Halide Ion Migration-Induced Instability in Metal Halide Perovskites

Generation of holes through above-bandgap excitation or through electrochemical injection increases ion migration and leads to phase segregation in metal halide perovskites (MHP). The thermodynamic and redox properties of halide perovskites provide a strong driving force for hole trapping and oxidation of iodide species in MHPs. However, mobile halide species within the perovskite lattice take time to migrate and generate halide-rich domains. When in contact with a non-polar solvent, the migration of iodine species is further extended to expulsion of iodine from the perovskite film. Thus, the mobility of halides and their susceptibility to hole-induced oxidation play a crucial role in determining the long-term stability of metal halide perovskites. Strategies to gain kinetic control over ion migration to slow phase segregation are needed to overcome these hurdles and achieve stable mixed halide perovskites. Modification of the perovskite composition through introduction of different cations, halide ions, or introduction of low-dimensional perovskite phases may suppress phase segregation will be discussed. Thus, in achieving stability and improving efficiency of perovskite solar cells and light emitting devices with minimal impacts, suppression of segregation remains the key factor.