Proceedings of MATSUS Spring 2024 Conference (MATSUS24)
DOI: https://doi.org/10.29363/nanoge.matsus.2024.386
Publication date: 18th December 2023
Understanding the performance limiting factors of organic solar cells (OSCs) with very small optical bandgaps is crucial for the development of novel tandem photovoltaics (PVs), such as OSC/OSC or perovskite/OSC tandems. For example, by combining a low-bandgap OSC ~ 1.1 eV with a high-bandgap absorber ~ 1.7 eV into a tandem solar cell, a power conversion efficiency (PCE) as high as 31.0% can be theoretically achieved, according to the detailed balance (DB) limit.
In this study, we investigate the loss mechanisms of ultra-low-bandgap OSCs based on the blend of the donor polymer PTB7-Th with the non-fullerene acceptor (NFA) COTIC-4F, with a bandgap of 1.15 eV, a promising candidate for tandem PV. In a conventional device structure, we reach a record PCE value of 8.55% with an enhanced short-circuit current density (JSC) and fill factor (FF) compared to previous records that relied on an inverted device geometry. While setting a record for this system, our PCE metrics still fall behind higher-bandgap OSC systems.
To guide further improvements, we investigate the various loss mechanisms and recombination processes using photoluminescence (PL) measurements, bias-dependent time-delayed collection field (TDCF) measurements, bias-assisted charge extraction (BACE), fluence-dependent photoinduced absorption (PIA), as well as light intensity-dependent open-circuit voltage (VOC). Complemented with optical and electrical device simulations, we show that JSC loss can be largely attributed to inefficient exciton dissociation in combination with geminate recombination of the charge transfer state. Further, we identify the fairly high bimolecular recombination coefficient as the main reason for the poor performance, while surface recombination is shown to mainly affect VOC. Finally, our simulations show that the simultaneous reduction of bimolecular recombination (e.g. by ternary blends), surface recombination (e.g. by self-assembled monolayers), exciton and charge transfer recombination (e.g. by vapor annealing) would enable efficiencies of >15% in the PTB7-Th:COTIC-4F system.
M.P., S.S., and D.N. acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through the project Extraordinaire (Project Number 460766640). F.L. acknowledges funding by the Volkswagen Foundation via the Freigeist Program. M.S. acknowledges the Heisenberg program from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for funding, Project Number 498155101. G.H. acknowledges funding from the Kommission für Gleichstellung und Frauenförderung (GFK, Commission of Equal Opportunity) of Universität Potsdam.
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