Unexpected yellow-blue electrochromism of ITO powders at modest potentials in aqueous electrolytes

Tin-doped indium oxide (ITO) powders. both commercial and laboratory samples. have been incorporated into diffuse-reflecting. interdigitated electrochromic displays using aqueous electrolytes. A surprising yellow-blue electrochromism has been observed which has the features of operation at low potential, minimal charge consumption (< 10 mC cm(-2)). reasonable switching speed (ca. 1s) and impressive lifetime (> 500 000 cycles). The observed electrochromic effect is attributed to a combination of changing the effective ITO band gap and populating/depopulating the conduction band. Smaller primary ITO crystallites were shown to lead to an enhanced effect, as doss increasing tin dopant level to 25 mol%. Increases in dopant level beyond 25% gave no further improvement. Cyclic voltammetry showed no obvious Faradaic process associated with the color changes observed. In addition to obtaining diffuse reflectance spectra, which show that contrast ratio for these displays actually changes direction at about 500 nm, we also measured CIE b*-values, which give a quantitative indication of observed blue-yellow shift. These powders appear to be promising candidates for low cost, low information content displays.