Regulatingthe Crystallizationand Morphology of PbI₂ Precursor Films for the Growth ofHigh-Quality PerovskiteFilms via Vapor–Solid Reaction

Developing vapor–solid reaction methods to prepare organic–inorganic hybrid perovskite thin films is highly compatible with processes in crystalline silicon solar cells and the thin-film photovoltaic industries, facilitating rapid industrialization. In the vapor–solid reaction, the crystallization quality of perovskite thin films is widely influenced by the crystallinity and microstructure of lead iodide (PbI₂) precursor films. During the thermal evaporation process of preparing the PbI₂ precursor films, we observed that PbI₂ tends to develop a disordered surface morphology and exhibits high crystallinity, which significantly hinders the uniform diffusion of the organic amine salt vapor during the subsequent vapor–solid reaction. This results in a high defect concentration within the perovskite crystals. In this study, we propose a strategy to optimize the PbI₂ precursor to address this issue. By employing mercaptoethylammonium Iodide (ESAI) vapor treatment on the PbI₂ precursor films, we found that the thiol groups (-SH) in ESAI can bond with uncoordinated Pb²⁺ ions, effectively regulating the crystallinity of the PbI₂ films and forming an ordered surface morphology. The resulting perovskite films exhibited significantly larger grain sizes. Additionally, we discovered that ESAI interacts with formamidinium iodide (FAI) to form hydrogen bonds, stabilizing FA⁺ ions and effectively passivating defects in the perovskite. By increasing the grain size and reducing the defect density in perovskite thin films, we achieved an inverted device with a maximum power conversion efficiency (PCE) of 20.79%. The unencapsulated device maintained 92% of their initial efficiency after being exposed to air for 1562 h, demonstrating excellent stability.