Strain-induced anisotropic ion migration in single-crystal cesium lead halide perovskites

Ion migration adversely affects perovskite solar cell (PSC) performance by upsetting film stability, inducing hysteresis, and light-induced halide segregation. Ion migration is also substantially influenced by residual strains, which are prevalent in fabricated PSCs. This study thus utilizes molecular dynamics simulations to analyze the influence of uniaxial, biaxial, and isotropic states of compressive and tensile stresses on ion migration in single-crystal cesium lead iodide (CsPbI₃) and bromide (CsPbBr₃) perovskites. Furthermore, nudged elastic band simulations are used to reveal energy barriers associated with ion migration under mechanical deformation. Tensile and compressive strains are observed to intensify and inhibit ion migration, respectively, in these metal halide perovskites (MHPs), with the extent of this alteration becoming more drastic on progressing from uniaxial to isotropic state of stress. We also reveal that uniaxial and biaxial states of stresses induce anisotropic ion migration in CsPbI₃, while ion migration remains isotropic under these loading conditions in CsPbBr₃. The heightened ion migration under tension in these MHPs is deciphered to arise from lowering of the energy barrier, while migration inhibition under compression arises from increase in barrier height. This study thus provides direct evidence of tensile and compressive strains influencing ion migration in MHPs and highlights that methods such as lattice-strain tailoring that are aimed at mitigating this phenomenon may need to be customized to the MHP of interest.