  The family of organic-inorganic tri-halide perovskites including MA
(MethylAmmonium)PbI$_{3}$, MAPbI$_{3-x}$Cl$_{x}$, FA (FormAmidinium)PbI$_{3}$
and FAPbBr$_{3}$ are having a tremendous impact on the field of photovoltaic
cells due to their ease of deposition and efficiencies, but device performance
can be significanly affected by inhomogeneities. Here we report a study of
temperature dependent micro-photoluminescence which shows a strong spatial
inhomogeneity related to the presence of microcrystalline grains, which can be
both light and dark. In all of the tri-iodide based materials there is evidence
that the tetragonal to orthorhombic phase transition observed around 160K does
not occur uniformly across the sample with domain formation related to the
underlying microcrystallite grains, some of which remain in the high
temperature, tetragonal, phase even at very low temperatures. At low
temperature the tetragonal domains can be significantly influenced by local
defects in the layers. In FAPbBr$_{3}$ a more macroscopic domain structure is
observed with large numbers of grains forming phase correlated regions.
