High‐Performance, Air‐Stable and Eco‐Friendly Terpene‐Processed Organic Solar Cells via Solvent‐Composition‐Controlled Film Formation Kinetics

ABSTRACT Terpene‐based solvents offer renewable and environmentally benign alternatives to halogenated solvents for sustainable solution processing of organic solar cells (OSCs). However, terpene‐processed OSCs still require improvements in power conversion efficiency (PCE). A key challenge is to ensure sufficient material solubility while controlling film‐formation kinetics to produce a favorable blend morphology. Here, we report efficient (PCE = 18.66%) and air‐stable terpene‐processed OSCs by controlling the composition of a eucalyptol (Eu):tetralin (Tet) binary solvent system. Using PM6:BTP‐eC9 as a model high‐performance donor–acceptor system, we systematically vary the Eu:Tet ratio from 100:0 to 0:100 (v/v) and reveal that solvent composition governs solution‐state aggregation, film‐formation kinetics, and blend morphology. The optimized Eu:Tet = 50:50 (v/v) composition enables balanced solvation and film formation, promoting nanoscale donor–acceptor phase separation and a favorable vertical composition gradient while retaining sufficient crystalline ordering. Consequently, Eu‐50‐processed OSCs achieve PCEs of 18.66% under nitrogen and 18.40% under ambient conditions. Moreover, the unencapsulated Eu‐50 devices retain ∼94% of their initial efficiency after 2110 h in air, whereas conventional chloroform‐processed devices lose more than 20% of their initial efficiency under identical conditions. Thus, this study establishes solvent‐composition control as a practical strategy for achieving efficient and air‐stable terpene‐processed OSCs.