Halide Ion Exchange Mechanisms in 2D Ruddlesden-Popper Perovskites: Diffusion- vs Reaction-Limited

Halide ion exchange provides a distinctive route to modulate the bandgap and optoelectronic properties. However, the mechanistic understanding of halide ion migration and exchange seen in 2D perovskites, compared to 3D counterparts, remain elusive thus far. Although numerous spacer ligands are employed for constructing 2D/3D perovskites or quasi-2D perovskites, exact role and impact of intercalated spacer ligand structure (such as aromaticity, alkyl chain length, and intermolecular interactions) on the halide ion mobility is unknown. By tracking the absorption changes of physically paired 2D bromide and iodide perovskite films upon thermal stress, thermally driven bidirectional halide ion movement occurs, and corresponding halide ion diffusion coefficient across the 2D halide perovskite interfaces are determined. Depending on the halide ion diffusion coefficient (m² sec^-1) governed by type of spacer ligands (aliphatic vs. aromatic) and their glass transition temperature (T_g), halide ion exchange undergoes with different exchange mechanisms along with formation of different kinetic intermediate of heterogeneous vs. homogeneous halide alloying.