  Recent rapid progress in efficiencies for solar water splitting by
photoelectrochemical devices has enhanced its prospects to enable storable
renewable energy. Efficient solar fuel generators all use tandem photoelectrode
structures, and advanced integrated devices incorporate corrosion protection
layers as well as heterogeneous catalysts. Realization of near thermodynamic
limiting performance requires tailoring the energy band structure of the
photoelectrode and also the optical and electronic properties of the surface
layers exposed to the electrolyte. Here, we report a monolithic device
architecture that exhibits reduced surface reflectivity in conjunction with
metallic Rh nanoparticle catalyst layers that minimize parasitic light
absorption. Additionally, the anatase TiO2 protection layer on the photocathode
creates a favorable internal band alignment for hydrogen evolution. An initial
solar-to-hydrogen efficiency of 19.3 % is obtained in acidic electrolyte and an
efficiency of 18.5 % is achieved at neutral pH condition (under simulated
sunlight).
