Zinc oxide (ZnO) is a wide band-gap semiconductor that is of interest for application in dye sensitized solar cells (DSSCs) because of similarity of its properties to TiO2. Unlike TiO2, ZnO can readily be grown in a wide variety of morphologies, using inexpensive, simple, and low temperature methods. Recent research on ZnO-based DSSCs focuses on modifying the ZnO layer morphology in order to maximize surface area, and enhance the electron collection by providing fast electron transport. It is expected that the improvement in cell performance by morphology modification is due to higher dye loading, increased electron lifetime and fast electron transport. However, ZnO properties may be affected by various synthesis methods. It is difficult to make a conclusion whether the change of performance are attributed to change of morphology or a change in the defect types and/ or defect concentrations. In this study, the influence of ZnO nanoparticle properties on cell performance has been investigated. Commercial ZnO nanoparticles with different sizes and optical properties were utilized. It was found that there is a complex relationship between native defects, dye loading, charge transport and photovoltaic performance. In particular, the presence of non-radiative defects was found to be detrimental to photovoltaic performance. In addition, with the similar defect emission intensities, sample exhibiting orange-red defect emission showed better performance than the samples emitting green defect emission. Nanoparticle properties and their relationship between dye adsorption, electron injection, electron lifetime and electron transport, and photovoltaic performance will be discussed. / published_or_final_version / Physics / Master / Master of Philosophy

Excitonic photovoltaic devices, including organic, hybrid organic/inorganic, and dye-sensitized solar cells, are attractive alternatives to conventional inorganic solar cells due to their potential for low cost and low temperature solution-based processing on flexible substrates in large scale. Though encouraging, they are currently limited by the efficiency from not yet optimized structural and material parameters and poor overall knowledge regarding the fundamental details. This dissertation aims to achieve improved performance of hybrid solar cells by enhancing material property and designing new device architecture. The study begins with the addition of XD-grade single-walled carbon nanotube (XDSWNT) into poly(3-hexylthiophene) (P3HT) to improve the current density. By having a weight ratio of XDSWNT and P3HT equaled to 0.1:1, short-circuit current was quadrupled from 0.12 mA cm-2 to 0.48 mA cm-2 and solar cell efficiency was tripled from 0.023% to 0.07%, compared to devices with pure P3HT as a hole transport material. Secondly, a significant improvement in device efficiency with 250 nm long ZnO nanorod arrays as photoanodes has been achieved by filling the interstitial voids of the nanorod arrays with ZnO nanoparticles. The overall power conversion efficiency increased from 0.13% for a nanorod-only device to 0.34% for a device with combined nanoparticles and nanorod arrays. The higher device efficiency in solid-state DSSCs with hybrid nanorod/nanoparticle photoanodes is originated from both large surface area provided by nanoparticles for dye adsorption and efficient charge transport provided by the nanorod arrays to reduce the recombinations of photogenerated carriers. Followed by the novel layer-by-layer self-assembly deposition process, the hybrid photoanode study was extended to the longer ZnO nanorod arrays. The best performance, 0.64%, was achieved when the thickness of the photoanodes equaled to 1.2 ?m. Finally, the photovoltaic devices were modified by adding ZnO nanoarpticles into P3HT to increase interfacial area between ZnO and P3HT. The efficiency was enhanced from 0.18% to 0.45% when the ZnO nanorod arrays were 625 nm in length. Our successful design of the device morphology significantly contributes to the performance of solid-state hybrid solar cells.

