The space industry is on the verge of rapid expansion into real-time earth observation, internet communication and space exploration. Initiatives are not only coming from government space agencies, but also from Silicon Valley entrepreneurs. This expansion calls for appropriate power generation in space, where solar energy will remain the premier technology for many years to come.
Solar cells for space missions are mostly based on multi-junction technology. The cells have multiple p–n junctions made of different semiconductor materials stacked on top of each other. Each p-n junction will convert a different part of the light spectrum into electricity, improving overall conversion efficiency.
Triple-junction cells – consisting of indium gallium phosphide (InGaP), indium gallium arsenide (InGaAs) and germanium – are fabricated using metal-organic vapour deposition (MOCVD) on germanium wafers. Conversion efficiencies of triple-junction cells have been demonstrated above 30% under AM0 spectrum. Triple-junction solar cells are the selection of choice for space applications thanks to their high efficiency, high resistance to cosmic radiation and low temperature coefficient.
Since the mid-nineties Umicore has been the recognized market leader in the supply of epi-ready, dislocation-free germanium substrates for III-V multi-junction solar cells. Germanium is the preferred substrate as it offers high strength at minimal thickness, cosmic radiation hardness, and active contribution to the cell’s overall performance. Advanced triple-junction solar cells on germanium offer the best lifetime performance for a given weight and size ratio.