonne Universités, UPMC Univ. Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, 4 place Jussieu, 75005 Paris, France
Abstract

Pushing nanocrystals optical features toward the infrared range can be a material challenge. Two strategies can be explored. Either the use of narrow band gap materials for the design of narrow interband transitions, either the use of doped semiconductor presenting intraband transition in the mid infrared [1]. Here, I will focus on this second strategy while using mercury chalcogenides compounds as optically active material.

At first, I will present our strategy to explore the synthesis of HgSe [2] and HgTe [3] self-doped nanocrystals which has been used to tune absorption up to the THz range, typically from 3 µm to 60 µm for peak absorption and up to 200µm for cut-off wavelength.

Because the doping plays a key role in this material, I will discuss the origin of self-doping and how surface chemistry can be used to tune accurately its magnitude [4].

In the last part of the talk, I will present some results relative to the use of this intraband transition for mid IR photodetection. I will show how an heterostructure made of HgSe and HgTe can be used to uncouple the problem of absorption and charge transport [5]. This paves the way for the design of even more complex colloidal heterostructure on the model of quantum cascade system develloped for III-V material.

[1] Emergence of intraband transitions in colloidal nanocrystals, A. Jagtap, C. Livache, B. Martinez, J. Qu, Audrey Chu, C. Gréboval, N. Goubet, E. Lhuillier, Opt. Mater. Express 8(5), 1174-1183 (2018)

[2] Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz, E. Lhuillier, M. Scarafagio, P. Hease, B. Nadal, H. Aubin, X. Z. Xu, N. Lequeux, G. Patriache, S. Ithurria, B. Dubertret, Nano Lett 16, 1282 (2016)

 [3] Terahertz HgTe nanocrystals: beyond confinement, N. Goubet, A. Jagtap, C. Livache, B. Martinez, H. Portales, X. Zhen Xu, R.P.S.M. Lobo, B. Dubertret, E. Lhuillier, J. Am. Chem. Soc. 140, 5053 (2018).

[4] Surface Control of Doping in self-doped Nanocrystals, A. Robin, C. Livache, S. Ithurria, E. Lacaze, B. Dubertret, E. Lhuillier, ACS Appl. Mat. Interface 8, 27122−27128 (2016).

 [5] Wavefunction engineering in HgSe/HgTe colloidal heterostructures to enhance mid infrared photoconductive properties, N. Goubet, C. Livache, B. Martinez, X. Z. Xu, S. Ithurria, S. Royer, H. Cruguel, G. Patriarche, A. Ouerghi, M. Silly, B. Dubertret, E. Lhuillier, Nano Lett 18 (2018)

16:30 - 17:00
S4.6-I1
Cheyns, David
imec
Quantum Dot Based Imagers: the Route to Wafer Scale Production
David Cheyns
imec, BE
Authors
David Cheyns a, Epimitheas Georgitzikis a, b, Pawel Malinowski a
Affiliations
a, Imec vzw, Kapeldreef 75, B-3001 Leuven
b, Department of Electrical Engineering, KULeuven, Kasteelpark Arenberg 10,Heverlee, 3001, BE
Abstract

State-of-the-art imagers use silicon circuitry for pixel readout, in combination with either a silicon absorber (visible region) or flip-chip bonded III-V materials (infrared region). Thin-film layers show a promise to replace these absorber layers, as the optical cross-talk can be reduced, and a heterogeneous integration on silicon chips enables further downscaling of the pixel size. In this talk we will discuss our progress and the challenges to incorporate colloidal quantum dot materials into a fab compatible process flow. Challenges lay in translating the chemical vocabulary and incorporating the silicon production fab restrictions into the device optimization. We will show initial results of incorporating infrared PbS based materials (950 nm and 1450 nm quantum peak absorption) into a silicon fab compatible stack. From a device aspect, the focus lays on high EQE values in combination with low noise (= dark current limited). From an integration aspect, the available contact materials are limited, and all layers need patterning using photo-lithography to enable processing on 200 or 300 mm wafers. These two aspects (device and integration) should be looked at jointly. A screening and optimization method will be presented, including a full opto-electronic characterization to determine the optimal stack constitution. This method enables a quick uptake of next generation quantum dot (or other thin-film) materials into wafer scale imager production.

SPMEn S10.3
Chair not set
14:30 - 15:00
S10.3-I1
Teichert, Christian
Montanuniversitaet Leobe
Advanced AFM Techniques to Study Photoconductivity of Inorganic and Organic Semiconductor Nanostructures
Christian Teichert
Montanuniversitaet Leobe, AT

Christian Teichert studied Physics in Halle, Germany; Ph.D. in 1992; 1992/93 Postdoc (Alexander von Humboldt fellowship) Research Center Juelich, Germany; 1993-1996 Postdoc UW Madison, U.S., 1996/97 Postdoc, Max Planck Institute of Microstructure Physics, Halle, Germany; 1997 Assistant Professor, University of Leoben, Austria, Head of Scanning Probe Microscopy Group Leoben; since 2001 Associate Professor, University of Leoben.

2002: Gaede Prize of the German Vacuum Society. 2014: reactivated fellowship of the Alexander von Humboldt Foundation.

Areas of expertise: Scanning Probe Microscopy based nanostructure research with focus on structure and electrical and mechanical properties of inorganic and organic semiconductors, two-dimensional materials, and cellulose based materials.

Organizer of several International Nanoscience Workshops and Symposia. Currently, he is the elected vice-chair of the Nanometer Structure Division of the International Union of Vacuum Science, Technology and Application (IUVSTA).

Authors
Christian Teichert a
Affiliations
a, Montanuniversitaet Leoben
Abstract

Besides morphological characterization, atomic-force microscopy (AFM) based techniques can also successfully be employed to study electrical and optoelectronic properties on the nanometer scale via conductive atomic-force microscopy (C-AFM) and Kelvin Probe Force Microscopy [1]. This will be demonstrated for upright standing ZnO nanorods [2,3] radial junction Si solar cells [4]. With respect to photovoltaic applications, the operation of these techniques under simultaneous illumination with white or monochromatic light - which are called photoconductive AFM (PC-AFM) and photo-assisted KPFM (PA-KPFM) is demonstrated [3,4]. For crystalline needles composed of small organic semiconductor molecules grown on graphene, the light-induced  charge spreading is measured by electrostatic force microscopy.

 

Work has been performed in collaboration with A. Andreev, I. Beinik, A. Nevosad, A. Matković, M. Mirkowska, M. Kratzer, K. Gradwohl, A. Matković, (Leoben), Y. Kozyrev, S. Kondratenko  (Kiev), M. Müller, A. Hývl, A. Vetushka, M. Ledinský, A. Fejfar (Prague), and B. Vasić, R. Gajić (Belgrade).

 

[1] C. Teichert, I. Beinik, in “Scanning Probe Microscopy in Nanoscience and Nanotechnology”, Vol. 2, Edited by B. Bhushan, (ISBN 978-3-642-10496-1) (Springer-Verlag, Berlin, 2011), pp. 691-721.

[2] I. Beinik, et al., J. Appl. Phys. 110 (2011) 052005.

[3] I. Beinik, et al., Beilstein J. Nanotechnol. 4 (2013) 208.

[4] M. Müller, et al., Jap. J. Appl. Phys. 54 (2015) 08KA08.

15:00 - 15:15
S10.3-O1
Simolka, Matthias
University of Applied Sciences Eslingen
Local Analysis of Li-ion Concentration and Diffusion-Migration Coefficients in Lithium-Silicon Electrodes
Matthias Simolka
University of Applied Sciences Eslingen
Authors
Matthias Simolka a, Christopher Heim b, K. Andreas Friedrich b, Renate Hiesgen a
Affiliations
a, University of Applied Sciences Esslingen, Kanalstrasse 33, 73728 Esslingen, Germany
b, German Aerospace Center, Institute of Engineering Thermodynamics, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany
Abstract

Electrochemical strain microscopy (ESM) and its derivative methods are an essential set of tools to study ionic solid materials and to help understand the difficulties in the design of new materials for next generation battery systems and their aging behavior.

A variation of the standard ESM proposed by Balke et al. was developped. We studied the diffusion-migration behavior of Li ions in nanostructured silicon anodes. In standard ESM, the Li-ions in the volume underneath the AFM tip are excited by an alternating electrical field and cause a movement of the sample surface. Contrary to the standard ESM our approach applies a longer voltage step of several milliseconds with an AC voltage overlaid. This does not only lead to a vibration of the ions but also to a change in ion concentration in the vicinity of the AFM tip. The correlated volume expansion and the amplitude in sample height increase are extracted for every image point using Matlab. The Silicon anodes were cycled against Lithium. 

Electrodes before and after cycling were analysed. In the ESM images, phases with different diffusion-migration coefficients appear. High initial diffusion-migration coefficiants and Li-concentration are assumed to depend on the crystal orientation. The fresh electrode has areas with high Li-concentration and well separated areas with no signal, whereas in the aged sample the Li-concentration dropped significantly. In conclusion, the observed capacity loss can be explained by the loss of Lithium ions.

15:15 - 15:45
S10.3-I2
Glatzel, Thilo
University of Basel
Dye Precursor Molecules on NiO(001) Studied by Non-Contact Atomic Force Microscopy
Thilo Glatzel
University of Basel, CH
Authors
Sara Freund a, Antoine Hinaut a, Edwin C. Constable b, Ernst Meyer a, Catherine Housecroft b, Thilo Glatzel a
Affiliations
a, Department of Physics, University of Basel, Switzerland
b, Department of Chemistry, University of Basel, Switzerland
Abstract

The properties of NiO, such as charge transport or optoelectronic characteristics, can be modified by functionalization with organic molecules. These kinds of organic/inorganic surfaces are of great interest, in particular, for the design of hybrid devices like dye sensitized solar cells [1]. However, a key parameter in the design of optimized interfaces is not only the choice of the compounds but also the properties of adsorption. Thus, fundamental studies of such hybrid systems at the nanoscale are desirable. So far, characterization of adsorbates at ambient temperature through spectroscopy techniques, such as x-ray photoelectron spectroscopy, has been limited to large agglomerates or self- assembled molecules. Recently, first studies of the adsorption properties of single molecules on NiO measured by force microscopy at low temperatures have been published [2]. This limit can be stretched to the level of individual adsorbates measured by means of non-contact atomic force microscopy at room temperature.

We investigated the deposition of a 2,2´-bipyridine based molecule, functionalized with carboxylic acid anchoring domains on a NiO(001) single crystal surface [3]. Depending on the coverage, single molecules, groups of adsorbates with random or recognizable shapes, or islands of closely packed molecules could be identified. Single molecules and self assemblies, as visible in the image on the right side, are resolved with submolecular resolution showing that they are lying flat on the surface with the 2,2´-bipyridine in a trans-conformation. Only in the close-packed form was a measurable charge transfer from the NiO to the molecular layer of 0.3 electrons per molecule observed independent on the molecular orientation of the islands.

[1] C. Wood et al., Phys. Chem. Chem. Phys. 18 (2016) 10727.
[2] A. Schwarz et al., J. Phys. Chem. C 117 (2013) 1105.
[3] S. Freund et al., submitted to ACS Nano (2017).

