Silicon-based solar cells dominates today’s solar energy industry, while this technology has difficulty in bringing the conversion efficiency beyond 23% for commercialized solar cells in mass production. In order to break the efficiency limit, many solar companies and institutes are devoting to development of various kinds of solar cell technologies.

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE (Fraunhofer ISE) and the Austrian company EV Group (EVG) have collaboratively improved the efficiency of a silicon-based multi-junction solar cell with two contacts to 30.2%, breaking the limit of the theoretical efficiency of a pure silicon solar cell, 29.4%.

The record-breaking solar cell connects subcells made of of GaInP, GaAs and Si materials to form an integrated silicon-based multi-junction structure that is able to absorb wider range of the sun’s spectrum.

The key is a “direct wafer bonding” process that transfers a few micrometers of III-V semiconductor material to silicon. After plasma activation, the subcell surfaces are bonded together in vacuum by applying pressure. The atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a monolithic device.

The researchers successfully realized a conversion efficiency of 30.2% for the III-V / Si multi-junction solar cell with 4cm^2 area, and the efficiency record was measured at Fraunhofer ISE’s calibration laboratory.

Current-voltage characteristic of the GaInP / GaAs / Si solar cell, measured at the Fraunhofer ISE calibration laboratory. (Source: Fraunhofer ISE)

“Key to the success was to find a manufacturing process for silicon solar cells that produces a smooth and highly doped surface which is suitable for wafer bonding as well as accounts for the different needs of silicon and the applied III-V semiconductors,” explains Dr. Jan Benick, team leader at Fraunhofer ISE.

The III-V / Si multi-junction solar cell costs a lot to produce at present, while it is possible to cut the costs by improvements in III-V epitaxy and the connecting technology with silicon. The ultimate objective of the researchers at Fraunhofer ISE is to make high efficiency solar PV modules with efficiencies of over 30 percent possible in the future.

Similar technology by Sharp, applied on satellites and chairs

Sharp is another solar company that has developing solar cells with multi-junction technology. This May, Sharp and Japan’s NEDO together launched a 31.17% efficiency solar cell using compound triple-junction technology, marking the world’s record.

Sharp’s compound triple-junction solar cell connects layers of GaInP, GaAs and InGaAs to absorb a wider range in the sunlight spectrum with a better absorbing effectiveness.

The 31.17% conversion efficiency was realized on a compound triple-junction solar cell with a surface area of 986cm^2. In advanced to this record, Sharp had realized an efficiency of 37.9% with the same technology on a solar cell with a small surface area of 1.047cm^2.

Sharp used to apply the compound triple-junction solar cell on satellites. It was impressive that Sharp has integrated this technology with battery and USB charging port onto a chair for offering solar-powering solution in people’s daily lives. Customers can sit on these ultra-high-tech chairs in three Starbucks shops in Tokyo, Japan.