On the Origin of Hysteresis in Perovskite Solar Cells

The origin of hysteresis behavior is probed in perovskite solar cells (PSCs) with simultaneous measurements of cell open circuit voltage (V_oc) and photoluminescence intensity over time following illumination of the cell. It is shown, for the first time, that the transient changes in terminal voltage and luminescent intensity do not follow the relationship that would be predicted by the generalized Plank radiation law. A mechanism is proposed based on the presence of a resistive barrier to majority carrier flow at the interface between the perovskite film and the electron or hole transport layer, in combination with significant interface recombination. This results in a decoupling of the internal quasi-Fermi level separation and the externally measured voltage. A simple numerical model is used to provide in-principle validation for the proposed mechanism and it is confirmed that mobile ionic species are a likely candidate for creating the time-varying majority carrier bottleneck by its reduced conductivity. The findings show that the V_oc of PSCs may be lower than the limit imposed by the cell luminescence efficiency, even under steady-state conditions.