Over the past decade, intensive academic and commercial interests have been paid on compounds possessing photochemical properties, namely for their preparation, chemical properties, high efficiency and potential low-cost. Compounds having intense photochemical properties gained great interest due to wide range of potential applications. The sensitizers are one of the key components for high power-conversion efficiency in the dye sensitized solar cells (DSSCs). They are the core components in the organic light-emitting devices (OLEDs) due to their ability to emit light with the wavelengths largely red- shifted from their absorption wavelength. Ruthenium based sensitizers have been tagged “molecular light switches” because, although the fluorescence of these complexes in aqueous solutions is negligible, it increases of greater than 10000 fold in the presence of DNA. Many polypyridyl and dipyrido phenazine ruthenium complexes have achieved high power conversion efficiencies and therefore are of practical interest. Several research groups stated that the dipyrido phenazine ligand may be thought of as comprising two components: a bipyridyl unit and a phenazine unit. These two subunits behave essentially separately, with many molecular orbitals being localised over only one subunit and a redox properties of central phenazine moiety in the dipyrido phenazine ligand are important for the photochemical applications. Therefore a phenazine ligand was selected as a model for the present investigation. The chemistry of phenazine ligand is mostly limited to the late transition metal and f - element complexes. Our laboratory has a rich backgroung in the aluminum and early transition metal chemistry. The aluminum chemistry and early transition metal chemistry are of great interest since aluminum and early transition metal complexes are environmentally friendlier and cheaper than the late transition metal compounds. Another drawback of the ruthenium-based sensitizers is the lack of absorption in the red region of the visible spectrum, and also low molar extinction coefficients. An essential requirement for efficient conversion of solar energy is the good spectral match of the sensitizer absorption to the emission spectrum of solar radiation. In this regard, the ruthenium sensitizers’ spectral response in the lower energy regions is not sufficient. The current project has three parts. In the first part we collected and reviewed known literature regarding the certain classes of non-innocent ligands containing the six-membered carbon- nitrogen heterocycles and regarding the ligands potentially important for the photochemical applications. We also reviewed all available to the data information about the complexes supported by the phenazine ligand. In the second part we have investigated interaction of alkylaluminum compounds and phenazine and observed reduction of phenazine accompanied by formation of dialuminum cage type compounds containing two formally mononegative phenazine ligand. The derivatization of phenazine has been also observed. It resulted in formation of compounds having a stable organic radical. In a third part of our project we have explored interaction of phenazine or thiophenazine with the alkylaluminum compounds and chromium dichloride. The reaction in the three component system resulted in reduction of phenazine ligand and lead to the heterometallic Cr(II) - aluminum complexes containing a formally dinegative phenazine or thiophenazine ligands. When a large excess of triethylaluminum was taken, reduction of phenazine and chromium has been observed leading to the heterometallic multinuclear Cr(I) - aluminum complex containing a formally dinegative phenazine ligands and two chromium atoms in one complex in the rare oxidation state one.

Thesis (Ph. D.)--University of Hong Kong, 2010. / Includes bibliographical references. Also available in print.

Thesis (M.Sc.)--Victoria University of Wellington, 2009. / Includes bibliographical references.

Thesis (Ph. D.)--University of Idaho, 2005. / Also available online in PDF format. Abstract. "July 2005." Includes bibliographical references.

The depletion of fossil fuels and the increasing energy demand for energy has led to the search for better and improved technologies with special focus renewable energy, especially solar cells. The first generation solar cells based on silicon are expensive, hence dye sensitized solar cells come in as a better alternative as these solar cells are environmental friendly, they have moderately good conversion efficiency and they are relatively cheap to produce. Dithiocarbamate ligands have been widely used in many research fields, as these are versatile ligands. Coordination of dithiocarbamates with metals such as ruthenium has produced high conversion efficiency and have the ability to extend the MLCT absorptions, and this can further extend their wavelength. In this study five dithiocarbamate sodium salt ligands were prepared and were coded as FL1= Aniline, FL2= p- toluidine, FL3= p- anisidine, FL4=dibenzyl, FL5=diphenyl. These ligands were used to synthesize Ru(II) metal complexes which were formulated as [Ru(FLx)(dcbpy)(NCS)] and [Ru(FLx)2(dcbpy)] where FLx is the dithiocarbamate ligand and dcbpy is 2,2-bipyridine-4,4’-dicarboxylic acid and the complexes were coded as FCx. The synthesized compounds were characterized using techniques such as the melting point, molar conductivity, FT-IR and NMR spectroscopy. For spectroelectrochemical studies of the metal complexes, techniques such as UV-Vis and photoluminescence spectroscopy were carried out. Furthermore, redox properties of the complexes were analyzed using cyclic and square wave voltammetry. The FT-IR displayed all the expected peaks of interest both in the dithiocarbamate ligands and in the metal complexes. The electronic spectra confirmed the successful coordination of ligand to the metal centre, the electronic spectra of the complexes also confirmed the six coordinate octahedral geometry of the complexes. The complexes exhibited some photoluminescence properties that are suitable for dye sensitization. The cyclic voltammogram of the complexes displayed more reduction potentials that could be attributed to the π-conjugation in the ligands incorporated during synthesis. The square wave voltammogram of the complexes is in agreement with the results obtained in cyclic voltammetry.

