220 9 Optical Properties
material loses coloration. Electrochromic material changes color when connected
to a source of electricity. When not connected to an electrical field, their color is
white. To change color, in most cases only small electric charges are necessary.
Both photochromic and electrochromic materials have a huge economic potential
for applications. Most important applications are found, for example, in windows,
which are either self-darkening by light or that are controlled during darkening
with an electronic control system.
However, in technical applications, both types of materials suffer a few crucial
problems: They change color, which means they get a new distinct color, in most
cases blue or green. However, most of the customers in large-volume technical
applications prefer the change from white, respectively, colorless to gray or even
black. The second problem is the time constant of the color change. Generally, the
consumer wants an immediate reaction to a change of external conditions. This
may be the change in sunshine intensity in the case of photochromic windows or
just the necessity of changing the illumination level in rooms by the use of electrochromic materials. In view of environmental protection, both types of materials
have a high potential to save energy as they can regulate illumination levels, as
well as glare, heat gain or loss. Buildings supplied with these windows use less
energy for air conditioning, save money and reduce air pollution associated with
energy production. Further applications include large-scale electrochromic display
panels, front and rear windows, and mirrors for cars and trucks. In particular, the
latter applications need fast response of the material to changing conditions, which
is, until now, not available. For broader applications in electronic display systems,
materials or a combination of materials leading to red and yellow colors are
missing. However, besides all these problems, windows based on the photochromic and especially electrochromic effect are already commercially available.
Photochromic and electrochromic materials apply the fact that in a few oxides,
such as WO 3 or MoO 3 , the metal ion changes easily its valency. As long as these
oxides consist of ions with the valency 4
+ and 6
+ only, they are white, respectively,
colorless; In the case of the appearance of 5
+ ions, these oxides are colored. (Strictly
speaking: Photochromic and electrochromic devices use these oxides always in the
hypostoichiometic state, therefore, the exact formula is MeO 3−x . As the deviation
from ideal stoichiometry is small, for reasons of brevity, in the following text,
generally the shorter version MeO 3 is used.) The photochromic or electrochromic
properties of these oxides are related to electron–hole pairs directly connected to
deviations from the perfect stoichiometry. Both types of materials differ only in
the way how the change in stoichiometry is provoked.
9.9.2
Photochromic Materials
The best-known photochromic materials are WO 3 , MoO 3 and Nb 2 O 5 . Whereas
WO 3 and Nb 2 O 5 change from white to blue, MoO 3 changes from white to green.
The presence of water in the atmosphere around the photochromic device or at
the surface is a necessary prerequisite for the change of color. When photochromic
material loses coloration. Electrochromic material changes color when connected
to a source of electricity. When not connected to an electrical field, their color is
white. To change color, in most cases only small electric charges are necessary.
Both photochromic and electrochromic materials have a huge economic potential
for applications. Most important applications are found, for example, in windows,
which are either self-darkening by light or that are controlled during darkening
with an electronic control system.
However, in technical applications, both types of materials suffer a few crucial
problems: They change color, which means they get a new distinct color, in most
cases blue or green. However, most of the customers in large-volume technical
applications prefer the change from white, respectively, colorless to gray or even
black. The second problem is the time constant of the color change. Generally, the
consumer wants an immediate reaction to a change of external conditions. This
may be the change in sunshine intensity in the case of photochromic windows or
just the necessity of changing the illumination level in rooms by the use of electrochromic materials. In view of environmental protection, both types of materials
have a high potential to save energy as they can regulate illumination levels, as
well as glare, heat gain or loss. Buildings supplied with these windows use less
energy for air conditioning, save money and reduce air pollution associated with
energy production. Further applications include large-scale electrochromic display
panels, front and rear windows, and mirrors for cars and trucks. In particular, the
latter applications need fast response of the material to changing conditions, which
is, until now, not available. For broader applications in electronic display systems,
materials or a combination of materials leading to red and yellow colors are
missing. However, besides all these problems, windows based on the photochromic and especially electrochromic effect are already commercially available.
Photochromic and electrochromic materials apply the fact that in a few oxides,
such as WO 3 or MoO 3 , the metal ion changes easily its valency. As long as these
oxides consist of ions with the valency 4
+ and 6
+ only, they are white, respectively,
colorless; In the case of the appearance of 5
+ ions, these oxides are colored. (Strictly
speaking: Photochromic and electrochromic devices use these oxides always in the
hypostoichiometic state, therefore, the exact formula is MeO 3−x . As the deviation
from ideal stoichiometry is small, for reasons of brevity, in the following text,
generally the shorter version MeO 3 is used.) The photochromic or electrochromic
properties of these oxides are related to electron–hole pairs directly connected to
deviations from the perfect stoichiometry. Both types of materials differ only in
the way how the change in stoichiometry is provoked.
9.9.2
Photochromic Materials
The best-known photochromic materials are WO 3 , MoO 3 and Nb 2 O 5 . Whereas
WO 3 and Nb 2 O 5 change from white to blue, MoO 3 changes from white to green.
The presence of water in the atmosphere around the photochromic device or at
the surface is a necessary prerequisite for the change of color. When photochromic