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Let the Sun Shine In

March 30, 2010
by Steven Bodzin

Tiny conductors could lead to high-energy solar power

After years of research and development, chemistry doctoral students Matt Law and Lori Greene shined a light on a cluster of 1×2-centimeter red squares, and, almost immediately, produced electricity. The jolt wasn’t much at first, but after continuous efforts, they coaxed the little chips into producing an electrical current equivalent to 1.5 percent of the solar energy hitting them. That’s only one-tenth the power they could have gotten from high-efficiency commercial solar panels at the hardware store. But what’s important is that these tiny panels were composed of nanomaterials, made primarily of zinc oxide, the same material found in sun block. What’s more, these panels produced 100 times more power than any previous solar cells made with nanomaterials.

The innovative solar panels are just one of associate chemistry professor Peidong Yang’s nanowire-related projects, but he is especially excited about the prospect of what he believes will be “low-cost, environmentally benign energy conversion with reasonable efficiency.” Such materials, he says, could eventually be used for solar power-capturing voltaic paint. The nanowires themselves form a matte white surface thinner than a sheet of paper.

Sandwiched between conductive polymers and soaked in red dye, the parallel nanowires absorb sunlight, energizing electrons that are then conducted down the wires to metal plates, creating a negative charge. The holes they leave behind migrate upward to a clear conductor at the top of the cluster. Nanowires are highly conductive, eliminating the bottleneck of forcing electrons to hop around. That increases the cells’ efficiency more than a hundredfold.

Previous work with nanowires was stymied by the difficulty of growing crystal wires that were long, thin, and parallel to one another. Greene developed a method using a polymer that bonds to the sides of the crystal but not to the tips, allowing the growing crystals to remain thin. The lab has grown crystals as long as 30 microns but no more than 200 nanometers across—as much as 200 times as long as they are wide, comparable to a half-inch long human hair.

Nanowires grow into what looks like a grassy lawn, except that 35 billion of them fit on a single square centimeter of treated glass. This gives them plenty of surface area to absorb sunlight. Because the wires are made of nontoxic, cheap materials and can be fabricated at room temperature, they could meet Yang’s goal of providing a cheap, plentiful, environmentally benign energy source. Hurdles remain, though. The clear conductor on which the wires grow contains expensive indium and tin. Yang says as researchers improve clear conductors, prices will drop. He says commercialization is still several years off. For now, though, the lab is experimenting with titanium and silicon nanowires in search of ever-higher efficiencies.

From the January February 2006 Chinafornia issue of California.

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