A multi-ring, ladder-type low band-gap polymer (PIDTCPDT-DFBT) is developed to show enhanced light harvesting, charge transport, and photovoltaic performance. It possesses excellent planarity and enhanced effective conjugation length compared to the previously reported fused-ring polymers. In order to understand the effect of extended fused-ring on the electronic and optical properties of this polymer, a partially fused polymer PIDTT-T-DFBT is also synthesized for comparison. The fully rigidified polymer provides lower reorganizational energy, resulting in one order higher hole mobility than the reference polymer. The device made from PIDTCPDT-DFBT also shows a quite promising power conversion efficiency of 6.46%. Its short-circuit current (14.59 mA cm−2) is also among the highest reported for ladder-type polymers. These results show that extending conjugation length in fused-ring ladder polymers is an effective way to reduce band-gap and improve charge transport for efficient photovoltaic devices.