Large-Area Chromogenics for Smart Windows
The concept of large-area chromogenic optical switching devices for glazings is introduced. Devices can be used for dynamic control of light through building, and vehicle glazings and certain information display applications. Also, chromogenic glazings can be used for the modulation of reflector surfaces for automotive mirrors. Both visible and solar control can be obtained from these devices. Switching ranges can be from as high as 80% to 5-10% transmittance in the visible region. A selection of optical switching technologies and their properties will be compared with emphasis on electrochromic and dispersed liquid crystal devices. The science of selected electrochromic devices is covered. The optical and electrochemical properties of tungsten oxide and nickel oxide devices are detailed. The general process of electrochromism is based on the reversible dual injection of ions and electrons which induce large optical changes due to both color center formation and electronic changes in the material. Longterm memory can be maintained for several hours without a holding potential. Technical issues concerning large-scale chromogenic devices are discussed.