Photoluminescence characterizations of highly ambient-air-stable CH₃NH₃PbI₃/PbI₂ heterostructure

CH₃NH₃PbI₃ perovskite material has demonstrated great promise in high-performance solar cells and light-emitting devices (LEDs). In this work, we investigated the impact of the coexistence of PbI₂ and CH₃NH₃PbI₃ perovskite on photoluminescence (PL) properties. In absorbance and PL measurements performed at room temperature, we observed an emission peak at 780 nm, which is consistent with the band-edge absorption of CH₃NH₃PbI₃. On the top surface, we observed dissolved PbI₂, which could serve as a passivation species for improving PL stability upon exposure to ambient conditions. Specifically, dual-peak PL spectra were observed at room temperature. The peak at 780 nm originates from the free-carrier transition of CH₃NH₃PbI₃ and the peak at 796 nm originates from the PbI₂-related recombination. Based on time-resolved PL and X-ray diffraction measurements, we can conclude that unconverted and dissolved PbI₂ in CH₃NH₃PbI₃ forms a type-II band alignment at the CH₃NH₃PbI₃/PbI₂ interface. This type II hetero-structure indeed influences the quality and PL performance of perovskites. To check this structure or quality of perovskite, x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS) are common ways. Here, we demonstrated a cheaper, faster and more convenience method, PL measurement, to check the quality of CH₃NH₃PbI₃ perovskite films. It should be a useful way to check the quality perovskite-based LEDs and solar cells merely by observing whether the dual peaks exist or not in the PL spectra.