Moderate Alkylthio Side-Chain Engineering Enables an Optimal Balance between Dielectric Enhancement and Morphological Order in Organic Solar Cells

Maintaining a balance among the multiple factors that govern organic solar cells (OSCs) is crucial, and understanding how the dielectric constant (εr) of the active layer influences device performance is of particular importance. Introducing alkylthio substituents into the polymer backbone represents an effective strategy to increase εr. Herein, three donor polymers-PBDT-T-BDD, PBDT-AST-BDD, and PBDT-DST-BDD-bearing 0, 2, and 4 alkylthio side chains per repeating unit, respectively, were designed to systematically modulate εr. With the gradual addition of alkylthio side chains, the polymers exhibited a slight blue shift in absorption, a gradual deepening of the HOMO energy level, and a pronounced increase in εr from 3.91, 4.36, to 5.13. When blended with the non-fullerene acceptor Y6, the optimal device based on PBDT-AST-BDD achieved a power conversion efficiency (PCE) of 15.19%, surpassing those of PBDT-T-BDD (13.58%) and PBDT-DST-BDD (11.80%). A similar trend was observed using L8-BO as the acceptor, with the PBDT-AST-BDD-based device attaining a high PCE of 17.09%. The superior performance of PBDT-AST-BDD devices originated from simultaneously enhanced short-circuit current density (JSC) and fill factor (FF), attributed to improved charge generation and suppressed bimolecular recombination, as evidenced by higher saturation current densities, enhanced carrier lifetimes, and reduced trap-assisted recombination. Comprehensive analyses revealed that achieving an optimal trade-off among εr enhancement, charge transport, and morphological stability requires precise control over the degree of side-chain sulfidation. These results establish precise control over the side-chain sulfidation degree as an effective molecular design principle for high-performance donor polymers in OSCs.