15:45 - 16:15
S10.3-I3
Wittstock, Gunther
Carl von Ossietzky University of Oldenburg
Scanning Electrochemical Microscopy of Energy Materials
Gunther Wittstock
Carl von Ossietzky University of Oldenburg, GE

Gunther Wittstock studied chemistry at the University of Leipzig and obtained a PhD in Analytical Chemistry. After stays at the University of Cincinnati (1992-1993) and at the Technical University of Munich, he prepared his habilitation at the Wilhelm-Ostwald-Institute of the University of Leipzig. In 2001 he became full professor of Physical Chemistry at the Carl von Ossietzky University of Oldenburg where he runs an electrochemistry group. His research interest is focused on localized interfacial charge transfer reaction which he investigates within a larger variety of application. This includes biomimetic interfaces, functional organic thin films on the basis of self-assembled monolayers, patterned organic thin films, organic-inorganic functional materials, nanoparticle assemblies at interfaces, localized electrocatalytic reactions in particular oxygen reduction reaction in different media. Recently, there has been a particular emphasis on molecular reaction in energy conversion systems. He uses scanning electrochemical microscopy which is complemented by surface spectroscopies and other microscopic techniques. Among others, his achievements have been recognized by a grant of the Alexander von Humboldt Foundation and the Klaus Jürgen Vetter Award of the International Society of Electrochemistry (ISE). Currently he is a member of the Executive Council and Treasurer of ISE.

Authors
Gunther Wittstock a
Affiliations
a, Carl von Ossietzky University of Oldenburg, 26129, Oldenburg, Germany, GE
Abstract

Energy materials provide a large multitude interfaces with inhomogeneously distributed reaction rates often directly dictating their functional properties.1 Local characterization for deriving structure reactivity relationships can be achieved by scanning electrochemical microscopy (SECM) but poses a set of challenges:

Composite materials make preparation of smooth uniform surfaces impossible. SECM studies have to deal with rough and even porous electrodes as in the case of dye-sensitized solar cells2 and gas-diffusion electrodes for fuel cells and lithium-oxygen batteries.3

Shielding reactive surfaces from a typical laboratory environment may be mandatory.1 Selective passivation has been studied negative electrodes in lithium-ion batteries (pyrolytic graphite,4 graphite composite5 and lithium).6

SECM can be used to distinguish between different parallel reaction pathways as has been demonstrated for gas diffusion electrodes.

 

Entirely new opportunities are enabled by liquid-liquid interfaces which can be chemically polarized to support light driven reaction. Photochemically active materials can be arranged and regenerated at these soft interfaces.7

 

Important contributions of my PhD students and Postdocs H. Bülter, P. Schwager, E. dos Santos Sardinha, S. Scarabina, I. Schmidt, I. Plettenberg, S. Rastgar and cooperation partners F. Peters, D. Fensker, J. Schwenzel (IFAM), M. Stenard, M. Wilkening (TU Graz) are gratefully acknowledged. Funding: DFG, State of Lower Saxony, Humboldt Foundation and the Conselho Nacional Brazil.

 

1     Bülter, Schwager, Fenske, Wittstock, Electrochim. Acta, 2016, 199, 366.

2     Shen, Nonomura, Schlettwein, Zhao, Wittstock, Chem. Eur. J., 2006, 12, 5832; Tefashe, Nonomura, Vlachopoulos, Hagfeldt, Wittstock, J. Phys. Chem. C, 2012, 116, 4316; Ellis, Schmidt, Hagfeldt, Wittstock, Boschloo; J. Phys. Chem. C 2015, 119, 21775; Schmidt, Plettenberg, Kimmich, Ellis, Witt, Dosche, Wittstock, Electrochim. Acta, 2016, 222, 735.

3     Schwager, Dongmo, Fenske, Wittstock, Phys. Chem. Chem. Phys., 2016, 18, 10774; Schulte, Liu, Plettenberg, Kuhri, Lüke, Lehnert, Wittstock; J. Electrochemcial Soc. 2017, 164, F873.

4     Bülter, Peters, Wittstock, Energy Technol. 2016, 4, 1486.

5     Bülter, Peters, Schwenzel Wittstock, Angew. Chem., Int. Ed., 2014, 53, 10531–10535; Schwager, Bülter, Plettenberg, Wittstock; Energy Technol. 2016, 4, 1472.

6     Bülter, Peters, Schwenzel, Wittstock; J. Electrochem. Soc. 2015, 162, A7024.

7     Rastgar, Pilarski Wittstock, Chem. Commun., 2016, 52, 11382–11385; Rastgar, Wittstock, J. Phys. Chem. C 2017, 121, 25961.

 

16:15 - 16:30
S10.3-O2
Palacios-Lidon, Elisa
Universidad de Murcia
Nanoscale Photogenerated Charge-Transfer Study of P3HT Nanoparticles /Graphene Oxide Complexes.
Elisa Palacios-Lidon
Universidad de Murcia, ES
Authors
Elisa Palacios-Lidón a, Emin Istif b, Ana Benito b, Wolfgang K. Maser b, Jaime Colchero a
Affiliations
a, Universidad de Murcia, (Campus Espinardo) Universidad de Murcia, Murcia, ES
b, Instituto de Carboquimica ICB-CSIC, Miguel Luesma Castan 4, Zaragoza, 50018, ES
Abstract

Solution processed graphene oxide (GO), has gained increased interest as optoelectronic device platform due  to its ability to act as amphiphilic macromolecule, favoring  the formation  of  complexes with conjugated polymers, that can be used in  improved optoelectronic devices. Recently, charge transfer has been demonstrated in complexes of poly(3-hexylthiophene nanoparticles (P3HT NPs)  and GO sheets (P3HTNPs – GO hybrids) prepared by self-assembly in-situ reprecipitation [1]. In this work, Kelvin Probe Force Microscopy (KPFM) is used to investigate the P3HTNPs–GO nanohybrids formation. By mapping the local surface potential (SP) in darkness, the polymer chain aggregrate structure is resolved. In addition, surface photovoltage (SPV) measurements on individual nanoparticles under super-band-gap illumination shed light on the photoinduced charge generation and charge recombination mechanisms at the different interfaces. This is an important step to understand the rather overall functionality thin film layers composed of individual nanoscale objects and for further optimizing the optoelectrical performance of thin film devices.

 

[1] E. Istif, J. Hernández-Ferrer, E. P. Urriolabeitia, A. Stergiou, N. Tagmatarchis, G. Fratta, M. J. Large, A. B. Dalton, A. M. Benito, and W. K. Maser, “Conjugated Polymer Nanoparticle–Graphene Oxide Charge-Transfer Complexes”, Adv. Funct. Mater., 1707548, 2018.

16:30 - 17:00
S10.3-I4
Mikkelsen, Anders
Department of Physics, Lund University
Atomic Scale Surface and Crystal Phase Control in Nanowires for Novel Electronics and Photonics.
Anders Mikkelsen
Department of Physics, Lund University, SE
Authors
Anders Mikkelsen a
Affiliations
a, Lund University, Department of Physics & NanoLund, Lund, Sweden
Abstract

The III-V nanowire (NW) technology platform has reached a level of advancement that allows atomic scale control of crystal structure and surface morphology as well as flexible device integration. In particular, controlled axial stacking of Wurtzite(Wz) and Zincblende(Zb) crystal phases is uniquely possible in the NWs. We explore how this can be used to affect electronic, optical and surface chemistry with atomic scale precision opening up for 1D, 2D and 3D structures with designed local properties.

We previously demonstrated atomically resolved Scanning Tunneling Microscopy/Spectroscopy (STM/S) on a wide variety of these III-V NWs and on operational NW devices[1-4]. We now study atomic scale crystal phase changes, their impact on local electronic properties and demonstrating atomic resolution STM during device operation[5-7]. We explore the surface alloying of Sb into GaAs NWs with controlled axial stacking of Wz and Zb crystal phases[5] demonstrating a simple processing-free route to compositional control at the monolayer level. Using 5K STM/S we measure local density of states of Zb crystal segments in Wz InAs NWs down to the smallest possible atomic scale crystal change[6]. The general Zb electronic structure is preserved locally in even the smallest possible segments and signatures of confined states are found.  We demonstrate a novel device platform allowing STM/S with atomic scale resolution across a III-V NW device simultaneously with full electrical operation and high temperature processing in reactive gases[7].

Using 5-15 femtosecond laser pulses combined with PhotoEmission Electron Microscopy (PEEM) we explore local dynamic response of carriers in the Wz and Zb crystal phases down to a few femtoseconds temporally and a few tens of nanometer spatially. We demonstrate that spatial control of multiphoton electron excitations is possible in semiconductor NWs by changing the crystal phase, orientation, and light polarization[8]. The control and understanding of multiphoton excitations could be used in the design of optoelectronic components that use hot electrons or photoelectrons for functionality.

[1] E. Hilner etal., Nano Lett., 8 (2008) 3978; M. Hjort et al., ACS Nano 6 (2012) 9679

[2] M. Hjort etal., Nano Lett., 13 (2013) 4492; M. Hjort et al., ACS Nano, 8 (2014) 12346

[3] J.L. Webb, etal Nano Lett. 15 (2015) 4865

[4] O. Persson etal., Nano Lett. 15 (2015) 3684

[5] M. Hjort etal Nano Lett., 17 (2017) 3634

[6] J.V. Knutsson etal ACS Nano, 11 (2017) 10519

[7] J.L. Webb etal, Sci.  Rep. 7 (2017) 12790

[8] E. Mårsell etal, Nano Lett. 18 (2018) 907

Sol2D S6.4
Chair: Christian Klinke
14:30 - 15:00
S6.4-I2
Vanmaekelbergh, Daniel
Universiteit Utrecht
2-D Silicene Honeycomb Superlattices from PbSe Nanocrystals by Nanocrystal Assembly at an Interface Followed by Oriented Attachment
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, Joep Peters a, Maryam Alimoradi Jazi a, Sophia Buhbut-Sinai a, Sara Bals a, Giuseppe Soligno a
Affiliations
a, Universiteit Utrecht, Princetonplein 1, Utrecht, 3584, NL
Abstract

It has been reported that drop casting of a suspension of PbX (X=S, Se, Te) nanocrystals on an ethylene glycol liquid substrate results in the formation of atomically coherent 2-D sheets of a nanocrystal monolayer in thickness. The Klinke group reported the formation of PbS NC sheets. We reported PbSe (S, Te) sheets with a superimposed honeycomb and square geometry.

I will present here our recent progress in this field which is based on extremely slow solvent evaporation under a constant (nearly saturated) solvent vapor phase. We present silicene type honeycomb sheets with lateral dimensions in the 100 micrometer range. We also prepared silicene structures that extend in the vertical dimension over several unit cells. Finally, we studied the "atomic-like" and orientation defects in these systems.

The mechanism of this remarkable self-assembly process was studied by in-situ GISAX and GIWAX and by molecular dynamic simulations.

Low-Dimensional Semiconductor Superlattices Formed by Geometric Control over Nanocrystal Attachment." Nano Letters 13(6): 2317-2323.

Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices." Science 344(6190): 1377-1380.

In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals." Nature Materials 15(12): 1248-1254.

Mono- and multilayer silicene-type honeycomb lattices by oriented attachment of PbSe nanocrystals: synthesis, structural characterization, and analysis of the disorder.

