Higher Efficiencies in Tandem

In his doctoral thesis, Markus Feifel researched a novel solar cell concept in the field of tandem photovoltaics. While conventional silicon solar cells use only one absorber material to convert sunlight into electricity, the tandem technology combines several different absorber materials, or subcells, to optimally convert a wider range of the sun’s spectrum into electricity.  Each subcell has a thickness of only a few micrometers and selectively absorbs only that part of the light that it can convert into electricity with comparatively low losses, thus enabling higher efficiencies than conventional single-junction solar cells. Due to the different crystallographic properties of the semiconductor materials used, the manufacturing processes of multi-junction solar cells used up to now are complex and costly. Therefore, these cells have been used almost exclusively in space applications. 

In the approach that Markus Feifel explored in his work, the solar cell layers are grown directly on low-cost silicon substrates. By using a new electron microscopy method, it was possible for the first time to detect numerous crystal defects that lower the performance of the overall structure. "By specifically analyzing the individual defects and their cause, we succeeded in increasing the efficiency of a GaInP/GaAs/Si triple-junction solar cell from 19.7 % to 25.9 % in several steps. I am very pleased to be awarded an honorary certificate for outstanding achievements by the Gips-Schüle Foundation for this research work and the results," says Markus Feifel. He adds: "The record value achieved is already within the range of the best silicon single-junction solar cells. However, there is still great potential for future improvement."

The findings from the research are to be used to strengthen Europe in its role as a research and development location for the photovoltaic industry and to effectively bring on the energy transition. Projects with the aim of setting up a pilot line for the production and further development of this tandem cell concept are already underway.