Flexible solid-state photoelectrochromic windows

Photoelectrochromic "smart" window technology is extended to include the use of flexible substrates and solid-state electrolytes. This should facilitate their application as retrofit modifications of office windows, where, by blocking incoming solar irradiation, they could substantially lower air-conditioning costs. These devices are based on a dye-sensitized TiO2 electrode coupled with a 500 nm thick WO3 electrochromic counter electrode, separated by a cross-linked polymer electrolyte containing LiI. A novel method for preparing conducting nanoporous TiO2 films is described that allows for the construction of these devices on flexible organic substrates. Colloidal solutions of TiO2 free of surfactants were spin-coated onto indium-tin oxide coated polyester substrates, resulting in highly transparent films ranging from 100 nm to 1 pm in thickness. Upon annealing at 100 degrees C, these films were strongly adherent and displayed excellent photoconductivity as shown by their current-voltage characteristics. The devices typically transmit 75% of visible light in the bleached state. After a few minutes of exposure to white light (75 mW/cm(2)), the windows turn dark blue, transmitting only 30% of visible light. They spontaneously bleach back to their initial noncolored state upon removal of the light source. (C) 1999 The Electrochemical Society. S0013-4651(99)99-03-065-7. All rights reserved.