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The exponential growth of the internet mandates the development of energy-efficient "green" photonics technologies. Currently, tele- and data-com laser technology is centred on quantum well (QW) lasers grown on InP. Despite their widespread deployment, and the improvements in efficiency enabled by QW band structure engineering, strong non-radiative losses, and inter-valence band absorption (IVBA) in InP-based 1.3-1.55 µm QW lasers remains a persistent issue. At room temperature, up to 80% of the threshold current density is due to Auger recombination (AR). The strong temperature sensitivity of AR creates a threshold current density with strong temperature dependence. These properties mandate temperature control via thermoelectric coolers to maintain operational stability, thereby increasing power consumption. There is a long-standing goal to develop 1.3-1.55 µm semiconductor lasers in which AR and IVBA are suppressed, creating energy-efficient, temperature-stable lasers. Significant research has centred on extending the wavelength range accessible to GaAs-based lasers towards the 1.3-1.55 µm tele- and data-com windows. GaAs-based QW lasers incorporating strained InyGa1−yAs QWs are already well established at wavelengths close to 1 µm. Extending the wavelength range associated with GaAs-based lasers to 1.55 µm offers significant advantages over incumbent InP-based technologies, including enhanced band offsets and refractive index contrast for improved carrier and optical confinement, the potential to exploit the benefits associated with vertical-cavity architectures, and potential integration with GaAs-based microelectronics. Critical thickness limitations preclude the growth of strained InyGa1−yAs QWs and thus unconventional material systems must be considered to target long-wavelength GaAs-based QW lasers. These unconventional material systems include QWs formed using highly-mismatched dilute nitride or bismide alloys with type-I band offsets, metamorphic QWs based on lattice-mismatched AlxInyGa1−x−yAs QWs with type-I band offsets grown on relaxed InzGa1−zAs buffer layers, and "W" QWs with type-II band offsets and based on conventional InyGa1−yAs/GaAs1−xSbx or highly-mismatched GaAs1−xBix/GaNyAs1−y alloys.

This thesis studies quaternary chalcogenide nanocrystals and their photovoltaic applications. A temperature-dependent phase change between two distinct crystallographic phases of stoichiometric Cu2ZnSnS4 is investigated through the development of a one pot synthesis method. Characterisation of the Cu2ZnSnS4 nanocrystals was performed using absorption spectroscopy, transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). An investigation was conducted into the effects of using hexamethyldisilathiane (a volatile sulphur precursor) in the nucleation of small (<7nm), mono-dispersed and solution-stable quaternary Cu2ZnSnS4 nanocrystals. A strategy to synthesize high quality thermodynamically stable kesterite Cu2ZnSnS4 nanocrystals is established, which subsequently enabled the systematic study of Cu2ZnSnS4 nanocrystal formation mechanisms, using optical characterization, XRD, TEM and Raman spectroscopy. Further studies employed scanning transmission electron microscopy (STEM) energy dispersive x-ray (EDX) mapping to examine the elemental spatial distributions of Cu2ZnSnS4 nanocrystals, in order to analyse their compositional uniformity. In addition, the stability of nanocrystals synthesised using alternative ligands is investigated using Fourier transform infrared spectroscopy, without solution based ligand substitution protocol is used to replace aliphatic reaction ligands with short, aromatic pyridine ligands in order to further improve Cu2ZnSnS4 colloid stability. A layer-by-layer spin coating method is developed to fabricate a semiconductor heterojunction, using CdS as an n-type window, which is utilised to investigate the photovoltaic properties of Cu2ZnSnS4 nanocrystals. Finally, three novel passivation techniques are investigated, in order to optimise the optoelectronic properties of the solar cells to the point where a power conversion efficiency (PCE) of 1.00±0.04% is achieved. Although seemingly modest when compared to the performance of leading devices (PCE>12%) this represents one of the highest obtained for a Cu2ZnSnS4 nanocrystal solar cell, fabricated completely under ambient conditions at low temperatures.

