  We develop techniques to study the temperature dependent localization,
thermalization, and the effects of phonon scattering on the excitons in halide
perovskites from the analysis of the emission spectra. The excitonic Urbach
edge, when inferred from the low energy tails of the temperature dependent
luminescence spectra, is shown to be sensitive to the electron distribution and
thermalization. A method to observe the Urbach focus is devised for halide
perovskites where the temperature dependence of the excitonic gap is anomalous.
The value of the zero-point bandgap renormalization is inferred to be about 33
meV. This small value of the bandgap renormalization rules out the formation of
small polarons and points to weak electron-phonon coupling. The experiments are
performed on the nanosheets of the archetypal halide perovskite, CsPbBr$_3$.

Materials Science