Researchers exceed theoretical efficiency limit for Si solar cells

November 14, 2016 // By Christoph Hammerschmidt
The solar cell technology is evolving rapidly, new efficiency records are set almost every month. Mostly, these achievements are bound to some exotic materials or configurations. This time, researchers have spiraled up the efficiency for silicon-based multi-junction cells: 30.2 percent is the current record. What makes their result so extraordinary: They exceeded the theoretical limit of silicon solar cells.

For this achievement, researchers from the Fraunhofer Institute for Solar Energy ISE (Freiburg, Germany) and semiconductor process company EV Group (St. Florian, Austria) used a direct wafer bonding process to transfer a few micrometers thin layer 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 efficiency achieved by the researchers presents a first- time result for this type of fully integrated silicon-based multi-junction solar cell. The complexity of its inner structure is not evident from its outer appearance: the cell has a simple front and rear contact just as a conventional silicon solar cell and therefore can be integrated into photovoltaic modules in the same manner. A conversion efficiency of 30.2 percent for the III-V / Si multi-junction solar cell of 4 cm2 was measured at Fraunhofer ISE’s calibration laboratory. In comparison, the highest efficiency measured to date for a pure silicon solar cell is 26.3 percent, and the theoretical efficiency limit is 29.4 percent.

The III-V / Si multi-junction solar cell consists of a sequence of subcells stacked on top of each other. Tunnel diodes internally connect the three subcells made of gallium-indium-phosphide (GaInP), gallium-arsenide (GaAs) and silicon (Si), which span the absorption range of the sun’s spectrum. The GaInP top cell absorbs radiation between 300 and 670 nm. The middle GaAs subcell absorbs radiation between 500 and 890 nm and the bottom Si subcell between 650 and 1180 nm, respectively. The III-V layers are first epitaxially deposited on a GaAs substrate and then bonded to a silicon solar cell structure. Subsequently the GaAs substrate is removed, and a front and rear contact as well as an antireflection coating are applied. “Key to the success was to find a manufacturing process for silicon solar cells that produces a