Effects of Ligand Chemistry on Ion Transport in Two-Dimensional Hybrid Organic-Inorganic Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites are potentially promising materials as passivation layers that can enhance the efficiency and stability of perovskite photovoltaics. The ability to suppress ion transport has been proposed as a stabilization mechanism, yet effective characterization of relevant modes of halide diffusion in 2D perovskites is nascent. In light of this knowledge gap, we combine molecular dynamics simulations with enhanced sampling and experimental validation to systematically characterize how ligand chemistry in seven (R-NH3)2PbI4 systems impacts halide diffusion, particularly in the out-of-plane direction. We find that increasing stiffness and length of ligands generally inhibits ion transport, while increasing ligand polarization generally enhances it. Structural and energetic analyses of the migration pathways provide quantitative explanations for these trends, which reflect aspects of the disorder of the organic layer. Overall, this mechanistic analysis greatly enhances the current understanding of halide migration in 2D hybrid organic-inorganic perovskites and yields insights that can inform the design of future passivation materials.