  The anomalous hysteresis is reported in photo current density-voltage (J-V)
curves of perovskite solar cells. The origins responsible for hysteresis are
primarily attributed to ferroelectricity, trapping/de-trapping, and ions
migration. Meanwhile, a switchable photovoltaic effect is disclosed in lateral
structure perovskite solar cells, where a p-i-n structure is formed after
poling the device with a reverse bias. Considering the normal sandwiched
structure of perovskite solar cells, i.e. the intrinsic perovskite layer is
situated in an electric field built by p-type (Spiro-OMeTAD) and n-type (TiO2)
layers, the poling process also works in such devices. The migration of ions
will induce a new electric field with opposite direction as inherent built-in
electric field. As a consequence, less collection of photogenerated carriers is
predicted due to the built-in electric field is partially screened. Here, the
ions induced electric field is experimentally demonstrated by exploring the
external quantum efficiency (EQE) spectra under various forward voltage biases.
The anomalous hysteresis observed on J-V curves is explained based on the
insight of ions migrations under bias voltage. The external applied voltage
presents significant influence on the stability of solar cells. Meanwhile, a
novel phenomenon of negative capacitance is initially detected at low
frequency, which is ascribed to the position reversal of negatively and
positively charged ions under forward bias.