Topological insulators (TIs) are recently predicted, and much studied, new quantum materials. These materials are characterised by their unique surface electronic properties; namely, behaving as band insulators within their bulk, but with spin-momentum locked surface or edge states at their interface. These surface/edge crossing states are protected by the underlying time-reversal symmetry (TRS) of the bulk band structure, leading to a robust topological surface state (TSS) that is resistant to scattering from impurities which do not break TRS. Their surface band dispersion has a characteristic crossing at time reversal invariant momenta (TRIM) called a Dirac cone. It has been predicted that the introduction of a TRS breaking effect, through ferromagnetic order for instance, will open a band-gap in this Dirac cone. It can be seen that magnetic fields are not time reversal invariant by considering a solenoid. If time is reversed, the current will also reverse in the solenoid and so the magnetic field will also be reversed. So it can be seen that magnetic fields transform as odd under time reversal, the same will be true of internal magnetisation. By manipulating this gapped surface state a wide range of new physical phenomena are predicted, or in some cases, already experimentally observed. Of particular interest is the recently observed quantum anomalous Hall effect (QAHE) as well as, e.g., topological magneto-electric effect, surface Majorana Fermions and image magnetic monopoles. Building on these novel physical effects, it is hoped to open new pathways and device applications within the emerging fields of spintronics and quantum computation. This thesis presents an investigation of the nature of magnetic doping of the chalcogenide TIs Bi2Se3 and Bi2Te3 using 3d transition-metal dopants (Mn and Cr). Samples were grown by molecular beam epitaxy (MBE), an ideal growth method for the creation of high-quality thin film TI samples with very low defect densities. The grown films were investigated using a range of complementary lab-based and synchrotron-based techniques to fully resolve their physical structure, as well as their magnetic and electronic properties. The ultimate aim being to form a ferromagnetic ground state in the insulating material, which may be expanded into device applications. Samples of bulk Mn-doped Bi2Te3 are presented and it is shown that a ferromagnetic ground state is formed below a measured TC of 9-13 K as determined by a range of experimental methodologies. These samples are found to have significant inhomogeneities within the crystal, a problem that is reduced in MBE-grown crystals. Mn-doped Bi2Se3 thin films were grown by MBE and their magnetic properties investigated by superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD). These reveal a saturation magnetisation of 5.1 μB/Mn and show the formation of short-range magnetic order at 2.5 K (from XMCD) with indication of a ferromagnetic ground state forming below 1.5 K. Thin films of Cr-doped Bi2Se3 were grown by MBE, driven by the recent observation of the QAHE in Cr-doped (Bi1−xSbx)2Te3. Investigation by SQUID shows a ferromagnetic ground state below 8.5 K with a saturation magnetisation of 2.1 μB/Cr. Polarised neutron reflectometry shows a uniform magnetisation profile with no indication of surface enhancement or of a magnetic dead layer. Further studies by extended x-ray absorption fine structure (EXAFS) and XMCD elucidate the electronic nature of the magnetic ground state of these materials. It is found that hybridisation between the Cr d and Se p orbitals leads to the Cr being divalent when doping on the Bi3&plus; site. This covalent character to the electronic structure runs counter to the previously held belief that divalent Cr would originate from Cr clusters within the van der Waals gap of this material. The work overall demonstrates the formation of a ferromagnetic ground state for both Cr and Mn doped material. The transition temperature, below which ferromagnetic order is achieved, is currently too low for usable device applications. However, these materials provide a promising test bed for new physics and prototype devices.

