Semprius Inc of Durham, NC, USA, which designs and makes high-concentration photovoltaic (HCPV) solar modules, has manufactured the first four-junction, four-terminal stacked solar cell using its proprietary micro transfer printing process (published online on 28 April 2014 in Nature Materials; doi:10.1038/nmat3946). Semprius worked in collaboration with professor John Rogers and his team in the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign as well as researchers at partner Solar Junction Corp of San Jose, CA, USA, which makes III-V multi-junction solar cells for CPV based on dilute-nitride materials.
The new stacked solar cell consists 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%. The process is capable of achieving solar cell efficiencies greater than 50% in the near future, reckons Semprius.
A key achievement of the 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 hence 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 its energy yield under normal operation in the field.
Semprius is a graduate of the US Department of Energy's SunShot Incubator Program. "This achievement is notable because it establishes a straightforward path to significant future increases in conversion efficiency," comments 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."
In 2012, Semprius announced the first mass-produced photovoltaic module that exceeded 33% efficiency. In September 2013, it increased this record to 35.5%, as confirmed by Germany's Fraunhofer Institute for Solar Energy Systems ISE.
"Because the process we used is fully compatible with our current production processes, we believe this demonstration can be easily transferred to manufacturing," says VP of technology Scott Burroughs. Over the past two years, Semprius has deployed systems with strategic customers in six US states and eight countries around the world.