Submitted

15:00 - 15:30
S6.4-I3
Schaller, Richard
Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
Colloidal Quantum Wells for Energy Manipulations on Fast Timescales
Richard Schaller
Department of Chemistry, Northwestern University, Evanston, IL 60208, USA

Since 2010, Richard D. Schaller has held a joint appointment as both a research scientist in the Center for Nanoscale Materials at Argonne National Lab and as an assistant professor in the Department of Chemistry at Northwestern University. Schaller’s research focuses on spectroscopy and physical chemistry of semiconductor nanomaterials From 2002 to 2010, Schaller was a Reines Distinguished Postdoctoral Fellow and then a permanent technical staff member at Los Alamos National Lab with Dr. Victor Klimov. Schaller obtained his PhD in physical chemistry from UC Berkeley in 2002 with Prof. Richard Saykally in nonlinear optics and near-field optics. In 2012, he was selected by the National Academy of Sciences as a Kavli Fellow participant.

Authors
Richard Schaller a, b
Affiliations
a, Argonne National Laboratory, Lemont, Illinois, United States
b, Northwestern University, Evanston, Illinois, United States
Abstract

The rapid development of colloidally synthesized, two-dimensional nanoplatelets with precise thickness-tunable, narrow band-edge absorption and photoluminescence unlock several classes of investigations. We have examined charge and energy transfer involving these structures for purposes of energy capture and conversion and lighting. Particularly fast transfer of excitations between self-assembled, co-facial arrangements of thinner donor and thicker acceptor structures is found and modeled. Electron transfer rates for four isoenergetic donor–acceptor pairs comprising a well-known molecular electron acceptor and controlled lateral extents of nanoparticles, examined via ultrafast photoluminescence, relate a dependence of charge transfer rate on the spatial extent of the electron–hole pair wave function explicitly. A nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

The rapid development of colloidally synthesized, two-dimensional nanoplatelets with precise thickness-tunable, narrow band-edge absorption and photoluminescence unlock several classes of investigations. We have examined charge and energy transfer involving these structures for purposes of energy capture and conversion and lighting. Particularly fast transfer of excitations between self-assembled, co-facial arrangements of thinner donor and thicker acceptor structures is found and modeled. Electron transfer rates for four isoenergetic donor–acceptor pairs comprising a well-known molecular electron acceptor and controlled lateral extents of nanoparticles, examined via ultrafast photoluminescence, relate a dependence of charge transfer rate on the spatial extent of the electron–hole pair wave function explicitly. A nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

15:30 - 15:45
S6.4-O3
Klepzig, Lars
Leibniz Universität Hannover
Thermoelectric Properties of Aerogels of PbS Nanoplatelets
Lars Klepzig
Leibniz Universität Hannover, DE
Authors
Lars Klepzig a, Jan Poppe a, Eugen Klein b, Christian Klinke b, c, Armin Feldhoff a, Nadja C. Bigall a
Affiliations
a, Institute of Physical Chemistry and Electrochemistry, Leibniz Universität Hannover, Callinstr. 3A, 30167 Hannover, Germany
b, Institute of Physical Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
c, Department of Chemistry, Swansea University – Singleton Park, Swansea SA2 8PP, United Kingdom
Abstract

A solid-state assembly of nanoparticles for the use as a thermoelectric generator1 is presented. The colloidal synthesis of the two-dimensional lead sulphide nanosheets was performed by oriented attachment of PbS nanoparticles2. A self-supporting network of these anisotropic nanoplatelets was formed by slow destabilization of the colloidal solution by altering the pH value. Supercritical drying of such gel-type networks resulted in aerogels. These aerogel materials exhibit extremely low density, high surface to mass ratio and mesoporosity. The physical properties were characterized by transmission electron microscopy, scanning electron microscopy and N2-physisorption measurements. The thermoelectric properties around room temperature were measured with a hot probe setup utilising multifunctional probes. Thus, the application of a temperature difference and the simultaneous measurement of the temperature difference and the induced thermovoltage was possible3. The material features the nanostructural benefits of aerogels while retaining the Seebeck coefficient of the bulk phase, enabling promising approaches for the use in thermoelectrical devices.

15:45 - 16:00
S6.4-O2
Schlosser, Anja
Leibniz Universität Hannover
3D Assemblies of CdSe Nanoplatelets for Application in Photoelectrochemical Sensing
Anja Schlosser
Leibniz Universität Hannover, DE
Authors
Anja Schlosser a, Jan Frederick Miethe a, Franziska Lübkemann a, Jan Gerrit Eckert a, Lea Celiné Meyer a, Nadja-Carola Bigall a
Affiliations
a, Institute of Physical Chemistry and Electrochemistry, Leibniz-Universität Hannover, Callinstr. 3A, D-30167 Hannover
Abstract

Semiconductor nanoparticle (NP) based photoelectrochemical sensors exhibit several advantages over other sensor types, such as a wide analyte range, fast responses, and high sensitivies.1 For the development of systems for multi-analyte detection, not only a spatial structuring of the photoelectrodes, but also the improvement of their sensitivity and their detection range is necessary. Due to their high surface-to-volume ratio, semiconductor nanoplatelets (NPLs) are promising candidates for the application in (multi-analyte) photoelectrochemical sensing devices. The high surface area of the NPLs potentially increases the absolute number of surface trap states per particle, which may lead to an enhanced photoresponse of the NP covered electrode.3 In addition, CdSe NPLs were already shown to assemble into highly porous non-ordered network structures with large surface areas.2 Our work reports on the preparation of different CdSe NPL based 3D assemblies on conductive glass electrodes and the photoelectrochemical characterization of the charge transfer processes across these structures. Electron microscopy revealed that porous NPL gels with different morphologies were obtained via the applied gelation processes. Photocurrents more than one magnitude larger than for simple particle monolayers were detected and the transport of charge carriers was proven by means of intensity modulated photocurrent spectroscopy (IMPS).4

 

References:

(1) Yue Z. et al., ACS Appl. Mater. Interfaces 2013, 5, 2800−2814.

(2) Naskar, S. et al., Chem. Mater. 2016, 28 (7), 2089–2099.

(3) Spittel, D. et al., ACS Nano 2017, 11 (12), 12174–12184.

(4) Jan F. Miethe, Anja Schlosser et al., submitted.

16:00 - 16:30
S6.4-O1
Rossinelli, Aurelio
ETH Zurich
Color-Tunable CdSe-Based Core/Shell Nanoplatelets
Aurelio Rossinelli
ETH Zurich, CH
Authors
Aurelio Rossinelli a, Andreas Riedinger b, Philippe Knüsel a, Patricia Marqués Gallego a, Felipe Antolinez a, David Norris a
Affiliations
a, 4Optical Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland
b, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
Abstract

Colloidal nanoplatelets (NPLs) are quasi-two-dimensional nanocrystals with atomically precise thickness in one dimension. Due to their highly anisotropic shape, they offer favorable optical properties such as narrow emission linewidths and large absorption cross-sections.1,2 However, as-synthesized, NPLs exhibit poor photo- and chemical stability. Thus, strategies have been sought to improve their properties by adding a shell on the NPLs. For spherical quantum dots, recently developed recipes for growing high-quality shells are performed at high temperatures. However, to date, this strategy has not been extended to NPLs because of their low thermal stability compared to quantum dots.3 Here, we present a method for obtaining CdSe/CdS core/shell NPLs in which the shell is added at high temperatures (~300 °C).4 This enables the growth of uniform and thick CdS shells, which is not possible with existing continuous-growth protocols. We further extend this protocol to produce alloyed CdxZn1-xS shells with varying composition and thickness. We obtain high-quality monodisperse and stable core/shell NPLs with narrow emission linewidths, high QYs exceeding 70-80%, and suppressed blinking. Such samples exhibit tunable emission peaks that can result in improvements for a wide range of applications in optics and optoelectronics relying on efficient and narrow emitters.

 

1)  S. Ithurria et al., Nat. Mater., 10, 936 (2011)

2)  A. Yeltik et al., J. Phys. Chem. C, 119, 26768 (2015)

3)  A. Riedinger et al., Nat. Mater., 16, 743 (2017)

4)  A. A. Rossinelli et al., Chem. Commun., 53, 9938 (2017)

16:30 - 17:00
S6.4-I1
Perez, Emilio M.
IMDEA Nanoscience
From Liquid-Phase Exfoliated 2D Materials to Functioning Devices
Emilio M. Perez
IMDEA Nanoscience, ES
Authors
Emilio Perez a
Affiliations
a, IMDEA Nanociencia, C/ Faraday, 9, Cantoblanco, 28049
Abstract

One of the most attractive areas of research in Nanoscience is the combination of 2D materials through van der Waals forces to form heterostructures (Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. Science 2016, 353, DOI: 10.1126/science.aac9439). Here, we present strategies beyond dispersion forces to interface 0D (molecules) and 2D materials.

We will show examples of 0D-2D heterostructures connected through covalent and noncovalent chemistry. In the first example, we will show a method to functionalize graphene covalently with exquisite atomic selectivity and yield (NanoLett. 2016, Chem. Commun. 2017). In the second example, we will show that the photoresponse of MoS2 photodetectors can be enhanced by simple and reversible supramolecular functionalization with organic dyes (Chem. Commun. 2016). We will also show the example of a naturally occurring van der Waals heterostructure, franckeite, an air stable, p-doped semiconductor with photoresponse in the far IR (Nat. Comms. 2017). Finally, we will present strategies for the self assembly of liquid-phase exfoliated 2D materials from suspension into functioning electronic devices (Chem. Commun. 2017 and Nanoscale 2018).

WatSpl S2.4
Chair: Krishnan Rajeshwar
16:00 - 16:30
S2.4-I1
May, Matthias
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany
Challenges and Opportunities of Water Splitting with Multi-Junction Solar Absorbers
Matthias May
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany

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 M. May a, b
Affiliations
a, University of Heidelberg, Heidelberg
b, Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
Abstract

Fossil fuels need to be replaced quickly and on a large scale to render our energy system compatible with current greenhouse gas emission targets. The scale of the transition is enormous and makes efficiency a key factor for its feasibility. The required pace of the transition, on the other hand, renders the modification of materials, that are already established in photovoltaics, for use in solar water splitting devices attractive.

In the case of light-driven water splitting, multi-junction absorbers are essential to provide enough photovoltage and -current for efficiencies beyond 10%. Challenges arising from the distribution of the solar spectrum over multiple sub-cells include appropriate internal band alignment, spectral shaping by (metallic) co-catalysts and the electrolyte as well as the reduction of reflection losses. In this talk, I will discuss current developments in the field of high-efficiency, immersed water splitting systems of currently up to 19% solar-to-hydrogen and outline the route towards efficiencies beyond 20%. Furthermore, I will outline the impact of efficiency on the potential of photoelectrochemical systems as a negative emission technology.

16:30 - 16:45
S2.4-O1
Pedesseau, Laurent
INSA, FOTON, UMR CNRS 6082
GaP Template on Si for Solar Water Splitting: Surface Energy Engineering
Laurent Pedesseau
INSA, FOTON, UMR CNRS 6082, FR

He obtained his MSc in condensed matter from the University of Montpellier in 2001. In 2004, he received his PhD in physics from the University of Toulouse for atomistic empirical simulations applied to Civil Engineering materials. In 2013, he was appointed as assistant professor at FOTON laboratory (INSA Rennes) to work on III–V semiconductor nanostructures for silicon photonics, hybrid-perovskites for photovoltaics, and optoelectronic device simulations for optical telecommunication.

