The performance of organic solar cells (OSCs) is critically governed by the fibrillar organization of bulk-heterojunction active layers, yet controlling long-range, directional fibrillar morphology under nonequilibrium processing remains challenging. Here we report that two-dimensional cobalt phosphosulfide (CoPS3) nanocrystals act as kinetic templates that reprogram molecular self-assembly during film formation. Through collective electronic interactions with conjugated polymer donors and small-molecule acceptors, CoPS3 modulates molecular densification and phase separation pathways, enabling refined, long-range fibrillar networks without increasing crystallinity. The resulting morphology enhances exciton dissociation and charge transport, and inhibits recombination, delivering a power conversion efficiency of 21.0%. This templating strategy is effective across multiple OSC systems, simultaneously improving efficiency and storage stability, offering a promising route for kinetic morphology control in organic optoelectronics. Controlling long-range, directional fibrillar morphology is important for organic solar cells (OSC). Here, the authors show that cobalt phosphosulfide nanocrystals can template film formation in OSCs, improving charge generation and transport, and achieving a power conversion efficiency of 21.0%.
The solution‐state aggregation of conjugated polymers critically determines the morphology and performance of organic solar cells (OSCs), yet processing optimization remains largely empirical. Here, we establish the sol–gel transition temperature (Tsol–gel), determined by rheology, as a transferable descriptor linking solution aggregation to film formation and device performance. Using cryo‐electron microscopy (CEM), small‐angle neutron scattering (SANS), and rheology, we reveal that the high‐performance donor polymer D18 in chlorobenzene evolves from dissolved wormlike chains to a weak gel and then to a strong gel upon cooling. Importantly, processing near the Tsol–gel temperature yields weak‐gel aggregates, which transform into a double fibril network during film formation, enabling enhanced charge transport, optimized phase separation, and uniform large‐area coating. Under this condition, D18:L8‐BO achieves a power conversion efficiency of 19.6% in small‐area devices and 17.1% in 17.6 cm2 mini‐modules. More importantly, this Tsol–gel‐guided strategy is further validated in multiple conjugated polymers in OSCs, including PM6, PffBT4T‐2OD, and D18 processed from o‐xylene, where the optimal performance consistently occurs near the corresponding sol–gel transition. These results identify weak‐gel pre‐aggregation near Tsol–gel as a general processing window for constructing favorable fibrillar morphologies and provide a broadly applicable framework for morphology control in high‐performance OSCs.
