GaInP, for example, has an ideal band-gap engineering potential and is commonly grown as a top photoabsorber in record photovoltaic layer structures with either lattice-matched stoichiometry (Ga0.51In0.49P) to GaAs(100) on Ge(100) substrates, or to an underlying GaInAs bottom solar cell. But even more than the intrinsic properties of the individual layers, their interfaces might govern the overall properties of the device. Therefore, investigating the properties of such buried interfaces, such as band alignment and space charge layers induced by defect formation or chemical modifications, is crucial for addressing the efficiency losses of these devices.