Authors
Laurent Pedesseau a, Ida Lucci a, Simon Charbonnier b, Maxime Vallet c, Pascal Turban b, Yoan Leger a, Tony Rohel a, Nicolas Bertru a, Antoine Létoublon a, Jean-Baptiste Rodriguez d, Laurent Cerutti d, Eric Tournié d, Anne Ponchet c, Gilles Patriarche e, Charles Cornet a
Affiliations
a, Univ Rennes, INSA Rennes, CNRS, Institut FOTON - UMR6082, F-35000 RENNES
b, Institut de Physique de Rennes, CNRS, Université de Rennes 1, 35042 Rennes, France
c, CEMES-CNRS Université de Toulouse UPS 29 rue Jeanne Marvig BP 94347 Toulouse, Cedex 04, France
d, IES Univ. Montpellier CNRS 860 Rue St - Priest, 34090 Montpellier, France
e, Centre de Nanosciences et de Nanotechnologies site de Marcoussis CNRS Université Paris Sud Université Paris Saclay route de Nozay 91460 Marcoussis, France
Abstract

The hydrogen production will play a major role for the energy transition. Recently, for water splitting context [1], [2], a demonstration of the efficiency enhancement of BiVO4 photoanodes has been shown in PEC devices[3] by simply a texturation of surfaces. Moreover, in the study of the semiconductor photocatalyst materials[4], GaP semiconductor appears to be a good candidate for photoelectrode in PEC devices[5]. One strong argument is to have at least 1.73 eV photopotential requirements for water splitting and its bandgap is larger and about 2.26 eV.

In this aim of water splitting applications, we propose a surface energy engineering for a large scale textured GaP template monolithically integrated on Si [6]. Based on experimental analysis and theory, the stability of the {114} facets is scrutinized by scanning tunneling microscopy images and also supported by density functional theory calculations. We then show that change of the surface energy for experimentally promoting the GaP(114) surface texturation can be achieved through (i) destabilizing the GaP(001) surface by using a vicinal Si substrate or through (ii) favoring the {114} facets formation by changing the group-V atmosphere above the surface on a miscut-free GaP substrate.

This work is supported by the French National Research Agency project ANTIPODE (Grant no. 14-CE26-0014-01) and Région Bretagne. The ab initio simulations have been performed on HPC resources of CINES under the allocation 2017-[x2017096724] made by GENCI (Grand Equipement National de Calcul Intensif).

[1] M. G. Walter et al., ‘Solar Water Splitting Cells’, Chem. Rev., vol. 110, no. 11, pp. 6446–6473, Nov. 2010.

[2] A. Fujishima and K. Honda, ‘Electrochemical Photolysis of Water at a Semiconductor Electrode’, Nature, vol. 238, no. 5358, p. 37, Jul. 1972.

[3] J. Zhao et al., ‘High-Performance Ultrathin BiVO4 Photoanode on Textured Polydimethylsiloxane Substrates for Solar Water Splitting’, ACS Energy Lett., vol. 1, no. 1, pp. 68–75, Jul. 2016.

[4] A. Kudo and Y. Miseki, ‘Heterogeneous photocatalyst materials for water splitting’, Chem. Soc. Rev., vol. 38, no. 1, pp. 253–278, 2009.

[5] E. E. Barton, D. M. Rampulla, and A. B. Bocarsly, ‘Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell’, J. Am. Chem. Soc., vol. 130, no. 20, pp. 6342–6344, May 2008.

[6] I. Lucci at al., ‘A Stress‐Free and Textured GaP Template on Silicon for Solar Water Splitting’, Adv. Funct. Mat. Hot Topic: Water Splitting, 1801585, 2018

16:45 - 17:00
S2.4-O2
Cendula, Peter
University of Zilina
Elucidation of Photovoltage Enhancements and Charge Transport in Multijunction Cu2O Photocathode through Semiconductor Simulations
Peter Cendula
University of Zilina, SK
Authors
Peter Cendula a, Matthew T. Mayer b, c, Jingshan Luo d, Linfeng Pan e, Michael Grätzel b
Affiliations
a, University of Zilina, kpt. Nalepku 1390, L.Mikulas, SK
b, Laboratory of Photonics and Interfaces (LPI), Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
c, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, Berlin, DE
d, Nankai University, 94 Weijin Road, Nankai District, Tianjin 300071, CN
e, Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne
Abstract

Unassisted water splitting by this approach requires efficient, stable and low-cost photoelectrode materials. Of the material candidates for photocathodes, Cu2O stands out as best performing oxide for hydrogen production currently available. Although numerous experimental investigations greatly enhanced performance and stability of Cu2O photocathode, there is lack of detailed theoretical understanding of charge 
transport mechanism in Cu2O buried junction photocathodes. On the macroscopic scale within the drift-diffusion approximation of charge transport in semiconductors, device simulation studies so far studied Cu2O photovoltaics. The charge transport in photoelectrochemical Cu2O buried junctions has not been addressed so far by device simulation study.

In our work, we discuss existing belief that separation of the valence band of p-type Cu2O to conduction band of n-type buffer layer limits the available photovoltage, providing example calculation with Al:ZnO buffer layer, which does not follow this rule. To explain the measured low photovoltage obtained with Al:ZnO buffer layer, we identify recombination at defect-rich Al:ZnO/Cu2O interface as a responsible mechanism. Furthermore, our device simulations of onset voltage of TiO2/Ga2O3/Cu2O photocathode correspond well with the measured value. We describe how the two energy barriers for electron transport at Cu2O/Ga2Oand at TiO2/Ga2Ointerfaces cause anodic shift of the onset voltage. Numerical quantification of the drift and diffusion currents throughout the TiO2/Ga2O3/Cu2O photocathode brings us to conclusion that electron diffusion in Cu2O bulk close to Cu2O/Ga2O3 dominates the electron current at the onset voltage. Variation of electron affinity, mobility and doping of Ga2O3 buffer layer in our model result in important improvements on onset voltage up to 1.65 V vs RHE and ratiometric power-saved of the Cu2O photocathode, providing future ideas for experimental developments.

17:30 - 19:00
Poster session
 
Fri Oct 26 2018
Plenary session 6
Chair: Sascha Sadewasser
09:00 - 09:30
6-K1
Leite, Marina
University of Maryland
Probing Solar Cells at the Nanoscale through Real-Time Functional Imaging
Marina Leite
University of Maryland, US

Leite is an Assistant Professor in Materials Science and Engineering, and in the Institute for Research in Electronics and Applied Physics (IREAP) at UMD. Her group investigates materials for energy harvesting and storage, from their nano-scale structural, electrical, and optical properties to their implementation in devices. Before joining UMD, Leite worked for two years at NIST and was a post-doctoral scholar at Caltech (Department of Applied Physics and Materials Science). She received her PhD in physics from Campinas State University in Brazil and the Synchrotron Light Source Laboratory. Leite's work has been recognized on the cover of 13 scientific journals, by the presentation of >85 invited talks, by the 2016 APS Ovshinsky Sustainable Energy Fellowship from the American Physical Society (APS) and the 2014 Maryland Academy of Sciences Outstanding Young Scientist Award. Leite’s research is currently funded by: NSF-ECCS (16-10833), NSF-DMR (16-09414), Army Research Lab, and the American Physical Society.

Authors
Marina Leite a
Affiliations
a, University of Maryland, 2123 Chemical and Nuclear Engineering Building, College Park, 20742, US
Abstract

Our constantly increasing society’s need for energy has triggered a pressing need for the development of new materials for renewable sources. Concerning materials for energy harvesting, the most promising approaches for high-performance and low-cost photovoltaics rely in inhomogeneous compounds, such as perovskites and polycrystalline thin films (e.g. CIGS and CdTe). Thus, resolving their electrical and optical behavior at the nanoscale is imperative to advance their understanding. In this talk, I will share our scientific findings to image and quantify the local voltage response of nano- and mesoscale inhomogeneities in perovskites [1,2], CIGS [3], and CdTe through a variant of KPFM and NSOM [4-6]. By submitting the samples to illumination and humidity treatments under controlled conditions, we map the dynamic physical behavior of MAPI and triple-cation perovskites.

 

References:

[1] J. M. Howard et al. J. Phys Chem Letters, in press (2018)

[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).

[3] E. M. Tennyson et al. ACS Energy Letters 1, 899 (2016).

[4] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review

[5] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective

[6] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).

Sol2D S6.5
Chair: Sandrine Ithurria
09:00 - 09:15
S6.5-O3
Schäfer, Philip
neaspec GmbH
Scanning near-field optical microscopy and spectroscopy elucidates growth characteristics and free charge carrier distributions in nano-platelets with 10 nm spatial resolution
Philip Schäfer
neaspec GmbH
Authors
Philip Schäfer a
Affiliations
a, neaspec GmbH
Abstract

Scattering-type scanning near-field optical microscopy (s-SNOM) has emerged as a key technology to gain chemical information, study structural properties and observe charge carrier distributions on the 10 nm length-scale. s-SNOM employs a metallic AFM tip which is illuminated to create a 10 nm small optical hot-spot at the tip apex independent from the wavelength of incident light. The concentrated light in the optical hotspot interacts with the sample surface below the tip and is modified by the local dielectric properties (absorption, reflection) of the sample. Detection of the elastically scattered light simultaneous to AFM imaging yields near-field images and broadband near-field spectra (nano-FTIR) with <20 nm spatial resolution. [1]

Using infrared s-SNOM imaging, the free carrier distribution in Bi2Se3 plates could be determined and it could be revealed that polyol-synthesized plates feature a non-uniform distribution of Se vacancies which cannot be cured by post-annealing. CVD-grown Bi2Se3 plates on the contrary don’t show such inhomogeneities. [2]

Furthermore s-SNOM images, recorded using mid-IR light, reveals the highly symmetric optical pattern of solvothermally grown Sb2Te3 hexagonal nanoplatelets. The superordinate optical patterns of the spiral growth patterns are shown to represent domains of distinct electronic properties, resulting from so-called growth twins with different densities of antisite defects. [3]

References:

[1] F. Huth et al., Nano Lett. 2012, 12, 3973

[2] X. Lu et al., Adv. Electron. Mater., 2018, 4, 1700377

[3] B. Hauer et al., Nano Lett., 2015, 15 (5), 2787

09:15 - 09:30
S6.5-O4
Li, Fu
Institute of Physical Chemistry, University of Hamburg
Shape and Size Control of the Synthesis of 2D Tin Sulfide (SnS) Nanosheets and Electronic Application
Fu Li
Institute of Physical Chemistry, University of Hamburg
Authors
Fu Li a, Mohammad Mehdi Ramin Moayed a, Christian Klinke a, b
Affiliations
a, Institute of Physical Chemistry, University of Hamburg, 20146 Hamburg, Germany
b, Department of Chemistry, Swansea University - Singleton Park, Swansea SA2 8PP, United Kingdom
Abstract

Colloidal two-dimensional (2D) tin (II) sulfide (SnS) nanocrystals are now emerging as potential and promising nanomaterials for electronic and optoelectronic applications. This is due to lower toxicity compared to other metal chalcogenides, such as PbS, PbSe, CdS, CdSe. We present a new and simple method for the preparation of colloidal 2D SnS nanosheets with large size, tunable thickness and single-crystallinity. The synthesis is performed by using tin (II) acetate as precursor to replace the common used tin halides (e.g. tin chloride) and harmful precursor (bis[bis(trimethylsilyl)amino] tin(II). The lateral size of synthesized square nanosheets can be tuned from 150 nm to 500 nm, and the thickness in a range of 25 to 30 nm. In addition, hexagonal shaped SnS nanosheets can also be synthesized (lateral size: 200-1700 nm, thickness: 15-50 nm). We control the shape and size by varying the amounts of ligands and precursors, which is also supported by DFT simulations. The crystal phase can also be optimized from the mixture of pseudotetragonal structure (PT, from nanoparticle byproducts) and orthorhombic structure (OR, from main product nanosheets) into single-crystalline OR structure. The optoelectronic measurements show their impressive conductivity and highly sensitivity to light. These materials are thus promising regarding electronic and optoelectronic applications.

