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This thesis is concerned with the influence of nanoscale boundaries and interfaces upon the electronic processes that occur within the inorganic semiconductors. Inorganic semiconductor nanowires and their blends with semiconducting polymers have been investigated using state-of-the-art ultrafast optical techniques to provide information on the sub-picosecond to nanosecond photoexcitation dynamics in these systems. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising nanowires in hybrid organic photovoltaic devices, revealing the performances to date. The experimental methods used during the thesis are detailed in Chapter 3. Chapter 4 describes the crucial roles of surface passivation on the ultrafast dynamics of exciton formation in gallium arsenide (GaAs) nanowires. By passivating the surface states of nanowires, exciton formation via the bimolecular conversion of electron-hole plasma can observed over few hundred picoseconds, in-contrast to the fast carrier trapping in 10 ps observed in the uncoated nanowires. Chapter 5 presents a novel method to passivate the surface-states of GaAs nanowires using semiconducting polymer. The carrier lifetime in the nanowires can be strongly enhanced when the ionization potential of the overcoated semiconducting polymer is smaller than the work function of the nanowires and the surface native oxide layers of nanowires are removed. Finally, Chapter 6 shows that the carrier cooling in the type-II wurtzite-zincblend InP nanowires is reduced by order-of magnitude during the spatial charge-transfer across the type-II heterojunction. The works decribed in this thesis reveals the crucial role of surface-states and bulk defects on the carrier dynamics of semiconductor nanowires. In-addition, a novel approach to passivate the surface defect states of nanowires using semiconducting polymers was developed.

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 improves our understanding of the charge-transfer dynamics in organic materials employed in dye-sensitized and nanotube-thiophene solar cells. For the purpose of this work, a femtosecond transient absorption spectroscopy setup was built. Additionally, microsecond transient absorption spectroscopy was utilised to explore dynamics on a longer time-scale. In the first study, the dependence of dye regeneration and charge collection on the pore- filling fraction (PFF) in solid-state dye-sensitized solar cells (DSSCs) is investigated. It is shown that while complete hole transfer with PFFs as low as ~30% can be achieved, improvements beyond this PFF are assigned to a stepwise increase in the charge-collection efficiency in agreement with percolation theory. It is further predicted that the chargecollection efficiency saturates at a PFF of ~82%. The study is followed by an investigation of three novel hole-transporting materials for DSSCs with slightly varying HOMO levels to systematically explore the possibility of reducing the loss-in-potential and thus improving the device efficiency. It is shown that despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. Furthermore, it is demonstrated that the design of the HTM has an additional impact on the electronic density of states present at the TiO2 electrode surface, and hence influences not only hole- but also electron-transfer from the sensitizer. Finally, a study on a polymer-single-walled carbon nanotube (SWNT) molecular junction is presented. Results from femtosecond spectroscopic techniques show that the polymer poly(3-hexylthiophene) (P3HT) is able to transfer charges to the SWNT within 430 fs. Addition of excess P3HT polymer leads to long-lived free charges making these materials a viable option for solar cells.

This thesis is concerned with ultrafast photophysics and excitation energy transfer in biomimetic and biosensing molecular nanomaterials, which have been investigated using timeresolved photoluminescence spectroscopy. Artificially-synthesised fully pi-conjugated nanorings with 6-24 units of porphyrins incorporated have demonstrated remarkable full delocalisation of the absorbing excited state across the entire ring on an ultrafast timescale. The largest ring of 24 porphyrin units with a diameter of ~10 nm is the largest man-made molecular nanoring at present. The constraint that the lowest electronic transition is forbidden due to their high molecular symmetries is released by static and dynamic distortions in the solution. These findings provide promising opportunities for using these man-made light-harvesting materials that match the efficiency of their natural counterparts in technological applications. On the other hand, self-assembled organic fluorescent nanoparticles (NPs) formed of amphiphilic monomers with targeting ligands exhibit potential to be used in bio-sensing applications. Homotransfer and heterotransfer of excitation energy in nanoparticles have been investigated, where mannose covalently linked on Amp1-B monomers provide binding sites for Concanavalin A which is attached with energy acceptors. Increasing the molar ratios of Amp1-B-mannose and Amp3-B can increase the binding sites and forming directional energetic cascades towards the NP surface, respectively. However, the amount of these two monomers must be optimised in order to reach the highest donor-acceptor energy transfer rate in the system.

