(135g) Anionic Diffusion in Two-Dimensional Halide Perovskite Heterostructures

The soft crystal lattice of hybrid halide perovskites facilitates facile anionic diffusion which impacts material stability, optoelectronic properties and solid-state device performance. Two-dimensional (2D) halide perovskites with bulky organic barriers have been used for improving the extrinsic stability as well as suppressing intrinsic anionic diffusion. With this strategy, devices with enhanced stability and reduced hysteresis have been achieved. Nevertheless, a fundamental understanding of the role of organic cations in inhibiting anionic diffusion across the perovskite-ligand interface is missing. Here, we demonstrate a quantitative investigation of the anionic inter-diffusion in atomically sharp and flat 2D heterojunctions between two arbitrarily determined phase-pure halide perovskite single crystals. Stark differences are observed in anion diffusion across 2D halide perovskite lateral and vertical heterostructures. Halide inter-diffusion in lateral heterostructures is found to be similar to the classical Fickian diffusion featuring continuous concentration profile evolution. However, vertical heterostructures show a “quantized” layer-by-layer diffusion behavior governed by a local free energy minimum and ion-blocking effects of the organic cations. Surprisingly, halide inter-diffusion coefficients for 2D perovskites capped with short aliphatic organic cations (e.g. butylammonium) are only 1~2 order of magnitudes smaller than that of 3D perovskites, suggesting the ineffectiveness of these cations in blocking the anionic migration. Furthermore, we found that bulkier and more rigid π-conjugated organic cations inhibit halide inter-diffusion much more effectively (D < 10^-20 m²/s) compared to aliphatic cations. These results offer significant insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform for heterostructure assembly and device integration.