18.6% Efficiency All‐Polymer Solar Cells Enabled by a Wide Bandgap Polymer Donor Based on Benzo[1,2‐d:4,5‐d′]bisthiazole

Abstract The limited selection of wide bandgap polymer donors for all‐polymer solar cells (all‐PSCs) is a bottleneck problem restricting their further development and remains poorly studied. Herein, a new wide bandgap polymer, namely PBBTz‐Cl, is designed and synthesized by bridging the benzobisthiazole acceptor block and chlorinated benzodithiophene donor block with thiophene units for application as an electron donor in all‐PSCs. PBBTz‐Cl not only possesses wide bandgap and deep energy levels but also displays strong absorption, high‐planar structure, and good crystallinity, making it a promising candidate for application as a polymer donor in organic solar cells. When paired with the narrow bandgap polymer acceptor PY‐IT, a fibril‐like morphology forms, which facilitates exciton dissociation and charge transport, contributing to a power conversion efficiency (PCE) of 17.15% of the corresponding all‐PSCs. Moreover, when introducing another crystalline polymer acceptor BTP‐2T2F into the PBBTz‐Cl:PY‐IT host blend, the absorption ditch in the range of 600–750 nm is filled, and the blend morphology is further optimized with the trap density reducing. As a result, the ternary blend all‐PSCs achieve a significantly improved PCE of 18.60%, which is among the highest values for all‐PSCs to date.