When two US government agencies claim to have broken the solar cell efficiency record set by Spectrolab in 2006, it begs the question: Who is measuring what and how? In August, the US National Renewable Energy Lab (NREL), which is operated for the US Department of Energy, announced that it had set a world record for photovoltaic (PV) cell efficiency by converting 40.8 percent of the light that hits the cell into electricity. But this was more than a year after a DARPA-funded group led by researchers from DuPont of Wilmington, Del., and from the University of Delaware in Newark announced that it had achieved a record-breaking efficiency of 42.8 percent “from sunlight at standard terrestrial conditions.” So what is really going on here?PV panels at work. Photo courtesy of Shell Solar.To begin with, it is important to understand just what is being tested. The NREL device is an inverted metamorphic triple-junction cell that uses gallium indium phosphide and gallium indium arsenide to split the solar spectrum into three equal parts, each of which is absorbed by the cell’s three junctions. The cell is grown on a gallium arsenide wafer, flipped over and the wafer removed. The device is thin and light and designed especially for the space satellite market and for terrestrial concentrated PV arrays.The very high efficiency solar cell (VHESC) module being developed by the DuPont-University of Delaware consortium is a solar power module that will achieve high performance through a novel lateral optical concentrating system that splits solar light into different energy bins and directs the light onto solar cells of various light-sensitive materials to cover the solar spectrum. The approach allows the use of whatever type of solar cell offers the best performance and cost (interestingly, NREL cells were among those submitted by this project for the initial test).At the core of the debate over who really holds the record is the fact that this is really an “apples and oranges” situation. NREL had a single solar cell that was tested at 300 suns. The DuPont-University of Delaware program had a collection of cells fabricated at several labs that could be assembled into a module using the unique optics design to achieve the 42.8 percent figure. Key is the fact that the reported efficiency of 42.8 percent is a sum of the solar cell efficiencies and not a measurement of the module. The combined solar cell efficiency of 42.8 percent is a milestone for this new architecture.The collection of unassembled cells was tested at 20 suns. (Suns, by the way, indicates the brightness of real sunlight; therefore, 300 suns would be 300 times the brightness of the sun.) To further complicate matters, neither test took losses in the optical or the electrical system into account.Friendly rivalryThe two groups may be challenging each other’s record, but they’re really engaged in what we hope is friendly – and, ultimately, highly beneficial – competition.Keith Emery, manager of the photovoltaics cell and module performance section of NREL in Golden, Colo., conducted the tests. He indicated that the DuPont-University of Delaware results were not really a record because the measurements were not made under standard conditions. He conceded, however, that the consortium’s number is a “milestone.” He said the beauty of it is that you can use “the best of all technologies and then add a spectral splitter and get numbers like what they’ve shown.”Each group is continuing its work on very high efficiency solar cells, striving for the government’s stated goal of 50 percent efficiency. At this point, the 42.8 percent achieved by the DuPont-University of Delaware group is a summation of individual cell efficiencies, not the efficiency of a single cell. Rather than a “record,” it’s more of a hypothetical example of how such a system would perform. Module results have since been obtained from the DuPont-University of Delaware program and will be reported on shortly, according to Emery. The next step for all groups working toward advancing solar efficiency is to present a working, assembled system for test.NREL continues work on concentrated solar (also called solar thermal) and on PV research to advance solar technology as an energy source. The DuPont-University of Delaware Solar Power Consortium was launched to develop commercial applications of the technologies explored in the earlier phase of the program. Figuring out how to squeeze the most energy from the sun at the lowest cost is no small or short-term task, so we expect to see more friendly rivalries pop up now and then as we move toward a common goal.
Photosensitizers