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Low stability of organic-inorganic perovskite (CH3NH3PbI3) solar cells inhumid air environments is a serious drawback which could limit practicalapplication of this material severely. In this study, from real-timespectroscopic ellipsometry characterization, the degradation mechanism ofultra-smooth CH3NH3PbI3 layers prepared by a laser evaporation technique isstudied. We present evidence that the CH3NH3PbI3 degradation in humid airproceeds by two competing reactions of (i) the PbI2 formation by the desorptionof CH3NH3I species and (ii) the generation of a CH3NH3PbI3 hydrate phase by H2Oincorporation. In particular, rapid phase change occurs in the near-surfaceregion and the CH3NH3PbI3 layer thickness reduces rapidly in the initial 1-hair exposure even at a low relative humidity of 40%. After the prolonged airexposure, the CH3NH3PbI3 layer is converted completely to hexagonal plateletPbI2/hydrate crystals that have a distinct atomic-scale multilayer structurewith a period of 0.65 nm. We find that conventional x-ray diffraction andoptical characterization in the visible region, used commonly in earlier works,are quite insensitive to the surface phase change. Based on results obtained inthis work, we discuss the degradation mechanism of CH3NH3PbI3 in humid air.

Degradation mechanism of CH3NH3PbI3 perovskite materials upon exposure to humid air