An operator inspects a photolithography tool used to manufacture high-efficiency Solar Junction concentrator solar cells. NREL’s pioneering multijunction work led to the Solar Junction SJ3 solar cell with tunable bandgaps, lattice-matched architecture, and ultra-concentrated tunnel junctions. Credit: Daniel Derkacs/Solar Junction

Award-winning PV cell pushes efficiency higher

January 2, 2013 by Bill Scanlon

(—It takes outside-the-box thinking to outsmart the solar spectrum and set a world record for solar cell efficiency. The solar spectrum has boundaries and immutable rules. No matter how much solar cell manufacturers want to bend those rules, they can’t.

So how can we make a solar cell that has a higher efficiency than the rules allow?

That’s the question scientists in the III-V Multijunction Photovoltaics Group at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) faced 15 years ago as they searched for materials they could grow easily that also have the ideal combinations of band gaps for converting photons from the sun into electricity with unprecedented efficiency.

A band gap is an energy that characterizes how a semiconductor material absorbs photons, and how efficiently a solar cell made from that material can extract the useful energy from those photons.

“The ideal band gaps for a solar cell are determined by the solar spectrum,” said Daniel Friedman, manager of the NREL III-V Multijunction Photovoltaics Group. “There’s no way around that.”

But this year, Friedman’s team succeeded so spectacularly in bending the rules of the solar spectrum that NREL and its industry partner, Solar Junction, won a coveted R&D 100 award from R&D Magazine for a world-record multijunction solar cell. The three-layered cell, SJ3, converted 43.5% of the energy in sunlight into electrical energy—a rate that has stimulated demand for the cell to be used in concentrator photovoltaic (CPV) arrays for utility-scale energy production.

Last month, that record of 43.5% efficiency at 415 suns was eclipsed with a 44% efficiency at 947 suns. Both records were verified by NREL. This is NREL’s third R&D 100 award for advances in ultra-high-efficiency multijunction cells. CPV technology gains efficiency by using low-cost lenses to multiply the sun’s intensity, which scientists refer to as numbers of suns.

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