28 June 2012
The SJ3 solar cell developed by the US Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) with industrial partner Solar Junction has been named one of 2012’s most significant innovations by R&D Magazine. The cell uses tunable bandgaps, lattice-matched architecture and ultra-concentration tunnel junctions to achieve a world-record conversion efficiency of 43.5% with potential to reach 50%.
The prestigious award is an indicator for the immense impact this new SJ3 cell could potentially have on the near future of solar energy. “The high performance and other attractive features of this cell will contribute to the lowering of costs for concentrating photovoltaic (CPV) electricity to grid parity levels,” says Dr. Daniel Friedman, manager of NREL’s III-V Multijunction Materials and Devices group. “The space photovoltaics industry is likely to find this cell attractive as well.”
The S3J, a high-efficiency triple junction solar cell for CPV applications, captures different light frequencies throughout the day, ensuring optimal conversion of photons to electrons. Friedman explains the cell’s unique design: “The cell's record-setting efficiency is based on a "lattice-matched" design which does not require thick buffer layers or complex processing, providing low cost and high reliability.” The 43.5% efficiency occurred under lens-focused light with 418 times the intensity of the sun. The record PV cell performance was achieved at little additional cost “by replacing the bottom germanium layer of the three-junction cell with gallium and a dash of a dilute-nitride alloy,” according to NREL information. This small change increases the bottom band-gap from an insufficient 0.67 electron volts (eV) to an ideal 1.0 eV.
“Solar Junction has developed a proprietary platform technology enabling the manufacture of high-quality dilute nitride semiconductors for use in multijunction solar cells. The world record was achieved by using this technology,” says John Herb, Vice President of Product and Program Management at Solar junction, based in San Jose, California (US). “NREL did much of the fundamental science, while Solar Junction solved the product development problems. The fundamental breakthrough enabling the commercialization of dilute nitrides and the product development was done at Solar Junction.”
Going forward, Herb is confident his team “will increase cell efficiency to 50% by combining the dilute nitride materials with other known solar materials to build cells with still higher-efficiency four- and five-junction designs.” At this time, his company is supplying the S3J cells only to a limited set of customers. “Currently, Solar Junction’s primary focus is expanding manufacturing for the current S3J cell family,” Herb says. “We are also actively working on the next generation of cell design that will deliver about 45% cell efficiency. Our early results with a four-junction cell design are very promising.”
Written by Sandra Henderson, Research Editor, Solar Novus Today