Preparation And Properties Of Novel WO₃ Electrochromic Devices

Electrochromism is the phenomenon of certain materials reversibly changing their optical properties(reflectance, transmittance, absorption rate, etc.) via redox reactions under the influence of an external electric field. It has found applications in many fields including smart window, automobile anti-glare rearview mirror, display, camouflage and so on. Typically, the basic structures of electrochromic devices contain a transparent electrode, an electrochromic layer, an electrolyte layer, an ion storage layer and the other counter electrode. Among them, the most commonly used electrode and electrolyte are ITO and organic solution of lithium salt. However, ITO electrodes are high-temperature preparation, high cost and poor flexibility. Organic electrolytes of lithium salt are sensitive to water, toxic and polluting the environment. Anather, the electrolytes have strongly effect on stability of electrochromic devices. To solve these problems, this thesis focuses on the research work of both the electrodes and electrolyte by utilizing the classic electrochromic material WO3.In the field of electrode, we use electron beam deposition method to prepare WO3/Ag/WO3(WAW) film at room temperature, which serves as both transparent conductive electrode and electrochromic layer. We systematically study the electrochromic properties of WAW film both on rigid and flexible substrates. The main results are as follows:(1) By regulating WAW film thickness of each layer, we obtain highly transparent conductive WAW film with average transmittance in the visible region of 80 % and the surface resistance of 12.2 Ω/□. We found that the electrochromic properties are mainly determined by the outer WO3 layer, and the inner main functions is to raise the film transmittance(bleached state).(2) Compared with the ITO/WO3, the WAW films display a higher coloration efficiency(136 vs. 45 cm2 C-1), a shorter average response time(10.8 vs. 12.5 s) and a higher contrast(53 vs. 24%) under the same condition. These results indicate that this bifuctional laminated films are promising candidates for high-performance electrochromic devices.(3) We further prepared flexible WAW films on PET substrates, which have excellent resistance to bending peformance, the sheet resistance almost unchanged after 1600 bending cycles. The flexible WAW film also showed good electrochromic properties, with an average response time of 23 s, the maximum contrast of 25 %, and coloration efficiency of 138 cm2C-1. Compared with rigid WAW, relatively poorer electrochromic properties of flexible WAW film probably due to its high resistance(23 vs. 12.2 Ω/ □) and high surface roughness(5.9 vs. 0.9 nm).(4) To more efficiently evaluate the electrochromic properties of materials under different preparation or test conditions, we presents a overall performance parameter-quality factor Γ(λ).In the field of electrolyte, we use a low-cost, water-alcohol soluble carbon dots to instead the traditional salt electrolyte, which blend with WO3 sol-gel to get an electrochromic sol. We obtain a simple, environment-friendly, three-layered electrochromic device with the structure of ITO/electrochromic sol/ITO, which has a fluorescence emission, light yellow and green reversible color change due to carbon nano-dots. The structures enrich the color and function of WO3 electrochromic device and have a potential application in the field of security and display.