Impedance Spectroscopy of Spin-Cast and Electrochemically Deposited PEDOT:PSS Films on Microfabricated Electrodes with Various Areas

We have examined the electrochemical impedance spectroscopy (EIS) of PEDOT:PSS-coated gold electrodes in various electrolytes as a function of temporal frequency (from 0.1 Hz to 10^4 Hz) using a custom-designed microfabricated substrate. The electrode areas were systematically varied over a broad range in nominal size from 10 x 10 um (100 um^2) to 500 x 500 um (2.5 x 10^5 um^2). Comparisons were made between spun-cast PEDOT:PSS crosslinked with GOPS and electrochemically deposited PEDOT:PSS films with similar thicknesses. The impedance spectra of the PEDOT:PSS-coated electrodes with various sizes could all be reasonably well described by a two-element equivalent circuit model with a solution resistance Rs and a film capacitance C. These two parameters together define a characteristic temporal frequency fc = 1/(2piRsC). By normalizing the impedance with respect to Rs (Zn = |Z|/Rs) and the frequency with respect to fc (fn=f/fc), we found that all of the experimental Bode curves could be collapsed onto a single master plot of Zn vs. fn. In addition, analytical formulas that allow the estimation of the film impedance magnitude and phase φ were derived and experimentally validated. These results are of fundamental interest and should be useful in designing films for coating biomedical electrodes.