U.S. researchers have created a triple-junction solar cell that has achieved an efficiency of 39.5%, a world record for conventional solar lighting. Although the concept is based on materials and processes that are still too expensive for commercial use, it may soon find practical applications for operating satellites and other space technologies.
According to pv magazine international.
A group of researchers led by the US Department of Energy’s National Renewable Energy Laboratory (NREL) has set a new world efficiency record for a solar cell with normal light without a concentration unit. Their cell based on three layers of III-V materials achieved a conversion efficiency of 39.5% – which exceeded the previous NREL record of 39.2%.
The research team was also able to simplify the unit significantly by using three nodes instead of the previous six by resorting to various innovations that have taken place in recent years. For the record, the previous record for a triple junction solar cell was set by Sharp Corporation in 2013 with a conversion rate of 37.9%.
The cell is described in detail in the article “Triple-junction solar cells with 39.5% terrestrial and 34.2% space efficiency enabled by thick quantum well superlattices”, recently published in Joule. The group combined layers of gallium-indium-arsenide (GaInAs), gallium-arsenide (GaAs) and gallium-indium-phosphide (GaInP) to create a unit capable of absorbing a wide segment of the solar spectrum.
The key to this approach is NREL’s recent work on “quantum wells”, which has made it possible to better calibrate each layer to absorb a different part of the solar spectrum. “Although GaAs is an excellent material and is typically used in III-V multijunction cells, it does not have the right band gap for a three-junction cell, which means that the balance of photocurrent between the three cells is not optimal,” he said. Ryan France, senior scientist and cell designer at NREL. “Here we have changed the band gap while maintaining excellent material quality using quantum wells, enabling this device and potentially other applications.”
Quantum wells are fine nanostructures inserted into the cell layer to modify the forbidden band and other properties. These quantum wells allowed the group to increase the band gap in the central layer of the GaAs cell to increase its performance and calibrate it with the other two. If the phenomenon of quantum wells is not new, the difficulties encountered in working with this type of material on a nanometer scale have so far limited its practical benefits.
The group recognized that the processes and materials it is working on for these cells are at least now too complex and too expensive for most common solar energy applications. Although NREL is working on various methods that can drastically reduce these costs, currently operating satellites and other space technologies – where space constraints mean that costs can come next to efficiency – are the only likely uses for any type of solar cell based on these. “III-V” materials, named after their group in the Periodic Table.
NREL also evaluated its triple-junction quantum well cells below the light spectrum they would be exposed to outside the Earth’s atmosphere, and arrived at a starting value of 34.2%. According to NREL, this is also a world record for a triple junction cell under this spectrum.
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