Capacitive and inductive effects in perovskite solar cells: the different roles of ionic current and ionic charge accumulation

TL;DR

This paper introduces an equivalent circuit model for perovskite solar cells that clarifies the roles of ionic current and charge accumulation in hysteresis effects, linking capacitive and inductive phenomena to recombination current modulation and aiding degradation understanding.

Contribution

It provides a unified framework connecting different models of ionic effects in PSCs through an equivalent circuit and experimental validation via electrochemical impedance spectroscopy.

Findings

Recombination currents cause capacitive and inductive effects in PSCs.

Electrochemical impedance spectroscopy confirms the model predictions.

The framework aids in understanding and mitigating degradation mechanisms.

Abstract

Dynamic hysteresis effects have been long known to occur in the J-V characteristics of perovskite solar cells (PSCs), with the ionic migration being identified as the primary factor. The hysteretic effects impacted early studies by the uncertainty in the evaluation of the power conversion efficiency, while currently, potential links to degradation mechanisms are in the focus. Therefore, understanding ion migration is a central goal, typically addressed by performing a combined large- and small signal analysis. The reported large capacitive and inductive effects created controversies with respect to the underlying mechanisms, yielding essentially two classes of models, one based on large accumulation capacitances and the other based on ionic modulation of the collected current. We introduce here an equivalent circuit model and interpret these phenomena in terms of recombination current modulation, identifying the distinct contributions from ion current and ionic charge accumulations. These contributions to the recombination current are associated with capacitive and inductive effects, respectively, and we corroborate the numerical simulations with electrochemical impedance spectroscopy (EIS) measurements. These show the role of the recombination currents of photogenerated carriers in producing both capacitive and inductive effects as the illumination is varied. Moreover, we provide a bridging point between the two classes of models and suggest a framework of investigation of defect states based on the observed inductive behavior, which would further aid the mitigation of the degradation effects.