Development of large band-gap conjugated copolymers for efficient regular single and tandem organic solar cells.

We demonstrated the synthesis and characterization of two conjugated copolymers, PBDTFBZO and PBDTFBZS, consisting of dialkylthiol substituted benzo[1,2-b:4,5-b']dithiophene donor and monofluorinated benzotriazole acceptor blocks. The resulting copolymers show large band gaps, deep HOMO and LUMO energy levels. Improved V(oc), J(sc), and FF were obtained at the same time to increase overall efficiencies of their single and tandem polymer solar cells. The enhanced V(oc) can be ascribed to a low-lying HOMO energy level by incorporating dialkylthiol and fluorine substituents on the polymer backbone. The improvement in J(sc) and FF are likely due to high carrier mobility, suppressed charge recombination, and fine nanostructure morphology. A 7.74% PCE was achieved from the regular single device based on PBDTFBZS:PC71BM blend film with 3% 1,8-diiodooctane (DIO) additive. In combination with low band gap diketopyrrolopyrrole (DPP)-based copolymer, tandem devices based on PBDTFBZS exhibited high PCE up to 9.40%. The results indicate that PBDTFBZO and PBDTFBZS are promising polymer donor materials for future application of large-area polymer solar cells.