Over 20% Efficiency in As-Cast Organic Solar Cells Enabled by Flory-Huggins Parameter Engineering.

The realization of high-efficiency as-cast organic solar cells (OSCs) represents a key step toward simplified processing and scalable fabrication for commercialization. However, achieving high-performance as-cast devices remains challenging because it demands delicate balance between donor-acceptor self-aggregation and miscibility. Herein, we fabricate as-cast OSCs through Flory-Huggins parameter (χ) engineering that precisely regulates donor-acceptor miscibility during solution casting. The chloroform-carbon disulfide (CF:CS2) mixed solvent, with tailored solubility and evaporation kinetics, dynamically modulates χ to form an optimized bulk heterojunction featuring desirable phase separation and a well-balanced fibrillar network. This design affords efficient exciton dissociation, facilitated charge transport, and suppressed recombination in the blend. Consequently, as-cast D18:L8-BO device processed with the CF:CS2 solvent system attained a PCE of 20.11% (certified as 19.69%) alongside enhanced photostability, representing the highest efficiency reported for as-cast devices. Besides, high PCEs of 18.89% and 15.12% are achieved in thick-film (300 nm) and large-area (20.17 cm2) as-cast devices. Moreover, a quantitative correlation between PCE and the interaction parameter χ is established, providing a quantitative guide for narrowing the miscibility window and identifying the optimal composition. These findings validate the efficacy of χ-parameter-mediated miscibility engineering in regulating blend phase behavior and provide a framework for advancing efficient as-cast OSCs.