  Two-dimensional (2D) organic-inorganic hybrid perovskites have been
intensively explored for recent years, due to their tunable band gaps and
exciton binding energies, and increased stability with respect to
three-dimensional (3D) hybrid perovskites. There were fascinating experimental
observations suggesting the existence of localized edge states in 2D hybrid
perovskites which facilitate extremely efficient electron-hole dissociation and
long carrier lifetimes. The observations and explanations of the edge states
are not quite converging implying that there can be multiple origins for the
edge state formation. Using first principles calculations, we demonstrate that
layer edge states are stabilized by internal electric fields created by
polarized molecular alignment of organic cations in 2D hybrid perovskites when
they are two layers or thicker. Our study gives a simple physical explanation
of the edge state formation, and it will pave the way for designing and
manipulating layer edge states for optoelectronic applications.
