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New Record Efficiency for Tandem Solar Cell Technology

In the field of photovoltaics, intensive research is being carried out to continually improve solar cell efficiency. Increasingly, the focus is on tandem photovoltaics, in which high-performance solar cell materials are brought together in various combinations in order to use the solar spectrum even more efficiently when converting light into electrical energy. Fraunhofer ISE is now reporting a new record efficiency of 25.9 percent for a III-V/Si tandem solar cell grown directly on silicon. This cell was produced on a low-cost silicon substrate for the first time – marking an important milestone on the way to economical solutions for tandem photovoltaics.

The Fraunhofer Institute for Solar Energy Systems ISE has been working for many years on multi-junction solar cells, in which two or three sub-cells stacked on top of another absorb different portions of the solar spectrum and convert it into electricity. Silicon is a suitable absorber for the infrared part of the spectrum. Several micrometer-thin layers of III-V compound semiconductors made of elements from groups III and V of the periodic table are deposited on top of silicon. These sub-cells efficiently convert the light from the ultraviolet, visible and near-infrared parts of the spectrum into electricity. III-V semiconductor solar cells on Germanium are often used in space as well as in concentrator photovoltaics. Through the development of better and more cost-effective processes in combination with the use of silicon as the bottom sub-cell, the tandem technology can become accessible for wide-range use in almost all photovoltaic applications in the future. However, there is still much work to be done before this objective can be achieved.

25.9 Percent for III-V/Si Tandem Solar Cells Grown Directly on Silicon

Several different approaches for producing combinations of III-V and silicon solar cells exist. For example, Fraunhofer ISE holds the world record efficiency of 34.1 (new 34.5) percent for a tandem solar cell in which the III-V semiconductor layers are transferred from a gallium arsenide to a silicon substrate and connected by wafer bonding. This technology is very efficient but also expensive. For this reason, Fraunhofer ISE has been working for many years on more direct manufacturing processes in which the III-V layers are deposited, or grown directly onto a silicon solar cell. In this process it is crucial that all layers maintain a high crystal quality, and this presents a major challenge. Now a new world record efficiency of 25.9 percent has been achieved for a III-V/Si tandem solar cell grown directly on silicon. The researcher Markus Feifel of Fraunhofer ISE recently presented his successful results at the 47th IEEE Photovoltaic Specialists Conference, which was held online as many conferences presently are. At this conference his work was distinguished with the Student Award in the Category of Hybrid Tandem Solar Cells. “The complex inner structure of the cell is not visible from the outside, since all of the separate absorbers are internally interconnected by additional crystal layers and electrically series connected,” explains the young solar cell researcher, who was able to improve his results from 24.3 to 25.9 percent in less than a year. “This was achieved by replacing a single thin layer within the multi-junction cell,” he explains. “A careful analysis of our cells showed that this layer created a potential barrier for electrons moving through the crystal structure. “



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