Comparison Study of the Chlorination Positions in Wide Band Gap Donor Polymers

Chlorination is a simple and effective molecular engineering strategy to downshift energy levels and increase band gap of polymer donors and thus can maximize the open-circuit voltage and light-harvesting for organic solar cells (OSCs). Building upon our previously reported efficient polymer donor PDZ-2, with chlorines on flanked thienyl of the benzodithiophene units, two new D-A type wide band gap donor polymers, PDZ-out and PDZ-in, have been synthesized by substituting chlorine atoms on alpha' and beta' positions of the pi-bridge thiophenes, respectively, to investigate the effect of chlorination on device performances. Density functional theory simulation and the UV-vis absorption spectrum reveal that chlorination on the pi-bridge thiophenes introduces steric hindrance between the bridge thiophenes and the FTAZ units, leading to increased twist angles in the backbone and a reduced conjugation length, especially at the alpha' position. Chlorination on the backbone of the polymer indeed lowers energy levels compared to that of unchlorinated J52. When blended with NFA ITIC in OSCs, PDZ-out:ITIC and PDZ-in:ITIC devices exhibit power conversion efficiencies (PCEs) of 7.6 and 0.35%, respectively, which are lower than that of PDZ-2:ITIC (PCE of 10.9%). The OSC performance difference is attributed to the relatively lower charge carrier mobility, insufficient exciton dissociation, and shorter charge carrier lifetime for PDZ-out:ITIC and PDZ-in:ITIC devices. The reduced planarity and photovoltaic performance, which is based on the polymer donors with chlorination on the pi-bridge thiophenes, highlight the importance of substituent positions when rational-designing new chlorinated donors.