34.1%: Monolithic Triple-Junction Solar Cell Efficiency

• Researchers at the Fraunhofer ISE have broken their own records to achieve higher efficiency levels for silicon based solar cells
• Breaking previous record of 33.3% for silicon-based multi-junction solar cell, the team achieved 34.1% efficiency this time around using direct wafer bonding method
• Using the technique of depositing III-V semiconductor layers on silicon solar cells while reducing the chance of defects in the layers, Fraunhofer ISE overachieved the previous record of 22.3%
• The team says efficiency value of 36% for monolithic multi-junction solar cells is possible which would substantially exceed the physical limit of 29.4% offered by a pure silicon solar cell

Fraunhofer ISE has achieved a new record efficiency level for silicon based solar cells, reaching 34.1% for a monolithic triple-junction solar cells. It also announced reaching  24.3% for III-V semiconductor layers deposited directly on the silicon, saying that the combination of multiple absorber materials for multi-junction PV cells help exploit the energy from the solar spectrum significantly better than conventional silicon solar cells.

34.1% efficiency

The German institute broke its own previous record of 33.3%  achieved for silicon-based multi-junction solar cells it announced in April 2018 (see Fraunhofer 33.3% Si-MJ Cell Efficiency Record). For the 34.1% efficiency record, Fraunhofer ISE's team deployed direct wafer bonding to create a monolithic multi-junction solar cell, depositing III-V layers on a gallium arsenide substrate. It improved deposition conditions and introduced a new cell structure for the uppermost sub-cell made of gallium indium phosphide. This helped better visible light conversion and proves the immense potential of this technology, said Dr Frank Dimroth, Head of Department III-V Photovoltaics and Concentrator Technology at Fraunhofer ISE.

24.3% efficiency

Creating multi-junction PV cells by directly depositing III-V semiconductor layers on silicon solar cells is less time consuming than the path involving wafer bonding. It also avoids the use of expensive GaAs substrates proving its potential for industrial implementation of the technology.

The team reduced the number of defects in the semiconductor layers which adversely impacts the cell efficiency. "We were able to make major progress in this area — current generation in the three sub-cells is now barely affected by these defects, which has enabled us to realize 24.3% efficiency for this technology for the first time anywhere in the world," said Dr Dimroth. "The potential is comparable to that of the wafer-bonded cells. We've got our work cut out for us in the coming years in order to prove that this is the case."

This efficiency level is an improvement over Fraunhofer ISE's previous record of 22.3% reported earlier for such a solar cell (see Fraunhofer ISE Reports 22.3% Efficiency).

Looking ahead for industrial mass production of monolithic multi-junction solar PV cells, the research team of Fraunhofer ISE sees challenges in finding an affordable process for manufacturing the III-V semiconductor layers and requirements of new deposition machines with higher throughput and deposition area. That's something, ISE says, it will pursue over the coming years.

"Monolithic multi-junction solar cells are a source of hope for the further development of the silicon solar cells dominating the field today because they can lead to significantly higher efficiency values when converting sunlight into electrical power," added Dr. Andreas Bett, Director of Fraunhofer ISE. "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."