Metal halide perovskites are a new class of semiconductor material which show great promise for thin film solar cells. Although, they have efficiencies competitive with monocrystalline silicon, the advantages of flexibility, low cost, and easy processing, makes them superior candidates over conventional inorganic solar cells [1]. Perovskite based thin film solar cells have now reached an efficiency of 25.6% as of April 2021 [2]. Perovskite materials are distinguished by tuneable bandgap [3], ambipolar charge transport [4], long carrier lifetimes [5], and long charge diffusion lengths [6]. At present, the most studied organic-inorganic perovskites are represented by the formula CH₃NH₃PbX₃ where X is a halide ion (Cl, Br, or I).

The major obstacle facing perovskite solar cells is their limited service life under operating conditions owing to the deterioration of the absorber material in contact with humidity, heat, and light [7, 8]. Understanding the degradation mechanism in perovskites is of crucial importance for device design and optimization. The degradation mechanism of perovskites has been a topic of extensive exploration [9-16]. Although various degradation pathways have been proposed, the exact route is still under debate. A few XPS studies have shown the presence of metallic lead in the perovskite films but the pathway of lead precipitation is not very well understood [17].

In this study, we aim to compare the thermal degradation mechanism of the two main lead halide perovskites: CH₃NH₃PbI₃ (MAPbI₃) and CH₃NH₃PbBr₃ (MAPbBr₃) using X-ray photoelectron spectroscopy (XPS). The samples were heated to around 200 °C and XPS data was recorded before and after heating. We observed that the perovskite peak in the MAPbI₃ C1s spectra changes to MA⁺ after heating, whereas the perovskite peak still remains after heating in the case of the MAPbBr₃ sample. This suggests that the perovskite phase in the MAPbI₃ sample fully degraded upon heating but the perovskite phase in the MAPbBr_­­3 sample only partially degraded. Similarly, in the N 1s spectra of the MAPbI₃ sample the perovskite peak disappeared after heating and a new peak corresponding to the binding energy of MAI was observed post heating. In contrast, the perovskite peak was seen before and after heating in MAPbBr₃ sample as observed in the C1s spectra. We also observed metallic lead peaks (as has been previously reported) in both the samples before and after heating. However, the intensity of the peaks increased post heating indicating that thermal stress accelerated the precipitation of metallic lead. We observed a decrease in the concentration of iodine and nitrogen after heating and an increase in the surface lead concentration in the MAPbI₃ sample. In contrast, only slight changes in concentration were observed in the MAPbBr₃ spectra. Based on these observations we propose a three-step degradation mechanism of perovskites. The degradation mechanism is used to explain the observed concentration changes and the role of halide ions in the structural stability of perovskites.