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 JSC 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 VOC. 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.