published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Thesis (Ph.D.)--Northwestern University, 2008. / Adviser: Hupp, Joseph T. Includes bibliographical references.

Dye sensitized solar cells (DSSCs) have received extensive attention among solar cells in recent years as the production cost is comparatively low and photovoltaic performance is good. Apart from TiO2, SnO2-based DSSCs are of great interest since SnO2 has a wide band gap and high mobility. Though the conversion efficiency of SnO2-based DSSCs is still not comparable to TiO2-based DSSCs, there is room for improvement to fabricate an efficient device. In this study, different commercial SnO2 nanoparticles have been compared. The number of SnO2 layers and paste formulation have been optimized. The effects of TiCl4 and TTIP treatments have been investigated. In order to further optimize the performance of SnO2-based DSSCs, different strategies have been adopted to increase dye loading, facilitate electron transport and enhance photon absorption. Different dopants (Zn, Mg and Ag) have been introduced to SnO2 pastes. It is found that cells with Zn dopants perform the best with increased dye uptake. SnO2 nanorods have been synthesized and mixed with SnO2 nanoparticles. More nanorods result in faster electron transport and hence increase the conversion efficiency. In addition, different gold nanostructures (nanostars, nanorods and nanocubic Au) have been synthesized and incorporated into SnO2 photoanodes to study the plasmonic effects. It can be observed that nanocubic Au demonstrates the largest improvement in conversion efficiency. The obtained results will be discussed in detail. / published_or_final_version / Physics / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 81-87) Also available in print.

Quantum dot sensitized solar cells (QDSCs) constitute one of the most promising low-cost solutions that are explored for the world’s needs of clean and renewable energy. Efficient, low-toxic and stable QDSCs for large-scale applications have formed the subject for the solar cell research during recent years. This circumstance also forms the motivation for this thesis, where the results of my studies to improve the efficiency and stability of green QDSCs are presented and discussed. The surface condition of quantum dots (QDs) is always crucial to the performance of QDSCs, since surface ligands can influence the loading amount of QDs, and that the surface defects can induce charge recombination in the solar cells. In this thesis work, a hybrid passivation approach was firstly utilized to improve the photovoltaic performance of CdSe QDs. After hybrid passivation by MPA and iodide ions, the loading efficiency of the QDs was increased with the ligands of MPA, and the surface defects on the QDs were reduced by the iodide ions, both contributing to an enhancement in the efficiency of the CdSe based QDSCs. This hybrid passivation strategy was then employed for low-toxic CuInS2 QDs, and was also demonstrated as an effective way to modify the surface state of the CuInS2 QDs and improve the performance of the QDSCs based on CuInS2 QDs.   To improve the stability of the QDSCs, solid state quantum dot sensitized solar cells (ss-QDSCs) based on CuInS2 QDs were investigated. In addition to the hybrid passivation, increasing the pore size of the TiO2 active layer and changing the composition of the CuInS2 QDs were also found to be useful approaches to improve the performance of the ss-QDSCs based on CuInS2 QDs. Furthermore, for the most used hole transport material- Spiro-OMeTAD- in solid state solar cells, silver bis(trifluoromethanesulfonyl)imide was shown to be an effective p-type dopant to increase its conductivity and to improve the performance of the solar cells based on it. / <p>QC 20160516</p>

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As fossil fuels, the major source of energy used today, create the greenhouse gas carbon dioxide which causes global warming, alternative energy sources are necessary in the future. There is a need for different types of renewable energy sources such as hydropower, windpower, wave- power and photovoltaics since different parts of the world have different possibilities. The sun is a never ending energy source. Photovoltaics use the energy of the sun and converts it into electricity. There are different types of photovoltaics and a combination of them could provide humankind with energy in a sustainable way. In this thesis dye-sensitized solar cells are investigated. Materials for the counter electrode have been investigated and resulting in a polymer based cathode outperforming the traditionally used platinized counter electrode in a cobalt-based redox mediator system (paper I). The sensitizer of the TiO2 was investigated, in this study by modifications of the π-linker unit in an organic donor-linker-acceptor based dye. Four new dyes were synthesized, all four showing extended absorption spectra compared to the reference dye. However, it was found that increasing the absorption spectrum does not neces- sarily increase the power conversion efficiency of the solar cell (paper II). In the last part of this thesis, water-based electrolyte dye-sensitized solar cells were investigated. A hydrophilic dye with glycolic chains close to the center of regeneration was synthesized. The results show increased wettability by water-based electrolyte for the sensitized surface, increased regenera- tion and performance for the hydrophilic dye compared to a hydrophobic dye (paper III). The glycolic chains complex with small cations such as Na+ and K+ in the electrolyte, this proba- bly facilitate the regeneration of the hydrophilic dye even further (paper IV). In this thesis new materials for a more environmentally friendly dye-sensitized solar cell are investigated.

Ruthenium (II) complexes are used extensively in photoelectrochemical and photophysical devices, such as Dye-Sensitized Solar Cells (DSSCs). The use of Cu(I) as a possible replacement for Ru(II) has to date had limited exploration, but has obvious advantages in terms of low cost and high abundance. However, Cu(I) typically undergoes conformational change from tetrahedral towards square planar upon oxidation or MLCT excitation, often leading to reduced stability, reduced electron transfer rates and reduced excited state lifetime, thus impairing useful function. Typically, steric constraints are used to prevent this; however these can often be synthetically intensive, involving multi-step and low yielding synthetic pathways. In this work, we explore “blocking” functionality using two different ligands combined with a range of bipyridyl ligands with varying substituent groups. The study has looked into the synthesis of heteroleptic Cu(I) complexes of the general formula: [Cu(POP)(bipyridyl)][BF4], where POP = bis[2-(diphenylphosphanyl)phenyl] ether, and [Cu(pmppE)(bipyridyl)], where pmppE = hydrazono pyrazol-5-thiones(one). The work presented in this thesis focuses on the synthesis, and subsequent photoelectrochemical and photophysical characterisation of Cu(I) complexes, yielding results that open new avenues for design of functional Cu(I) systems. Solar cell testing also revealed photovoltages comparable to those of existing Cu(I) DSSC sensitisers. An extensive spectroscopic study of [Cu(POP)(dmbpy)]+ and [Cu(POP)(tmbpy)]+ has revealed the latter to have the significantly larger quantum yield: 65 % and 4% respectively in PMMA at 300 K. A complimentary computational investigation was carried out in order to gain a better understanding of how structural rigidity affects emission properties.

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