Hansen Parameter‐Engineered Binary Solvents Enable 19.3% Efficient Organic Solar Cells with Green Processability

Abstract The widespread use of toxic halogenated aromatic solvents in organic solar cells (OSCs) poses severe environmental and health hazards, yet their replacement with eco‐friendly alternatives remains challenging due to poor solubility of high‐performance photoactive materials. Herein, a Hansen solubility parameters (HSPs)‐guided binary solvents strategy is reported to unlock non‐aromatic, halogen‐free solvent systems for sustainable OSCs manufacturing. By blending dihydropyran (DHP), 2‐methyl tetrahydrofuran (MeTHF), and cyclopentyl methyl ether (CPME) with carbon disulfide (CS₂), precise HSPs alignment is achieved, enabling dissolution of the polymer donor PM6 and non‐fullerene acceptor L8‐BO. As the solvent mixture transitions from DHP:CS₂ to MeTHF:CS₂ to CPME:CS₂, the resulting PM6:L8‐BO blend films exhibit progressively enhanced crystallinity, optimized phase separation, leading to more efficient exciton separation, improved charge transport and collection and reduced non‐geminate recombination, ultimately achieving power conversion efficiencies (PCEs) of 13.94%, 17.15%, and 18.51%, respectively. Further optimization via a quaternary blended photoactive layer processed with CPME:CS₂ raised the PCEs to 19.31%, representing one of the highest efficiencies reported for OSCs processed with halogen‐free and non‐aromatic solvents. This work underscores the importance of employing a binary‐solvent strategy to discover novel, eco‐friendly solvent systems, thereby advancing the development of high‐performance OSCs.