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Thinned Germanium Substrates for III-V Multijunction Solar Cells

Multijunction solar cells are usually grown on Ge substrates. This implies several disadvantages that hinder the performance of the whole multijunction and limit their possible applications. The drawbacks caused by the substrate are: heavier devices, higher operation temperatures, lower performance and lack of photon confinement. In this work we propose thinning the substrate as a valid solution to the aforementioned challenges. The influence of the substrate thickness on the Ge subcell performance inside a multijunction is simulated using 2D TCAD tools. Simulation results point to the back surface recombination as the key parameter to enhance the development of thinned Ge subcells. Ge substrates have been thinned down, achieving 115μm thick samples. Finally, solar cells have been manufactured out of the thinned substrates proving a limited degradation and showing the feasibility of this process to manufacture Ge subcells thinned down up to 115μm.