Tungsten Trioxide Based Electrochromic Films And Devices

In this dissertation, tungsten trioxide (WO3) based thin films for electrochromic applications are developed using various approaches including sputtering, anodic oxidation, hydrothermal method, templating method, hybridization, etc. The main purpose is to improve the electrochromic properties of WO3 based materials such as spectrum modulation, switching speed and coloration efficiency. The electrochromic effect of WO3 thin films in infrared band is also investigated. Finally, an all-solid-state electrochromic device based on NiO/WO3 complementary structure and solid hybrid polyelectrolyte is fabricated.Self-organized macroporous WO3 films were grown by an anodic oxidation in a NaF electrolyte from a DC-sputtered tungsten layer on ITO glass. Well-structured films adhere to the entire surface under optimized experimental conditions. The as-anodized films show an amorphous structure and are poorly transparent. After annealing, the films adopt an orthorhombic structure and become transparent. The anodized WO3 films on ITO glass with macroporous structure exhibit excellent electrochromic properties, including faster switching speed and larger color contrast. WO3 microbowl array films have been electrodeposited on ITO glasses using PS spheres as template. The films show a connected network of monodispersed pores with average size of 600 nm after the template is removed. Comparing to dense films prepared without PS template for the same deposition time, the porous WO3 films deposited with PS template show enhanced electrochromic properties, i.e. high switching speed and fast coloration efficiency. Especially for the WO3 microbowl array film deposited for 300 s, a coloration efficiency as high as 68 cm2C-1, and fast coloration (bleaching) speeds of 3.6 s (1.0 s) are obtained. Mean while the cyclic stability has no significant difference between the WO3 films prepared with and without PS template.WO3 nanotree films were prepared by hydrothermal oxidation of W substrate. The film thickness can be controlled by adjusting the hydrothermal duration and a 550 nm-thick nanotree film was obtained after hydrothermal process for 5 h. The as-prepared film was of hexagonal structure and kept its crystal structure until it was annealed up to 500℃. The nanotree film annealed at 400℃exhibits remarkable electrochromic properties with an optical reflectance modulation of 30% at 500 nm. The coloration efficiency value as high as 43.6 cm2 C-1 is achieved for this film. This is due to the porous structure of nanotree film and its hexagonal and trigonal tunnels of h-WO3, into which Li+can be intercalated and deintercalated readily. Hexagonal WO3 nanowire array film was obtained using a template-free hydrothermal method by adding ammonium sulfate as capping agent. The WO3 nanowires grown vertically on FTO-coated glass substrate are woven together at the surface of the film, forming well-aligned arrays at the bottom part and a porous surface morphology. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that each nanowire is a hexagonal single crystal and the long axis of it oriented toward [0001] direction. Due to the highly porous surface, good contact with conductive substrate and large tunnels of hexagonal structured WO3, fast switching speed of 7.6 and 4.2 s for coloration and bleaching, respectively, and high coloration efficiency of 102.8 cm2 C-1 are achieved for the WO3 nanowire array film.Nanostructured polyaniline (PANI)-WO3 hybrid thin films were synthesized via a molecular assembling route in a solution of aniline using peroxotungstic acid (PTA) as the dopant and ammonium persulfate as the oxidant. The films show a porous morphology with nanorod arrays on the surface, and WO3 is uniformly incorporated into the polymer network. Electrochemical and electrochromic tests including cyclic voltammetry, chronoamperometry and corresponding in situ transmittance of PANI-WO3 hybrid films comparing with neat PANI film and sol-gel WO3 film were conducted in 0.5 M sulfuric acid solution. The hybrid films, being a dual electrochromic material, varied from royal purple to green, pale yellow and finally dark blue as the applied potential was scanned from 0.8 V to-0.5 V. Compared to sulfate doped PANI film, the high colouration efficiency and comparable durability are obtained in the PANI-WO3 hybrid films. The PANI-WO3 hybrid films also show faster switching speed and better durability than WO3 film. The enhanced electrochromic properties such as faster switching speed and better durability are mainly attributed to the combining of advantages of both materials and the formation of the donor-acceptor system.WO3 films were deposited by reactive dc magnetron sputtering at different substrate temperatures. With the increasing of the deposition temperature, the diffusion coefficient of H+ ions in the WO3 films decrease. The infrared reflectance modulation and color efficiency first increase and then decrease with the deposition temperature, and maximum values of 40% and 18.5 cm2 C-1, respectively, are achieved at 250℃and 9μm. The WO3 films with reflectance modulation (higher than 30%) in infrared band can be fabricated into devices, which have considerable applications in thermal control and infrared camouflage. An all-solid-state electrochromic device based on NiO/WO3 complementary structure and solid polyelectrolyte was manufactured for modulating the optical transmittance. The device consists of WO3 film as the main electrochromic layer, single-phase hybrid polyelectrolyte as the Li+ ion conductor layer, and NiO film as the counter electrochromic layer. Indium tin oxide (ITO)-coated glass was used as substrate and ITO film act as the transparent conductive electrodes. The effective area of the device is 5×5 cm2. The device showed an optical modulation of 55% at 550 nm and achieved a coloration efficiency of 87 cm2 C-1. The response time of the device is found to be about 10 s for coloring step and 20 s for bleaching step. The electrochromic mechanism in the NiO/WO3 complementary structure with Li+ion insertion and extraction was investigated by means of cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS).