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Our group has investigated the use of Self Assembled Monolayers (SAMs) made of semiconductor hole transport organic molecules to replace the most common p-type contact, PEDOT:PSS, in perovskite based solar cells. The SAMs can be easily incorporated into the devices by simple solution processing by dipping the TOC ( transparent conducting oxide glass) in a solution containing the SAM. The SAMs are transparent, do not have influence over the ion migration mechanismes that have been found in perovskite solar cells and are very robust. The SAM molecule leads to a solar to energy conversion efficiency always higher than PEDOTPSS. In some cases over 17% which is among the highest efficiencies reported to date for PiN perovskite solar cells. The present finding highlights the potential of semiconductor based SAMs to fabricate stable and high performing planar PSCs. Moreover, intial measuements of carrier losses on this type of solar cells will be also presented.
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Head of the Young Investigator Group Dr. Antonio Abate
Photovoltaic solar cells can directly convert the sunlight into electric energy by making use of the photovoltaic effect in semiconductors. Halide perovskites are emerging crystalline semiconducting materials with among the strongest light absorption and the most effective electric charge generation needed to design the highest efficient photovoltaic solar cells. Their efficiency has increased very rapidly from 3.8% in 2009 to over 22.1% nowdays. The long-term stability of these cells is crucial for their economic viability, and yet this criterion is still barely studied. Antonio Abate’s goal is to develop stable perovskite solar cells with an operating life exceeding 25 years.
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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.
Both transient and steady state photoluminescence PL have been frequently used to analyze the properties of halide perovskite films[1] and recently also layer stacks, i.e. films with interfaces.[2-4] Here, we present our current level of understanding of how to analyze the data. In the case of films, long decays in transient PL correlate well with strong steady state PL. The shape of the decays allows us to determine bimolecular and monomolecular recombination coefficients, the former of which is clearly affected by photon recycling.[1, 5] In the case of films with one interface, we show that high luminescence is still beneficial for high open-circuit voltages in devices and still correlates with long photoluminescence decays.[2] We show by simulation how the combination of steady state PL with tr-PL can be used to better understand band alignment at interfaces and how it provides an estimate of the surface recombination velocities. Finally, we discuss the case of layer stacks with two contacts and of full devices. Here, additional effects such as the conductivity and capacitance of contact layers become important. In addition, the question arises on how to compare purely optical techniques with techniques that use electrical detection. One example is the comparison between transient PL and transient photovoltage. The difference between the two is that transient PL measures the internal voltage, i.e. the quasi-Fermi level splitting, and transient photovoltage measures the external voltage that builds up at the external terminals of the cell. While both decays are affected by the contact layers, the impact is substantially different. The external voltage first has to be built up by charging up the capacitance of the interfacial layers, the internal voltage peaks immediately after the laser pulse and then decays fairly quickly.
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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 . Physics of Solar Cells: Perovskites, Organics, and Photovoltaics Fundamentals (CRC Press). His h-index 95, 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 2014-2019 list of ISI Highly Cited Researchers.
The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying new physical behaviours that may lead to developing new applications. These responses are the outcome of complex interactions of electronic and ionic carriers in the bulk and at interfaces. Based on a systematic application of frequency modulated techniques and time transient techniques to the analysis of kinetic phenomena, we present a picture of the dominant effects governing the kinetic behaviour of halide perovskite devices. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases. The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We also show the identification of the impedance of ionic diffusion by measuring single crystal samples. Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in defects of the structure.1 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.2 We describe important insights to the measurement of EQE in frequency modulated conditions, which shows that the quantum efficiency can be variable at very low frequencies. The combination of IMPS and Impedance Spectroscopy is able to provide a detailed picture that explains low frequency characteristics, influencing the fill factor of the solar cell. As a summary we suggest an interpretation of the effects of charge accumulation, transport, and recombination. on current-voltage characteristics and time transient properties, and we suggest a classification of the time scales for ionic/electronic phenomena in the perovskite solar cells.
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Prof. Dieter Neher studied physics at the University of Mainz. In 1990 he gained his PhD with Prof. G. Wegner. From 1990-1992 he was a research associate at the Optical Sciences Centre, Tucson, Arizona and at the Centre for Research in Electrooptics and Lasers, Orlando, Florida with Prof. G. Stegeman. 1992 he joined again Prof. G. Wegner at the MPI-P, heading the group Electrooptical Phenomena in Polymers. Following his habilitation in November 1998, he became Professor of Soft Matter Physics at the Institute for Physics and Astronomy at the University of Potsdam. Current research interests are electrical, optical and optoelectronic processes in conjugated materials.
