Fabrication of Electrochromic Device Using Nano-particle Deposition System and Its Response Time Modeling for Large-area(1m2 Class) Application

Abstract

Electrochromism refers to a color change under an externally applied voltage. Increasing interest in this phenomenon has led to new manufacturing processes for electrochromic devices (ECDs). Herein, major processes are introduced and compared in terms of their process parameters. The six representative fabrication processes discussed in this paper are electrodeposition, sol-gel, spray pyrolysis, chemical vapor deposition (CVD), thermal evaporation deposition and sputtering. Commercialization of ECDs requires a consideration of environmental issues, cost, performance and scale of manufacture. Therefore, in this study, we fabricated ECDs using a novel nanoparticle deposition system (NPDS), a seventh fabrication process. It is a low-cost eco-friendly process. The possibility of commercialization of the NPDS is discussed. Deposition of tungsten oxide (WO3) thin films on fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) substrates was conducted using the NPDS. This is a deposition approach based on low-vacuum air-spraying at room temperature. The structure of the WO3 films was characterized using X-ray diffraction, and the surface morphology was investigated using scanning electron microscopy and atomic force microscopy. The electrochemical properties of the films were examined using cyclic voltammetry and chronocoulometry. The ECD was fabricated using the deposited WO3 film, a counter electrode and an electrolyte. When a predetermined voltage (3 V) was applied, the color of the prepared WO3 films changed from transparent yellow to dark blue, demonstrating electrochromism. The WO3 film exhibited an optical contrast of up to 50% at a wavelength of 800 nm. After confirming the feasibility of using the NPDS for fabricating ECDs, we constructed a large-area NPDS and used it to produce large-area electrochromic windows (ECWs). The conventional response-time model showed a large difference from the actual value with increasing ECW active area. To overcome this problem, a new electrochromic response-time model based on the transient response of a resistance–capacitor (RC) direct-current (DC) circuit was developed. This model provided an estimation of the coloration time of ECWs as a function of the size of the active area. Five samples of different sizes were prepared: 10 x 10 mm2, 50 x 50 mm2, 300 x 300 mm2, 500 x 500 mm2 and 1 x 1 m2. We then measured the current and transmittance variation of the fabricated ECWs during coloration. The response time was defined in terms of the current difference. The response model defined the RC value as proportional to the length of one side of the active area of the ECW. The estimates derived using the response model were in good agreement with measured data for relatively large ECWs. A plasma process was introduced into a conventional NPDS to improve the contrast of the ECDs. Plasma is the fourth material state in which gas molecules are ionized

it is used for electrically or chemically modifying object surfaces. Plasma treatment of WO3 particles decreased the contact angle of the WO3-deposited surface. This meant that the contact area between the WO3-deposited surface and the electrolyte had increased. Moreover, the WO3 surface crystal structure became more amorphous. These effects improved the contrast from 50 to 65.9%.