Semprius, Inc. has manufactured the first four-junction, four-terminal stacked solar cell using its proprietary micro transfer printing process. In this effort, Semprius worked in collaboration with Professor John Rogers and his team at the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign and researchers at Solar Junction, Inc., a leading III-V high-efficiency solar cell manufacturer and Semprius partner. The results of this project will be published this week in the journal Nature Materials.
The new stacked solar cell is comprised of a three-junction microcell that is stacked on top of a single-junction germanium microcell using Semprius' high-speed micro transfer printing process, which enables the simultaneous formation of thousands of stacked microcells with very high yields. By using four junctions, the stacked cell is able to capture light across a broader portion of the solar spectrum and therefore achieve efficiencies much higher than conventional silicon and thin-film single-junction solar cells. Initial trials yielded solar cells with measured efficiencies up to 43.9 percent. This process is capable of achieving solar cell efficiencies greater than 50 percent in the near future.
A key achievement of this project was the development of a new interfacial material that is placed between the top and bottom cell to minimize optical losses within the stack and thereby optimize overall conversion efficiency. In addition, the new stacked cell has four terminals, rather than the standard two. This reduces the spectral dependence of the solar cell and increases the solar cell's energy yield under normal operation in the field.
Semprius is a graduate of the U.S. Department of Energy's SunShot Incubator Program. "This achievement is notable because it establishes a straight-forward path to significant future increases in conversion efficiency," said Dan Friedman, manager of the National Renewable Energy Laboratory (NREL) III-V Multijunction Photovoltaics Group. "Increasing efficiency is critical to reducing the cost of solar energy because it helps drive down not only module costs, but also many other costs, including the cost of land, labor and wiring."
Semprius has been at the forefront of high-efficiency, HCPV solar module development for the past seven years. In 2012, Semprius announced the first mass-produced photovoltaic module that exceeded 33 percent efficiency. In September 2013, Semprius increased this record to 35.5 percent, as confirmed by the Fraunhofer Institute for Solar Energy Systems ISE in Germany.
"We would like to thank our collaborators at the University of Illinois and Solar Junction for their help in this project," said Scott Burroughs, vice president of Technology at Semprius. "Because the process we used is fully compatible with our current production processes, we believe this demonstration can be easily transferred to manufacturing."