This thesis examines the electro-optic, electric and electrochemical properties of liquid crystal (LC) materials in self-assembly systems, that is, liquid crystal-polymer electrolyte composites (LC-PEs), LC binary mixtures, and their potential application in dye-sensitised solar cells (DSSCs). The birefringence of LCs causes light modulation, which can be controlled by an applied voltage and electric field. In particular, the LCs are used as one of the components for the electrolyte redox couple which is responsible for charge transfer mechanism in DSSCs. In this work, LC-PEs were developed by dissolving LCs in polymer electrolytes; using a homologous series of cyanobiphenyls in a range of concentrations, alkyl chain lengths and dielectric permittivities. We found that doping the polymer electrolyte with 15% 4'-cyano-4'-pentylbiphenyl (5CB) improved ionic conductivity by up to 13 % compared to pure polymer electrolyte. Materials with positive dielectric permittivity and shorter alkyl chain length have been identified to be compatible with iodide/triiodide (I^-/I_3^-)-based polymer electrolytes. In DSSCs, 15% 5CB and 15% E7 LC-PEs exhibited the best efficiencies of 3.6 % and 4.0 %, respectively. In addition to LC-PEs, the self-assembly properties of smectic phase LCs were also utilised as templates for controlling the polymer structure in polymer electrolytes. A porous polymer network was prepared using various techniques including self-assembly, by applying an electric field and using a polyimide (PI) alignment layer. We found that the electrochemical and photovoltaic properties of these materials strongly correlated to the morphology/structure with the self-assembled structure, thus showing the best photovoltaic performance (5.9 %) even when compared with a reference solar cell (4.97 %). Finally, this thesis explores the interaction of LCs with graphene (Gr) in DSSC device architectures. Gr-based DSSCs were fabricated using different processing conditions, with the result being that Gr improved the performance of the DSSCs. The highest efficiency obtained was 5.48 % compared to the 4.86 % of a reference DSSC. The incorporation of LC-PEs in Gr-based DSSCs improved the performance of DSSCs was observed in devices with low concentrations of LCs due to the Gr inducing planar alignment of LCs. These results suggest a new strategy to improve DSSC efficiency by incorporating LC materials in the polymer electrolyte component. Even though these LCs are highly insulating, their self-assembly and dielectric polarisability help enhance ionic conductivity and optical scattering when doped into polymer electrolytes. This work can be extended in a fundamental way to elucidate the ionic conduction mechanism of LC-based electrolyte systems. Furthermore, it would be interesting if the benefits of using LC-PEs and smectic-templated polymer electrolytes (Sm-Pes) can be translated further in commercial electrochemical energy conversion systems.

This thesis presents a study of hybrid solar cells, specifically looking at various methods which can be employed in order to increase the power conversion efficiency of these devices. The experiments and results contained herein also present a very accurate picture of how rapidly the field of hybrid solar cells has progressed within the past three years. Chapters 1 and 2 present the background and motivation for the investigations undertaken, as well as the relevant theory underpinning solar cell operation. Chapter 2 also gives a brief review of the literature pertinent to the main types of devices investigated in this thesis; dye-sensitised solar cells, semiconductor sensitized solar cells and perovskite solar cells. Descriptions of the synthetic procedures, as well as the details of device fabrication and any measurement techniques used are outlined in Chapter 3. The first set of experimental results is presented in Chapter 4. This chapter outlines the synthesis of mesoporous single crystals (MSCs) of anatase TiO₂ as well as an investigation of its electronic properties. Having shown that this material has superior electronic properties to the conventionally used nanoparticle films, they were then integrated into low temperature processed dye-sensitised solar cells and achieved power conversion efficiencies of > 3%, exhibiting electron transport rates which were orders of magnitude higher than those obtained for the high temperature processed control films. Chapter 5 further investigates the use of MSCs in photovoltaic devices, this time utilising a more strongly absorbing inorganic sensitiser, Sb₂S₃. Utilising the readily tunable pore size of MSCs, these Sb₂S₃ devices showed an increase in voltage and fill factor which can be attributed to a decrease in recombination within these devices. This chapter also presents the use of Sb₂S₃ in the meso-superstructured configuration. This device architecture showed consistently higher voltages suggesting that in this architecture, charge transport occurs through the absorber and not the mesoporous scaffold. Chapters 6 and 7 focus on the use of hybrid organic-inorganic perovskites in photovoltaic devices. In Chapter 6 the mixed halide, lead-based perovskite, CH₃NH₃PbI_3-xCl_x is employed in a planar heterojunction device architecture. The effects of Lewis base passivation on this material are investigated by determining the photoluminescence (PL) lifetimes and quantum efficiencies of treated and untreated films. It is found that passivating films of this material using Lewis bases causes an increase in the PLQE at low fluences as well as increasing the PL lifetime. By globally fitting these results to a model the trap densities are extracted and it is found that using these surface treatments decreases the trap density of the perovskite films. Finally, these treatments are used in complete solar cells resulting in increased power conversion efficiencies and an improvement in the stabilised power output of the devices. Chapter 7 describes the materials synthesis and characterisation of the tin-based perovskite CH₃NH₃SnI₃ and presents the first operational, lead-free perovskite solar cell. The work presented in this thesis describes significant advances in the field of hybrid solar cells, specifically with regards to improvements made to the nanostructured electrode, and the development and implementation of more highly absorbing sensitizers. The improvements discussed here will prove to be quite important in the drive towards exploiting solar power as a clean, affordable source of energy.

