Temperature‐Driven Weak Gel Aggregation of Conjugated Polymers Enables Double‐Fibril Networks for High‐Performance Organic Solar Cells
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 ( T sol–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 T sol–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 cm 2 mini‐modules. More importantly, this T sol–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 T sol–gel as a general processing window for constructing favorable fibrillar morphologies and provide a broadly applicable framework for morphology control in high‐performance OSCs.