  Two-dimensional (2D) perovskites with general formula $APbX_4$ are attracting
increasing interest as solution processable, white-light emissive materials.
Recent studies have shown that their broadband emission is related to the
formation of intra-gap color centers; however, the nature and dynamics of the
emissive species have remained elusive. Here we show that the broadband
photoluminescence of the 2D perovskites $(EDBE)PbCl_4$ and $(EDBE)PbBr_4$ stems
from the localization of small polarons within the lattice distortion field.
Using a combination of spectroscopic techniques and first-principles
calculations, we infer the formation of ${Pb_2}^{3+}$, $Pb^{3+}$, and ${X_2}^-$
(where X=Cl or Br) species confined within the inorganic perovskite framework.
Due to strong Coulombic interactions, these species retain their original
excitonic character and form self-trapped polaron-excitons acting as radiative
color centers. These findings are expected to be applicable to a broad class of
white-light emitting perovskites with large polaron relaxation energy.
