Abstract
Perovskites have proven to be a promising candidate for highly efficient solar cells, light-emitting diodes, and x-ray detectors, overcoming limitations of inorganic semiconductors. However, they are notoriously unstable. The main reason for this instability is the migration of mobile ions through the device during operation as they are mixed ionic-electronic conductors. Here, we show how measuring the capacitance in both the frequency and the time domain can be used to study ionic dynamics within perovskite-based devices, quantifying activation energy, diffusion coefficient, sign of charge, concentration, and the length of the ionic double layer in the vicinity of the interfaces. Measuring the transient of the capacitance furthermore allows for distinguishing between ionic and electronic effects.
Highlights
Measuring the temperature-dependent capacitance as a function of frequency or time is a well-established technique in experimental physics to quantify electronic defect states in semiconductors.[1,2,3,4] The most famous examples are deep-level transient spectroscopy (DLTS) and thermal admittance spectroscopy (TAS).[5,6] These techniques allow for quantifying activation energy, attemptto-escape frequency, sign, and concentration of electronic defect states.Due to the intriguing properties of perovskites such as high charge-carrier mobilities, long diffusion lengths, strong absorption, and low exciton binding energies, perovskites have been successfully used in many optoelectronic applications including lightemitting diodes, lasers, and x-ray detectors.[7,8,9,10] Both DLTS and TAS have been used to study perovskite-based devices.[11,12,13,14,15] To measure electronic defect states, these techniques rely on the depletion approximation, assuming that the depletion region is free of mobile carriers
Based on the example of a PEABr0.2Cs0.4MA0.6PbBr3 quasi-2D/3D perovskite layer typically used for light-emitting diodes,[24,25] we show how both measurements can be used to quantify the properties of mobile ions such as activation energy, diffusion coefficient, sign of scitation.org/journal/jcp charge, concentration, and the length of the ionic double layer
Capacitance techniques must be applied with caution to mixed ionic–electronic conductors because the measured capacitance features can be caused by both mobile ions and electronic defect states
Summary
Measuring the temperature-dependent capacitance as a function of frequency or time is a well-established technique in experimental physics to quantify electronic defect states in semiconductors.[1,2,3,4] The most famous examples are deep-level transient spectroscopy (DLTS) and thermal admittance spectroscopy (TAS).[5,6] These techniques allow for quantifying activation energy, attemptto-escape frequency, sign, and concentration of electronic defect states.Due to the intriguing properties of perovskites such as high charge-carrier mobilities, long diffusion lengths, strong absorption, and low exciton binding energies, perovskites have been successfully used in many optoelectronic applications including lightemitting diodes, lasers, and x-ray detectors.[7,8,9,10] Both DLTS and TAS have been used to study perovskite-based devices.[11,12,13,14,15] To measure electronic defect states, these techniques rely on the depletion approximation, assuming that the depletion region is free of mobile carriers. We review the difference in studying mobile ions by capacitance measurements in the frequency and in the time domain. To illustrate the difference between impedance spectroscopy and transient ion drift, we measure a perovskite-based device illustrated in Fig. 3(a) (details of the device are found in Sec. S2 in the supplementary material).
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