  The ideality factor determined by measuring the open circuit voltage (VOC) as
function of light intensity is often used as a means to identify the dominant
recombination mechanism in solar cells. However, applying this Suns-VOC
technique to perovskite cells is problematic since the VOC evolves with time in
a way which depends on the previously applied bias (Vpre), the light intensity,
and the device architecture/processing. Here we show that the dominant
recombination mechanism in two structurally similar CH3NH3PbI3 devices
containing either mesoporous Al2O3 or TiO2 layers can be identified from the
signature of the transient ideality factor following application of a forward
bias, Vpre, to the device in the dark. The transient ideality factor, is
measured by monitoring the temporal evolution of VOC at different light
intensities. The initial values of the transient ideality were consistent with
corresponding photoluminescence vs intensity as well as electroluminescence vs
current density measurements. Time-dependent simulations of the measurement on
modelled devices, which include the effects of mobile ionic charge, show that
Shockley Read Hall (SRH) recombination through deep traps at the charge
collection interfaces is dominant in both devices. Using transient photovoltage
measurements superimposed on the evolving VOC of bifacial devices we further
show that the charge collection interface extends throughout the mesoporous
TiO2 layer, consistent with a transient ideality signature corresponding to SRH
recombination in the bulk of the film. This information could not be inferred
from an ideality factor determined from only steady-state VOC values. The
method we have developed will be useful for identifying performance bottlenecks
in new variants of perovskite devices by comparison with the transient ideality
signatures we have predicted for a range of possible recombination schemes.
