Watching Polarons Move in the Energy and Frequency Domains Using Color Impedance Spectroscopy

The hybrid electronic-ionic transport property of π-conjugated polymers enables new (opto)electrochemical device constructs for energy conversion and storage and biosensing applications. One major challenge is separating the energy and frequency dependence of Faradaic events-those involving charge transfer and the redox processes of the conjugated backbone from the non-Faradaic components, such as ionic motion. Herein, we combine optical spectroscopy with electrochemical impedance spectroscopy (EIS) to resolve the frequency response of ionic-electronic coupling as a function of electrochemical doping potential. First, using EIS, we identify two different frequency regimes resulting in potential-dependent capacitive elements on the order of ∼10 μF/cm2 in a high-frequency regime and ∼50-150 μF/cm2 in a low-frequency regime. Given the larger magnitude and greater potential dependence, we posit that polaronic motion is more likely to occur at low frequencies (<1 kHz) and overlaps with ionic motion. The use of color impedance spectroscopy (CIS) enables observation of polaronic motion with frequency modulation. We observe that higher doping potentials show a greater motion of polarons above the DC-bias baseline concentration for onset in electrochemical doping, but all potentials considered demonstrate a critical frequency at which the polaronic motion is "frozen"(∼40 Hz). This critical information obtained from CIS in highly dielectric environments offers a unique figure of merit for future studies on electronic-ionic coupling by which to compare across polymer/electrolyte interfaces, including the role of a charge-supporting electrolyte, a solvent, and alternative Faradaic processes (e.g., electrocatalysis).