Perovskite semiconductors demonstrate a large potential for commercial applications in single and multijunction solar cells, detectors or LEDs. A key for these applications is their highly fluorescent nature due to low defect densities and their simple processability from solution. However, complete devices suffer from losses to due non-radiative losses. This talk presents recent advances in understanding and suppressing non-radiative recombination in perovskite solar cells. Quantification of the quasi-Fermi level splitting through precise measurements of the photoluminescence quantum yield (PLQY) on perovskite films with and without attached charge transport layers pinpoint the origin of these recombination losses. These studies highlighted the role of interfacial recombination in limiting the internal voltage in the absorber layer [1-3]. By performing these measurements as function of light intensity allowed us to experimentally assess the efficiency potential of any neat perovskite film on glass, with or without attached transport layers [4]. We find that properly-passivated triple cation perovskite films exhibit exceptionally high implied PCEs >28%. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency towards the thermodynamic limit.
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Incident photon to current efficiency (IPCE) measurement is used to characterize solar cells or photoelectrodes. For the IPCE measurements, a chopper based setups with Xe arc lamp is normally used. Such setup provides unstable light illumination which changes with time so no two IPCE measurements can be truly compared. Besides, these setups require long warm-up time, cause thermal heating of the solar cell/photoelectrode and have low power efficiencies. In this work, an advanced setup is used for the IPCE measurements. This setup provides a stable and reproducible LED light source. The light from LEDs is controlled electrically and provides an extensive frequency range for the IPCE measurement. IPCE measurements on such extensive frequency range are not possible with a mechanical chopper. With the advanced IPCE setup, complementary information is also obtained, i.e., phase information for real sine wave excitation, which provides further insight into the charge transfer kinetics of the system under investigation.
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Richard Friend holds the Cavendish Professorship of Physics at the University of Cambridge. His research encompasses the physics, materials science and engineering of semiconductor devices made with carbon-based semiconductors, particularly polymers. His research advances have shown that carbon-based semiconductors have significant applications in LEDs, solar cells, lasers, and electronics. His current research interests are directed to novel schemes – including ideas inspired by recent insights into Nature’s light harvesting – that seek to improve the performance and cost of solar cells.
Management of the Coulomb interaction in organic LEDs and solar cells
The physics of organic semiconductors is often controlled by large electron-hole Coulomb interactions and by large spin exchange energies. I will discuss recent strategies that allow these interactions to be harnessed for efficient device operation. For LEDs, 3:1 statistical formation of triplet:singlet excitons through electron-hole recombination limits efficiency if the triplet state is non-emissive, but is recovered for systems with reduced exchange energy and strong spin-orbit coupling in organo-metallic systems or for system where triplet-triplet collisions produce singlet excitons efficiently. We have recently demonstrated that π-conjugated radical materials with doublet ground states can operate with high efficiency in the doublet manifold. For organic PV systems, long-range charge separation from the donor-acceptor heterojunction must overcome a substantial Coulomb barrier, which we find always sets up a measurable optical Stark shift that we measure to be 200 meV or more. Whether this separation is ultrafast (sub-picosecond) or slow (>10 psec) depends on the ‘excess’ energy provided by the photogenerated exciton to the separating electron-hole pair, both for fullerene acceptor systems and those with non-fullerene acceptors. Longer time bimolecular recombination should show similar spin statistics to LED operation, causing significant non-radiative recombination for triplet formation where there is easy access to a low-lying localized triplet states. I will present evidence that though fullerene systems generally show rapid bimolecular triplet formation, this can process can be very strongly reduced in some non-fullerene acceptor systems.
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Multiexciton generation through singlet fission has the potential of exceeding Shockley–Queisser limit in photovoltaic devices. However, only very few materials suitable for singlet fission are available at present and the mechanism of inter- and intra-molecular singlet fission are not fully understood. Detailed knowledge regarding the processes is crucial for developing new materials. In this talk, I will present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character. This emergent mechanism should motivate new time-resolved experiments for spin-state tracking in SEF-active materials, while bringing into focus the use of chromophoric bridges, which can simultaneously act as antennas for sensitizing iSEF and fine-tune the properties that enable fission.
