Publication date: 25th September 2020
Electrochemical impedance spectroscopy (EIS) and related analysis have successfully guided efforts for the improvement of many electrochemical and photoelectrochemical systems. For perovskite solar cells (PSC), EIS analysis has shown some complications due to stability issues and particular shapes of the obtained spectra that cannot be explained by standard models.
Here, I will present results showing a fully validated series of impedance spectra that represents a reproducible EIS timeseries at open circuit voltage (VOC) for more than 20 hours, with a total of 150 analysed spectra. Significant changes in resistance and capacitance are observed and will be discussed, with the additional observation that these changes are almost 100% reversible if the devices are kept in the dark for only one day. The tested devices are full PSC devices that have proven to be stable over more than 500 hours. We compare those results to our latest generation of PSC that feature a self-assembled monolayer (SAM) of a fullerene derivative that have shown less than 1% of degradation over 1000 hours under light.
The second part of this talk will focus on the low frequency hook that is frequently encountered in EIS measurements on PSC. This feature has been called: (low-frequency) inductive loop, curl-back or negative loop and is also found in Lithium Ion batteries, proton exchange fuel cells (PEFC), organic light emitting diodes (OLED), thin film model electrodes, and in the field of corrosion, for example. The hook will be explained from a systems theory perspective, and a very basic explanation on the basis of a change in resistance (ΔR) of a main process will be given, considering the respective timescales. The most common equivalent circuit model (ECM) for such a low frequency hook is reviewed and the relation to a general empirical low-pass filter type model is highlighted. Thereby, a new viewpoint on the low frequency hook in EIS is presented that can facilitate the understanding and the parameter identification for the impedance analysis of spectra with such features.
In sum, this talk is designed to help researchers to get a better understanding and to create an awareness of the important aspects for an accurate, meaningful, and conclusive impedance analysis on PSC.
This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks: “Solid Oxide Interfaces for Faster Ion Transport”. Funding from the Kakenhi Grant-In-Aid for young scientists, grant numbers 18K13993 and 20K15028, is also acknowledged.
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