Philosophiae Doctor - PhD / The first report of a functioning photo-electrochemical solar cell in 1991 attracted a lot of interest from scientists and industrial groups. From an industrial point of view these so-called dye-sensitised solar cells (DSCs) offered the promise of moderate efficiency devices at ultra-low costs, owing to simple processing methods and the use of inexpensive materials. From an academic viewpoint, DSCs raised important scientific questions around the fundamental processes governing their operation and how these processes influence the photon-to-electron conversion efficiency of the cell. Major successes have since been achieved in understanding these processes, however the conversion efficiency of the best manufactured DSCs remains around 11%, significantly lower than that of their silicon photovoltaic counterparts. In traditional DSCs, charge generation is achieved by ultrafast electron injection from a photo-excited ruthenium-based dye molecule into the conduction band of a film of TiO₂ nanoparticles, subsequent dye regeneration by an I⁻ /I⁻₃ containing redox electrolyte and finally hole transportation to a platinum-coated counter electrode. The low DSC efficiencies are attributed to scattering of electrons at the interface between two TiO₂ nanoparticles leading to recombination with holes present in the redox electrolyte. Recent studies have shown that the application of films of highly ordered TiO₂ nanotubes instead of nanoparticles has the potential to improve the overall conversion efficiency of the cell. This is ascribed to the one-dimensional nature of nanotubes, which provides a linear transportation route for electrons generated during operation of the DSC. As a result the recombination probability of the electrons with nearby holes in the device is decreased. This work investigated the synthesis of Al₂O₃-coated TiO₂ nanotubes via the anodisation technique for application in DSCs. TiO₂ nanotube arrays with an average length of 15 μm, diameter of 50 nm and wall thickness of 15 nm were synthesised via anodisation using an organic neutral electrolyte consisting of 2 M H₂O + 0.15 M NH₄F + ethylene glycol (EG) at an applied voltage of 60 V for 6 hours. In addition, scanning electron microscope (SEM) micrographs showed that anodisation at these conditions yields nanotubes with smooth walls and hexagonally shaped, closed bottoms. X-ray diffraction (XRD) patterns revealed that the as-anodised nanotubes were amorphous and as such were annealed at 450 °C for 2 hours in air at atmospheric pressure, which yielded crystalline anatase TiO₂ nanotubes. Highresolution transmission electron microscope (TEM) images revealed that the nanotube walls comprised of individual nano-sized TiO₂ crystallites. Photoluminescence (PL) spectroscopy showed that the optical properties, especially the bandgap of the TiO₂ nanotubes are dependent on the crystallinity, which in turn was dependent on the structural characteristics, such as the wall thickness, diameter and length. The PL measurements were supplemented by Raman spectra, which revealed an increased in the quantum confinement of the optical phonon modes of the nanotubes synthesised at low anodisation voltages, consequently yielding a larger bandgap The annealed nanotubes were then coated with a thin layer of alumina (Al₂O₃) using a simple sol-gel dip coating method, effectively used to coat films of nanoparticles. Atomic force microscopy (AFM) showed that the average nanotube diameter increased post sol-gel deposition, which suggests that the nanotubes are coated with a layer of Al₂O₃. This was confirmed with HR-TEM, in conjunction with selected area electron diffraction (SAED) and XRD analyses, which showed the coating of the nanotube walls with a thin layer of amorphous Al₂O₃ with a thickness between 4 and 7 nm. Ultraviolet-visible (UVvis) absorbance spectra showed that the dye-adsorption ability of the nanotubes are enhanced by the Al₂O₃ coating and hence is a viable material for solar cell application. Upon application in the DSC, it was found by means of photo-current density – voltage (I – V) measurements that a DSC fabricated with a 15 μm thick layer of bare TiO₂ nanotubes has a photon-to-light conversion efficiency of 4.56%, which increased to 4.88% after coating the nanotubes with a layer of alumina. However, these devices had poorer conversion efficiencies than bare and Al₂O₃-coated TiO₂ nanoparticle based DSCs, which boasted with efficiencies of 6.54 and 7.26%, respectively. The low efficiencies of the TiO₂ nanotube based DSCs are ascribed to the low surface area of the layer of nanotubes, which yielded low photocurrent densities. Electrochemical impedance spectroscopy (EIS) showed that the electron lifetime in the alumina coated nanotubes are almost 20 times greater than in a bare layer of nanoparticles. In addition, it was also found that the charge transfer resistance at the interface of the TiO₂/dye/electrolyte is the lowest for an Al₂O₃-coated TiO₂ layer.

