A range of titanium dioxide (titania) samples provided by Huntsman Pigments and Additives were investigated for their suitability for use in various optoelectronic devices, specifically dye-sensitized solar cells (DSSCs). Five of the titania samples are 1-20 micrometre size spherical 3D porous beads made up of titania nanoparticles and a further six samples are porous titania nanoparticle clusters of no specific shape, all of which possess high surface areas from 85 to 276 m2g-1. The samples were compared to commercially available nanocrystalline TiO2 powders and paste. All of the samples were initially assessed for suitability in DSSC devices by investigating various properties such as crystal phase, particle size, band gap, morphology and N719 dye adsorption, both as a powder sample and as a sintered film, employing techniques such as powder x-ray diffraction, UV/Vis spectroscopy and scanning electron microscopy. Different methods of formulating the samples into pastes for application to a substrate were attempted and electrochemical properties of a selection of films were also compared. The more promising titania samples were formulated into dye-sensitized solar cells and cell efficiencies calculated. DSSC devices were also fabricated with low temperature (125oC) sintering of the titania layer to assess the suitability of the samples for use in devices with flexible substrates. Initial devices incorporating the Huntsman TiO2 samples provided low efficiencies (< 0.1%). The samples were then modified with pre-sintering treatment prior to paste formulation to optimize crystallinity, particle size, porosity and surface area. The modified titania bead samples showed great promise in low temperature sintered devices, providing device efficiencies of 2.8%, more than double that of those incorporating the standard P25 TiO2 (1.3%). After sample modification a superior solar cell performance (3.2%) was also observed in 510oC sintered devices when compared to the standard P25 TiO2 devices (2.9%), with higher photocurrent and open circuit voltage than devices fabricated from commercially optimized TiO2 paste. Devices were also fabricated using pre-sensitized titania in an attempt to reduce device manufacturing time. The modified samples again showed good performance, providing working devices with efficiencies comparable to the equivalent pre-sensitized P25 devices.

To this day, fossil fuels still make up over 80% of the earth’s energy production. Many sources of renewable energy are available, but photovoltaics is the only source with the capacity proven to meet the increasing world energy needs. Third generation devices such as dye-sensitized and organic solar cells have gained much interest due to their cost effectiveness and flexibility but have yet to become commercially viable. Here methods have been studied to improve these devices with the use of Gold nanowire arrays. These additions provide plasmonic and light scattering enhancements in dye-sensitized solar cells. Different TiO2 deposition methods have been studied to protect the gold from the redox couple in the electrolyte. Several novel methods have been undertaken to incorporate gold nanowire arrays in organic solar cells with some success. Structural characterization shows the proposed architecture is achieved, but working devices met suffered from low success rate.

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 204-217).

Solar energy is the foremost power source of our planet. Driving photosynthesis on our planet for 3 billion years the energy stored in the form of fossil fuels also originates from the sun. Consumption of fossil fuels to generate energy is accompanied with CO2 emission which affects the earth's climate in a serious manner. Therefore, alternative ways of converting energy have to be found. Solar cells convert sunlight directly into electricity and are therefore an important technology for future electricity generation. In this work solar cells based on the inorganic semiconductor titanium dioxide and hole-transporting dyes are investigated. These type of solar cells are categorized as hybrid solar cells and are conceptually related to both dye-sensitized solar cells and organic solar cells. Light absorption in the bulk of the hole-transporting dye layer leads to the formation of excitons that can be harvested at the organic/inorganic interface. Two design approaches were investigated: 1) utilizing a multilayer of a hole-transporting dye and 2) utilizing a hole-transporting dye as light harvesting antenna to another dye which is bound to the titanium dioxide surface.  Using a multiple dye layer in titanium dioxide/hole transporting dye devices, leads to an improved device performance as light harvested in the consecutive dye layers can contribute to the photocurrent. In devices using both an inteface-bound dye and a hole-transporting dye, excitation energy can be transferred from the hole-transporting dye to the interface dye.

