Quinoxaline-based pi-conjugated donor polymers with different substituent groups (phenyl, p-fluorphenyl, p-methoxyphenyl, methyl, or hydrogen) on the quinoxaline unit were synthesized and characterized, focusing on the effects of these substituent groups on the performance of bulk heterojunction (BHJ)-based polymer solar cells fabricated with phenyl C-71 butyric acid methyl ester ([70]PCBM) as an acceptor. Polymers with phenyl groups or p-methoxyphenyl groups showed nanometer-scale phase separation or uniform morphology in BHJ film, together with high field effect transistor (FET) hole mobility. On the other hand, polymers with methyl or hydrogen and a polymer with fluorine exhibited micrometer-scale domains in BHJ and low hole mobility. It is suggested that the mostly steric hindrance of the substituents regulate the polymer's aggregation properties and compatibility with [70]PCBM, which probably affects the hole transport properties and BHJ morphologies. We also investigated the effects of substituent groups on highest occupied molecular orbital energy level and optical band gap of the polymers. (C) 2011 Elsevier B.V. All rights reserved.