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In the field of organic photovoltaics, efficiencies beyond 17% have recently been achieved by combining low bandgap conjugated polymers with small-molecule non-fullerene acceptors (NFAs). To understand what differentiates non-fullerenes from conventional fullerenes in terms of the charge separation and transport processes, we have used ultrafast transient absorption spectroscopy (TAS), electro-modulated differential absorption spectroscopy (EDA) and terahertz measurements (THz). Combinations of different polymers (J61, P3HT, PCDTBT) with the m-ITIC acceptor were investigated. We have thus demonstrated how charge generation and recombination processes depend on parameters such as the charge-transfer driving force, the short-range charge mobility and the morphology. Moreover, to simplify the complexity of the processes caused by the phase morphology of bulk heterojunction blends, we have worked with bilayers of the donor polymer with the NFA, and with blends containing dilute concentrations of the acceptor. We show for example that hole-transfer processes can be very fast in spite of negligible driving force, and that free charge generation is strongly impacted by THz mobilities.
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Two-dimensional hybrid metal-halide perovskites are quantum-well like structures that exhibit large exciton binding energies (200 – 400 meV) and have garnered considerable attention for many emerging quantum opto-electronic applications. We investigated the peculiar spectroscopic signatures that make the excitons in these materials distinct from conventional 2D semiconductors. The most striking one of them is the spectral finestructure with an energetic spacing of about 35-40 meV that is ubiquitously present at the excitonic transition in their linear absorption spectra[1]. Via a combination of linear and coherent non-linear optical spectroscopies, we show that (a) there are at least four distinct excitonic states that are non-adiabatically coupled via phonon-driven fluctuations[1,2], (b) Each one of them is distinctly dressed by the lattice optical phonons[3], (c) they are subjected to diverse elastic exciton-exciton and exciton-phonon scattering processes[4] and (d) Stable biexcitons are present, yet they differ in the presence of both attractive and repulsive inter-excitonic interactions[5]. Given the direct consequences of phonon interactions in the finestructure characteristics and due to the fundamental polaronic nature of excitations in ionic metal-halide perovskite lattices, we propose that exciton-polarons are primary photo-excitations in these material architectures.
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The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices can change dramatically during solution deposition and subsequent annealing processes. As solvent vaporizes or as heat is applied, molecules aggregate and rearrange themselves, changing their electronic coupling. This can dramatically change the exciton dynamics in the material and thus the suitability of the material for electronic devices. The exciton dynamics of molecules in solution and in films of aggregates can be measured using transient absorption spectroscopy. However, the progression of exciton dynamics during film formation and annealing is unknown since measurements typically cannot be performed quickly enough to collect accurate transient absorption spectra of these species. The exciton dynamics of evolving material systems can be measured by increasing the speed of data collection. A novel implementation of transient absorption spectroscopy is introduced that can measure transient spectra with up to a 60 ps pump-probe time delay in one shot. The exciton dynamics of intermediate aggregation states are revealed during the formation and annealing of an organic film. The information gained using this technique can be used to modify environmental parameters during the film formation process to kinetically trap aggregates with exciton dynamics tailored for particular types of electronic devices.
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Aldo Di Carlo is Director of the Institute of Structure of Matter of the National Research Council and Full Professor of Optoelectronics and Nanoelectronics at the Department of Electronics Engineering of the University of ROme "Tor Vergata". His research focuses on the study and fabrication of electronic and optoelectronic devices, their analysis and their optimization. Di Carlo founded the Center for Hybrid and Organic Solar Cells (CHOSE) which nowadays involve more than40 researchers dealing with the development of III generation solar cells (DSC, OPV and Perovskite) and on scaling-up of these technologies for industrial applications. CHOSE has generated 6 spin-off companies and a public/private partnership. Di Carlo is author/coauthor of more than 500 scientific publications in international journals, 13 patents and has been involved in several EU projects (three as EU coordinator)
Perovskite photovoltaic technology can be scaled to large area modules and panels by using printing processes and laser pattering. In this talk we will present the progresses made to scale up from small area solar cells to modules and panels with dimension of 0.5 sqm. In particular we show the successful application of 2D materials, i.e., graphene [1], functionalized MoS2 [2] and MXenes[3], in perovskite solar modules (PSMs) and panels (PSPs) by interface engineering the standard mesoscopic n-i-p structure. The use of 2D materials has the dual role to improve both the stability and the overall power conversion efficiency (PCE) of the PSMs compared to standard devices. Moreover, 2D materials-based PSMs show reproducible performance over large module number and remarkable stability under prolonged thermal stress test at 85°C. By applying the 2D interface technology, we are able to fabricate large area modules (136 cm2 active area) with efficiency of 14.7%. Several 0.5 m2 panel panels were realized and tested in outdoor condition demonstrating a power conversion efficiency (PCE) exceeding 10% on active area.
