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···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···O­(carbonyl)
interaction. Consequently, TETOR-based polymer (TffBT-TETOR) has greatly
lower-lying FMOs, stronger aggregation, closer π-stacking, and
better miscibility with fullerenes versus the TRTOR-based counterpart
(TffBT-TRTOR). These characteristics are attributed to the additional
S···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 Voc of 0.76 V, and a higher Jsc of 18.30 mA cm–2 than the
TffBT-TRTOR-based PSCs. These results demonstrate that optimizing
intramolecular noncovalent S···O interactions by incorporating
electron-withdrawing ester groups is a powerful strategy for materials
invention in organic electronics.