Solar Cells Info

Source: SemiconductorToday.com,  23 September 2008
http://www.semiconductor-today.com/news_items/2008/SEPT/FRAUNHOFER_230908.htm

After in mid-July announcing a European record of 37.6% for solar cell efficiency, the Fraunhofer Institute for Solar Energy Systems (ISE) of Freiburg, Germany has achieved a new record of 39.7% (closer to the world record of 40.8% set in August by the US Department of Energy’s National Renewable Energy Laboratory).  The new cell uses the same metamorphic (lattice mismatched) triple-junction structures consisting of more than 30 layers (including Ga0.35In0.65P, Ga0.83In0.17As and Ge) grown on a germanium substrate by metal-organic chemical vapor deposition (MOCVD) using a reactor from Aixtron of Aachen, Germany. However, the higher efficiency has been achieved by improving the contact structures of the solar cells through using a front-side network of thin metal wires that transport large currents but with low resistance, according to Frank Dimroth, head of the III-V – Epitaxy and Solar Cells Group.

For concentrator photovoltaic (CPV) systems, optimal efficiency of multi-junction solar cells must often be achieved at a concentration factor of 300-600 suns. At different concentration factors, the metallization on the front side of the cell makes a major difference. In the front grid, current is conducted through a network of thin wires (see figure 1) from the middle of the solar cell to the edge, where it is then picked up by a 50µm gold wire. Under concentrated sunlight in particular, the structure of this metal network is critical. On the one hand, the metal wires must be big enough to transport, with low resistance, the large currents that are generated under concentrated sunlight. On the other hand, the wires must be as small as possible, since sunlight cannot penetrate through the metal and thus the cell area covered by the metal cannot be used for converting sunlight to electricity.

For the past two years, sponsored by the European Union research project Fullspectrum (SES6-CT-2003-502620), Fraunhofer ISE has been working on a program for the theoretical calculation of the optimal contact structures. Based on this work, the new cells are especially suited to inhomogeneous radiation, as occurs for concentration factors of 300-600 suns. The solar cells have been installed in concentrator modules of FLATCON type, both at Fraunhofer ISE and at spin-off firm Concentrix Solar GmbH, among others.

“We are very pleased to have advanced a further decisive step in such a short amount of time,” says Dr Andreas Bett, department head at Fraunhofer ISE. Higher conversion efficiencies will help the new technology to become market competitive and to further cut the costs of solar electricity generation in the future, he adds.