Design, Preparation And Application Of One-Dimensional TiO₂ And ZnO Arrays

Ordered nanomaterial arrays, especially arrays of free-standing nanorods and nanotubes on conducting substrates have been applied in many different fields. When implemented in photovoltaics, these structures can increase the surface area between the charge-generating and charge-transporting layers, improving the efficiency of charge extraction. In this study, we choose TiO₂ and ZnO which are two of the most extensively studied wide band gap metallic oxide semiconductors. The nanorod and nanotube arrays of the two materials were prepared by different techniques. The applications of these arrays in nanodevices, especially in organic-inorganic hybrid solar cells were researched in detail.Porous anodic aluminum oxide (AAO) membrane/ITO glass composite templates were prepared by anodizing the magnetron sputtered Al on the ITO glass. For avoiding the delaminating and cracking of the AAO at the bottom of the pores during anodization, Ti and W layers were sputtered before the magnetron sputtering of Al. Annealing was carried out to eliminate the inner stress of the sputtered layers, improving the connection between these layers. During anodization, the faster developing pores would oxidize the W barrier layer when it reached the bottom. The oxidation of the W slowed down the development of the pores, preventing the cracking of the AAO membrane and ensuring the completion of the whole array of the nanopores.Arrays of TiO₂ nanorods and nanotubes were embedded in AAO/ITO template by sol-gel electrophoresis. The structure of the TiO₂ changed from nanorod to nanotube with the sol aging time. The W ring electrode was the precondition to the formation of different structures. The competition between the diffusion rate of the charged particles and the deposition rate caused the formation of different structures. The as-prepared TiO₂ arrays were fabricated in hybrid solar cells by spin coating P3HT in the arrays. The conversion efficiencies were up to 0.38% and 0.48% for the TiO₂ nanorod and nanotube solar cells. The efficiency of the nanotube array solar cell was improved by 26% compared to the nanorod cell due to the larger specific surface area.TiO₂ arrays were also prepared in free-standing AAO template by magnetron sputtering and annealing. The parameters of the porous AAO template were highly influential in determining the nanostructure of the sputtered film. The template with a 200 nm pore diameter would result in the formation of nanotubes, while the double-sided through hole template with a 80 nm pore diameter would result in the formation of nanorods, and the one-sided blind hole template with a 80 nm pore diameter would result in the formation of closed-end tubes. The XRD measurement showed the annealed TiO₂ nanotubular film was polycrystalline anatase phase without preferred orientation. The photoluminescence (PL) spectrum revealed the as-prepared TiO₂ nanostructural film was an indirect bandgap semiconductor with oxygen vacancy defects that exist more in rods than in tubes. Then, the TiO₂ nanotubular film were removed from the AAO and transferred onto an ITO glass. A layer of P3HT was spin coated into the TiO₂ nanotube array to fabricate a hybrid solar cell. By adding a TiO₂ connecting layer between TiO₂ nanotubular film and ITO glass, the performance of the solar cell improved obviously, the conversion efficiency was increased 3.5 times and up to 0.34%. After mixing fullerene derivative PCBM with P3HT, due to the efficient separation of the electron-hole pair, the short circuit current density was improved 8 times and up to 9.98 mA cm-2, and the conversion efficiency was increased 6 times and up to 2.07%.ZnO nanotube arrays and Cu@ZnO coaxial nanocables were fabricated in free-standing AAO template by electrodeposition and then annealing. During the PL test, a green emission band was excited from the Cu@ZnO coaxial nanocable array, indicating that the ZnO was doped by Cu at the interface of the coaxial heterojunction. ZnO free-standing arrays of nanorods and nanotubes on ITO glass were prepared using similar technique. The carrier density of the ZnO array was 2.67×1020 cm-3, indicating the great potential of application in transparent conducting oxides.ZnO nanorod arrays were prepared by a simple one-step electrodeposition from aqueous solution. The 55?deposition produced a dense ZnO film, while the 85?deposition produced a nanorod array, indicating that the morphology of the deposited film depended on the temperature. The ITO/ZnO/P3HT:PCBM/Ag solar cell fabricated with the 85?deposited ZnO array had a conformal morphology with the array. For the solar cell using the unannealed ZnO array, the direct contact between the polymer and the dominant defect region at the bottom would cause serious current leakage. By eliminating those defects through annealing, or preventing the direct contact using a denser array, the leakage could be avoided, in turn the open circuit voltage could be increased. The current was mainly generated by the P3HT:PCBM polymer blend. Thus, the effective way to increase the short circuit current density was to increase the density of ZnO rods so as to increase the load of the polymer blend and the interface between the ZnO rods and the polymer. The TiO₂ shell deposited on the ZnO by ALD decreased the rate of recombination between electrons and holes, in turn improved the conversion efficiency, which was up to 2.10% and was higher than all the ZnO array cells without TiO₂ shell.