Theoretical investigation on the effect of fluorine and carboxylate substitutions on the performance of benzodithiophene-diketopyrrolopyrrole-based polymer solar cells

Three donor–acceptor (D–A) polymers 2–4 were designed and investigated based on the reported polymer 1 with benzo[1,2-b:4,5-b′]dithiophene (BDT) as D fragment and diketopyrrolopyrrole (DPP) as A fragment. The fluorine substitutions on the BDT unit in molecule 2 have less influence on the lowest unoccupied molecular orbital (LUMO) compared with the carboxylate substitutions on the BDT unit in 3 and 4. The charge transfer rate (kinter-CT) of molecule 4 is the largest, which determines that molecule 4 has a priority in the interfacial process among these investigated molecules with the same acceptor PC 61 BM. The designed molecules 2–4 show larger open-circuit voltages (Voc), relatively narrower bandgaps and higher value of kinter-CT/kinter-CR than 1. Moreover, the results demonstrate that fluorine and carboxylate substitutions on molecule 4 show a synergistic effect on the FMO energy levels and electron interfacial process, which is expected to help the further understanding of the design rules for polymer donor materials in polymer solar cells.