Fraunhofer ISE Researchers Achieve 39.7% Solar Cell Efficiency

At 39.7% efficiency for a multijunction solar cell, researchers at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg have exceeded their own efficiency record of 37.6%, which they achieved in July of this year. The result was reached using multijunction solar cells made out of III-V semiconductors. III-V semiconductor multijunction solar cells are used in concentrating photovoltaic (PV) 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.”

In these concentrating PV applications, optimal efficiency is 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, lead image, top of page) 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 that 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 comparison, figure 2, below, shows a solar cell wafer with different concentrator solar cell structures.)

These cells are especially suitable for situations of inhomogeneous radiation, as occurs in the case of concentrated sunlight. The same solar cells are installed in the concentrator FLATCON modules manufactured at spin-off company, Concentrix Solar GmbH, among others.

Concentrix Solar recently announced that it moved its manufacturing facilities to Freiburg, where it will begin industrial-scale production of its concentrator photovoltaic modules. The company said that the relocation was made necessary by the company’s rapid growth and the commencement of industrial-scale manufacture for the FLATCON technology.

The company says that high efficiency rates and the refinements applied to the technology in readiness for volume manufacture are a result of experience Concentrix Solar has accumulated in two years of pilot production. CTO Dr. Andreas Gombert is optimistic regarding further quality gains, “We work constantly with our R&D department and our local cluster of R&D partners to improve product design, production quality and efficiency, and on further reductions in cost.”

“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 multijunction solar cells with highest efficiencies. One emphasis has been 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 vapor-phase epitaxy (MOVPE).

Today such multijunction III-V semiconductor solar cells achieve among the highest conversion efficiency worldwide. Due to the large material and manufacturing costs, however, they are only used in concentrating PV systems and in space. These current developments of the Fraunhofer researchers may facilitate more cost-efficient use of these types of cells for additional applications in the near future, especially in countries with a large fraction of direct solar radiation.

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