Six-Junction Solar Cell Sets Two World Records for Efficiency

Scientists at the National Renewable Energy Laboratory (NREL) have fabricated a solar cell that set the world record for the highest solar conversion efficiency at 47.1%, which was measured under concentrated illumination (the equivalent of 143 suns). A variation of the same cell also set an efficiency record under one-sun illumination at 39.2%.

To construct the cell, the team used III-V materials, a group of semiconductors so named because they fall in columns III and V on the periodic table. They exhibit a wide range of light absorption properties. Each of the cell’s six junctions (the photoactive layers) is captures light from a specific part of the solar spectrum. The device contains about 140 total layers of various III-V materials, yet is one-third the width of a human hair.

Based on their high efficiencies and costs, these types of cells are most often used to power satellites. On Earth, however, the six-junction solar cell is well-suited for use in concentrator photovoltaics, said Ryan France, an NREL scientist.

“One way to reduce cost is to reduce the required area,” France says. “That can be done by using a mirror to capture more light and focus it down to a point. Then you can use a hundredth or even a thousandth of the material used in flat-plate silicon cells. 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 is impossible, thanks to fundamental limits imposed by thermodynamics.

Currently the main hurdle to topping 50% efficiency is reducing resistive barriers inside the cell that impede current flow of current. Still, NREL is trying to lower the cost of III-V solar cells to open up new markets for the highly efficient cells.