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. Yet, their potential is held back by a key weakness: phase instability. In compounds like MAPbI_3-xBr_x, exposure to light drives halide segregation, splitting the material into 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 MAPbI_1.5Br_1.5 powders via mechanochemical routes using different synthesis strategies, starting with precursor powders MAX and PbX₂ (X=I or Br) or parent perovskite powders MAPbI₃/MAPbBr₃ as educts for the ball milling procedure. 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. 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 optical and solid-state NMR spectroscopy. We found that the different synthesis approaches lead to systematic changes in crystallite size and defect density among otherwise compositionally similar powder samples.

To investigate phase segregation behavior, we performed in-situ XRD measurements under illumination. We find that the segregation kinetics are primarily governed by the shallow defect density, where a higher concentration of shallow traps, presumably halide vacancies, leads to an accelerated halide segregation process. In contrast to this, the extent of segregation at equilibrium under illumination, i.e. the thermodynamics, is dictated by crystallite size. In particular, we find less segregation in samples with smaller crystallites. A simple model based on the surface-to-bulk ratio describes the experimentally found correlation well, indicating the segregation process is driven primarily by carriers within the bulk. Furthermore, the model also reveals a critical size threshold of 38 nm, below which halide segregation under illumination is predicted to be suppressed entirely. 

These findings provide deeper experimental insight into the structural factors governing halide segregation in mixed halide perovskites. Understanding the interplay between crystallite size, defect density and segregation behaviour offers a potential pathway toward enhancing the long-term stability of perovskites for photovoltaic applications.