Understanding liquefaction in halide perovskites upon methylamine gas exposure

Methylamine (CH₃NH₂, MA) gas-induced fabrication of organometal CH₃NH₃PbI₃ based perovskite thin films are promising photovoltaic materials that transform the energy from absorbed sunlight into electrical power. Unfortunately, the low stability of the perovskites poses a serious hindrance for further development, compared to conventional inorganic materials. The solid-state perovskites are liquefied and recrystallized from CH₃NH₂. However, the mechanism of this phase transformation is far from clear. Employing first principles calculations and ab initio molecular dynamics simulations, we investigated the formation energy of primary defects in perovskites and the liquefaction process in CH₃NH₂ vapor. The results indicated that defect-assisted surface dissolution leads to the liquefaction of perovskite thin films in CH₃NH₂ vapor. Two primary defects were studied: one is the Frenkel pair defect (including both negatively charged interstitial iodide ion (I_i ^-) and iodide vacancy (V_I ⁺) at the PbI₂-termination surface, and the other is the Schottky defects (methylammonium vacancy, V_MA) at the MAI-termination surface. Moreover, the defect-induced disorder in the microstructure reduces the degeneration of energy levels, which leads to a blue shift and broader absorption band gap, as compared to the clean perovskite surface. The mechanism of how defects impact the surface dissolution could be applied for the further design of high-stability perovskite solar cells.