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What's Behind Record-Breaking Solar Cell Efficiencies, Part 1

In solar, it's hard to go a month without hearing news about conversion efficiencies. In September, for example, Oerlikon Solar and its partner, Corning, said they broke the world efficiency record for a lab-created tandem-junction amorphous-silicon cell. The cell, which was tested by the U.S. National Renewable Energy Laboratory, delivered 11.9 percent stabilized efficiency.

Meanwhile, Conergy said its new selective-emitter technology could boost solar-cell efficiency from one of its German factories by "up to 0.5 percentage points." And scientists at Yonsei University and the Massachusetts Institute of Technology announced new technologies that could one day enhance cell efficiencies by up to 65 percent, in the case of Yonsei, and that could double cell efficiencies, in the case of MIT

"Everyone's working on efficiency," said Paul Wormser, senior director of product engineering and system solutions at Sharp Electronics' solar division. "You would be hard pressed to find a manufacturer that hasn't stated publicly at least once a year something about an efficiency improvement program."

Martin Green, a professor and executive research director at the University of New South Wales' Photovoltaics Centre of Excellence, said he's seen cell efficiency improvements in the labs accelerate in the last few years, with about 10 new "highest confirmed efficiencies" for different photovoltaic cell and module technologies every six months.

Commercial cells lag behind the efficiencies achieved by labs in so-called "champion cells," or best of breed cells, and companies don't expect cells produced in high volume to reach the efficiencies of those top lab cells. But the efficiency gains in the labs have translated into commercial gains as well: First Solar, for example, reported that its panels grew from 10.9 percent efficiency to 11.2 percent efficiency from the second quarter in 2009 to the same quarter this year, Green pointed out.

The upcoming October edition of Progress in Photovoltaics, which tracks the highest confirmed lab efficiencies for solar cells and panels, cites new records for the top concentrator cell, the top large-area crystalline cell, the top copper-indium-gallium-diselenide (CIGS) cells and the top tandem-junction amorphous-silicon cell, among others.  Let's take a look at the best efficiencies that the labs – and factories – have produced so far:

Crystalline Silicon

For crystalline-silicon technologies, efficiencies suddenly become far more crucial starting around 2005, during a worldwide shortage of solar-grade silicon that lasted until 2008. Silicon supplies were limited and expensive, and that gave manufacturers a huge incentive to eke as much power as possible out of each panel. "Pre 2008, getting silicon at all was a challenge, so you needed to squeeze as much as you could out of the silicon," said Jenny Chase, lead solar analyst for Bloomberg New Energy Finance. Because solar panels are sold per watt of peak capacity, not per panel, manufacturers that boosted their efficiencies could grow their production capacity – and profits – without having to make more panels or access more silicon.

Looking at the natural limits of the materials, crystalline silicon could reach a theoretical efficiency of 28, 29 or 30 percent, scientists say. Theoretical efficiencies are based on lab conditions that will never be found in commercial production, warned Lars Waldmann, director of public relations for Schott Solar. But in the labs, researchers are three to four percentage points away.

The University of New South Wales, which holds the record for the most efficient crystalline-silicon cell, created a cell with an efficiency of 25 percent. Sandia National Laboratories tested the cell in 1999, and it was also used in a record-setting solar panel with 22.7 percent efficiency, according to the university's School of Photovoltaic and Renewable Energy Engineering.

Meanwhile, SunPower makes the most efficient silicon panels on the market with 19.3 percent efficiency, according to Photon's annual module overview, which came out in February. (Some efficiency is always lost when companies combine cells into panels.) In May, the company announced a new line of panels rated for up to 19.5 percent efficiency, and in June, SunPower announced it had set a new world record for large-area silicon solar cells with a conversion efficiency of 24.2 percent measured by the National Renewable Energy Laboratory.

The drive toward higher efficiencies is less critical now than it was a few years ago, during the worldwide solar silicon shortage, Chase said. "Now companies can get as much silicon as they need at reasonable prices." Silicon prices were $59 to $60 per kilogram in September, according to Bloomberg New Energy Finance. Companies were rumored to be paying spot prices as high as $400 per kilogram in 2007, according to the Prometheus Institute at that time.

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But when the silicon shortage ended in 2008 and panel prices started falling, companies had another reason to boost efficiencies: to lower costs. If manufacturers can increase their panel efficiencies, the same factory can produce more megawatts worth of panels without having to add production lines. Aside from the potential to lower the panels' cost per watt, higher efficiencies can also reduce the cost of installation – including the pieces such as racking and mounting, wiring and inverters – because fewer panels are needed to deliver the same amount of power, Chase said. And the highest-efficiency cells and panels can also sell at a premium, a major advantage as manufacturers' profit margins are being slashed.

