Six-junction solar cell sets world records for efficiency

Scientists at the US National Renewable Energy Laboratory (NREL) have fabricated a six-junction solar cell that now holds the world record for the highest solar conversion efficiency at 47.1%, as measured under concentrated illumination. A variation of the same cell also set the efficiency record under one-sun illumination at 39.2%.

As described in the journal Nature Energy, the researchers constructing the device relied on III-V materials — so called because of their position on the periodic table — that have a wide range of light absorption properties. Each of the cell’s six junctions (the photoactive layers) is specially designed to capture light from a specific part of the solar spectrum. The device contains about 140 total layers of various III-V materials to support the performance of these junctions, yet is three times narrower than a human hair.

Due to their highly efficient nature and the cost associated with making them, III-V solar cells are most often used to power satellites, which prize III-V’s high performance. On Earth, however, the six-junction solar cell is well suited for use in concentrator photovoltaics, said study co-author Ryan France.

“One way to reduce cost is to reduce the required area, and you can do that by using a mirror to capture the light and focus the light down to a point,” France said. “Then you can get away with a hundredth or even a thousandth of the material, compared to a flat-plate silicon cell. You use a lot less semiconductor material by concentrating the light. An additional advantage is that the efficiency goes up as you concentrate the light.”

France described the potential for the solar cell to exceed 50% efficiency as “actually very achievable”, but acknowledged that 100% efficiency cannot be reached due to the fundamental limits imposed by thermodynamics. Lead author John Geisz added that the main hurdle to topping 50% efficiency is to reduce the resistive barriers inside the cell that impede the flow of current.

Geisz added that NREL is also heavily engaged in reducing the cost of III-V solar cells, enabling new markets for these highly efficient devices that demonstrate “the extraordinary potential of multijunction solar cells”.

Image caption: John Geisz and Ryan France fabricated a solar cell that is nearly 50% efficient. Image credit: Dennis Schroeder, NREL.

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