Design of Terpene‐Based Eco‐Friendly Solution Process for High‐Performance Organic Solar Cells

Terpene‐based solvents, derived from natural products, have emerged as promising candidates for enabling truly eco‐friendly solution processing of organic solar cells (OSCs). However, achieving high power conversion efficiency (PCE) with these green solvents remains challenging due to unfavorable blend morphologies in terpene‐processed active layers. In this study, we report high‐performance, terpene‐processed OSCs with a PCE of 17.3% by designing a new green co‐solvent system consisting of eucalyptol (Eu) and 2‐methyltetrahydrofuran (MeTHF). To improve blend morphology, we introduce MeTHF, another green solvent, into the terpene (Eu) system. The addition of MeTHF enhances molecular ordering during film formation, promoting the development of crystalline fibrillar networks within the active layer. Furthermore, its significantly lower boiling point (78 °C) compared to Eu (178 °C) accelerates solvent evaporation and suppresses excessive phase separation. These synergistic effects lead to well‐developed donor–acceptor interfaces and efficient charge transport pathways, resulting in improved charge generation and reduced charge recombination. Consequently, OSCs processed with the optimized Eu:MeTHF (50:50 v/v) cosolvent achieve a high PCE of 17.3%, surpassing those processed with Eu or MeTHF alone. To the best of our knowledge, this PCE is the highest value reported to date for terpene‐processed OSCs. Thus, this work provides valuable insights into the rational design of terpene‐based solvent systems toward the scalable and sustainable production of high‐performance OSCs.