9.8
Photochromic and Electrochromic Materials
9.8.1
General Considerations
Photochromic materials change color reversibly as a function of light intensity. In the
dark, they are usually white or colorless, whereas in sunlight or UV radiation they
change color. In most cases, the intensity of coloration is a function of the light
intensity and, after removal of the light source, the material loses its coloration.
Electrochromic materials change color when connected to a source of electricity; to
change color, in most cases, only small electrical charges are needed.
Both photochromic and electrochromic materials, may have some economic
potential for applications. However, from a technical viewpoint they suffer from
one crucial problem, namely that when they do change color the new color is
distinct. Yet, in many large-volume technical applications a change from white (or
colorless) to gray or even black would be adequate, and even preferred. A typical
example of such an application would be that of sun-protecting windows, which are
already on the market.
A second problem is the time constant of the color change. Ideally, the consumer
requires an immediate reaction to a change of external conditions, whether to
sunshine intensity for photochromic windows or simply to change illumination
levels in rooms by the use of electrochromic materials. Both types of material also
have great potential to save energy as they can regulate not only illumination levels,
but also glare and heat gain or loss. Windows with incorporated photochromic or
electrochromic coated glasses save energy by keeping the heat out as they gradually
darken when the sun rises. However, in the morning and evening, the windows
should remain transparent while the sun is low in the sky. Hence, buildings fitted
with these windows use less energy for air conditioning, and consequently save
money and reduce air pollution associated with energy consumption. Further
applications include large-scale electrochromic display panels, and front/rear windows and mirrors for cars and trucks, although the latter applications require the
material to respond rapidly to changing conditions. An additional problem is that,
for broader applications in electronic display systems, materials (or combinations of
materials) that produce red and yellow colors are, at present, unavailable.
9.8.2
Photochromic Materials
The best-known photochromic materials are WO 3 , MoO 3 , and Nb 2 O 5 . WO 3 and
Nb 2 O 5 change from white to blue, whereas MoO 3 changes from white to green. An
explanation for the described phenomena is found in the ability of these oxides to
change stoichiometry. Considering the examples of WO 3 and MoO 3 , it is known that
in the colorless, bleached state, the metal ions are in the valency states 6
þ and 4
þ
,
while in the colored state ions of valency 5
þ are present. (Strictly speaking,
9.8 Photochromic and Electrochromic Materials j257
Photochromic and Electrochromic Materials
9.8.1
General Considerations
Photochromic materials change color reversibly as a function of light intensity. In the
dark, they are usually white or colorless, whereas in sunlight or UV radiation they
change color. In most cases, the intensity of coloration is a function of the light
intensity and, after removal of the light source, the material loses its coloration.
Electrochromic materials change color when connected to a source of electricity; to
change color, in most cases, only small electrical charges are needed.
Both photochromic and electrochromic materials, may have some economic
potential for applications. However, from a technical viewpoint they suffer from
one crucial problem, namely that when they do change color the new color is
distinct. Yet, in many large-volume technical applications a change from white (or
colorless) to gray or even black would be adequate, and even preferred. A typical
example of such an application would be that of sun-protecting windows, which are
already on the market.
A second problem is the time constant of the color change. Ideally, the consumer
requires an immediate reaction to a change of external conditions, whether to
sunshine intensity for photochromic windows or simply to change illumination
levels in rooms by the use of electrochromic materials. Both types of material also
have great potential to save energy as they can regulate not only illumination levels,
but also glare and heat gain or loss. Windows with incorporated photochromic or
electrochromic coated glasses save energy by keeping the heat out as they gradually
darken when the sun rises. However, in the morning and evening, the windows
should remain transparent while the sun is low in the sky. Hence, buildings fitted
with these windows use less energy for air conditioning, and consequently save
money and reduce air pollution associated with energy consumption. Further
applications include large-scale electrochromic display panels, and front/rear windows and mirrors for cars and trucks, although the latter applications require the
material to respond rapidly to changing conditions. An additional problem is that,
for broader applications in electronic display systems, materials (or combinations of
materials) that produce red and yellow colors are, at present, unavailable.
9.8.2
Photochromic Materials
The best-known photochromic materials are WO 3 , MoO 3 , and Nb 2 O 5 . WO 3 and
Nb 2 O 5 change from white to blue, whereas MoO 3 changes from white to green. An
explanation for the described phenomena is found in the ability of these oxides to
change stoichiometry. Considering the examples of WO 3 and MoO 3 , it is known that
in the colorless, bleached state, the metal ions are in the valency states 6
þ and 4
þ
,
while in the colored state ions of valency 5
þ are present. (Strictly speaking,
9.8 Photochromic and Electrochromic Materials j257