# Investigation of Open Air Stability of CsPbBr3 Thin-Film Growth on Different Substrates
## 3 Results and Discussion
### Surface Chemical Evolution  
Due to the small probing depth and the capability of chemically discerning different species of the same element, XPS was employed to monitor the chemical evolution of the surface of the samples as a function of the aging time. In particular, the lead XPS chemical shift provided very interesting clues  
Figure 5: UV–Vis spectra in reflected light as a function of the time of exposure: (**a**) silica glass, (**b**) ITO, (**c**) soda-time glass, and (**d**) gold. about the perovskite degradation process. Figure 6 depicts the XPS regions characteristic of the most intense and diagnostic XPS peak of lead (6\(f\) core transition), which was characterized by a doublet due to the spin-orbit coupling. Hereafter, we refer to the principal peak of the \(f\) transition as the right one. Lead peaks compatible with CsPbBr\({}_{3}\) were always present in the pristine samples (binding energy of the principal peak equal to 138.5 eV, doublet marked with red lines in the spectra reported in Figure 6). It is well known that the broadening of these peaks or the growth of secondary ones is related to the presence of lead in new chemical environments [12; 28]. We observed this on the samples deposited on ITO, gold, and, to a minor extent, soda-line glass (see Figure 6) upon the exposure to atmospheric moisture. On the other hand, on silica glass, the characteristic peak of lead remained almost identical throughout the stability test (Figure 6a). The presence of a peak at a lower binding energy with respect to CsPbBr\({}_{3}\) was reasonably attributable to the formation of reduced species such as Pb(0). It is also evident that the presence of these compounds was strictly related to the exposure to air, since it was never observed in the \"as-prepared samples\" and its intensity increased as a function of the exposure time. The XPS peaks of cesium and bromine did not present evident binding energy shifts and, therefore, did not provide useful information about the chemical evolution of the surface. However, the relative intensity of XPS peaks is related to the atomic composition of the sample; therefore, it is possible to monitor the surface compositional trends as a consequence of the exposure time. Figure 7 displays the semi-quantitative elemental analysis for lead, cesium, and bromine as determined by XPS, employing tabulated sensitivity factors [28]. The expected composition of perovskite constitutes lead and cesium (20 at.%; one in every five atoms) and bromine (60 at.%; three in every five atoms). For the sake of clarity, in Figure 7, two red dashed lines are added: one at 20%, related to the expected amount of cesium and lead, and the other at 60%, related to the expected amount of bromine. As already observed, soda-line and quartz glass presented the most stable and similar compositional profiles, while the surface of the perovskite thin films deposited on the other  
Figure 6: X-ray photoelectron spectroscopy (XPS) characteristic peaks of lead: (**a**) silica glass, (**b**) ITO, (**c**) soda-line glass and (**d**) gold. Solid lines = main peaks; dashed lines = secondary peaks. two substrates was characterized by much more variable compositional trends indicating, on the whole, a less stable behavior in air. The surface of the film deposited on silica and soda-lime glass resulted enriched in Cs and depleted in Pb and Br, especially after the degradation tests. These data, together with XRD and UV-Vis results, suggest that the degradation of these samples caused a migration of lead bromide close to the surface of the film, bringing about a sort of passivation of the films that reduced their further degradation.