Publication date: 16th July 2025
Halide migration limits the stability of hybrid halide perovskites for optoelectronic applications. A common approach is the use of additives, such as ionic liquids, to suppress halide diffusion in the material. However, quantitative analyses of halide diffusion and the influence of additives appears complex.
Therefore, we develop an approach to quantitatively evaluate the halide exchange of I⁻ and Br⁻ to form MAPbIxBr3-x from neat perovskite powders from in-situ XRD data of our previous work.[1] We base our evaluation on an effective interdiffusion model,[2,3] which uses the halide concentration profiles to extract average effective interdiffusion coefficients and halide penetration depths. We expand the model to obtain time-dependent diffusion coefficients which are decreasing due to the decreasing concentration gradient during the halide exchange process. As such, we can quantitatively extract diffusion coefficients for both ions during the complete halide exchange process.
We further compare the findings for halide interdiffusion between perovskite powders with and without the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4). Without the additive Br⁻ diffuses one magnitude faster into the MAPbI3 particles than I⁻ into the MAPbBr3 particles. We attribute this to higher barriers for Br⁻ vacancy formation, which restricts the I⁻ penetration into the MAPbBr3 phase, while Br⁻ can diffuse easily into and through MAPbI3. With the additive the diffusion coefficients are accelerated and equalized for both halides. A preferred interaction between the BMIM+ cation and Br⁻, may ease Br⁻ vacancy formation. Additionally, the high mobility of the additive may allow for shuttling of halides between the particles and thus results in the overall increase of the diffusivity.
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