Two efficiency records set for monolithic silicon triple solar cells
The researchers at Fraunhofer ISE improved the world record for a monolithic multiple-junction solar cell produced by wafer bonding to 34.1 %. A new efficiency record of 24.3 % was set for a silicon solar cell with directly deposited semiconductor layers.
Monolithic multiple-junction solar cells are considered to be promising for the further development of the silicon solar cells that dominate today. This is because they can be used to achieve significantly higher efficiencies for converting sunlight into electricity. Andreas Bett, director of Fraunhofer ISE, believes that an efficiency of 36 % is possible. According to Bett, the physical limit of a pure silicon solar cell of 29.4 % would thus be significantly exceeded. The high efficiency makes it possible to generate more power per area and thus save materials for solar cells and module materials.
For the highly efficient multi-junction solar cell, thin layers of III-V semiconductors a few micrometers thick are applied to a silicon solar cell. In order to optimally use the energy of sunlight, the different layers absorb different spectral ranges of sunlight: gallium indium phosphide between 300 – 660 nm (visible light), aluminum gallium arsenide between 600 – 840 nm (near infrared light) and silicon between 800 – 1200 nm (longer wavelength light). Like normal silicon solar cells, the new triple solar cells are equipped with contacts on the front and back so that the solar cells can be easily integrated into solar modules.
For the monolithic multiple-junction solar cell, the direct wafer bonding process known from microelectronics production is used. In a first step, the III-V layers are deposited on a gallium arsenide substrate. The surfaces are then deoxidized in a chamber under high vacuum with the aid of an ion beam and pressed together under pressure. The atoms of the III-V semiconductor layers form bonds with the silicon and form a unit. The subcells of GaInP, AlGaAs and silicon stacked on top of each other are interconnected by tunnel diodes. Subsequently, the GaAs substrate is wet-chemically removed and a nanostructured back contact as well as an anti-reflective coating and a contact grid are applied to the front.
Compared to earlier experimental arrangements, the deposition conditions were improved and a new cell structure was introduced for the uppermost gallium-indium phospid subcell, which better converts the visible light. “At 34.1%, the cell demonstrates the enormous potential of this technology,” explains Dr. Frank Dimroth, Head of Department III-V Photovoltaics and Concentrator Technology at Fraunhofer ISE. The previous world record for this cell class was 33.3%.
Another possibility for the realization of multiple solar cells is the direct deposition of III-V semiconductor layers (GaInP/GaAs) onto the silicon solar cell. This process requires significantly fewer process steps than wafer bonding and avoids the use of the more expensive GaAs substrate, which is why it is advantageous for industrial implementation of the technology. However, the atomic structure must be very well controlled so that the gallium and phosphorus atoms at the interface to silicon occupy the correct lattice locations. Defects in the semiconductor layers can also impair the efficiency of the solar cells. “Here we were able to make important progress – the current generation in the three subcells hardly suffers from these defects, so that we were able to achieve an efficiency of 24.3 % for this technology for the first time worldwide,” says Dimroth. “The potential corresponds to that of wafer-bonded cells, and here we still have some development work ahead of us in the coming years to demonstrate this. In December 2018, Fraunhofer ISE presented such a solar cell with an efficiency record of 22.3 %.
On the way to industrial mass production of monolithic multi-junction solar cells, the Fraunhofer ISE researchers see challenges especially in a cost-effective process for the production of III-V semiconductor layers. They currently regard direct growth on silicon as the most promising approach. However, research is also being conducted into methods in which the GaAs substrates are recycled many times after the semiconductor layers have been transferred to silicon. For cost-efficient throughput in solar cell production, new equipment must also be developed to achieve deposition on larger substrates in a shorter time. The ISE researchers will pursue these approaches in the coming years.
In addition to Fraunhofer ISE, the partners involved were semiconductor equipment supplier Aixtron SE, TU Ilmenau and the university of Marburg.
More information: https://www.ise.fraunhofer.de/en.html
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