Methylammonium lead iodide (MAPbI₃) thin films were grown at low temperatures on glass substrates via 3-zone chemical vapor deposition. Lead iodide (PbI₂) and lead bis (dipivaloylmethanate) [Pb(dpm)₂] precursors were used as lead sources. Due to the high sublimation temperature (~400℃) of the PbI₂ precursor, a low substrate temperature could not be constantly maintained. Therefore, MAPbI₃ thin films degraded into the PbI₂ phase. In contrast, for the Pb(dpm)₂ precursor, a substrate temperature of ~120℃ was maintained because the sublimation temperature of Pb(dpm)₂ is as low as 130℃ at a high vapor pressure. As a result, high-quality MAPbI3 thin films were successfully grown on glass substrates using Pb(dpm)₂. The rms (root-mean-square) roughness of MAPbI3 thin films formed from Pb(dpm)₂ was as low as ~19.2 nm, while it was ~22.7 nm for those formed using PbI₂. The grain size of the films formed from Pb(dpm)₂ was as large as approximately 350 nm.

Methylammonium lead triiodide (MAPbI₃)-based perovskite solar cells potentially have potential advantages such as high efficiency and low-cost manufacturing procedures. However, MAPbI₃ is structurally unstable and has low phase-change temperatures (30°C and 130°C); it is necessary to solve these problems. We investigated the crystal structure and phase separation using real-time temperature-change X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. MAPbI₃ has a tetragonal structure, and at about 35°C the c-axis contracts, transforming MAPbI₃ into the related cubic crystal structure. In addition, at 130°C, phase separation occurs in which CH₃NH₂ and HI at the center of the unit cell of the perovskite structure are extracted by gas, leavingand only PbI₂ of the three-component structure, is produced as the final solid product.