Electrochromic properties of atmospheric CVD MoO3 and MoO3-WO3 films and their application in electrochromic devices

The transition metal oxides exhibit electrochromism, defined as color change caused by applied voltage. The electrochromic (EC) effect can be stronger in mixed oxides because of the higher optical absorption, due to electron transitions between two kinds of metal sites with different valences. Thin films Of MoO3 and mixed Mo/W oxides were obtained by atmospheric pressure chemical vapor deposition (CVD), using pyrolytic decomposition of metal hexacarbonyls. IR and Raman spectra are presented and compared with data for WO3 from previous investigation. Sputter auger spectroscopy was used to determine the chemical composition of the mixed films. The visible transmittance of films deposited on glass substrate was about 80%; on conductively coated glass (i.e. SnO2:Sb/glass) the visible transmittance drops to around 60%. After annealing, the optical transmittance of the single oxide films slightly improves, while for the mixed films it slightly decreases. The electrochromic properties Of MoO3 and mixed Mo/W oxide films were investigated by cyclic voltammetry performed in a conventional potentiostatic three-electrode configuration. Current and transmittance were measured simultaneously during the potential sweep at different wavelengths over the spectral range of 400-800 nm. Deep blue coloring of all the electrochromic films (MoO3, WO3 or Mo/WO3) appears only in the cathodic range, minimizing visible light transmittance. At positive potentials, the EC film starts bleaching (for about 3 min) and the film returns its initial transparency. It was found that the CVD deposited MoO3 and Mo0.07W0.93O3 thin films performed well as electrodes in electrochromic devices. We made devices based on Mo0.07W0.93O3/LiClO4 + propylene carbonate + polyvinyl alcohol + PVA/SnO2:Sb. The devices exhibited good cyclic reversibility (colored/bleached state) and good coloration efficiencies. We measured a transmittance change (600 nm) of 48.4% switching to 17.4% for our best device. (c) 2005 Published by Elsevier B.V.