Three-Dimensional Design Leads to Better Solar Cells

By borrowing a trick from optical-fiber technology, a startup makes cells that trap light to create more electricity.

• BY PRACHI PATEL

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A Santa Barbara, California, company called Solar3D plans to make silicon solar cells that are more efficient than conventional cells by borrowing the light-trapping concept behind optical-fiber technology. The company claims that its three-dimensional design will funnel light into silicon and keep it trapped—giving the material more time to convert it into electricity.

Silicon has a theoretical maximum light-to-electricity conversion efficiency of 29 percent, but panels on the market today are only 15 to 18 percent efficient. Solar3D does not have exact efficiency numbers for its design yet, but CEO Jim Nelson says, "We think it's going to approach silicon's theoretical efficiency. We won't get to 29 percent—nobody's going to get that high with silicon—but we're hoping to get as close as possible."

Solar3D's design will tackle two factors that bring down solar efficiency. First, 30 percent of the light hitting solar panels is reflected and lost. Second, many of the electrons created when light hits silicon are reabsorbed by the material before they reach the external circuit.

The new design has channels in its top light-collecting layer, which will be made of silicon dioxide or another similar material; these direct light downward, helping to eliminate reflection, Nelson says. The lower layer is an array of three-dimensional structures, each a few micrometers wide, which trap light by emulating the waveguides used in optical fibers. Optical fibers contain two cylindrical layers with different refractive indices that continuously reflect light back into the core. Nelson says this 3D structure will allow the light to bounce around until the photons have yielded as many electrons as possible. "We'll also put contacts very close to where that happens so that the electrons don't have to travel very far," he says.

Many other light-trapping concepts exist. Another that borrows the technology of optical fibers is a fiber-optic solar cell that Georgia Tech researchers have made by wrapping dye-sensitized solar cells around optical fibers. But, says Nelson, Solar3D's cells use conventional silicon materials, so they could be produced on existing manufacturing equipment and be dropped into existing modules.

Solar3D is at a very early stage and is starting out at a time when venture capitalists are hesitant to fund solar technologies with high startup costs. But the company is not seeking venture capital. Instead, it is funded by private investors, including Nelson, who come from a finance background but have a passion for renewable energy. Analysts say the company's success will hinge on whether its manufacturing costs will compete with conventional crystalline silicon technology. "It's very difficult for new emerging technologies to gain a foothold in the market," says Matthew Feinstein, an analyst with Lux Research. "Bankability is often a major concern, as these technologies don't have a proven track record."

Technologies such as Solar3D's that promise higher efficiencies are good because they will make solar power less expensive, says Georgina Benedetti, an analyst with Frost & Sullivan. However, she says, "one main challenge for the industry in general is competition from Chinese manufacturing." Cheaper solar panels from China combined with troubles controlling costs are what led to the downfall of Solyndra, the Fremont, California, solar-panel manufacturer that recently declared bankruptcy after having been backed by a U.S. Department of Energy loan guarantee.

But Nelson seems undeterred. He is confident in Solar3D's design and its compatibility with existing solar infrastructure. "Our whole approach is made with mass production in mind," he says. "Our whole purpose is to make it competitive." The company expects to have a prototype device made by the end of 2011.