  Methylammonium lead halide perovskites have been developed as highly
promising materials to fabricate efficient solar cells in the past few years.
The real impact to energy applications relies on the understanding and
controlling of the stability of the material. We investigated the degradation
of CH3NH3PbI3 by air exposure using x-ray diffraction (XRD), x-ray
photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The
stoichiometric sample was grown with co-evaporation of PbI2 and CH3NH3I on a Au
coated Si wafer. It was found that the perovskite thin film gradually turned to
PbI2 in air, accompanied with complete removal of N and substantial reduction
of I. It was also observed that PbI2 crystallization roughened the film and
resulted in a partial exposure of the Au substrate.
