Photoluminescence characterizations of highly ambient-air-stable CH 3 NH 3 PbI 3 /PbI 2 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.