Fraunhofer Bests Own Solar-Cell Number

Charles Troy

Researchers at Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany, have attained 39.7 per cent efficiency for a multijunction solar cell, beating their own European record of 37.6 per cent achieved in March. III-V semiconductor multijunction solar cells are used in photovoltaic concentrator technology for solar power stations.

“We have improved the contact structures of our solar cells,” said Frank Dimroth, head of the III-V–Epitaxy and Solar Cells Group at Fraunhofer ISE. “As a result, using the same semiconductor structures, we now achieve the higher efficiency when converting sunlight into electricity.”

For use in photovoltaic concentrator systems, the optimal efficiency of multijunction solar cells should run between 300 and 600 suns; i.e., at a sunlight concentration factor of 300 to 600. The metallization of the front side marks the main difference for various concentration factors.

In the front grid, the current is conducted through a network of thin wires from the middle of the solar cell to the edge, where it is picked up by a 50-μm gold wire. Under concentrated sunlight, the structure is decisive. For one, 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 because the sunlight cannot penetrate metal and, thus, the cell area covered by metal cannot be used for the electrical conversion.

For the past two years, work has been performed on a new program for the theoretical calculation of optimal contact structures. Based on this work, sponsored by the EU Project Fullspectrum (SES6-CT-2003-502620), solar cells holding the latest record efficiencies were developed. The cells are especially suitable for situations of inhomogeneous radiation, as occurs in the case of concentrated sunlight. These solar cells are installed in the concentrator modules of the Flatcon type at Fraunhofer ISE and at the spin-off company Concentrix Solar GmbH, among others.

“We are very pleased to have advanced a further decisive step in such a short amount of time,” said Dr. Andreas Bett, department head at Fraunhofer ISE. “Highest conversion efficiencies help the young technology to become market competitive and to further sink the costs of generating electricity from the sun for the future.”

For more than 10 years, researchers at Fraunhofer ISE have been developing multijunction solar cells with highest efficiencies. One emphasis here is on the so-called metamorphic (lattice mismatched) triple-junction solar cells made out of Ga0.35In0.65P, Ga0.83In0.17As and Ge, which have an especially high theoretical efficiency potential. The solar cell structures consist of more than 30 single layers, which are deposited on a germanium substrate by means of metallorganic vapour-phase epitaxy (MOVPE).

Today such multijunction III-V semiconductor solar cells achieve the highest conversion efficiency worldwide by far. Because of the large material and manufacturing costs, however, they are used only in concentrating photovoltaic systems and in space.