Colorimetric properties and structural evolution of cathodic electrochromic WO3 thin films

Cathodic amorphous tungsten trioxide (WO3) thin films have been deposited by reactive direct current magnetron sputtering and have been studied for their colorimetric and electrochromic properties. Those studies were carried out under two different potential cycling process: (i) switching mode (the response between coloration and bleaching in increasing potential steps) and (ii) modulation mode (the stepwise modulation to coloration with increasing potential and back to bleaching). Optical measurements, performed as a function of applied potential, showed excellent transmittance contrasts (∼80%) between colored and bleached states. The color stimuli and the changes that take place upon reversible switching or modulation were recorded based on the Commisson International de l’Éclairage (CIE) system. It was found that, under various potentials, significant changes occurred in the hue and saturation for WO3, as exhibited by the CIE 1931 xy chromaticity coordinates. As WO3 was reduced (W6+ + e– → W5+), a sharp decrease in luminance was observed. Excellent reversibility is demonstrated not only by colorimetric properties, but also by the corresponding intrinsic structures of the films, as investigated by µ-Raman spectroscopy. Furthermore, the WO3 films displayed a fast response time and good long-term cycling durability, which was attributed to their amorphous nature.