The charge transfer dynamics between adsorbate molecules and surfaces are important for a variety of different technologies but especially for dye-sensitized solar cells. The main aim of this thesis was to study charge transfer between organic molecules and surfaces, especially relating to the situation observed in dye sensitized solar cells. This broad aim can be split into two distinct research objectives. One of these was to study the charge transfer between a Au (111) surface and a variety of different molecules using synchrotron-based photoemission spectroscopy. Resonant photoemission spectra of a C60 monolayer on Au (111) showed distinctive superspectator features which were not observed for the multilayer or clean gold spectra. These features were determined to be resultant from spectator decay involving electrons transferred from the gold substrate to the adsorbed molecule, either in the ground state or during the timescale of the core-hole lifetime. These features were also found for monolayers of bi-isonicotinic, isonicotinic, nicotinic and picolinic acid on gold, but not for the dye molecule, N3, on gold. This suggests that, although charge transfer occurs between the surface and the ligand molecules that constitute N3, no charge transfer occurs between the N3 dye molecule and the gold. The other objective was to determine whether the core-hole clock technique, previously only used in photoemission spectroscopy, could be adapted for resonant inelastic x-ray scattering. For this, bi-isonicotinic acid on TiO2 was studied because this system had already been explored using photoemission spectroscopy. The charge transfer times were measured using the relative decrease in the elastic peaks for the LUMO and LUMO+1 photon energies of the multilayer and monolayer. This gave a similar result to the photoemission studies providing more confidence for using this adaptation in situations where photoemission would be impossible, such as buried interfaces.

The development of mesoporous titania (meso-TiO2) films is a considerable research goal in the field of mesoporous material development due to their proven applicability in solar cells and phtocatalysts. In this work, the meso-TiO2 films were fabricated through different methods and these home-made titania structures were applied in DSSCs and photocatalysts. Meso-TiO2 powders were first prepared from ethanol/water or ethanol solvent. The meso-TiO2 made from the ethanol/water solvent did not have an ordered mesostructure, but that made from ethanol solvent had 2D-hexagonal mesostructure. Films were prepared by adding ordered meso-TiO2 particles into paste formulations of P25 nanoparticles with weight proportion ranging from 0 to 100%. These were used to form films by doctor blading, and the influence of paste composition on film structure, morphology, porosity, optical properties and cell performance were investigated. Secondly, ordered meso-TiO2 films were fabricated by dip coating from aqueous or ethanol solvent. Both films had cubic mesostructures, but the film coated from aqueous solvent was not uniform. The film formed from ethanol solvent was doped with sulphur. The effects of doping on the mesostructure, morphology, structure, optical properties and photocatalytic activity were studied. The thickness of films was increased by repeated coating. The number of layers had an influence on the mesostructure, morphology, optical properties and cell performance when these films were applied in DSSCs Finally, a novel method was adopted to prepared meso-TiO2 films. Molecular titania precursors or titania colloidal seeds were used as the titania source. Both of them can be used to prepare free-standing hybrid films at air-water interface by a self-assembly method, however the one synthesised from the molecular titania precursor did not contain very much titania and became a powder after calcination. In contrast, after calcination, the films formed from the colloidal titania solution remained intact, and were composed of mixtures of TiO2 nanoparticles and nanowires with mesopores arising from interparticle porosity. These films were applied in DSSCs. This interfacial method was also successfully extended to prepare free-standing ZnO films from a molecular precursor. After calcination, the free-standing ZnO films were found to be composed of rough spheres formed by flocculation of smaller nanoparticles.

A fluorescent solar collector (FSC) is an optoelectronic waveguide device that can concentrate both diffuse and direct sunlight onto a solar cell which is then converted to electricity. Fluorescent collectors offer the potential to reduce the cost of crystalline silicon (c-Si) solar cells, but so far their effectiveness has been demonstrated only theoretically. The major problems in the device obtaining high practical efficiency are photon transport losses and material instability. This aim of this research is to increase the fundamental understanding of photon transport losses in fluorescent collectors, and to explore the method in overcoming the losses in fluorescent solar collectors. This thesis presents the theoretical and experimental results obtained during the development and characterisation of fluorescent collectors, a thin film c-Si solar cell used to detect photons from the fluorescent solar collector. A method for preparing fluorescent collector plates was by spin-coating dye-doped PMMA (Polymethylmethacrylate) on glass slides (BK7 glass). An optical characterisation technique for determining reabsorption loss of the fluorescent collectors was developed and used to evaluate the performance of the fluorescent collector base on laser dyes. The validity of this approach was verified by comparing the results with theoretical solutions, derived using a model adapted from the Weber and Lambe, and the modified Weber and Lambe, theories. Different losses in the FSC are studied in this work and we investigate the effect of surface scattering in a realistic FSC. To characterise the photon transport inside the collector, we monitor the angular distribution of a collimated light beam, which enters the collector from the edge, after propagation and total internal reflection. We find that the surface scattering process is described well by Fraunhofer diffraction at surface inhomogeneities of a size, roughly 11 μm. The major losses in FSC are found to be caused by the surface roughness, which increases the probability of the escape cone losses in FSC from the top surface. This loss can be suppressed to less than 2% using an index matching planarisation layer. The influence of the bulk and surface scattering on the performance of the FSC are presented in this work, which were compared with the experiment and theory. The angular dependence of the edge fluorescence was studied in this work, and it showed the edge fluorescence decreases by increasing the detection angle, and the value of reabsorption also decreases by increasing the detection angle, up to 20°, before increasing again. It was shown, also, that that the quasi blackbody function agrees well with angular dependence of the edge fluorescence spectral, divided by the cos(detection angle) region where the absorption and fluorescence band of the dye overlap. Also, the modified Weber and Lambe theory for the angular dependence of the fluorescence agreed well with the experimental results.

