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

Mixed\ntin–lead halide perovskite materials, with\na bandgap\nof ∼1.2–1.3 eV, are promising absorber materials for\nsolar cells, with their bandgap ideally matched to the solar spectrum,\nand for the low-bandgap part of all-perovskite tandem solar cells.\nHowever, tin(II) is not very stable and can be oxidized to tin(IV),\nresulting in the decomposition of the perovskite. Here, we evaluate\nthe reaction products and reaction kinetics of the decomposition of\nFA_0.75Cs_0.25­Pb_0.5­Sn_0.5I₃ thin films in response to exposure to oxygen,\nmoisture, and illumination using in situ measurements of optical transmittance,\nX-ray diffraction, and UV-vis-NIR spectroscopy. We found the decomposition\noccurs by a dry oxidation pathway (1 × 10^–9 mol/m²·s at 25 °C in air) and a water-accelerated\noxidation pathway (3 × 10^–9 mol/m²·s at 25 °C in 50% RH air). An analytical kinetic rate\nexpression for the decomposition is derived and validated with a mean\ntest error of 18%. Further, we develop a predictive model of the decay\nof the ambipolar diffusion length, in which the chemical decomposition\nrate expression is the most dominant feature. The results highlight\nthe importance and utility of quantitative measurements of perovskite\ndegradation.