End Group Tuning In Acceptor-Donor-Acceptor Nonfullerene Small Molecules For High Fill Factor Organic Solar Cells

High fill factors have only recently become commonplace in nonfullerene-based organic solar cells, with the balance of charge carrier mobilities often cited as the contributing factor. Here an end-group modification to a commonly used nonfullerene acceptor (O-IDTBR) is reported, in which the rhodanine end groups are replaced with dicyano moieties, resulting in the acceptor O-IDTBCN. This new acceptor affords significant improvement in the fill factor (73%) and photocurrent (19.8 mA cm(-2)) in organic solar cells with the low bandgap polymer PTB7-Th. A narrowing of the bandgap, as a result of greater push-pull hybridization, allows complementary absorption to the donor and thus improved photon harvesting. Additionally, the measurement of charge carrier mobilities and lifetimes in both systems reveal that the PTB7-Th:O-IDTBCN blend possesses more balanced charge carrier mobilities, and longer lifetimes. Morphology studies reveal a slightly greater degree of molecular mixing of the O-IDTBCN when blended with PTB7-Th, despite the greater and more balanced charge carrier mobilities in this blend.