Siloxane-Terminated and Fluorinated Thiophene Side Chains Enable 20.28% Efficiency in Organic Solar Cells

Two-dimensional (2D) side-chain engineering can effectively extend the π-conjugation of nonfullerene acceptors (NFAs), but often induces severe steric hindrance, necessitating voltage-sacrificing fluorination strategies to optimize the morphology of organic solar cells (OSCs). Herein, we resolve this trade-off by developing a siloxane-terminated side-chain strategy to synthesize two NFAs, Th–Si and FTh–Si. The highly flexible siloxane tail acts as a molecular lubricant to alleviate steric hindrance and drive ordered lamellar packing. To further synergize the photoactive blend morphology, we introduce a siloxane-based additive (c-5Si) to promote fibrillation via “like-dissolves-like” interactions, and the second acceptor ITIC to induce a “parallel-model”-like morphology by thermodynamic incompatibility. Empowered by this optimized interpenetrating network, the nonfluorinated Th–Si successfully overcomes steric barriers to achieve an exceptional balance of overall photovoltaic parameters. The resulting ternary OSCs exhibit an impressive power conversion efficiency (PCE) of 20.28%, establishing siloxane engineering as a robust pathway for high-performance OSCs.