WINDOWS could soon be providing us with power, as well as letting in light. A team of chemists led by Michael Gr盲tzel at the Swiss Federal Institute of Technology in Lausanne, have made a breakthrough in their pioneering design of a transparent solar cell so that it now converts 10 per cent of the energy in sunlight into electricity.
Although it cannot yet match the 17 per cent efficiency of the latest commercial solar panels, which are made from silicon-based cells, Gr盲tzel argues that his version will cost only a tenth as much.
Both types of panel rely on the photoelectric effect. The current they produce is created when electrons bound within atoms are liberated by incoming photons of light. In conventional solar cells, this process takes place within silicon structures; Gr盲tzel鈥檚 panel, however, relies on the combination of a number of different substances trapped between two panes of glass.
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The inside surfaces of both panes are coated with a film of tin oxide, which convey liberated electrons to and from the external circuitry. One of these 鈥渃ontacts鈥� is coated with a film of titanium dioxide, and on top of this is another film, this time of a photosensitiser chemical 鈥� a substance that exhibits the phoroelectric effect. 鈥淭he photosensitiser harvests sunlight and the titanium dioxide harvests electrons,鈥� says Gr盲tzeI.
All the films are so thin that they are transparent. The remaining space between the panes is filled with an iodine-based electrolyte. The iodine ions in the solution ferry the electrons across to complete the cell鈥檚 internal circuit (see Diagram).
Gr盲tzel and his colleagues have already produced a window that is 8 per cent efficient (Technology, 26 October 1991). They have now increased this figure still higher by using a new chemical 鈥渁nchor鈥�, which binds the photosensitiser to the titanium dioxide film about a thousand times more strongly than before. With weaker anchors, any air or water that seeps between the panes can destroy the electrical contact between the layers. The new anchor is also versatile, increasing the number of photo-sensitiser chemicals that the researchers can choose from.
At one end of the anchor, a phosphonate group binds to the titanium dioxide film. The other end consists of rings of carbon and nitrogen atoms that bind to almost any transition metal complex 鈥� some of which are photosensitisers. 鈥淓ffectively, the photosensitiser is irreversibly adsorbed on the titanium dioxide,鈥� says Gr盲tzel.
The researchers have now tested a photosensitiser that contains a ruthenium complex. This releases electrons in response to photons of light at most visible wavelengths. To increase the efficiency further, the team is now looking for complexes that respond to infrared radiation as well. According to Gr盲tzel, infrared radiation 鈥渁ccounts for almost half the radiation reaching the Earth鈥檚 surface鈥�.
Bob Hill, director of the Newcastle Photovoltaics Applications Centre at the University of Northumbria, says Gr盲tzel鈥檚 device is a 鈥渟ignificant development鈥�, but argues that it cannot yet compete with its silicon-based cousins because of its lower efficiency.
Gr盲tzel believes that because his cell is much cheaper it could hold huge potential for poorer countries with sunny climates. Small versions of the panels have already been built into small devices, to create solar-powered watches, for example. But Gr盲tzel says that because they are transparent, they should be able to double up as windows.
The latest breakthrough by Gr盲tzel and his colleagues is published in this month鈥檚 edition of the Journal of the Chemical Society, Chemical Communications, p 65.
