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Environmental

Using silver nanoparticles in thin-film solar cells greatly increases their efficiency, and allows them to capture infrared energy

Prior posts have noted that there is a lot of infra-red energy in the energy spectrum, but that it is not powerful enough to knock electrons free. Traditional solar cells work because visible and ultraviolet light knock electrons free from atoms, and the electrons form a useful direct current. Prior posts have noted that one approach being researched to make use of infra-red energy relies on the ability of infra-red energy to cause atoms to vibrate; the current impediment to this approach is that there is not in existence a rectifier that could capture this form of vibrational energy and turn it into current. Two researchers have taken a different approach. First, they start with thin-films (1-2 microns of silicon in depth rather than the standard 200-300 microns), which if used in the standard manner are less efficient than the standard cell; though less expensive (less silicon, a key cost component), thin films produce 20% less electricity per unit of area, which off-sets to some significant degree their cost advantage. Now the clever part. The researchers add nanoparticles of silver to the thin-film silicon. When struck by a broad range of "light", including infra-red energy, the electrons in an atom of silver vibrate in a way that causes them to radiate small amounts of light themselves known as a surface plasmon (see http://en.wikipedia.org/wiki/Plasmon). The plasmon runs parallel to the surface of the material that is propagating it, rather than penetrating this material. By travelling horizontally, a plasmon passes through more of the solar cell's silicon than any incident beam from the sun could; in effect, the cell has been turned on its side and made much thicker. Such travel gives the plasmon an opportunity to be absorbed and thereby converted into electricity. The system increases the absorption of infrared "light" tenfold, bringing the efficiency of thin cells much closer to that of the traditional solar cells. Of course, silver increases the cost, but so little is used that the new technique would add only a few cents to the price of a solar panel. The new approach is discussed by the researchers at http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-26-21793.