Solvent vapor diffusion-driven multiscale pre-aggregation of non-fullerene acceptors enables high-performance organic solar cells.

Precise control of active layer morphology is crucial for improving organic solar cell efficiency but remains challenge. Here, we report a solvent vapor diffusion (SVD) method that creates a vertical solvent gradient by diffusing benzene vapor into a toluene-based acceptor solution before layer-by-layer fabrication. This process tunes multiscale pre-aggregation of non-fullerene acceptors, forming hierarchical domains that facilitate efficient exciton dissociation and charge transport. This strategy also optimizes the vertical donor/acceptor composition profile, aligning exciton generation to minimize recombination and enhance charge collection. Morphological, spectroscopic, and device analyses show enhanced molecular ordering, improved phase separation, and superior carrier dynamics compared with conventional processing. Solvent vapor diffusion strategy demonstrates universality across diverse systems, yielding consistent performance gains. Devices based on D18/L8-BO achieved an efficiency of 20.18%, while D18 (1% PYIT)/L8-BO-C4 reached 20.71%, establishing the strategy as a powerful approach for structural engineering in high-performance OSCs.