Publishing their results in Nature Energy under the title "III–V-on-silicon solar cells reaching 33% photoconversion efficiency in two-terminal configuration", the researchers report they demonstrated a III–V/Si cell reaching similar performances to standard III–V/Ge triple-junction solar cells.
Back in November 2016, the solar researchers in Freiburg together with their industry partner EVG had demonstrated an efficiency of 30.2 percent, increasing it to 31.3 percent in March 2017. Now they have succeeded once again in greatly improving the light absorption and the charge separation in silicon, thus achieving a new record of 33.3 percent efficiency.
The "direct wafer bonding" process used to transfer III-V semiconductor layers only 1.9 micrometres thick to silicon was performed with EVG's EVG580 ComBond chamber after the surfaces had been deoxidized under high vacuum with an ion beam. Then in a pressurized environment, the atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a monolithic device. The researchers addressed key issues of multi-junction cells such as III–V/Si interface recombination and silicon's weak absorption by using poly-silicon/SiOx passivating contacts and a novel rear-side diffraction grating for the silicon bottom cell.