Silver doping effect on the electrochromic performance of the WO3 thin films produced by thermal evaporation in vacuum

Abstract

Ag doped WO3 thin films are successfully fabricated by thermal evaporation method in vacuum as precursor, and then annealed at 450 °C in nitrogen gases atmosphere. Cyclic voltammetry, repeating chronoamperometry, chronocolumetry and optical transmittance are recorded in 1 M LiClO4/propylene carbonate electrolyte. The color of the WO3 thin films changes from transparent to the blue under the applied potential of -1.8 and +1.9 V. XRD studies show that all the films have a polycrystalline structure of WO3. The molecular formation is also confirmed by Raman and X-Ray photoelectron spectroscopy (XPS). The EDS elemental mappings clearly confirm the homogeneous distribution of Ag, W and O elements into the WO3 network. From the optical studies, indirect band gap energy of the WO3 decreases as Ag doping level increases. The best coloration and bleaching times (2.33 and 1.25 s) are obtained for the Ag(2 %):WO3 thin films, compared to the pure WO3 (3.79 and 1.96 s). The produced WO3 films exhibit high reversibility (39.6/69.1 %), high coloration efficiency values (26.3/141.2 cm2/C), and good cycling stability. From the EIS measurements, the charge-transfer resistance is Ag(2 %):WO3 < the pure WO3 < Ag(5 %):WO3 < Ag(8 %):WO3, which means that Ag(2 %):WO3 has the biggest electrochemically active surface area. Electronic energy levels of the pure WO3 and the Ag:WO3 thin films are also given. From the Mott-Schottky studies, it is determined that the donor concentration of the materials is of the order of 1014–1015 cm−3. In the literature, there are very limited studies related to electrochromic thin films by thermal evaporation in vacuum. In this sense, the reported results in this study are promising for the electrochromic applications.