Performance-Limiting Factors in Ultralow-Bandgap PTB7-Th:COTIC-4F-Based Organic Solar Cells

Understanding performance-limiting factors of organic solar cells (OSCs) with very small optical bandgaps is crucial for the development of tandem photovoltaics. Here, we investigate ultralow-bandgap (1.15 eV) OSCs based on the blend of the donor polymer PTB7-Th with the nonfullerene acceptor COTIC-4F. In a conventional device structure, we reach record PCE values of 8.55%. However, this PCE still falls behind that of higher-bandgap OSCs. To guide further improvements, we investigate various loss and recombination processes. Complemented with optical and electrical simulations, we show that J SC loss can be largely attributed to inefficient exciton dissociation and geminate recombination of the charge transfer state. Further, we identify a high bimolecular recombination coefficient as the main reason for the poor performance, while surface recombination predominantly affects V OC . Finally, our simulations show an efficiency potential >15% upon simultaneous reduction of bimolecular, surface, exciton, and charge transfer recombination in the PTB7-Th:COTIC-4F system.