[Ferro-Alloys.com]The organisation says the conversion efficiency of its InGaP/GaAs cell sets a world record for a two-junction solar cell measured under one-sun illumination

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

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

The tandem cell, made of an InGaP cell atop a GaAs cell, has an area of about 0.25 square centimetres and was measured under the AM1.5 global spectrum at 1,000 W/m2.

The cell 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 atNREL 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