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Focusing Light on Silicon Beads

Placing tiny spheres of silicon in reflective trays could be the key to cheap, efficient solar cells.
November 13, 2007

A company in Japan has developed a novel way of making solar cells that cuts production costs by as much as 50 percent. The photovoltaic (PV) cells are made up of arrays of thousands of tiny silicon spheres surrounded by hexagonal reflectors.

Silicon spheres: Making solar cells out of tiny spheres of silicon (top image, black) can reduce the amount of the material used to just one-fifth. Hexagonal reflectors (middle image) ensure that most of the light hits the spheres. A flexible foil base means that the solar cells can be shaped for different applications (bottom image).

The key advantage of the system is that it reduces the total amount of silicon required, says Mikio Murozono, president of Clean Venture 21 (CV21), based in Kyoto, Japan. “We use one-fifth of the raw silicon material compared with traditional PV cells,” he says.

This can make a huge difference to the overall cost of producing solar cells, says Howard Branz, principal scientist at the National Renewable Energy Laboratory’s National Center for Photovoltaics, in Golden, CO. “About 20 to 30 percent of the cost of a solar-cell module is in the cost of the raw silicon,” he says.

CV21 started production of its cells in October; the first of its 10-kilowatt modules go on sale this month. While these modules will initially cost about the same as the traditional variety, the price is set to drop by 30 percent in 2008, as production increases in May from 1,000 cells a day to 60,000 cells a day, says Murozono. The ultimate goal is to make them 50 percent cheaper than existing cells by 2010, he says.

Spherical solar cells were originally proposed by Texas Instruments about 30 years ago, says Branz. But while they had the potential to reduce the amount of silicon used, they brought with them a host of new problems. Their curved surfaces, for example, can cause more light to be reflected, which reduces their efficiency. What’s more, only half of the sphere ends up actually being exposed to light. Significant gaps also tend to form between the spheres when arranged in arrays, which can further reduce the efficiency of the solar cell.

CV21’s solution was to place each of the one-millimeter-diameter silicon spheres in its own hexagonal aluminium reflector. These work like car headlights but in reverse, ensuring that any light hitting the reflector is directed toward the sphere. When this approach is used, even the underside of the sphere is utilized. The hexagonal shape of the reflectors allows them to be slotted together without dead space between them. “Effectively, these are mini-concentrators,” says Branz.

The spheres themselves consist of a positively doped (p-type) ball of silicon. The ball’s surface is treated to make it negatively doped (n-type), and an antireflective coating is also added. These two outer layers form the basis of the photovoltaic semiconductor material. The spheres are then bonded to an electrode on a flexible foil substrate via a hole at the bottom of the reflector.

The flexible foil base means that the modules can be shaped for different applications. “You could install them on top of hybrid electric cars and curved tiles on rooftops,” says Murozono.

“I think the technology works,” says Branz. But the question is whether the company can make the solar cells more efficient. The cells currently being produced have efficiencies of only about 10 percent, he says. “Right now, most solar cells are 14 to 15 percent efficient.”

Reducing the cost by 30 percent does not help if 30 percent more cells are needed to produce the same amount of electricity, says Branz.

Murozono says that there are ways to make the cells more efficient–for example, by improving the purity and quality of the silicon. Reducing the size of the spheres to 0.8 millimeters should also improve performance, while reducing costs even further by using 20 percent less silicon. “We are going to improve the efficiency to 13 percent within 2008, and 15 percent by 2010,” Murozono says.

“There’s a worldwide shortage in terms of high-quality silicon,” says Charles Cromer, a researcher at the Florida Solar Energy Center, in Cocoa, FL. This shortage has largely been driven by the growth of demand for integrated circuits and solar cells, and has only served to push up the price of silicon-intensive photovoltaics. So a solar cell that uses far less silicon than its competitors should give CV21 a real edge in terms of reducing costs. “But just because they can make them cheaper doesn’t mean they will be selling them half price to consumers,” Cromer says.

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