Electron beam induced current (EBIC) and atom probe tomography (APT) were used in this study to determine electrical activities and impurity compositions at extended defects in multicrystalline silicon (mc-Si) samples. The results provide, for the first time, information regarding the chemical species present at defects whose electrical activity has previously been measured. A new APT specimen fabrication process was developed with the ability to select a specific defect for APT analysis. Development of the APT specimen fabrication process proceeded by first selecting and optimising the preferential etching for nano-scale defect delineation. Three etchants were evaluated, namely Secco, Sirtl and Dash, from which the Secco etch was selected. Three parameters were optimised to produce etch pits with geometries that meet the requirements imposed by APT specimen fabrication methods. The optimum parameters were 0.05M potassium dichromate concentration, 20°C etch temperature, and 30sec etch time. In the second stage, marking techniques were developed in order for the defects to be located throughout the APT specimen fabrication process. However, it became apparent that the conventional APT specimen fabrication method could not be used to fabricate APT specimens containing selected defects in a mc-Si sample. This led to the development of a novel APT specimen fabrication approach which allowed APT specimens to be fabricated, reproducibly, containing grain boundaries and isolated dislocations. In order to evaluate accurately iron contamination in mc-Si, four atom probe parameters were optimised to maximise detection sensitivity: the evaporation rate, the laser beam energy, the pulse repetition rate and the specimen temperature. The optimisation process can be divided in to two parts. In the first part, a matrix of pre-sharpened single-crystal silicon specimens was subjected to a variety of experimental parameters. The optimised parameters were determined to be 0.3% evaporation rate, 0.5nJ beam energy, 160kHz repetition rate and 55K specimen temperature. The second part was to determine the iron detection efficiency –the percentage of detected Fe ions that can be correctly identified as Fe– and sensitivity using these parameters to analyse a specially prepared iron calibration specimen. The values were determined to be a detection efficiency of about 35% and sensitivity of 54ppm or 2.70x1018 atom/cm3. The APT specimen fabrication process and the optimised APT analysis parameters were used to analyse four extended defects in mc-Si samples subjected to three different processing conditions, namely gold-contaminated, as-grown and phosphorus diffusion gettering (PDG). The important aspects of the analysis are listed below: • Gold was not detected at the grain boundary and its associated dislocations in the gold-contaminated specimen. The binding enthalpy of gold to such defects is thus less than 0.63eV. • Iron was not detected in any specimen. • Copper was observed at the grain boundary in the as-grown specimen in the form of individual atoms as well as clusters with diameters ranging between 4nm and 9nm. The electrical activity of the grain boundary was about 58%. • Nickel and carbon were detected at the grain boundary in the post-PDG specimen with the former having platelet structures with diameters and thicknesses ranging between 4nm-7nm and 2nm-4nm, respectively. The recombination strength of the defect was about 22%. • Two nickel clusters were found at the isolated dislocation in the post-PDG specimen. The clusters were spherical with an average diameter of 10nm. The distance between the two clusters was 35nm. The recombination strength of the defect was about 4%.

This thesis studies lead suphide (PbS) colloidal quantum dots and their photovoltaic applications. Different sizes of PbS QDs were synthesised and characterised using absorption spectroscopy and transmission electron microscopes. PbS QD Schottky junction devices were fabricated with AM1.5 power conversion efficiency up to 1.8 %. The Schottky junction geometry limits the device performance. A semiconductor heterojunction using ZnO as an electron acceptor was built and the device efficiency increased to 3%. By studying the light absorption and charge extraction profile of the bilayer device, the absorber layer has a charge extraction dead zone which is beyond the reach of the built-in electric field. Therefore, strategies to create a QD bulk heterojunction were considered to address this issue by distributing the junction interface throughout the absorber layer. However, the charge separation mechanism of the QD heterojunction is not clearly understood: whether it operates as an excitonic or a depleted p-n junction, as the junction operating mechanism determines the scale of phase separation in the bulk morphology. This study shows a transitional behaviour of the PbS/ZnO heterojunction from excitonic to depletion by increasing the doping density of ZnO. To utilise the excitonic mechanism, a PbS/ZnO nanocrystal bulk heterojunction was created by blending the two nanocrystals in solution such that a large interface between the two materials could facilitate fast exciton dissociation. However, the devices show poor performance due to a coarse morphology and formation of germinate pairs. To create a bulk heterojunction where a built-in electric field could assist the charge separation, a TiO2 porous structure with the pore size matching with the depletion width was fabricated and successfully in-filled by PbS QDs. The porous device produces 5.7% power conversion efficiency, among one of the highest in literature. The enhancement comes from increased light absorption and suppression of charge recombination.

