  The previously developed bistable amphoteric native defect (BAND) model is
used for a comprehensive explanation of the unique photophysical properties and
for understanding the remarkable performance of perovskites as photovoltaic
materials. It is shown that the amphoteric defects in donor (acceptor)
configuration capture a fraction of photoexcited electrons (holes) dividing
them into two groups: higher energy bright and lower energy dark electrons
(holes). The spatial separation of the dark electrons and the dark holes and
the k-space separation of the bright and the dark charge carriers reduce
electron hole recombination rates, emulating the properties of an ideal
photovoltaic material with a balanced, spatially separated transport of
electrons and holes. The BAND model also offers a straightforward explanation
for the exceptional insensitivity of the photovoltaic performance of
polycrystalline perovskite films to structural and optical inhomogeneities. The
blue-shifted radiative recombination of bright electrons and holes results in a
large anti-Stokes effect that provides a quantitative explanation for the
spectral dependence of the laser cooling effect measured in perovskite
platelets.
