At 39.7% efficiency for a multi-junction solar cell, researchers at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg have exceeded their own European record of 37.6% which they achieved just a short time ago. III-V semiconductor multi-junction solar cells are used in photovoltaic concentrator technology for solar power stations.

“We have improved the contact structures of our solar cells,” says 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 the utilization in photovoltaic concentrator systems, the optimal efficiency of multi-junction solar cells must often be achieved between 300 - 600 suns, that is, at a sunlight concentration factor of 300 – 600. The metallization of the front side makes the main difference for different concentration factors. In the front grid the 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 50 µm gold wire. Particularly under concentrated sunlight, the structure of this metal network is decisive. For one, the metal wires must be big enough to transport, with low resistance, the large currents which are generated under concentrated sunlight. On the other hand, the wires must be as small as possible since the sunlight cannot penetrate through metal and thus the cell area covered by metal cannot be used for the electrical conversion.

For the past two years at Fraunhofer ISE, work is being 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 newest record efficiencies were developed. These 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 type FLATCON® 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,” says 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 ten years, researchers at Fraunhofer ISE have been developing multi-junction 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 metal-organic vapour-phase epitaxy (MOVPE). Today such multi-junction III-V semiconductor solar cells achieve the highest conversion efficiency worldwide by far. Due to the large material and manufacturing costs, however, they are only used in concentrating PV systems and in space.