Stability of Organic Cations in Solution-Processed CH3NH3PbI3 Perovskites: Formation of Modified Surface Layers

We report on a combined X-ray and UV photoemission spectroscopy study (XPS and UPS) of organic inorganic perovskites prepared from a solution of lead chloride (PbCl2) and methylammonium iodide (CH3NH3I). The XPS intensities are consistent with a pure iodide perovskite (CH3NH3PbI3), with no detectable chloride left. However, we found that the elimination of chloride results in residual methylamine molecules (CH3NH2) trapped within the perovskite crystal lattice. Furthermore, we show that vacuum annealing or sputtering induces the formation of a thin PbI2 layer at the crystal surface which acts as a surface barrier blocking electron transfer from the underlying perovskite film.