Cyclic Voltammetry and Spectroelectrochemistry of Two Common Thiophene Polymers Reveals Ion Diffusion and Polaron Wave Function Extent

Abstract

Cyclic voltammetry is a conventional characterization method for understanding the electronic structure of organic electronic materials. Despite the maturity of the field, and ubiquity of the technique, a fundamental understanding of the processes occurring during the collection of a typical cyclic voltammogram is still a matter of debate. We have collected scan rate dependent cyclic voltammograms using two common mixed ion-electron transporting polymers while varying the position of the electrical contact to the film. When electrical contact to the film is opposite the side in contact with the electrolyte, it may be possible for electric fields emanating from the contact to draw ions across the interface. When contact is made on the same side as the electrolyte, these fields are not present, and the ion transport is expected to be diffusive. Scan rate dynamics are independent of the position of the contact, providing clear evidence that drift under an applied field is not an appropriate model of ion transport. The technique also rules out carrier transport as the rate-limiting step in these materials. Instead, we have found that Fick’s law diffusion is sufficient to model the majority of the observations and extract density of states information. Comparison with steady-state UV–vis spectroelectrochemistry data analyzed using multivariate curve resolution (MCR) shows the voltage dependence of polaronic and neutral species in the film. Combining the two measurements allows the size of the charge carriers to be estimated in terms of the number of monomer units responsible for the UV–vis absorption features observed.