Preparation And Electrochromic Properties Of Two-layer Tungsten Oxide Thin Films

Electrochromic materials could be applied widely in the field of the militarycamouflage and spacecraft thermal control due to their ability of changing color andheating absorption with the applied voltage. Tungsten oxide (WO3) exhibits greatphotoelectric properties and plays a significant role in developing adaptive camouflage.In this paper, the WO3thin films were deposited on the ITO conducting glassthrough direct current reactive magnetron sputtering. The influences of oxygen partialpressure, sputtering pressure, and temperature on the composition, morphologies andelectrochromic properties of the one-layer WO3thin films were systematicallyinvestigated. In order to improve the electrochromic properties of the WO3thin film, thetwo-layer WO3thin films were synthesized on the basis of the one-layer thin films. Theeffects of distributing and component on the electrochromic properties of the tow-layerfilms were studied. The component, surface morphology, spectra and electrochemicalproperties were measured by X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), scanning electron microscope (SEM), atomic force microscopy(AFM), UV-visible spectrophotometer and electrochemical working station.The results show that the deposited WO3thin films are unstoichiometric, namelyO/W <3. The increasing of oxygen partial pressure can lead to the enhancement of theO/W ratio of the films. The WO3thin film appeared the outstanding electrochromicproperties when the oxygen partial pressure was85%. The research on varyingsputtering pressure from50W to100W during deposition illustrate that the increasingsputtering pressure results in denser microstructure and higher roughness. All theamorphous WO3thin films prepared in the condition of85%oxygen partial pressure androom temperature, with the sputtering pressure of100W and120W, exhibited greatdifference in surface morphologies and show good electrochromic properties. Themaximum transmittance contrast between the colored and bleached state of the WO3thin films were74%and86%, while the colored/bleached response time and coloringefficient were9.6s/2.9s and45.07cm2·C-1,9.3s/3.9s and43.11cm2·C-1，respectively.The transmittance modulation and coloring efficient of the one-layer WO3thin filmsboth reach an advanced level, which can satisfy the requirement of adaptive camouflage.The WO3thin film sputtered under350℃was monoclinic crystalline. With theincreasing of sputtering temperature, the O/W ratio of the crystal film was smaller thanthe amorphous films produced at the same conditions. After200cycles，the cyclestability of the crystal film could still remain99%.According to experiments of the one-layer thin films, three kinds of two-layer WO3thin films were sputtered with different component and structure. The analysis resultsindicate that the surface morphologies and component of thin films could influence the electrochromic properties of the films directly. Among the three kinds of films, thetwo-layer electrochromic thin film with loose upper layer and dense underlayer got thebest electrochromic properties. The loose structure is in favor of the moving of ionswhich could reduce the response time, while the dense stricture could improve theability of ion storage. The cycle stability, transmittance modulation, coloring efficientand colored/bleached response time of the two-layer film are84%,74%,19.86cm2·C-1and64.6s/99.7s, respectively.