Modeling Ion Diffusion in Mixed-Halide Perovskites

Licentiatavhandling, 2026

Halide perovskites have attracted significant attention for optoelectronic applications due to their high efficiency and tunable optoelectronic properties. However, their long-term stability remains limited, largely due to the soft and polarizable lattice, which facilitates defect formation and ion migration. Understanding defect-mediated ion transport is therefore essential for improving material stability. In this thesis, ion transport in inorganic halide perovskites is investigated across multiple length scales. At the atomic level, density functional theory (DFT) combined with the nudged elastic band (NEB) method is used to study defect-mediated ion migration in CsPb$X_3$ ($X$ = I, Br, Cl). The results demonstrate that migration barriers depend not only on intrinsic material properties but also on computational methodology, including migration pathways, exchange--correlation functionals, defect charge state, and spin--orbit coupling. Structural factors such as lattice relaxation, volume, and the local bonding environment are found to play a central role. At larger length and time scales, ion diffusion in mixed compositions Cs$_{1-x}$Rb$_x$PbBr$_{3(1-y)}$I$_{3y}$ is investigated using molecular dynamics simulations based on neuroevolution-based machine-learned interatomic potential. Diffusion coefficients are evaluated across a range of temperatures and compositions and analyzed using Arrhenius behavior, enabling direct comparison of activation energies and prefactors. The results show that ion transport is governed by both energetic and dynamical factors and is strongly influenced by composition and lattice flexibility, linking atomistic migration mechanisms to macroscopic diffusion behavior.