Crystallization Kinetics Directed by Additive Symmetry for Morphology Control in High‐Efficiency Organic Solar Cells

ABSTRACT The nanoscale morphology of the active layer critically governs the performance of organic solar cells (OSCs), where the molecular packing and phase organization of small‐molecule acceptors play a decisive role. Although solvent additives are widely used to regulate morphology, their volatility often compromises reproducibility and long‐term stability. Here, we systematically investigate four positional isomers of dibromonaphthalene (DBN): 1,8‐, 1,5‐, 2,7‐, and 2,6‐DBN, as solid‐state additives in PM6:Y6‐based OSCs. Despite identical chemical compositions, the DBN isomers exhibit distinct molecular symmetry and crystallization kinetics, leading to fundamentally different impacts on acceptor aggregation and phase organization. Among them, 2,6‐DBN, featuring a highly symmetric configuration and moderate supercooling, enables balanced crystallization kinetics during thermal annealing. This kinetically accessible ordering promotes compact π–π stacking, favorable vertical phase distribution, and reduced energetic disorder in the active layer. Consequently, charge carrier mobility is enhanced, non‐radiative recombination losses are suppressed, and balanced charge transport is achieved, yielding a champion power conversion efficiency of 19.65% in PM6:Y6 devices. These results highlight crystallization kinetics, rather than additive crystallinity alone, as a key determinant for morphology control, providing a generalizable strategy for rational solid additive design in high‐performance OSCs.