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Record cell sets IMEC's III-V solar agenda

Scientists at the European research center hope to make the most of industry-leading germanium junctions, and intend to deploy an unusual cell design to do so.

Belgian researchers have produced a record-breaking single-junction GaAs solar cell, thanks to high quality germanium substrates that they aim to fully exploit in future triple-junction cells.

Made at the Interuniversity Microelectronics Center (IMEC) in Leuven, using a high-quality germanium substrate, the cell achieved 24.7 percent conversion efficiency under one sun concentration of the standard terrestrial spectrum.

IMEC s compatriots Umicore had developed the high-performance substrates under a research project with the European Space Agency, called ESA-IMAGER.

“They asked us to make standard single-junction cells on those substrates to check the quality, which turned out to be very good,” said Giovanni Flamand, leader of the IMEC III-V solar team.

After confirming the improved performance of the standard cells with lower defect levels at the US National Renewable Energy Laboratory in November 2007, Flamand hopes to translate this to non-standard triple-junction cells for use in concentrating photovoltaics.

The IMEC team intends to monolithically grow InGaP and GaAs subcells on one germanium substrate, which would be removed prior to mechanical bonding with a high-efficiency germanium subcell. This allows the bonded germanium layer to be connected in parallel rather than in series with the other subcells, in an assembly the same thickness as existing triple-junction cells.

“We can, in that construction, make full use of the current generated by the germanium bottom cell. This is not fully used in a monolithic stack, because it is limited to the lowest current generated by one of the top cells that are connected in series,” Flamand said.

“By stacking them mechanically on top of each other we can win another 1.5-2 percent in overall efficiency and thereby attain a higher efficiency than can be reached with present state of the art.”

Flamand concedes that achieving this will take some time, but he still hopes to produce competitive III-V solar cells using this method for IMEC to ultimately license out for commercial sale.

“The 24.7 percent cell that we have now, it s a very nice result for us, of course, but in itself, it s not something that you can commercialize,” he said. “It cannot compete with triple junction cells that go to 35 or 40 percent, so it's just an intermediate step in the further development of our multi-junction approach.”

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