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NREL hails 31.1% PV cell landmark

The US’ National Renewable Energy Lab (NREL) has created a two-junction PV cell with a world record 31.1% conversion efficiency.

The 0.25-square-centimetre tandem cell, made of a gallium-indium phosphide cell atop a gallium-arsenide cell, was measured under the AM1.5 global spectrum at 1,000 W/m2.

The thin-film cell, covered on the front with a bilayer anti-reflection coating and on the back with a highly reflective gold contact layer, was grown inverted, similar to the NREL-developed inverted metamorphic multi-junction solar cell – and flipped during processing.

"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 optimising variables such as the bandgaps and the layer thicknesses," says NREL scientist Myles Steiner.

“But internal optics plays an under-appreciated 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.”

The new cell improves on traditional single-junction gallium-arsenide designs by enhancing the “photon recycling" that allows a PV cell to create electron-hole pair, boosting voltage and conversion efficiency.

The cell was built under the aegis of the Department of Energy's Foundation Program to Advance Cell Efficiency (F-Pace), a project with the Obama administration’s cost-cutting SunShot Initiative.

The F-Pace project, jointly run by NREL, University of California, Berkeley and Spectrolab, has the mission of developing a 48%-efficient concentrator cell.

US technology company Alta Devices held the previous record for a two-junction 30.8% efficient cell.

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