A Rigid Polymer Donor Based on a Simple Benzodithiophene–Thiazole Backbone for Organic Photovoltaics

We report a new polymer donor, PBDTTz , featuring a simple benzo[1,2‐ b :4,5‐ b ′]dithiophene–thiazole backbone for organic photovoltaics (OPVs). Introduction of thiazole units induces intramolecular N···S interactions that lock the backbone conformation, yielding a rigid and coplanar structure. This design not only enhances intermolecular π–π interactions but also balances crystallinity with miscibility. As a result, PBDTTz exhibited a deeper highest occupied molecular orbital energy level, stronger π–π stacking, and improved blend morphology with the nonfullerene acceptor L8‐BO, significantly outperforming its thiophene analog PBDTT in OPV performance. Optimized cells achieved a power conversion efficiency of 15.3%, which is quite high considering its simple chemical structure. These results highlight thiazole incorporation as an effective strategy to simultaneously control backbone rigidity, crystallinity, and miscibility, providing a straightforward route to high‐performance polymer donors for OPVs.