Bill Scanlon, NREL
January 02, 2013 | 3 Comments
If the material isn't good, though, "you have to introduce an electric field to sweep the carriers out before they recombine and are lost," Friedman said.
But to do that, virtually all impurities would have to be removed. And the only way to remove the impurities would be to use a different growth technique.
Using Molecular Beam Epitaxy to Virtually Eliminate Impurities
Solar cells are typically grown using metalorganic vapor-phase epitaxy, or MOVPE.
"It works great, except you always get a certain level of impurities in the material. That's usually not a problem, but it would be an issue for this novel material, with the gallium arsenide diluted with nitrogen," Friedman said.
A different growth technique, molecular beam epitaxy (MBE), is done in such an ultra-high vacuum — 10 to the minus 13 atmospheres — that it can lower the impurities to the point where an electric field can be created in the resulting photovoltaic junction. And that would make the otherwise promising gallium-arsenide-dilute-nitride material work as a solar cell.
"The only problem was that there was no one in the entire world manufacturing solar cells by MBE," Friedman said.
But that was soon to change.
Partnering with a Startup out of Stanford University: Solar Junction
A Stanford University research group with expertise in the use of MBE for other electronic devices saw an opportunity, and around 2007, they spun out a startup company they named Solar Junction.
Because Solar Junction was a mix of enthusiastic recent Ph.D.s and experienced hands from outside the established solar cell field, "they weren't tied to the constraints of thinking this couldn't be done, that the only economically viable way to make solar cells was with MOVPE," Friedman said.
The federal lab and the startup got together. Solar Junction won a $3 million DOE/NREL Photovoltaic Technology Incubator contract to develop a commercial multijunction cell using dilute nitrides, and also received more than $30 million of venture-capital funding for this commercialization effort. To see more about NREL's Incubator projects, see the NREL news release.
"So Solar Junction had this good idea. But now they had to prove that you could actually make a high-efficiency solar cell with this," Friedman said. "Otherwise, who cares? People can make a lot of claims, but it's very simple to know whether you have a good solar cell or not — you just measure it."
It didn't take that long, Friedman said. By 2011, NREL had certified a new efficiency record for Solar Junction's SJ3 cell. The cell achieved an efficiency of 43.5% under concentrated sunlight, a significant step beyond the previous multijunction efficiency record of 41.6%, and far beyond the maximum theoretical efficiency of 34% for traditional one-sun single-junction cells.
Dilute-Nitride Junction Eliminates Need for Heavy Germanium Layer
With the new dilute-nitride junction, the germanium layer, which constitutes about 90% of the weight of the cell, is no longer needed. That may not be a big deal when it's part of a huge fixed utility-scale array. But when solar cells are used to power satellites, reduction in weight means a smaller rocket is needed to launch into space, potentially reducing costs significantly. The lighter weight is also essential for the military, which is increasingly asking soldiers to carry backpacks that include solar devices to power electronics.
Serendipitously, if the germanium substrate is retained, it has essentially the ideal band gap of 0.7 eV for a fourth junction, perfect for capturing longer wavelengths of the solar spectrum. That paves the way for a 50%-efficient solar cell in the not-distant future.
The cost to manufacture the SJ3 cell is competitive with that of the industry-standard GaInP/GaAs/Ge cell, according to Solar Junction. Its greater efficiency translates to significant cost-of-energy savings.
According to a report released this fall from IMS Research, the CPV market is forecast to double in 2012 and reach almost 90 megawatts. The World Market for Concentrated PV (CPV) — 2012 predicts installations of CPV will grow rapidly over the next five years to reach 1.2 gigawatts by 2016.
Because of its design and size, SJ3 is an instant plug-in replacement for the standard cell now used by the space and CPV industries. So, for example, if a 40%-efficient cell were replaced with a 44%-efficient cell, this would instantly increase the entire system power output by close to 10%.
"This is really a classic example of NREL developing something and then industry picking it up and running with it and making it a great commercial success," Friedman said. "We started with some very basic materials research. We took it to the point where it made sense for industry to take over and take it to the marketplace."
"We conceived the cell, demonstrated the individual parts, and let the world know about it," Friedman said. "But Solar Junction put all the parts together with record-breaking results, made it work with MBE, and commercialized it at a time when no one else seemed to be interested in or able to do it."
And now, utilities are ordering the SJ3 cells so fast that Solar Junction has depleted its pilot-scale stock and gone into partnership with manufacturer IQE to ramp up to full manufacturing scale.