In an efficient solar-driven water-splitting photo-electrochemical (PEC) cell, the individual process steps can be divided into three different steps: (i) generation of appropriate electrochemical potentials by max. light absorption, (ii) non-dissipative transport of charge carriers and (iii) efficient charge separation and high catalytic activity at the solid liquid interface.  Consideration of all energetic and kinetic processes leads to the conclusion that only individual absorber materials are suitable with band gaps, which are too large for an efficient exploitation of the sun light and, therefore, efficient water splitting. Alternatively, tandem layer structures are capable to create a sufficient photo voltage, i.e. a sufficient splitting of the quasi-Fermi levels, and in addition to explore the solar spectrum most efficiently [1]. When using tandem cells, STH efficiencies can be achieved clearly exceeding 20% [2,3]. The difficulty arises to achieve a stable performance and to describe the microscopic processes at the challenging solid-liquid interface. Based on surface chemistry observed in model experiments [4], we firstly applied interfacial processing sequences for the functionalization of highly efficient III-V-semiconductor tandem absorbers [5], secondly studied in situ interfacial chemistry on the atomic scale, and thirdly prepared well-defined surfaces to explore different surface reconstructions on their initial interaction with water and oxygen. Different atomic surface reconstructions of InP and GaP, [100] and [111], prepared by metal-organic vapor epitaxy (MOVPE) and transferred in inert gas to the photo-electrochemical cell were investigated. In order to realize cost-competitive tandem device structures, low-defect III-V semiconductor integration into the mature silicon technology can be accomplished with the involvement of graded layer growth of GaAs_xP_1-x. For that, we studied graded GaAsP(001) growth in situ with reflection anisotropy spectroscopy (RAS). With increasing As supply, a characteristic spectral fingerprint of the surface reconstruction shifts towards lower photon energies, which is well observable at growth temperature and for a broad range of As concentrations. Within a simplified empirical model, this shift depends approximately linearly on the As content in the GaAsP layer. The As content of individual GaAsP layers can be quantified in situ during growth which is beneficial for process control. It is shown how to control in situ atomic scale preparation and epitaxial growth of efficient device structures for water splitting.