Photoinduced Degradation of Lead Halide Perovskite Thin Films in Air

As an excellent photoelectric material, metal halide perovskites have been rapidly developed in the photovoltaic field. The power conversion efficiency of solar cells based on perovskite materials now exceeds 24%, which is close to the conversion efficiency of silicon-based solar cells. However, organic-inorganic hybrid perovskite materials are sensitive to light, oxygen, and moisture, particularly when combined in the ambient environment, limiting their commercial application in perovskite devices due to their poor environmental stability. Therefore, a comprehensive understanding of the degradation mechanism is the key for development of an effective method to inhibit the degradation of perovskite materials. Herein, the photo-induced degradation process of CH3NH3PbI3 films in air was studied by conventional optical and structural characterization methods, including ultraviolet-visible (UV-Vis) absorption spectroscopy, X-ray diffraction (XRD) and advanced transmission electron microscopy (TEM) equipped with a probe spherical aberration corrector. The CH3NH3PbI3 films were first decomposed into hexagonal PbI2 and amorphous phase, and subsequently oxidized to the amorphous phase under the combined effects of light and oxygen. The molecular formula of the amorphous phase was further confirmed as PbI2-2xOx(0.4 < x < 0.6) via X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Further analysis showed that the film degradation is mainly related to superoxide (O-2(center dot-)) formed by combination of oxygen molecules and photoelectrons in the perovskite film. The organic part of the CH3NH3PbI3 is oxidized by O-2(center dot-) and CH3NH3PbI3 is decomposed to form volatile products, such as CH3NH2 and I-2, then degraded into PbI2, and oxidized to form the amorphous PbI2-2xOx. Therefore, during the initial degradation of film under light soaking in air, the degradation sites are mainly located at the interface between CH3NH3PbI3 and air. Many pores were observed on the film surface due to the large loss of volatile decomposition products during the initial degradation. The films then converted to a honeycomb hollow morphology due to the continuous consumption of material under light soaking, reducing the mass of the film as well. Finally, the entire film was oxidized to form an amorphous structure. Herein, for the first time, we report that the formation of amorphous oxides is accompanied by the degradation of perovskite film. This study presents a new understanding of the photo-induced degradation mechanism of perovskite films in air and provides novel theoretical guidance to promote the long-term stability of perovskite solar cells.