Chemical and Electronic Structure Characterization of Lead Halide Perovskites and Stability Behavior under Different Exposures—A Photoelectron Spectroscopy Investigation

The past few years, two perovskite materials have attracted much attention in the solar cell community: CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3– x Cl x . While these materials are usually characterized using their structure (via X-ray diffraction (XRD)) and performance within solar cell communities, not so much attention has been devoted to their surface chemical composition and, specifically, the surface composition. Photoelectron spectroscopy (PES) can easily fulfill this task, and, in addition to chemical information, PES provides an overall picture of the electronic structure of the perovskite and its relation to mesoporous TiO 2 when studied with hard X-rays. In this work, CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3– x Cl x have been compared with each other and also to CH 3 NH 3 PbCl 3, and it appears that, despite very different morphologies and kinetics of formation, the two former materials present a very similar electronic structure and chemical composition (i.e., no chlorine is observed in the final CH 3 NH 3 PbI 3– x Cl x materials). Nevertheless, chlorine is very important during the preparation, because it affects the formation of crystalline CH 3 NH 3 PbI 3 . We have also exposed the classical CH 3 NH 3 PbI 3 to various environments, such as water, temperature, and long-time storage in air and argon, and followed changes of the surface composition with PES. The main result of the different exposures is that the perovskite is decomposed into PbI 2, but an important point is that this degradation seems to occur already at 100 °C and is not only related to large humidity. Indeed, even in an inert atmosphere such as argon, a slow degradation to PbI 2 is observed. The results obtained are crucial for a better understanding of this material and will help to improve not only the post-conditioning of the cells but also their synthesis.