(256f) Molecular Modeling of Halide Diffusion in 2D Organic-Inorganic Hybrid Perovskites

Organic-inorganic hybrid perovskites are promising semiconductor materials for a wide range of optoelectronic applications due to their unique properties, including high carrier mobility, solution processibility, and strong light absorption. However, facile halide diffusion is a source of instability for 3D perovskites that leads to rapid degradation of device performance. In contrast, 2D perovskites exhibit relatively suppressed halide diffusion that is hypothetically further improvable via surface cation design. Nonetheless, the halide diffusion mechanism in 2D perovskites remains largely unknown and is challenging to experimentally characterize. To understand the microscopic origin of the diffusion and the role of organics cations, we have employed the molecular dynamics (MD) simulation to explore different halide migration paths at the atomic scale and the role of surface cation in suppressing diffusion. Steered MD and thermodynamic integration were used to characterize the free energy profiles of each migration path at surface/bulk layers for various systems. In comparison to 3D perovskites, we observe that 2D perovskites exhibit relatively high diffusion activation energies and a large the asymmetry across the accessible channels, which accounts for the suppression of halide diffusion. In addition, the change in the energy barrier when applying different organic cations and the number of inorganic layers offers guidelines for molecular engineering of perovskite materials toward long-term stability.