Role of Mobile Ions in Mixed Conducting Devices Close to Equilibrium and Under Light or Voltage Bias

Improving physical and chemical understanding of mixed conductors is necessary to design advanced solutions for energy conversion and storage, as well as information technology.¹ In some of these fields, such as halide perovskite based photovoltaics, it is essential to establish models describing ions and electrons close to equilibrium as well as under non-equilibrium conditions. In particular, the behavior of ions and their interplay with electronic charges under applied light and/or voltage is an increasingly important yet largely unexplored question. In practice, such models should be able to (i) reproduce the electrical response of devices based on mixed conducting materials, and (ii) trace such behavior back to the transport, storage and reaction of electronic and ionic charge carriers.
In this contribution, questions (i) and (ii) above are addressed, using halide perovskites as a convenient model system. First, various bulk and interfacial properties of these materials close to equilibrium are presented. These can be explained based on established defect chemical models.^2–4
Secondly, some of the peculiar features in the response of mixed conducting devices under bias are discussed (e.g. hysteresis, inductive behavior). These can be reproduced using drift-diffusion simulations that consider ions as inert species, pointing towards an electrostatic influence of ionic defects on the electronic response of devices. An equivalent circuit modeling approach is proposed. The resulting model generalizes the concept of ionic-to-electronic current amplification, and it provides a physically meaningful tool to carry out small-signal analysis of mixed conducting devices under bias.^5,6
Finally, more complex interactions between electronic and ionic charge carriers under bias imply that ions influence device response beyond considerations related to electrostatics. To explore this question, we extend the defect chemical treatment relevant to photo-active mixed conductor at equilibrium⁷ to the situation where the system is exposed to light. The results highlight guidelines for the processing of mixed conducting materials as well as the design of advanced energy conversion and storage devices.
References
1. J. Maier, Physical chemistry of ionic materials: ions and electrons in solids, 2023, John Wiley & Sons
2. A. Senocrate et al., (2017) Angewandte Chemie 129 (27), 7863-7867
3. D. Moia, J. Maier, Mater. Horizons. 10 (2023) 1641–1650.
4. M. Jung et al. (2024) Advanced Materials Interfaces, 2300874
5. D. Moia, arXiv:2405.10477 [physics.app-ph] (2024)
6. D. Moia, et al., Energy Environ. Sci. 12 (2019) 1296–1308.
7. A. Senocrate, et al., Solid State Ionics. 321 (2018) 69–74.