Synergistic regulation of donor–acceptor aggregation and morphology by isomeric solid additives for high-performance organic solar cells with over 20% efficiency

Molecular aggregation and phase morphology of the active layer in bulk-heterojunction (BHJ) solar cells are crucial to attain efficient and stable organic solar cells (OSCs). Most studies of solid additives in high-efficiency OSCs have primarily focused on the impact of these additives on the acceptors, while largely neglecting the synergistic effects of additives on donor and acceptor. Herein, we introduce a synergistic morphology regulation approach by utilizing two isomeric solid additives (4-bromobenzothiadiazole (4-BBT) and 5-bromobenzothiadiazole (5-BBT)). 4-BBT or 5-BBT promotes both the crystallinity and π–π stacking of the polymer donor PM6 while effectively suppressing excessive aggregation of the acceptor L8-BO, which leads to a favorable phase morphology. When mixed additives are loaded simultaneously, synergistic regulation can be achieved, enabling finer nanoscale phase separation with enhanced donor–acceptor miscibility and well-ordered packing. Further analyses indicate that the mixed additives effectively slow down the film formation and charge relaxation dynamics, thereby prolonging crystallization time and enhancing π–π stacking while effectively suppressing recombination losses. Consequently, modified by the mixed additives, the PM6:L8-BO device demonstrates high efficiency of 19.32%, coupled with improved short-circuit current (J_SC) and fill factor (FF). Besides, the D18:L8-BO-C4-based devices treated with 4-BBT+5-BBT delivered a remarkable efficiency of 20.13%, with an outstanding FF of 83.01%. Furthermore, the optimized device shows excellent photostability and thermal stability. This study provides a versatile and effective strategy for accurate regulation of the molecular aggregation and phase morphology through synergistic isomeric solid additive engineering, thereby offering insights into the rational design of efficient and stable organic photovoltaic materials.