Germany - With an efficiency of 37.6%, researchers at the Fraunhofer Institute ( http://www.ise.fhg.de ) for Solar Energy Systems convert sunlight into electrical energy, thus setting a new European record. The result was reached using so-called multi-junction solar cells made out of III-V semiconductors.
Up to now such solar cells have been used primarily for space applications. The current developments of the Fraunhofer researchers facilitate the more cost-efficient use of these types of cells for terrestrial applications. For this purpose, sunlight is concentrated up to 2000 times and focused onto a multi-junction solar cell having an area of only a few square millimeters.
The concentrator technology promises that the costs of generating electricity from sunlight can be considerably decreased already in the near future, especially in countries with a large fraction of direct solar radiation.
In multi-junction solar cells made out of gallium indium phospide (GaInP), gallium indium arsenide (GaInAs) and germanium (Ge), the solar spectrum is divided into three wavelength ranges in order to convert it very efficiently into electrical energy. The achieved efficiency of 37.6% was reached by the Fraunhofer researchers under very high radiation intensities with a concentration ratio of 1700.
“For more than ten years now at Fraunhofer ISE, we are working intensively on the development of multi-junction solar cells for use in concentrator systems because this technology promises by far the highest efficiencies for solar energy conversion,“ explains Dr. Frank Dimroth, head of the group “III-V – Epitaxy and Solar Cells” at Fraunhofer ISE.
Decisive advances in the material development have lead to the record efficiency we achieved. The most important goal was that the solar cells function reliably even when exposed to very high intensity levels.
“Especially the quality of the tunnel diodes with a thickness of only 30nm and with which the three regions of the cell are connected, plays a decisive role,” says Frank Dimroth. The work is supported through the EU research project Fullspectrum.
Multi-junction solar cells are made up of complex structures with about 30 single layers of III-V compound semiconductors all of which must have a very high crystalline quality. For producing the solar cell structures, the researchers at Fraunhofer ISE use a metal organic gas phase epitaxy (MOVPE) reactor from the company Aixtron in Aachen.
“For the first time we could show that multi-junction solar cells with such high efficiencies and larger energy yields can be manufactured,” says Frank Dimroth pleased. Because we carry out our research on production systems, we can transfer our new results to the market within several months with our cooperation partner AZUR SPACE in Heilbronn.