Tandem junction (n–p+-Si/ITO/WO3/liquid) core–shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. The tandem devices exhibited open-circuit potentials of Eoc = −1.21 V versus E0′(O2/H2O), demonstrating additive voltages across the individual junctions (n–p+-Si Eoc = −0.5 V versus solution; WO3/liquid Eoc = −0.73 V versus E0′(O2/H2O)). Optical concentration (12×, AM1.5D) shifted the open-circuit potential to Eoc = −1.27 V versus E0′(O2/H2O) and resulted in unassisted H2 production during two-electrode measurements (anode: tandem device, cathode: Pt disc). The solar energy-conversion efficiencies were very low, 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H2, respectively, due to the non-optimal photovoltage and band-gap of the WO3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes.