Photovoltaic cells are a promising solution to the current energy crisis. Among the different photovoltaic cell technologies developed, dye-sensitised solar cells (DSSC) are emerging as viable low-cost alternatives to Si PV technology. This thesis presents studies on fabrication and characterisation of TiO2 based dye-sensitised solar cells. Chapter 1 gives an overview of different photovoltaic cell technologies and a review of the state-of-the art DSSC technology. Chapter 2 describes the techniques used for characterisation of DSSCs. Chapter 3 describes the fabrication of TiO2 based dye-sensitised solar cells. Chapter 4 presents the analysis of measurements obtained by the characterisation techniques. Finally chapter 5 summarises the work done and suggests directions for future work.

This thesis is concerned with measuring ultrafast electron dynamics taking place in dye-sensitized mesoporous semiconductor films employed as working electrodes in dye-sensitized solar cells (DSCs). An understanding of these ultrafast charge transfer mechanisms is essential for designing efficient photovoltaic (PV) devices with high photon-to-current conversion efficiency. Optical-pump terahertz-probe (OPTP) spectroscopy is a sub-picosecond resolution, non-contact, photoconductivity measurement technique which can be used to directly measure charge carrier dynamics within nanostructured materials without the need for invoking complex modelling schemes. A combination of OPTP and photovoltaic measurements on mesoporous TiO2 films show an early-time intra-particle electron mobility of 0.1 cm2/(Vs). This value is an order of magnitude lower than that measured in bulk TiO2 and can be partly explained by the restricted electron movement because of geometrical constraints and increased trap sites in the nanostructured material. In addition, the mesoporous film behaves like a nanostructured composite material, with the TiO2 nanoparticles embedded in a low dielectric medium (air or vacuum), leading to lower apparent electron mobility. THz mobility measured in similar mesoporous ZnO and SnO2 films sensitized with the same dye is calculated to be 0.17 cm2/(Vs) for ZnO and 1.01 cm2/(Vs) for SnO2. Possible reasons for the deviation from mobilities reported in literature for the respective bulk materials have been discussed. The conclusion of this study is that while electron mobility values for nanoporous TiO2 films are approaching theoretical maximum values, both intra- and inter-particle electron mobility in mesoporous ZnO and SnO2 films offer considerable scope for improvement. OPTP has also been used to measure electron injection rates in dye-sensitized TiO2, ZnO and SnO2 nanostructured films. They are seen to proceed in the order TiO2 >SnO2 >ZnO. While the process is complete within a few picoseconds in TiO2/Z907, it is seen to extend beyond a nanosecond in case of ZnO. These measurements correlate well with injection efficiencies determined from DSCs fabricated from identical mesoporous films, suggesting that the slow injection components limit the overall solar cell photocurrent. The reasons for this observed difference in charge injection rates have been explored within. It is now fairly common practice in the photovoltaic community to apply a coating of a wide band-gap material over the metal-oxide nanoparticles in DSCs to improve device performance. However, the underlying reasons for the improvement are not fully understood. With this motivation, OPTP spectroscopy has been used to study how the conformal coating affects early-time mechanisms, such as electron injection, trapping or diffusion length. The electron injection process is unaffected in case of TiCl4-treated TiO2 and MgO-treated ZnO, while it becomes much slower in case of MgO-treated SnO2. Finally, a light-soaking effect observed in SnO2-based solid-state DSCs has been examined in detail using THz spectroscopy and transient PV measurement techniques. It is concluded that continued exposure to light results in a rearrangement of charged species at the metal-oxide surface. This leads to an increase in the density of acceptor states or a lowering of the SnO2 conduction band edge with respect to the dye excited state energy level, ultimately leading to faster electron transport and higher device photocurrents.

