Researchers at the Fraunhofer Institute for Solar Energy Systems ISE have once again succeeded in raising the efficiency value of monolithic triple-junction solar cells made of silicon and III-V semiconductor materials. Using a combination of multiple absorber materials, these multi-junction photovoltaic cells exploit the energy from the solar spectrum significantly better than conventional silicon solar cells. The world record for a monolithic multi-junction solar cell manufactured by wafer bonding has been increased to 34.1% and an efficiency record of 24.3% achieved for a solar cell with the III-V semiconductor layers deposited directly on the silicon.
“With monolithic multi-junction solar cells, we believe that we can achieve efficiency values of 36%, which would substantially exceed the physical limit of 29.4% offered by a pure silicon solar cell,” explains Dr. Andreas Bett, Institute Director of Fraunhofer ISE. The high efficiency allows for more output per surface area, thus creating a savings of solar cell and module materials.
For the production of multi-junction photovoltaic cells, thin III-V semiconductor layers only a few micrometers thick are deposited on a silicon solar cell. In order to optimally exploit the sun’s rays, the different layers absorb light from different spectral ranges: gallium indium phosphide in the 300–660 nm range (visible light), aluminum gallium arsenide in the 600–840 nm range (near infrared light) and silicon in the 800–1200 nm range (long-wavelength light).
In heading toward the industrial mass production of monolithic multi-junction photovoltaic cells, Fraunhofer ISE researchers see challenges in finding an affordable process for manufacturing the III-V semiconductor layers. For cost-effective solar cell production, new deposition machines with higher throughput and deposition area will be required. These are all methods that researchers at ISE will pursue in the coming years.
Work on wafer-bonded solar cells is funded by the German Federal Ministry for Economic Affairs and Energy. Work on directly grown cells, in which partners Aixtron SE, TU Ilmenau and Philipps-Universität Marburg were involved, was funded by the German Federal Ministry of Education and Research.
Triple-junction solar cells made of III-V semiconductors and silicon have the potential to take photovoltaics to a new level of efficiency. © Fraunhofer ISE