A series of low-bandgap alternating copolymers consisting of electron-accepting quinoxaline derivatives and electron-donating carbazole or fluorene were synthesized viaa 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. [ABSTRACT FROM AUTHOR]