<p>För att undersöka skillnad i prestationsförmåga mellan celler sensiterade med växtextraktsbaserad färg, och celler sensiterademed ruteniumkomplex-baserad färg, samt huruvida presskraften påverkar en cells prestationsförmåga, tillverkades icke-slutna fotoelektrokemiska färg-sensiterade solceller med tunnfilmsfotoelektroder av pressad, nanoporös titandioxid.</p><p>Cellerna pressades med tre olika presskrafter och sensiterades med växtextraktsfärg från rödkål, rödbeta, viol och henna, samt en ruteniumkomplex-baserad färg som fick utgöra kontrollbetingelse. För varje cell uppmättes IPCE- och iV-värde och motsvarande fyllnadsgrad (fill factor) och dessa jämfördes.</p><p>Ingen signifikant skillnad kunde fastställas mellan celler pressade med olika presstryck. Bland cellerna sensiterade med växtextraktbaserad färg presterade rödbeta bäst. Cellen med högst effektivitet hade fyllnadsgraden 70%. Emellertid uppvisade de växtfärgade cellerna genomgående sämre effektivitet än de rutenium-sensiterade och fotoströmmarna var mycket låga. IPCE-värdena var allmännt låga: den bäst presterande cellen hade ett IPCE-värde på något över 0,06 i våglängdsintervallet 440-470 nm. En förklaring till detta är de övriga ämnen som förutom pigment återfinns i de växtbaserade färgerna. Dessa hindrar pigmentmättnad och förhindrar att växtfärgen når ruteniumfärgens intensitet. En annan anledning består i svårigheten att passa ihop energinivåerna i cellens elektrolyt-halvledarsystem med energinivåerna hos pigmentet i växtfärgen.</p> / <p>Non-sealed photoelectrochemical dye sensitised solar cells (DSSC) with pressed nanoporous TiO2 thin film photoelectrodes were manufactured for the purposes of finding out whether plant extractbased dye sensitised cells can perform as well as ruthenium complex-based dye sensitised cells and whether the pressing force affects the cell performance.</p><p>The cells were pressed with three different pressing forces and sensitised with plant extracts from red cabbage, beetroot, violet and henna, as well as with a ruthenium complex-based dye for comparison. The IPCE and iV values and the corresponding fill factors of the cells were evaluated and compared.</p><p>No significant difference between the cells pressed with different pressing forces could be established. Among the plant extract-based dye sensitised cells the ones sensitised with beetroot extract performed best. The cell that achieved the highest efficiency had a fill factor of 70%. Compared to the ruthenium-sensitised cells the overall performance of the plant dye sensitised cells were very poor and the produced photocurrents very low. The IPCE values were generally low: one of the best-performing cells had an IPCE value of slightly over 0.06 in the 440-470 nm wavelength ranges. One reason for this is that it is difficult to obtain a plant extract dye as intense and deep in colour as ruthenium complex-based dyes, since pigment saturation is obstructed by the presence of other chemical compounds in the plant extracts. Another is that it is a delicate and difficult matter to match the energy levels in the electrolyte-semiconductor system with the energy levels of the pigments in the plant extract dye.</p>

Recent studies suggest that the performance of dye-sensitised solar cells (DSC) has appeared to have reached a limit, therefore solar cells based on semiconductor materials, such as extremely thin absorber (ETA) solar cells and tandem solar cells are currently the subject of intense research in the framework of low-cost photovoltaic devices as sources of harvesting sunlight to generate electricity. Generally, semiconductor solar cells have been divided into two different types, namely anodic and cathodic type solar cells. Extensive research and development work has been focused on anodic semiconductor sensitised solar cells to date. In contrast, the cathodic semiconductor sensitised solar cells have received no attention