Fabrication and Characterization of Aligned Titania Nanowire Films for Solar Cell Applications Public Deposited

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Last Modified
  • March 19, 2019
Creator
  • Ren, Zheng
    • Affiliation: College of Arts and Sciences, Department of Applied Physical Sciences, Materials Science Graduate Program
Abstract
  • A new hydrothermal method has been developed to grow foldable free-standing TiO2 films which are composed of highly aligned rutile nanowires. The rutile phase TiO2 nanowires are grown in the [001] direction. The free-standing film possesses a very high percentage of (110) surface, which is thermodynamically the most stable and well-studied TiO2 surface. Different growth parameters have been investigated and a new growth mechanism of the free-standing film is proposed. The free-standing film shows its stability in many extreme environments. For example, it can maintain the same morphology after high temperature annealing, hydrogen treatment and surface coating. In addition, the free-standing nanowire film can be directly attached to different substrates, some of which have not been reported to support the growth of vertical TiO2 nanowire structures, hence providing another method of fabricating nanowire structures on various substrates. The new process can also avoid damage to the conducting metal oxide layer during the growth process and other treatments. Vertically grown TiO2 nanowires array is a very promising electrode structure for dye-sensitized solar cells (DSSCs), because of its advantages of large surface area, high diffusion coefficient, short electron transport pathway and less trapping sites than nanoparticle film. Because the surface area of the semiconductor porous film is essential for the performance of DSSCs, morphology control is fundamentally important for nanowire material growth techniques. By stacking nanowire films layer by layer, a multilayer assembly photoanode has been built. The multi-layer TiO2 assembly can precisely control the thickness and the surface area of semiconductor photoanode in a DSSC. DSSCs using these multiple-layer electrodes were built and tested. By enhancing the contact between nanowires and the substrate and improving the electrical properties with high temperature annealing, 3.8% efficiency was achieved for a single layer electrode. After attaching three layers together, 4.8% solar energy conversion efficiency was shown for a three-layer of TiO2 film photoanode, demonstrating an efficiency boost of 25% compared with the single layer electrode. The multi-layer assembly photoanode shows a new way to increase and control the roughness of a TiO2 nanowire array, which also helps to improve the performance of a TiO2 electrode in many applications such as solar cell and water splitting. A new direct-deposit method was developed to build a flexible photoanode utilizing the free-standing foldable TiO2 nanowire film. The new approach provides good electrical interconnection in the TiO2 electrode and obviates the need for high temperature treatment, allowing the device to be constructed on a plastic substrate. A flexible photoanode was built. A DSSC with this design shows a high short circuit current density (8 mA/cm2) and open circuit voltage (0.75 V). An efficiency of 1.5% DSSC was achieved using this photoanode. The TiO2 free-standing film is a promising material to control the morphology and surface properties of electrodes in many applications. The mechanism of growing the free-standing nanowire film is expected to extend to other metal oxide nanowire growth techniques and achieve better morphology control for crystal growth.
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  • In Copyright
Advisor
  • Qin, Lu-Chang
Degree
  • Doctor of Philosophy
Graduation year
  • 2012
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