Ab Initio Thermodynamic Study of PbI2 and CH3NH3PbI3 Surfaces in Reaction with CH3NH2 Gas for Perovskite Solar Cells

Hybrid organic-inorganic halide perovskites for photovoltaics have attracted research interest due to their unique material properties, but suffered from poor stability. In this work, we investigated the surface phase diagrams of PbI$_2$ and cubic CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) a better understanding precursor effect on perovskite synthesis via solid-gas reaction, by density functional theory calculations combined with thermodynamics. Using devised slab models PbI$_2$(001) MAPbI$_3$(100), (110) (111) surfaces perfect various vacant defect terminations, calculated formation energies adsorption CH$_3$NH$_2$ molecule under different conditions temperature, pressure pH chemical potentials species. Our revealed that can be facilitated including HI or NH$_4$I dissociation additive CH$_3$NH$_3^+$ cation NH$_3$-CH$_3$NH$_3^+$ complex, which is beneficial conversion MAPbI$_3$ reaction. Furthermore, found among surfaces, MA-terminated MAI-terminated (100) are placed thermodynamically stable region at values 1 6, being agreed well experimental findings. We believe work gives fundamental reaction high-crystallinity towards solar cells improved