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

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, PC₇₁BM, 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.