Frank–van der Merwe-Mediated Sequential Thermal Evaporation for Stable FASnI 3 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.