Investigation of x-ray photoelectron spectroscopic (XPS), cyclic voltammetric analyses ofWO3 films and theirelectrochromic response in FTO/WO3/electrolyte/FTO cells

Abstract

Electrochromic thin films of tungsten oxide(WO3)were prepared on transparent conducting oxide substrates, i.e., fluorine doped tin oxide coated (FTOor SnO2:F) glass and microscopic glass substrates by the electron beam evaporation technique using pureWO3 (99.99%) pellets at various substrate temperatures (i.e.,Tsub = room temperature(RT, 30 °C), 100 °C and 200 °C). The films were prepared under vacuum of the order of1 × 10−5 mbar. The room temperature prepared films were further post-heat-treated(Tanne) at200 and 300 °C for about 1 h in the vacuum environment. The prepared films are in monoclinic phase. The chemicalcomposition has been characterized by using the XPS technique. The W 4f and O 1s core levels ofWO3 films have been studied on the samples. The obtained core level binding energies revealed theWO3 films containedsix-valent tungsten (W6+). The electrochemical nature of the films was studied bya three-electrode electrochemical cell in the configuration ofFTO/WO3/H2SO4/Pt,SCE, using the cyclic voltammetry (CV) technique. Electrochromic devices (ECDs) of the generaltype FTO/WO3/electrolyte/FTO were studied. The films produced at higher substrate temperature show smaller modulation ofthe visible spectrum, compared with the films produced at lower temperatures. The significantchemical bonding nature associated with the coloring/bleaching process which follows theH+ ionincorporation in the film is studied by FTIR analysis. The W–O–W framework peak was observedat 563 cm−1 and confirms the stability of the films in the electrochemical analysis. The resultsobtained from cyclic voltammetry technique and ECD cell characterization are usedto emphasize the suitability for some applications of the solar control systems.