Effect of Fused Thiophene Bridges on the Efficiency of Non-Fullerene Polymer Solar Cells made with Conjugated Donor Copolymers Containing Alkyl Thiophene-3-Carboxylate

New wide-bandgap donor-acceptor-π type conjugated polymers, PBTTT and PBTDTT, containing different π-extended conjugated bridges between benzodithiophene (BDT) as a donating unit and 3-carboxylthiophene (3CT) as a weak electron-withdrawing group, were successfully synthesized and employed as donor polymers for polymer solar cells. Thienothiophene (TT) and dithienothiophene (DTT) were selected as fused thiophene bridges for inclusion in the polymer backbone instead of a single thiophene. As a control donor polymer, PBTT containing a single thiophene bridge was also prepared. Its properties were compared with those of PBTTT and PBTDTT. As the conjugation length of the p-extended bridge (i.e., from thiophene to DTT) increased, the bandgap of the polymer slightly decreased, and the highest occupied molecular orbital level shifted to a higher-lying level. Compared to PBTT, PBTTT and PBTDTT exhibited relatively low solubility and red-shifted absorption spectra in solution states. PBTTT and PBTDTT showed strong aggregation between polymer chains, even in solution states, which were similar to the film state. Among the three copolymers, PBTTT-based non-fullerene polymer solar cells (NF-PSCs) with ITIC-Me showed a high power conversion efficiency of 7.30% with a high short circuit current density of 14.05 mA/cm2. The superior performance of the NF-PSCs based on PBTTT may be attributed to their predominant face-on orientation of polymer chains and relatively more favorable surface morphology. [graphic not available: see fulltext]