Oxide-Based Electrochromics: Advances in Materials and Devices
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Oxide-Based Electrochromics: Advances in Materials and Devices C. G. Granqvist, İ. Bayrak Pehlivan, S. V. Green, P. C. Lansåker and G. A. Niklasson Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P. O. Box 534, SE-75121 Uppsala, Sweden
ABSTRACT Electrochromic (EC) devices are able to vary their throughput of visible light and solar energy by the application of a voltage. They are of much interest for “smart” windows in buildings and are able to create energy efficiency, occupant well being, and security. This paper gives a survey over oxide-based EC device technology and also presents some recent advances regarding EC thin films of mixed metal oxides, nanoparticle-containing electrolytes to join these films, and metal-based transparent electrical conductors needed to apply the voltage. INTRODUCTION Electrochromic (EC) devices consist of a number of superimposed layers and are able to change their optical properties—reversibly and persistently—under the action of a voltage [1, 2]. They show many analogies to electrical batteries, and the degree of optical absorption depends on the level of charging. This optical functionality is interesting for a number of applications; the largest one, in terms of surface area and societal importance, is in “smart” architectural windows that are able to impart energy efficiency, occupant well being, and security. Today EC technology stands out as one of the key green nanotechnologies for sustainability and energy in the built environment [3]. EC technology was introduced for display applications in the late 1960s and early 1970s [4]. The potential of the technology for energy efficient buildings was realized in the 1980s [5, 6], and this application has been instrumental for EC development during many years especially with regard to oxide-based technology. The energy savings can be huge, and it was stated recently that highly insulated “smart” windows applied in the residential and commercial sectors in the USA could diminish the annual energy expenditure by as much as 4.5 % [7]. Interestingly, display applications may be returning in the context of paper-like displays and printed electronics [8], but then the devices typically utilize organic EC materials rather than the more rugged inorganic and oxide-based technology of most interest for fenestration and discussed below. OXIDE ELECTROCHROMICS: A PRIMER Operating principles Figure 1 illustrates an EC device and shows five superimposed layers on a single transparent substrate or positioned between two such substrates [1]. The layered materials in the
EC device are of three kinds: (i) the electrolyte is a pure ion conductor and separates the two EC films (or a single EC film from an optically inactive ion storage film), (ii) the EC films are mixed conductors of both ions and electrons, and (iii) the transparent conductors conduct nothing but electrons. The optical functionality is caused by the EC film(s), which alter their optical absorption upon ion insertion / extraction from the electrolyte jo
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