Negative Capacitance and Inverted Hysteresis: Matching Features in Perovskite Solar Cells

TL;DR

This study investigates the origins of negative capacitance and inverted hysteresis in perovskite solar cells using impedance spectroscopy, revealing ion/vacancy interactions that affect device performance.

Contribution

It introduces the Surface Polarization Model to explain the low-frequency responses and links ion dynamics to the observed hysteresis and capacitance features.

Findings

Ion/vacancy interactions have a time constant of 10^0 - 10^2 s.

Negative capacitance correlates with decreased recombination resistance.

Inverted hysteresis reduces fill factor and photovoltage.

Abstract

Negative capacitance at the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, where the origin is still under discussion. Here we use Impedance Spectroscopy to analyse these responses in methylammonium lead bromide cells treated with lithium cation at the electron selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the Surface Polarization Model, we obtain a time constant associated to the kinetics of the interaction of ions/vacancies with the surface, {\tau}kin, in the range of 10^0 - 10^2 s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding to a flattening of the cyclic voltammetry. As consequence of these results we find that that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values.