Temperature-dependent photoluminescence of Co-evaporated MAPbI₃ ultrathin films

The hybrid organic–inorganic perovskite (PVK) thin films have attracted amounts of interest in the past decade due to the thickness-depended quantum confinement effects and tunable band gap as well as the demands of device miniaturization. Vacuum deposition is a promising method to realize precise control of film thickness and composition, low-temperature processing, the good compatibility with dry processing and compatible with the modern silicon-based semiconductor industry. Here, we report on the temperature-dependent photoluminescence properties of MAPbI₃ ultrathin films with long-term stability and high quality fabricated by the co-evaporation. XPS, AFM and XRD results reveal that an excess MAI vapor environment plays a key role in sample preparation. PL is more sensitive than XPS to detect trace MAPbI₃-like materials with strong luminescence properties. Temperature- dependent PL measurements show that the bandgap of MAPbI₃ films red-shifted with the temperature decreasing in the range from 298 to 78 K, which is opposite to that of the traditional semiconductors and can be attributed to the interaction between the electron–phonon and the thermal expansion induced band gap renormalization. Gradual phase transition from tetragonal one to orthorhombic one is observed to start between 108 and 118 K, which is lower than the reported sudden phase transition temperature of 150 K for bulk MAPbI₃ and suggests the important role of quantum confinement in ultrathin films. Our findings provide a way to fabricate PVK ultrathin films with long-term stability for novel light emitting devices.