Publication date: 16th July 2025
Mixed halide perovskites have emerged as leading contenders for multi-junction solar cells, thanks to their tunable band gaps—controlled by halide composition—and excellent optoelectronic properties. However, their potential for application suffers from phase instability. In compounds like MAPbI3-xBrx, exposure to light drives halide segregation leading to the formation of iodine- and bromine-rich domains that ultimately compromise device performance. Despite extensive research, the mechanisms behind this light-induced instability remain poorly understood, as they are shaped by a complex interplay of a multitude of different factors.
In this study, we synthesized a suite of MAPbI1.5Br1.5 powders via mechanochemical routes[1] using different synthesis strategies. Additionally, we prepared mixed halide perovskite powders incorporating varying amounts of the ionic liquid BMIMBF4, which has been shown to act as a passivating agent[2]. We characterized the structural and optical properties of the samples using a range of complementary methods, such as SEM, X-ray diffraction (XRD), time-resolved photoluminescence (TRPL) as well as solid-state NMR spectroscopy. The results showed that the different MAPbI1.5Br1.5 perovskite powders exhibit varying crystallite sizes and defect densities but otherwise similar properties.
To investigate phase segregation behavior, we performed in-situ XRD measurements under illumination. We find that the segregation kinetics are primarily governed by the defect density, whereas the extent of segregation, reflecting the thermodynamic constraints of the process, is dictated by crystallite size. In particular, we observed less segregation in powders with small crystallites, and we predict that segregation even completely ceases below a threshold of around 10-15 nm. The recovery behavior of the segregated samples in the dark, was governed by different factors than the segregation process. Specifically, the presence of BMIMBF4 decreases the segregation rate, yet enhances (re-)mixing rates.
Together, these findings provide more profound experimental insight on the kinetics and thermodynamics of halide segregation and its correlation to material structure, which might offer a pathway toward more stable perovskite materials for photovoltaic applications in the future.
We thank the DFG (Deutsche Forschungsgemeinschaft) for funding within project numbers 506642499 (SPP 2196/2).
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