Investigations On The Growth Of ZnO Nanostructures, Properties And Its Application In Dye Sensitized Solar Cells

ZnO is a transparent semiconductor with a wide band gap of3.37eV and large exciton binding energy of60meV which can be utilized for blue and ultraviolet light emitting diodes, laser diodes, and solar cells. The critical setbacks to the fulfillment of utilizing ZnO in such devices are the absence of reliable and reproducible p-type ZnO and controlling the morphology of ZnO nanostructures. In this dissertation, the main discussion and conclusions on ZnO are as following:(1) Nitrogen substitution of lattice oxygen has been recognized as a potentially effective method. However, it remains difficult to achieve good quality p-type conduction in N-doped ZnO. Since the electrical properties of N-doped ZnO are sensitive to the sites of nitrogen in the host lattice, studying the influence of N-doping on the lattice dynamics of ZnO will help to better understand the doping mechanism. In addition, the origin of the additional Raman mode at275cm-1remains controversial to this point. An understanding of this vibration mode would help to clarify the N-doping mechanisms in ZnO. A systematic investigation on the optical properties of N-doped ZnO thin films was performed in order to understand the origin of an additional Raman mode at275cm-1. This Raman peak was observable only at N2pressures lower than30Pa during PLD deposition. Its intensity decreased with an increase of N2pressures and eventually vanished at pressures above30Pa. N substitution of O (No) was identified by photoluminescence and x-ray photoelectron spectroscopy and correlated well with the Raman intensity. The electrical measurements showed significant changes in resistivity, charge carrier concentration, and mobility due to the presence of N acceptors. Investigations on undoped ZnO films grown in Ar without N2further confirm that N doping plays a key role in the Raman scattering. The experimental data indicate that the Raman mode originates from NO related complexes, likely in the form of Zni-No. These investigations help to understand the doping mechanisms and underlying physics of the additional Raman mode in the ZnO films.(2) A unique approach has been developed to deposit ZnO with tunable morphology and physical properties by introducing methanol in an electrochemical process. As the methanol increases in the aqueous electrolyte, the growth mode of ZnO transforms from thin films to nanostructures, corresponding to crystalline orientation change from (002), to (102), and then to (101). These structural changes are accompanied by significant variations of optical and electrical properties, including the widening of band gap from3.31eV to3.53eV, increase of resistivity, and decrease of charge carrier concentration. Compositional analysis indicates that the samples grown at higher methanol concentrations contain Zn(OH)2which is likely responsible for the band gap change. The growth mechanism is discussed in terms of the impact of methanol on the chemical reaction, i.e. the change of growth mode results from the adsorption of nitrate ions on the polar surface of ZnO. This finding helps to grow specific ZnO nanostructures with desired morphologies and properties for device applications.(3) The advancement of nanomaterials and nanotechnology has provided huge potential to improve solar cell performance with an expectation of reducing the cost and improving the cell efficiency. Significant research has been done on nanorod based solar cells in recent years. In order to increase surface area and improve the light trapping effect for cell efficiency enhancement,3-D hierarchical ZnO nanostructures and ZnO/TiO2core-shell structures including nanorods, nanosheets, and nanoshrubs have been developed by using a solution based, two-step deposition process. The extremely large surface area is expected to improve dye loading and charge injection along with the improved light trapping, all of which helps to increase the energy conversion efficiency. Dye-sensitized solar cells were demonstrated based on the core-shell hierarchical nanostructures. The efficiency of the nanoshrub structures was increased by40%. This improved performance is attributed to the larger surface area and light trapping effects in nanoshrub core-shell structures. First, the enhanced photon absorption associated with the augmented surface area results in increased dye loading and light absorption, leading to an increase in JSC. Second, the network of crystalline ZnO nanorods and their direct contact with the electrode increase the efficiency of electron collection, which also helps to reduce charge recombination at the same time. Finally, the hierarchical nanostructures increase the light-harvesting efficiency by light scattering enhancement and trapping.