Effect of ambient humidity on the crystallization and microstructural evolution of multi-step processed CsPbBr₃ thin films for solar cell applications

Understanding how moisture influences the crystallization and electronic quality of all-inorganic perovskites is crucial for glovebox-free fabrication, yet its mechanistic role remains insufficiently understood. Here, we systematically investigate how relative humidity (RH) governs the multi-step crystallization and phase evolution of CsPbBr₃ thin films deposited on SnO₂ by spin coating. Films were prepared under four RH levels: dry (∼0%), moderate (30–40%), ambient (50–70%), and high (80–90%)—to correlate humidity-dependent morphological, structural, and optical properties with photovoltaic performance. SEM shows that RH imprints porosity from the PbBr₂ precursor into the developing film: lower RH produces relatively compact layers, whereas higher RH generates more isolated crystallites and increased voids. During CsBr conversion, morphology evolves more slowly under dry conditions, while elevated RH accelerates grain growth and surface reorganization. X-ray diffraction confirms a humidity-driven phase progression from PbBr₂ to CsPb₂Br₅ and finally to cubic CsPbBr₃, with sharper CsPbBr₃ peaks and reduced residual phases at ambient RH. UV–Vis spectra show enhanced absorption at intermediate humidity, whereas PL intensities are strongest for mixed-phase films, indicating radiative recombination promoted by secondary phases. HTM-free, carbon-based devices fabricated under ambient RH deliver the highest performance (PCE 5.20%, Voc 1.461 V, Jsc 5.03 mA cm⁻², FF 70.6%), while extremely dry and humid conditions yield lower efficiencies due to incomplete conversion and non-ideal microstructure. These results demonstrate that moisture level modulates distinct crystallization pathways that determine film connectivity, phase purity, and electronic quality in CsPbBr₃, ultimately influencing device performance.