A series of cyclopentadithiophene-based low band gap conjugated polymers with varied side-chain patterns and F-substituents were synthesized. By replacing the shorter 2-ethylhexyl (EH) side-chain with the longer 3,7-dimethyloctyl (DMO) side-chain, it resulted in significant changes to the optical, electrochemical, and morphological properties of the polymers, as well as the subsequent performance of devices made from these materials. Solar cells fabricated from polymer with 2-ethylhexyl (EH) side-chain and monofluoro substituent exhibits increased open circuit voltage, short circuit current and fill factor, resulting in the highest power conversion efficiency (5.5%) in this series of polymers. This finding provides valuable insight for making more efficient low band gap polymers.