Isomerization‐Controlled Aggregation in Photoactive Layer: An Additive Strategy for Organic Solar Cells with Over 19.5% Efficiency

Morphology control of the photoactive layer is crucial for achieving high‐performance organic solar cells (OSCs), yet it remains a significant challenge in this field. One effective approach is the additive strategy, which fine‐tunes the morphology of the photoactive layer. However, the underlying mechanisms governing the impact of different types of additives from liquid, solid, to volatile solid, on the bulk heterojunction morphology and device performance are not fully understood. Herein, we present an aggregation regulation strategy for acceptor molecules by incorporating three novel isomeric additives: 4‐bromo‐1,2‐dichlorobenzene (LCB), 1‐bromo‐2,4‐dichlorobenzene (SCB), and 2‐bromo‐1,4‐dichlorobenzene (VCB) into the blend active layer. Our results reveal that these additives induce stepwise regulation of acceptor molecule aggregation during film formation. The liquid additive LCB primarily extends solvent evaporation time, effectively preventing excessive aggregation, while the solid additive SCB significantly shortens the aggregation period during the film evolution, resulting in the most compact molecular π‐π stacking. Furthermore, the volatile solid additive VCB fine‐tunes the intermolecular interactions and crystallization within the active layer, promoting optimal molecular self‐assembly and aggregation for ideal molecular stacking. Consequently, the power conversion efficiencies of 19.33% and 19.51% were achieved for the VCB‐processed D18:L8‐BO‐ and PM6:L8‐BO‐based OSCs, respectively, outperforming the LCB‐processed and SCB‐processed devices.