Abstract

So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative ‘capacitance’ dependent on the sign of its gradient. The ‘capacitance’ appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon related to the hysteresis in the current–voltage curve. The apparent negative capacitance remains elusive in the impedance analysis of metal halide perovskite solar cells. Here Ebadi et al. show that it can be attributed to slow transients in the injection current instead of classical capacitive effect, i.e. charge accumulation.

Highlights

  • -called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells

  • We show that negative capacitance in perovskite diodes is of the same nature as the slow positive response, which was originally ascribed to a ‘giant dielectric constant’[17], and later to accumulation of electronic and ionic charge at interfaces[18]

  • These devices, reaching efficiencies up to 19%, employ a solar-cell architecture consisting of Fluorine-doped tin oxide (FTO)/TiO2 (50 nm)/mesoporous TiO2 (150 nm)/perovskite (500 nm)/doped spiro-MeOTAD (150 nm)/Au (80 nm) with a nominal perovskite composition of Cs0.1FA0.9Pb(Br0.1I0.9)[3]

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Summary

Introduction

-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. We show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. Whereas resistive and capacitive elements can be directly related to transport, accumulation, and recombination of charges in common semiconductor devices, inductive features in the impedance are more puzzling. This is because they require the establishment of a considerable magnetic field, which is not present in the materials and geometries employed. This interpretation relates to the coupled electronic-ionic impedance introduced by Pockett et al.[20] and in that study attributed to ion-induced modified recombination rates

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