Dye-sensitized solar cell (DSSC) has attracted increasing interest as a promising hybrid organic-inorganic solar cell. At the heart of the device is a photosensitizer, which is anchored onto a wide-bandgap semiconducting metal oxide. It harvests solar light and transfers the energy via electron transfer to a suitable material (e.g. TiO2) to produce electricityas opposed to chemical energy in plant. The topic of this thesis focuses on the design and synthesis of metal-free organic dyes for applications in DSSCs. Specific attention has been paid to the correlation between the molecular structures and physical properties, as well as their performances in DSSCs. Chapter 1 presents the major components and working principle of DSSC, following by a brief overview of the development of organic dyes and their application in DSSCs. In chapter 2, we have designed two types of new phenothiazine-based dyes to investigate the positioning effect a donor group on the cell performance. The structural features of a donor aryl group at the C(7) position of phenothiazine core extend the π-conjugation of the chromophore and efficiently suppress the dye aggregation on TiO2 film. As a result, Type 1 dyes have better light harvesting properties in contact with TiO2 films, and give much better photovoltaic performance than Type 2 dyes. Chapter 3 presents the synthesis and characterization of a series of simple phenothiazine-based dyes, in which, a linear electron-rich (4-hexyloxy)phenyl group at C(7) of the phenothiazine periphery as the donor, and an alkyl chain with different length at N(10). The dye molecules show a linear shape which is favorable for the formation of a compact dye layer on the TiO2 surface, while their butterfly conformations can sufficiently inhibit molecular aggregation. Moreover, the alkyl substituents with different chain length at N(10) could further optimize the performance through complete shielding the surface of TiO2 from the Iˉ/I3ˉ electrolyte. Under simulated AM 1.5G irradiation, the PT-C6 based DSSC produces a short-circuit photocurrent of 15.32 mAcm−2, an open-circuit photovoltage of 0.78 V, a fill factor of 0.69, corresponding to a power conversion efficiency (PCE) of 8.18%. Moreover, we designed a stepwise approach for co-adsorption of the organic dye PT-C6 with a porphyrin dye (ZnP) for DSSCs. Upon optimization, the device made of the PT-C6 + ZnP system yielded Jsc = 19.36 mA cm-2, Voc =0.735 V, FF = 0.71 and η = 10.10%. In chapter 4, we further developed five organic dyes appended with T, TT, E, ET, or EE (T and E denote thiophene and 3,4-ethylenedioxythiophene (EDOT), respectively) on the C(7) atom of phenothiazine core as electron donors. We have also analyzed the structure-performance corelations of dye molecules in the aspect of dye aggregation, electron injection, dye regeneration and interfacial charge recombination of electrons with electrolytes and/or oxidized dye molecules, through DFT calculation, impedance analysis and transient photovoltage studies. In chapter 5, we extended our studies by using phenothiazine as a building block to construct 3D bulky organic dyes. We systematically investigated the influence of 3D bulky substituents on dye aggregation and charge recombination, as well as photovoltaic performance of DSSCs. The molecular design strategy demonstrates that high Voc can be realized by employing 3D-phenothiazine dyes featuring a bulky substituent, such as, hexylcarbazole and dihexylfluorene units. Impressively, the co-adsorbent-free DSSCs based on dye TP3 exhibits a photovoltaic performance with efficiency up to 8.00 %. In order to realize a panchromatic absorption and further enhance the energy conversion efficiency of DSSCs, we also designed a stepwise approach for co-adsorption of the organic dye TP3 with a NIR dye YR6 for co-sensitized DSSCs. Upon optimization, the device made of the TP3 + YR6 system yielded Jsc = 19.18 mA cm-2, Voc =0.721 V, FF = 0.712 and η = 9.84 %. The power-conversion efficiency is the highest reported efficiency for a squaraine dye-based co-sensitized panchromatic DSSCs. From chapters 6 and 7, a series of new simple panchromatic dyes based on thiadiazolo[3,4-c]pyridine (PyT) have been designed for panchromatic DSSCs. These new organic dyes exhibit broad absorption spectrum in the range of 300~850 nm and high molar extinction coefficients. The electrochemical analyses demonstrate that the incorporation of the auxiliary electron-deficient thiadiazole[3,4-c]pyridine unit can fine-tune the HOMO and LUMO energy levels and red-shift the absorption spectra to NIR region. The overall conversion efficiencies of liquid-electrolyte DSSCs based on these sensitizers range from 0.46 to 6.30 %. We draw some conclusions in chapter 8 together with the outlooks in DSSCs

