CHAPTER 9: Epitaxial III-V Thin Film Absorbers: Preparation, Efficient InP Photocathodes and Routes to High Efficiency Tandem Structures

The application of III-V semiconductor compounds allow the design of highly efficient absorbers for light-induced water splitting due to their outstanding properties and flexibility. This includes the possibility to tune magnitude and nature of band gaps as well as lattice constants enabling well-defined homo- and heteroepitaxial growth by metal-organic vapor phase epitaxy. Reflection anisotropy spectroscopy enables optical in-situ growth control facilitating the preparation of abrupt interfaces, which are essential for high-efficiency applications. Following this approach, InP(100) absorbers were prepared featuring a stable and efficient photocathode. Surface functionalization could be achieved by in situ photoelectrochemical preparation of an n-type indium oxide layer, which contains phosphates and phosphites, creating a p-n⁺ junction. In addition to stabilizing the cathode against corrosion, the junction enables efficient charge transfer to the Rh electrocatalyst. Our recent progress in III-V heteroepitaxy on silicon substrates extends the parameter space aiming for a water splitting tandem structure on Si. With Si as bottom cell, lattice-matched GaPN is a promising candidate for the top cell. To gain a better understanding of initial surface transformations induced by the electrolyte, we employ model adsorption experiments with GaP(100), the material basis for GaPN. Furthermore, we describe the concept for quantum-well, nanostructured absorbers.