The Pivotal Electronic Contribution of Thioether Groups in Asymmetric Side Chains to Improved Photovoltaic Performance of Nonfused-Ring Acceptors

Asymmetric side-chain engineering is an effective strategy to improve the photovoltaic performance of nonfused-ring electron acceptors (NFREAs), yet the heteroatom effects of chalcogen-based ether side chains remain underexplored. In this work, two NFREAs, 3TT–C4–O (with an oxygen ether) and 3TT–C4–S (with a thioether), which differ only in the central chalcogen atom of the asymmetric side chain, were designed and synthesized. Their photophysical, electrochemical, and photovoltaic properties were systematically investigated. The results show that 3TT–C4–S exhibits a higher molar extinction coefficient, deeper energy levels, and enhanced molecular polarity compared with 3TT–C4–O, which can be attributed to the larger atomic radius, higher polarizability, and lower electron density of the sulfur atom. When using PM6 as the donor, the photovoltaic device based on 3TT–C4–S achieved a power conversion efficiency (PCE) of 13.25%, outperforming its oxygen analogue 3TT–C4–O (PCE = 11.98%) as a result of the higher short-circuit current density (J_SC) and a higher fill factor (FF). Charge dynamics and morphology studies further revealed that 3TT–C4–S-based devices enable more efficient exciton dissociation and charge collection with appropriate phase separation and tighter molecular packing, resulting in relatively less charge recombination. This work elucidates the electronic contribution of sulfur in asymmetric side chains and provides insight into the molecular design of high-performance NFREAs.