Novel electrochromic devices based on complementary nanocrystalline TiO2 and WO3 thin films

Electrochromic devices were elaborated based on two complementary electrodes made of a nanocrystalline metal oxide thin film deposited on conducting glass. The first electrode holds a 5 mu m thick nanocrystalline TiO2 film derivatized by a monolayer of a phosphonated triarylamine which can be rapidly oxidized by electron transfer to the conducting support followed by charge percolation inside the monolayer. The oxidation in accompanied by a blue coloration due to the absorption band at 730 nm of the stable triarylamminum radical cation. The second electrode bears a 0.2 mu m thick nanocrystalline WO3 film which turns from colorless to blue by reduction and lithium ion insertion. The former electrode reaches an absorbance of at least 3 between 700 and 730 nm after full oxidation (16 mC/cm(2)) at 1.0 V vs. NHE while for the second? complete reduction at - 1.3 V (74 mC/cm(2)) leads to A = 2.4 at 774 nm. An electrochromic device comprising both electrodes separated by an electrolytic solution of 0.1 Li+ in 4,7-dioxaoctanitrile reaches an absorbance of 2.2 at 700 nm, 4 s after a voltage step to 1.5 V. The system was shown to sustain at least 14400 coloration-discoloration cycles without degradation. (C) 1999 Elsevier Science S.A. All rights reserved.