  Ionic migration has been proposed as a possible cause of photovoltaic
current-voltage hysteresis in hybrid perovskite solar cells. However, a major
objection to this hypothesis is that hysteresis can be reduced by changing the
interfacial contact materials, which are unlikely to significantly influence
the behaviour of ionic defects within the perovskite phase. Here we use
transient optoelectronic measurements, combined with device simulations, to
show that the primary effects of ionic migration can in fact be observed both
in devices with hysteresis, and with hysteresis free type contact materials.
The data indicate that electric-field screening, consistent with ionic
migration, is similar or identical in both high and low hysteresis CH3NH3PbI3
cells. Simulation of the transients shows that hysteresis requires the
combination of both mobile ionic charge and recombination near the contacts.
Passivating contact recombination results in higher photogenerated charge
concentrations at forward bias which screen the ionic charge, reducing
hysteresis.
