FUNDAMENTAL INSIGHTS INTO THE CHARGE TRANSFER PROCESSES IN DYE-SENSITIZED SOLAR CELLS: HOLE TRANSFER, REGENERATION, CHARGE RECOMBINATION, AND ELECTRON INJECTION
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There has been an increasing demand for clean and renewable energy worldwide in the 21st century because of foreseeable shortage of fossil fuels and the environmental concerns of using them. Converting solar energy into electricity or chemical bonds for energy storage has been demonstrated to be a viable way to a sustainable energy future. The scope of this thesis involves understanding fundamental aspects of dye-sensitized solar cells (DSSCs) that use molecular visible light chromophores chemically anchored on mesoporous nanocrystalline anatase TiO2 thin films to harvest the sun light and convert to electrical energy. Challenges and solutions are discussed for designing DSSCs with higher solar energy conversion efficiencies. Chapter 2 describes the characterization of the redox properties of TiO2 interfaces sensitized to visible light by a series of cyclometalated ruthenium polypyridyl compounds bis-ligated to a terpyridyl ligand, with three carboxylic acid/carboxylate or methyl ester groups for surface binding, and a tridentate cyclometalated ligand with a conjugated triarylamine (NAr3) donor group. These unique molecules with two coupled redox active centers showed non-Nerstian redox behavior at TiO2 interfaces. They also created disparate dipole moments and open circuit photovoltage with different quantum yield of intramolecular hole transfer upon illumination. A special intermolecular hole transfer event was observed spectroscopically and modeled by Monte Carlo simulation that rendered an average hole hopping frequency on TiO2 surface. Chapter 3 presents two donor-acceptor organic dyes differing only by a two-heteroatom change from oxygen to sulfur within the donor unit. Despite similar optical and redox properties for the two dyes, a consistently higher open-circuit voltage (Voc) was measured for the sulfur containing dye relative to the oxygen containing one in both I˗/I3˗ or CoIII/II electrolytes. The improved efficiency observed with the sulfur containing dye in an iodide redox mediator is against the commonly held view that sulfur atoms promote charge recombination attributed to inner-sphere interactions. Detailed mechanistic studies revealed that this was a consequence of a 25-fold enhancement of the regeneration rate constant that increased the regeneration yield under open circuit conditions. The study showed that a high short circuit photocurrent does not imply optimal regeneration efficiency as was often assumed. The two organic dyes described in Chapter 3 were further investigated in regards to their aggregation effect on charge recombination to cobalt redox mediators, Chapter 4. Chenodeoxycholic acid was found to reduce aggregation effectively and help slow down charge recombination to the oxidized cobalt redox mediator. Chapter 5 provides the first spectroscopic evidence for heavy heteroatom, selenium, enhancing charge recombination between TiO2 electrons and oxidized iodide species in DSSCs. This led to a deleterious lower Voc value for the ruthenium sensitizer with a selenium heteroatom in an iodide/triiodide electrolyte. This implies that the design of sensitizer molecules must take into account the position of heteroatoms that may interact with the iodide species in the electrolyte and facilitate faster charge recombination pathway. Finally, in chapter 6, three panchromatic ruthenium dipyrrinate coordination compounds were tested in DSSCs. Excitation wavelength dependent electron injection yields were observed for higher energy dipyrrin ligand localized excited state than the lower energy MLCT state. These ruthenium sensitizers were found to have MLCT excited states at lower energy with respect to the TiO2 acceptor states. Dye molecules with higher MLCT excited state energy were synthesized to achieve higher electron injection yield without sacrificing Voc.