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

摘要

The steady-state photoelectrochemical 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 (Φ_b), 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 Φ_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 nanowire photoelectrodes are discussed.