Electrochromic safety glass for smart-window applications

SPIE Newsroom recently carried a brief overview of the current status and underlying science of electrochromics by Granqvist. Although research in this area dates back to the 1960s, no reliable large-area electrochromic (EC) product for smart window applications has been brought to market. This is mainly due to issues involving cost, performance, and the stability of prospective devices and production methods. We have developed a relatively low-cost process that can be used to manufacture a device that provides both high switching range and long-term stability. It is based on the use of two complementary inorganic EC layers prepared by electrodeposition, together with polyvinyl butyral (PVB) as an ion-conducting polymer electrolyte interlayer. Inorganic EC materials are inherently more stable than organic ones. The use of two complementary layers instead of combining one layer with so-called ion-storage film makes it possible to switch between highermaximum and lower-minimum transmittance with enhanced coloration efficiency. The glass industry has employed PVB to produce laminated safety glass for about 60 years. Its unique properties include adjustable adhesion, high impact strength, resistance to light and temperature, and excellent optical transparency, toughness and flexibility. By taking the same techniques used to produce conventional PVB layers and laminated safety glass, and applying them to electrochromic materials, similar physical properties can be achieved. A relatively thick, solid polymer electrolyte also has advantages over extremely thin inorganic solid electrolytes in allceramic EC devices. The absence of self-discharge due to current leakage, a common issue with thin inorganic electrolytes, is a major benefit. The structure of Gesimat’s laminated EC glass is shown in Figure 1. Two sheets of transparent conducting oxide (TCO) glass are coated with complementary layers of tungsten oxide Figure 1. The Gesimat electrochromic device has a multilayer structure.