Anisotropic cation migration and strain driving compositional segregation in FACsPbI3

Formamidinium-cesium lead iodide (FACsPbI3) is among the most promising absorber materials for high-efficiency perovskite solar cells, and A-site cation mixing of FA and Cs represents a crucial strategy to enhance the long-term stability of devices. However, inhomogeneous spatial distribution of A-site cations can adversely impact photovoltaic performance. In this work, we employ density functional theory to investigate the influence of cation mixing on migration behavior and to elucidate the origin of compositional inhomogeneity. We find that the incorporation of Cs not only suppresses halide anion migration but also significantly inhibits the migration of both FA and Cs cations, with a particularly pronounced hindering effect on Cs migration under strain-free conditions. The spherical Cs ion exhibits anisotropic migration within the hybrid perovskite lattice, preferentially moving along the C-H axis of neighboring FA cation. In contrast, planar FA cations tend to migrate along pathways perpendicular to their own plane due to hydrogen-bond interactions, exhibiting behavior characteristic of a coupled rotational–translational motion. During the initial solidification stage of the hybrid perovskites, thermodynamically favorable Cs-aggregated domains readily form; subsequent lattice strain drives anisotropic migration of both Cs and FA, leading to spatially inhomogeneous cation distributions. These findings deepen the understanding of cation migration mechanisms in mixed-cation perovskites and reveal key factors responsible for compositional inhomogeneity, offering theoretical guidance for experimental strategies aimed at mitigating phase segregation in perovskites.