Table 3
Diffusion coefficient of H " in WO3 coatings at room temperature [01]
tteat treatment (°C) Diffusion coefficient (cm2/s)
As deposited (amorphous) 57 × 10 ~1
350°C (crystalline) 2.4 × Ill ~
400°C (crystalline) 0.52 × l(/ i~
3.1.1. Effects of dopants on electrochromism in tungsten oxide
The dopant levels for electrochromic applications are usually high so that
mixed-cation oxides may be a more suitable way to describe the composition of the
films. These doped coatings have several advantages as illustrated by the examples
below. For coatings with multiple cations stoichiometry control is difficult and
expensive to process by physical vapor deposition but could be made by sol-gel
with little or no added cost. The examples cited in this section have not been made
by sol-gel, but this method could be used to exploit the effect of dopants even
further.
Table 4 shows the effect of doping tungsten oxide with molybdenum. The
results show that the coloring efficiency of the films increases considerably when
the dopant level is at 0.8 atomic percent. Coloring efficiency is the slope of the
straight line obtained by plotting the optical density on the y-axis and the charge
consumed along the x-axis. The change due to the doping primarily occurs because
the peak of the absorption spectrum after doping shifts from 1.3 eV (955 nm) to
about 1.7 eV (730 nm), resulting in a deeper color in the visible part of the
spectrum.
Polycrystalline films of cerium/tungsten oxide ((Cs20)0.44W206) with pyrochlore
crystalline structure were examined. This structure has "tunnels" that allow ions to
diffuse quickly through the coating. The films in the colored state were blue for
tungsten oxide and grey-blue for the pyrochlore. In addition, the diffusion coeffi-
cient of protons changed from 10-10 cmZ/s in the former to 7 x 10-9 cmZ/s in the
latter, due to the tunnel structure of the pyrochlore [64]. Pyrochlores with 3-D or
hexagonal channels have also been reported when Na, K, Rb, Ag and NH 4 are
used instead of cerium [65,66]. The tunnel network is also present in hex-
Table 4
Coloration efficiency (cm2/C) of mixed MocW 1 _cO3 films at various wavelengths (A) [63]
c A = 700 nm A = 640 nm A = 600 nm A = 520 nm
0 65 57 47 25
0.008 110 95 83 56
0.13 70 72 71 68
0.25 67 67 67 60
0.8 65 60 55 45
1 77 73 63 38