Photoelectrochromic smart windows

Electrochromic films have been combined with photoelectrochemical electrodes to make a self-powered smart window. These ''photoelectrochromic'' smart windows combine the advantages of photochromic films, namely that they are self-powered by the incident light, with the advantages of electrochromic windows, such as the ability to control the state of coloration externally when desired. When the windows are short-circuited, the observable process is photochromism, but the mechanism is unique and has several potential advantages over conventional photochromic films. The light absorbing process is physically separate from the coloration process, allowing each to be individually optimized. The materials constraints are greatly relaxed compared to single-component photochromic films in which one material must meet all criteria (color change, switching speed, photostability, etc.). Furthermore, since the coloration process in a PEC cell requires an external electrical current between the two electrodes, a particular state (transparent, absorbing, or imaged) can either be stored when the electrodes are at open circuit, or can be changed when the electrodes are connected. The light-absorbing function in the PEC cell is performed by a dye-sensitized semiconductor electrode that produces a photovoltage sufficient to color the electrochromic film deposited on the counterelectrode. We describe the dye sensitization process, its advantages over conventional photovoltaic devices in applications such as smart windows, and recent developments in photoelectrochromic smart windows.