The limits of solar efficiency are being pushed higher by the newest multi-junction PV cell from Soitec and its partners, Fraunhofer Institute for Solar Energy Systems ISE, CEA-Leti and the Helmholtz Center Berlin. The group announced in late September that they reached a PV conversion efficiency of 44.7 percent for a solar cell.
This puts them one step closer to 45 percent and knocking on the door of 50 percent conversion efficiency. The record was achieved under a concentration of 297 suns. The new champion cell is able to absorb light across the spectrum from ultraviolet to infrared. “This is a major step towards further reducing the costs of solar electricity and continues to pave the way to the 50 percent efficiency roadmap,” a representative from Soitec stated.
“We are incredibly proud of our team which has been working now for three years on this four-junction solar cell,” said Frank Dimroth, Department Head and Project Leader in charge of the development work at Fraunhofer ISE. “This four-junction solar cell contains our collected expertise in this area over many years.”
“Besides improved materials and optimization of the structure, a new procedure called wafer bonding plays a central role,” Dimroth explained. “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.”
The new record surpasses the 44.4 percent efficiency mark set by Sharp Solar earlier this year. That multi-junction cell reached the former record under the concentration of 302 suns. Both the Sharp cell and the new Soitec and partners cell are designed to be used in concentrating photovoltaics (CPV), which magnify the amount of sunlight that hits each individual cell through optic devices like Fresnel lenses. CPV modules can concentrate sunlight on the cells by hundreds of times, if not more. The cells themselves use more exotic, expensive materials like gallium and arsenide than conventional PV cells do, but they’re also far more efficient.
This new record holder uses a new solar cell structure with four solar subcells, according to Soitec. 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 realize highest efficiencies for the conversion of sunlight to electricity. 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.
The group has been able to ramp up the efficiency levels quickly. “Back in May 2013, the German-French team of Fraunhofer ISE, Soitec, CEA-Leti and the Helmholtz Center Berlin had already announced a solar cell with 43.6 percent efficiency,” a spokesperson for Soitec stated.
“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,” added André-Jacques Auberton-Hervé, Soitec 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.”
The cells will be used in Soitec’s Concentrix CPV systems, which are manufactured in San Diego, Calif. Soitec already has CPV installations in 18 different countries including Italy, France, South Africa and the United States. Earlier this year, the company announced its CPV modules were producing electricity at 31.8 percent efficiency.
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