Organic solar cells based on chlorine functionalized benzo[1,2- b :4,5- b ′]difuran-benzo[1,2- c :4,5- c ′]dithiophene-4,8-dione copolymer with efficiency exceeding 13%

Benzo[1,2-b:4,5-b′]dithiophene (BDT) has been widely used to construct donor-acceptor (D-A) copolymers in organic solar cells (OSCs). However, benzo[1,2-b:4,5-b′]difuran (BDF), an analogue of BDT, has received less attention than BDT. The photovoltaic performance of BDF copolymers has lagged behind that of BDT copolymers. Here, we designed and synthesized two BDF copolymers, PBF1-C and PBF1-C-2Cl. PBF1-C-2Cl, which is composed of BDF and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione connected by a chlorinated thiophene π-bridge, displays a low-lying highest occupied molecular orbital energy level, which helps in yielding a high open-circuit voltage (Voc) in OSCs. As a result, when blended with Y6, PBF1-C-2Cl-based devices showed a high Voc of 0.83 V and a power conversion efficiency (PCE) of 13.10%. To the best of our knowledge, the PCE of 13.10% is among the highest efficiency values for OSCs based on BDF copolymers.