This thesis studies parameters influencing both the performance and stability of solid state sensitized solar cells (ssSSCs). ssSSCs benefit from their low materials and manufacturing processing costs, a consequence of using solution processed materials. However, solution processed materials are often structurally and electronically disordered. By characterizing fully operational ssSSCs and their charge transport properties, this thesis elucidates the factors limiting charge transport and proposes routes towards both improved photovoltaic conversion efficiency and long-term stability. Chapter 2 provides an explanation of the operation of ssSSCs, while Chapter 3 discusses the basic methods used in this thesis. Having set this background, Chapter 4 explores the interaction between atmospheric oxygen and charge doping mechanisms in the organic semiconductors used in ssSSCs. To understand the implications of the findings presented in Chapter 4, a new technique, “transient mobility spectroscopy”, was developed to understand the evolution of balanced charge transport behaviour of disordered semiconductors at different operating conditions in ssSSCs. This technique is presented in full in Chapter 5. The understanding gained in Chapters 4 and 5 suggest that alternative light absorbers with higher extinction coefficients may be beneficial to improving the performance of ssSSCs. Chapter 6 discusses the use of an organometal trihalide perovskite, as light absorber in ssSSCs. Using time resolved techniques, the charge transport and recombination mechanisms in various device architectures are explored, allowing suggestions to be made towards future improvements. Chapter 7 uses the technique presented in Chapter 5 to understand a rapid degradation mechanism of working ssSSCs. Particular focus is placed on the titanium dioxide charge-transporting layer. Building on this newfound understanding, two methods for attaining stable photovoltaic performance are provided, a great step forward for this technology.

The aim of this thesis is to develop a deeper understanding and the technology in the nascent field of solid-state organic-inorganic perovskite solar cells. In recent years, perovskite materials have emerged as a low-cost, thin-film technology with efficiencies exceeding 16% challenging the quasi-paradigm that high efficiency photovoltaics must come at high costs. This thesis investigates perovskite solar cells in more detail with a focus on incorporating plasmonic nanostructures and perovskite film formation. Chapter 1 motivates the present work further followed by Chapter 2 which offers a brief background for solar cell fabrication and characterisation, perovskites in general, perovskite solar cells in specific, and plasmonics. Chapter 3 presents the field of plasmonics including simulation methods for various core-shell nanostructures such as gold-silica and silver-titania nanoparticles. The following Chapters 4 and 5 analyze plasmonic core-shell metal-dielectric nanoparticles embedded in perovskite solar cells. It is shown that using gold@silica or silver@titania NPs results in enhanced photocurrent and thus increased efficiency. After photoluminescence studies, this effect was attributed to an unexpected phenomenon in solar cells in which a lowered exciton binding energy generates a higher fraction of free charge. Embedding thermally unstable silver NPs required a low-temperature fabrication method which would not melt the Ag NPs. This work offers a new general direction for temperature sensitive elements. In Chapters 6 and 7, perovskite film formation is studied. Chapter 6 shows the existence of a previously unknown crystalline precursor state and an improved surface coverage by introducing a ramped annealing procedure. Based on this, Chapter 7 investigates different perovskite annealing protocols. The main finding was that an additional 130°C flash annealing step changed the film crystallinity dramatically and yielded a higher orientation of the perovskite crystals. The according solar cells showed an increased photocurrent attributed to a decrease in charge carrier recombination at the grain boundaries. Chapter 8 presents on-going work showing noteworthy first results for silica scaffolds, and layered, 2D perovskite structures for application in solar cells.

