Electrochemical threshold conditions during electro-optical switching of ionic electrophoretic optical devices

Electro-optical modulation by electrophoresis of dye ions is a promising technique for applications such as electronic paper displays and nonmechanical beam steering devices. To achieve a sufficient response rate in these devices, the transition time between two different optical states can be decreased by increasing the magnitude of the voltage applied across the electrodes, but this also leads to irreversible and undesirable electrochemical reactions. An electron tunneling model has been developed to describe the electrochemical reaction and to better understand the conditions determining its onset. The model gives rise to three predictions that were subsequently confirmed experimentally: the magnitude of the applied surface charge density should determine the rate of electrochemical activity, the bulk concentration of ions in the solution should shift the threshold voltage at which electrochemical reactions occur, and the reaction rate should be substantially enhanced around nanometer-sized bumps on the electrode surface. Applying this new understanding, the transition time of a device incorporating porous zinc antimonate (ZnSb2O6) electrodes and a solution of Methylene Blue dye in methanol was reduced by a factor of approximately 20.