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.
