Wafer bonding enables 44.7 percent efficiency record solar cell

24th September 2013
The modified III-V structure has four solar subcells and was grown using a new technology. The process allows the connection of two semiconductor crystals, which otherwise cannot be grown on top of each other with high crystal quality

The Fraunhofer Institute for Solar Energy Systems ISE, Soitec, CEA-Leti and the Helmholtz Centre Berlin have achieved a new world record for the conversion of sunlight into electricity.

They used a new solar cell structure with four solar subcells.



World record solar cell with 44.7 percent efficiency, made up of four solar subcells based on III-V compound semiconductors for use in concentrator photovoltaics (©Fraunhofer ISE)

Surpassing competition after only over three years of research, and entering the roadmap at world class level, a new record efficiency of 44.7 percent was measured at a concentration of 297 suns. This indicates that 44.7 percent of the solar spectrum's energy, from ultraviolet through to the infrared, is converted into electrical energy. This is a major step towards reducing further the costs of solar electricity and continues to pave the way to the 50 percent efficiency roadmap.



IV-characteristic for the current best four-junction solar cell under AM1.5d ASTM G173-03 spectrum at a concentration of 297 suns. The measurements were carried out at the Fraunhofer ISE CalLab (©Fraunhofer ISE)

Back in May 2013, the German-French team of Fraunhofer ISE, Soitec, CEA-Leti and the Helmholtz Centre Berlin had already announced a solar cell with 43.6 percent efficiency. Building on this result, further intensive research work and optimisation steps led to the present efficiency of 44.7 percent.

These solar cells are used in concentrator photovoltaics (CPV), a technology which achieves more than twice the efficiency of conventional PV power plants in sun-rich locations. The terrestrial use of so-called III-V multi-junction solar cells, which originally came from space technology, has prevailed to realise highest efficiencies for the conversion of sunlight to electricity.



External quantum efficiency of the four-junction solar cell. The measurements were carried out at the Fraunhofer ISE CalLab

In this multi-junction solar cell, several cells made out of different III-V semiconductor materials are stacked on top of each other. The single subcells absorb different wavelength ranges of the solar spectrum.

“We are incredibly proud of our team which has been working now for three years on this four-junction solar cell,” says Frank Dimroth, Department Head and Project Leader in charge of this development work at Fraunhofer ISE.

“This four-junction solar cell contains our collected expertise in this area over many years. Besides improved materials and optimisation of the structure, a new procedure called wafer bonding plays a central role. With this technology, we are able to connect two semiconductor crystals, which otherwise cannot be grown on top of each other with high crystal quality. In this way we can produce the optimal semiconductor combination to create the highest efficiency solar cells,” he explains.

“This world record increasing our efficiency level by more than 1 point in less than 4 months demonstrates the extreme potential of our four-junction solar cell design which relies on Soitec bonding techniques and expertise,” says André-Jacques Auberton-Hervé, Soitec’s Chairman and CEO.

“It confirms the acceleration of the roadmap towards higher efficiencies which represents a key contributor to competitiveness of our own CPV systems. We are very proud of this achievement, a demonstration of a very successful collaboration.”

“This new record value reinforces the credibility of the direct semiconductor bonding approaches that is developed in the frame of our collaboration with Soitec and Fraunhofer ISE. We are very proud of this new result, confirming the broad path that exists in solar technologies for advanced III-V semiconductor processing,” says Leti CEO Laurent Malier.

Concentrator modules are produced by Soitec (started in 2005 under the name Concentrix Solar, a spin-off of Fraunhofer ISE). This particularly efficient technology is employed in solar power plants located in sun-rich regions with a high percentage of direct radiation. Presently Soitec has CPV installations in 18 different countries including Italy, France, South Africa and California.



 

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