It’s been a hot year for gallium-based PV cells, with new test cells producing higher efficiency levels than ever before. Recently Sharp announced it broke the record for overall efficiency of any PV cell, reaching 44.4 efficiency (under concentration) with a triple-junction PV cell. And Last week the National Renewable Energy Lab announced a new world record of 31.1 percent conversion efficiency for a two-junction PV cell without concentration. Besting a record in the category that was set just 2 months ago.

Such PV cells are far more expensive than traditional silicon PV cells, since they use more expensive materials like gallium, indium and arsenide. But by applying multiple layers of materials, the cells respond to a wider range of the sun’s spectrum and can produce much more power for a given space. Using inexpensive Fresnel lenses, the sun can also be concentrated on them—sometimes by hundreds of times—to get even more energy out of the sun’s rays.

The new champion cell is part of DOE’s Foundational Program to Advance Cell Efficiency (F-PACE) to 48 percent efficiency, which was launched in 2011. At that time NREL’s best single-junction gallium-arsenide solar cell was 25.7 percent efficient. Since then Alta Devices has set a series of records, from 26.4 percent in 2010 to 30.8 percent efficient, which was achieved just two months ago. “It brings us one step closer to the 48 percent milestone,” said NREL Principal Scientist Sarah Kurtz, who leads the F-PACE project. “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.”

The new tandem cell consisted of a gallium indium phosphide cell atop a gallium arsenide cell. “Historically, scientists have bumped up the performance of multi-junction 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,” said NREL Scientist Steiner. “The scientific goal of this project is to understand and harness the internal optics,” he said. The researchers were able to improve cell efficiency by enhancing the photon recycling, using a gold back contact to reflect uncaptured photons into the cell and by allowing luminescence from the upper cell to couple into the lower junction.