The previous record of 30.8 percent efficiency was held by Alta Devices.
The tandem cell was made of a gallium indium phosphide cell atop a gallium arsenide 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 DOE'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 gallium-arsenide solar cell was 25.7 percent 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 percent in 2010 to 28.8 percent 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 percent 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 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 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.