Electrochromic devices change color and light transmissivity upon application of an electric charge. As such they should prove valuable as "smart" windows that can reduce air-conditioning costs by being darkened to absorb sunlight and reject unwanted solar heat. The biggest drawback is the cost of providing a power source and controls for the windows. NREL researchers have worked on various ways of self-powering electrochromic windows with solar cells or photovoltaic (PV) films (also available for licensing). A recent development is a new form of self-controlled and self-powered photoelectrochromic system that integrates the photovoltaic and electrochromic elements into a single device.
Recent advances in the field of photoelectrochemistry have produced technology for liquid-junction solar cells that could potentially compete with solid-state semiconductor PV technologies. Nanocrystalline titanium dioxide film, normally only responsive to ultraviolet light, is coated with a dye that both reduces corrosion of the TiO2 electrode and sensitizes it to a broad spectrum of visible light. In response to light, this dye injects electrons into the TiO2, replacing them with electrons from an iodine solution electrolyte. The iodine absorbs electrons at the counterelectrode, completing the electrical circuit and maintaining a stable chemical balance.
Seeing that this technology could be complementary to electrochromics, NREL researchers coated the counterelectrode with tungsten trioxide electrochromic film and added lithium (electrochromic darkening occurs when an electrical charge drives the small lithium cations into the lattice of the WO3) to the electrolyte solution. The result is a single-element device requiring no external power source or sensors that darkens when exposed to sunlight and clears when no longer sunlit. Unlike simple photochromic reactions, such as photogrey glasses, however, there are two separate processes involved, and both the photovoltaic and electrochromic processes can be optimized and adjusted. Also, because both darkening and clearing occur as a result of a complete electrical circuit, either state can be preserved by breaking the circuit with an external switch.
The TiO2 electrode and WO3 electrochromic film can be easily applied to commercially available tin-oxide-coated (electrically conductive) glass, so that NREL's photoelectrochromic technology should lend itself to inexpensive mass-production processes. With an electrolyte layer that is thin relative to the cell area, the electrochromic reaction is confined to areas that are directly illuminated. This makes it possible to use the technology for optical displays or optical switches as well as for smart windows. NREL is seeking industrial partners — most likely glass or thin-film manufacturers — to help perfect and commercialize this highly promising technology.
The Photoelectrochromic Cell turns from its bleached, largely transparent state (top) to its dark blue sunlight-blocking state within about a minute of exposure to bright light, and reverts to its bleached state within a few minutes after the light stops. Disconnection of the circuit with an external switch would allow, for example, the opaque state to be maintained for a day or the bleached