Two-Dimensional Conjugated Benzo[1,2-b:4,5-b′]diselenophene-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 (V_oc) 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 V_oc and excellent PCE simultaneously in selenophene-based OSCs.