Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.

Performance of dye-sensitized solar cells ͑DSSCs͒ based on ZnO nanorods prepared by hydrothermal and vapor-deposition methods has been investigated. In spite of their inferior optical properties, DSSCs based on hydrothermally grown rods exhibit higher power conversion efficiency, which can be attributed to the higher dye adsorption. Hydrothermally grown and vapor deposited nanorods also exhibit different dependence of photovoltaic performance on the annealing conditions of the rods, indicating significant effect of the native defects on the achievable photocurrent and power conversion efficiency. Efficiency of 0.22% is obtained for both as grown hydrothermally grown nanorods and vapor deposited nanorods annealed in oxygen at 200°C.

Heterojunction NiO∕ZnO light emitting diodes have been fabricated using low temperature solution-based growth methods. While negligible light emission has been obtained for the as-grown NiO film, devices with annealed NiO film exhibit room-temperature electroluminescence (EL), which was attributed to the detrimental effects of nickel oxide hydroxide in as-grown NiO layers. The device performance can be further modified by insertion of the organic layers between NiO and ZnO and the EL spectra exhibited dependence on the bias voltage. For higher bias voltages, strong UV-violet emission peak can be obtained in spite of the dominance of defect emission in the photoluminescence spectra.

One dimensional ͑1D͒ ZnO nanostructures are of interest for applications in dye-sensitized solar cells ͑DSSCs͒ since they exhibit significantly improved electron transport compared to that in porous films. However, 1D nanostructures also have a significantly lower surface area than the porous films. Thus, the achieved solar cell efficiencies are typically much lower in spite of the improved charge transport. In this work, we investigated DSSCs based on ZnO tetrapods to achieve an increased surface area compared to that of 1D nanostructures. The cell performance as a function of the tetrapod film thickness and the dye used was studied. To further increase the surface area, mixed morphologies ͑tetrapods with nanoparticles͒ were also investigated. Under optimal conditions, an AM 1.5 power conversion efficiency of ϳ1.2% was achieved.