which is very surprising. Developing the cathodic semiconductor sensitised solar cell concept is very important in the development of tandem solar cells as well as other new solar cell configurations. The main reason for the lack of research in this area was due to the rarity of p-type semiconductor materials, which made it difficult to find suitable materials to match the energy band edges for cathodic semiconductor sensitised solar cells (CSSC) as well as solid-state cathodic semiconductor solar cells (SS-CSSC). The primary aim of this thesis was to construct cathodic semiconductor sensitised solar cells as well as their solid-state analogues (SS-CSSC). The work conducted within this doctoral study presents state-of-art materials and thin film processing/preparation methods, their characterisation and developing CSSCs and SS-CSSCs employing such films in cascade configurations. No reports have been published in the literature on SS-CSSC to date. The first stage of this thesis is focused on optimising the morphology and the texture (porosity) of the CuI and NiO semiconductor photocathode, by the introduction of new deposition methods namely, pulsed-electrodeposition (PED) and Aerosol-Assisted Deposition (AAD) and Aerosol-Assisted Chemical Vapour Deposition (AACVD). The electrodes prepared by employing the methods mentioned above and controlling the deposition parameters systematically, we have achieved significant improvement in the film morphology and the texture of the deposited films. The resulting electrodes showed excellent improvement in the photoelectrochemical performance which made it suitable for application in construction of both CSSC and SS-CSSC. The photoelectrochemical performance of the electrodes can be seen clearly through the photocurrent density data. For the case of bare CuI, the PEC performance of electrode prepared by the AAD and PED compared against that of continuous-electrodeposition (ED) electrodes. The photocurrent density achieved for the electrodes prepared by AAD and PED was reported around 175 and 75 µAcm-2 respectively which are way higher than the ED case. At the second stage of this study, the work focused on fabrication and characterisation of the CSSCs. Cathodic sensitised PEC solar cells (CuI/Cu2S/(Eu2+/Eu3+) and NiO/Cu2S/(I3-/I-)) were fabricated by deposition of p-Cu2S on the texture controlled CuI and NiO photocathodes. The morphological properties of the photocathode, in particular layer thickness, particle size and film porosity, play an important role in the PEC performance of CSSCs. Optimisation of these parameters led to increased adsorption of the Cu2S light harvester on the photocathode s surface. As a result, the charge injection from Cu2S to the wide band gap photocathode material (CuI and NiO) was significantly improved. Due to this, the CSSC performance showed significant improvement as semiconductor sensitised cathodic solar cells (CSSC). The IPCE and photocurrent density of the CSSC achieved in this study was around (19 and 7 %) and (1 and 0.5 mAcm-2) for the CuI/Cu2S and NiO/Cu2S electrodes respectively. Finally, the SS-CSSC has been fabricated by employing n-Fe2O3 electron transport layer. The construction of SS-CSSC for the first time using the n-Fe2O3 electron transport layer (CuI/Cu2S/Fe2O3 and NiO/Cu2S/Fe2O3) allowed us to study the materials, optical and photoelectrochemical properties of this device. Under AM 1.5 illumination, the SS-CSSC shows a photocurrent density of 6 and 9 µAcm-2 for CuI/Cu2S/Fe2O3 and NiO/Cu2S/Fe2O3 solar cells, respectively. The results of this work indicated low performance for both SS-CSSC compared to CSSC results, due to the lack of adsorption between the absorber and Fe2O3 electrode. However, this study proved the concept of SS-CSSC based on semiconductor material, which is valuable for the future work of cathodic semiconductor sensitised solar cells as well as solid-state tandem solar cells.

