Identifying Dominant Recombination Mechanisms in Perovskite Solar Cells by Measuring the Transient Ideality Factor

Abstract

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 evolution of VOC as a function of time at different light intensities. The initial values of ideality found using this technique 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 superimposed transient photovoltage measurements of the evolving VOC on 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 the steady-state VOC values alone. We anticipate that the method we have developed will be useful for identifying performance bottlenecks in new variants of perovskite solar cells by comparison with the transient ideality signatures we have predicted for a range of possible recombination schemes.

Interpreting Ideality Factors for Planar Perovskite Solar Cells: Ectypal Diode Theory for Steady-State Operation

“…Combined experimental and theoretical studies, which seek to understand the performance limitations of state-ofthe-art PSCs, such as Refs. [3,4,16], conclude that current techniques for analysis do not provide reliable conclusions. Almora et al [3] and Calado et al [4] attribute the difficulty in determining a unique value for the ideality factor to the interference of undesirable hysteresis often observed in J -V measurements [26][27][28][29].…”

“…It is therefore crucial to be able to correctly evaluate and interpret the value of the ideality factor. However, in the case of metal halide perovskite solar cells (PSCs), this approach has led to some unexpected results, which cast doubt on its validity in assessing this promising photovoltaic technology [2][3][4][5][6]. Perovskite solar cells have recorded a rapid rise in power conversion efficiency over a short but intense period of research effort [7,8].…”

“…Using the hypothesis that ion migration is the origin of J -V hysteresis, Calado et al [4] explain that it is the evolving profile of the electric potential across a PSC that hampers the measurement of a cell's V OC , and thus the ideality factor, by conventional means. It is noted that the interpretation of a steady-state ideality factor should take into account the homogeneity of charge-carrier densities across the perovskite layer.…”

“…In PSCs, a high density of mobile ionic charge is predicted to exist within the perovskite material [36]. As a result, the distribution of the electric field is controlled by the position of the mobile ions and evolves on a slower timescale than that of the charge-carrier dynamics [4,33,37]. At steady state, ionic accumulation or depletion at the edges of the perovskite absorber layer (PAL) screens the electric field from the bulk of the PAL and causes potential drops to form across the interfaces between the PAL and each of the transport layers (TLs).…”

Suppressing Vacancy Defects and Grain Boundaries via Ostwald Ripening for High‐Performance and Stable Perovskite Solar Cells

“…The n value for pristine PVSCs is calculated as 1.46, implying a trap‐dominated recombination. By contrast, n value for GdF 3 –aminobutanol‐based device is 1.15, indicating that the trap‐assisted non‐radiative recombination is suppressed …”