AVS 60th International Symposium and Exhibition
    Thin Film Thursday Sessions
       Session TF+PS-ThM

Paper TF+PS-ThM5
Ultrasonic Spray Deposition of Mesoporous WO3 Films Displaying 100% Optical Modulation

Thursday, October 31, 2013, 9:20 am, Room 102 C

Session: Advanced CVD Methods
Presenter: C.A. Wolden, Colorado School of Mines
Authors: C.-P. Li, Colorado School of Mines
C. Engtrakul, National Renewable Energy Laboratory
R.C. Tenent, National Renewable Energy Laboratory
C.A. Wolden, Colorado School of Mines
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Smart windows employing electrochromic materials to reversibly modulate optical transmission and reflection can significantly improve building energy efficiency. However, further improvements in both performance and cost reduction are required for widespread implementation of electrochromic windows. Mesoporous films comprised of nanocrystalline domains can provide both high performance and good durability. Sol-gel chemistries combined with sacrificial templating agents is a well-established technique to introduce controlled porosity, but conventional techniques for film formation such as spin/dip coating are not amenable to large scale production. Ultrasonic spray deposition (USD) is an attractive technique for large scale, low cost thin film manufacturing of nanostructured thin films. USD provides high materials utilization and the use of benign solvents enables deposition under ambient conditions. Piezoelectric nebulizer technology addresses the quality and uniformity issues that were a concern with conventional pressure driven sprays. In this work mesoporous WO3 films were deposited by adapting sol-gel chemistry to ultrasonic spray deposition. The sol was formed by dissolving WCl6 and an organic templating agent (P123) in ethanol. The dynamics of both sol preparation and hydrolysis were investigated by UV-Vis spectroscopy. Electrochromic performance is strongly correlated to both the annealing conditions and the sol composition. Optimized films produced world record coloration efficiency (68 cm2/C), resulting in never before achieved 100% optical modulation across the visible spectrum during cycling. The films also displayed good switching kinetics, producing a 75% change in absolute transmission in 4 and 19 s during coloration and bleaching, respectively. Achievement of optimum electrochromic performance could be achieved by controlling the sol concentration and/or the number of spray passes in this robust process. The films were smooth and uniform, and the excellent performance is attributed to the nanocrystalline nature of the films, which provides high specific surface area (>100 m2/g) for efficient lithium ion intercalation.