För att undersöka skillnad i prestationsförmåga mellan celler sensiterade med växtextraktsbaserad färg, och celler sensiterademed ruteniumkomplex-baserad färg, samt huruvida presskraften påverkar en cells prestationsförmåga, tillverkades icke-slutna fotoelektrokemiska färg-sensiterade solceller med tunnfilmsfotoelektroder av pressad, nanoporös titandioxid. Cellerna pressades med tre olika presskrafter och sensiterades med växtextraktsfärg från rödkål, rödbeta, viol och henna, samt en ruteniumkomplex-baserad färg som fick utgöra kontrollbetingelse. För varje cell uppmättes IPCE- och iV-värde och motsvarande fyllnadsgrad (fill factor) och dessa jämfördes. Ingen signifikant skillnad kunde fastställas mellan celler pressade med olika presstryck. Bland cellerna sensiterade med växtextraktbaserad färg presterade rödbeta bäst. Cellen med högst effektivitet hade fyllnadsgraden 70%. Emellertid uppvisade de växtfärgade cellerna genomgående sämre effektivitet än de rutenium-sensiterade och fotoströmmarna var mycket låga. IPCE-värdena var allmännt låga: den bäst presterande cellen hade ett IPCE-värde på något över 0,06 i våglängdsintervallet 440-470 nm. En förklaring till detta är de övriga ämnen som förutom pigment återfinns i de växtbaserade färgerna. Dessa hindrar pigmentmättnad och förhindrar att växtfärgen når ruteniumfärgens intensitet. En annan anledning består i svårigheten att passa ihop energinivåerna i cellens elektrolyt-halvledarsystem med energinivåerna hos pigmentet i växtfärgen. / Non-sealed photoelectrochemical dye sensitised solar cells (DSSC) with pressed nanoporous TiO2 thin film photoelectrodes were manufactured for the purposes of finding out whether plant extractbased dye sensitised cells can perform as well as ruthenium complex-based dye sensitised cells and whether the pressing force affects the cell performance. The cells were pressed with three different pressing forces and sensitised with plant extracts from red cabbage, beetroot, violet and henna, as well as with a ruthenium complex-based dye for comparison. The IPCE and iV values and the corresponding fill factors of the cells were evaluated and compared. No significant difference between the cells pressed with different pressing forces could be established. Among the plant extract-based dye sensitised cells the ones sensitised with beetroot extract performed best. The cell that achieved the highest efficiency had a fill factor of 70%. Compared to the ruthenium-sensitised cells the overall performance of the plant dye sensitised cells were very poor and the produced photocurrents very low. The IPCE values were generally low: one of the best-performing cells had an IPCE value of slightly over 0.06 in the 440-470 nm wavelength ranges. One reason for this is that it is difficult to obtain a plant extract dye as intense and deep in colour as ruthenium complex-based dyes, since pigment saturation is obstructed by the presence of other chemical compounds in the plant extracts. Another is that it is a delicate and difficult matter to match the energy levels in the electrolyte-semiconductor system with the energy levels of the pigments in the plant extract dye.

Supramolecular chemistry has asserted itself as a significant multidisciplinary field concerned with molecular effects afforded through non-covalent molecular interactions. The increased interest in the literature towards nanoscale devices, through modulation of molecular function, has seen the renaissance of supramolecular chemistry as function progresses from solution to surface. Heterosupramolecular chemistry follows the architectural principles of supramolecular chemistry and embraces both covalent and non-covalent interactions of condensed phase surfaces and molecular components. A modular approach to device architecture was applied as a novel method of performing photocatalysis under visible light illumination. The application of heterosupramolecular assembly to the design of photoelectrochemical cells capable of visible light induced charge separation allowed the study of interfacial processes by means of electrochemical observations. Preparation of a series of supramolecular components was undertaken as specific molecular species within a photochemical system. Starting from a synthesised bidentate ligand that incorporated an acidic functional group, 4,4'-bis(methyl)phosphonate-2,2'-bipyridine (dmpbpy) as its ethyl ester, was chelated to give the surface sensitisers, bis-(2,2'-bipyridine)-(4,4'-bis(methyl)phosphonato-2,2'-bipyridine)ruthenium(II) dichloride ([Ru(bpy)2(dmpbpy)]Cl2) and cis-bis-(4,4'-bis-(methyl)phosphonato-2,2'-bipyridine)(2,2'-bipyridine)ruthenium(II) dichloride ([Ru(dmpbpy)2(bpy)]Cl2). An electron relay moiety with an acidic functional group, 1-ethyl-1'-(2-phosphonoethyl)-4,4'-bipyridinium dichloride (EVP), was also prepared using a procedure developed by the candidate. The electronic properties of the prepared photosensitisers were examined by theoretical quantum chemical TD-DFT calculations on the molecular structures and singlet excitations were discussed in relation to experimental data. This identified that the lowest lying LUMO states were consistently occupied by 2,2'-bipyridine (bpy) and this was