Phase Homogeneity and Photothermal Stability in Fully Vacuum-Processed Perovskite Solar Cells

Vacuum-deposited lead halide perovskite thin films enable solvent-free fabrication, eliminating residual processing solvents that might compromise the long-term stability. Here, we investigate the stability of thermally evaporated mixed-cation compositions FA_0.8Cs_0.2PbI₃ and FA_0.8MA_0.2PbI₃ (FA⁺ = formamidinium and MA⁺ = methylammonium) under thermal and light stress. Although from a thermodynamic perspective the phase stability hierarchy is typically described as MA⁺ < FA⁺ < Cs⁺, with Cs-based perovskites expected to be the most stable, both compositions exhibit thermal robustness, retaining their structural, optical, and morphological properties after continuous heating at 85 °C for over 500 h. Under continuous illumination, however, distinct degradation pathways emerge: FA_0.8Cs_0.2PbI₃ shows the largest morphological and optical changes. This is attributed to chemical inhomogeneities caused by CsI-rich segregations during crystallization, which make point defects effective triggers for photodegradation. Film homogeneity improves by partially replacing iodide with bromide. Based on these results, we selected FA_0.8MA_0.2PbI₃ and FA_0.8Cs_0.2Pb-(I_0.8Br_0.2)₃ for device fabrication and evaluated their operational stability. The resulting perovskite solar cells maintain their performance after four months of outdoor operation and withstand 900 h under continuous sun-equivalent indoor illumination at room temperature. These results demonstrate how a high-quality crystallization process can reveal the potential of MA-containing perovskite formulations for long-lived perovskite photovoltaics.