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Here, the structural, electronic and optical properties of the GaP 1-x Sb x /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 (Γ-band) cross-over is found to occur at 30% Sb content. Especially, at a Sb content of 32%, the GaP 1-x Sb x 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 GaP 1-x Sb x 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. • Assessment of GaP1-xSbx/Si tandem materials association is given for the development of efficient III-V/Si photoelectrodes. • The ideal 1.7 eV/1.1 eV bandgap combination can be achieved by using monolithic integration of direct bandgap GaPSb. • Direct bandgap Direct. • Band alignement between GaPSb, Si and water redox potentials is discussed for photoanode or photocathode operation. • The GaPSb/Si materials association is interesting both for hydrogen evolution and oxygen evolution reactions. [ABSTRACT FROM AUTHOR]