Loren Kaake1
Simon Fraser University1
There are three major classes of models for ion transport in organic electrochromic materials. The first is based on equivalent circuits, the second uses the drift-diffusion equation, and the third emphasizes the morphological heterogeneity of organic electronic materials. Of these models, the drift-diffusion equation provides the appropriate balance of rigor and ease of understanding. A simple model for describing the dynamics of electrochromic devices that accounts for in-situ spectroscopic observations and frequency dependent cyclic voltammetry will be presented. The importance of ion drift versus ion diffusion remains a point of contention. Clearly, including drift is a more general perspective, but it will be shown that ion transport does not depend on the applied field, demonstrating that in at least one common conducting polymer, diffusion alone is the appropriate metric for quantifying the rate of ion movement. As is typical of diffusive processes in solids and liquids, the rate of ion diffusion in organic electrochromic materials depends on the solubility of the ion in the electrochromic material. When the charge density of the organic semiconductor is included in the expression for solubility, quantitative fits can be obtained that describe the voltage (not field) dependence of the rate of ion diffusion. We find that the fastest switching electrochromic materials can be obtained by increasing the solubility of the ion in the electrochromic material, which includes the use of hydrated ions for hydrophilic polymers.