NREL reports world record 31.1 percent efficiency for III-V solar cell

Jun 25, 2013

(Phys.org) —The Energy Department's National Renewable Energy Lab has announced a world record of 31.1% conversion efficiency for a two-junction solar cell under one sun of illumination.

NREL Scientist Myles Steiner announced the new record June 19 at the 39th IEEE Photovoltaic Specialists Conference in Tampa, Fla. The previous record of 30.8% efficiency was held by Alta Devices.

The tandem cell was made of a gallium cell atop a cell, has an area of about 0.25 square centimeters and was measured under the AM1.5 global spectrum at 1,000 W/m2. It was grown inverted, similar to the NREL-developed inverted metamorphic multi-junction (IMM) solar cell – and flipped during processing. The cell was covered on the front with a bilayer anti-reflection coating, and on the back with a highly reflective gold contact layer.

The work was done at NREL as part of DOE's Foundation Program to Advance Cell Efficiency (F-PACE), 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%-efficient concentrator cell, NREL's best single-junction gallium-arsenide solar cell was 25.7% efficient. This efficiency has been improved upon by other labs over the years: Alta Devices set a series of records, increasing the gallium-arsenide record efficiency from 26.4% in 2010 to 28.8% in 2012. Alta's then-record two-junction 30.8% 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% milestone," said NREL Principal Scientist Sarah Kurtz, who leads the F-PACE project in NREL's National Center for Photovoltaics. "This joint project with the University of California, Berkeley and Spectrolab has provided us the opportunity to look at these near-perfect 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 said. 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," he said.

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 gallium-arsenide 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 . 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 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%. 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 said.

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VENDItardE
1 / 5 (7) Jun 25, 2013
sure they have....righhhhhhhhhhhhhhhhhhhhhhttttt
DirtySquirties
1 / 5 (8) Jun 25, 2013
Woop dee doo. Everyone's breaking solar efficiency records everyday because there are 400 million types of solar panels and each one of them has a different efficiency percentage at the top. These percents mean squat to me because I am an idiot and I want to think that a solar collector/panel/whatever is just a solar collector/panel/whatever. I ain't got a clue what the difference between a III-V, IV, MXCVII, and a II solar panel is and I'm tired of people spouting about how some scientist broke records and how it's a big deal in the solar industry when we see a handful of them every day.
packrat
1 / 5 (7) Jun 25, 2013
yada yada yada,,....... I'll believe it when I see them advertised for sale until then it's just more fud.....
VendicarE
not rated yet Jun 26, 2013
Maybe this success is why the Filthy Republicans have been working hard to destroy the NREL

During the Ronald Reagan administration, the institute's budget was cut by some 90%; many employees 'reduced in force' and the laboratory's activities were reduced to R&D. In later years, renewed interest in the energy problem improved the institute's position, but funding has fluctuated. In 2006 funding had dropped to the point that NREL was forced to lay off 32 workers, and in 2011 anticipated congressional budget shortfalls led to a voluntary buyout program for 100 to 150 staff reductions.