The new four junction space solar cell requested developing a number of new metamorphic materials and structures, in particular InGaAsP and InGaP sub-cell materials, optically transparent tunnel diodes and bond layers as well as metamorphic buffer to grow III/V cell stack of Ge substrate. It is noteworthy that additionally to high lattice mismatch to Ge, the requested composition of quaternary InGaAsP material was earlier predicted to be in a miscibility gap for As- and P-containing compounds, thus, is hardly achievable using typical growth parameters of a metalorganic epitaxy process. Further, the parts of the developed cell structure were directly bonded together, for which a corresponding chemical-mechanical polishing as well as wafer bonding processes were developed and optimised. Here, higher roughness and bow of metamorphic layers and high Indium content were remarkable challenges for CMP and wafer bonding, respectively. After multiple development rounds, each with several design optimisations, a 4J SBT cell with an BOL efficiency of 31% and an EOL efficiency of 25,5% (after 3E15 cm2 1MeV electron irradiation) could be demonstrated.
It is to note that the EOL efficiency could be confirmed after the formal end of the project due to being out of schedule for EOL characterization. With certain improvements identified in the project, the practical BOL efficiency has the potential to be further increased reaching the target of 32-33% for this cell design. These values are very close to the theoretically predicted cell performance potential for this particular cell design. The 4J SBT cells achieved in the project showed very high radiation hardness and successfully passed thermal cycling according to typical space requirements and, thus, demonstrated their technical potential for implementation into a future cell product, especially focussing the operation in radiation reach environments.
In parallel, crystal growth of dislocation free germanium ingots with large diameters was achieved and a pilot-line for wafer manufacturing was successfully realised. The surface quality of resulting 200 mm wafers achieves the grade of commercial 150 mm substrates. The 200 mm epi-ready Ge wafers were used for realisation of 3G30 solar cell structures used as a benchmark. High wafer-to-wafer and run-to-run reproducibility regarding the epitaxy process as well as excellent device performance with an average cell efficiency of 29.9% was for the first time achieved confirming industrial feasibility of both, wafer and cell manufacturing with 200mm wafer size. The cell reliability was justified by engineering tests, in particular thermal cycling performed on cell coupon level. Finally, the growth of solar cell structures on two separate 200 mm Ge substrates and their subsequent merging by the direct semiconductor bonding technology was demonstrated to confirm the potential of cost reduction for this promising technology by using large area substrates.
Project results were disseminated in 6 publications in scientific journals (incl. conference proceedings) and covered all main fields of the project. 4 of these publications were joint publications involving multiple partners. Moreover, the achieved results were communicated in 7 conferences and workshops without proceedings. At the end of the project, a scientific workshop on space solar cells was organised and held virtually.