A process has been developed to make the dielectric layer for organic thin-film transistors (OTFTs) in a roll-to-roll vacuum web coater environment. This dielectric layer combined with an organic semiconductor layer and metal layer deposited in vacuum allows a solvent-free process to make organic/inorganic multilayer structures for thin-film electronic devices on a flexible substrate at, potentially, high speed. The polymeric gate dielectric layers were fabricated by flash evaporation of acrylic monomers onto a polymer film with pre-patterned metal gates followed by radiation curing by electron beam, ultra-violent light (UV) or plasma. With a non-polar dielectric surface, charge carrier mobility (μ) of 1 cm2-V-1s-1; on/off curren ratio of 108, sub-threshold swing (SS) of 0.3 V/decade and saturated output curve were routinely achieved in dinaphtho-[2,3-b:2'3'-f]thieno[3,2-b]thiophene (DNTT) transistors with dielectric layer of tripropylene glycol diacrylate (TPGDA) of ~400 nm. Apart from the TPGDA, monomer formulas including 1,6-Hexanediol diacrylate (HDDA) as well as several commercial acrylic resins have been used to make the dielectric layer. The highest areal capacitance of 41nF-cm-2 was achieved with a pin-hole free film of less than 100 nm made of an acrylate mixture resin. A non-polar dielectric surface treatment layer has been developed based on flash evaporation of lauryl acrylate and HDDA mixture. The transistors with the buffer layer showed constant performance and a mobility fivefold greater than those of untreated samples. The effect of humidity, oxygen, and light during switching cycles of both pentacene and DNTT transistors were studied. Water and oxygen/illumination had a distinct effect on both pentacene and DNTT transistors. Oxygen leads to acceptor-like charge traps under illumination, which shifted the turn-on voltage (Vto) to more positive values. In contrast, water in transistors gave rise to donor-like charge traps, which shifted the Vto and the threshold voltage (VT) more negatively. The DNTT devices showed good stability in dry air without encapsulation, while pentacene transistors degraded with either repeating measurement or long term storage. A DNTT transistor with a PS-coated TPGDA dielectric layer showed stable drain current (Id) of ~105A under bias stress of the gate voltage (em>Vg) of -20V and the drain voltage (em>Vd) of -20V for at least 144 hours. The Vto shift after the stress was less than 5 V and was recoverable when the device was kept in dry air for a few days. Possible reasons for the Vto shift have been discussed.

Experimental investigations of the optical properties of GaN nanostructured light emitting diode (LED) arrays are presented. Microphotoluminescence spectroscopy with pulsed and continuous wave lasers was used to probe the carrier dynamics and emission mechanisms of nanorod LED arrays fabricated by a top down etching method. Results show a possible reduction in internal electric field as nanorod diameter decreases. Localisation effects were also observed, affecting the spectral shape of the nanorod emission. Under two-photon excitation, quantum dot-like sharp spectral peaks in the PL spectra are found to exist in abundance amongst all the nanorod samples. The optical properties of these localised states, which are shown to be associated with the nanorod free-surfaces, are characterised using non-linear and time resolved spectroscopy. An investigation into spatially resolved single nanorods was also carried out. Single nanorods were isolated, and characterised using pulsed lasers. The etching is shown to increase the carrier decay life-time at extended intervals over several hundred ns. The temporal evolution and excitation power density dependence of the quantum dot-like states are also presented for the first time. The long lived localised states are thought to arise from surface effects, in particular Fermi-surface pinning, causing localisation and spatial separation of carriers. Additional work on nano-pyramid array LEDs, with quantum wells on semi-polar surfaces is also presented. Optical properties using micro-photoluminescence are compared to cathodoluminescence studies. An uneven distribution of emission wavelengths across the pyramid facet is thought to lead to an emission mechanism involving carriers transferring between multiple spatially localised states. Finally, experimental techniques and fabrication methods for future work are documented in detail.

