An electrochromic device (ECD) controls optical properties such as optical transmission, absorption, reflectance and/or emittance in a continual but reversible manner on application of voltage (electrochromism). This property enables an ECD to be used for applications like smart glass, electrochromic mirrors, and electrochromic display devices.
The history of coloration goes back to 1704 when Diesbach discovered Prussian blue (hexacyanoferrate), which changes the color from transparent to blue under oxidation of iron. In the 1930s, Kobosew and Nekrassow first noted electrochemical coloration in bulk tungsten oxide. While working at Balzers in Lichtenstein, T. Kraus provided a detailed description of electrochemical coloration in thin film of tungsten trioxide (WO3) on 30 July 1953. In 1969, S. K. Deb demonstrated electrochromic coloration in WO3 thin films. Deb observed electrochromic color by applying electric field of the order of 104 Vcm−1 across WO3 thin film. In fact, the real birth of the EC technology is usually attributed to S. K. Deb’s seminal paper of 1973, wherein he described the coloration mechanism in WO3. The electrochromism occurs due to the electrochemical redox reactions that take place in electrochromic materials. Various types of materials and structures can be used to construct electrochromic devices, depending on the specific applications.
Electrochromic (sometimes called electrochromatic) devices are one kind of electrochromic cells. The basic structure of ECD consists of two EC layers separated by an electrolytic layer. The ECD works on an external voltage, for which the conducting electrodes are used on the either side of both EC layers. Electrochromic devices can be categorized in two types depending upon the kind of electrolyte used viz. laminated ECD are the one in which liquid gel is used while in solid electrolyte EC devices solid inorganic or organic material is used. The basic structure of electrochromic device embodies five superimposed layers on one substrate or positioned between two substrates in a laminated configuration. In this structure there are three principally different kinds of layered materials in the ECD: The EC layer and ion-storage layer conduct ions and electrons and belong to the class of mixed conductors. The electrolyte is a pure ion conductor and separates the two EC layers. The transparent conductors are pure electron conductors. Optical absorption occurs when electrons move into the EC layers from the transparent conductors along with charge balancing ions entering from the electrolyte.
Laminated electrochromic devices are the ones in which a liquid gel is used as the electrolyte.
Mode of operation
Typically, ECD are of two types depending on the modes of device operation, namely the transmission mode and reflectance mode. In the transmission mode, the conducting electrodes are transparent and control the light intensity passing through them; this mode is used in smart-window applications. In the reflectance mode, one of the transparent conducting electrodes (TCE) is replaced with a reflective surface like aluminum, gold or silver, which controls the reflective light intensity; this mode is useful in rear-view mirrors of cars and EC display devices.
Electrochromic windows, also known as smart windows, are a new technological arrangement for achieving energy efficiency in buildings, with variable transmittance of light and solar energy. These ‘‘smart windows’’ can automatically control the amount of light and solar energy passing through the windows which subsequently improves indoor comfort; for example, electrochromic glass provides better glare resistance than fritted glass in most direct sunlight applications. The efficiency of these windows will vary depending on their placement, size, and local climate conditions since these factors influence the amount of sunlight that comes in contact with these windows. Electrochromic windows generally achieve their control over light and heat through their layered design. These layers within the window allow for tinting of the glass in response to increases in incoming sunlight as well as protection against UV radiation. An example of this layered design is the electrochromic glass developed by Gesimat, where multiple layers of material (i.e. tungsten oxide, polyvinyl butyral, and Prussian Blue) are sandwiched by two dual layers of glass and fluorine-doped tin oxide-coated glass. Together, the tungsten oxide and Prussian Blue layers form complementary electrochromic layers; essentially, this means they form the positive and negative ends of a battery using the incoming solar energy. The polyvinyl butyral (PVB) forms the central layer in this configuration, and it serves as a polymer electrolyte (this allows for the flow of ions which, in turn, generates a current).
Smart windows are currently being used in an increasing number of buildings and vehicles. SAGE Electrochromics developed SageGlass, which is a glass that can tint on-demand in order to reduce glare and incoming heat. Owing to its energy efficiency and ability to scale to large window sizes, SageGlass has found an application in many commercial buildings employing a modernist architectural style. The Boeing Company provides electrochromic windows for the passenger cabin of the all-new 787 Dreamliner, which will allow passengers to electronically shade their windows, replacing the plastic screens found on today's airplanes. The Ferrari 575M Superamerica is also the very first production car to adopt an innovative rotating roof incorporating electrochromic technology developed exclusively for large glass surfaces, which transforms it from a coupe to a convertible in just a few seconds.
Electrochromic reflecting surfaces are employed as self darkening mirrors that regulate reflections of flashing light from following vehicles at night so that a driver can see them without discomfort.
Gentex first introduced a rear-view mirror that automatically adjusted to glare conditions in 1982. These mirrors have forward- and rearward-facing sensors to measure the ambient light level and glare of approaching vehicles, respectively.
Electrochromic display operating in either reflecting or transmitting mode. Advantages of EC display is Low power consumption and cheap.
However, there are various other display applications where ECD can be used viz. electrochromic paper, electrochromic goggles, motorcycle helmet-visors. Electrochromic paper, which on touching it with a stylus electrode can create an image, has been prepared by incorporating ECs in paper. Furthermore, visors of helmets and goggles can be colored to a chosen degree at day times and bleached during the night.
Electrochromic device manufacturers
- SAGE Electrochromics
- View, Inc.
- PPG Aerospace a business unit of PPG Industries
- prelonic technologies
- S. K. Deb, Appl. Opt., 8(S1) (1969) 192
- S. K. Deb, Phil. Mag., 27 (1973) 801
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- "PPG Selected to Provide Dimmable Windows for Boeing 787"