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Halide diffusion in mixed-halide perovskites and heterojunctions

Viren Tyagi, Mike Pols, Geert Brocks, Shuxia Tao

TL;DR

This work addresses halide demixing in mixed-halide perovskites by quantifying defect diffusion with ns-scale MD enabled by an Allegro neural-network potential trained on DFT. The study finds that diffusion of halide vacancies and interstitials is 2–3× faster in CsPb(I_xBr_{1-x})_3 than in the pure phases, with two distinct diffusion channels corresponding to Br- and I-specific migration and activation energies in the range $E_a \\approx 0.13$–$0.22$ eV, yielding room-temperature diffusivities around $D_{300K} \\sim 10^{-7}$–$10^{-6}$ cm^2 s^{-1}. Interface structure governs diffusion across CsPb(I_xBr_{1-x})_3 heterojunctions: Br-rich interfaces significantly block vacancy diffusion, while I-rich interfaces are permeable to both vacancies and interstitials, facilitating intermixing as shown by nanodomain simulations and defect-occupancy analyses. These insights provide a mechanistic understanding of halide transport and phase-separation kinetics, offering guidance for design strategies to suppress demixing in perovskite devices and delivering quantitative diffusion parameters at technologically relevant temperatures.

Abstract

Migration of halide defects guides ion transport in metal halide perovskites and controls the kinetics of halide mixing and phase separation. We study the diffusion of halide vacancies and interstitials in \ce{CsPb(I_{x}Br_{1-x})_{3}} and \ce{CsPbI_{3}}/\ce{CsPbBr_{3}} heterojunctions by molecular dynamics simulations using neural network potentials trained on density functional theory calculations. We observe enhanced diffusion of both vacancies and interstitials in the mixed halide compounds compared to the single halide ones, as well as a difference in mobility between Br and I ions in the mixed compound. Diffusion across heterojunctions is governed by the interface structure, where a Br-rich interface blocks migration of vacancies in particular, but an I-rich interface is permeable.

Halide diffusion in mixed-halide perovskites and heterojunctions

TL;DR

This work addresses halide demixing in mixed-halide perovskites by quantifying defect diffusion with ns-scale MD enabled by an Allegro neural-network potential trained on DFT. The study finds that diffusion of halide vacancies and interstitials is 2–3× faster in CsPb(I_xBr_{1-x})_3 than in the pure phases, with two distinct diffusion channels corresponding to Br- and I-specific migration and activation energies in the range – eV, yielding room-temperature diffusivities around – cm^2 s^{-1}. Interface structure governs diffusion across CsPb(I_xBr_{1-x})_3 heterojunctions: Br-rich interfaces significantly block vacancy diffusion, while I-rich interfaces are permeable to both vacancies and interstitials, facilitating intermixing as shown by nanodomain simulations and defect-occupancy analyses. These insights provide a mechanistic understanding of halide transport and phase-separation kinetics, offering guidance for design strategies to suppress demixing in perovskite devices and delivering quantitative diffusion parameters at technologically relevant temperatures.

Abstract

Migration of halide defects guides ion transport in metal halide perovskites and controls the kinetics of halide mixing and phase separation. We study the diffusion of halide vacancies and interstitials in \ce{CsPb(I_{x}Br_{1-x})_{3}} and \ce{CsPbI_{3}}/\ce{CsPbBr_{3}} heterojunctions by molecular dynamics simulations using neural network potentials trained on density functional theory calculations. We observe enhanced diffusion of both vacancies and interstitials in the mixed halide compounds compared to the single halide ones, as well as a difference in mobility between Br and I ions in the mixed compound. Diffusion across heterojunctions is governed by the interface structure, where a Br-rich interface blocks migration of vacancies in particular, but an I-rich interface is permeable.
Paper Structure (6 sections, 2 equations, 5 figures, 3 tables)

This paper contains 6 sections, 2 equations, 5 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Methods
  3. Results
  4. Diffusion in CsPb(I_0.5Br_0.5)_3
  5. Diffusion across interfaces
  6. Summary and Conclusions

Figures (5)

  • Figure 1: Energies along CI-NEB migration paths for I-mediated interstitial (a) and vacancy (b), and for Br-mediated interstitial (c) and vacancy (d) in CsPb(I_0.5Br_0.5)_3 from DFT and NNP calculations. Zero is set at the energy minima.
  • Figure 2: Temperature-dependent diffusion coefficients of halide interstitials ($\mathrm{I^{-}}$) and vacancies ($\mathrm{V^{+}}$) in pure CsPbI3 (I), randomly mixed CsPb(I_0.5Br_0.5)_3 (X), and pure CsPbBr3 (Br). The filled square and open circle symbols represent halide interstitials and vacancies, respectively. The dashed lines represent fits to an Arrhenius expression, and the error bars represent the standard error in mean at each point.
  • Figure 3: a) Halide species decomposed temperature-dependent diffusion coefficients of halide interstitials and vacancies in CsPb(I_0.5Br_0.5)_3. Blue is for I, and red is for Br. The filled square symbols represent halide interstitials, and the open circle symbols represent halide vacancies. The dashed lines represent the fits to an Arrhenius expression, and the error bars represent the standard error in mean at each point. Schematic representation of the diffusion paths for (b) halide interstitial and (c) halide vacancy defects in CsPb(I_0.5Br_0.5)_3. The arrows represent the migration directions of the ions.
  • Figure 4: (a) Bulk interface and layers away from the interface in $16\times6\times6$ cubic supercell of CsPb(I_xBr_1-x)_3, with 2 Br-rich interfaces along the periodic boundaries. Defect occupancy at different layers ($\mathrm{N_{Frames}/N_{Total}}$) of (b) halide vacancy for I-rich interfaces, (c) halide vacancies for Br-rich interfaces, (d) halide interstitials for I-rich interfaces, and (e) halide interstitials for Br-rich interfaces.
  • Figure 5: Final frames from the MD simulations along all three axes for (a) halide interstitial and (b) vacancy in Br cubic domain, and (c) halide interstitial and (d) vacancy in I cubic domain. The non-domain halide species atoms are omitted for the sake of clarity.