  Halide perovskites are semiconductors that exhibit sharp optical absorption
edges and small Urbach energies allowing for efficient collection of sunlight
in thin-film photovoltaic devices. However, halide perovskites also exhibit
large nuclear anharmonic effects and disorder, which is unusual for efficient
optoelectronic materials and difficult to rationalize in view of the small
Urbach energies that indicate a low amount of disorder. To address this
important issue, the disorder potential induced for electronic states by the
nuclear dynamics in various paradigmatic halide perovskites is studied with
molecular dynamics and density functional theory. We find that the disorder
potential is dynamically shortened due to the nuclear motions in the
perovskite, such that it is short-range correlated, which is shown to lead to
favorable distributions of band edge energies. This dynamic mechanism allows
for sharp optical absorption edges and small Urbach energies, which are highly
desired properties of any solar absorber material.