This thesis describes the synthesis and characterization of a series of photosensitizers, transition metal-containing polymers and small organic molecules for dye-sensitized solar cells and organic solar cells. To begin with, a brief overview on the background of dye-sensitized solar cells (DSSCs) and organic solar cells was presented in Chapter 1. In Chapter 2, a series of novel donor-acceptor-π-acceptor bithiazole-based and fluorenone-based organic dyes for dye-sensitized solar cells were successfully synthesized and fully characterized. We discovered that the performance of the photovoltaic devices depends significantly on the nature and strength of the electron-donating end group along the conjugated main. Some of the materials have been found to show higher power conversion efficiency of 4.71% (Voc = 565 mV, Jsc = 11.71 mA cm–2, FF = 0.71) under AM 1.5 irradiation (100 mW cm–2). In Chapter 3, ten novel donor-donor-π-acceptor organic dyes for dye-sensitized solar cells have been synthesized and applied for the fabrication of DSSCs, including six dibenzothiophene-based photosensitizers and carbazole-based photosensitizers. All the dyes have efficient charge injection from the excited sensitizer molecule to TiO2 conduction band and can provide ample driving force for efficient dye regeneration best overall light to electricity conversion efficiency of 5.28% (Voc = 0.70 V, Jsc = 11.06 mA cm–2, FF = 0.68) under AM 1.5 irradiation, which reached 73% with respect to that of an N719-based device fabricated under similar fabrication conditions. Besides, nine novel di-anchoring organic sensitizers employing two different electron-donating cores, which are the fluorene and carbazole units, and two symmetrical anchoring cyanoacrylic acid (acceptor) termini have been synthesized and studied for their applications in DSSCs in Chapter 4. In Chapter 5, four new platinum polyyne polymers were prepared via the Sonogashira-type dehydrohalogenation reaction between the ethynyl precursor and trans-Pt(PBu3)2Cl2. All of the polymers are air-stable and well characterized by different spectroscopic methods and photophysical measurements. Their photovoltaic behaviors were fully investigated. Their model compounds were also prepared and studied. In Chapter 6, a series of new organic small molecules were designed and synthesized comprising head-to-head coupled heylthiophene, dithienosilole and dithienogermole units. They exhibited broad absorption peaks with favorable spectral overlap with the solar spectrum. These seven small molecules have been applied to presented. Among these molecules, the highest PCE of 4.93% was achieved with a Voc = 0.79 V, Jsc = 1.22 mA cm-2 and FF = 0.51 under illumination of an AM 1.5 solar cell simulator. Finally, Chapters 7 and 8 present the concluding remarks and the experimental details of the work described in Chapters 2−6

In this study a presentation of the technology behind dye-sensitized solar cells, their design as well as the role of the different parts of the cell. The characterization of the cell is divided into four sections namely: the characterization of the paste required to make the TiO2 film and its optical properties using SEM-EDX and XRD analytical techniques; Analysis of the various absorptions of three Ru-based dyes using UV-Vis spectroscopy, Photoluminescence and Fourier Transform Infra-Red spectroscopy; the characterization and the analyses of the entire cell using Electrochemical Impedance Spectroscopy. The nine cells were prepared by examining RuL2(CN)2 , RuL2(NCS)2 or N3 dye and RuL2(NCS)2 TBA+ or N719 dye. [L = 2,2'-bipyridyl-4,4'-dicarboxylic acid ;TBA = tetra-butyl ammonium] were combined with three electrolytes namely: Z–150 , AN–50 and PN–50. The Iodolyte PN–50 is an iodide based low viscosity electrolyte with 50 mM of tri-iodide dissolved in a solvent called propionitrile (PN). The Iodolyte AN–50 is an iodide based low viscosity electrolyte with 50 mM of tri-iodide dissolved in a solvent called acetonitrile (AN). The Iodolyte Z–150 is an iodide based low viscosity electrolyte with 150 mM of tri-iodide dissolved in a solvent called 3-methoxypropionitrile (MPN) and with additives such an ionic liquid, malkylbenziimidazole and guanidine thiocyanate. A solar simulator was utilized with which the standard solar irradiation can be created in laboratory conditions. The fill factors as well as overall performance efficiencies of the these cells are quite low < 1.0%,.