In Situ Postsynthesis of 2D/3D Bilayer Structured All-Inorganic Perovskites: Elucidating the Cation Exchange Mechanism

Constructing 2D/3D bilayer structured all-inorganic perovskites through cation exchange is critically challenging. So far, only a few reports have claimed 2D/3D heterostructure formation via in situ surface reconstruction or cation interdiffusion. Yet, the underlying mechanism remains elusive and a fundamental understanding is still lacking from both thermodynamic and mechanistic perspectives: why and how organic cations displace Cs⁺ ions. This work presents a detailed mechanistic study encompassing molecular design, experimental validation, and theoretical verification to elucidate the cation exchange mechanism behind this surface reconstruction process. We have specifically developed a novel ammonium iodide salt, namely, DMA-BzAI, by incorporating a strong electron-donating dimethylamino moiety on the para position of the benzene ring in the most commonly used benzylammonium iodide (BzAI). This design aims to decrease the polarization force of the spacer cation toward octahedral inorganic slabs, providing stronger driving forces for ionic substitution. In situ X-ray scattering analysis confirms the dynamic evolution of n = 1 2D perovskites on CsPbI₂Br perovskites by treating with DMA-BzAI, contrasting sharply to the case of BzAI. A comprehensive theoretical investigation, including Bader charge analysis, formation energy, and nudged elastic band calculations, further demonstrates that both thermodynamic favorability and low activation barriers allow DMA-BzA⁺ cations to go through cation exchange reactions to substitute the strongly bound Cs⁺ ions in the inorganic perovskite lattice, leading to in situ formation of 2D/3D bilayer structure, in alignment with experimental observations. These mechanistic results provide fundamental insights into the cation exchange mechanism behind 2D/3D heterojunction formation in inorganic perovskites, offering rational ligand design principles for future research.