Year

2013

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

This thesis presents a study on the modification of titanium dioxide (TiO2) nanoparticle preparation through two hydrolysis routines: sol – gel and hydrothermal and the use of dopants for dye-sensitized solar cell (DSSC) application. Certain TiO2 characteristics, such as particle size, morphology, surface area, and phase structure, are crucial for obtaining superior power conversion efficiency in dye sensitized solar cells. Due to the agglomeration problem of the sol – gel process, this method has been found to make it difficult to control particle sizes with high surface area. In this work, we report a simple approach to improve the DSSC by controlling the degree of aggregation through zeta potential analysis. We found that different aqueous colloidal conditions, i.e., potential of hydrogen (pH), water/titanium alkoxide (titanium isopropoxide) ratio, and surface charge, obviously led to different particle sizes in the range of 10 to 500 nm. The stable sol solution was used for the blocking layer as well. Power conversion efficiency of 7.15% was obtained by using anatase TiO2 optimised to 10-20 nm in particle size with a compact blocking layer and a scattering layer. The scattering layer was made of particles with an average size of 100-200 nm, which were obtained through the sol – gel method by controlling the reaction parameters.

Using the stable sol solution, one-dimensional (1D) TiO2 nanostructures were prepared using the electrospinning method to verify their potential for use as the photoelectrode of DSSCs. The achieved 1D mesoporous nanofibers were 100 nm in diameter and 10-20 μm in length, and were composed of aggregated anatase nanoparticles 20-30 nm in size. The employment of these novel 1D mesoporous nanofibers significantly improved the dye loading and light scattering of the DSSC photoanode, and resulted in conversion cell efficiency of 6.64%, corresponding to a ~65% enhancement over the Degussa P25 reference photoanode. Electrochemical impedance spectroscopy was used to investigate the electron transfer through the photoanode, and it showed improved charge transport and electron diffusion through the electrospun TiO2 nanofibers.

The effects of phase structure on the photovoltaic performance were also investigated. To investigate the phase structure effects, it is very important to have the different phase structures in the same particle sizes. In this work, we controlled the phase transfer rate by controlling the synthesis parameters. The obtained nanoparticles were of the same size, but with different phase structures. The nanoparticles containing 75% anatase phase with 25% brookite phase showed the best photovoltaic performance, with power conversion efficiency of 6.8 % for operation with a TiO2 blocking layer.

The hydrothermal synthesis method was used to synthesize oriented, single crystalline, one-dimensional TiO2 nanostructures. In this study, a precisely controlled procedure was used to synthesize 3D dendritic and 1D TiO2 nanostructures by tuning the hydrolysis rate of the titanium precursor. Based on this innovation, oriented 1D rutile TiO2 nanostructure arrays with continually adjustable morphologies, from nanorods (NRODs) to nanoribbons (NRIBs), and then nanowires (NWs), as well as transition state morphologies, were successfully synthesized. The photovoltaic performance tests showed that the photoanode constructed from the oriented NRIB arrays possessed not only a high surface area for sufficient dye loading and better light scattering in the visible light range than for the other morphologies, but also a wider band-gap and a higher conduction band edge, with more than 200% improvement in power conversion efficiency in dye-sensitized solar cells (DSSCs) compared with the NROD morphology.

Titanium dioxide nanoparticles doped with nitrogen were prepared through the sol – gel method. Compared to undoped pure anatase titanium dioxide nanoparticles with the same particle size, the nitrogen doped titanium dioxide showed improved photovoltaic performance. Electrochemical impedance spectroscopy of cells with N doped TiO2 and pure TiO2 indicated that the charge transport of the photoelectrode was improved after doping with nitrogen. As a result, a photoelectric conversion efficiency of 6.8% was obtained for N doped TiO2 photoanode.

In summary, the results show the systematic influence that the synthesis conditions have on the crystalline structure of titanium dioxide in such aspects as particle size, phase structure, surface area, and morphology. Greater attention to the synthesis of TiO2 for DSSCs showed how significantly the synthesis conditions can improve the photovoltaic performances.

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