Influence of side chains on low optical bandgap copolymers based on 2,1,3-benzoxadiazole for polymer solar cells

Two copolymers based on 2,1,3-benzoxadiazole (BO) as the acceptor unit and quaterthiophene as the donor unit, which possess different steric hindrance in the backbone, were designed and synthesized. The backbone conformations and electronic structures as well as the theoretically optimal structures of both copolymers were calculated. The influences of steric hindrance on photophysical and electrochemical properties of the polymers were investigated. With the steric hindrance of the polymers decreased, the optical bandgaps were decreased, and the HOMO levels upshifted while the LUMO levels downshifted. The weight ratio and annealing temperature on the performances of the polymer-based devices were evaluated in detail. The best device based on the polymer exhibited a moderate power conversion efficiency of 4.44%.