09:30 - 10:00
S6.5-I1
Siebbeles, Laurens
TU Delft
Ultrafast Dynamics of Charge Carriers and Excitons in 2D Metal Chalcogenide Materials
Laurens Siebbeles
TU Delft, 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, Opto-electronic materials section, Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology
Abstract

Two-dimensional metal chalcogenide (CdSe, PbSe, PbS) materials were synthesized by processing from solution. Typical thicknesses range from one to more than ten nanometers. We studied the photogeneration, mobility and decay dynamics of charge carriers and excitons in: 1) CdSe nanosheets, 2) superlattices of connected PbSe QDs with square or honeycomb geometry, and 3) PbS nanosheets. 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.

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.

10:00 - 10:15
S6.5-O1
Salzmann, Bas
Utrecht University
CdSe Nanorings: Synthesis and Opto-Electronic Properties
Bas Salzmann
Utrecht University
Authors
Bas Salzmann a, Christiaan Post a, Daniel Vanmaekelbergh a
Affiliations
a, Universiteit Utrecht, Princetonplein 1, Utrecht, 3584, NL
Abstract

Colloidal CdSe nanoplatelets (NPLs), also known as colloidal quantum wells, exhibit interesting properties such as narrow emission lines and atomic controlled thicknesses. To selectively tune the electrical and optical properties of these highly interesting nanomaterials, several methods have been developed to yield either core/shell or core/crown NPLs. Moreover, it has recently been shown that those NPLs can also be converted into nanorings with a toroidal topology.1 It is hypothesized that this new geometry exhibits interesting properties such as the presence of magnetoexcitonic states and terahertz absorption. Until know, not much knowledge has been obtained about this new geometric shape of CdSe.

In our research, we optimize the synthesis procedure of NPLs and subsequently etch the NPLs to obtain the characteristic ring-like geometry of nanorings. To reveal information about the obtained particles, we use a wide range of techniques such as time-resolved PL spectroscopy, single dot spectroscopy, AFM and TEM. From these experiments, the electrical and optical properties can be determined. As such, we take the first steps to extend the knowledge about this unexplored geometry of CdSe.

1.             Fedin, I.; Talapin, D. V., Colloidal CdSe Quantum Rings. J Am Chem Soc 2016, 138 (31), 9771-4.

10:15 - 10:30
S6.5-O2
Peters, Joep
Universiteit Utrecht
Mono- and Multilayer Silicene-Type Honeycomb Lattices by Oriented Attachment of PbSe Nanocrystals: Synthesis, Structural Characterization, and Analysis of the Disorder.
Joep Peters
Universiteit Utrecht, NL
Authors
Joep peters a, Thomas Altantzis b, Sara Bals b, Daniel Vanmaekelbergh a
Affiliations
a, Universiteit Utrecht, Princetonplein 1, Utrecht, 3584, NL
b, 2 EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Abstract

Nanocrystal (NC) solids are commonly prepared from non-polar organic suspensions, with preservation of the original capping. On the other hand, NC superstructures with a direct crystalline connection between the NCs have been reported too. Here, we present large-scale uniform superstructures of attached PbSe NCs with a silicene-type honeycomb geometry, resulting from solvent evaporation under nearly reversible conditions. Those superstructures were prepared by self-assembly of the nanocrystals on a liquid substrate. The nanocrystals are subsequently oriented attached to form a superstructure with preferred geometry. Besides  the hoenycomb lattice, we also prepared multilayered silicene honeycomb structures by using larger amounts of PbSe NCs. Using HAADF-STEM tomography we show that the bilayer and multilayered silicene structures of attached PbSe nanocrystals form slices of the simple cubic superstructure. We describe the disorder in the silicene lattices in terms of the nanocrystals position and their atomic alignment. The silicene honeycomb sheets are large enough to be used in transistors and opto-electronic devices.

WatSpl S2.5
Chair: Roland Marschall
09:30 - 10:00
S2.5-O1
Peter, Laurence
University of Bath
Understanding the Role of Nanostructuring in Photoelectrode Performance for Light-Driven Water Splitting
Laurence Peter
University of Bath, GB

Laurie Peter received his B.Sc. and Ph.D. from the University of Southampton (UK). After a period working at the Fritz Haber Institute in Berlin in the group of the late Heinz Gerischer, he returned to the Southampton before moving to the University of Bath, where he has been Professor of Physical Chemistry since 1993. He partially 'retired' in 2009 and is currently spending 6 months at the Ludwig Maximillian University in Munich working in the group of Professor Thomas Bein. Laurie Peter's interest sinclude fundamental studies of dye-sensitized solar cells (DSCs), research into inorganic thin film solar cells based on sustainable materials such a copper zinc tin sulfide and photoelectrochemical water splitting. He has developed a number of experimental techniques such as intensity modulated photocurrent spectroscopy (IMPS) and charge extraction that are used to characterize DSCs and water splitting systems. He has also developed in situ microwave methods for investigating photoelectrochemical reactions. At present he is attempting to understand the kinetics and mechanisms of light-driven oxygen evolution at iron oxide electrodes.

Authors
Laurence Peter a, - Gurudayal c, Lydia Helena Wong c, Fatwa Abdi b
Affiliations
a, Department of Chemistry, University of Bath, Claverton Down, University of Bath, Bath,UK, BA2 7AY, GB
b, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels
c, School of Materials Science and Engineering Nanyang Technological University, Singapore
Abstract

 

Understanding water splitting at nanostructured electrodes presents some formidable challenges. Here, regular nanostructured photoelectrodes are considered using hematite nanorod arrays as an example. Since Mott Schottky plots are often reported for nanostructured electrodes, we revisit the effects of the cylindrical nanorod geometry on the capacitance-voltage behaviour. The limiting case of complete depletion is discussed in terms of the residual geometric capacity at the base of the nanorods. Since nanorod arrays generally leave areas of the substrate exposed, it is also necessary to consider the parallel capacitance associated with the fraction of uncovered surface. We then turn to the enhancement of external quantum efficiency (EQE) achieved by nanostructuring, again using hematite nanorod arrays as experimental examples. We show that, although very substantial EQE enhancement should be achieved by simple geometric effects, the performance of nanostructured hematite electrodes in the visible region of the spectrum is considerably lower than predicted if all charge carriers generated in the space charge region (SCR) were collected. Further analysis reveals that the internal quantum efficiency increases with photon energy, suggesting that the probability of generating free, rather than bound, electron-hole pairs in hematite depends on the excess energy hν - Egap.

  

10:00 - 10:15
S2.5-O2
Selim, Shababa
Imperial College London
Investigating the Influence of Nanostructuring on Photoanode Performance
Shababa Selim
Imperial College London, GB
Authors
Shababa Selim a, Laia Francas a, Camilo Mesa a, Andreas Kafizas a, Dongho Lee b, Kyoung-Shin Choi b, James R. Durrant a
Affiliations
a, Imperial College London, Department of Chemistry, South Kensington Campus, London, SW7 2AZ
b, Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706, USA
Abstract

The performance of semiconducting materials as photoanodes is continually optimised overtime. With the aid of nanostructuring, noteworthy performance enhancements for the water oxidation reaction are obtained when coupled with electrocatalysts such as FeOOH/NiOOH and Co-Fe Prussian Blue.1,2 In the longstanding compromise between the light penetration depth and charge diffusion lengths, nanostructuring provides an effective tool. However, it is not often efficacious unless coupled with co-catalyst or in presence of hole scavengers. This highlights the presence of significant surface recombination that is especially critical for kinetically demanding reactions such as the four hole oxidation of water, requiring the accumulation of holes on the surface.

In this talk, we will discuss the structure performance relationship between densely packed and mesoporous structures of bismuth vanadate alongside other n-type materials such as titania and hematite. Using techniques such as transient photocurrent measurements (TPC) and photo induced absorption spectroscopy (PIAS), we can directly probe the influence of porosity and increased surface area on electron transport and photo-catalysis.

 

References:

1         T. W. Kim and K.-S. Choi, Science (80-. )., 2014, 343, 990–994.

2         F. S. Hegner, I. Herraiz-Cardona, D. Cardenas-Morcoso, N. López, J. R. Galán-Mascarós and S. Gimenez, ACS Appl. Mater. Interfaces, 2017, 9, 37671–37681.

10:15 - 10:30
S2.5-O3
Xi, Fanxing
Helmholtz Center Berlin for Materials and Energy
Activation of amorphous MoSx as a hydrogen evolving catalyst in aqueous electrolysis
Fanxing Xi
Helmholtz Center Berlin for Materials and Energy, DE
Authors
Fanxing Xi a, Peter Bogdanoff a, Sebastian Fiechter a
Affiliations
a, Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, Berlin, DE
Abstract

Solar water splitting is a sustainable and in principle environmentally sound approach to solve the high energy demand of mankind using renewable energies[1], because of the high energy density of hydrogen and no CO2 emission when used as a fuel. To provide hydrogen in large quantities, nontoxic, earth abundant and cheap catalysts are in demand to replace costly platinum for reducing the overpotential in the hydrogen evolving reaction (HER) process[2].

In our study, molybdenum sulfide layers of different crystallinity and morphology have been prepared by reactive magnetron sputtering and tested as HER catalyst. Best performance was obtained starting from an amorphous MoSx layer deposited at room temperature on FTO (η10=180mV at pH 0 sulfuric acid electrolyte), which in Raman spectroscopy shows similar vibrations as the highly efficient HER catalyst, (NH4)2Mo3S13[3]. However, during electrochemical measurement, production of hydrogen was proved along with the release of H2S especially during the first 20min of CV (activation step). Afterwards, only hydrogen was evolved from the electrode. A structural change of the material from amorphous MoSx to MoS2 layered structure happens during CV was proved by Raman measurement with the disappearance of terminal and bridging [S2]2- units and the emergence of the characteristic vibration mode A1g (out of plane S bonding) from layer structured MoS2. Mo atoms at the edges of formed MoS2 nano-islands are oxidized to Mo(VI) after removal from the electrolyte. From the ratio of Mo (IV) to Mo(VI) in the XPS spectra, the size of MoS2 nano-islands could evaluated. The high number of this Mo atoms at the edges explains the good performance of the HER electrode. For stability test, room temperature sputtered MoSx electrode was measured under CV conditions for 10h exhibiting an increase in overpotential of 46 mV.

