Chemical Reaction Kinetics of the Decomposition of Low-Bandgap Tin–Lead Halide Perovskite Films and the Effect on the Ambipolar Diffusion Length

Mixed tin–lead halide perovskite materials, with a bandgap of ∼1.2–1.3 eV, are promising absorber materials for solar cells, with their bandgap ideally matched to the solar spectrum, and for the low-bandgap part of all-perovskite tandem solar cells. However, tin(II) is not very stable and can be oxidized to tin(IV), resulting in the decomposition of the perovskite. Here, we evaluate the reaction products and reaction kinetics of the decomposition of FA0.75Cs0.25Pb0.5Sn0.5I3 thin films in response to exposure to oxygen, moisture, and illumination using in situ measurements of optical transmittance, X-ray diffraction, and UV-vis-NIR spectroscopy. We found the decomposition occurs by a dry oxidation pathway (1 × 10–9 mol/m2·s at 25 °C in air) and a water-accelerated oxidation pathway (3 × 10–9 mol/m2·s at 25 °C in 50% RH air). An analytical kinetic rate expression for the decomposition is derived and validated with a mean test error of 18%. Further, we develop a predictive model of the decay of the ambipolar diffusion length, in which the chemical decomposition rate expression is the most dominant feature. The results highlight the importance and utility of quantitative measurements of perovskite degradation.