Enhancing Polymer Photovoltaic Performance via Optimized Intramolecular Ester-Based Noncovalent Sulfur ••• Oxygen Interactions

Head-to-head (HH) bithiophenes are typically avoided in polymer semiconductors since they engender undesirable steric repulsions, leading to a twisted backbone. While introducing electron-donating alkoxy chains can lead to intramolecular noncovalent S center dot center dot center dot O interactions, this comes at the cost of elevating the HOMOs and compromising polymer solar cell (PSC) performance. To address the limitation, a novel HH bithiophene featuring an electron-withdrawing ester functionality, 3-alkoxycarbonyl-3'-alkoxy-2,2'-bithiophene (TETOR), is synthesized. Single crystal diffraction reveals a planar TETOR conformation (versus highly twisted diester bithiophene), showing distinctive advantages of incorporating alkoxy on promoting backbone planarity. Compared to first-generation 3-alkyl-3'-alkoxy-2,2'-bithiophene (TRTOR), TETOR contains an additional planarizing (thienyl)S center dot center dot center dot O(carbonyl) interaction. Consequently, TETOR-based polymer (TffBT-TETOR) has greatly lower-lying FMOs, stronger aggregation, closer pi-stacking, and better miscibility with fullerenes versus the TRTOR-based counterpart (TffBT-TRTOR). These characteristics are attributed to the additional S center dot center dot center dot O interaction and electron-withdrawing ester substituent, which enhances backbone planarity, charge transport, and PSC performance. Thus, TffBT-TETOR-based PSCs exhibit an increased PCE of 10.08%, a larger V-oc of 0.76 V, and a higher J(sc) of 18.30 mA cm(-2) than the TffBT-TRTOR-based PSCs. These results demonstrate that optimizing intramolecular noncovalent S center dot center dot center dot O interactions by incorporating electron-withdrawing ester groups is a powerful strategy for materials invention in organic electronics.