Effect of chain curvature on the performance of diketopyrrolopyrrole-based polymer solar cells

Two novel polymers, PDTBO and PD2TBO, containing diketopyrrolopyrrole and alkoxyl-substituted benzothiadiazole were designed and synthesized for polymer photovoltaics. The introduction of thiophene and bithiophene as different pi-bridge units results in good planarity of the polymer backbone but a different curvature in polymer chains. The inclusion of bithiophene in the repeat unit leads to a potentially larger curvature and a zigzag conformation for PD2TBO. This increased curvature does not damage the thin-film crystallinity. Instead, high hole mobility is recorded for PD2TBO which is almost two orders of magnitude higher than that of linear PDTBO. Furthermore, high power conversion efficiency (PCE) of 5.3% is obtained for the PD2TBO/PC71BM blend film ascribed to good miscibility while the linear polymer PDTBO exhibits a moderate PCE of 2.1%. Our work demonstrates that the modulation of the chain curvature is an efficient approach to improve the performance of polymer solar cells.