California, USA -- 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:
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.