  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.