[1] A. Agresti et al. ACS Energy Lett. 2019, 4, 8, 1862-1871
[2] L. Najafi et al ACS Nano 2018, 12, 11, 10736-10754
[3] A. Agresti, et al., Nature Materials volume 18, pages1228–1234(2019)
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Dr. Hongwei Han is Professor at Huazhong University of Science and Technology (HUST) / Wuhan National Laboratory for Optoelectronics (WNLO), and Distinguished Professor of ‘ChangJiang Scholars Program’. He obtained his bachelor degree from the College of Chemistry and Molecular Science in 2000 and his doctor degree from the School of Physics and Technology in 2005 at Wuhan University. And then, Dr. Han continued his research work at Monash University of Australia as Postdoc. After that he joined HUST and WNLO in 2008 and began to establish his group of Printable Mesoscopic Photovoltaics & Optoelectronics. Since 2000, Dr. Han has worked on the fully printable mesoscopic solar cells. The characteristic of such device is to print nanocrystalline layer, spacer layer and counter electrode layer on a single conductive substrates layer-by-layer, and then sensitized with dye and filled with electrolyte (or filled with perovskite materials directly). In 2015 his group fabricated 7m2 fully printable mesoscopic perovskite solar module. His more than 60 peer-reviewed publications in Science、 Nature Chemistry、 Nature Communications、J. Am. Chem. Soc.、Energy Environ. Sci. et al. have been published and 15 Patents have been applied within past five years.
Over the past few years, organic–inorganic hybrid perovskites have attracted significantly attention as light absorbers in efficient photovoltaics. While impressive power conversion efficiencies exceeding 25% have been attained within a period of only a few years, concerns have been raised about the viability of this class of photovoltaics as a scalable and long-term reliable energy source. In our group the printable perovskite solar cells were developed with triple mesoscopic layers1. The conjugated or non-conjugated bifunctional molecules were introduced into the perovskite materials to enhance their stability and efficiency2. The results indicated that the triple mesoscopic structure and functional molecular shows a very important role in the inhibition of ionic migration and material decomposition. The fully printable mesoscopic perovskite solar cell presents no obvious decay within over 8000h light soaking and high certified efficiency of more than 18%. WThe crystals of the perovskite films are reconstructed by post-treating with methylamine gas and allows the regeneration of the photodegraded PSCs via the crystal reconstruction and the PCE can recover to 91% of the initial value after two cycles of the photodegradation-recovery process. Meanwhile, the characterization of the mesoscopic perovskite solar cells under the UV light soaking condition was performed. A 110 m2 mesoscopic perovskite solar system3,4 was exhibited. These results offer a promising prospect for its commercial application.
References
(1) Z. L. Ku, Y. G. Rong, M. Xu, T. F. Liu, H. W. Han, Full Printable Processed Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells with Carbon Counter Electrode, Scientific Reports 2013, 3, 3132.
(2) A. Y. Mei, X. Li, L. F. Liu, Z. L. Ku, T. F. Liu, Y. G. Rong, M. Xu, M. Hu, J. Z. Chen, Y. Yang, M. Gratzel, H. W. Han, A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability, Science 2014, 345, 295.
(3) Yaoguang Rong, Yue Hu, Anyi Mei, Hairen Tan, Makhsud I. Saidaminov, Sang Il Seok, Michael D. McGehee, Edward H. Sargent, Hongwei Han, Challenges for commercializing perovskite solar cells, Science 2018, 361,
(4) Yue Hu, Yanmeng Chu, Qifei Wang, Zhihui Zhang, Yue Ming, Anyi Mei, Yaoguang Rong, Hongwei Han, Standardizing Perovskite Solar Modules beyond Cells, Joule 2019, 3, 2076.
