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.1 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.2–7
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 higher-maximum 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.8
A relatively thick, solid polymer electrolyte also has advantages over extremely thin inorganic solid electrolytes in all-ceramic EC devices.9 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 (about 800nm thick) for cathodic coloring, and Prussian blue (about 500nm thick) for anodic coloring. Both layers are applied using cathodic electrodeposition from aqueous solutions. Currently, our laboratories can homogenously coat areas up to 1.2m × 0.8m.
The ion-conducting polymer interlayer (0.76mm thick) is produced by the extrusion of a mixture of PVB and plasticizer, a salt that contains Li ions, and some common polymer additives. Then, the two TCO-coated glass panes with tungsten oxide and Prussian-blue overlayers are laminated together via the ion-conducting PVB sheet. This is performed under elevated temperature and pressure.
The dynamic change in the transmission spectra of the device during switching is shown by the colored and bleached states described in Figures 2. and 3.. The change in visible light transmittance can range from about 8% (colored) to 77% (bleached). Solar transmittance can be changed from 6% to 56%. Each intermediate state between the fully colored and bleached state can be adjusted.
Switching, which alters reflectance of the electrochromic device only slightly, is performed by applying voltages between about 0.5 and 2.5V DC. With tungsten oxide on the negative terminal and Prussian blue on the positive terminal, the device is colored blue: by changing polarity, the device bleaches (becomes transparent).
Electrical energy is required only during switching, not for maintaining the color state. If the voltage is turned off while switching, the glass retains the color attained. This is due to the battery-like behavior of the EC device.10 Energy consumption for one full coloring cycle, or for minimum to maximum bleaching, is approximately 200W/m2.
Electrochromic glass produced by this process has passed harsh durability tests, including 40,000 switching cycles at room temperature, simulating a 20-year lifetime with five cycles per day, temperature-related tests, and standard trials for laminated safety glass. It is now ready for pilot production.
Part of this work was funded by the German Ministry of Economics and Labour, Contract no. 0327233F.
