Proceedings of nanoGe Fall Meeting 2021 (NFM21)
DOI: https://doi.org/10.29363/nanoge.nfm.2021.090
Publication date: 23rd September 2021
The device performance of organic bulk heterojunction (BHJ) solar cells is ultimately derived from processes taking place between excitons, charge-transfer (CT) states, and free charge carriers. In fullerene-acceptor-based BHJ systems, characterized by large donor-acceptor energy level offsets, the photovoltage is typically limited by the energetics and kinetics of CT states.[1] However, this picture has been challenged with the emergence of nonfullerene-acceptor-based systems with low energy offsets,[2] where the role of excitons becomes important. In this work, the interplay between excitons, CT states and free charge carriers is investigated.[3] Based on detailed balance considerations, we derive an analytical framework describing the photocurrent quantum yield, the charge-carrier recombination rate constant, and the open-circuit voltage in low-offset BHJ systems. Furthermore, the essential conditions for mutual equilibrium between the different species to occur are clarified. Finally, we find that the photovoltaic parameters are not only limited by the energetics but also critically determined by the relative kinetics between the different species. These findings provide insights into the operation of state-of-art non-fullerene organic solar cells with low offsets, paving the way for power conversion efficiencies exceeding 20%.
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