High‐Performance All‐Polymer Solar Cells Enabled by n‐Type Polymers with an Ultranarrow Bandgap Down to 1.28 eV

Abstract Compared to organic solar cells based on narrow‐bandgap nonfullerene small‐molecule acceptors, the performance of all‐polymer solar cells (all‐PSCs) lags much behind due to the lack of high‐performance n‐type polymers, which should have low‐aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to the near‐infrared region. Herein, two novel polymer acceptors, DCNBT‐TPC and DCNBT‐TPIC, are synthesized with ultranarrow bandgaps (ultra‐NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm² V⁻¹ s⁻¹ obtained for DCNBT‐TPC. Blended with a polymer donor, PBDTTT‐E‐T, the resultant all‐PSCs based on DCNBT‐TPC and DCNBT‐TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short‐circuit currents up to 19.44 and 22.52 mA cm⁻², respectively. This is the first example that a PCE of over 10% can be achieved using ultra‐NBG polymer acceptors with a photoresponse reaching 950 nm in all‐PSCs. These results demonstrate that ultra‐NBG polymer acceptors, in line with nonfullerene small‐molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all‐PSCs.