Template-assisted epitaxial growth of (111)-dominated FAPbI₃ for enhanced photovoltaic performance and stability

The mixed cation strategy significantly improves the device performance and stability of metal halide perovskites. However, phase separation during the fabrication and ageing process degrades their efficiency and stability. For FA-based perovskites, (111)-dominated films show exceptional stability against thermal stress and moisture. (111)-dominated FAPbI₃ with a single cation (FA⁺) is difficult to prepare directly. In this study, (111)-dominated FAPbI₃ was obtained through ion exchange utilizing a (111)-dominated FA_0.85MA_0.15PbI₃ template. The DFT results revealed that the ion exchange mechanism occurs along the (111) facet. This transformation enabled a transition in its conductivity type from n-type to p-type. The (111)-dominated p-FAPbI₃ film and device exhibit low defect density and excellent thermal and humidity stability. The (111)-dominated FAPbI₃ device retained 95% of its initial PCE after being stored in a N₂ atmosphere at 85 °C and 90% initial PCE after exposure to ambient conditions of 50-60% RH at room temperature for 30 d.