Electrochromics: finally a technology for large-scale applications?

Windows that can change their throughput of visible light and solar energy have long been a dream of architects and automobile manufacturers. Such windows can provide comfort and energy efficiency: simultaneously! The materials that enable the variable transmittance are often referred to as ‘chromogenic’ (a term introduced in a book¹ published by SPIE in 1990). The optical change is induced by different factors in different materials: ultraviolet irradiation in photochromic materials, a temperature change in thermochromic materials, exposure to a reducing or oxidizing gas in gasochromic materials, an electrical voltage in electrochromic (EC) materials, etc. The EC materials² allow easy user control and have the widest range of application. They can be used not only for ‘smart windows’ in buildings and cars, but also in information displays, eyewear, and in many other applications.

What is an EC device? Figure 1 shows a five-layer structure on a transparent substrate (or between two such substrates).² The center layer is an electrolyte, usually a polymer layer or a thin film of a hydrous oxide. On one side is a thin film of an EC layer, typically an oxide (with WO₃ being widely used) or a suitable organic film. The other side of the electrolyte has a thin film serving as ion storage, with or without electrochromism. Again one can use an oxide (with NiO or IrO₂ being two candidates with particularly good properties) or an organic film. Thin films of a transparent and electrically-conducting material (such as Sn-doped In₂O₃, often referred to as ITO) are on the two sides of the central three-layer stack. Applying a voltage of only 1–2V dc to the ITO films moves charge into or out of the EC film, for which the optical absorption is then changed. If everything else in the device is transparent, or if the ion storage darkens at the same time as the EC film does, then the transparency of the whole device will change. The device has open circuit memory, i.e., the voltage needs to be applied only when the optical properties are to be altered. The change takes place, typically, in a few seconds in a cm²-sized device and minutes for those measured in square meters.

Buildings with ‘smart windows’ based on electrochromism have been tried by the glass and coatings industry for decades, and results have been reported, for example, by Pilkington in the UK, Saint Gobain in France, Asahi in Japan, and OCLI in the US. For one reason or another, these ‘smart windows’ did not make it to the market. Today, however, the scene seems to be changing and SAGE Electrochromics³ in the US are selling EC roof windows. Another hot development is by Saint Gobain Sekurit in Herzogenrath, Germany, who make EC car roofs⁴ on a limited commercial scale: for instance, the new Ferrari Super America, a model limited to 550 units, will have this feature. With these roofs one can combine the feeling of space afforded by a glass with the protection from thermal discomfort caused by too much solar irradiation. The cost of these roofs may be prohibitive for all but a small group of wealthy enthusiasts, but new EC materials and manufacturing techniques may turn them into common products in the near future.

All of the EC devices mentioned above make use of glass substrates, which implies that large substrates and expensive coating units must be used. But cheap production of EC devices is definitely necessary if the technology is to be used other than in niche markets. An alternative route, using web coating via roll-to-roll manufacturing, has now been pioneered by a spin-off company from Uppsala University in Sweden, ChromoGenics Sweden AB. This company, which collaborates with DuPont Ventures and Volvo Technology Transfer, is currently setting up a pilot plant for polyester-based EC foil. The foil uses the design in Figure 1 with WO₃ as the EC film and a NiO-based film as ion storage.⁵ The centrally-positioned electrolyte also serves as a lamination material. Using a polymer-based foil, rather than glass, opens new applications for EC devices. One of them is illustrated in Figure 2: a visor with variable transmittance gives a biker the comfort provided by a dark visor in sunlight and the safety provided by a clear visor in the dark.⁶Figure 3 shows how the visor colors and bleaches: the visual experience is that the change takes place in a few seconds. The clear-state transmittance was deliberately set to 50%, but new EC materials⁷ can give more than 80% transmittance.

What about the future of electrochromics? Consider first what is between the cold emptiness of outer space and your own warm heart. How many skins do you have for protection? More than you think! We have first the earth's atmosphere, limiting the temperature range we are exposed to some -50 to + 50°C. Then we use buildings to give us a livable ambience, and we have clothes. Finally, of course we have our human skin. This makes four skins all together. But perhaps we can make good use of a fifth skin to meet the challenges of energy efficient, yet comfortable, housing in the future. These possibilities may arise by merging membrane architecture⁸ with EC foil technology to place light-weight houses with little embedded energy inside huge membranes that allow the flow of visible light and solar energy to be controlled and optimized. The possibilities offered by such membranes (although based on conventional glass technology) were in fact pointed out more than 50 years ago by the great visionary Buckminster Fuller.⁹ Perhaps this vision will come true, thanks to electrochromics.