Regulating Intramolecular Charge Transfer and Resonance Effects to Realize Ultrawide Bandgap Conjugated Polymer for High‐Performance All‐Polymer Solar Cells

Abstract All‐polymer solar cells (APSCs) have attracted increasing attention due to the superior stability and mechanical flexibility. However, power conversion efficiencies (PCEs) of APSCs fall behind ascribed to limited efficient polymer donors (PDs). Particularly, there are rare reports of high‐performance PDs with ultrawide bandgaps, which are indispensable to maximally harvest high‐energy photons for better light utilization. Herein, the significance of intramolecular charge transfer and quinoid resonance effects in regulating bandgaps of donor‐acceptor conjugated materials is highlighted, and a novel polymer donor PBPD based on phenyl/pyridyl fused phenanthridine (PD) moiety is reported. Consequently, PBPD receives an ultrawide optical bandgap up to 2.24 eV and strong absorptivity at 487–523 nm, which exactly follows the strongest radiation of solar spectrum. The amplified light utilization of high‐energy photons endows PBPD:PY‐IT based APSCs with high power conversion efficiency of 15.29%, which ranks the best among APSCs with ultrawide bandgap PDs and challenges typical PDs with bandgaps of ≈1.80 eV. Thanks to the unique light responses and molecular orientations of PBPD, the PBPD‐contained ternary APSCs receive further optimized photon utilization and refined PCEs over 17%. This study sheds light on the design principles of ultrawide bandgap polymers and demonstrates the great promise of large bandgap PDs for efficient APSCs.