“We are working on methods to surpass the theoretical limits of silicon solar cells,” says Dr. Frank Dimroth, department head at Fraunhofer Institute for Solar Energy Systems (ISE).
This is another example of the seemingly limitless innovation in photovoltaic materials and systems worldwide.
Researchers at ISE worked with the Austrian company EV Group (EVG) to build a silicon-based multijunction solar cell with a conversion efficiency of 30.2 percent. 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, according to the scientists.
They used a “direct wafer bonding” process to transfer a few micrometers of III-V semiconductor material to silicon, a familiar process in the microelectronics industry. After plasma activation, the subcell surfaces are bonded together in a vacuum by applying pressure. The atoms on the surface of the III-V subcell form bonds with the silicon atoms to create a monolithic device. The cell has a simple front and rear contact just like a conventional silicon solar cell, making it easier to integrated into today’s photovoltaic module assemblies.
The announcement provides more details on how three layers in the multijunction cell work together to achieve such high efficiency:
“The III-V / Si multi-junction solar cell consists of a sequence of subcells stacked on top of each other. So-called “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.”
“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.