<p>The properties relevant for electron transfer processes between dye molecules and semiconductor substrates, titanium dioxide (TiO₂) and zinc oxide (ZnO), have been studied by means of photoelectron spectroscopy, PES, near edge X-ray absorption spectroscopy, NEXAFS, and resonant photoemission, RPES.</p><p>For dye-sensitized solar cells, the currently used dyes are ruthenium polypyridine complexes adsorbed to the semiconductor via carboxyl linker groups. A series of such complexes has been investigated, and the most efficient dye so far, cis-bis(4,4'-dicarboxy-2,2'-bipyridine)-bis(isothiocyanato)ruthenium(II), RuL'₂(NCS)₂, was studied in more detail. The results revealed a high content of thiocyanate orbitals in the highest occupied molecular orbital, HOMO, of this complex, which partly explains its efficiency in the solar cell. The thiocyanate ligands were found to be highly influenced by the substrate when the dye is adsorbed onto ZnO, which is not the case for the corresponding TiO₂ system. </p><p>A bridge bonding between TiO₂ and the L' ligand was proposed, where the carboxyl groups are deprotonated and all oxygens interact with surface titanium ions. For ZnO, the results indicate a different bonding geometry, involving protonated carboxyl groups.</p><p>For the display system a dye molecule, which shifts color upon electrochemical treatment, was adsorbed on TiO₂ and studied in its reduced and oxidized states. The major electronic difference between the two states was shown to occur on the nitrogen atom. In addition, a reversible photoreduction process during the measurements was observed.</p>

This thesis focuses on liquid redox electrolytes in dye-sensitized solar cells (DSCs). A liquid redox electrolyte, as one of the key constituents in DSCs, typically consists of a redox mediator, additives and a solvent. This thesis work concerns all these three aspects of liquid electrolytes, aiming through fundamental insights to enhance the photovoltaic performances of liquid DSCs. Initial attention has been paid to the iodine concentration effects in ionic liquid (IL)-based electrolytes. It has been revealed that the higher iodine concentration required in IL-based electrolytes can be attributed to both triiodide mobility associated with the high viscosity of the IL, and chemical availability of triiodide. The concept of incompletely solvated ionic liquids (ISILs) has been introduced as a new type of electrolyte solvent for DSCs. It has been found that the photovoltaic performance of ISIL-based electrolytes can even rival that of organic solvent-based electrolytes. And most strikingly, ISIL-based electrolytes provide highly stable DSC devices under light-soaking conditions, as a result of the substantially lower vapor pressure of the ISIL system. A significant synergistic effect has been observed when both guanidinium thiocyanate and N-methylbenzimidazole are employed together in an IL-based electrolyte, exhibiting an optimal overall conversion efficiency. Tetrathiafulvalene (TTF) has been investigated as an organic iodine-free redox couple in electrolytes for DSCs. An unexpected worse performance has been observed for the TTF system, albeit it possesses a particularly attractive positive redox potential. An organic, iodine-free thiolate/disulfide system has also been adopted as a redox couple in electrolytes for organic DSCs. An impressive efficiency of 6.0% has successfully been achieved by using this thiolate/disulfide redox couple in combination with a poly (3, 4-ethylenedioxythiophene) (PEDOT) counter electrode material under full sunlight illumination (AM 1.5G, 100 mW/cm2). Such high efficiency can even rival that of its counterpart DSC using a state-of-the-art iodine-based electrolyte in the systems studied.The cation effects of lithium, sodium and guanidinium ions in liquid electrolytes for DSCs have been scrutinized. The selection of the type of cations has been found to exert quite different impacts on the conduction band edge (CB) of the TiO2 and also on the electron recombination kinetics, therefore resulting in different photovoltaic behavior. / QC 20120124

The properties relevant for electron transfer processes between dye molecules and semiconductor substrates, titanium dioxide (TiO2) and zinc oxide (ZnO), have been studied by means of photoelectron spectroscopy, PES, near edge X-ray absorption spectroscopy, NEXAFS, and resonant photoemission, RPES. For dye-sensitized solar cells, the currently used dyes are ruthenium polypyridine complexes adsorbed to the semiconductor via carboxyl linker groups. A series of such complexes has been investigated, and the most efficient dye so far, cis-bis(4,4'-dicarboxy-2,2'-bipyridine)-bis(isothiocyanato)ruthenium(II), RuL'2(NCS)2, was studied in more detail. The results revealed a high content of thiocyanate orbitals in the highest occupied molecular orbital, HOMO, of this complex, which partly explains its efficiency in the solar cell. The thiocyanate ligands were found to be highly influenced by the substrate when the dye is adsorbed onto ZnO, which is not the case for the corresponding TiO2 system. A bridge bonding between TiO2 and the L' ligand was proposed, where the carboxyl groups are deprotonated and all oxygens interact with surface titanium ions. For ZnO, the results indicate a different bonding geometry, involving protonated carboxyl groups. For the display system a dye molecule, which shifts color upon electrochemical treatment, was adsorbed on TiO2 and studied in its reduced and oxidized states. The major electronic difference between the two states was shown to occur on the nitrogen atom. In addition, a reversible photoreduction process during the measurements was observed.

