Dynamic Ion Migration in 2D Halide Perovskites

Two-dimensional (2D) and quasi-2D layered perovskites, composed of alternating spacer cations and inorganic layers, have emerged as promising alternatives to conventional three-dimensional (3D) perovskites due to their suppressed halide ion mobility and improved ambient stability. Nevertheless, both halide anion and spacer cation migration can still occur in these reduced-dimensional perovskites, and ion migration remains a critical challenge for perovskite optoelectronic applications. Under photoirradiation and electrochemical bias, intrinsic iodine electrochemistry drives defect-mediated halide ion migration, further promoting halide segregation in mixed halide systems. Several effective strategies have been proposed to mitigate such dynamic ion migration, including controlling the inorganic layer number (n), crystallographic phase (Ruddlesden-Popper or Dion-Jacobson), crystal orientation, and, most importantly, the molecular structure of the intercalated spacer cations. However, a mechanistic understanding that links these structural parameters (spacer, A-, B-, and X-site composition) to lattice stability and ion migration remains limited. The ion migration processes discussed in this review provide insights into the thermodynamic and kinetic factors governing ion migration and offer design principles for improving the long-term operational stability of perovskite-based devices.