Mastering the behavior of mobile ionic defects in photo-active mixed ionic-electronic conductors exposed to light and/or voltage bias is one of the most exciting frontiers in solid state ionics. It represents both a conceptual challenge, as it requires combining knowledge on the defect chemical and optoelectronic properties of materials, as well as a promising perspective, as it may pave the way to novel devices in the field of energy and information technology.[1][2] Progress in this direction relies on the development of appropriate models that can provide an accessible, yet accurate, interpretation of experimental results in terms of material properties and device function. In practice, this means designing tools able to predict and analyze (i) the behavior (such as the electrical response) of devices based on mixed conducting materials, and (ii) trace such behavior back to the transport, storage and reaction of defects, described as function of the relevant thermodynamic parameters and bias condition. In this contribution, we address these aspects.

First, we propose an equivalent circuit model that can describe the electrical response of devices based on mixed conductors under bias. The circuit is a generalized version of previously reported models combining the established transmission line approach[3] with the concept of ionic-to-electronic current amplification.[4] We demonstrate the analytical accuracy of the model, by comparing it with drift-diffusion simulations of mixed conducting devices. We focus on hybrid halide perovskites as model systems. This material class presents excellent optoelectronic properties, and also shows significant ionic conduction even at room temperature.[5] Our analysis of calculated impedance spectra based on the proposed model allows the clarification of several anomalous experimental observations, including inductive behavior and multiple low frequency features reported for perovskite solar cells.[6]

Secondly, we focus on questions related with the quasi-equilibrium of photo-active mixed conductors under light, in terms of their defect behavior. By combining defect chemical models describing hybrid perovskites[7][8] with the relevant description of their optoelectronic properties, we are able to explore the effect of light on the steady-state defect concentrations. We present a systematic analysis of the key parameters in our kinetic model. Specifically, we consider the role of ionic disorder, electron-hole generation and recombination, redox and surface reactions in the system. The results highlight guidelines for the design of materials used in solar cells. More in general, they point towards the potential of controlling the defect chemistry of active materials (e.g. via control of component partial pressures) to optimize the performance of solar energy conversion devices.

These findings will aid the progress towards a complete description of the role that light plays in the already complex interplay between ionic and electronic defects in the solid state. The proposed models can be applied to the investigation of mixed conducting devices employed in applications ranging from energy conversion and storage to computing and sensing.