Customized Molecular Design of a Novel Wide‐Bandgap Polymer Donor Based on Benzo[1,2‐ b :3,4‐ b ′:6,5‐ b ″]Trithiophene Unit with Over 20% Solar Cell Efficiency

Abstract In the realm of high‐performance organic solar cells (OSCs), the scarcity of wide‐bandgap polymer donor materials poses a significant challenge. To tackle this issue, an innovative breakthrough is made by introducing a chlorine atom and an ester group into the benzo[1,2‐ b :3,4‐ b ′:6,5‐ b ″]trithiophene unit, namely BCE. Using BCE as the electron‐accepting unit, benzo[1,2‐ b :4,5‐ b ’]dithiophene derivative (BDTT) as the electron‐donating unit, and alkyl‐substituted thiophene as the bridging unit, a new class of D–A type alternating conjugated polymers are successfully synthesized. Subsequently, after systematic optimizations of the side chain length of the BDTT unit and the proportion of fluorinated BDTT, a polymer donor material, PBCE‐2 is ultimately obtained, which exhibits excellent solution processability, suitable energy levels, and temperature‐dependent aggregation characteristics. The binary OSC based on PBCE‐2 and the well‐known acceptor L8‐BO demonstrates an impressive power conversion efficiency (PCE) of 19.2%. When a fullerene acceptor, PC 71 BM, is incorporated to construct a ternary OSC, the corresponding PCE is further elevated to 20.4%. This achievement marks PBCE‐2 as another promising polymer donor that has joined the exclusive “20% Efficiency Club,” following in the footsteps of notable polymer donors such as PM6, D18, and their derivatives.