Preparation of Dip-coated Electrochromic Thin Film and Its Application to Photoelectrochromic Systems

Abstract

Electrochromic and photochromic materials with reversible transmittance in response to an applied voltage and illumination, respectively, are the most common materials for smart windows. An alternative, the photoelectrochromic cell (PECC) was first demonstrated in 1996. For PECC, coloration is attained by the photon-electron conversion with no need for external power sources. In this study, tungsten oxide (WO3) thin film was fabricated by the sol-gel method with film thickness control by repeated dip coating process. Electrochemical analysis was conducted in a three electrode half-cell using a lithium-based electrolyte and an optical transmittance change in colored/bleached state could be shown by using He-Ne laser (λ =633 nm). From a materials analysis perspective, WO3 thin films fabricated by the sol-gel method showed better adhesion with FTO glass after the annealing process when compared with samples without annealing. In addition, WO3 films with amorphous phases exhibited better electrochromic performance than crystalline WO3 films and film thickness control was possible by repeated dip coating. These results suggest that these modifications would be advantageous when assessing the electrochromic performance. Consequently, the optimum thickness for the best electrochromic performance was decided for employing to photoelectrochromic systems. After that, we fabricated two types of WO3 employed PECC. One is a layered structure composed of dye-sensitized TiO2 nanoparticle (NP) on the crystalline WO3 electrochromic film as the photoanode and employed the Pt counter electrode as a photocathode. The other is a separated structure composed of amorphous or crystalline WO3 electrochromic film electrode as a photocathode and dye-sensitized TiO2 NP as a photoanode. In addition, we apply WO3 thin film with different thickness at two types of PECCs for understanding the transmittance change tendency when illuminated with time on cell compared to that of solely electrochromic sytems. UV-vis spectra measurement results showed the sample with a thinner film of WO3 and with amorphous phases in separated structure exhibited the best coloration switching kinetics compared with others over illumination time and absolute transmittance change value was also quite remarkable. Furthermore, WO3 thin film in separated structure exhibited faster optical transmittance switching kinetics with illumination time than that of in layered structure. Consequently, this results show that WO3 employed two type of the photoelectochromic system exhibited same tendency compared to crystalline phase and film thickness dependent electrochromic properties. Lastly, for enhancing the optical properties of photoelectrochromic systems, which were dip-coated WO3 electrochromic thin film was employed, more transparent dyes on TiO2 NP have to be used for higher initial transmittance value (as-prepared state) in two types of photoelectrochromic systems.