Semprius Inc. has manufactured a four-junction stacked solar cell using a proprietary microscale transfer printing process that has achieved a conversion efficiency of 43.9 percent.
The stacked solar cell consists of a three-junction compound semiconductor microcell that is stacked on top of a single-junction germanium microcell. 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 measured efficiencies up to 43.9 percent but the process is capable of achieving efficiencies greater than 50 percent in the near future, the company claimed. The transfer system allows the formation of thousands of stacked microcells simultaneously with high yield, Semprius (Durham, North Carolina) said.
The development reflects the trade-off between pursuit of high-efficiency solar cells that are more complex and potentially more expensive to manufacture and simpler silicon- or organic-based solar cells that are lower in efficiency but low cost to make.
To achieve its high efficiency result 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. (San Jose, Calif.) which makes III-V multi-junction solar cells for photovoltaic concentrator systems (CPV) based on dilute-nitride materials.
A key achievement of the project was development of an 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 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 was established in 2005 to develop a transfer printing process that could be applied to applications as diverse as OLED display backplanes and concentrator photovoltaic modules.
In 2012, Semprius announced the first mass-produced photovoltaic module that exceeded 33 percent efficiency and in September 2013, it increased this record to 35.5 percent, 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," said Scott Burroughs, vice president of technology at Semprius. Over the past two years, Semprius has deployed systems with strategic customers in six US states and eight countries around the world.
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