German solar cell reaches 40-percent efficiency

Freiburg, Germany-based Fraunhofer Institute for Solar Energy Systems said its researchers have been able to boost solar efficiency to a record level of 39.7 percent for a III-V multi-junction crystalline silicon cell.

The researchers said they were able to break their previous European record of 37.6 percent, which they achieved in March. Those solar cells are used at Concentrix Solar, which spun out from Fraunhofer.

Fraunhofer researchers said the III-V semiconductor multi-junction solar cells are used in photovoltaic concentrator technology for solar power stations. To date, the type of solar cells that have achieved the highest conversion efficiencies are limited to applications in concentrating photovoltaic systems. That’s because these systems typically have high material and manufacturing costs (see Spectrolab solar cell breaks 40% efficiency barrier).

The researchers said the improvement in solar efficiencies are central in helping make solar more market competitive (see Striking up the intermediate band).

The Fraunhofer Institute said that for more than 10 years researchers have been developing highly efficient, multi-junction solar cells. The researchers said 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.

In terms of solar utilization in these concentrator systems, the researchers said the optimal efficiency of multi-junction solar cells should range between the equivalent of 300 to 600 times the power of the sun.

The researchers said the structure of the metal network on the front side of the concentrator figures centrally into improving efficiency. That’s because metal wires integrated to the front side of the concentrator system conducts current from the middle of the solar cell to the edge.

And these wires are critical to performance. Metal wires need to be big enough to transport electrons, but thin so as not to create too much resistance.

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