An Optimized Fibrous Network by Collaborative Slowing Aggregation Kinetics and Enhancing Intermolecular Interactions via a Co‐Solvent Combined With Solid Additive Strategy Enables 20% Efficient Organic Solar Cells

A well-defined fibrous network is important for efficient exciton dissociation and charge transport in organic solar cells (OSCs). However, forming such fibrous morphology is difficult to control in polymer:non-fullerene acceptor (NFA) systems, mainly because of the relatively weak intermolecular interactions between components and fast film formation dynamics. Herein, we propose a co-solvent combined with solid additive strategy to synchronously regulate film formation kinetics and tune the intermolecular interaction in D18:L8-BO systems. The high-boiling-point co-solvent, trichloroethylene (TCE) slows evaporation and promotes uniform aggregation, while the high-melting-point solid additive, 5,6-dibromo-2,1,3-benzothiadiazole (BBT) enhances intermolecular interactions and prolongs crystallization. In situ characterizations reveal that both the liquid-phase stage and liquid-to-solid transition stage of D18 and L8-BO in the blend are prolonged effectively. As a result, an optimized fibrous network with a wider fiber diameter (10.94 nm) is formed. The enlarged coherence lengths of D18:L8-BO in both out-of-plane (010) and (100) directions indicate enhanced molecular packing and structural ordering. These optimized fibrous ordered structures enhance exciton dissociation, suppress charge recombination, and enable more balanced charge transport. Consequently, the optimized devices achieve a power conversion efficiency (PCE) of 20.18%.