Impedance spectroscopy for perovskite solar cells: Characterisation, analysis, and diagnosis

Impedance spectroscopy (IS) has great potential to become a standard technique for the characterisation, analysis, and diagnosis of perovskite solar cells (PSC). However, the interpretation of IS data from PSC is still challenging due to the large number of dynamic processes which are not yet fully understood. Here we provide a general framework for IS analysis on PSC. We begin by reviewing the timescales reported for electrical and electrochemical dynamics, many of which are too fast or too slow to monitor with IS, and yet they have a significant impact on the impedance spectrum of PSC. To account for this, we review guidelines for obtaining high quality impedance spectra, including suitable measurement settings as well as the relevant device physics at different solar cell operating points. Based on this, we propose a universal equivalent circuit model (ECM) that exploits the fact that impedance spectra from perovskite solar cells ubiquitously demonstrate high and low frequency signatures that are separated by several orders of magnitude. We show that the high frequency signatures are consistent with fast electronic processes, while the low frequency signatures are consistent with electrochemical processes such as ion drift and diffusion and electrochemical reactions. This allows us to propose a simple, robust, and adaptable ECM that can be used to parameterise relevant material parameters as well as monitor loss mechanisms for all PSC materials and architectures.