A Twin-Cell Solar Panel
by Kevin Bullis
Technology Review, February 15, 2011
A new design has funding through a White House initiative to develop cheap solar panels.
A start-up called Stion will receive $1 million from the National Renewable Energy Laboratory (NREL) to develop a new type of low-cost, high-efficiency solar panel. The company will use the new funding to make solar panels that combine two types of solar cells, which will allow the panels to efficiently convert a wide range of the solar spectrum into electricity.
Stion already makes thin-film solar panels, a type of solar panel that is generally less efficient than conventional, crystalline silicon solar panels, but that can cost much less to manufacture -- in some cases half as much. The new panels are meant to be just as efficient as conventional silicon ones, but still significantly cheaper to manufacture. The funding is part of the Obama administration's recently announced Sun Shot initiative, which has the goal of reducing the cost of installed solar panels by 75 percent, to make solar power competitive with fossil fuels.
Stion's existing panels convert 12 percent of the energy in sunlight into electricity. This is a high figure for thin-film solar panels, which typically have efficiencies that range from 6 to 11 percent. The new panels use a tandem-solar-cell design to increase efficiencies to 15 to 18 percent, says founder and chief technology officer Howard Lee.
Lee won't say exactly what it costs Stion to make its first-generation solar panels, but he says the company is now "close to being competitive" with First Solar, the world's biggest thin-film solar-panel maker. First Solar's low costs and relatively high efficiencies have made it the only thin-film manufacturer to rival the production capacity of the largest conventional solar-panel makers. (The panels are cheaper than conventional silicon panels, but less efficient.)
First Solar makes cadmium-telluride thin-film solar panels, while Stion makes a type of solar panel made of copper, indium, gallium, and selenium (CIGS). This material is just starting to be commercialized, and many experts believe it can achieve higher efficiencies than cadmium-telluride solar cells.
But two things set Stion's technology apart from other CIGS solar panel manufacturers. First, it doesn't require specialized manufacturing equipment -- instead it uses conventional sputtering equipment that is used now to make thin films for semiconductor chips and hard drives.
Lee says that will make it easy for it to expand its manufacturing worldwide. "You can use equipment that you can buy off the shelf," he says.
Second, Stion's panels are tuned to efficiently convert different parts of the solar spectrum than other CIGS panels. Its first-generation product is tuned absorb light more toward the red end of the solar spectrum, which makes it a good match with a solar cell -- made of materials similar to CIGS -- designed for the blue end of the spectrum.
Pairing the cells increases the overall solar-panel efficiency and can drive down costs by reducing the number of panels that need to be installed. (Tandem thin-film solar cells made of amorphous silicon have been made by other manufacturers, but they are far less efficient than the ones Stion is developing.)
Stion, which was founded in 2006, has raised about $115 million to date. It makes solar panels in San Jose, California, and is building another factory in Mississippi with the help of an additional $75 million loan. Its goal under the NREL funding is to produce, within a year, a version of its tandem panels that could be mass-produced.
The NREL program is meant to fund companies that are taking "radically different" approaches to solar panels that could lead to the production of solar panels at "below 50 cents per watt," says Martha Symko-Davies, who manages the PV Technology Incubator project at NREL. Researchers have experimented with CIGS solar cells for decades, but she says companies such as Stion have made great strides in the last three to four years in bringing CIGS to market. As a result, making a tandem cell should now be relatively easy. "If you've got your CIGS working, doing what Stion is doing shouldn't take much more," she says.
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