Resource-efficient epitaxial processes for highly efficient III-V tandem solar cells

One major restriction to the broader commercial use of III-V solar cells fabricated by metalorganic vapor phase epitaxy (MOVPE) is costs, partly caused by time-intensive processes. To produce cost-competitive hydrogen directly from solar irradiation a key objective of my PhD is to develop faster and cheaper processes for manufacturing multi-junction solar cells. This not only includes significantly higher growth rates but also a more efficient use of precursor materials. In previous work at Fraunhofer ISE, it was already demonstrated for the first time that the growth rate of GaAs could be increased from 4 μm/h to 280 μm/h. Hereby, several challenges must be overcome, as in general the material quality suffers due to promoted non-radiative recombination. Analyzing the defects that occur at high growth rates and understanding how they are formed is of high scientific importance and will be done in cooperation with TU Ilmenau. Thus, the conditions under which crystals of high quality can be grown fast will be developed.

In the next step of my thesis, the gained knowledge and the optimized processes will be adapted to a new MOVPE system. This system will comprise a reactor especially designed for high throughput. It will be the first of its kind, for instance due to ultra-fast heating and cooling, as well as squared wafers for higher precursor efficiency. Consequently, the full epitaxial growth of a solar cell is shortened from hours to a couple of minutes. Another innovation involves the incorporation of plasma into the reaction chamber. For instance, plasma can be used to deoxidize sample surfaces and clean the reaction chamber quickly and efficiently. The study will also focus on completely new capabilities of plasma ignition during growth. A more efficient precursor decomposition coupled with lower temperatures is expected to enhance deposition.

Eventually, a major objective of my PhD is to fabricate a GaInP/GaAs tandem solar cell with an efficiency above 30% grown in less than five minutes. Combining this novelty with other parts of the project will pave the way for highly efficient and green production of hydrogen directly from solar irradiation at competitive costs.