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

摘要

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/cm in a high-frequency regime and ~50-150 μF/cm~2 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).