Breaking the rules of the sun with solar
Towards the end of 2012 researchers at the National Renewable Energy Laboratory, working with Solar Junction, developed a PV cell that was 44 percent efficient at converting sunlight into electricity. That shatters the maximum solar conversion efficiency of conventional solar cells which, according to the Shockley-Queisser limit is at 33.7 percent efficient. While other multi-junction PV cells for concentrating PV applications have surpassed the limit NREL and Solar Junction were awarded a R&D 100 award in 2012 from R&D Magazine for the record—NREL’s third in the field, that and Solar Junction’s work to commercialize the technology shows how important state-funded research can be in terms of moving technologies forward.
The 33.7 percent limit was limited to a solar cell with a single p-n junction, which can only react to a certain bandgap of sunlight, of and based on research published in 1961. "The ideal band gaps for a solar cell are determined by the solar spectrum," said Daniel Friedman, manager of the NREL III-V Multijunction Photovoltaics Group. "There's no way around that." While it holds true today, by investigating more materials and their properties, researchers have been able to surpass the limit with other materials and cells that integrate multiple p-n junctions. NREL and Solar Junction won the award for their unique approach.
“Using tunable bandgaps, lattice-matched architecture, and ultraconcentration tunnel junctions, the SJ3 achieves 43.5 percent,” R&D said in announcing the award. That was in August. The group recently beat that record recorded at concentration levels of 415 suns with a 44 percent efficiency at 947 suns.
“Recognizing that currently used lattice-matched alloys are not optimal for maximum photovoltaic conversion, NREL scientists used a semi-empirical model to design a dilute-nitride form of a gallium indium nitride arsenide crystal, which was then grown. Solar Junction engineers established a molecular-beam epitaxy (MBE) approach for depositing this material,” R&D explained.
"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."
The way that Solar Junction has designed the unique cell, matching the design and size of other CPV cells, the SJ3 could replace standard multi-junction cells now used in space and CPV industries, boosting the device’s efficiency without completely replacing the whole device.