Zhengqi Liu, Mingyan Zhan, Yanan Wei, Mingxu Zhou, Jiali Weng, Yongni Su, Xingjie Liu, Xubing Bai, Xiaotao Hao, Zheng Tang, Laju Bu, Guanghao Lu, Yinhua Zhou, Jianqi Zhang, Xin Zhang, Yunhao Cai, Hui Huang
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.