Focusing Light on Silicon Beads

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

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). Credit: Clean Venture 21

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.

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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.