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The terahertz absorption coefficient, index of refraction, and conductivity of nanostructured ZnO have been determined using time-resolved terahertz spectroscopy, a noncontact optical probe. ZnO properties were measured directly for thin films and were extracted from measurements of nanowire arrays and mesoporous nanoparticle films by applying Bruggeman effective medium theory to the composite samples. Annealing significantly reduces the intrinsic carrier concentration in the ZnO films and nanowires, which were grown by chemical bath deposition. The complex-valued, frequency-dependent photoconductivities for all morphologies were found to be similar at short pump-probe delay times. Fits using the Drude-Smith model show that films have the highest mobility, followed by nanowires and then nanoparticles, and that annealing the ZnO increases its mobility. Time constants for decay of photoinjected electron density in films are twice as long as those in nanowires and more than 5 times those for nanoparticles due to increased electron interaction with interfaces and grain boundaries in the smaller-grained materials. Implications for electron transport in dye-sensitized solar cells are discussed.

ZnO nanowires, grown on transparent conducting oxide substrates from aqueous solutions of methenamine and Zn(NO3)2, were integrated as the wide band gap semiconductor into dye-sensitized solar cells. ZnO nanowires and their growth mechanisms were studied using electron microscopy, x-ray diffraction and photoluminescence measurements. The solution growth method forms dense arrays of long nanowires oriented normal to the substrate surface because nanowires growing at off-normal angles are prevented from growing further when they run into neighbouring wires. Dye-sensitized solar cells with ZnO nanowires were assembled and characterized using optical and electrical measurements. Short circuit current densities of 1.3 mA cm−2, and overall power conversion efficiencies of 0.3% were achieved with 8 µm long nanowires. Photocurrent and efficiency increase with increasing nanowire length and improved light harvesting. Low surface area and a shunt that appears under light illumination limit the solar cell performance. Internal quantum efficiencies were similar for nanowires of all lengths, indicating that electron transport is not limited by the nanowire dimensions for aspect ratios less than 70.

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