A pair of donor acceptor polymers PBDThDTBT and PBDTchDTBT are synthesized, which share the same conjugated backbone, but are designed with hexyl and cyclohexyl side chains, respectively. The stronger steric hindrance of cyclohexyl endows PBDTchDTBT a deeper lying HOMO energy level of -5.39 eV compared to -5.22 eV for PBDThDTBT. However, PBDThDTBT and PBDTchDTBT exhibit a similar optical bandgap around 1.72 eV and a hole mobility around 10(-5) cm(2)V(-1) s(-1). Interestingly, the PBDTchDTBT/PC71BM blends exhibited higher hole mobility than PBDThDTBT/PC71BM after DIO was added. The higher hole mobility and fibrillar network in the active layer endows PBDTchDTBT higher power conversion efficiency of 7.9%, together with simultaneously improved open-circuit voltage of 0.80 V, short-circuit current density of 13.50 mA cm(-2), and fill factor of 72.74% after a systemic study of their solar cell devices.