Achieving high‐efficiency organic solar cells through halogenated benzene solvent‐mediated regulation of active layer aggregation behavior

Abstract Controlling nanoscale morphology is essential for boosting organic solar cells (OSCs) performance, especially as highly conjugated fused‐ring small molecule acceptors tend to aggregate excessively due to strong π–π interactions. Here, we present a straightforward morphology tuning approach by leveraging electrostatic interactions between the solvent and the non‐fullerene acceptor BTP‐eC9. By replacing chlorobenzene with bromobenzene, characterized by a higher boiling point and a more negative electrostatic potential, the solvent–acceptor interactions are strengthened and the film formation process is delayed. This modification extends the film formation time window, effectively suppresses excessive acceptor crystallization, and promotes favorable phase separation with appropriate domain size, thereby reducing recombination losses and improving carrier extraction. Consequently, PM6: BTP‐eC9‐based devices achieve a power conversion efficiency of 18.5%, with a short‐circuit current density of 28.2 mA cm −2 and a fill factor of 76.7%. Our results reveal the critical role of solvent electrostatics in shaping active layer morphology and demonstrate a scalable, additive‐free strategy for enhancing OSC performance.