  The past few years have witnessed unprecedented rapid improvement of the
performance of a new class of photovoltaics based on halide perovskites. This
progress has been achieved even though there is no generally accepted mechanism
of the operation of these solar cells. Here we present a model based on
bistable amphoteric native defects that accounts for all key characteristics of
these photovoltaics and explains many idiosyncratic properties of halide
perovskites. We show that a transformation between donor-like and acceptor-like
configurations leads to a resonant interaction between amphoteric defects and
free charge carriers. This interaction, combined with the charge transfer from
the perovskite to the electron and hole transporting layers results in the
formation of a dynamic n-i-p junction whose photovoltaic parameters are
determined by the perovskite absorber. The model provides a unified explanation
for the outstanding properties of the perovskite photovoltaics, including
hysteresis of J-V characteristics and ultraviolet light-induced degradation.
