  Organic-inorganic halide perovskites recently have emerged as a promising
material for highly effective light-emitting diodes (LEDs) and solar cells
(SCs). Despite efficiencies of both perovskite SCs and LEDs are already among
the best, the development of a perovskite dual functional device that is
capable of working in these two regimes with high efficiencies is still
challenging. Here we demonstrate that the dual functional device based on mixed
halide perovskite CH3NH3PbBr2I can be switched from SC to LED with low
threshold voltage Vth < 2 V by exposing to Sun at open circuit Voc or at small
bias voltage of Vpol ~ 1 - 2 V. Such photo-poling creates in-situ p-i-n
junction via methylammonium (CH3NH3+, MA+) and I-/Br- ions migration to
interfaces, lowering charge injection barriers, and self-balancing injection
currents in perovskite LED. We show that before the photo-poling, the
electroluminescence (EL) is highly unstable in LED regime, whereas after the
photo-poling, stabilized EL exhibits unusual dynamics, increasing with time and
poling cycle number, while Vth and injection current decrease with cycling
runs. Additionally, photo-induced and current-induced halide segregation
accumulates with cycling, that is found beneficial for LED, increasing its
efficiency and brightness, but reversibly degrading photovoltaic (PV)
performance, which can be easily recovered.