In this thesis we synthesize two organic materials, one is discotic liquid crystal Acid-6, and the other is novel discotic liquid crystal polymer DLC-PAM. After demonstrating the molecular structures of Acid-6 and DLC-PAM by FT-MS, 1H-NMR and FT-IR, we use the two materials as photo-sensitized dyes for dye-sensitized solar cells(DSSCs) and manufacture two kinds of cells. We use polyacrylamide(PAM) as main chain of the novel discotic liquid crystal polymer DLC-PAM and graft the discotic liquid crystal monomer Acid-6 onto PAM by chemical synthesis. DLC-PAM belongs to side-chain liquid crystal polymer, and it can show the properties of it¡¦s discotic liquid crystal function. One of the properties is absorption of visible light. By observing the UV-Vis spectrum, we can realize the absorption band is located between 200 ~ 450 nm and confirm that it is able to be a photo-sensitized dye. Another property of discotic liquid crystal is the self-assembly ability, the moleculars can assemble into hexagonal columnar structure by themselves, and the property enable discotic liquid crystal to have better mobility. In this part, we can demonstrate DLC-PAM and Acid-6 really have hexagonal columnar structure by X-ray diffractmeter. After qualitative demonstrating and optical analysis, we use DLC-PAM and Acid-6 as photo-sensitized dyes for DSSCs and manufacture two kinds of cells successfully. The more photocurrent occur when the two DSSCs are woking. Besides, the two DSSCs have good performance on power conversion efficiency which can achieve 0.047 % for DLC-PAM and 0.364 % for Acid-6. Therefore, in this research we prove that DLC-PAM and Acid-6 are able to be photo-sensitized dyes for DSSCs and successfully demonstrate that using the two materials to manufacture DSSCs is feasible.

Dye-sensitized solar cells (DSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Although efficiencies exceeding 10% in full sunlight have been presented, major improvements of the system are however limited. Electron transport is one of the processes in the cell and is of major importance for the overall performance. It is further a complex process because the transport medium is a mesoporous film and the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, present models describing electron transport include simplifications, which limit the practical use, in terms of improving the DSC, because the included model parameters usually have an effective nature. This thesis focuses in particular on the influence of the mesoporous film on electron transport and also on the influence of electron-ion interactions. In order to model diffusion, which is assumed to be the transport process for electrons in the DSC, Brownian motion simulations were performed and spatial restrictions, representing the influence of the mesoporous film, were introduced by using representative models for the structure. The simulations revealed that the diffusion coefficient is approximately half the value for electrons and ions in mesoporous systems. To study the influence of ions, a simulation model was constructed in where electric fields were calculated with respect to the net charge densities, resulting from the different charge carrier distributions. The simulations showed that electron transport is highly dependent on the nature of the ions, supporting an ambipolar diffusion transport model. Experimentally, it was found that the transport process is dependent on the wavelength of the incident light; we found that the extracted current was composed of two components for green light illumination, one fast and one slow. The slow component showed similar trends as the normal current. Also we found that the transport coefficient scaled linearly with film thickness for a fixed current, which questions diffusion as transport process. Other experiments, investigating various effects in the DSC, such as the effect of different cations in the electrolyte, are also presented. / QC 20100708

During the last decade, the dye sensitised solar cell (DSC) has attracted much attention. The technology has a potential to act as a new generation of photovoltaic device, it has also increased our knowledge within the field of photoelectrochemistry. The materials used in the DSC have been used in other technologies, such as electrochromic displays. This thesis examines how such systems can be analysed to understand their properties from their components. Both of the considered device technologies consist of a thin mesoporous semiconductor film immersed in an electrolyte. The study starts by investigating some of the fundamental properties of the mesoporous semiconductor and its interface with the electrolyte. This gives rise to the charge-voltage relationship for the devices, which is related to the chemical capacitance and electronic energy levels for the materials. In particular,special attention is given to the DSC and the properties of the charge carriers in the semiconductor. For the DSC, several techniques have been developed in order to understand the processes of transport and recombination for the charge carriers in the semiconductor film, which are vitally important for performance. In this thesis, particular focus is given to light modulation techniques and electrical analysis with impedance spectroscopy. The transportproperties show for both techniques a nonlinear behaviour, which is explained with the trapping model. The DSC solar cell is analysed in order to interpret the transport measurements for film thickness optimisation. DSC cells with new semiconductor materials, such as ZnO, were analysed with impedance measurements to provide new insights into the optimisation of the performance of the photoelectrochemical solar cell technology. / QC 20100804

