Fraunhofer ISE and EVG set 31.3% efficiency for silicon multi-junction solar cell

The Austrian manufacturer of semiconductor production equipment EV Group (EVG) and Germany’s Fraunhofer Institute for Solar Energy Systems ISE have announced a new efficiency record for silicon-based multi-junction solar cells.

Researchers from both entities claim they have achieved an efficiency of 31.3% for this kind of cells, thus improving their previous record of 30.2%, which was announced last November.

The scientists have used a direct wafer bonding process to transfer a few micrometers of III-V semiconductor material to silicon. Through this process, which is implemented after plasma activation, the subcell surfaces are tied together in vacuum by applying pressure. As a result, Fraunhofer ISE explains, atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a monolithic device.

The cell consists of a sequence of three subcells made of gallium-indium-phosphide (GaInP), gallium-arsenide (GaAs)and silicon (Si), which are stacked together and span the absorption range of the sun’s spectrum. The III-V layers are epitaxially deposited on a GaAs substrate and then bonded to a silicon solar cell structure.

Although it is extremely complex in its inner working, the cell has a common external appearance. This, the researchers claim, gives hope that it can soon be integrated into a standard PV module using the simple front and rear contact seen in any ordinary silicon solar cell.

In early 2016, researchers at Australia’s University of New South Wales managed to reach a conversion efficiency of 34.5% using a 28 cm-squared four-junction mini-module that utilized a hybrid four-junction receiver to maximize the amount of electricity extracted from sunlight – a result that challenged perceptions on theoretical efficiency limits in solar.