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

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.