Periodic arrays of n-GaAs nanowires have been grown by selective-area metal-organic chemical-vapor deposition on Si and GaAs substrates. The optical absorption characteristics of the nanowire-arrays were investigated experimentally and theoretically, and the photoelectrochemical energy-conversion properties of GaAs nanowire arrays were evaluated in contact with one-electron, reversible, redox species in non-aqueous solvents. The radial semiconductor/liquid junction in the nanowires produced

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... -unity external carrier-collection efficiencies for nanowire-array photoanodes in contact with nonaqueous electrolytes. These anodes exhibited overall inherent photoelectrode energy-conversion efficiencies of $8.1% under 100 mW cm À2 simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590 AE 15 mV and short-circuit current densities of 24.6 AE 2.0 mA cm À2 . The high optical absorption, and minimal reflection, at both normal and off-normal incidence of the GaAs nanowire arrays that occupy <5% of the fractional area of the electrode can be attributed to efficient incoupling into radial nanowire guided and leaky waveguide modes. Broader context Due to the voltage requirements to produce fuels from sunlight, water, and CO 2 as the inputs, two light-absorbing materials, with band gaps of 1.7 eV and 1.1 eV, respectively, are attractive as the foundation for high-efficiency articial photosynthesis. The integration of materials with 1.7 and 1.1 eV band gaps is, however, very challenging. Accordingly, a nanowire-growth strategy has been developed to integrate single crystal III-V nanowires (e.g. GaAs) with highly mismatched Si substrates. In this work, GaAs nanowire arrays grown on Si were studied using a non-destructive contact method involving non-aqueous photoelectrochemistry. The approach has allowed us to understand the interplay of nanowire growth with the optical absorption and electrical properties of such systems, and will aid in the design and optimization of nanowire-based systems for solar energy-conversion applications.