Photovoltaic (PV) technologies could provide abundant, clean and secure energy through the conversion of sunlight into electricity, but currently are too expensive to compete with conventional sources of power. Novel PV devices incorporating nanostructured materials, such as the dye-sensitized solar cell (DSC), have been identified as viable, low-cost alternatives to traditional solar cell designs. In spite of technological progress in the field over the last twenty years, the underlying physics governing DSC operation is still not well understood. In this thesis, first-principles (i.e. parameter-free) calculations are performed with the aim of connecting experimentally-measured photoemission data to the underlying atomistic and electronic structure of interfaces found in DSCs. The principal system under study is the interface between anatase titanium dioxide (TiO₂) and the "N3" dye molecule, one of the most widely-investigated device designs in DSC research. Atomistic models of the interface are determined within density-functional theory. Core-level spectra of these interface models are then calculated using a ∆SCF approach. Comparison of the calculations to published experimental data finds that intermolecular interactions have a significant effect on the spectra. Next, the electronic structure of bulk TiO₂ and of isolated N3 molecules is calculated using the GW approximation and ∆SCF method respectively. For the former, it is shown that including Hubbard U corrections in the initial Hamiltonian reduces the GW gap by 0.4 eV. These calculations are then used to determine the valence photoemission spectrum of the full interface. By including image-charge effects, thermal broadening and configurational disorder, quantitative agreement with experimentally-measured spectra is demonstrated. In addition to the N3/TiO₂ system, calculations of the core-level spectra of the interfaces between TiO₂ and H₂O and bi-isonicotinic acid are also presented. The thesis concludes with a study of the X₂Y₃/TiO₂ interfaces (X=Sb, Bi; Y=S, Se) found in recently-developed semiconductor-sensitized solar cells.

The main goal of this thesis is to investigate and develop the physics of dye-based photovoltaic physics through molecular and photonic routes. Numerous photovoltaics devices have been fabricated through the course of this work to study their characteristics, performance, physical composition and action. The relative youth of the field of dye-based optoelectronics provides extensive scope for new research which provides fascinating opportunities in terms of physical processes.This thesis is divided into two main projects; exploring the adaption of conventional dye-sensitized solar cells via starkly different routes which serendipitously culminated in striking similarities at their conclusion. The first route is through incorporating spectrally complementary dye molecules with the intention of extending the range of light absorption of the final devices. This initial aim was achieved and was then furthered by the realisation of an unexpected and unprecedented energy transfer process occurring, imparting enhanced photocurrent generation in both the near-IR and visibile region. The second route involves investigating the effect on dye-sensitized solar cell physics and performance of the inclusion of metallic nanoparticles with the expectation of inducing plasmonic interactions. Novel systems were designed and implemented, devices were made which display significant performance enhancement, attributed to plasmonic coupling into the dyes and thereby increasing photocapture. A number of other investigations are documented whose current completion does not sufficiently warrant independent chapters but whose scientific interest is evident.

Models for electron transport and back reaction in dye-sensitized nanocrystalline solar cells were investigated by developing novel measurement techniques and the results were used to test two complementary models; diffusive electron transport within the TiO2 medium and the quasi-static approximation to deal with non steady-state conditions where trapping plays a role. These will be shown to be partly correct and the shortfalls highlighted and discussed. In the end it was found that more knowledge of the parameters governing the behaviour of electrons is required to further test and develop the models. The incorporation of a secondary sensing electrode allowed the internal quasi-Fermi level (QFL) within the TiO2 to be probed. The behaviour of the voltage measured by the secondary sensing electrode was in accordance with diffusive electron transport in the TiO2. This was confirmed by measuring the QFL along the current-voltage curve of the cell, and by the temperature dependence of the measured QFL. Discrepancies concerning the behaviour of the ideality of the open-circuit voltage (and hence the electron lifetime) between experiment and modelling are highlighted and discussed throughout. Assuming an Arrhenius relationship simple expressions for the temperature dependence of the open-circuit voltage were derived and experimentally tested. The trapped electron density was measured along the current-voltage curve. With the inclusion of the secondary sensing electrode and measuring the trap distribution, the way the trapped charge varied could be modelled and compared to experiment. This provided an important link between the free and trapped electron density profiles but again highlighted shortcomings of the applied models. The quasi-static approximation was tested against a full numerical solution (continuum model) to determine the phase space in which it is applicable. Knowing this, an almost ideally behaving cell was used to test the quasi-static approximation. Having shown that it was valid for the given cell, the quasi-static approximation was used to determine how the conduction band electron lifetime varied with temperature, resulting in an Arrhenius dependence of the back reaction rate of electrons. A strong temperature dependence of the electron lifetime, and hence a strong temperature dependence of the electron diffusion length was demonstrated.

