Vacuum-Controlled Solvent Evaporation for Morphological Engineering of Wide-Bandgap Perovskite Films

In this study, we applied the vacuum-assisted solution process (VASP), a scalable method for large-area perovskite film formation, to wide-bandgap (WBG) perovskites. The effects of various key process parameters on film morphology and quality were systematically investigated, and their impact on device performance was also analyzed. By monitoring the time-dependent chamber pressure, we revealed that faster vacuum depressurization enhances dimethylformamide evaporation, yielding larger grains and more uniform perovskite films, consistent with the LaMer nucleation model. These findings were further validated by theoretical calculations. Optimization of vacuum retention time showed that prolonged delays (>45 s) increased nucleation density, reduced N-methyl-2-pyrrolidone content in the intermediate phase, and induced wrinkled textures associated with iodide-rich domains, thereby degrading film uniformity and optical quality. These textures indicate the occurrence of halide segregation in WBG perovskites, which we experimentally confirmed this behavior. Molarity variation experiments demonstrated that increasing precursor concentration thickened the films, but excessive thickness at 1.5 M promoted wrinkling, phase segregation, and performance loss despite higher absorbance. Device characterization confirmed that optimal performance was achieved at a 15 s vacuum retention time (60 mTorr) and 1.4 M precursor concentration with high open-circuit voltage and photocurrent. These results provide quantitative evidence for the critical role of solvent evaporation kinetics and morphological control in VASP, offering practical guidelines for the scalable production of high-quality WBG perovskite layers for tandem solar cell applications.