pH-Independent, 520 mV Open-Circuit Voltages of Si/Methyl Viologen2+/+ Contacts Through Use of Radial n+p-Si Junction Microwire Array Photoelectrodes

The effects of introducing an n+-doped emitter layer have been evaluated for both planar Si photoelectrodes and for radial junction Si microwire-array photoelectrodes. In contact with the pH-independent, one-electron, outer-sphere, methyl viologen redox system (denoted MV2+/+), both planar and wire array p-Si photoelectrodes yielded open-circuit voltages, Voc, that varied with the pH of the solution. The highest Voc values were obtained at pH) 2.9, with Voc) 0.53 V for planar p-Si electrodes and Voc) 0.42 V for vapor-liquid-solid catalyzed p-Si microwire array samples, under 60 mW cm-2 of 808 nm illumination. Increases in the pH of the electrolyte produced a decrease in Voc by approximately-44 mV/pH unit for planar electrodes, with similar trends observed for the Si microwire array electrodes. In contrast, introduction of a highly doped, n+ emitter layer produced Voc) 0.56 V for planar Si electrodes and Voc) 0.52 V for Si microwire array electrodes, with the photoelectrode properties in each system being essentially independent of pH over six pH units (3 < pH < 9). Hence, formation of an n+ emitter layer not only produced nearly identical photovoltages for planar and Si microwire array photoelectrodes, but decoupled the band energetics of the semiconductor (and hence the obtainable photovoltage) from the value of the redox potential of the solution. The formation of radial junctions on Si microwire arrays thus provides an approach to obtaining Si-based photoelectrodes with high-photovoltages that can be used for a variety of photoelectrochemical processes, including potentially the hydrogen evolution reaction, under various pH conditions, regardless of the intrinsic barrier height and flat-band properties of the Si/liquid contact. I