Researchers at the Germany-based Fraunhofer Institute for Solar Energy Systems (ISE), together with the Austrian company EV Group (EVG), have claimed a new record after successfully manufacturing a silicon-based multi-junction solar cell with 30.2% efficiency. According to Fraunhofer ISE, that efficiency exceeds the theoretical limit of silicon solar cells.
For this achievement, the researchers used a “direct wafer bonding” process to transfer a few micrometers of III-V semiconductor material to silicon, a well-known process in the microelectronics industry. 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.
According to Fraunhofer ISE, 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, Fraunhofer ISE adds.
“We are working on methods to surpass the theoretical limits of silicon solar cells,” says Dr. Frank Dimroth, department head at Fraunhofer ISE. “It is our long-standing experience with silicon and III-V technologies that has enabled us to reach this milestone today.”
A conversion efficiency of 30.2% for the III-V / Si multi-junction solar cell of 4 cm2 was measured at Fraunhofer ISE’s calibration laboratory. In comparison, the researchers say the highest efficiency measured to date for a pure silicon solar cell is 26.3%, and the theoretical efficiency limit is 29.4%.
More details are available here.
Photo courtesy of Fraunhofer ISE