14 April 2020
NREL six-junction solar cell sets efficiency records of 47.1% for 143-sun and 39.2% for 1-sun illumination
In research funded by the US Department of Energy (DOE) Solar Energy Technologies Office, the National Renewable Energy Laboratory (NREL) has fabricated a monolithic, series-connected six-junction inverted metamorphic solar cell with record solar energy conversion efficiency of 47.1%, measured under concentrated illumination (operated under the direct spectrum at 143-suns concentration). When tuned to the global spectrum, a variation of the same cell also set the efficiency record under one-sun illumination at 39.2% (‘Six-junction III-V solar cells with 47.1% conversion efficiency under 143 suns concentration’, Nature Energy (2020) 11).
“This device really demonstrates the extraordinary potential of multi-junction solar cells,” says lead author John Geisz, a principal scientist in NREL’s High-Efficiency Crystalline Photovoltaics Group. Geisz’s co-authors are NREL scientists Ryan France, Kevin Schulte, Myles Steiner, Andrew Norman, Harvey Guthrey, Matthew Young, Tao Song, and Thomas Moriarty.
Each of the cell’s six junctions (photoactive absorbing layers) is specially designed to capture light from a specific part of the solar spectrum. The device contains a total of about 140 layers of various III-V materials to support the performance of these junctions.
Due to their highly efficient nature and the cost associated with making them, III-V solar cells are most often used to power satellites. On Earth, however, the six-junction solar cell is well-suited for use in concentrator photovoltaics, says co-author Ryan France of NREL’s III-V Multijunctions Group.
“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. 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,” he adds. “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 that 100% efficiency cannot be reached due to the fundamental limits imposed by thermodynamics.
Geisz says that currently the main research hurdle to topping 50% efficiency is to reduce the series resistive barriers inside the cell that impede the flow of current. Meanwhile, he notes that NREL is also heavily engaged in reducing the cost of III-V solar cells, enabling new markets for the highly efficient devices.