The solar-to-hydrogen (STH) efficiency limits, along with the maximum efficiency values and the corresponding optimal band gap combinations, have been evaluated for various combinations of light absorbers arranged in a tandem configuration in realistic, operational water-splitting prototypes. To perform the evaluation, a current-voltage model was employed, with the light absorbers, electrocatalysts, solution electrolyte, and membranes coupled in series, and with the directions of optical absorption, carrier transport, electron transfer and ionic transport in parallel. The current density vs. voltage characteristics of the light absorbers were determined by detailed-balance calculations that accounted for the ShockleyQueisser limit on the photovoltage of each absorber. The maximum STH efficiency for an integrated photoelectrochemical system was found to be $31.1% at 1 Sun (¼1 kW m À2, air mass 1.5), fundamentally limited by a matching photocurrent density of 25.3 mA cm À2 produced by the light absorbers. Choices of electrocatalysts, as well as the fill factors of the light absorbers and the Ohmic resistance of the solution electrolyte also play key roles in determining the maximum STH efficiency and the corresponding optimal tandem band gap combination. Pairing 1.6-1.8 eV band gap semiconductors with Si in a tandem structure produces promising light absorbers for water splitting, with theoretical STH efficiency limits of >25%. Broader contextAn integrated system that allows for the direct production of fuels from sunlight would provide a scalable, sustainable source of clean fuels for grid storage as well as for use in the transportation sector. Because all of the components of such a complete system must operate under mutually compatible conditions, an assessment of the required materials properties necessitates a systems-level analysis of the operational conditions of an integrated solar-fuel generator. Accordingly, the optimal system efficiency has been calculated for various solar-fuel generator system geometries and component dimensions, as a function of the band gaps of the light absorber components that serve to capture and convert sunlight into chemical fuels. Dual band gap light absorber congurations have been evaluated, with the dual band gap, tandem structure, providing optimal efficiency and thus a preferred approach, to effect direct solar-fuel production. The assessment has incorporated a variety of systems-level parameters that are present in an actual operating solar-fuel generator system, including the thermodynamic constraints on the light absorbers as given by the Shockley-Queisser detailed-balance limit, the overpotentials of earth-abundant catalysts for water oxidation and reduction reactions, and the effects of solution resistance and light absorber quality on the overall conversion process of sunlight into chemical fuels.

Several application fields can benefit from solar-hydrogen technologies via specific short-term and long-term pathways.