Micro-Raman spectroscopic characterization of a tunable electrochromic device for application in smart windows

An asymmetric electrochromic (EC) device based on an active EC tungsten oxide-titanium oxide (WO3-TiO2) layer was constructed. The EC active layer consisted predominantly of monoclinic WO3 nanocrystallites with a minor additional component of hexagonal WO3 and amorphous TiO2. Detailed micro-Raman spectroscopic studies of the intercalation and deintercalation of lithium in the EC active layer of the EC device as a function of the applied voltage were performed. Three significant structural stages occur upon intercalating Li into the WO3-TiO2 layer when coloration potentials of 1.0, 1.5, 2.0, and 3.0 V are applied to the EC device. In the first stage (applied potential of 1.0 V), the m-Li-x WO3 phase is retained. In the second stage, (applied potential of 1.5 and 2.0 V) the m-Li-x WO3 transforms to a tetragonal phase. In the third stage, (applied potential of 3.0 V) the Raman spectrum exhibits no spectral bands, showing that Li-x WO3 has attained the highest-symmetry cubic phase. This phase sequence is confirmed by the X-ray diffraction (XRD) measurement. These phase transitions can be reversed and, upon complete deintercalation, m-WO3 with traces of h-WO3 is recovered. Optical transmission studies were performed in conjunction with Raman and XRD studies. A shift of the optical transmittance peak position from 639 to 466 nm and reduction in the width of the transmittance curve with increasing applied potential opens up the possibility of smart window applications for the nanocrystalline WO3-based EC device. Copyright (c) 2008 John Wiley & Sons, Ltd.