Light Induced Halide Segregation in Mixed-Halide Perovskites

P. Machovec¹, L. Horák¹, M. Dopita¹, V. Holý^1,2

¹Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic,

²Institute of Condensed Matter Physics, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic,

petr.machovec@matfyz.cuni.cz

Mixed-halide perovskites (MHPs) exhibit tunable band gaps, making them attractive for tandem photovoltaic applications. However, under illumination, halide ions migrate and segregate into iodine- and bromine-rich regions, reducing device efficiency. Here, we present a quantitative X-ray diffraction (XRD) approach for resolving the spatial distribution of halide ions during and after illumination. We present a model linking local composition fluctuations to strain fields, atom displacements, and diffuse scattering, enabling fitting of measured diffraction profiles from polycrystalline FA₀.₈₃Cs₀.₁₇Pb(I₀.₆Br₀.₄)₃ thin films and FA₀.₈₃Cs₀.₁₇Pb(I₀.₈₅Br₀.₁₅)₃ single crystals.

Illumination experiments were conducted using a solar simulator at 1 Sun equivalent, with diffraction patterns measured before and after 10 min and 30 min light exposures, followed by relaxation in darkness for up to two days. The concentration of Br within the sample was modelled by a random function with a correlation function  and the experimental data were fitted using model of x-ray scattering, with parameters including the root mean square (rms) Br concentration deviation σ, correlation length , grain radius R, and asymmetry factor α.

In pristine polycrystalline samples, diffraction peaks were symmetric, consistent with a cubic perovskite lattice with mean grain radius of 50 nm. Illumination induced asymmetric broadening toward higher diffraction angles, increasing with scattering angle, and accompanied by slight peak shifts to lower 2θ. The fits revealed a significant rise in σ during illumination, indicating enhanced fluctuations in local composition, followed by slow partial relaxation in darkness within tens of hours. The asymmetry factor α remained consistently > 5, which is the limit of sensitivity of our model to this parameter, indicating the formation of highly bromide-rich regions embedded in a slightly iodine-rich matrix—an observation not previously observed by optical probes such as photoluminescence, which reported I-rich domains. The correlation length was found to be ≥ 15 nm and unaffected by illumination cycles.

For the single crystal samples reciprocal space maps were measured before and after 30 minutes of light soaking. We attempted fitting the data with the same correlation function as the polycrystalline samples, but the shape of the diffraction maxima can’t be properly fitted. A distribution of the Br concentration consisting of Br-rich spheres in slightly I-rich volume was used to achieve good fit.

The results suggest that illumination drives preferential outward migration of I⁻ ions from nucleation sites such as grain boundaries or defects, leading to the observed microstructure. The method provides quantitative, bulk-sensitive insight into light-induced halide segregation, complementing surface-sensitive optical techniques. It also highlights the incomplete reversibility of segregation. This quantitative diffraction-based approach offers a new pathway to investigate ionic migration and microstructural evolution in perovskite optoelectronic materials.