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).