Enhanced device performance of polymer solar cells by planarization of quinoxaline derivative in a low-bandgap polymer

A series of low-bandgap alternating copolymers consisting of electron-accepting quinoxaline derivatives and electron-donating carbazole or fluorene were synthesized via a Suzuki coupling reaction. For the purpose of improving the molecular packing of polymer chains and to enhance the charge carrier mobility in the packing direction, a new quinoxaline derivative, 10,13-bis-(4-octyl-thiophene-2-yl)-dibenzo[a,c]phenazine, which is expected to have planar polycyclic structure, was synthesized and introduced as a new building block of alternating copolymers instead of frequently-used 5,8-dithien-2-yl-2,3-diphenylquinoxaline. Copolymers with the planar quinoxaline derivative exhibited better optical and structural properties compared to copolymers with a less planar quinoxaline derivative. A power conversion efficiency was achieved up to 3.8% when the copolymer with planar quinoxaline derivative was blended with [6,6]-phenyl-C71-butyric acid methyl ester as an active layer material in bulk heterojunction solar cells.