Enhanced photovoltaic performance of A-D-A'-D-A type non-fused ring electron acceptors via side chain engineering

Featuring simplified synthesis and flexible chemical alteration, non-fused ring electron acceptors (NFREAs) are the ideal candidates for constructing low-cost organic solar cells (OSCs). Herein, we report three A-D-A'-D-A type NFREAs, namely ffBTz-BO, ffBTz-EH, and ffBTz-C4, where difluorinated benzotriazole (ffBTz) with different side chain lengths were employed as the weak electron-deficient A' core. Compared with ffBTz-BO and ffBTz-C4, ffBTz-EH with appropriate side chain length strikes a balance between the enhanced molecular crystallinity and the favorable face-on orientation, resulting in high charge mobilities. Consequently, ffBTz-EH-based OSC delivered the highest power conversion efficiency (PCE) of 12.96% enabled by its efficient charge transport and most suitable phase separation, which represents one of the highest efficiencies among A-D-A'-D-A type NFREAs. This work demonstrates that alkyl side chain on the central A' core plays a critical role in tuning molecular crystallinity, active layer morphology, and further device performance, which provides a meaningful perspective for designing highly efficient A-D-A'-D-A type non-fused ring electron acceptors in the future.