Overview of Engineered Germanium Substrate Development for Affordable Large-Volume Multijunction Solar Cells

New massive markets for space multijunction solar cells are being discussed globally. For such an explosive increase in demand to materialize, a more sustainable and affordable Ge substrate technology is required. To this end, lithography-based Ge-on-Nothing and electrochemical process-based porous Ge wafers were developed. Both approaches yield uniform and smooth monocrystalline Ge-on-Ge engineered substrates after annealing, of which the top layer is weakly attached to the parent substrate. High-quality space solar cells were grown on them, followed by successful foil detachment and surface reconditioning. These results clearly demonstrate the feasibility of the reusable Ge substrate concept.