Almost 50 years of thermophotovoltaic (TPV) research from various sectors has resulted in a variety of potential applications and TPV technology options. In this work the potential of commercial TPV applications is assessed with specific reference to the UK. The assessment considers competing technologies for electricity generation, namely solar photovoltaics, external and internal heat engine generators, electro¬chemical cells and direct heat-to-electricity conversion devices. Electricity generation by TPV conversion from waste heat of industrial high-temperature processes is identified as one of the most suitable TPV applications. This market is examined in more detail using three specific high-temperature processes from the iron and steel and the glass sectors. Results are extrapolated to the entire UK high-temperature industry and include potential energy and CO2 savings. This work gathers knowledge from TPV and other literature sources and evaluates the technological options for the heat source, the radiator and the PV cell for a TPV system. The optical control in terms of the angular, spatial and in particular spectral radiation distributions in cavities is identified as a specific factor for TPV conversion and critical for a system design. The impact of simultaneous radiation suppression above and below the PV cell bandgap on an ultimate efficiency level is examined. This research focuses on fused silica (SiO2) in TPV cavities and examines the aspects of radiation guidance by total internal reflection and spectral control using coupled radiative and conductive heat transfer. Finite volume modelling and experimental work have examined the radiator-glass-air-PV cell arrangement up to a SiO2 thickness of 20 cm. Both show that the efficiency improves for an increased SiO2 thickness. Finally, the novel concept of a TPV cavity consisting of a solid dielectric medium is assessed.

Harnessing solar energy has become a promising clean and renewable energy source alternative to fossil fuels since the development of low-cost dye sensitized solar cells (DSSC) and organic photovoltaic solar cell devices. Their power-conversion efficiencies, below 13% and 9% respectively, still limit the economic viability of these technologies. The geometry and optical properties of photonic crystals can be used to improve the absorption and charge collection efficiencies of these devices. This thesis describes the fabrication of TiO2 DSSC and ZnO-polymer solar cell devices based on a three-dimensional photonic crystal structure. Photonic crystal polymer structures were produced by holographic lithography and thermally stabilized in order to be used as templates for atomic layer deposition (ALD) of various metal oxides. For this purpose, an ALD apparatus was built and ALD processes for the growth of TiO2, ZnO, Al2O3, ZnO:Al, and Zr3N4 were established and deposited on photonic crystal templates. After ALD, the template was removed by calcination at 500°C, at which ZnO:Al films lost their conductivity of 250 S/cm preventing their use as transparent conducting oxide (TCO) electrodes. The produced 90 nm TiO2 photonic crystal shell DSSC and TiO2 inverse replica devices based on the dye N-719 and iodine/iodide redox electrolyte provided power-conversion efficiencies of 0.9% and 0.49% respectively and their diffusion lengths were 2× and 3× longer than that of a nanocrystalline reference device respectively. ZnO-polymer devices, comprising a P3HT layer as absorber and PEDOT:PSS film as hole-transporter, were also investigated.

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.

This thesis describes studies on nanohybrid systems consisting of single-walled carbon nanotubes (SWNTs) with monolayer coatings of semiconducting polymers. Steady-state and time-resolved optical and high-resolution microscopy experiments were used to investigate the blends. These materials show promise for use in organic photovoltaics (OPVs) owing to the high carrier mobilities and large aspect ratios of SWNTs, the controllable solubilisation of tubes with various polymers and the broad light-harvesting abilities of organic polymers. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising carbon nanotubes in OPV devices, revealing poor performances to date. The experimental methods used during the thesis are detailed in Chapter 3 and the solution processing techniques used to prepare the polymer–nanotube blend samples are described in Chapter 4. Chapter 5 describes a study on a nanotube blend with a thiophene polymer, a system previously unsuccessfully implemented into OPV devices. Ultrafast spectroscopic measurements showed that electrons can transfer on a 400 fs time scale from the polymer to nanotubes and the conditions to allow long-lived free charges to be produced were found. The study is extended in Chapter 6 to show that nanostructures consisting of a nanotube coated in one polymer can then be coated by a second polymer and that these nano-engineered structures could be implemented into OPV devices. The use of a competition binding process to isolate purely semiconducting nanotubes dispersed with any desired polymer is then described in Chapter 7. Finally, Chapter 8 introduces systems consisting of chains of porphyrin units, nature’s light-harvesting systems, bound to nanotubes and the blends were found to exhibit the required electronic alignment for use in OPVs. The work described in this thesis provides an explanation for the poor device behaviour of nanotube–polymer blends to date and, in particular, demonstrates several nanohybrid systems that show particular promise for improved OPV applications.