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Catching the Sun

May 1, 2006

No matter what type of technology they use, all electronic musicians depend on one thing: electric power. That power comes mostly from burning fossil fuels, but environmental issues aside, the supply will eventually run out. Until then, it will continue to increase in cost until it becomes prohibitively expensive. Nuclear power is also a common power source, but its critics cite lack of safety and negative impact on the environment as potential risks.

FIG. 1: Titanium dioxide nanotubes could dramatically decrease the energy cost of fabricating solar cells, and their efficiency will increase if the nanotubes can be grown longer than the 360 nm structures shown here.

Among the alternatives are solar cells, which convert sunlight directly into electricity with no polluting by-products. Solar cells have been around for decades, but the high energy cost of manufacturing has kept them from becoming widespread. By many estimates, it takes 2 to 5 gigajoules per square meter to make silicon-based solar cells, which is arguably more energy than they can ever generate. (One joule is defined as the work done when one ampere of electric current passes through a resistance of one ohm for one second. It is also equivalent to 0.2389 calories.)

New research that is currently being conducted by a team at Pennsylvania State University ( under the direction of Craig Grimes, professor of electrical engineering and materials science and engineering, could decrease that energy cost dramatically. Up until now, solar cells have been fabricated from silicon-based semiconductor material. A more recent approach uses nanoparticles and photosensitive dyes. However, Grimes notes, “Nanoparticle solar cells are the gold standard of this new approach, but because of certain limitations, it appears they have gotten as good as they are going to get.” So he and his team decided to modify the idea by using highly ordered arrays of nanotubes instead of particles to act as a substrate for the dye.

The fabrication process begins with a piece of glass that has first been coated with fluorine-doped tin oxide, which is then followed by a layer of titanium that is 500 nanometers thick. Placing the layer in an acidic bath undergoing a mild electric current causes parallel nanotubes of titanium dioxide to form and grow to a length of about 360 nm. The tubes are then heated in an oxygen-rich environment to crystallize them. The process transforms the opaque titanium coating into a highly ordered array of transparent titanium dioxide nanotubes (see Fig. 1). Finally, the nanotube array is coated with a commercially available photosensitive dye, forming the negative electrode of the cell.

When light strikes the dye, electrons are knocked free of its molecules. Many of these electrons recombine before passing out of the cell. But the tube structure of the titanium dioxide allows ten times more electrons to make it out than with particulate coatings. Initial experiments have yielded an efficiency of only 3 percent because of the length of the nanotubes. (Silicon solar cells are around 21 percent efficient at converting sunlight into electricity.) Grimes expects to do much better when they are able to apply a thicker coat of titanium and thus grow longer tubes, which would allow more electrons to escape.

“There is still a great deal of optimization of the design that needs to be done,” says Grimes. “If we get about 3 percent conversion with 360 nanometers, what we could get with 4 microns [4,000 nm] is an exciting question we hope to answer soon. I think we can reach a 15 percent conversion rate with these cells, using a relatively easy fabrication system that is commercially viable.” Grimes expects to decrease the energy required for fabrication by a factor of at least 100, compared with silicon solar cells.

The new solar-cell technology could be a big boon for electronic musicians, especially those who take their equipment into the field, where there are few if any electrical outlets. Even homebound musicians could benefit from reduced dependence on the power company. Commercialization of this technology is still years away, but the future looks bright.

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