Mobile ionic defects are key protagonists in the function of solid state energy conversion and storage devices. During charging and discharging of solid-state batteries, ions are shuttled between two electrodes via a solid state electrolyte (SSE). In mixed conducting solar cells, such as the ones based on metal-halide perovskites, mobile ions do not directly participate in the photo-generation of electronic charges and their extraction, yet they represent a decisive factor for the efficiency of some of the relevant processes. As a result, much of the experimental analysis and optimization of these devices relies on knowledge of the ionic properties of their components. The investigation of ionic conductivities in ionic and mixed conductors via electrochemical methods is well established. [1] On the other hands, simple methods that allow determination of the density and mobility of mobile ionic defects are lacking. While the quantification of ionic conductivity is often a good starting point for material screening and basic device testing, the density (N_ion) and mobility (u_ion) of mobile ions are fundamental input parameters required to build more accurate models describing device behaviour, and to inform future steps in research.

Here, we investigate and compare frequency- and time-domain electrochemical methods for the estimation of ion density and mobility. [2] We use the electrical device simulation software Setfos [3] to generate synthetic impedance and transient current data, based on known input parameters. The transient current technique involves the analysis of the current signal produced by a voltage step from a condition of steady-state out-of-equilibrium to the equilibrium state. We test the validity of analytical formulas used to extract ion density and mobility from such transient data. First, we review the charge extraction method to evaluate mobile ion density and the relevant parameter validity window for the case of both SSEs and mixed conducting solar cells. We stress the importance of understanding the space charge situation under the applied voltage as well as at the equilibrium condition, in order to extract appropriate estimates of N_ion. This applied to both SSEs and to the mixed conducting active layer of solar cells.

Based on these results, we explore simplifying assumptions that may allow independent extraction of mobility values from current transient data. We study under which circuimstances a “time-of-flight” approach, while not justified based on typical experimental conditions, can be used to obtain meaningful results. Finally, we discuss the application of these models to the analysis of measured data for the SSE LiPON and halide perovskite based devices.

This study demonstrates the potential of drift-diffusion simulations for the diagnostics of method development in the field of energy devices involving ion conduction.