Electrochemical aspects of display technology based on nanostructured titanium dioxide with attached viologen chromophores

Progress in recent years in the field of electrochromic displays based on viologen modified high-surface area TiO2 electrodes (Vio2+/TiO2) has moved the technol. towards commercialization. Viologen mols. (Vio2+), derivatized with phosphonic acid attachment groups can be chemisorbed on nanostructured TiO2 layers of thickness 2-10 μm. Characterization by cyclic voltammetry, spectroelectrochem. and impedance spectroscopy demonstrates that colorless Vio2+/TiO2 is reversibly reduced to the strongly colored cation radical species Vio+•TiO2. This system can constitute the working electrode of an electrochromic display with a capacitive doped SnO2 electrode as counter electrode, the latter coated by an electrochem. inert white light-reflecting layer. Such a device is stable upon repeated coloration-bleaching cycles with a bleached-to-colored state contrast ratio exceeding 5. Multicolor displays can be achieved by patterning different electrochromophores onto different areas of one working electrode.