Photooxidation reaction kinetics of mixed-cation mixed-halide perovskite

Abstract

In this work, we use in situ absorbance measurements to study the kinetics of photooxidation of a representative mixed-cation mixed-halide perovskite composition, FA_0.8Cs_0.2Pb(I_0.83Br_0.17)₃. We identify two dominant mechanisms of degradation in the presence of oxygen: a dry photooxidation pathway (with surface reaction rate of (0.89 ± 0.29) × 10⁻⁹ mol m⁻² s⁻¹ at 25 °C in dry air under 1 sun equivalent photon flux) and a water-accelerated photooxidation pathway (with a total surface reaction rate of (1.8 ± 0.9) × 10⁻⁹ mol m⁻² s⁻¹ at 25 °C in 50% relative humidity air under 1 sun equivalent photon flux). Notably, water vapor is found to increase the total decomposition rate at lower temperatures but decrease the rate at higher temperatures compared to dry conditions. We propose a temperature-dependent hydrate formation pathway to explain this behavior and outline a possible degradation mechanism for both the dry and water-accelerated photooxidation pathways. A mathematical expression for the initial decomposition rate as a function of temperature, ambient partial pressures of water vapor and oxygen, and above bandgap photon flux is derived and fitted to observed rates based on the mechanistic analyses. These results provide key insights into the kinetics of decomposition for commercially relevant perovskite absorbers and may serve as a foundation for future device lifetime predictions when coupled with time-dependent mass transfer simulations.