Programme
Season 2025/2026
21 January 2026
Paul Drude Institute and University of Potsdam, DE
Organic solar cells have recently exceeded 20% power-conversion efficiency, prompting a key question: how much further can we push performance? Despite rapid advances, progress is limited by photophysical loss channels that are not yet described within a unified framework. At the heart of the problem lies the intricate excited-state dynamics at the donor–acceptor interface, where excitons, charge-transfer states and fully separated charges are in constant interconversion, governed by the materials’ energetic landscape. A central challenge is to understand and mitigate loss pathways involving singlet and triplet charge-transfer states and local triplet excitons, which ultimately constrain the open-circuit voltage. This talk will outline recent insights into these processes and discuss how mastering them could unlock the next efficiency gains in organic photovoltaics.
In this talk, we share our experimental data and kinetic model that, for the first time, explicitly incorporates the formation and re-splitting of local triplet excitons. Fully parameterised by the interfacial energy offset, this unified framework reproduces key photovoltaic observables – such as the charge-generation efficiency, photoluminescence, electroluminescence and the Langevin reduction factor. Our results show that ~the~ triplet-state dynamics may govern device performance. In systems with short triplet lifetimes, triplet decay emerges as the dominant recombination pathway, reconciling long-standing experimental findings, including those in benchmark systems like PM6:Y6. In systems with long triplet lifetimes, triplets can be recycled to mitigate this loss channel. The model further offers a mechanistic explanation for the empirically observed link between energy offset, radiative singlet-exciton decay and reduced-Langevin recombination ~as well as a correlation~, and accurately predicts the device efficiency across different material systems.
By connecting excited-state kinetics with macroscopic device metrics, our work provides a unified mechanistic picture of the photophysics in organic semiconductors.
Eswaran Jayaraman
SDU Centre for Advanced Photovoltaics and Thin-film Energy Devices (CAPE), DK
Sheet-to-Sheet and Roll-to-Roll Processing Large-Area Organic Photovoltaics and Their Lifetime Performance
In recent years, there have been notable successes in commercializing organic photovoltaics (OPV) for new applications. In the laboratory, their power conversion efficiency has exceeded 20% on the cell level using the spin-coating technique. However, the still relatively low performance for large OPV modules, with limited thermal and light stability, has remained a bottleneck for wider adoption. We focused on using a Sheet-to-Sheet and Roll-to-Roll compatible slot-die coating method to fabricate organic photovoltaic cells and modules in ambient conditions, achieving performance similar to that of spin-coated devices. The compatibility of the coating technique was studied across different OPV architectures by fabricating devices using a commercially available Indium tin oxide transparent electrode. The long-term thermal stability of different OPV architectures was studied to identify the degradation pathways. Additionally, we showed long-term light-soaking stability for over 800 hours. Further, to unlock the full Roll-to-Roll compatibility, we adopted a hybrid approach that combines the advantages of R2R vacuum and solution coating methods for fabricating organic solar modules on glass and flexible polyethylene terephthalate (PET) in a top-illumination configuration. The opaque bottom electrodes were developed using R2R sputtering to achieve low sheet resistance and reduced surface roughness. The remaining layers in the devices, including the top transparent silver nanowire (AgNWs) anodes, were optimized using an R2R-compatible slot-die coating method at ambient conditions with greener solvents. We achieved the best PCE of 11.5 % for an ITO-free flexible mini-module. We believe our findings will pave the way for the development of greener, low-cost, and stable organic photovoltaics.
18 February 2026
Eva Unger
Helmholtz-Centre Berlin, DE
TBD
TBD
Hayley Gilbert
University of Cambridge and Diamond Light Source UK
Linking the Chemical and Optoelectronic Properties of Alloyed Perovskites Using Optical and X-ray Spectro-microscopy
Mixed composition lead halide perovskites have emerged as successful light-absorbing materials for next-generation thin-film solar cell devices. For example, state-of-the-art triple cation Cs0.05MA0.17FA0.78Pb(I0.83Br0.17)3 perovskite solar cells have achieved higher efficiency and reproducibility than their single cation counterparts. The tunability of the band gap through compositional engineering is highly desirable for tandem devices and has helped such devices exceed efficiencies of 30%. However, the alloyed nature of these materials makes the thin films used in devices chemically and structurally heterogeneous, which can reduce device performance and long-term stability, creating a bottleneck in the development of perovskites into commercial devices. Additionally, increasing the band gap by increasing the bromide content has been shown to lead to halide segregation occurring upon illumination, reducing efficiency. Despite being integral to long term efficiency and stability, the mechanism behind halide segregation is not fully understood.
To elucidate the mechanism of halide segregation and the heterogeneous nature of alloyed perovskites thin films, we use photoluminescence (PL) spectroscopy, hyperspectral PL microscopy, and synchrotron X-ray techniques including X-ray absorption near edge spectroscopy (XANES) and XANES mapping under operando illumination conditions. This suite of approaches allows us to probe the optoelectronic performance, ion migration, elemental composition, and local chemical environment and structure both macroscopically and as a function of position across the thin film samples. By correlating these metrics, we gain a deeper understanding of the driving forces of performance-limiting halide segregation in these materials beyond what any single technique could deliver alone.
