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 technology towards commercialisation. Viologen molecules (Vio2+), derivatised with phosphonic acid attachment groups can be chemisorbed on nanostructured TiO2 layers of thickness 2–10 μm. Characterisation by cyclic voltammetry, spectroelectrochemistry and impedance spectroscopy demonstrates that colourless Vio2+/TiO2 is reversibly reduced to the strongly coloured 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 electrochemically inert white light-reflecting layer. Such a device is stable upon repeated colouration–bleaching cycles with a bleached-to-coloured state contrast ratio exceeding 5. Multicolour displays can be achieved by patterning different electrochromophores onto different areas of one working electrode.