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.