16 September 2020
Record 25.9% efficiency for III-V multi-junction solar cells on silicon
As part of the funded MehrSi project, the Fraunhofer Institute for Solar Energy Systems ISE of Freiburg, Germany, in cooperation with the Technical University of Ilmenau, the Philipps University of Marburg and deposition equipment maker Aixtron SE of Herzogenrath, Germany, has achieved record efficiency of 25.9% for a multi-junction solar cell grown directly on a silicon substrate. This was done through optimizing the layer structure and technology.
“For the first time we have now been able to realize a tandem solar cell based on a silicon wafer with such high efficiency,” says Dietmar Schmitz, Aixtron’s vice president corporate technology transfer. Until now, the production of III-V multi-junction solar cells has been based on more expensive compound semiconductor substrate material.
“The gallium and phosphorus atoms must occupy the correct lattice positions at the interface with silicon. To achieve this, we have to control the atomic structure very well. This requires exceptionally high precision,” notes Schmitz. “In addition, to achieve the necessary high quality of the epiwafers, it is crucial that a high crystal quality of all layers is achieved during epitaxial growth,” he adds. “This was achieved in the project thanks to the improved system technology developed by Aixtron and the good cooperation with the project partners.”
In tandem photovoltaics, different combinations of high-performance solar cell materials are arranged in layers on top of each other in order to use the different wavelengths of sunlight more efficiently when converting light into electrical energy. Silicon is suitable for absorbing the infrared portion of the solar spectrum. Layers of different III-V semiconductor compounds, a few microns thick, are then applied to the silicon base to convert the ultraviolet, visible and near-infrared light more efficiently into electricity. Due to tandem photovoltaics using a silicon substrate, the multi-junction solar cells are more cost-effective than those based on other substrates.
After establishing the technological base, the project partners are now working to further increase efficiency and reduce manufacturing costs. To this end, layer deposition is to be realized even faster, with higher throughput and thus more cost-optimization.
“Specifically, the dislocation density in the III-V solar cell layers is to be reduced from 108cm-2 to the range of 1-5x106cm-2 in order to increase the efficiencies to more than 30%,” says professor Dr Michael Heuken, VP corporate R&D at Aixtron. “Last but not least, the cost-effectiveness of the epitaxy processes is to be further optimized,” he adds.
By means of more cost-effective processes, in combination with silicon as the lowest sub-cell, tandem technology is to be made accessible for broad-based photovoltaics in the future. Aixtron says that it has already been working on such III-V multi-junction solar cells on silicon with several partners for many years.
The project SiTaSol for the joint development of layer packages that can be produced as quickly and cheaply as possible is funded by the European Union (EU). In SiTaSol, Aixtron has built and tested a specially optimized metal-organic vapor phase epitaxy (MOVPE) system in its own laboratory.
SiTaSol is funded by the EU (funding number: 727497) and MehrSi is funded by Germany’s Federal Ministry of Education and Research BMBF (funding number: 03SF0525D).