Thin-Film Solar Cells on Flexible Steel Sheets

09 June of 2009 by

xunlight Thin Film Solar Cells on Flexible Steel Sheets

Normally conventional silicon solar panels, are bulky and rigid, but a startup in Toledo, Ohio, called Xunlight, has developed a way to make solar panels more flexible and larger.

It has developed a roll-to-roll manufacturing technique that formsthin-film amorphous silicon solar cells on thin sheets of stainlesssteel. Each solar module is about one meter wide and five and a halfmeters long.

These lightweight, flexible sheets could easily be integrated intoroofs and building facades or on vehicles. Such systems could be moreattractive than conventional solar panels and be incorporated moreeasily into irregular roof designs.

They could also be rolled up and carried in a backpack, says thecompany’s cofounder and president, Xunming Deng. “You could take itwith you and charge your laptop battery,” he says.

Amorphous silicon thin-film solar cells can be cheaper thanconventional crystalline cells because they use a fraction of thematerial: the cells are 1 micrometer thick, as opposed to the150-to-200-micrometer-thick silicon layers in crystalline solar cells.

However the price that is paid for being this thin is efficiency,which at the moment is a maximum of 7% versus the 20% or more of thebest crystalline cells. To boost their efficiency, Xunlight madetriple-junction cells, which use three different materials, amorphoussilicon, amorphous silicon germanium, and nanocrystalline silicon, eachof which is tuned to capture the energy in different parts of the solarspectrum. Conventional solar cells use one primary material, which onlycaptures one part of the spectrum efficiently.

Despite this innovation they still only achieve efficiences of 8%,so Xunlight’s large modules produce only 330 watts, whereas an array ofcrystalline silicon solar panels covering the same area would produceabout 740 watts.

United Solar Ovonic,based in Auburn Hills, MI, is already selling flexible PV modules. Thecompany also uses triple-junction amorphous silicon cells, and itsmodules can be attached to roofing materials. But Xunlight’s potentialadvantage is its high-volume roll-to-roll technique. “If theirroll-to-roll process allows them to go to lower cost and larger area,that’s the central advantage,” says Johanna Schmidtke, an analyst withLux Research, in Boston. “But they have to prove it with manufacturing.”

Other companies, notably Heliovolt and Nanosolar, are in a race tomake thin-film panels using copper indium gallium selenide (CIGS)cells. These have shown efficiencies on par with crystalline siliconand can be made on flexible substrates. In comparison with amorphoussilicon, CIGS is a relatively difficult material to work with, and noone has been able to create low-cost products consistently in largequantities, says Ryan Boas, an analyst with Photon Consulting, inBoston.

Building integrated photovoltaics (BIPV), especially rooftopapplications, would be the biggest market for flexible PV technology,Boas says. That’s because flexible products are inherently very light,in addition to being quick and easy to install. “Imagine carrying aroll of flexible product on the roof and unrolling it,” he says.“Workers are already used to unrolling roofing material.”

But there are hidden risks and costs associated with BIPV, Schmidtkesays. “BIPV is often touted as low cost,” she says, “but in actuality,you’ve got greater risk in terms of a watertight system [for roofingmaterials] or fire risk, and that increases total installation cost.”However, BIPV does have the advantage of being more aestheticallypleasing, which is important to consumers, she says.

So far, Xunlight has raised $40 million from investors. In December,the state of Ohio gave the company a $7 million loan to speed up theconstruction of a 25-megawatt production line for its flexible solarmodules. The company expects to have commercial products available in2010.
Image Credit: Xunlight – Xunming Deng, cofounder of Xunlight, holds his company’s flexible solar modules.

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