Hybrid perovskite/silicon tandem solar cells have demonstrated strong potential to surpass the power conversion efficiency limits of single-junction devices while maintaining low production costs, with power conversion efficiencies reaching up to 34.6%. However, hybrid perovskites are currently not stable enough for large-scale deployment. Photo-induced degradation remains a major obstacle to the use of perovskites in solar cells, affecting their long-term stability. Current photochemical degradation models are still limited, particularly regarding the impact of halide composition and environmental conditions on perovskite stability. Our study aims to address these limitations by proposing a kinetic model investigating mixed-halide triple-cation perovskites, with a focus on the degradation kinetics associated with iodine and formamidinium losses, as well as the production of metallic lead (Pb(0)) under illumination [1].

First, we used X-ray photoelectron spectroscopy (XPS) to monitor the degradation of our perovskite material (FAPbI_1-xBr_x, where FA = formamidinium) induced by white-light exposure. This degradation is characterized by the formation of metallic lead in the zero oxidation state (Pb(0)) [1–3]. We then developed a kinetic model describing the evolution of lead, halide (I, Br), and formamidinium concentrations. To build this model, we assumed that the photodegradation process occurs in two steps. In the first step, the perovskite material degrades into lead iodide with the release of FAI in gaseous form. In the second step, lead iodide further decomposes into metallic lead (Pb(0)) while releasing gaseous I₂. By considering these two steps as first-order chemical reactions, we established differential equations describing the time evolution of Pb, FA, and I concentrations. The results of our model show good agreement with the evolution measured experimentally.

Using atomic force microscopy (AFM) together with our kinetic model [3], we provide insights into the environmental effects on perovskite durability (ultra-high vacuum versus N2 environment), confirming that light-induced degradation occurs in both environments but through distinct mechanisms. In the literature, the absence of a Pb(0) signal in XPS is often associated with improved perovskite stability. However, through local probe microscopy measurements, we demonstrate here that the perovskite material can still degrade without generating a detectable Pb(0) signal in XPS.

Our results also reveal that an increased bromine content (x = 5 to 20%) improves perovskite stability while highlighting distinct degradation pathways between nitrogen (N2) and ultra-high vacuum (UHV) environments. This finding contradicts previous claims of stability under N2 atmosphere. Overall, these results emphasize the importance of understanding degradation processes in order to optimize perovskite materials for future applications.