Past Events
19 November 2025
Luis Huerta Hernandez
Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), SA
The mixed ionic-electronic transport properties of Sn-based perovskites
The mixed ionic-electronic conduction in metal halide perovskites is responsible for several phenomena observed in perovskite devices, including hysteresis in current-voltage (IV) curves, formation of charge extraction barriers at interfaces with contacts, and detrimental chemical reactions between mobile ions and contacts. In Sn-based perovskites, an enhancement in the electronic conductivity is induced by a high hole density [p] and the facile formation of Sn vacancies (VSn2-). However, how the ion transport is influenced by the [p] and VSn2- remains elusive. In this talk, we discuss the link between electronic and ionic transport in Sn-based perovskites. Our results indicate that ionic and electronic conductivities concomitantly rise with higher Sn content. We demonstrate that the mobile ion density is enhanced at higher [p] and VSn2-, resulting in improved lateral ion migration and ionic conductivity. We further identify iodide as the most mobile ionic species in Sn-based perovskites. Our theoretical calculations prove that [p] and VSn2- jointly reduce the energy barrier for iodide migration from 0.38 to 0.12 eV. Hence, our results suggest that holes and VSn2- facilitate the transport of mobile ions in Sn-based perovskites. Considering the intrinsic high [p] typically found in this class of materials, the ionic-electronic coupling has major implications for the performance and stability of Sn-based perovskite devices
Kai Brinkmann
Institute of Electronic Devices and Wuppertal Center for Smart Materials and Systems, University of Wuppertal, DE
How Crystallization Agents Govern Halide Perovskite Grain Growth
Perovskite solar cells are approaching market readiness, yet large-scale commercialization demands precise control over film quality. A major source of uncertainty in processing arises from the elusive role of external influences and crystallization additives. While additive engineering has certainly improved reproducibility and performance, the underlying mechanisms are still debated, as common explanations using heterogeneous nucleation frequently fall short on explaining the observation from the lab. As a result, perovskite deposition often relies on heuristic approaches, guided by experience rather than by fundamental understanding, which can cause considerable frustration among researchers and engineers when small variations lead to unpredictable outcomes.
After a year-long jorney and an unprecentably broad collaboration amongs various disciplins, we can now unravel that the decisive influence of typical crystallization additives lies not in the nucleation phase but in promoting grain coarsening through enhanced ion mobility across grain boundaries. In-situ and ex-situ spectroscopy, diffraction, and microscopy were combined with device performance analysis, phase-field simulations, and DFT calculations to build a coherent mechanistic picture that holds across various perovskite compositions and additive types.
Our results indeed suggest that a plethora of polular crystallization additives interact with grain boundaries forming ion-conductive pathways during annealing – which in turn facilitates grain coarsening, that is the roop cause the outcome of successful additive engineering. Indeed it seems like most approaches of additive engineering follows identical mechanisms like several post-processing approaches, which is why we are now able to present a unified framework that seems to apply both for additive engineering and post-process grain engineering, which we believe will improve the comminates control over the perovskite formation.
15 October 2025
Philip Schulz
IPVF - Institut Photovoltaïque d'Île-de-France, Fr
Beyond the Surface: Advanced XPS Approaches for Perovskite Solar Cell Interface Analysis
Metal halide perovskites (MHPs) represent a versatile class of semiconductors that have redefined modern optoelectronics, most prominently through their application in perovskite solar cells. Over the past decade, the power conversion efficiency of these devices has risen dramatically. However, further improvements in both performance and operational stability critically depend on precise control of the interfaces between the MHP absorber and the adjacent charge transport layers.
Photoemission spectroscopy (PES) offers a powerful means to probe the chemical and electronic structure of these buried interfaces, though its application remains challenging due to the complex reactivity of perovskite materials under investigation. In my talk, I will discuss the combined use of synchrotron- and laboratory-based X-ray photoelectron spectroscopy (XPS) to elucidate the interfacial chemistry between MHP films and adjacent oxide charge transport or pre-encapsulation layers. Using hard X-ray photoelectron spectroscopy (HAXPES), we specifically examine atomic-layer-deposited (ALD) SnO₂ and NiO layers on a double-cation mixed-halide perovskite, revealing the formation of new chemical species and interfacial energy level shifts that can impair device performance.
I will conclude with a broader perspective on the application of PES methods for the characterization of MHP materials, highlighting the dual role of X-ray irradiation as both a source of material degradation and a probe to uncover intriguing self-healing phenomena such as the light- and radiation-induced recovery observed in formamidinium lead bromide films.
Matteo Degani
Department of Chemistry, University of Pavia, IT
Large Organic Cations as Enablers of Record-Breaking Efficiency Through Passivation of Perovskite Solar Cells
The use of large organic ammonium cations has emerged as a game-changing strategy in the field of perovskite photovoltaics, unlocking record-breaking efficiencies and unprecedented operational stability. When deposited onto the surface of three-dimensional (3D) perovskite layers, these bulky cations serve a dual function: they effectively passivate surface and interfacial defects, and they can promote the formation of distinct layers at the interface. Depending on the nature of the cation and the processing conditions, two main scenarios can occur:
the assembly of a thin molecular monolayer, or the crystallization of a low-dimensional perovskite capping layer. While both configurations have demonstrated beneficial effects on device performance, their underlying mechanisms differ significantly and are often misunderstood or generalized in literature. This talk aims to shed light on the fundamental differences between two distinct passivation strategies employed in perovskite solar cells: molecular layer formation and low-dimensional perovskite capping. By examining critical parameters—including cation size, solvent dynamics, and precursor stoichiometry. Representative examples will be presented for both conventional (n-i-p) and inverted (p-i-n) device architectures, highlighting how each passivation approach influences photovoltaic performance and stability.
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