How can solar energy be used even better in the future to provide affordable and clean energy for everyone? An interdisciplinary research team at TU Ilmenau has now come a lot closer to answering this question: As part of many years of research, the scientists from the departments of Fundamentals of Energy Materials and Theoretical Physics, together with international partners, have deciphered a previously unknown atomic structure on an industrially prepared silicon surface that plays a crucial role in the production of solar cells and has occupied researchers worldwide for many decades. The decoding of the newly discovered motif on silicon surfaces could significantly increase the efficiency of silicon-based, so-called tandem solar cells and make solar energy much cheaper in the future. The researchers have published their findings in the Journal of Applied Surface Science.
With the 2030 Agenda, the global community has set itself 17 goals - the Sustainable Development Goals (SDGs) - for sustainable development. Solar cells play a crucial role in achieving SDG 7, affordable and clean energy, as they convert sunlight into clean energy. According to the Federal Network Agency, solar cells with a total output of 14.1 gigawatts were installed in Germany in 2023 alone - twice as much as in the previous year. In the same year, solar cells with a total output of 81.7 gigawatts were installed in Germany, which corresponds to a contribution to the renewable electricity mix of almost 50 percent (47.8).
"However, the usable area for photovoltaics on earth is often limited," explains Chris Yannic Bohlemann, research associate at the Fundamentals of Energy Materials Group at TU Ilmenau and lead author of the study: "It is therefore important to convert sunlight into electrical power with the highest possible efficiency, especially since the costs of individual solar modules are already only a fraction of the costs of open-air solar parks."
In principle, electricity from photovoltaics can be generated using various semiconducting materials. The semiconductor material silicon is currently used for almost all solar modules used on earth. "At currently around 25 percent for industrially manufactured modules, however, the theoretical maximum efficiency of silicon-based solar modules of around 30 percent has almost been reached, and the possibilities for improving the efficiency of conventional silicon solar cells are reaching their natural limits," says Bohlemann.
Scientists worldwide are therefore conducting intensive research into combining silicon with other semiconductor materials, such as materials from the group of III-V semiconductors, in so-called multiple absorber or tandem solar cells, in which the light is better utilized by stacking several semiconductor materials on top of each other. Efficiency levels of almost 50 percent can already be achieved with concentrated sunlight. "However, III-V tandem solar cells are still very expensive compared to purely silicon-based cells. III-V semiconductor structures in combination with silicon would be the ideal solution - high efficiency at low cost. The difficulty arises during application, where so-called defects often form in the semiconductor, which reduce efficiency," explains Bohlemann.
Less raw materials required and cheaper production
For a total of five years, the researchers at TU Ilmenau therefore worked with international partners on how such defects can be avoided in the future when growing III-V semiconductor structures on low-cost silicon. They combined various experimental methods with theoretical findings and used their own sample transfer system. This enabled them to examine the surfaces with a scanning tunneling microscope without any impurities on an atomic scale.
This means that we can 'press pause' at various points in the growth of III-V semiconductor materials on silicon and are thus the only research group in the world to analyze exactly what happens during these growth processes.
To do this, the scientists used infrared spectroscopy, among other things, in experimental investigations and were initially able to detect hydrogen and, in further measurements, silicon and arsenic on the surface, which together formed a 'mixed structure'. Based on these results, the scientists then carried out simulations to determine the chemical composition of these elements on the surface. Chris Yannic Bohlemann: "We found several chemically stable configurations, one of which was completely new. This new configuration fitted very well with the atomic images we observed and agreed well with the experimental results."
According to the researchers, these findings should make it possible in future to minimize defects in the growth of III-V semiconductors on silicon and thus increase the efficiency of silicon-based tandem solar cells: "The aim is now to be able to combine reliable tandem solar cells with high efficiency with the already highly developed and cost-effective silicon photovoltaics. Other optoelectronic III-V components with the highest performance characteristics, such as light-emitting diodes, could also be produced on silicon substrates in future," explains Bohlemann:
'This would reduce the raw material requirements of rare materials such as indium and make production even cheaper.
The research results of the interdisciplinary team led by Professor Thomas Hannappel were produced as part of several projects funded by the Federal Ministry of Education and Research and the German Research Foundation (DFG) and in long-term cooperation with the Group of Theoretical Physics I led by Professor Erich Runge as well as international partners such as Dr. Oleksandr Romanyuk from the Academy of Sciences in the Czech Republic.
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Chris Yannic Bohlemann
Fundamentals of Energy Materials