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The ideality factor determined by measuring the open circuit voltage (VOC) asfunction of light intensity is often used as a means to identify the dominantrecombination mechanism in solar cells. However, applying this Suns-VOCtechnique to perovskite cells is problematic since the VOC evolves with time ina way which depends on the previously applied bias (Vpre), the light intensity,and the device architecture/processing. Here we show that the dominantrecombination mechanism in two structurally similar CH3NH3PbI3 devicescontaining either mesoporous Al2O3 or TiO2 layers can be identified from thesignature of the transient ideality factor following application of a forwardbias, Vpre, to the device in the dark. The transient ideality factor, ismeasured by monitoring the temporal evolution of VOC at different lightintensities. The initial values of the transient ideality were consistent withcorresponding photoluminescence vs intensity as well as electroluminescence vscurrent density measurements. Time-dependent simulations of the measurement onmodelled devices, which include the effects of mobile ionic charge, show thatShockley Read Hall (SRH) recombination through deep traps at the chargecollection interfaces is dominant in both devices. Using transient photovoltagemeasurements superimposed on the evolving VOC of bifacial devices we furthershow that the charge collection interface extends throughout the mesoporousTiO2 layer, consistent with a transient ideality signature corresponding to SRHrecombination in the bulk of the film. This information could not be inferredfrom an ideality factor determined from only steady-state VOC values. Themethod we have developed will be useful for identifying performance bottlenecksin new variants of perovskite devices by comparison with the transient idealitysignatures we have predicted for a range of possible recombination schemes.

Identifying dominant recombination mechanisms in perovskite solar cells by measuring the transient ideality factor