Design Considerations for Nanowire Heterojunctions in Solar Energy Conversion/Storage Applications

The steady-state photoelcctrochemical responses of semiconductor nanowire arrays in a nonaqueous regenerative photoelectrochemical cell were analyzed. Experimental and numerical simulation data were collected to determine the extent that dopant density levels, N-D, have on the efficiency of semiconductor nanowire photoelectrodes with radii (r) comparable to the width of the depletion region (W). Films of Si nanowires (r < 40 nm) were prepared by metal-assisted chemical etching of single-crystalline Si(111) substrates with known bulk optoelectronic properties and utilized as photoelectrodes in a methanolic electrolyte containing dimethylferrocene (dmFc) and dimethylferrocenium (dmFc(+)). This photoelectrochemical system featured definable values for the rate of heterogeneous charge-transfer, the interfacial equilibrium barrier height (4:01,), and the rate of surface recombination. Under white light illumination, the photocurrent potential responses of Si nanowire arrays were strongly influenced by the ratio between the nanowire radius and the depletion region width (r/W). Lightly doped Si nanowire arrays consistently showed lower light-saturated photocurrents than heavily doped Si nanowire arrays despite having hole diffusion lengths, L-p, that were larger by a factor of 2. Measurement of the wavelength-dependent external quantum yields for the Si nanowire arrays separated out the effects from the underlying Si substrate and confirmed that carrier-collection was either significantly enhanced or suppressed by the Si nanowires depending on the value of r/W established by the Phi(b), and N-D. Digital simulations of nanowire heterojunctions using a two-dimensional semiconductor analysis software package (TeSCA) and known system parameters are presented that further explore the quantitative interplay between r/W and collection efficiency for nanowire photoelectrodes. The implications for designing low-cost semiconductor photoelectrodes using nanowire-based heterojunction architectures are examined, and tolerances for control over doping levels in semiconductor nanowirc photoelectrodes are discussed.