Evidence for ion migration in hybrid perovskite solar cells with minimal   hysteresis
  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. 
Philip Calado, Andrew M. Telford, Daniel Bryant, Xiaoe Li, Jenny   Nelson, Brian C. O'Regan, Piers R. F. Barnes
cond-mat.mtrl-sci