A new solar efficiency record story has been making the rounds the past few days – the lofty 44 percent figure achieved by Solar Junction. Funny thing is, this happened in October; or, at least, that’s when Solar Junction put out a press release announcing the record.
The fresh flurry of stories about this not-so-new record was apparently prompted by a news release from the National Renewable Energy Laboratory, which makes the important point that this U.S. Department of Energy lab – that is, the government – has been a partner of Solar Junction’s on this important research into multijunction photovoltaics.
Yeah, that government that so often appears dysfunctional might just be doing some things right.
According to NREL, the road to the 44 percent efficiency record has been a long one, dating back some 15 years, to a time when scientists at the lab began looking for ways to take maximum advantage of the energy offered by the sun.
Because photons of sunlight arrive in a differentiated range, conventional solar cells with a single characteristic band gap energy are quite limited in their efficiency. As the NREL explained: “Higher-energy photons give up their excess energy to the solar cell as waste heat, while lower-energy photons are not collected by the solar cell, and their energy is completely lost.”
The way around this problem? Solar cells with multiple junctions, each of which has its own band gap energy.
NREL soon came up with “the first, practical, commercial multijunction solar cell.” Further research revealed that using gallium arsenide with nitrogen added to it could drive efficiency even higher by producing an ideally sized band gap in the third junction. But making the material work would require a different growing technique, one that would produce a cleaner material, lowering impurities “to the point where an electric field can be created in the resulting photovoltaic junction.”
And this is where Solar Junction, a startup out of Stanford University, came into the picture.
The scientists at Solar Junction were becoming quite handy with a technique called molecular beam epitaxy (MBE). Hooking up with NREL through the DOE/NREL Photovoltaic Technology Incubator program brought the researchers $3 million to work with, and venture capitalists poured in 10 times that amount.
And from there, the efficiency records have tumbled.
“We conceived the cell, demonstrated the individual parts, and let the world know about it,” Daniel Friedman, manager of the NREL III-V Multijunction Photovoltaics Group, said in a statement. “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.”
This SJ3 cells is aimed at utility-scale projects that use concentrating photovoltaics, in which lenses are used to ratchet up the intensity of the light hitting the solar cells. “In regions of clear atmosphere and intense sunlight, such as the U.S. desert Southwest, CPV has outstanding potential for lowest-cost solar electricity,” NREL said. “There is enough available sunlight in these areas to supply the electrical energy needs of the entire United States many times over.”