Illumination Intensity Dependence of the Recombination Mechanism in Mixed Perovskite Solar Cells

Recombination mechanisms in solar cells are frequently assessed through the determination of ideality factors. In this work we report an abrupt change of the value of the "apparent" ideality factor (n(AP)) in high-efficiency FA(0.71)MA(0.29)PbI(2.9)Br(0.1) based mesoscopic perovskite solar cells as a function of light intensity. This change is manifested as a transition from a regime characterized by n(AP)similar to 1.8-2.5 at low light intensities (<10 mWcm(-2)) to one characterized by n(AP)similar to 1. This transition is equally observed in the recombination resistance extracted from open-circuit impedance measurements. We use drift-diffusion simulations with explicit consideration of ion migration to determine the origin of this transition. We find that a change ofrecombination mechanism concurrent with a modification of the concentration of ionic vacancies is the most likely explanation of the observed behaviour. In the drift-diffusion simulations we show that the apparent ideality factor is in fact affected by the ion vacancy concentration so it is not the optimal parameter to assess the dominant recombination mechanism. We argue that a procedure based on a recently derived "electronic" ideality factor obtained from the high frequency feature of the impedance spectrum is better suited to determine the recombination route that dictates the photovoltage.