Measuring Photo-Enhanced Ionic Conductivity
Thin-film measurement is a great tool to characterize new materials with respect to their electronic, ionic, magnetic, dielectric or optical properties under well-controlled fabrication and test conditions. In particular, impedance spectroscopy offers the most detailed access to the electronic properties.
The MFIA Impedance Analyzer and the MFLI Lock-in Amplifier with the MF-IA option are perfectly suited to measure the often very large in-plane resistance of thin films in a reliable and fast manner.
This blog post introduces work carried out by Thomas Defferriere, Dino Klotz, Juan Carlos Gonzalez-Rosillo, Prof. Jennifer L. M. Rupp and Prof. Harry L. Tuller from MIT and Kyushu University. Co-author Dino Klotz joined Zurich Instruments shortly after publishing this work.
Figure 1. Dino with the Zurich Instruments MFIA Impedance Analyzer and the April 2022 issue of Nature Materials showing an artistic rendering of the surface of the thin film with its different grains separated by grain boundaries in black, where the rays of light “enhance” the ionic current blocking grain boundaries.
Content of the Study
In this work, the influence of UV light on the ionic conductivity of the grain boundary resistance of a 3% Gd-doped CeO2 (GDC) thin film was analyzed. GDC is one of the most prominent oxygen ion conductors used in high-temperature fuel cells and electrolyzers. However, at lower doping levels, it suffers from a large grain boundary resistance that makes polycrystalline samples two to three orders of magnitude more resistive than single crystals. The additional resistance is caused, in large part, by the space-charge zone and associated potential barrier due to the positive charge of the grain boundary core.
The study demonstrated that it is possible to decrease the grain boundary resistance by 72% at 250°C under UV light illumination, whose photon energy is slightly above the band-gap of GDC. Further, it could be shown that this effect is not caused by heat, electronic conductivity, surface catalysis or other effects. The findings are based on electrochemical impedance spectroscopy (EIS) measured with the MFIA.
Figure 2. (Top left) EIS in the dark; (top right) EIS under UV light; (bottom) single-frequency impedance transients (SFIT) monitoring the evolution of the sample resistance when the light source is turned on (t1).
Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) is a very powerful method for analyzing electrochemical systems in steady state. Characteristics such as conductivity, charge transfer and diffusion can be measured in operando. The small-signal excitation ensures that the DUT remains in its predefined operating point.
However, this study required some more insights about the mechanisms behind the observed light effect. The dynamic evolution of relevant parameters during the transition when the light is switched on can help to provide the additional information required for a more incisive analysis.
Classic EIS is then difficult to align with such measurements because the steady-state condition is one of the fundamental requirements for EIS. In the field of semiconductors, deep level transient spectroscopy (DLTS) is an established technique to determine the energy levels of certain defects by applying voltage pulses and monitoring the evolution of the high-frequency capacity as a function of time.
Single-Frequency Impedance Transients (SFIT)
In this study, a comparable technique has been used: single-frequency impedance transients (SFIT), where the impedance at one specific frequency is measured continuously. For example, high-frequency SFIT measurements have been used to determine the temperature evolution in a fuel cell or battery by relating the Ohmic resistance to the electrolyte temperature [1, 2].
Since it is impossible to measure a full impedance spectrum while the DUT undergoes a transition, an alternative is to monitor the impedance at a specific frequency as a function of time and analyze its dynamics in the time domain. Resistance or capacitance can both be used as indicative parameters for a specific process or behavior.
The method presented in Figure 2 nicely compliments a classic EIS analysis, where spectra recorded in two different static operating points represent the characteristics of the system in steady state, respectively. The evolution of relevant parameters, such as capacitance and resistance at a certain frequency, can provide additional information about the dynamics of the changes and thereby provide more information on the exact mechanism and the driving force.
Conclusion of the Study
In this work, it was concluded that photo-generated electrons travel to the grain boundary where they get trapped and compensate the positive core charge and thereby mitigating the potential barrier at the grain boundary in the dark. The reverse process, the resistance increase when the light is switched off, is slightly slower. It is limited by the diffusion time for holes from the bulk to the grain boundary where they have to recombine with the trapped electrons to revert the grain boundary to its initial state.
The work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks: "Solid Oxide Interfaces for Faster Ion Transport", Department of Energy, Basic Energy Sciences and the Swiss National Science Foundation.
Here is the link to the full paper: Defferriere, T. et al. Photo-enhanced ionic conductivity across grain boundaries in polycrystalline ceramics. Nat. Mater. 21, 438 (2022).
References
1. Njodzefon, J.-C., Klotz, D., Kromp, A., Weber, A. & Ivers-Tiffée, E. Electrochemical modeling of the current-voltage characteristics of an SOFC in fuel cell and electrolyzer operation modes. J. Electrochem. Soc. 160 F313 (2013).
2. Schmidt, J.P., Manka, D., Klotz, D. & Ivers-Tiffée, E. Investigation of the thermal properties of a Li-ion pouch-cell by electrothermal impedance spectroscopy. Journal of Power Sources 196, 8140-8146 (2011).