Monolithic Solar Water-Splitting Systems: Towards a Sustainable Hydrogen-Energy Future

On of challenging routes to convert solar energy in storable fuels is the light-induced water-splitting in hydrogen and oxygen on integrated tandem-systems based on the assembly of stable and efficient semiconducting photoelectrodes. The working of such a system requires a convenient band-alignment of the n- and p-type semiconducting absorbers, which constitute the base for the photoanode and –cathode, respectively, to fulfill the thermodynamic and kinetic reaction requirements. The water-splitting efficiency depends on the individual performance of photoanodes and photocathodes, which in turn depend on (i) efficient light harvesting including the excess photonic energy, (ii) low recombination losses and (iii) high electron transfer rates of photo-generated minority charge carriers at the different generated materials interfaces. These preconditions are pursued in a system consisting essentially of electro-catalytic centers embedded in a passivated semiconductor substrate having an extremely low concentration of surface reaction sites. Condition (i) requires high absorptivity of the semiconducting material, whereas condition (ii) requires control of the electronic properties of the various interfaces and (iii) implies a better understanding and steering of the electro-catalytic process occurring at the surface of reactive centers that convert sunlight directly to fuels. After an introductory part in the main electronic und energetic aspects of the conversion process in these type of systems, preliminary and model experiments concerning the use of silicon and technologically advanced IV and III-V semiconductors for halfcell configurations are presented. The future implementation of surface modified materials into tandem structures is discussed and future directions concerning the exploitation of photonic effects at metal arrangements of plasmonic materials and the implementation of bio-electrocatalysts in advanced devices is outlined.