NREL sets new solar cell efficiency record with ‘quantum well’ design reaching 39.5%

Researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL) have created a solar cell with a record efficiency of 39.5% under 1 sun global lighting. This is the highest-efficiency solar cell of any type, measured under standard 1 sun conditions.

“The new cell is more efficient and has a simpler design that could be useful for a variety of new applications, such as very space-constrained or low-radiation space applications,” said Myles Steiner, senior scientist at NREL’s High-Efficiency. Crystalline Photovoltaics (PV) Group and principal investigator of the project. He has collaborated with NREL colleagues Ryan France, John Geisz, Tao Song, Waldo Olavarria, Michelle Young and Alan Kibbler.

Details of the development are described in the document “Triple-junction solar cells with 39.5% terrestrial and 34.2% space efficiency enabled by thick quantum well superlattices” which appears in the May issue of Joule magazine.

Scientists at NREL set a record earlier in 2020 with a 39.2% efficient six-node solar cell using III-V materials.

Several of the best recent solar cells are based on the reverse metamorphic multijunction (IMM) architecture invented at NREL. This newly improved triple junction IMM solar cell has now been added to the Best Research Cell Efficiency Chart† The chart, showing the success of experimental solar cells, includes the previous IMM record with three nodes of 37.9% set in 2013 by Sharp Corporation of Japan.

The efficiency improvement followed research into ‘quantum well’ solar cells, which use many very thin layers to change the properties of solar cells. The scientists developed a quantum well solar cell with unprecedented performance and implemented it in a three-node device with different band gaps, with each node tuned to capture and use a different part of the solar spectrum.

The III-V materials, so named because of where they fall on the periodic table, span a wide variety of energy bandgaps that allow them to target different parts of the solar spectrum. The top junction is made of gallium-indium phosphide (GaInP), the middle of gallium arsenide (GaAs) with quantum sources, and the bottom of lattice mismatched gallium-indium arsenide (GaInAs). Each material has been highly optimized over decades of research.

“An important element is that while GaAs is an excellent material and generally used in III-V multijunction cells, it does not have quite the right bandgap for a three-junction cell, meaning the balance of photocurrents between the three cells is not optimal,” said France, senior scientist and cell designer. “Here we have adjusted the bandgap while maintaining excellent material quality by using quantum sources, enabling this device and potentially other applications.”

The scientists used quantum wells in the middle layer to widen the GaAs cell’s bandgap and increase the amount of light the cell can absorb. Importantly, they developed optically thick quantum source devices without major voltage drop. They also learned how to anneal the GaInP top cell during the growth process to improve performance and how to minimize the threading dislocation density in mismatched GaInAs lattices, discussed in separate publications. Altogether, these three materials make up the new cell design.

III-V cells are known for their high efficiency, but the manufacturing process has traditionally been expensive. Until now, III-V cells have been used to power applications such as space satellites, unmanned aerial vehicles and other niche applications. Researchers at NREL have been working to dramatically reduce the manufacturing cost of III-V cells and provide alternative cell designs, making these cells economical for a variety of new applications.

The new III-V cell was also tested for how efficient it would be in space applications, especially for communications satellites, which are powered by solar cells and where high cell efficiency is crucial, and came in at 34.2% for a start of – life measurement. The current cell design is suitable for low-radiation environments, and higher radiation applications may be enabled by further development of the cell structure.

NREL is the U.S. Department of Energy’s premier national laboratory for research and development in renewable energy and energy efficiency. NREL is managed for the Energy division by the Alliance for Sustainable Energy LLC.

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