Fraunhofer ISE achieves 30.2% efficiency for silicon multi-junction solar cell

Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) have confirmed today the successful creation of a silicon-based multi-junction solar cell that has achieved a record 30.2% efficiency.

In collaboration with Austrian company EV Group (EVG), the researchers exceeded the theoretical limit of silicon solar cells using a direct wafer bonding process to transfer a few micrometers of III-V semiconductor material to silicon.

This process, which is a well-known application in the microelectrics industry, bonds together subcell surfaces in a vacuum by applying pressure after the plasma activation stage. According to Fraunhofer, the atoms on the surface of the III-V subcell then form bonds with silicon atoms to create a monolithic device that, as a fully integrated cell, results in higher efficiencies.

Externally, the solar cell looks the same, its complexity restricted to its inner workings, which gives the researchers hope that it can soon be integrated into a standard PV module using simple front and rear contact like any ordinary silicon solar cell.

"We are working on methods to surpass the theoretical limits of silicon solar cells," said 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."

The 30.2% efficiency was confirmed at the Fraunhofer lab in Freiburg based on a 4 cm-square Si multi-junction solar cell. This far surpasses the current efficiency record for a pure silicon solar cell – 26.3% – and also exceeds the theoretical efficiency limit of 29.4% for silicon solar cells.

"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," added Jan Benick, team leader at Fraunhofer ISE.

Institute director Eicke Weber said: "I am pleased that Fraunhofer ISE has so convincingly succeeded in breaking through the glass ceiling of 30% efficiency with its fully integrated silicon-based solar cell with two contacts. With this achievement, we have opened the door for further efficiency improvements for cells based on the long-proven silicon material."

Earlier this year 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.