  Hysteresis in the current-voltage characteristics of hybrid organic-inorganic
perovskite-based solar cells is one of the fundamental aspects of these cells
that we do not understand well. One possible cause, suggested for the
hysteresis, is polarization of the perovskite layer under applied voltage and
illumination bias, due to ion migration within the perovskite. To study this
problem systemically current-voltage characteristics of both regular (light
incident through the electron conducting contact) and so-called inverted (light
incident through the hole conducting contact) perovskite cells were studied at
different temperatures and scan rates. We explain our results by assuming that
the effects of scan rate and temperature on hysteresis are strongly correlated
to ion migration within the device, with the rate-determining step being ion
migration at/across the interfaces of the perovskite layer with the contact
materials. By correlating between the scan rate with the measurement
temperature we show that the inverted and regular cells operate in different
hysteresis regimes, with different activation energies of 0.28+-0.04 eV and
0.59+-0.09 eV, respectively. We suggest that the differences, observed between
the two architectures are due to different rates of ion migration close to the
interfaces, and conclude that the diffusion coefficient of migrating ions in
the inverted cells is 3 orders of magnitude higher than in the regular cells,
leading to different accumulation rates of ions near the interfaces. Analysis
of VOC as a function of temperature shows that the main recombination mechanism
is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar
to what is the case for other thin film inorganic solar cells.