In this study, we used two types of materials as working electrode of DSSC, commercial TiO2 (P25) nanoparticle and ZnO nanotip. First part, we sintered TiO2 electrode in nitrogen and treat TiO2 electrode by UV light irradiation, in order to increase dye adsorption onto TiO2. Second part, we used three different buffer layers to grow ASD-ZnO nanotip as working electrode, sputtered-ZnO, sputtered-AZO and spin-coating TiO2. The hetrojuction is formed at the interface of ZnO nanotip/TiO2 buffer layers due to the different materials between ZnO and TiO2, which is beneficial for electron-hole separation. The morphology was measured by field emission scanning electron microscope (FE-SEM, Philip XL-40FEG). The sheet resistivity was measured by four-point probe. The crystallinity was examined by X-Ray diffraction (XRD, Simens D5000). Structural and spectral properties are characterized by ultraviolet-visible spectroscopy (UV-Vis) spectroscopy. The fourier transform infrared spectroscopy (FT-IR spectroscopy, BRUKER 66v/s) deals with the infrared region of absorption spectroscopy. And the Angilent B1500A is used for current-voltage (I-V) characterization of solar cells. In our results, we enhance the performance of TiO2 electrode DSSC, the open circuit voltage can reach to 0.56 V, the short circuit current density can reach to 16.65 mA/cm2, the conversion efficiency can reach to 4.6 % and the fill factor can reach to 49.2 %. On ZnO electrode, the open circuit voltage can reach to 0.57 V, the short circuit current density can reach to 4.43 mA/cm2, the conversion efficiency can reach to 1.21 % and the fill factor can reach to 47.9 %.

In this research, hills-like hierarchical structured TiO2 photoanodes for dye-sensitized solar cells (DSSCs) have been prepared. We expected these appropriately aggregated TiO2 clusters in photoanode layer could cause stronger light scattering and higher dye loading that increased efficiency of photovoltaics. For detailed light-harvesting study, different molecular weight of polyvinyl alcohol (PVA) polymers were used as binders for TiO2 nanoparticles (P-25 Degussa) aggregation. After preparing a series of TiO2 films with dissimilar morphology, the reflection of TiO2 films, absorbance of attached dye, amount of dye loading, and performance of fabricated DSSC devices were measured and investigated. An optimized device had higher dye loading and well light harvesting at the same time that induced a 23% increase of short-circuit current Jsc in DSSCs. Moreover, we found that electrolyte could penetrate or diffuse easily in this higher porous structure. We fabricated dye-sensitized solar cells with MPN-based liquid electrolyte and gel polymer electrolyte. From this structure, the short-circuit current Jsc was increased around 16% and 19% respectively compared to conventional layers with liquid electrolyte and gel polymer electrolyte devices. The increase in highly viscous system of gel polymer electrolyte is due to easier penetration by such hills-like hierarchical structure.

The thesis discusses how to utilize Sol-gel method to prepare nano-sized TiO2 films of photoelectrodes and the their use in Dye-Sensitized Solar Cells. The main goal is the study on the production of TiO2 photoelectrodes. When making nano-sized TiO2 films of photoelectrodes, we have to first produce TiO2 sol via the Sol-gel method optical thin films are then made by spin coating. After its spin coating, we study the crystalline phase and morphology of nano-sized TiO2 films of photoelectrodes in terms of two ways: Supercritical drying and oven drying. Among the Dye-Sensitized Solar Cells that are made with different drying methods, owing to higher porosity and surface area, the nano-sized TiO2 films of photoelectrodes made by supercritical drying adsorb more dye molecules and are thus more efficient.

In this study, we deposit titanium dioxide (TiO2) on the indium tin oxide (ITO/glass) substrate by a liquid phase deposition (LPD) method as a buffer layer and coat TiO&not;2 particles on LPD-TiO2 films by spin-coating method as anode of dye-sensitize solar cell (DSSC). In order to adjust the optical absorption edge of titanium dioxide to the visible light, we co-dope fluorine and nitrogen into TiO2 by LPD method and Ammonium Fluoride (NH4F). In our experiment, the morphology and thickness was characterized by scanning electron microscopy (SEM), structure was characterized by X-ray diffraction (XRD), chemical properties was characterized by electron spectroscope chemical analysis (ESCA), structural and spectral properties were characterized by ultraviolet-visible spectroscopy (UV-Vis) spectroscopy and current-voltage (I-V) characterization of solar cells was measured by B1500A. In our results, we enhance the performance of TiO2 as a DSSC`s anode, the open circuit voltage can reach to 0.71 V, the short circuit current can reach to 5.14 mA, the conversion efficiency can reach to 1.91 % and the fill factor can reach to 52.5 %.

<p>This thesis is divided into four parts with organic chromophores for dye sensitized solar cells as the common feature and an introduction with general concepts of the dye sensitized solar cells.</p><p>The first part of the thesis describes the development of an efficient organic chromophore for dye sensitized solar cells. The chromophore consists of a triphenylamine moiety as an electron donor, a conjugated linker with a thiophene moiety and cyanoacrylic acid as an electron acceptor and anchoring group. During this work a strategy to obtain an efficient sensitizer was developed. Alternating the donor, linker or acceptor moieties independently, would give us the tool to tune the HOMO and LUMO energy levels of the chromophores. The following parts of this thesis regard this development strategy.</p><p>The second part describes the contributions to the HOMO and LUMO energy levels when alternating the linker moiety. By varying the linker the HOMO and LUMO energy levels was indeed shifted. Unexpected effects of the solar cell performances when increasing the linker length were revealed, however.</p><p>The third part describes the investigation of an alternative acceptor group, rhodanine-3-acetic acid, in combination with different linker lengths. The HOMO and LUMO energy level tuning was once again successfully shifted. The poor electronic coupling of the acceptor group to the semiconductor surface proved to be a problem for the overall efficiency of the solar cell, however.</p><p>The fourth part describes the contributions from different donor groups to the HOMO and LUMO energy levels and has so far been the most successful in terms of reaching high efficiencies in the solar cell. A top overall efficiency of 7.1 % was achieved.</p>

Thesis (Ph.D.)--Bowling Green State University, 2009. / Document formatted into pages; contains xix, 184 p. : ill. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 76-82).