Electrochromics: Finally a Technology for Large-Scale Appli-cations?

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 book 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 materials 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). 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 Sndoped 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 Figure 1. Shown is the EC device design and how the ions move under an externally-applied electric field.