Studies on magnetron sputtered deposited nanocrystalline tungsten oxide films useful for electrochromic devices

Abstract

WO3 thin films were sputtered on to different substrates (viz., Si, quartz and ITO coated glasses) at different temperatures and partial pressures using DC magnetron sputtering. The influences partial pressures of O2 and Ar and substrate temperature (at constant sputtering power) on different characteristics viz., optical, structural, morphological, compositional and electrochromism of the films have been investigated. These characteristics were observed to be strongly reliant on deposition conditions. Thickness and roughness (r.m.s.) of the films were found to be in the ranges of 700–918 nm and 3.18–8.4 nm, respectively. The transmittance in the visible wavelength region of these films was found to in the range of 33–80% depending upon the deposition conditions and optical band gap is found to decrease with increase of substrate temperature from 400 K to 600 K at 16.3 mTorr. The X-Ray diffractogram of the films (deposited at 400 K at 16.3 mTorr) exhibited sharp peaks from the reflections of (0 2 1) and (4 2 1) planes of monoclinic crystalline phase of WO3 indicating the prepared films are of crystalline in nature. The structural arrangement of WO3 molecules in the films were analyzed using micro-Raman studies. The morphology studies (using FE-SEM, AFM and EDS techniques) of the films indicated reasonably good stoichiometry of WO3 films; these results further indicated that the films consist of nanoparticles of with sizes varying upon the deposition conditions. For studying the electrochromism of the films, they were immersed in an anhydrous electrolyte (mixture of 0.2 M lithium perchlorate and 50 ml propylene carbonate); coefficient of diffusion of Li+ ions and coloration efficiency were measured by cyclic voltammetry and chrono-coulometry methods, respectively. The partial oxygen pressure and substrate temperature used during deposition were found to influence the above mentioned parameters of the films to a larger extent.