Zinc oxide (ZnO) has remarkable optical properties with a wide bandgap (3.2 eV) and a large exciton binding energy (60 meV). In 2005, Law et al. (Law et al., 2005) reported the preparation of ZnO nanowire array on TCO glass by seed mediated liquid phase synthesis method. The experiment was purposely designed to grow ZnO nanowires with a high aspect ratio so as to attain a nanowire film with high density and sufficient surface area (Fig. 5a and b). A ~25-pm-thick film consisting of ZnO nanowires in diameter of ~130 nm was mentioned to be able to achieve a surface area up to one-fifth as large as a nanoparticle film used in the conventional DSCs. The superiority of ZnO nanowires for DSC application was firstly demonstrated by their high electron diffusion coefficient, 0.05-0.5 cm2 s-1, which is several hundred times larger than that of nanoparticle films. Larger diffusion coefficient means longer diffusion length. In other words, the photoanode made of nanowires allows for thickness larger than that in the case of nanoparticles. This can compensate for the insufficiency of surface area of the nanowire-based photoanode. DSC based on ZnO nanowire gave a power conversion efficiency of 1.5%. In the same year, Prof. Aydil group also successfully prepared ZnO nanowire array on TCO glass and then fabricated a DSC with a conversion efficiency of 0.5% (Baxter & Aydil, 2005).
Fig. 5. Nanowire dye-sensitized cell based on ZnO wire array. (a) Schematic diagram of the cell; (b) Typical scanning electron microscopy cross-section of a cleaved nanowire array on TCO Scale bar=5 pm. (Law et al., 2005).