With increasing pressure over energy supply and climate change, there has been growing interest in developing alternative and sustainable energy sources to move away from fossil energy. Solar energy is abundant and clean energy option. However to achieve high efficiency and low-cost solar power, continuing efforts to develop new solar technologies are needed. Since the pioneering work of Michael Grätzel on utilizing TiO2 nanoparticles in dye sensitized solar cell (DSSC), much research has been devoted to DSSC research. However, there are a number of challenging issues remaining, for example, insufficient sunlight harvesting by the sensitizer, inefficient electron transport in the photoelectrode and the stability concern. The aims of this thesis were to develop and investigate new nanostructured semiconductor materials as photoanodes for enhanced DSSC efficiency.
Specifically, the objectives of the research carried out in the thesis involve optimizing the DSSC using two different strategies:
1) Morphological design and control of novel photoanode materials to enhance visible to nearinfrared
sunlight harvesting (Chapter 3, 4 and 5).
2) Structural optimization to improve the electron transfer and suppresses the recombination
(Chapter 6, 7 and 8).
The first chapter is a general introduction to the motivation of this work and background of photovoltaics and dye-sensitized solar cells, such as the device structure, operating principles and the key research challenges and detailed objectives of the work. In Chapter 2, all the experimental details including the materials, device fabrication and characterization techniques are presented Chapter 3 presents a published work on shell-in-shell TiO2 hollow spheres featuring excellent light scattering properties, synthesized by a facile one-pot hydrothermal method and opted in a bilayer-structured photoelectrode as scattering layer. The role of different ratio of reactants sucrose to TiF4 in the reagents was investigated. A 19% enhancement in DSSC efficiency is observed after introducing the shell-in-shell TiO2 hollow spheres as a scattering layer in the solar cell. The results from this chapter reveal that the use of shell-in-shell TiO2 hollow spheres could offer new opportunities for the development of high-efficiency DSSCs.
Chapter 4 demonstrates a upconverter modified light scattering structure based on the shell-inshell TiO2 hollow spheres developed in Chapter 3 to further increase the efficiency of DSSC by improving near-infrared light harvesting. In this work, dual-function upconversion scattering upconverter doped TiO2 hollow shells were prepared by a simple one-pot hydrothermal method followed by calcination. The potential of upconverter doped TiO2 hollow shells to harvest and 3 utilize of the NIR light in DSSC is demonstrated. Particularly, the upconversion property and response mechanism of the upconverter doped TiO2 nanocomposites are determined, and the improved performance of the DSSCs based on such structure is demonstrated. Furthermore, highly crystallized ultralong mesoporous TiO2 microbelts have also been proved to be a potential candidate for DSSC due to its high surface area and effective light scattering property in Chapter 5. The property and the photovoltaic performance of TiO2 microbelts are investigated. Due to its high surface area for dye loading and large diameter for light harvesting, the DSSCs fabricated with the TiO2 microbelts have shown a notable increase in the overall conversion efficiency (7.84%) compared with the P25 cell (5.92%) in the same thickness.
Chapter 6 presents the published work on nanosized TiO2 single crystals with different percentages of exposed (001) facets in the presence of HF solution. The correlation between particle morphology, exposed (001) facets and photo-conversion efficiency of the nanosized anatase TiO2 single crystals is clearly demonstrated. Such results reveal that an enhancement in DSSC overall conversion efficiency is observed for the photoanode consisting of nanosized TiO2 single crystals with a higher percentage of exposed (001) facets. The reason for this was investigated by dark current scan and open-circuit voltage decay scan.
The effect of sodium on photovoltaic properties of DSSCs assembled with TiO2 nanosheets with exposed (001) Facets is presented in Chapter 7. The photovoltaic properties of NaOH-washed anatase TiO2 nanosheets with exposed (001) facets were investigated in comparison with the controlled anatase TiO2 nanosheets washed with deionized water. A decreased efficiency and increased recombination rate was observed for NaOH-washed TiO2 nanosheets in comparison to the H2O-washed counterpart, the reason for this has been further investigated and XPS confirmed the deleterious effect of the presence of sodium is responsible for this.
Mesoporous TiO2 nanocrystals have proved to effectively improve the DSSC efficiency due to the improved dye loading capacity and reduced charge recombination of its mesoporous network as presented in Chapter 8. Mesoporous TiO2 nanocrystals were synthesised by a one-step amino acid assisted synthesis method using L-lysine as a catalyst. The mesoporous structure of the closely packed TiO2 nanocrystals offers high surface area, small crystalline particle size and superior connectivity between particles. An 18% enhancement in DSSCs conversion efficiency was achieved using the Mesoporous TiO2 nanocrystals as photoanodes (7.66%) compared to that of the benchmark Degussa P25 TiO2 (6.49%).
Finally, conclusions and recommendations are presented in Chapter 9 summarising the key advances and achievements of this thesis work on DSSC. Both optimization strategies as hypothesised earlier have been demonstrated to be effective and will make a valuable contribution to the development of photoanodes materials used in high efficiency DSSCs.