While the lower panel prices may be a driver for efficiency, they also could limit the amount that efficiencies can grow. Companies will only take steps to raise efficiencies if those steps cost less than it would cost a developer to simply add more panels to achieve the same result.

CIGS

The new Progress in Photovoltaics edition includes two new results for copper-indium-gallium-diselenide cells, which have demonstrated the highest lab efficiencies of any thin film. In April, a CIGS cell on glass, made by NREL, tested at 19.6 percent efficiency, replacing an NREL record of 19.4 percent from January of 2008.

Meanwhile, ZSW Stuttgart produced a cell that delivered 20.3 percent efficiency when it was tested by Fraunhofer this summer. That cell had an aperture area of only 0.5 square centimeters, making it too small to be accepted as an outright record, Green said. Measurement errors are more likely to happen with small areas and the champion cells can also be less representative of the group because it's possible to make thousands on a single substrate and sort through them for the "flash in the pan," he said.

But the high efficiency – confirmed by Fraunhofer – won the ZSW Stuttgart cell a spot on the publication's "Notable Exceptions" chart, which notes highly efficient cells and panels that don't meet the standards for class records. The cell replaced another ZSW Stuttgart cell, tested by Fraunhofer just six months ago in April, with 20.1 percent efficiency.

The top commercial CIGS panels, made by Q-Cells using Solibro cells, get up to 11.2 percent efficiency, according to Photon's February overview.  Würth Solar also sells a copper-indium-diselenide cell, with no gallium, that delivers 11.8 percent efficiency, according to the overview.

Amorphous Silicon

In August, Oerlion Solar's above-mentioned 11.9 percent cell, measured by NREL, broke the previous record of 11.7 percent efficiency set by Kaneka back in 2004. Oerlikon's cell included a new, thin light-trapping glass from Corning. Chris O'Brien, head of market development at Oerlikon Solar, said the company also improved its tandem technology with a better (and cheaper) reflective backsheet -- the sheet at the back of the cell that reflects the photons that get past the silicon layers back into the silicon for another chance to convert those photons into electricity – and a thinner silicon layer, which boosts stabilized efficiency. And the technology has the potential for even higher efficiency: The cell didn't include the usual antireflective coating to enhance the capture of light, O'Brien added.

The top commercial amorphous silicon (a-si) and microcrystalline panels are made by Pramac, an Oerlikon customer, with 9.2 percent efficiency, according to Photon in February. Sharp – as well as IBC Solar using Sharp cells – makes the next most efficient panels at 9 percent efficiency, according to the overview. These types of cells have the potential to reach more than 11 percent efficiency in 2012, said Thomas Block, production manager in strategy and business development for Schott. Waldmann added that the technology could reach a panel efficiency of up to 12 percent in the coming years.

Meanwhile, in Progress in Photovoltaics' "Notable Exceptions" section, Uni-Solar produced a tandem-junction cell with 12.5 percent stabilized efficiency last year. The cell, tested by NREL in March 2009, had layers of a-si and nanocrystalline silicon. But it only had a designated illumination area of 0.27 square centimeters, which is why it isn't listed as the official record. Uni-Solar claims it also holds the world record for flexible a-si, with a cell that demonstrated 15.4 percent efficiency in the lab.

In production, Uni-Solar's current cell efficiency is 8.2 percent, with a panel efficiency of 6.7 percent. Photon's February overview lists the top amorphous-silicon panels as tied at 7 percent efficiency from Iscorn (using triple-junction Uni-Solar cells) and Viessmann (using single-junction Schott cells). Schott already has a tandem-junction a-si cell with more than 10 percent efficiency, Block said. Meanwhie, Uni-Solar plans to deliver commercial panels with  12 percent efficiency by 2012 and in June released a roadmap anticipating efficiencies could eventually rise above 20 percent, at a price of 95 cents per watt.

In Part 2 of this article, we’ll look at cadmium-telluride and multijunction-concentrator solar-cell conversion efficiencies and examine why efficiencies are both important and not-so-important in the marketplace.

Jennifer Kho is a freelance reporter and editor based in Oakland, Calif. Aside from RenewableEnergyWorld.com, her stories have appeared in The New York Times' Green blog, The Wall Street Journal, Los Angeles Times, AOL's DailyFinance, MIT's Technology Review, The Christian Science Monitor, Reuters.com, Earth2Tech and other publications. She has more than a decade of journalism experience and has been covering green technology since 2004.

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