1.            Lewis, N.S. and D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization. Proc Natl Acad Sci U S A, 2006. 103(43): p. 15729-35.

2.            McCrory, C.C., et al., Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J Am Chem Soc, 2015. 137(13): p. 4347-57.

3.            Kibsgaard, J., T.F. Jaramillo, and F. Besenbacher, Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate Mo3S13 (2-) clusters. Nature Chemistry, 2014. 6(3): p. 248-253.

Sol2D S6.6
Chair: Christophe Delerue
11:00 - 11:30
S6.6-I1
Mahler, Benoit
ILM - Institut Lumière Matière, CNRS
Colloidal Synthesis of WS2 Nanosheets: Chemical Control of Morphology and Crystal Structure.
Benoit Mahler
ILM - Institut Lumière Matière, CNRS, FR

Benoit Mahler is a CNRS researcher at the ILM (Light and Matter Institute) in Lyon (France). His research interests include the colloidal synthesis of semiconductor nanostructures and heterostructures, the growth of two-dimensional materials and their applications for light harvesting applications.

Authors
Benoit Mahler a
Affiliations
a, ILM - Institut Lumière Matière, CNRS, 10 rue Ada Byron, Villeurbanne, FR
Abstract

Semiconducting monolayers hold many promises for the development of optoelectronic and spintronic devices, but are comparatively less explored for light harvesting. This situation is partly due to the lack of adequate materials: current production strategies (exfoliation and epitaxial growths) are limited to explore the wide range of potential two-dimensional (2D) materials for light conversion applications. Using colloidal chemistry to synthesize monolayers opens up the possibility to achieve an unprecedented control over the size, edges nature, composition and structure of the nanosheets. This control would allow to precisely engineer their electronic and optical properties to produce highly efficient monolayer-based nanomaterials for light conversion applications.

Using tungsten disulfide as a prototype material, we explored the transition metal dichalcogenide (TMDC) colloidal synthesis, through a strategy relying on precursors decomposition in high boiling point organic solvent. After having identified conditions to reproducibly synthetize colloidal WS2 monolayers in the metallic 1T crystal structure,1 we systematically modify the reaction parameters. The obtained protocols allow us to tune the crystal structure as well as the shape and the aggregation of WS2 nanosheets. These first steps demonstrate the potential of colloidal synthesis to prepare TMDCs nanosheets of controlled crystal structure, shape and thickness. Other materials and alloys can also be prepared through this approach, as recently demonstrated by other groups for WSe22 and MoxW1-xS2.3

 

(1)           Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. J. Am. Chem. Soc. 2014, 136, 14121.

(2)           Jung, W.; Lee, S.; Yoo, D.; Jeong, S.; Miró, P.; Kuc, A.; Heine, T.; Cheon, J. J. Am. Chem. Soc. 2015, 137, 7266.

(3)           Sun, Y.; Fujisawa, K.; Lin, Z.; Lei, Y.; Mondschein, J. S.; Terrones, M.; Schaak, R. E. J. Am. Chem. Soc. 2017, 139, 11096.

11:30 - 11:45
S6.6-O1
Zhou, Pengshang
University of Gent
Synthesis of Colloidal Tungsten Diselenide (WSe2) Nanocrystals by Hot Injection Method
Pengshang Zhou
University of Gent, BE
Authors
Pengshang Zhou a, Shalini Singh a, Pieter Schiettecatte a, Zeger Hens a
Affiliations
a, Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium
Abstract

Following the discovery of graphene, a vibrant research area on two-dimensional (2D) transition metal chalcogenides (TMDs) layered materials has emerged in recent years due to their exciting and diverse properties.1-4 The existing approaches to make TMDs are mainly exfoliation, substrate growth and colloidal synthesis. Among them, colloidal wet-chemistry methods are particularly promising as the as-synthesized colloidal dispersions are directly fit for solution-based processing, opening a gateway for a straightforward and cost-efficient introduction into various technology platforms. However, compared to the other two preparation schemes, colloidal synthesis is relatively underdeveloped with only a few examples being known.2-4

In this contribution, we describe the preparation of WSe2 nanocrystals using a hot injection colloidal synthesis in the presence of a capping ligand as the solvent. We find that the choice of capping ligands influences the shape of the nanocrystals to great extent. Nano-flowers were obtained in the presence of oleic acid as capping ligand, whereas, nanosheets were obtained in the presence of oleylamine. Moreover, modification of the reaction temperature allowed to control the size of the nanocrystals. A thorough investigation of the reaction yield by means of quantitative XRF analysis enabled us to rationalize the reaction process. This synthesis strategy might provide a versatile approach to synthesize a wide range of TMDs such as WS2 and MoSe2.

[1] Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., ... & Firsov, A. A. (2004). Electric field effect in atomically thin carbon films. Science, 306(5696), 666-669.

[2] Mahler, B., Hoepfner, V., Liao, K., & Ozin, G. A. (2014). Colloidal synthesis of 1T-WS2 and 2H-WS2 nanosheets: applications for photocatalytic hydrogen evolution. Journal of the American Chemical Society, 136(40), 14121-14127.

[3] Jung, W., Lee, S., Yoo, D., Jeong, S., Miró, P., Kuc, A., ... & Cheon, J. (2015). Colloidal synthesis of single-layer MSe2 (M= Mo, W) nanosheets via anisotropic solution-phase growth approach. Journal of the American Chemical Society, 137(23), 7266-7269.

[4] Jin, H., Ahn, M., Jeong, S., Han, J. H., Yoo, D., Son, D. H., & Cheon, J. (2016). Colloidal single-layer quantum dots with lateral confinement effects on 2D exciton. Journal of the American Chemical Society, 138(40), 13253-13259.

11:45 - 12:00
S6.6-O2
Parkinson, Bruce
University of Wyoming
New 2D Nanoporous Covalent Organic Framework Materials with Functionalized
Bruce Parkinson
University of Wyoming
Bruce Parkinson received his BS in chemistry at Iowa State University in 1972 and his PhD from Caltech in 1977 and was a post-doctoral scientist at Bell Laboratories in 1978. He then spent time at the Ames Laboratory and the Solar Energy Research Institute (now known as the National Renewable Energy Laboratory). He moved to the Central Research and Development Department of the DuPont Company in 1985 and in 1991 he became Professor of Chemistry at Colorado State University until his recent departure to join the Department of Chemistry and the School of Energy Resources at the University of Wyoming. His current research covers a wide range of areas including materials chemistry, surface chemistry and photoelectrochemical energy conversion. He has more than 220 peer-reviewed publications and holds 5 US patents.
Authors
Bruce Parkinson a, John Hoberg a
Affiliations
a, Department of Chemistry, University of Wyoming, Laramie, WY, USA
Abstract

We have developed new families of highly ordered two dimensional covalent organic framework (2D-COF) materials. The materials have a hexagonal aromatic backbone with nanopores with controlled sizes that can be functionalized with any number of functional groups. We make them in high yields from condensation reactions from easy to obtain starting materials that in some cases produce very sharp x-ray diffraction patterns as well as showing triangular shaped crystallites with sharp diffraction spots in the TEM indicating a high degree of order. Single metal binding sites can also be designed within the framework structures. We have initially concentrated on putting charged functional groups in the pores to use as membrane materials and have demonstrated very specific ion charge and size selectivity with membranes made from these 2D-COFs. We have also produced 2D-COF materials that have less order but selectively absorb and release hydrogen at near room temperature. Other applications such as size exclusion filtration will also be shown.

12:00 - 12:30
S6.6-O3
Fong, James
Efficient Light Emission from MoS2 Flakes by in-solution Superacid Treatment
James Fong
Authors
Prabhuraj Balakrishnan a, Matthew J Fong a, Christopher S woodhead a, Ramon Bernardo Gavito a, Robert J Young a
Affiliations
a, Lancaster University, UK
Abstract

In recent years, monolayers of transition metal dichalcogenides have attracted a great deal of research interest. These two-dimensional (2D) materials typically have direct bandgaps in the visible or near-infrared, making them attractive for both classical and quantum optoelectronic applications [1]. Many different techniques have been developed to isolate monolayers from bulk; among them liquid assisted exfoliation is preferred by many, as it is simple, scalable, convenient and cost-effective [2]. However, the flakes produced using this class of technique are often not suitable for many applications, having poor uniformity and optical properties due to unintentional doping and other impurities.

In this work we employ ultrasonic bath sonication of bulk MoS2, starting with powders of grain sizes ranging from 6 to 40 µm, in an isopropanol/water mixture (70/30 vol %), following the procedure according to [3]. This produces suspensions of few-layer MoS2 flakes. Photoluminescence (PL) measurements from drop-cast samples on silicon substrates are found to have full widths at half maxima of 200 meV, indicating an average thickness of 2-3 layers. This agrees with independent atomic force microscopy measurements [4]. However, PL intensities are found to be weak – a typical result for flakes produced with this technique.

Super acid treatment of dry mechanically-exfoliated monolayers of MoS2 has been shown to vastly improve internal quantum efficiency, and with it, PL intensity [5] But it impractical to implement in a manufacturing process. In this work we will detail a new all-wet superacid treatment process for MoS2 flakes. We show that it enhances PL emission intensity by over 200 times. It thus represents a promising technique for the practical application of 2D inks for optoelectronics.

 

Reference:

1. Ryou et al. Scientific Reports 6, 29184 (2016)

2. Huo et al. Science Bulletin 60, 1994 (2015)

3. Bernal et al. 2D Materials, 3, 035014 (2016)

4. Bissett et al. ACS Omega, 2, 738 (2017)

5. Amani et al. Science, 350, 1065 (2015).

 

WatSpl S2.6
Chair: Matthias May
11:00 - 11:30
S2.6-O1
Rajeshwar, Krishnan
University of Texas, Arlington
New Families of Ternary Rare Earth Chalcogenides for Photoelectrochemical Applications
Krishnan Rajeshwar
University of Texas, Arlington
Authors
Krishnan Rajeshwar a
Affiliations
a, The University of Texas at Arlington
Abstract

Even after ca. four decades of R&D effort, we still do not have a "magic bullet" inorganic semiconductor to photoelectrochemically generate fuels or chemicals from sunlight in a sustainable, efficient and environment-friendly manner. While it is unlikely that a single semiconductor candidate will emerge that simultaneously satisfies all the optical, electrical, surface chemical, and electrochemical prerequisites for efficient solar conversion, complex oxides or chalcogenides (or derivatives thereof, e.g., oxynitrides) do provide a versatile framework for rational design of the "perfect beast" in a chemical architectural sense. Ultimately two or more such semiconductor compositions can be combined in a composite design much like complementary functionalities are combined in photosynthetic assemblies in Nature. In such designs, the semiconductor(s) and the photoactive junction can even be separated from the electrolyte and the electrocatalyst component in a "buried junction" design. In this vein, the author's laboratory has been engaged in the development of time- and energy-efficient methods for synthesizing new families of photoelectrode or photocatalyst materials. In this particular talk, the author will provide first a context for the key role that solid-state chemistry paradigms and principles can play in photoelectrode designs for driving multi-electron processes typical of solar fuel generation. A representative ternary semiconductor system, namely, M-Ln-X (M = divalent metal, e.g., Ba, Ln = lanthanide element, e.g., Ce, and X = chalcogen, e.g., S) will be discussed in this talk.

