  Mixed halide perovskites undergo phase segregation, manifested as spectral
red-shifting of photoluminescence spectra under illumination. In the
iodine-bromide mixed perovskites, the origin of the low-energy luminescence is
related to iodine-enriched domains formation. Such domains create favorable
bands for the induced carrier funneling into them. Despite the phase
segregation process is crucial for mixed halide perovskite-based
optoelectronics, numerous gaps exist within the understanding of this
phenomenon. One such gap pertains to the emergence of temporary and
intermediate photoluminescence peaks during the initial stages of phase
segregation. However, these peaks appear only within the first few seconds of
illumination. Nevertheless, the decreasing temperature may prolong these
initial stages. In this work, we carry out a detailed study of the temperature
dependence of anion segregation in MAPbBr_2I and MAPbBr2.5I0.5 halide
perovskites, to obtain a deeper comprehension of segregation processes,
particularly during their initial stages. The temporal evolution of
low-temperature photoluminescence reveals the undergoing of the intermediate
stage during the segregation process and temperature-related phase transition
from orthorhombic to tetragonal phase. To complement the phase segregation
study, the temperature dependence of time-resolved photoluminescence
spectroscopy is provided, allowing us to estimate the change in the
photoluminescence lifetimes for the initial and segregated peaks with
temperature.