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 &GT; 3&percnt;, 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, CH3NH3PbI3-xClx 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.

Hybrid organic-inorganic solar photovoltaic (PV) cells capable of directly converting sunlight to electricity have attracted much attention in recent years. Despite evident technological advancements in the PV industry, the widespread commercialisation of solar cells is still being mired by their low conversion efficiencies and high cost per Watt. Perovskites are an emerging class of semiconductors providing a low-cost alternative to silicon-based photovoltaic cells, which currently dominate the market. This thesis develops a series of studies on “all-solid state perovskite solar cells” fabricated via vapour deposition which is an industrially-accessible technique, to achieve planar heterojunction architectures and efficient PV devices. Chapter 2 presents a general outlook on the operating principles of solar cells, delving deeper into the specific operational mechanism of perovskite solar cells. It also explores the usual methods employed in the fabrication of perovskite thin films. Chapter 3 describes the experimental procedures followed during the fabrication of the individual components constituting the device from the synthesis of the precursors to the construction of the functioning perovskite PV devices. Chapter 4 demonstrates pioneering work involving the dual-source vapour deposition (DSVD) of planar heterojunction perovskite solar cells which generated remarkable power conversion efficiency values surpassing 15%. These significant results pave the way for the mass-production of perovskite PVs. To further expand the range of feasible vapour deposition techniques, a two-layer sequential vapour deposition (SVD) technique is explored in Chapter 5. This chapter focusses on identifying the factors affecting the fundamental properties of the vapour-deposited films. Findings provide an improved understanding of the effects of precursor compositions and annealing conditions on the films. Chapter 5 concludes with a comparison between SVD and DSVD fabricated films, highlighting the benefits of each vapour deposition technique. Furthermore, hysteretic effects are analysed in Chapter 6 for the perovskite PV devices fabricated based on different structural configurations. An interesting discovery involving the temporary functioning of compact layer-free perovskite PV devices suggests the presence of a built-in-field responsible for the hysteresis of the cells. The observations made in this chapter yield a new understanding of the functionality of individual cell layers. Combining the advantages of the optimum vapour deposition technique established in Chapter 4 and Chapter 5, with the enhanced understanding of perovskite PV cell operational mechanism acquired from Chapter 6, an ongoing study on an “all-perovskite” tandem solar cell is introduced in Chapter 7. This demonstration of the “all-perovskite” tandem devices confirms the versatility of perovskites for a broader range of PV applications.

In this thesis, two-dimensional materials such as graphene are tested for their suitability for opto-electronic applications using terahertz time domain spectroscopy (THz-TDS). This ultrafast all-optical technique can probe the response of novel materials to photoexcitation, and yield information about the dynamics of the material systems. Graphene grown by chemical vapour deposition (CVD) is studied using optical-pump THz-probe time domain spectroscopy in a variety of gaseous environments in Chapter 4. The photoconductivity response of graphene grown by CVD is found to vary dramatically depending on which atmospheric gases are present. Adsorption of these gases can open a local bandgap in the material, allowing stimulated emission of THz radiation across the gap. Semiconducting equivalents to graphene, molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), grown by CVD, are investigated in Chapter 5. These members of the transition metal dichalcogenide family show sub-picosecond responses to photoexcitation, suggesting promise for use in high-speed THz devices. In Chapter 6, an alternative production route to CVD is studied. Liquid-phase exfoliation offers fast, easy production of few-layer materials. THz spectroscopy reveals that the dynamics of these materials after photoexcitation are remarkably similar to those in CVD-grown materials, offering the potential of cheaper materials for future devices. Finally in Chapter 7, it is shown that carbon nanotubes can be used to make ultrafast THz devices. Unaligned, semiconducting single walled carbon nanotubes can be photoexcited to produce an ultrafast, dynamic THz polariser. The work in this thesis demonstrates the potential for these novel materials in future opto-electronic applications. THz spectroscopy is shown to be an important tool for the characterisation of new materials, providing information that can be used to understand the dynamics of materials, and improve production methods.