11:30 - 11:45
S2.6-O2
Schuhmann, Wolfgang
Ruhr Universität Bocum
Improving the Photoelectrocatalytic Activity of Metal-Doped BiVO4-Based Photoabsorbers by Means of Oxygen Evolution Co-Catalysts
Wolfgang Schuhmann
Ruhr Universität Bocum
Authors
Wolfgang Schuhmann a, Ramona Gutkowski a, Joao Junqueira a, Tim Bobrowski a, Olga Krysiak a
Affiliations
a, Analytical Chemistry–Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum, D-44801 Bochum, Germany
Abstract

The recombination of photogenerated electron-hole pairs is one of the major limiting factor in photo­electrocatalysts absorbing in the visible region of the solar spectrum. The performance as expressed by the incident photon-to-current efficiency (IPCE) strongly depends on the loading and thickness of the active material. Especially for BiVO4 the slow electron transport to the back contact facilitates charge carrier recombination and thus does not allow to exploit the entire potential of the photo­absorber. Hence, thin layers are favoured to avoid excessive charge carrier recombination, however, on the expense of a low absorption of the incident light with the result of low photocurrent output of such electrodes.

We present the development of different strategies to address the limitations evoked by charge carrier recombination of BiVO4-based photoabsorbers. The electrochemically-induced deposition of Pt-nanoparticles enhanced the contact between electrochemically deposited BiVO4 films and fluorine doped tin oxide (FTO) electrodes which showed significantly lower recombination rates during frontside illumination1. Modifications of BiVO4 films by doping with different metals showed enhancements of the IPCE by more than 30% at 1.2 V vs. RHE for Mo/Zn and Mo/B doped systems during light driven oxygen evolution reaction (OER)1. High-throughput screening showed highest improvement of photocurrents by a factor of 10 for Bi(V-Mo-X)O4 material libraries as compared to a BiVO4 reference material2. To further improve the performance of BiVO4-based photoabsorbers a systematic characterisation of different co-deposited OER catalysts onto pre-modified Mo-doped BiVO4 films was performed revealing the necessity for an optimal deposition technique and loading of the co-catalyst in question such as electrodeposition, photoassisted electrodeposition and photo­deposition. Moreover, an alternative technique for simple electrode preparation and modification based on an air brush-type spray-coater system is proposed which allows to easily tune the layer thickness of photoabsorber films and is able to deposit gradients of additives or OER co-catalysts. Using the spray-coating technique in combination with an optical scanning droplet cell allowed for a quick and easy but precise tuning of the investigated systems for further enhancing the performance of BiVO4-based photoabsorbers.

References

R. Gutkowski, D. Peeters, W. Schuhmann, J. Mater. Chem. A, 2016, 4, 7875–7882.

R. Gutkowski, C. Khare, F. Conzuelo, Y. Kayran, A. Ludwig, W. Schuhmann, Energy Environ. Sci., 2017, 10, 1213–1221.

Acknowledgements

The authors are grateful to the financial support of the DFG within the framework of the SPP1613 (SCHU929/12-1 and 12-2).

11:45 - 12:00
S2.6-O3
Gimenez Julia, Sixto
Institute of Advanced Materials (INAM), Universitat Jaume I
Water oxidation with metal oxide semiconductor materials
Sixto Gimenez Julia
Institute of Advanced Materials (INAM), Universitat Jaume I, ES

Sixto Gim�nez (1973, M. Sc. Physics 1996, Ph. D. Physics 2002) is researcher at Universitat Jaume I de Castell� (Spain). His professional career has been focused on the study of particulated materials. 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 (2003-2006) where he focused on the development of non-destructive and in-situ characterization techniques of the sintering behavior of metallic porous materials. In 2006-2007, he was responsible for a new research line on nanostructured particulated materials for magnetic applications at CEIT (Spain). 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 devices and biosensors based on nanoscaled materials, particularly studying the optoelectronic and electrochemical responses of the devices by electrical impedance spectroscopy. He has co-authored more than 30 papers in international journals and has been awarded with a Ramon y Cajal fellowship for 2008-2012.

Authors
Sixto Gimenez Julia a, MIguel García-Tecedor a, Drialys Cardenas-Moscoso a, Roser Fernández-Climent a
Affiliations
a, Institute of Advanced Materials (INAM), Universitat Jaume I, 12071 Castelló, Spain
Abstract

The development of photoelectrochemical strategies for the production of added-value chemicals and fuels using solar light is particularly attractive to overcome the dependence of fossil fuels at a global scale.[1] Specifically, the photoelectrochemical oxidation of H2O to produce solar H2 as a clean energy vector or valuable chemical precursor stands out as one of the most promising approaches in this direction. Different approaches have been followed to achieve competitive devices, targeting Solar To Hydrogen (STH) efficiency of 10%, durability of 10 years and cost of 2-4 $/kg of dispensed Hydrogen.[2] For this purpose, the use of low-cost, earth-abundant, stable materials synthesized by easily up-scalable methods is essential.[3] In the present talk, we will discuss about the suitability of earth-abundant metal oxides to achieve these targets. Different examples of the synergistic combination of metal oxides (Fe2O3[4], and BiVO4[5],[6]) with catalytic layers (Fe-Co Prussian Blue, Ag3PO4, etc…) will be described, 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.

12:00 - 12:30
S2.6-O4
Parkinson, Bruce
University of Wyoming
The Past, Present and Future of Solar Fuels
Bruce Parkinson
University of Wyoming
Bruce Parkinson received his BS in chemistry at Iowa State University in 1972 and his PhD from Caltech in 1977 and was a post-doctoral scientist at Bell Laboratories in 1978. He then spent time at the Ames Laboratory and the Solar Energy Research Institute (now known as the National Renewable Energy Laboratory). He moved to the Central Research and Development Department of the DuPont Company in 1985 and in 1991 he became Professor of Chemistry at Colorado State University until his recent departure to join the Department of Chemistry and the School of Energy Resources at the University of Wyoming. His current research covers a wide range of areas including materials chemistry, surface chemistry and photoelectrochemical energy conversion. He has more than 220 peer-reviewed publications and holds 5 US patents.
Authors
Bruce Parkinson a
Affiliations
a, Department of Chemistry, University of Wyoming, Laramie, WY, USA
Abstract

The interest in the direct storage of solar energy as a chemical fuel in a semiconductor based photoelectrochemical system started with the demonstration of solar photoelectrolysis of water with large band gap oxide semiconductor electrodes in the late 1970s. In the last decade or so there has been both and increased interest and increased funding towards achieving a goal of efficiently producing cost effective fuels from solar energy with either water or carbon dioxide as a feedstock. This talk will review the progress towards this goal considering recent developments. One of these developments is that the cost of photovoltaic systems has been decreasing rapidly to where currently the cost of the solar panels is now exceeded by balance of systems cost. The cost of electrolyzers will be also decrease as they are improved and scaled. Connecting these two existing technologies has the advantage of producing hydrogen where and when it is needed and at pressure. These facts mean that the window for direct solar photoelectrolysis is rapidly closing. One possible breakthrough is that a new stable, efficient, inexpensive, defect-tolerant and scalable new materials are identified that can quickly improve the efficiency of photoelectrolysis much like the hybrid perovskites are have done for photovoltaic devices. This talk will review the progress in combinatorial approaches to discover new materials for photoelectrolysis with some examples including one from the Solar Hydrogen activity Research Kit (SHArK) Project, a distributed science project that provides undergraduates and high school students with the resources to produce and screen metal oxide semiconductors for photoelectrolysis activity. In addition the reasons for producing hydrogen from water rather than direct carbon dioxide reduction to produce fuels will be reviewed. A new system for storing solar energy directly as redox equivalents, a solar chargeable redox flow battery, will also be introduced and its advantages and disadvantages compared to solar hydrogen generation will be discussed.

12:30 - 14:30
Lunch
Sol2D S6.7
Chair: Sandrine Ithurria
14:30 - 15:00
S6.7-O1
Dufour, Marion
ESPCI
Engineering Bicolor Emission in 2D Core/Crown CdSe/CdSe1–xTex Nanoplatelet Heterostructures Using Band-Offset Tuning
Marion Dufour
ESPCI, FR
Authors
Marion Dufour a, Violette Steinmetz b, Eva Izquierdo a, Thomas Pons a, Nicolas Lequeux a, Emmanuel Lhuillier b, Laurent Legrand b, Maria Chamarro b, Thierry Barisien b, Sandrine Ithurria a
Affiliations
a, Laboratoire de Physique et d’Etude des Matériaux, PSL Research University, CNRS UMR 8213, UPMC Sorbonne Université, ESPCI Paris, 10 rue Vauquelin, 75005 Paris, France
b, Sorbonne Universités, UPMC University Paris 06, CNRS-UMR 7588, Institut des Nanosciences de Paris, 4 place Jussieu, 75005 Paris, France
Abstract

Colloidal 2D hetero-nanoplatelets offer new possibilities in terms of heterostructure engineering. The growth can be done either in the confined direction2 or perpendicular to the confined direction3. This second type of heterostruture is called core/crown. It gives the opportunity to tune the composition and the lateral dimensions of each material while keeping a constant confinement (thickness).  We show that synthesizing CdSe/CdSe1-xTex core/crown nanoplatelets with the right composition and lateral extension enables bicolor emission at the single-nanoparticle level. The first transition at low energy comes from core/crown interface recombination (Xint) and is comparable to the one observed in CdSe/CdTe4. The second one, at higher energy, originates from a direct recombination of the exciton in the crown. It is only visible for Te compositions x close to 60% and more likely occurs as the crown dimensions increase. This bicolor emission results from a competition between the conduction band offset that attracts the electron in the core material and the exciton binding energy that retains it in the crown. For 60% of Te those energies are similar (~250meV) and allows the coexistence of the two recombination processes.

 

(1)         Dufour, M.; Steinmetz, V.; Izquierdo, E.; Pons, T.; Lequeux, N.; Lhuillier, E.; Legrand, L.; Chamarro, M.; Barisien, T.; Ithurria, S. Engineering Bicolor Emission in 2D Core/crown CdSe/CdSe1-xTex nanoplatelet Heterostructures Using Band-Offset Tuning. J. Phys. Chem. C 2017, 121, 24816–24823.

(2)         Ithurria, S.; Talapin, D. V. Colloidal Atomic Layer Deposition (c-ALD) Using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media. J. Am. Chem. Soc. 2012, 134, 18585–18590.

(3)         Tessier, D.; Spinicelli, P.; Dupont, D.; Patriarche, G.; Ithurria, S.; Dubertret, B. Efficient ExcitonConcentrators Built from Colloidal Core/Crown CdSe/CdS Semiconductor Nanoplatelets. 2014.

(4)         Pedetti, S.; Ithurria, S.; Heuclin, H.; Patriarche, G.; Dubertret, B. Type-II CdSe/CdTe Core/crown Semiconductor Nanoplatelets. J. Am. Chem. Soc. 2014, 136, 16430–16438.

