ZnO Nanowires and Their Application for Solar Cells

Nanowires (NW) are defined here as metallic or semiconducting particles having a high aspect ratio, with cross-sectional diameters « 1 ┤m, and lengths as long as tens of microns. Well-aligned one-dimensional nanowire arrays have been widely investigated as photoelectrodes for solar energy conversion because they provide direct electrical pathways ensuring the rapid collection of carriers generated throughout the device (Tang et al., 2008), as well as affording large junction areas and low reflectance owing to light scattering and trapping (Muskens et al., 2008). Solar energy conversion is a highly attractive process for clean and renewable power for the future. Excitonic solar cells (SCs), including organic and dye-sensitized solar cells (DSSC), appear to have significant potential as a low cost alternative to conventional inorganic photovoltaic (PV) devices. The synthesis and application of nanostructures in solar cells have attracted much attention. Metal oxide nanowire (NW) arrays with large surface area and short diffusion length for minority carriers represent a new class of photoelectrode materials that hold great promise for photoelectrochemical (PEC) hydrogen generation applications. Up to now, various metal oxide nanostructures such as TiO2, ZnO, Fe2O3, ZrO2, Nb2O5, Al2O3, and CeO2 have been successfully employed as photoelectrodes in SCs. Among the above-mentioned metal oxide nanostructures, the study of TiO2 and ZnO is of particular interest due to the fact that they are the best candidates as photoelectrode used in SCs. However, the advantage offered by the increased surface area of the nanoparticle film is compromised by the effectiveness of charge collection by the electrode. For DSSCs, the traditional nanoparticle film was replaced by a dense array of oriented, crystalline nanostructures to obtain faster electron transport for improving solar cell efficiency. A typical high-efficiency DSSC (Grätzel, 2009) consists of a TiO2 nanocrystal thin film that has a large surface area covered by a monolayer of dye molecules to harvest sunlight. Comparedwith TiO2, ZnO shows higher electron mobility with similar bandgap and conduction band energies. ZnO is a direct wide bandgap semiconductor (Eg = 3.4 eV) with large exciton binding energy (~60 meV), suggesting that it is a promising candidate for stable room temperature luminescent and lasing devices. Therefore, ZnO nanowires is an alternative candidate for high efficient SCs.