Side-chain engineering of regioregular copolymers for high-performance polymer solar cells processed with nonhalogenated solvents

Conjugated polymers for bulk heterojunction polymer solar cells (BHJ PSCs) should be processed with nonhalogenated solvents because of environmental concerns. Here, we report novel regioregular copolymers for high-performance PSCs processed with nonhalogenated solvents. The regioregular copolymers (i.e., rr-PfBT2f4T-2OD and rr-PfBT2f4T-2DT) consist of 3 '',4 '-difluoro-2,2 ':5 ',2 '':5 '',2"'-quaterthiophene (2f4T) with different side chains (2-octadodecyl (OD) and 2-decyltetradecyl (DT)) and 5-fluorobenzo[c][1,2,5]thiadiazole (fBT). The regioregular copolymers with controlled fBT orientation show high solubility in nonhalogenated solvents. Both regioregular copolymers possess suitable energy levels, leading to sufficient energy offsets with a nonfullerene acceptor, BTP-eC11. An rr-PfBT2f4T-2DT:BTP-eC11-blended film exhibited predominant face-on orientation compared to the rr-PfBT2f4T-2OD:BTP-eC11-blended film. In addition, the rr-PfBT2f4T-2DT:BTP-eC11-blended film showed much more balanced hole/electron mobility (mu(h)/mu(e) similar to 4.73) than rr-PfBT2f4T-2OD:BTP-eC11-blended film (mu(h)/mu(e) similar to 45.86). Therefore, rr-PfBT2f4T-2DT:BTP-eC11-based PSCs, processed with 1,2,4-trimethylbenzene, showed a power conversion efficiency of 13.21% which is 60% higher than rr-PfBT2f4T-2OD:BTP-eC11-based PSCs.