Frank-van der Merwe-Mediated Sequential Thermal Evaporation for Stable FASnI₃ Perovskite Solar Cells

Thermal evaporation provides a solvent-free and scalable process for thin-film deposition with high reproducibility, which is particularly attractive for fabricating oxygen- and solvent-sensitive tin-based halide perovskites. However, fully thermally evaporated FASnI₃ perovskites have been well underexplored, and the film quality is hindered by uncontrolled crystallization and high defect density. Herein, we develop a sequential thermal evaporation strategy, in which the different growth modes of SnI₂ and FAI thin films on the substrate are precisely regulated to guide perovskite crystallization for improving film quality. We found that the FASnI₃ film follows a Frank-van der Merwe growth mode owing to the lower surface free energy of the first evaporated SnI₂ underlayer, which further promotes the formation of high-quality perovskite thin films with a preferred out-of-plane orientation, homogeneous Sn distribution, and suppressed interfacial defects. Benefiting from the SnI₂ underlayer, the FASnI₃-based PSCs achieve a champion power conversion efficiency (PCE) of 5.51%. The unencapsulated devices exhibit negligible current-voltage hysteresis and impressive stability, retaining over 91.98% of the initial PCE after 2100 h in a nitrogen atmosphere. These findings provide valuable insights into the scalable fabrication of high-quality and stable lead-free perovskite thin films for high-performance optoelectronics.