3 April 2018

Fraunhofer ISE demonstrates two-terminal GaInP/GaAs/Si solar cell with efficiency of 33.3%

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Converts One-third of the Sunlight into Electricity: 33.3% silicon-based multi-junction Solar Cell
Fraunhofer Institute for Solar Energy Systems ISE of Freiburg, Germany – together with wafer-bonding equipment maker EV Group of St Florian, Austria – has developed a new silicon-based multi-junction solar cell with 1-sun AM1.5G solar energy conversion efficiency of 33.3% (Romain Cariou et al, Nature Energy, DOI: 10.1038/s41560-018-0125-0).

Silicon solar cells currently have a 90% share of the photovoltaic market, but they are nearing their theoretical efficiency limit. The efficiency of silicon single-junction solar cells is intrinsically constrained to 29.4%, and practically limited to around 27%. It is possible to overcome this limit by combining silicon with higher-bandgap materials, such as III–V compound semiconductors, in a multi-junction device. However, challenges associated with combining these materials have hindered the development of highly efficient III–V/Si solar cells.

Fraunhofer ISE achieved the silicon-based multi-junction solar cell’s high conversion efficiency by using the microelectronics manufacturing process of direct wafer bonding to transfer III-V layers just 1.9μm thick – which had been epitaxially deposited on a gallium arsenide (GaAs) substrate – onto a silicon solar cell.

The surfaces were first deoxidized in an EVG580 ComBond chamber under high vacuum with an ion beam and then bonded together under pressure. The atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a monolithic device. The GaAs substrate is subsequently removed.

The III-V/Si multi-junction solar cell specifically consists of a sequence of subcells stacked on top of each other. A gallium indium phosphide (GaInP) top cell absorbs visible sunlight with wavelengths of 300-670nm, a middle gallium arsenide (GaAs) subcell absorbs 500-890nm near-infrared radiation, and a bottom silicon subcell absorbs longer 650-1180nm wavelengths. Spanning the absorption range of the sun’s spectrum boosts the efficiency significantly.

The three subcells are internally connected by tunnel diodes, including a tunnel oxide passivated contact (TOPCon) applied to the front and back surfaces of the silicon. Also, the cell has a simple front and rear contact (as for a conventional silicon solar cell) and can therefore be integrated into photovoltaic modules in the same manner.

Fraunhofer ISE and industry partner EVG had already demonstrated 30.2% efficiency in November 2016, increasing it to 31.3% in March 2017. They have now again improved the light absorption and the charge separation in silicon, achieving a record of 33.3% efficiency. Specifically, a nanostructured diffraction-grating back-side contact is implemented on the silicon bottom cell to prolong the path length of light, and a front-side contact grid and anti-reflection coating are also applied. Performance is said to be similar to standard III–V/Ge triple-junction solar cells.

“Costs have decreased to such an extent that photovoltaics has become an economically viable competitor to conventional energy sources,” says Fraunhofer ISE director Dr Andreas Bett. However, for industrial manufacturing of III-V/Si multi-junction solar cells in particular, the costs of the III-V epitaxy and the connecting technology with silicon must be reduced. There are still great challenges to overcome in this area, which Fraunhofer ISE intends to solve through future investigation.

“The new result shows how material consumption can be reduced through higher efficiencies, so that not only the costs of photovoltaics can be further optimized but also its manufacture can be carried out in a resource-friendly manner,” says Bett.

Fraunhofer ISE’s new Center for High Efficiency Solar Cells (being constructed in Freiburg) will provide a setting for developing next-generation III-V and silicon solar cell technologies. The ultimate objective is to make high-efficiency solar PV modules with efficiencies of over 30% possible in the future.

Dr Roman Cariou, the young scientist and first author of the paper, was supported through the European Union with a Marie Curie Stipendium (HISTORIC, 655272). The work was also supported by the EU within the NanoTandem project (641023) as well as by Germany’s Federal Ministry for Economic Affairs and Energy BMWi in the PoTaSi project (FKZ 0324247).

See related items:

Fraunhofer ISE lays cornerstone for Center for High-Efficiency Solar Cells

Fraunhofer ISE and EVG set 30.2% efficiency record for silicon-based multi-junction solar cell

Fraunhofer ISE teams with EVG to enable direct wafer bonding for next-gen solar cells

EVG extends wafer bonding equipment and process solutions for covalent bonding technology

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