speculated to be a factor affecting quantum injection yields. The effect of microwave modification of colloidal TiO2 suspensions under extended periods of treatment was investigated. Nanoparticles of TiO2 were compared and contrast to similar convection hydrothermally treated TiO2 and a commercial titania product, namely Degussa P25, both of which are utilised in device fabrication. The investigation identified that extended periods of microwave hydrothermal treatment do not greatly enhance the crystallinity and primary grain size of TiO2. The heterosupramolecular assembly of a multi-component photochemical system was constructed from prepared molecular and condensed phase components. It was demonstrated that this device was capable of inducing a photochemical reaction in H2O under irradiation with  > 420 nm in the absence of an organic electron donor. Interpretation of the photocurrents obtained from this assembly provided understanding of photochemical reactions under low light intensities. Optimised conditions for the photochemical reaction was determined to be pH = 5 and illumination yielded = 0.0036% with an apparent quantum yield (AQY) = 1.6%. Photocatalytic decomposition of organic compounds in a dye-sensitised photoelectrocatalytic cell was investigated for the complete mineralisation of EDTA into CO2, H2 and simple amines and interpreted through photocurrent observations. This was extended to a broad range of organic compounds of various solution concentrations as a simulated industrial waste stream. Photooxidation gave unique photocurrent-time profiles which identified two distinct interfacial processes by mathematical treatment of photocurrent transients with a kinetic model. Kinetic parameters were proposed as a factor for qualitative discrimination of the organic compounds. The implications of these results for heterogeneous catalysis were discussed and the formation of Host-Guest complexes as a method of molecular sensing and as specific photocatalytic receptors was proposed.

Philosophiae Doctor - PhD / This work investigated the synthesis of Al2O3-coated TiO2 nanotubes via the anodisation technique for application in DSCs. TiO2 nanotube arrays with an average length of 15 μm, diameter of 50 nm and wall thickness of 15 nm were synthesised via anodisation using an organic neutral electrolyte consisting of 2 M H2O + 0.15 M NH4F + ethylene glycol (EG) at an applied voltage of 60 V for 6 hours. In addition, scanning electron microscope (SEM) micrographs showed that anodisation at these conditions yields nanotubes with smooth walls and hexagonally shaped, closed bottoms. X-ray diffraction (XRD) patterns revealed that the as-anodised nanotubes were amorphous and as such were annealed at 450 °C for 2 hours in air at atmospheric pressure, which yielded crystalline anatase TiO2 nanotubes. Highresolution transmission electron microscope (TEM) images revealed that the nanotube walls comprised of individual nano-sized TiO2 crystallites. Photoluminescence (PL) spectroscopy showed that the optical properties, especially the bandgap of the TiO2 nanotubes are dependent on the crystallinity, which in turn was dependent on the structural characteristics, such as the wall thickness, diameter and length. The PL measurements were supplemented by Raman spectra, which revealed an increased in the quantum confinement of the optical phonon modes of the nanotubes synthesised at low anodisation voltages, consequently yielding a larger bandgap The annealed nanotubes were then coated with a thin layer of alumina (Al2O3) using a simple sol-gel dip coating method, effectively used to coat films of nanoparticles. Atomic force microscopy (AFM) showed that the average nanotube diameter increased post sol-gel deposition, which suggests that the nanotubes are coated with a layer of Al2O3. This was confirmed with HR-TEM, in conjunction with selected area electron diffraction (SAED) and XRD analyses, which showed the coating of the nanotube walls with a thin layer of amorphous Al2O3 with a thickness between 4 and 7 nm. Ultraviolet-visible (UVvis) absorbance spectra showed that the dye-adsorption ability of the nanotubes are enhanced by the Al2O3 coating and hence is a viable material for solar cell application. Upon application in the DSC, it was found by means of photo-current density – voltage (I – V) measurements that a DSC fabricated with a 15 μm thick layer of bare TiO2 nanotubes has a photon-to-light conversion efficiency of 4.56%, which increased to 4.88% after coating the nanotubes with a layer of alumina. However, these devices had poorer conversion efficiencies than bare and Al2O3-coated TiO2 nanoparticle based DSCs, which boasted with efficiencies of 6.54 and 7.26%, respectively. The low efficiencies of the TiO2 nanotube based DSCs are ascribed to the low surface area of the layer of nanotubes, which yielded low photocurrent densities. Electrochemical impedance spectroscopy (EIS) showed that the electron lifetime in the alumina coated nanotubes are almost 20 times greater than in a bare layer of nanoparticles. In addition, it was also found that the charge transfer resistance at the interface of the TiO2/dye/electrolyte is the lowest for an Al2O3-coated TiO2 layer.