Halogen-Free Solvent and Additive-Processed Organic Solar Cells Achieving 20.50% Binary Efficiency.

Transitioning to halogen-free solvent processing is essential for the scalable manufacturing of organic solar cells (OSCs), yet it remains challenging due to severe molecular aggregation and unfavorable phase separation. Additive engineering can effectively modulate film morphology, but it always relies on halogen-based interactions with photoactive materials, compromising environmental sustainability. Herein, we developed three novel halogen-free additives, namely cyclohexylbenzene (ChB), 1,4-dicyclohexylbenzene (DChB), and 1,3,5-tricyclohexylbenzene (TChB), which are benzene derivatives substituted with different numbers of cyclohexyl groups. These additives enable precise regulation of molecular packing and crystallization through distinct steric hindrance effects and additive-acceptor interactions. Both theoretical and experimental investigations reveal that the bulky cyclohexyl groups preferentially adsorb onto the terminal units of L8-BO, creating steric spacers that effectively suppress excessive aggregation during the halogen-free solvent processing. Moreover, the additives serve as kinetic mediators during spin-coating to promote compact terminal packing of L8-BO, while their subsequent thermal volatilization further promotes well-ordered long-range crystallinity. The resulting microstructure significantly enhances charge utilization, enabling D18:L8-BO OSCs to achieve a record-breaking efficiency of 20.50% with improved operational stability. This work underscores the unique advantage of halogen-free additives in selectively modulating molecular aggregation, enabling dual eco-friendly fabrication and paving the way for commercialization of high-performance organic photovoltaics.