Modeling the FA and I losses in mixed-halide perovskite through chemical rate equations: insights into light-induced degradation

Abstract

Photo-induced degradation is a major obstacle for the practical use of perovskites in solar cells, and understanding this degradation is key to maximizing the potential of perovskite photovoltaics. This study addresses the lack of accurate models for photochemical degradation kinetics in perovskites by deriving rate equations for a cutting-edge triple-cation mixed halide perovskite using a two-step reaction model. Our model describes the temporal evolution of iodine and formamidinium losses, along with the generation of metallic lead (Pb(0)) under continuous white light. Applied to perovskite samples with varying bromine contents (5 to 20%), we found that increased Br content enhances stability under illumination, aligning with reports for Br contents below 25%. Additionally, our study shows that degradation pathways vary between nitrogen (N₂) and ultra-high vacuum (UHV) environments. UHV conditions accelerate Pb(0) formation, while no Pb(0) appears in N₂. Despite the absence of Pb(0) in N₂, atomic force microscopy data reveal light-induced degradation, contradicting previous claims of N₂ stability. This degradation includes perovskite's transformation into lead-iodide and granular structure development on the surface. These findings improve understanding of environmental impacts on perovskite stability and highlight XPS's limitations in detecting photodegradation in N₂.