Moisture Uptake Relaxes Stress in Metal Halide Perovskites at the Expense of Stability

Previous studies have shown that the degradation rate of metal halide perovskites in an ambient atmosphere increases with the amount of tensile stress, which primarily arises from the coefficient of thermal expansion mismatch with the substrate. In this work, we show the first evidence of tensile stress relaxation in perovskite films resulting from moisture uptake. Indeed, for multiple perovskite compositions we observe that tension relaxes rapidly in ambient conditions, as compared to inert conditions, with quartz crystal microbalance measurements showing a mass density increase on a similar time scale indicative of moisture uptake. The uptake of moisture at free surfaces, including grain boundaries, can reduce tension in a constrained film, similar to how adatom diffusion reduces residual stress following thin film formation. Unfortunately, the uptake of moisture can catalyze other degradation mechanisms such as PbI 2 formation or a transition to a nonperovskite structural phase. Stress-induced uptake of moisture is an especially important problem for all-inorganic perovskites because they are annealed at much higher temperatures, causing high tensile stress. It explains the unusually poor ambient stability of these perovskites. Using a diethyl ether antisolvent bath to attach CsPbI 2 Br to the substrate at a much lower temperature, we reduced the initial tensile strain from 0.43 ± 0.04% to 0.12 ± 0.05%, thus reducing the driving force for moisture uptake and improving its ambient phase stability by over a factor of 15.