The Role of Halogen-Free Solvents on the Stability of PTQ-2F:BTP-4F-Based Organic Solar Cells

The appeal of organic solar cells (OSCs) as a source of environmentally friendly electricity is often negatively impacted by the common use of hazardous materials in their manufacturing, such as halogenated solvents. This study explores the fabrication and operational stability of PTQ-2F:BTP-4F-based OSCs using halogenated (chloroform (CF), chlorobenzene) and nonhalogenated (ortho-xylene (oXY), 1,2,4-trimethylbenzene) solvents in a hot-solvent spin-coating process. Initial power conversion efficiencies (PCEs) of up to 12.2% (CF) and 10.0% (oXY) are achieved, with detailed morphological and performance analyses conducted via ex situ and operando grazing-incidence small-angle X-ray scattering (GISAXS). Ex situ measurements reveal significant differences in bulk-heterojunction nanostructures, with benzene-based solvents producing domain size distributions distinct from CF-processed films. Operando GISAXS connects real-time degradation kinetics to the domain size evolution, highlighting solvent-dependent kinetics. Halogenated solvents facilitate a gradual PCE decay, while nonhalogenated solvents exhibit a rapid initial burn-in phase followed by stabilization, with oXY-processed OSCs demonstrating superior long-term stability. Morphological stability in oXY films originates from the limited coalescence of small polymer domains, retaining a more fine-grained structure in the active layer. This study emphasizes the critical role of processing solvents in OSC performance and stability, positioning oXY as a sustainable candidate for scalable and eco-friendly OSC fabrication.