Materials have always had a large impact on society over the different ages. Piezoelectric materials are the often ‘invisible’ materials which find widespread use, unknown to the general public by large. Mobile electronics, automotive systems, medical and industrial systems are few of the key areas where ‘piezoelectricity’ is indispensable. The parking sensor of our car uses the effect and even the echo to image an unborn baby in a womb requires the exploitation of the piezoelectric effect. The work presented in this thesis investigates the piezoelectric effect in semiconductors, namely in III V, III N and II VI materials to have a better understanding and design potential applications in light emitting diodes (LEDs) and other electronic devices. The current work focuses on the non-linear behaviour in the strain of the piezo effect, which is manifested by the generation of electric field under crystal deformation. Previous works have already confirmed the reports of the existence of non-linear piezoelectric effects in zincblende III V semiconductors. Here, the same semiempirical approach using Density Functional Theory has been utilized to investigate the strain dependent elastic and dielectric properties of wurtzite III N materials. While we report the strong non-linear strain induced piezoelectric behaviour with second order coefficients, all spontaneous polarization terms are substantially smaller than the previously proposed values. We show that, unlike existing models, our calculated piezoelectric coefficients and nonlinear model provide a close match to the internal piezoelectric fields of quantum well and superlattice structures. Also, pressure dependence of the piezoelectric field in InGaN based LEDs predicts a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. The LED devices using the proposed structures including a metamorphic layer under the active region of the device are expected to increase their light output power by up to 10%. We also explored the impact of the non-linear piezo effect in nanowires and present a further theoretical computational study of single photon sources optimization in InGaN based wurtzite single quantum dots. We observed the light emission can be made by those single photon sources covering the entire visible spectrum through suitable change in the alloy composition.

This thesis investigates novel hole extraction interfaces and series interconnects for applications in organic photovoltaics, specifically in single junction solid-state dye-sensitized solar cells (DSSCs) and tandem DSSC/polymer bulk heterojunction solar cells. Improvements in hole extraction and device performance by using materials compatible with scalable deposition methods are presented, including tungsten- and molybdenum-disulphide (WS2 and MoS2), and p-type doped spiro-OMeTAD (2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene) nanoparticle dispersions. WS2 and MoS2 hole extraction layers increase averaged short circuit currents by 20% and 16% respectively, and power conversion efficiencies by 19% and 14% respectively when compared with control devices. Similarly, doped spiro-OMeTAD nano-particle layers improved short circuit current densities by 32% and efficiencies by 9%. Tandem device interconnects using these novel hole extraction formats have been fabricated, but although devices did exhibit rectification, overall performance was poor. Possible reasons for their limited success have been analysed. Dye-sensitized solar mini-modules are also reported. In order to assure the scalability of DSSC technology, these larger area devices were constructed using doctor blade coating to deposit the hole transporter material. As well as achieving a respectable maximum power conversion efficiency of 2.6%, it has also been shown that the extent to which hole transporter infiltrates the mesoporous photoanode of these devices may be tuned by altering substrate temperature during deposition. It was found that an optimal coating temperature of 70 degrees C produced the best efficiency, with a corresponding pore-filling fraction of 41%.

A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (VOC) and decrease in short circuit current (Jsc) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in VOC is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in Jsc primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in Jsc, due primarily to poor overall charge transport in the blend. VOC increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C70 solar cells. Due to graphene’s high transparency, graphene devices have enhanced Jsc, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. VOC is raised by using MoO3 as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C70 layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C70.