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Perovskites for solar cells have an ABX3 structure where the cation A is MA, FA, or Cs; the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities.
Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase” at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures suppress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase” of FAPbI3 that can be avoided using the also unstable CsPbI3). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibality and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5)
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The Voc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V.
Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that EA has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology.
The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6)
(1) Jeon et al. Nature (2015)
(2) Lee et al. Advanced Energy Materials (2015)
(3) McMeekin et al. Science (2016)
(4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016)
(5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016).
(6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture Science (2018)
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He obtained a Ph.D. (cum laude) in Nanoscience and Nanotechnology, University of Valencia (Spain), 2017. He has over 6 years of experience in third generation photovoltaics, obtained at internationally recognized institutions. He joined Saule Technologies in 2017, and since July 2018 he is the Director Of Knowledge Management. He is leading a team involved in a broad range of activities such as; Business Development, Marketing, IP and Project Management and currently has a particular focus on product development for IoT applications.
The internet of things (IoT) stands for the extension of the internet beyond computers and smartphones to a broader scope of devices that sense, gather, generate, and exchange information. Most of these smart sensors are in our homes, offices or in industrial environment, remote monitoring for example physical parameters such as temperature, pressure, humidity, sensing volatile organic compounds or detecting motion. Currently, more than 20 billion IoT devices are operating and the count of devices is expected to grow to about 75 billion by 2025, of which the majority will reside indoors. In order to pave the way for real world autonomous operation capabilities, power sources other than conventional batteries must be applied to efficiently harvest the photons from indoor light sources such as white light emitting diodes and fluorescent tubes. Organic-inorganic perovskites solar cells are ideal candidates for efficient indoor power generation owing to their inherent physical properties such as high absorption coefficient, long carrier diffusion length and exceptional defect tolerance. In addition, the band gap tunability of hybrid perovskites allows to seamlessly adjust for a given light source scenario. Saule Technologies has been developing a fully scalable inkjet printing process of perovskite solar cell modules on lightweight flexible substrates. Our perovskite solar cell solutions offer high power density at low light intensity conditions and at the same time exhibit long lifetime to guarantee autonomous operation for IoT devices far beyond typical battery lifetime.
It has been a long but exciting way from the first laboratory samples to the first commercial perovskite solar cell production line. The first applications of hybrid perovskite solar cells will include powering IoT devices and many more will follow soon, given that adequate production capacity has already been established.
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The capability to use optical model to precisely predict the light propagation property and charge generation rate within the devices allows us to design optimal device architectures with improved performance. In this talk, I will discuss how to apply high throughput optical model to rapidly screen more than 10 million device structures in order to identify the very best device design for extremely high performance tandem[1] and semitransparent polymer solar cells (ST-PSCs)[2]. In addition, I will highlight how to engineer the optical property of ST-PSC for smart greenhouse applications[3] and the design of multiple-function ST-PSC with both heat insulation and power generation properties[4].
Reference
M. Li, X. Wan, L. Ding, H.-L. Yip, Y. Cao, Y. Chen, et al, Science, 2018, 361, 1094
R. Xia, C. Brabec, H.-L. Yip, Y. Cao, Joule, 2019, 3, 2241
H. Shi, R. Xia, H.-L. Yip, et al, Adv. Energy Mater., 2018, 8, 1803438
C. Sun, R. Xia, H. Shi, F. Huang, H.-L. Yip, et al, Joule, 2018, 2, 1816
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Ji-Seon Kim is a Professor of Solid State Physics and Director of the EPSRC Centre for Doctoral Training in Plastic Electronic Materials (https://www.imperial.ac.uk/plastic-electronics-cdt/) at Imperial College London (UK). She also holds an Invited Professorship at Tokyo Institute of Technology (Japan) and held an Invited Visiting Professorship at KAIST (South Korea). She has previously taken up an EPSRC Advanced Research Fellowship at the University of Cambridge, obtained a PhD in Physics in 2000 at Cambridge.
Her research focuses on the basic science and technology of Soft Electronic Materials and Devices (organic semiconductors, organic/inorganic hybrids and bio-nanomaterials), exploring fundamental scientific issues related to using these materials for new optoelectronic applications (energy, display and healthcare).
