Enhancing Molecular Stacking Through “Strengthened Aggregation in Pseudo‐Dry Film” Strategy by Bromothiazol Additive for Efficient Organic Solar Cells

Abstract Regulating the morphology and molecular ordering of the active layer is crucial for developing high‐performance organic solar cells (OSCs). However, enhancing the molecular stacking is challenging as non‐fullerene acceptors (NFAs) are confined within the polymer network owing to the well donor/acceptor miscibility and fast solvent evaporation. Herein, the 2,5‐dibromothiazol (DBrTz) removable solid additive is chosen to optimize the film‐forming kinetics for enhancing molecular aggregation of the PM6:L8‐BO blends. During the film formation process, chloroform evaporates first, trapping DBrTz in the film (pseudo‐dry film state). Thereafter, DBrTz will gradually volatilize, persistently prompting the L8‐BO to stack and aggregate orderly until the volatilization of DBrTz is completed. The behavior is designated as “enhanced aggregation in pseudo‐dry film,” an occurrence hitherto unobserved in other solid additives. This results in more compact π – π stacking and orderly long‐range aggregation of L8‐BO. Furthermore, DBrTz facilitated increased face‐on orientations and improved vertical component distributions. This optimized morphology facilitates charge generation, transport, and extraction. Consequently, DBrTz‐processed PM6:L8‐BO OSCs achieved a power conversion efficiency (PCE) of 19.4%. This work elucidates the principles of solid additives and offers valuable insights for fostering the development of novel additives to improve the morphology and the efficiency of OSCs.