New Records for Triple-Junction Solar Cells

Triple-junction solar cells made of III-V semiconductors and silicon have the potential to take photovoltaics to a new level of efficiency. (Source: Fh. ISE)

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany, have once again succeeded in raising the effi­ciency value of monolithic triple-junction solar cells made of silicon and III-V semi­conductor materials. Using a combination of multiple absorber materials, these multi-junction photo­voltaic cells exploit the energy from the solar spectrum signi­ficantly better than conven­tional silicon solar cells. The world record for a monolithic multi-junction solar cell manu­factured by wafer bonding has been increased to 34.1% and an efficiency record of 24.3% achieved for a solar cell with the III-V semi­conductor layers deposited directly on the silicon.

“Monolithic multi-junction solar cells are a source of hope for the further development of the silicon solar cells dominating the field today because they can lead to signi­ficantly higher effi­ciency values when converting sunlight into electrical power. We believe that we can achieve effi­ciency values of 36%, which would sub­stantially exceed the physical limit of 29.4% offered by a pure silicon solar cell,” explains Andreas Bett, Institute Director of Fraunhofer ISE. The high efficiency allows for more output per surface area, thus creating a savings of solar cell and module materials – an important aspect in regard to the sustaina­bility of photo­voltaics.

For the production of multi-junction photo­voltaic cells, thin III-V semiconductor layers only a few micro­meters thick are deposited on a silicon solar cell. In order to optimally exploit the sun’s rays, the different layers absorb light from different spectral ranges: gallium indium phosphide in the 300–660 nm range (visible light), aluminum gallium arsenide in the 600–840 nm range (near infrared light) and silicon in the 800–1200 nm range (long-wavelength light). This enables signi­ficantly increased effi­ciencies compared to single-junction silicon solar cells. Like today’s conventional silicon solar cells, these cells each have a contact on the front and rear sides, which allows for easy integration in solar modules.

Already well established in micro­electronics, the process of direct wafer bonding is employed for creating a monolithic multi-junction solar cell. This involves depositing the III-V layers on a gallium arsenide substrate in an initial step, after which an ion beam is used to deoxidize the surfaces in a high-vacuum chamber before they are pressed together under pressure. The atoms in the III-V semi­conductor layers form a bond with the silicon, forming a single unit. Now stacked on top of each other, the GaInP, AlGaAs and silicon sub-cells are inter­connected via tunnel diodes. The GaAs substrate is subsequently removed using wet chemistry, a nano­structured rear-side contact is attached and an anti-reflection coating and a contact grid are applied to the front side.

“In contrast to earlier results, the depo­sition conditions were improved and a new cell structure was introduced for the uppermost sub-cell made of gallium indium phosphide which enables even better visible light conversion than before. With an effi­ciency of 34.1%, the cell demonstrates the immense potential of this tech­nology,” says Frank Dimroth, Head of Department III-V Photo­voltaics and Concen­trator Technology at Fraunhofer ISE. The former world record for this cell class was 33.3% efficiency.

Directly depositing the III-V semi­conductor layers (GaInP/GaAs) on the silicon solar cells is another method used to create multi-junction photo­voltaic cells. This procedure involves consi­derably fewer process steps than wafer bonding and avoids the use of expensive GaAs substrates, which means it is quite advan­tageous in the industrial implementation of this technology. Nonetheless, the atomic structure must be very carefully controlled to ensure that the gallium and phosphorous atoms are arranged on the correct lattice sites at the interface to the silicon material. Defects in the semi­conductor layers can also have an adverse effect on the solar cells’ efficiency. “We were able to make major progress in this area – current gene­ration in the three sub-cells is now barely affected by these defects, which has enabled us to realize 24.3% efficiency for this technology for the first time anywhere in the world,” Frank Dimroth says. “The potential is comparable to that of the wafer-bonded cells. We’ve got our work cut out for us in the coming years in order to prove that this is the case.” In December 2018, Fraunhofer ISE intro­duced this type of solar cell with an efficiency record of 22.3%.

In heading toward the industrial mass production of monolithic multi-junction photo­voltaic cells, Fraunhofer ISE researchers see challenges in particular in finding an affordable process for manu­facturing the III-V semi­conductor layers. Direct growth on silicon is currently the most promising approach, but other methods are being researched where the GaAs substrates can be recycled many times over after the semi­conductors are transferred to the silicon. For cost-effective solar cell production new deposition machines with higher throughput and depo­sition area will be required. These are all methods that researchers at ISE will pursue in the coming years. (Source: Fh.-ISE)

Link: III-V and Concentrator Photovoltaics, Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany

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