Lead-free tin halide perovskites, particularly formamidinium tin iodide (FASnI₃) and cesium tin iodide (CsSnI₃), represent a promising class of materials for sustainable photovoltaics due to their optimal bandgaps for tandem solar cell integration and their reduced environmental impact. The fabrication of uniform, compact, and defect-free thin films with precise stoichiometry is essential for achieving high device performance and long-term stability. Although solution-processing remains widely used at the laboratory scale, vacuum-based sequential physical vapor deposition (PVD) offers a solvent-free, industrially scalable alternative with superior control over film composition, thickness, and crystallization.

In this study, we investigate the growth dynamics of FASnI₃ and CsSnI₃ thin films via all-vacuum sequential deposition using in situ X-ray diffraction (XRD) to monitor real-time phase evolution during deposition and annealing. By comparing different precursor stacking orders, we explore how interdiffusion kinetics and crystallization behavior influence film morphology and stability.

For FASnI₃, two sequential deposition routes—FAI/SnI₂ and SnI₂/FAI—were examined. In both cases, initial perovskite formation was detected during deposition at room temperature, indicating rapid interfacial reactivity. FAI emerged as the primary diffusing species since post-annealing morphology varied significantly. The FAI/SnI₂ route producing perovskite at 160 °C exhibited notable voids and non-uniformity. In contrast, the SnI₂/FAI route yielded dense, void-free films. From this difference, we conclude that FAI is the main diffusing species. Nonetheless, both configurations showed thermal degradation above 180 °C, revealing the intrinsic thermal instability of FASnI₃.

For CsSnI₃, sequential deposition routes of CsI/SnI₂ and SnI₂/CsI were studied. The CsI/SnI₂ configuration required annealing above 100 °C to initiate 1:1:3 perovskite formation, but ultimately formed uniform, dense films upon annealing between 180–240 °C. The SnI₂/CsI sequence, in contrast, led to immediate perovskite formation upon CsI deposition at room temperature but produced films with prominent cavities after annealing. These results confirm that here SnI₂ is the main mobile species. Its inward diffusion into CsI yields controlled reaction fronts, whereas outward diffusion from SnI₂ layers can lead to void formation. Additionally, both sequences transiently exhibited the Cs₂SnI₆ phase and decomposed thermally above 240 °C, reverting to CsI.

Our findings establish that precursor stacking order is a critical factor governing crystallization pathways, phase purity, and film quality in all-vacuum deposited tin perovskites. Optimal sequences—SnI₂/FAI for FASnI₃ and CsI/SnI₂ for CsSnI₃—minimize residual precursors and suppress morphological defects. Given the observed thermal vulnerabilities, we also studied Cs-alloying in FASnI₃ and found complete mixed A-cation perovskite formation.

Keywords: halide perovskites, physical vapor deposition, in situ X-ray diffraction, formamidinium tin iodide, cesium tin iodide, diffusion model, perovskite solar cells.