  Here, the structural, electronic and optical properties of the GaP1-xSbx/Si
tandem materials association are determined in view of its use for solar water
splitting applications. The GaPSb crystalline layer is grown on Si by Molecular
Beam Epitaxy with different Sb contents. The bandgap value and bandgap type of
GaPSb alloy are determined on the whole Sb range, by combining experimental
absorption measurements with tight binding (TB) theoretical calculations. The
indirect (X-band) to direct ({\Gamma}-band) cross-over is found to occur at 30%
Sb content. Especially, at a Sb content of 32%, the GaP1-xSbx alloy reaches the
desired 1.7eV direct bandgap, enabling efficient sunlight absorption, that can
be ideally combined with the Si 1.1 eV bandgap. Moreover, the band alignment of
GaP1-xSbx alloys and Si with respect to water redox potential levels has been
analyzed, which shows the GaPSb/Si association is an interesting combination
both for the hydrogen evolution and oxygen evolution reactions. These results
open new routes for the development of III-V/Si low-cost high-efficiency
photoelectrochemical cells.
