Phase transitions of the WO3 layer in photoelectrochromic devices

Photoelectrochromic systems are a combination of a dye solar cell and an electrochromic material, usually WO3. They change their transmittance on illumination. We prepared a particularly advantageous configuration, which can be coloured and bleached under illumination, and bleached in the dark. During colouring, electrons and Li+ cations are intercalated into the electrochromic WO3 layer Due to this intercalation, the WO3 crystals change their structure from monoclinic through tetragonal to cubic. The intercalation and the phase transition are completely reversible. We investigated these phase transitions with the help of electrochemical potential measurement, IR spectroscopy and X-ray diffractometry applied to the separate layers. We discuss the effect of the phase transition on the colouring and bleaching characteristics of the photoelectrochromic device, i.e. the time dependence of the voltage and the optical density.