The Fraunhofer Institute for Solar Energy Systems ISE and Austrian company EV Group (EVG) today reported efficiency of 30.2% for a silicon-based multi-junction solar cell.
The researchers used a "direct wafer bonding" process to transfer a few micrometers of III-V semiconductor material to silicon. The atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a monolithic device.
The efficiency achieved is a first-time result for this type of fully integrated silicon-based multi-junction solar cell, according to the announcement. It noted that the highest efficiency measured to date for a pure silicon solar cell is 26.3%, and the theoretical efficiency limit is 29.4%.
The 30.2% conversion efficiency for the III-V / Si multi-junction solar cell of 4 sq cm (0.62 sq inches) was measured at Fraunhofer ISE's calibration laboratory. The cell comprises a sequence of subcells stacked on top of each other that are made of gallium-indium-phosphide (GaInP), gallium-arsenide (GaAs) and silicon (Si), and span the absorption range of the sun's spectrum.
The direct wafer-bonding process is already used to produce computer chips.
Fraunhofer ISE will work further to tackle the challenges on the way to industrial manufacturing of these solar cells, namely reducing the costs of the III-V epitaxy and the connecting technology with silicon.