Amorphous WO
3·nH
2O thin films were deposited at room temperature on Si and ITO/glass substrates. After mixing Na
2WO
4 solution and HCl solution, they were dripped immediately on a rotating substrate to produce a deposited film. This process is simple and applicable to a non-heat resistant substrate. Furthermore, it might be preferable for producing electrochromic (EC) properties of WO
3 thin film because the film obtained at room temperature contains OH group. The optimum film deposition parameters are as follows: Na
2WO
4 solution, 1.0 mol·dm
−3; HCl solution, 1.0 mol·dm
−3; flow rate, 2.0 mL·min
−1; pH: 0.2. The films obtained under the optimum parameters had a composition of WO
3·(2.6-2.7)H
2O and displayed good adhesion and scratch hardness (pencil method > 9H). The thin films obtained using this method exhibit excellent EC properties of color efficiency (45-50 cm
2·C
−1), and response time (3 V, 2 s, ΔT = 50%).
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