Halide Segregated Crystallization of Mixed-Halide Perovskites Revealed by In Situ GIWAXS.

Mixed-halide perovskites of the composition MAPb(BrI), which seem to exhibit a random and uniform distribution of halide ions in the absence of light, segregate into bromide- and iodide-rich phases under illumination. This phenomenon of halide segregation has been widely investigated in the photovoltaics context since it is detrimental for the material properties and ultimately the device performance of these otherwise very attractive materials. A full understanding of the mechanisms and driving forces has remained elusive.