Three polymers bearing a common carbazole thiophene quinoxaline thiophene backbone, but different side chains, were designed and synthesized in order to investigate the effect of side chains on their photovoltaic performance. Their photophysical, electrochemical, and photovoltaic properties were investigated and compared. The polymer EWC3, with the largest amount of side chains, showed the highest power conversion efficiency of 3.7% with an open-circuit voltage (V-oc) of 0.92 V. The atomic force microscopy images of the active layers of the devices showed that the morphology was highly influenced by the choice of the solvent and processing additive. It is worth noting that polymer solar cells (PSCs) fabricated from EWC3, with branched side chains on the carbazole units, gave a much higher V-oc than the devices made from EWC1, which bears the same electron-deficient segment as EWC3 but straight side chains on carbazole units. This study offered a useful and important guideline for designing 2,7-carbazole-based polymers for high-performance PSCs.