This thesis is concerned with energy transport processes in a series of low-bandgap copolymers, solution processable hybrid organic-inorganic perovskite materials, and donor-acceptor triad dyes which are used in photovoltaic cells and solar concentrator devices. These processes are investigated using time-resolved photoluminescence (PL) spectroscopy techniques which allow the investigation of transport processes with sub-ps time resolution. Two donor-acceptor-donor triad dyes composed of perylene bisimide units are compared, and rapid energy transfer (< 2 ps) from the donor to a central bay-substituted PBI unit is observed in both molecules. For the linear molecule, in which the transition dipole moments corresponding to the lowest singly excited state are all aligned along the molecule long-axis, such rapid energy transfer is shown to be consistent with predictions of a modified Förster model which takes into account both the delocalisation of the excitation and the short donor-acceptor separation. When the dipole moments of the donor units are perpendicular to that of the central acceptor however, this model is found to strongly underestimate the energy transfer. The energy transfer is found to arise due to a combination of both through-space (Förster type) and through-bond energy transfer, the latter of which is mediated by molecular torsions which break orthogonality and enable conjugation between the two units. This rapid energy transfer also coincides with either retention or rotation of polarisation between absorption and emission. These dyes are therefore shown to be promising candidates for use in luminescent solar concentrators (LSC), as rapid intramolecular energy transfer and control of the emission polarisation are two features which can help to reduce self-absorption and escape losses in LSC devices. The effect of chemical structure on the morphology, energy transport properties and overall photovoltaic device efficiency is determined for a series of low-bandgap polymers comprising benzodithiophene donor and benzothiadiazole acceptor units. Photovoltaic devices incorporating polymer:fullerene blends are found to yield devices with power conversion efficiencies of up to 6%, with the highest PCE observed in devices which form films exhibiting a very low degree of crystallinity in X-ray diffaction patterns and a corresponding low surface roughness in thin films. The influence of crystallite formation on energy transport is probed by time resolved PL quenching of polymer films on a TiO2 quenching layer. Exciton diffusion lengths in these films are standard for low-bandgap polymers, ranging from 4 to 7.5 nm. The diffusion length is found to be higher in films with a higher degree of crystallinity, however direct PL quenching measurements on polymer:PCBM films show however that the vast majority of generated excitons are found to reach an interface and dissociate within 1 ps, showing that exciton diffusion does not present a bottle-neck for device efficiencies. From these observations it is concluded that the boundaries between crystalline and amorphous domains may impede charge extraction at the charge densities found during photovoltaic operation. Finally, the distance over which electron-hole pairs can diffuse before decay or trapping is investigated in two organic-inorganic hybrid perovskite structures (CH3NH3PbI3-xClx) by monitoring the rate and degree of PL quenching in the presence of either an electron or hole acceptor material. The diffusion lengths observed in these materials are on the order of 100 nm for the triiodide perovskite (x = 0), and over 1 μm in the mixed halide material (x > 0). These diffusion lengths are extremely long compared with those observed in other solution processable materials and explain the very high power conversion efficiencies that have been reported in photovoltaic cells containing these and similar perovskite materials. The longer diffusion lengths in particular correlate with good power conversion efficiency when a planar device geometry is used. Similar cells using the triiodide material however show poor efficiencies, attributed to the smaller diffusion length in this material. Application of the PL quenching technique to determine diffusion lengths is therefore shown to be a useful and simple method by which the suitability of a given perovskite material for use in a planar PV cell can be determined.

This thesis is concerned with the influence of nanoscale boundaries and interfaces upon the electronic processes that occur within both organic and inorganic semiconductors. Photoluminescent polymers, highly conducting polymers and nanoscale inorganic semiconductors have been investigated using state-of-the-art ultrafast optical techniques, to provide information on the sub-picosecond photoexcitation dynamics in these systems. The influence of dimensionality on the excitation transfer dynamics in a conjugated polymer blend is studied. Using time-resolved photoluminescence spectroscopy, the transfer transients both for a three-dimensional blend film, and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of a SnS2 matrix have been measured. A comparison of the experimental data with a simple, dimensionality-dependent model is presented, based on point dipole electronic coupling between electronic transition moments. Within this approximation, the energy transfer dynamics are found to adopt a three-dimensional character in the solid film, and a two-dimensional nature in the monolayers present in the SnS2 -polymer nanocomposite. The time-resolved conductivity of isolated GaAs nanowires has been investigated by optical-pump terahertz-probe time-domain spectroscopy. The electronic response exhibits a pronounced surface plasmon mode that forms within 300 fs, before decaying within 10 ps as a result of charge trapping at the nanowire surface. The mobility has been extracted using the Drude model for a plasmon and is found to be remarkably high, being roughly one third of that typical for bulk GaAs at room-temperature and indicating the high quality and low bulk defect density in the nanowires studied. Finally, the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers, P3HT and MDMO-PPV, blended with PCBM are presented. The observed terahertz-frequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the π−π* absorption peak (560 nm for P3HT), and significantly below it (800 nm). The photoconductivity at 800 nm is unexpectedly high, which is attributed to the presence of a charge transfer complex. In addition, the excitation-fluence dependence of the photoconductivity is studied over more than four orders of magnitude. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that long-lived positive polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times (~ ps) the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least ~0.1cm2 V−1 s−1. At later times, a sub-linear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer timescale (> 1 μs).