The photophysical properties of a series of Zn (II) porphyrins adsorbed onto a semiconductor were investigated using steady-state absorbance and emission measurements. The ability of the porphyrins to undergo triplet-triplet annihilation (TTA), a photophysical process through which photons in the red and near-infrared (NIR) regions of the optical spectrum can be converted into higher energy photons (upconversion), was explored. Aggregation capabilities were determined to verify possibility of these molecules to undergo triplet-triplet annihilation (TTA). TTA has significant potential for increasing the efficiency of dye-sensitized solar cells (DSSCs) by upconverting photons in the energy rich NIR region of the solar spectrum. A key requirement for efficient TTA is aggregation of the sensitizer dye, and in this thesis, we have examined the aggregation of porphyrins in TiO2-based sol-gel films. Solution phase absorption and emission studies were conducted using zinc (II) tetraphenylporphyrin and three of its functionalized derivatives, tetra(4-aminophenyl)porphyrin Zn(II), tetra(4-carboxyphenyl)porphyrin Zn(II), and tetra(4-sulfonatophenyl)porphyrin Zn(II), to evaluate their potential as DSSC sensitizers on TiO2 thin films. Mesoporous TiO2 thin films were synthesized, using a polymer-templating sol-gel route, and characterized with tunneling electron microscopy (TEM), atomic force microscopy (AFM), and UV-Vis absorbance measurements. Spectroscopy measurements were also carried out on porphyrin-sensitized TiO2 thin films and compared to solution-based results. A simple DSSC was constructed and used to further explore the application of zinc (II) porphyrin sensitizers in photovoltaic applications.

El processament convencional de materials d'òxid de titani (TiO2) inherentment implica un consum energètic important i ha esdevingut una limitació tecnològica per a la fabricació de dispositius emprant substrats termolàbils així com per a la preparació de materials híbrids orgànic/TiO2. Aquesta tesi doctoral tracta del desenvolupament de processos químics per a la preparació de capes i pols de TiO2 d'acord amb principis de Química Verda, posant especial èmfasi en el processament a baixa temperatura. Plantejaments simples, benignes amb el medi ambient i de baix cost són els desitjats sota aquestes directrius. A més a més, els mètodes a baixa temperatura (idealment fins a un màxim proper a 100ºC) haurien de donar lloc a materials que exhibeixin propietats similars a aquells processats a alta temperatura o tractats solvotermalment. S'ha focalitzat especial interès en la recerca d'aplicacions pràctiques dels materials produïts en camps com la conversió d'energia solar i materials actuadors. / El procesamiento convencional de materiales de óxido de titanio (TiO2) inherentemente implica un consumo energético importante y ha resultado una limitación tecnológica para la fabricación de dispositivos utilizando substratos termolábiles así como para la preparación de materiales híbridos orgánico/TiO2. Esta tesis doctoral trata el desarrollo de procesos químicos suaves para la preparación de capas y polvos de TiO2 de acuerdo con principios de Química Verde, poniendo especial énfasis en el procesamiento a baja temperatura. Planteamientos simples, benignos con el medioambiente y de bajo conste son los deseados bajo estas directrices. Además, los métodos a baja temperatura (idealmente hasta un máximo próximo a 100ºC) deberían dar lugar a materiales que exhiban propiedades similares a aquellos procesados a alta temperatura o tratados solvotermalmente. Se ha centrado especial interés en la búsqueda de aplicaciones prácticas de los materiales producidos en campos como la conversión de energía solar y materiales actuadores. / Conventional high-temperature processing of titanium dioxide (TiO2) materials inherently implies important energy consumption and has became a technological limitation for fabricating devices employing thermolabile substrates as well as preparing hybrid organic/TiO2 materials. This PhD thesis deals with the development of chemical processes for preparing TiO2 films and powders fitting Green Chemistry principles, putting special emphasis on low-temperature processing. Simple, environmentally benign and low-cost approaches are desired upon these guidelines. Moreover, low-temperature methods (ideally up to a maximum closer to 100ºC) should give rise to materials exhibiting properties similar than those processed at high temperature or solvothermally treated. Special interest has been focused on finding practical applications of the as-prepared materials in fields such as solar energy conversion and actuating materials.

A novel ionic liquid crystal (ILC) system (C18IMCNBr) with a liquid crystal alignment used as an electrolyte for a dye-sensitized solar cell (DSSC) showed the higher short-circuit current density (Jsc) and the higher light-to-electricity conversion efficiency than the system using the non- alignment liquid crystalline ionic liquid (C18IMCNBr),due to the higher conductivity of liquid crystal alignment. The larger Jsc and efficiency value of liquid crystal alignment supported that the higher conductivity of liquid crystal alignment is attributed to the enhancement of the exchange reaction between iodide species. As a result of formation of the two-dimensional electron conductive pathways organized by the localized I3- and I- at liquid crystal alignment layers, the concentration of polyiodide species exemplified by Im- (m =5,7, ...) was higher in alignment C18IMCNBr. However, in the two-dimensional electron conductive pathways of C18IMCNBr, more collision frequencies between iodide species (I-,I3-, and Im-) could be achieved than that in the three-dimensional space of C18IMCNBr, which could lead to the promotion of the exchange reaction between iodide species, the contribution of a two-dimensional structure of the conductive pathway through the increase of collision frequency between iodide species was proposed.