Solar ♦ news digest
The work was done at NREL as part of the DOE’s Foundation Program to Advance Cell Efficiency (F-PACE). This is a project of the department’s SunShot Initiative that aims to lower the cost of solar energy to a point at which it is competitive with other sources including fossil fuels.
At the beginning of the F-PACE project, which aims to produce a 48 percent efficient concentrator cell, NREL’s best single-junction GaAs solar cell was 25.7 percent efficient.
The firm believes this efficiency has been improved upon by other labs over the years: Alta Devices set a series of records, increasing the GaAs record efficiency from 26.4 percent in 2010 to 28.8 percent in 2012. Alta’s then-record two-junction 30.8 percent efficient cell was achieved just two months ago.
The new record may not last long either, but “it brings us one step closer to the 48 percent milestone,” says NREL Principal Scientist Sarah Kurtz, who leads the F-PACE project in NREL’s National Centre for Photovoltaics.
Kurtz adds, “This joint project with the University of California, Berkeley and Spectrolab has provided us the opportunity to look at these near-perfect cells in different ways. Myles Steiner, John Geisz, Iván García and the III-V multijunction PV group have implemented new approaches providing a substantial improvement over NREL’s previous results.”
“Historically, scientists have bumped up the performance of multijunction cells by gradually improving the material quality and the internal electrical properties of the junctions - and by optimizing variables such as the bandgaps and the layer thicknesses,” NREL Scientist Myles Steiner continues.
But internal optics plays an underappreciated role in high-quality cells that use materials from the third and fifth columns of the periodic tables - the III-V cells. “The scientific goal of this project is to understand and harness the internal optics,” Steiner adds.
When an electron-hole pair recombines, a photon can be produced, and if that photon escapes the cell, luminescence is observed - that is the mechanism by which light-emitting diodes work. In traditional single- junction GaAs cells, however, most of the photons are simply absorbed in the cell’s substrate and are lost.
With a more optimal cell design, the photons can be re- absorbed within the solar cell to create new electron-hole pairs, leading to an increase in voltage and conversion efficiency. In a multijunction cell, the photons can also couple to a lower bandgap junction, generating additional current, a process known as luminescent coupling.
The NREL researchers improved the cell’s efficiency by enhancing the photon recycling in the lower, gallium- arsenide junction by using a gold back contact to reflect photons back into the cell, and by allowing a significant fraction of the luminescence from the upper, GaInP junction to couple into the GaAs junction. Both the open-circuit voltage and the short-circuit current were increased.
Silicon solar cells now dominate the world PV market, but researchers see opportunities for new materials. High-efficiency concentrator cells bolstered by lenses that magnify the power of the sun are attracting interest from utilities because the modules have demonstrated efficiencies well over 30 percent. And there may be commercial opportunities for one-sun or low- concentration III-V cells if growth rates can be increased and costs reduced.
The same cell should work well when lenses are added to multiply the sun’s power. “We expect to observe similar enhancements of the solar cell characteristics when measured under concentrated illumination,” Steiner concludes.
NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by the Alliance for Sustainable Energy, LLC.
Energizer and Ascent Solar to deliver 13 million hours of light
Using CIGS technology, the companies will unite to brighten up life for families living in the dark
According to the World Energy Outlook, International Energy Agency, every day as the sun sets around the world, 1.3 billion people without electricity either live in darkness or expose their families to the hazards of kerosene lighting.
This year Energizer and Ascent Solar will be changing that scene for many rural families as they team up to donate 13 million hours of solar light working with the global non profit One Million Lights.
Energizer chose Ascent Solar Technologies to be the solar panel provider for the donated Energizer lanterns and lights because of Ascent’s award-winning, flexible thin-film technology and interest in using their transformational technology to benefit families in
July 2013
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