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The influence of light soaking and heat stress on the degradation of indium tin oxide (ITO)/CH3NH3SnI3and SiO2/CH3NH3SnI3hybrid perovskite is studied. The measurements of X-ray photoelectron survey spectra (XPS) showed an increase in the concentration of substrate ions in the surface layer of ITO/CH3NH3SnI3and reduction of N:Sn and I:Sn ratios after 500 h of white light irradiation and 200 h of annealing at 90 °C. The high-energy-resolved XPS Sn three-dimensional (3d) spectra of ITO/CH3NH3SnI3indicated a consistent increase in the contribution of Sn4+ions at these exposure times under light soaking and heat stress. Scanning electron microscopy (SEM) measurements show a violation of the continuity of the perovskite layer with the exposure time and an increase in the size of the voids, which is consistent with the indicated changes in the XPS spectra. On the other hand, we consider the effect of irradiation and annealing on the diffusion of the substrate elements and their chemical interaction with the overlying perovskite layers. The XPS Sn 3d, O 1s, and valence band (VB) measurements and density functional theory (DFT) calculations reveal that at the initial stages of CH3NH3SnI3degradation, the SnI4phase separation occurs and with a further increase in exposure time, tin is oxidized to form a SnO2phase. In addition, we found that CH3NH3SnI3perovskite prepared on a SiO2substrate is more stable than on an ITO substrate, and the effect of the oxidation of tin atoms due to the migration of oxygen ions is very weak. The theoretical modeling demonstrates that the formation of SnI2defects in CH3NH3SnI3became irreversible due to the oxidation by the migration of the oxygen ions from the substrate. The calculated formation energy of an oxygen vacancy in ITO is about 2 times smaller than in SiO2that explains the instability of ITO/CH3NH3SnI3.

We analyze the electronic structure of several 5pbinary oxides (In2O3, SnO, SnO2, Sb2O3, Sb2O5, and TeO2) using density functional theory and soft X-ray spectroscopy. We find that there is bonding in hybridized cation 5s-O 2pstates at the bottom of the valence band in oxides with cations in both the formal 5s2and 5s0valencies. We similarly find that there is antibonding in hybridized cation 5d- or 5s-O 2pstates at the top of the valence band for oxides with cations in 5s2or 5s0valencies, respectively, and the density of O 2pstates in these antibonding states is much greater in the 5s0oxides than the 5s2oxides. The calculated and quasi-empirical band gaps for these oxides are tabulated, and we identify the hybridizations responsible for the spectral features in the measured XES.

This article features the potential and examples of the comprehensive use of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) to analyze the degradation processes in APbX3hybrid halide perovskites, where the X-site anion is I–and Br–and the A-site cation is represented by CH3NH3, CH(NH2)2, and Cs. The whole set of XPS options is used, including the measurements of survey, core levels (C 1s, N 1s, Pb 4f, I 3d and Br 3d), and valence bands spectra, which provides a complementary understanding of the electronic structure and chemical bonding of perovskites while excluding the random errors. It is found that stability of APbX3perovskites with respect to light soaking and heat stress is connected with the formation of PbX2degradation product, which can be monitored by high-energy resolved XPS Pb 4f, I 3d-spectra and valence band spectra. The obtained results are confirmed by DFT calculations of the formation energies of intrinsic PbX2-defects. Furthermore, our results show that the resistance of hybrid lead halide perovskites to stress factors (light or heat) is gradually increased while substituting the A-site cation going from CH3NH3+to CH(NH2)2+and then to Cs+.