Thiazolyl-Substituted Isomeric Benzodithiophenes Boost the Photovoltaic Performance of Polymer Donors

The two-dimensional side groups on benzodithiophene, especially thiophene derivatives, play a critical role in tailoring the bandgap, interchain packing, and photovoltaic outcomes of the most successful polymer donors. In light of the highly similar five-membered ring but vastly different electron-deficient properties, thiazole is expected to not only inherit the ability of the thiophene side group in conformation control but also be beneficial for constructing wide-bandgap polymer donors with deeper highest occupied molecular orbitals (HOMOs). Herein, two isomeric polymers (PBDT-oTz and PBDT-iTz) with different thiazolyl orientations on benzodithiophene are explored. A systematic investigation reveals that PBDT-oTz, featuring thiazolyl nitrogen far from benzodithiophene, achieves a deeper HOMO and appropriate molecular aggregation compared with its PBDT-iTz counterpart. Consequently, the PBDT-oTz-based device affords an excellent power conversion efficiency of 15.02%, much better than the 6.39% for PBDT-iTz. These results manifest the great effectiveness of thiazole in constructing high-performance wide-bandgap polymer donors.