Optical and Electrochromic Properties of Sol-Gel Deposited Doped Tungsten Oxide Films
The goal of this study is to investigate the effects of doping of tungsten oxide on its electrochromic properties. The work is directed toward the development of neutral coloring tungsten oxide materials with properties superior to undoped tungsten oxide. Two basic types of sol-gel formulations were used for experimentation. Type 1 used a WOCl4 precursor. Type 2 used a proprietary (Donnelly) tungsten complex precursor. The Type 2 precursor was used only for comparison to Type 1. Doping experiments were performed using the Type 1 chemistry. The dopants studied were Co, Cr, Nb, Ti, V and Y. The range of dopant was 1-12 mole %. Improved electrochromic behavior was observed for tungsten oxide films doped with V and Ti.
Analysis of the films included x-ray diffraction, ellipsometry, cyclic voltammetry and spectrophotometry. X-ray diffraction showed that all films heat treated at temperatures below 300°C were amorphous in structure. The refractive indices for undoped films were measured. We found the n, k values of the Type 1 films to be lower than the Type 2 films. The n and k values were n=1.79 and k=2.8x10-3, and n=2.08 and k=3.6x10-3 at 550 nm, for Type 1 and Type 2 films respectively. Both types of tungsten oxide films showed low absorption and high transparency in the visible range. As expected, we found that the film density, and hence the refractive index and extinction coefficient, depended on coating solution chemistry, hydration, and densification procedures. Undoped Type 1 films showed slightly higher lithium diffusion coefficients (DLi), compared to undoped Type 2 films, DLi=1.36x 10-9 cm2 s-1 and 1.31 x 10-9 cm2 s-1, respectively.
We also noted that the properties of the films could be reproduced for any coating chemistry and densification scheme. The electrochemical and optical behavior were determined by using an in-situ cuvette cell with a 1M LiCl04 / propylene carbonate electrolyte. Cyclic voltammetric measurements showed that doped Type 1 films exhibited electrochemical reversibility beyond 1200 cycles without change in charge capacity. A slight lowering of charge capacity was noted for the undoped films after cycling. The charge capacity for the V doped film was 16.9 mC/cm2 compared to undoped film, 9.6 mC/cm2. Spectrophotometry showed that doped films tended to exhibit a higher absorbance in their colored state compared to undoped films. Considerable improvement in the lithium diffusivity was noted for all the doped films. The greatest change was a factor of 20x for vanadium doping. Doping appeared to increase the cyclic durability of all the tungsten films out to 1200 cycles. Color changes by doping were noted for several dopants. The doped films with the best overall properties were about 8% vanadium and titanium tungsten oxide. The optimum concentration lies in the range of 7 to 12 mol%. The electrochromic color was a neutral brownish-blue for vanadium and grayish-blue for titanium doped tungsten oxide.