Progress in Oxide-Based Electrochromics: Towards Roll-to-Roll Manufacturing
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Progress in Oxide-Based Electrochromics: Towards Roll-to-Roll Manufacturing Claes G. Granqvist Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P. O. Box 534, SE-75121 Uppsala Sweden
ABSTRACT Electrochromics is a key “green” technology for massive energy savings in the built environment jointly with indoor comfort. This paper surveys basic electrochromic (EC) device designs, useful oxide materials and their nanostructures, and elements of a theoretical description of the EC phenomenon. It also outlines critical manufacturing technologies and their pros and cons. Focus is on EC foil technology, which is shown capable of mass fabrication via roll-to-roll web coating and continuous lamination. INTRODUCTION Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, under the action of a voltage [1, 2]. This functionality is of great interest for numerous applications, and device aspects rather than fundamental science have been the driving force to develop EC technology ever since the discovery of the phenomenon. Electrochromism was first found in tungsten oxide [3] and is now known to exist in numerous inorganic (mainly oxides) and organic materials; only the former group is discussed here. Applications in displays were first discussed for EC devices, and interest in this area remains strong. The uses of EC devices for energy efficient fenestration appeared in the 1980s [4, 5], and such windows are often referred to as “switchable” or “smart”. The relationship between energy efficient buildings and EC technology has grown over the years, and electrochromics is now considered one of the key “green nanotechnologies” for sustainability and energy in the built environment [6]. A number of concurrent factors are responsible for today’s large interest in EC windows. They include (i) the world wide concern for global warming, which is connected to CO2 emissions from energy production, (ii) the growing population in the world, and its increasing demands and aspirations, (iii) the fact that as much as 40% of the world’s primary energy is used in buildings, (iv) that the potential for energy savings in the built environment is huge [7], (v) that windows are “weak links” in the buildings’ energy systems, but they cannot be made small since this would have adverse effects on indoor comfort, (vi) that EC windows can lead to very substantial energy savings [8], and (vii) the fact that “green” building technology can bring financial advantages [9]. Oxide electrochromics, and its application to windows, has taken a long time to mature. One reason for this is that the technology is “difficult” in the sense that it requires mastering of a number of non-standard techniques, and that failure on one of leads to the demise of the entire technology. But now, after more than 25 years of sustained research and development, EC
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technology may finally be ready for implementation on windows [10]. Large-scale manufacturing is a key here, and this aspect is discussed
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