Electric Field Contributions to Ion Motion in Organic Electrochemical Devices

Organic electrochemical devices include biosensors, transistors, electrochromic displays, and light emitting electrochemical cells. One key step in device operation is the movement of ions within the electronically conductive π-conjugated material. The mechanisms of ion motion and the structure-property relationships that govern those mechanisms continue to be a source of keen interest. One important vantage point to understanding this motion is through the fundamental equations of mass transport (the drift-diffusion equation). Because both electronic charge carriers and ions are in motion, the details of this coupling can, in principle, be quite complex. In order to develop a first order model that encapsulates typical behavior in the most common systems, we performed spectroelectrochemical measurements on thin films of PEDOT:PSS, a common mixed conducting material. Specifically we examined what role, if any, applied electric fields have on the transport of ions in PEDOT:PSS. This was accomplished by comparing a device with potential applied at the electrolyte interface to a device where a potential is applied across the PEDOT:PSS thin film, in an attempt to use fields to draw the ions more rapidly through the bulk of the material. No discernable difference in the electrochemical charging dynamics was observed, demonstrating that macroscopic fields are not an important influence in ion motion. To better rationalize this result, finite element simulations were performed. These simulations highlight the importance of local fields as established by both the electronic charge carriers and the intermolecular forces between the ions and the polymeric medium in which it is embedded.