Theoretical and Experimental Assessment of Thinned Germanium Substrates for III-V Multijunction Solar Cells

Solar cells manufactured on top of Ge substrates suffer from inherent drawbacks that hinder or limit their potential. The most deleterious ones are heavy weight, high bulk recombination, lack of photon confinement and an increase of the heat absorption. The use of thinned Ge substrates is herein proposed as a possible solution to the aforementioned challenges. The potential of a thinned Ge subcell inside a standard GaInP/Ga(In)As/Ge triple-junction solar cell is assessed by simulations, pointing to an optimum thickness around 5-10 {\mu}m. This would reduce the weight by more than 90 %, whereas the available current for the Ge subcell would decrease only by 5 %. In addition, the heat absorption for wavelengths beyond 1600 nm would decrease by more than 85 %. The performance of such a device is highly influenced by the front and back surface recombination of the p-n junction. Simulations remark that good back surface passivation is mandatory to avoid losing power generation by thinning the substrate. In contrast, it has been found that front surface recombination lowers the power generation in a similar manner for thin and thick solar cells. Therefore, the benefits of thinning the substrate are not limited by the front surface recombination. Finally, Ge single-junction solar cells thinned down to 85 {\mu}m by wet etching processes are demonstrated. The feasibility of the thinning process is supported by the limited losses measured in the current generation (less than 6 %) and generated voltage (4 %) for the thinnest solar cell manufactured.