Abstract
Frequency resolved methods are widely used to determine device properties of perovskite solar cells. However, obtaining the electronic parameters for diffusion and recombination by impedance spectroscopy has been so far elusive, since the measured spectra do not present the diffusion of electrons. Here we show that intensity modulated photocurrent spectroscopy (IMPS) displays a high frequency spiraling feature determined by the diffusion-recombination constants, under conditions of generation of carriers far from the collecting contact. We present models and experiments in two different configurations: the standard sandwich-contacts solar cell device and the quasi-interdigitated back-contact (QIBC) device for lateral long-range diffusion. The results of the measurements produce the hole diffusion coefficient of Dp = 0.029 cm2/s and lifetime of τp = 16 μs for one cell and Dp = 0.76 cm2/s and τp = 1.6 μs for the other. The analysis in the frequency domain is effective to separate the carrier diffusion (at high frequency) from the ionic contact phenomena at a low frequency. This result opens the way for a systematic determination of transport and recombination features in a variety of operando conditions.
Highlights
Frequency resolved methods are widely used to determine device properties of perovskite solar cells
There have been presented a large number of evaluations of the diffusion length Ln = (Dnτn)1/2 measured by time transient methods in the archetype perovskite solar cells (PSC) with a methylammonium (MA) cation, namely, MAPbI3 and MAPbBr3.1,2 There have been abundant determinations of carrier mobilities by a range of techniques: space-charge limited-current (SCLC), Hall effect, THz frequency measurements, etc
These methods yield us the important advantage that they can be applied in full efficient devices avoiding effects of ionic polarization that plague other techniques, as it has been well described recently,[5] provided that the diffusion effect is observed at high frequency, far from the low frequency ionic polarization
Summary
The physical parameters for absorbance and extraction affecting the form of the spectra are the light absorption length, α−1, and the diffusion length, Ln, respectively It is noted that looping spectra producing a negative Q′ at high frequency (NHF) appear only when the absorption length is much shorter than the cell thickness. Another required condition for this feature is the diffusion length being longer than the absorption.
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