15:00 - 15:30
S6.7-O2
Lauth, Jannika
Carl-von-Ossietzky University Oldenburg
Colloidal Two-Dimensional PbS Nanosheets and Ultrathin PbS Nanoplatelets – High Mobility vs. Photoluminescence Properties
Jannika Lauth
Carl-von-Ossietzky University Oldenburg
Authors
Jannika Lauth a, Michele Failla b, Francisco Manteiga Vázquez b, Qianli Yu b, Eugen Klein c, Ryan Crisp b, Christian Klinke c, Sachin Kinge d, Arjan Houtepen b, Laurens Siebbeles b
Affiliations
a, Carl von Ossietzky University Oldenburg, Carl von Ossietzky Starß2 9-11, Oldenburg, 26129, DE
b, Department of Chemical Engineering, Delft University of Technology, van der Maasweg 9, 2629 HZ Delft, The Netherlands
c, Institute of Physical Chemistry University of Hamburg
d, Materials Research & Development, Toyota Motor Europe, Zaventem, Belgium
Abstract

Solution-processed two-dimensional (2D) semiconductors with tunable band gaps represent highly promising materials for next generation ultrathin electronics. Their dimensionality-dependent optoelectronic properties differ significantly from their zero-, one- and three-dimensional counterparts and can be tuned by colloidal chemistry methods for controlling the structures’ thickness.

We use optical pump-terahertz probe (OPTHzP) spectroscopy as a non-contact method to determine the thickness-dependent transient charge carrier mobility in 2D PbS nanosheets of different thickness (4 – 16 nm) and find high values ranging from 231 cm2/Vs in the thinnest, 4 nm thick sheets, up to 472 cm2/Vs and 427 cm2/Vs in 6 nm and 16 nm thick PbS nanosheets, respectively. Furthermore, we model the frequency dependent charge carrier mobility of 2D PbS nanosheets with a Drude-Smith behavior and reveal a growing contribution of photoexcited excitons in thinner PbS nanosheets due to their increased exciton binding energy.[1] 

We find that by carefully controlling the reaction kinetics, the thickness of colloidal 2D PbS layers can be reduced to < 2 nm to the formation of ultrathin PbS nanoplatelets. Ultrathin 2D PbS layers are particularly interesting due to their increasing carrier multiplication (CM) efficiency with decreasing nanosheet/nanoplatelet thickness.[2,3] We show that in thicker PbS nanosheets, free and mobile charges are generated under photoexcitation, whereas in ultrathin PbS nanoplatelets bound excitons are formed. A photoluminescence quantum yield of up to 20 % is obtained by surface passivation of the significantly blue-shifted PbS nanoplatelets (Abs: 683 nm, 1.8 eV, PL: 705 nm, 1.75 eV) and underpins their potential for NIR light-emitting applications.[4] Our work emphasizes the excellent usability of colloidal chemistry and spectroscopy methods for producing 2D tailor-made band gap materials for high mobility AND light emitting optoelectronics.

[1] Lauth, J., Failla, M, Klein, E., Klinke, C., Kinge, S., Siebbeles, L. D. A., submitted 2018.

[2] Bielewicz, T.; Dogan, S.; Klinke, C., Small 2015,11, 826-833.

[3] Aerts, M.; Bielewicz, T.; Klinke, C.; Grozema, F. C.; Houtepen, A. J.; Schins, J. M.; Siebbeles, L. D. A., Nat. Commun. 2014,5, 3789.

[4] Manteiga Vazquéz, F., Yu, Q., Crisp, R., Kinge, S., Houtepen, A. J., Siebbeles, L. D. A., Lauth, J., in preparation.

 

15:30 - 16:00
S6.7-I1
de Mello Donega, Celso
Universiteit Utrecht
Ultrathin Colloidal Binary and Ternary Copper Chalcogenide Nanosheets
Celso de Mello Donega
Universiteit Utrecht, NL

Celso de Mello Donega is an Associate Professor in the Chemistry Department of the Faculty of Sciences at Utrecht University in the Netherlands. His expertise is in the field of synthesis and optical spectroscopy of luminescent materials. His research is focused on the chemistry and optoelectronic properties of nanomaterials, with particular emphasis on colloidal nanocrystals and heteronanocrystals.

Authors
Celso de Mello Donega a, Anne Berends a, Ward van der Stam b
Affiliations
a, Universiteit Utrecht, Princetonplein 1, Utrecht, 3584, NL
b, Optoelectronic Materias, TU Delft, Julianalaan 136, Delft, 2628, NL
Abstract

Ultrathin colloidal semiconductor nanosheets with thickness in the strong quantum confinement regime are of particular interest, since they combine the extraordinary properties of 2D nanomaterials with versatility in terms of composition, size, shape, and surface control, and the prospects of solution processability. However, synthesis procedures for materials other than the prototypical Pb- and Cd- chalcogenides are still underdeveloped. Compound Cu-chalcogenides are an interesting class of materials, which have been attracting increasing attention as alternatives to Cd- and Pb-chalcogenides, since they have low toxicity, potentially lower costs, and a very wide range of compositions. This latter point makes them extremely versatile, capable not only of offering similar properties to those already demonstrated by Cd- and Pb-chalcogenide nanocrystals (such as PL tunability in the visible to NIR spectral range and high absorption coefficients), but also unprecedented features, such as plasmon resonances.

In this talk, we discuss recent work by our group on ultrathin (~2nm thick) nanosheets of both binary (Cu2-xA and Cu2-xA, A= S, Se) and ternary (CuInA2 and CuInA2)  Cu-chalcogenides, with well-defined shape (triangular or hexagonal) and dimensions in the ~100 nm to ~1 µm range. The binary nanosheets form through 2D-constrained stack-templated nucleation and growth. The 2D-constraints are imposed by halide-stabilized lamellar Cu-thiolate (or Cu-selenoate) supramolecular complexes that act as soft-templates. Covellite Cu-rich CuInS2 nanosheets form via self-organization and oriented attachment of chalcopyrite CuInS2 nanocrystals, which is induced by a sudden change in the composition of the nanocrystal building blocks due to preferential extraction of In3+ by in-situ generated H2S. Primary amines play several essential roles in the formation of these nanosheets. CuInA2 nanosheets are also obtained by partial self-limited cation exchange in Cu2-xSe or In2S3 template nanosheets. Moreover, the charge carrier dynamics in ultrathin Cu2-xS nanosheets is studied with THz spectroscopy.

  

16:00 - 16:15
S6.7-O3
Molina-Sanchez, Alejandro
University of Valencia
Excitonic States in Semiconducting Two-Dimensional Perovskites
Alejandro Molina-Sanchez
University of Valencia
Authors
Alejandro Molina-Sánchez a
Affiliations
a, Institute of Materials Science of the University of Valencia (ICMUV)
Abstract

Hybrid organic/inorganic perovskites have emerged as efficient semiconductor materials
for applications in photovoltaic solar cells with conversion efficiency above
20 %. In addition, recent experiments have shown the possibility of synthesizing ultra-thin two-dimensional (2D) organic perovskites. These 2D structures would have similar optical properties to others layered semiconductors such as the single-layer
transition metal dichalcogenides (MoS2, WSe2, etc.). For instance, a large exciton
binding energy, together with advantages such as a simple fabrication process with potentially low-cost and large-scale manufacture, and the possibility of having a wide range of optical bandgap values and exciton binding energies by changing the chemical identity of the constituent atoms.

Up to now, state-of-the-art simulations of the excitonic states have been limited
to the study of bulk organic perovskites. A large number of atoms in the unit cell together with the complex role of the molecules makes difficult and inefficient the use
of ab initio methods and the research on the excitonic states in 2D
perovskites have been mainly addressed with semi-empirical methods. In this work, we propose to define a simplified crystal structure to describe
2D perovskites, by replacing the molecular cations with inorganic atoms. Our intention is to apply state-of-the-art, parameter-free and predictive ab initio methods like the GW method and the Bethe-Salpeter equation to obtain the excitonic states of a simple unit cell which resembles a classic 2D material.

We find that inorganic 2D perovskites are stable and hence ultra-thin
2D materials based in all-inorganic perovskites could be synthesized. Moreover,
the optical activity  (like absorption or photoluminescence) is carried out at the bromine and lead atoms and therefore
the conclusions can be qualitatively exported to organic 2D perovskites. For
instance, optical properties of all-inorganic 2D
perovskites are strongly influenced by excitonic effects, with
binding energies up to 0.6 eV. Besides of conceiving a simple material to interpret optical
experiments in more complex 2D organic perovskites, we propose a new set of materials to
increase the family of 2D semiconductors.

16:15 - 16:30
S6.7-O4
Mule, Aniket Sandip
Synthesis and Isolation of Discrete-Growing CdSe Nanocrystals
Aniket Sandip Mule
Authors
Aniket S. Mule a, Simon F. Solari a, Aurelio A. Rossinelli a, Philippe N. Knüsel a, Sergio Mazzotti a, Marianne Aellen a, David J. Norris a
Affiliations
a, Optical Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland
Abstract

Quantum confinement in semiconductor nanocrystals leads to size-dependent optical properties. Conventionally, the growth of nanocrystal results in a gradual red-shift of the features in absorption spectra. However, in certain cases, the absorption features shift in discrete energetic steps with growth. Such discrete growth is often assigned to nanoplatelets or magic-sized clusters. Although the existence and growth of nanoplatelets is proven1,2, the understanding for the latter remains elusive. This is due in part to complicated synthetic routes that involve multiple ligands and coordinating solvents3,4 or specific complex pathways5 (e.g. templates), which make the investigation of such species difficult.  Here, we introduce a simple route to synthesize and isolate stable CdSe nanocrystals showing discrete evolution of absorption features, a subset of which have been previously assigned to magic-sized clusters. We grow them in a template-free regime using only one type of ligand in a non-coordinating solvent up to a size of 2.8 nm. Their isolation is confirmed by absorption, photoluminescence, photoluminescence excitation, and NMR studies. The particles are further analyzed using electron microscopy to evaluate their size and monodispersity. Finally, we provide mechanistic insight into the evolution of different sizes by conducting parametric studies and studying the growth in precursor-free conditions. This synthetic protocol allows us to investigate the early stages of nucleation and growth of nanocrystals in greater detail.

1.         Ithurria, S. et al. Colloidal nanoplatelets with two-dimensional electronic structure. Nat. Mater. 10, 936–941 (2011).

2.         Riedinger, A. et al. An intrinsic growth instability in isotropic materials leads to quasi-two-dimensional nanoplatelets. Nat. Mater. 16, 743–748 (2017).

3.         Kudera, S. et al. Sequential Growth of Magic-Size CdSe Nanocrystals. Adv. Mater. 19, 548–552 (2007).

4.         Cossairt, B. M. et al. CdSe Clusters: At the Interface of Small Molecules and Quantum Dots. Chem. Mater. 23, 3114–3119 (2011).

5.         Wang, Y. et al. Magic-Size II–VI Nanoclusters as Synthons for Flat Colloidal Nanocrystals. Inorg. Chem. 54, 1165–1177 (2015).

 

 
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Sustained Water Oxidation by Direct Electrosynthesis of Ultrathin Organic Protection Films on Silicon
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