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 (PbCl 2 ) and methylammonium iodide (CH 3 NH 3 I). The XPS intensities are consistent with a pure iodide perovskite (CH 3 NH 3 PbI 3 ), with no detectable chloride left. However, we found that the elimination of chloride results in residual methylamine molecules (CH 3 NH 2 ) trapped within the perovskite crystal lattice. Furthermore, we show that vacuum annealing or sputtering induces the formation of a thin PbI 2 layer at the crystal surface which acts as a surface barrier blocking electron transfer from the underlying perovskite film.