Fraunhofer Efficiency Record For Tandem Solar Cells
Record 22.3 percent efficiency for tandem of III-V layers on silicon
Scientists at Fraunhofer ISE in cooperation with partners have achieved a new efficiency record of 22.3 percent for a multi-junction solar cell made of silicon and III-V semiconductor materials. The achievement is that the III-V layers were directly grown on the silicon.
By combining different semiconductor materials, solar cell researchers are attempting to surpass the theoretical efficiency limit of 29.4 percent for a single-junction silicon solar cell, and convert sunlight into electricity even more efficiently.
A promising approach is to combine silicon with III-V semiconductor compounds like GaAs. To realise this, one method is to first deposit the III-V solar cell structures on GaAs substrates, then transfer to a silicon solar cell using semiconductor bonding technology and lastly etch away the GaAs substrate. Another less costly approach, however, is to directly grow the III-V layers on the silicon solar cell.
In the second approach, the atomic structure must be controlled extremely well during growth so that the gallium and phosphorous atoms arrange on the correct lattice sites at the interface to the silicon material. Also, the distance between the atoms in the crystal lattice must be increased in order to produce the GaAs material. Researchers have been working on these challenges for over ten years. Now they have been able to greatly reduce the defect density in the III-V semiconductor layers on the silicon and have successfully produced a III-V/Si tandem solar cell with a new efficiency record of 22.3 percent using this direct-growth approach.
The efficiency value was published in the internationally recognised table of the best research-cell efficiencies worldwide on December 25, 2018.
“We are delighted with this result for the direct growth of III-V semiconductors on silicon, an important research approach for tandem solar cells,” says Andreas Bett, institute director of Fraunhofer ISE. “In Freiburg, we are presently building a new research centre for high efficiency solar cells. Our work on tandem cells will be carried out in the new facilities upon its completion in 2020. With the improved technical infrastructure, we expect the developments in multi-junction solar cells based on silicon to accelerate rapidly.”
Within the MehrSi project over the past years, the junction between the silicon crystal and the first III-V semiconductor layer of GaP was investigated and continuously optimised in close cooperation with the research groups of Thomas Hannappel on the TU Ilmenau and of Kerstin Volz at the Philipps University Marburg. The defects in the crystalline structure were first made visible and then reduced step by step. “The record efficiency of our III-V/Si tandem solar cell demonstrates that we have achieved a very good understanding of the materials,” explains Frank Dimroth, coordinator of the MehrSi project. With the successful direct growth of III-V layers on silicon, we can avoid using expensive III-V substrates for epitaxy. This approach is, therefore, a key technology for the cost-effective manufacture of high efficiency tandem solar cells in the future.
The MehrSi project, in which the record multi-junction solar cell on silicon was created, was financed by the German Federal Ministry for Education and Research BMBF. The project partners were the TU Ilmenau, the Philipps University of Marburg and the system manufacturer Aixtron SE.