A series of quinoxaline-based copolymers, namely, poly[N-9 ''-heptadecanyl-2,7-carbazole-alt-5,5-(5',8'-di-2-thienyl-quinoxaline)] (P1), poly[N-9 ''-heptadecanyl-2,7-carbazole-alt-5,5- (5',8'-di-2-thienyl-2,3-bis (4-octyloxyl)phenyl)quinoxaline] (P2), and poly[N-9 ''-heptadecanyl-2,7-carbazole-alt-5,5-(5',8'-di-2-thienyl-2,3-bis(4-(3,7-dimethyloctyloxy)pheny)quinoxaline] (P3), were synthesized and characterized for use in polymer solar cells (PSCs). We describe the effect of modifying the alkyl group of the side chain of the quinoxaline derivatives on the electronic and optoelectronic properties of the polymers. The field-effect hole mobility as well as the electronic energy levels and processability of the materials for PSC applications were investigated. Among the studied quinoxaline-based copolymers, P2 showed the best photovoltaic performance with an open-circuit voltage (V(OC)) of 0.82 V, a short-circuit current density (J(SC)) of 9.96 mA/cm(2), a fill. factor (FF) of 0.49, and a power-conversion efficiency of 4.0% when a P2/PC(71)BM blend film was used as the active layer under AM 1.5 G irradiation (100 mW/cm(2)).