Photoanodic behavior of vapor-liquid-solid-grown, lightly doped, crystalline Si microwire arraysElectronic supplementary information (ESI) available: Additional information regarding the VLS growth of Si microwire arrays, four-point resistance measurements, photoelectrochemical measurements, concentration overpotential and resistance corrections, and optical measurements of Si wire array films. See DOI: 10.1039/c2ee03468a

Arrays of n-Si microwires have to date exhibited low efficiencies when measured as photoanodes in contact with a 1-1′-dimethylferrocene (Me 2 Fc +/0 )-CH 3 OH solution. Using high-purity Au or Cu catalysts, arrays of crystalline Si microwires were grown by a vapor-liquid-solid process without dopants, which produced wires with electronically active dopant concentrations of 1 × 10 13 cm −3 . When measured as photoanodes in contact with a Me 2 Fc +/0 -CH 3 OH solution, the lightly doped Si microwire arrays exhibited greatly increased fill factors and efficiencies as compared to n-Si microwires grown previously with a lower purity Au catalyst. In particular, the Cu-catalyzed Si microwire array photoanodes exhibited open-circuit voltages of ∼0.44 V, carrier-collection efficiencies exceeding ∼0.75, and an energy-conversion efficiency of 1.4% under simulated air mass 1.5 G illumination. Lightly doped Cu-catalyzed Si microwire array photoanodes have thus demonstrated performance that is comparable to that of optimally doped p-type Si microwire array photocathodes in photoelectrochemical cells. Both Au- and Cu-catalyzed Si microwire arrays grown with a high-purity catalyst demonstrate excellent performance as photoanodes in a non-aqueous regenerative electrochemical cell.