Two‐Dimensional Conjugated Benzodiselenophene‐Based Copolymer Donor Enables Large Open‐Circuit Voltage and High Efficiency in Selenophene‐based Organic Solar Cells

A two‐dimensional electron‐rich fused‐ring moiety (ClBDSe) based on benzo[1,2‐b:4,5‐b′]diselenophene is synthesized. Three copolymers (PBDT−Se, PBDSe−T, and PBDSe−Se) are obtained by manipulating the connection types and number of selenophene units on the conjugated main chains with two 2D fused‐ring units and two different π‐bridges, respectively. In comparison with PBDT−Se and PBDSe−Se, PBDSe−T with benzo[1,2‐b:4,5‐b′]diselenophene unit and thiophene π‐bridge exhibits the deepest HOMO energy level and the strongest crystallinity in neat films. The PBDSe−T:Y6 blend film exhibits the best absorption complementarity, the most distinctive face‐on orientation with proper phase separation, the highest carrier mobilities, and the lowest charge recombination among three blend films. Finally, the PBDSe−T:Y6‐based device delivers an impressive power conversion efficiency (PCE) of 14.50 %, which is higher than those of PBDT−Se:Y6 and PBDSe−Se:Y6. Moreover, a decent open‐circuit voltage (Voc) of 0.89 V with a remarkably small energy loss of 0.44 eV is achieved for PBDSe−T:Y6. The efficiency of 14.50 % is the highest value for selenophene‐containing copolymer‐based binary organic solar cells (OSCs). This study provides evidence that introduction of 2D‐benzo[1,2‐b:4,5‐b′]diselenophene as a fused electron‐rich unit with π‐bridging into copolymeric donors is a valid strategy for providing high Voc and excellent PCE simultaneously in selenophene‐based OSCs.