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Jenny Nelson is a Professor of Physics at Imperial College London, where she has researched novel varieties of material for use in solar cells since 1989. Her current research is focussed on understanding the properties of molecular semiconductor materials and their application to organic solar cells. This work combines fundamental electrical, spectroscopic and structural studies of molecular electronic materials with numerical modelling and device studies, with the aim of optimising the performance of plastic solar cells. She has published around 200 articles in peer reviewed journals, several book chapters and a book on the physics of solar cells.
In organic heterojunction devices, current generation results from the sequence of photon absorption, charge separation, and charge collection in competition with recombination. To understand and design organic PV devices, we need models of these processes that incoporate both the device architecture and the molecular nature of the materials. Device models work fairly well in describing charge collection and recombination, and resulting curent-voltage curves, but usually with some empirical form for the charge generation efficiency and recombination coefficients. A full description of microscopic processes such as interfacial charge transfer requires molecular scale models. For design purposes, we would like to be able to predict device behaviour from the properties of the molecular components, but it is challenging to combine these aspects in a single model. In this talk we will discuss the degree to which molecular level models and time-resolved device models can explain measurements both of charge carrier dynamics, and of overall device behaviour. We will then address the challenges in bringing the two approaches together into a single framework.
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Professor Meredith is the Sêr Cymru Research Chair in Sustainable Advanced Materials at Swansea University Department of Physics in the United Kingdom where he also leads the newly established Centre for Integrative Semiconductor Materials. He is an Honorary Professor at the University of Queensland in Australia, and formerly an Australian Research Council Discovery Outstanding Researcher Award Fellow. He was educated in the UK at Swansea, Heriot-Watt and Cambridge Universities, and also spent six years as a senior scientist at Proctor and Gamble. His current research involves the development of new high-tech materials for applications such as optoelectronics and bioelectronics. He has particular interests and expertise in next generation semiconductors, functional surface coatings, solar energy systems, sensing and photodetection. Professor Meredith has published >250 papers and 29 patents and is co-founder of several start-up companies including XeroCoat and Brisbane Materials Technology. He is the recipient of numerous awards including the Premier of Queensland’s Sustainability Award (2013), is a Fellow of the Learned Society of Wales, a Fellow of the Institute of Physics and is widely recognised for his contributions to innovation and the promotion of renewable energy. He has served on several advisory bodies and boards including the Queensland Renewable Energy Target Public Enquiry Expert Panel and the ARENA Solar R&D Program Technical Advisory Board. In 2020 he received an OBE for services to materials research and innovation and was also appointed to the EPSRC’s Strategic Advisory Network in 2021.
The emergence of so-called non-fullerene electron acceptors (or NFAs) have delivered a step change in the power conversion efficiencies of single junction organic solar cells with >18% recently reported. The development of these materials has evolved rather empirically, and the task at hand is now to develop consistent structure property relationships to guide further rational molecular and architectural design. The NFAs appear in several ways to be quite different to the fullerene acceptors that were the mainstay n-type organic semiconductors for several decades. In particular, we have been studying their electro-optical properties and have found a number of surprising features relating to charge transfer state energetics, low finesse cavity effects, and charge generation and recombination [1,2]. In my talk I will present some of our new findings on NFAs and pose the question as to whether these materials require a ‘re-write of the electro-optical rulebook’ of organic solar cells.
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The traditional concept for organic solar cells (OSC) suggests an offset in energy levels (Eoffset) to provide sufficient driving force to split excitons into free charge carriers. Eoffset is a most important Key performance Indicator (KPI), as a low Eoffset increases the open-circuit voltage (VOC), however, may also lead to a poor charge generation efficiency. Understand the factors limiting device operation at very small Eoffset is therefore of outmost importance. In this presentation we show that exciton splitting in highly efficient NFA systems at negligible EHOMO, offset still takes place, but on ultra-long timescales, even exceeding the exciton lifetime, which obviously becomes the ultimate limit for efficient systems. Moreover, we analyze the voltage losses and surprisingly, in systems where no charge transfer state is detected, we show that the non-radiative voltage losses still correlate with the small but non-negligible EHOMO offset until reaching the pristine materials´ limit.
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