This thesis details the development of a novel photovoltaic device based on organometal halide perovskites. The initial focus of this thesis begins with the study of lighttrapping strategies in solid-state dye-sensitised solar cells (detailed in chapter 3). While I report enhancement in device performance through the application of near and far-field light-trapping techniques, I find that improvements remain step-wise due to fundamental limitations currently employed in dye-sensitised solar cell technology— notably, the available light-sensitising materials. I found a promising yet under researched family of materials in the methyl ammonium tri-halide plumbate perovskite (detailed in chapter 4). The perovskite light-sensitiser was applied to the traditional mesoscopic sensitised solar cell device architecture as a replacement to conventional dye yielding world-record breaking photo-conversion e!ciencies for solid-state sensitised solar cells as high as 8.5%. The system was further developed leading to the conception of a novel device architecture, termed the mesoporous superstructured solar cell (MSSC), this new architecture replaces the conventional mesoporous titanium dioxide semiconductor with a porous insulating oxide in aluminium oxide, resulting in very low fundamental losses evidenced through high photo-generated open-circuit voltages of over 1.1 V. This development has delivered striking photo-conversion ef- ficiencies of 10.9% (detailed in chapter 6).

Electrically detected magnetic resonance (EDMR) is a sensitive spectroscopic technique, which can be used to readout few to single electron spins in semiconductor and carbon nanodevices for applications in solid state quantum information processing (QIP). Since only electrically active defects contribute to the EDMR signal, this technique can be used further to investigate defects and impurities in photovoltaic devices, in which they limit the sunlight-to-energy conversion efficiency significantly. Here, I employ X-band EDMR for semiconductor defect analysis and identify the most important recombination centres in Czochralski silicon with oxide precipitates, which can be intentionally grown to confine detrimental metallic impurities to inactive regions of the wafer in order to serve as a defect-free substrate for modern silicon photovoltaic devices. Those experiments show that oxide precipitation is accompanied by the formation of silicon dangling bonds. Furthermore, I describe a very promising route towards the fabrication and readout of few to single electron spins in carbon nanotube devices, which can be characterised structurally via transmission electron microscopy in order to relate their electrical and spin properties with their structure. Finally, I employ EDMR to read out electron spin states in donor-doped silicon field-effect transistors as a prerequisite for their application in QIP. I report on a novel cryogenic probe head for EDMR experiments in resonant microwave cavities operating at 0.35 T (9.7 GHz, X-band) and 3.34 T (94 GHz, W-band). This approach overcomes the inherent limitations of conventional X-band EDMR and permits the investigation of paramagnetic states with a higher spectroscopic resolution and signal intensity. Both advantages are demonstrated and discussed. I further report on a novel mechanism giving rise to the EDMR effect in donor-doped silicon field-effect transistors, which is capable of explaining why the EDMR signal intensities of the conduction electrons are enhanced by a factor of ∼100, while the donor resonance signals increase by a factor of ∼20 from X- to W-band only. The spin-relaxation and dephasing times are extracted from a series of pulsed-EDMR measurements and confirm this model. The author gratefully acknowledges funding from Trinity College Oxford, Department of Materials, EPSRC DTA, and Konrad-Adenauer-Stiftung e.V. (Begabtenförderung).

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.

Nanomaterials have attracted great interest due to the unique physical properties and great potential in the applications of nanoscale devices. Two dimensional atomic crystals, which are atomic thickness, especially graphene, have triggered the gold rush recently due to the fascinating high mobility at room temperature for future electronics. The crystal structure of nanomaterials will have great influence on their physical properties. Thus, this thesis is focused on developing the methods to control the crystal structure of nanomaterials, namely quantum dots as semiconductor, boron nitride (BN) as insulator, graphene as semimetal, with low cost for their applications in photonics, structural support and electronics. In this thesis, firstly, Mn doped ZnSe quantum dots have been synthesized using colloidal synthesis. The shape control of Mn doped ZnSe quantum dots has been achieved from branched to spherical by switching the injection temperature from kinetics to thermodynamics region. Injection rates have been found to have effect on controlling the crystal phase from zinc blende to wurtzite. The structural-property relationship has been investigated. It is found that the spherical wurtzite Mn doped ZnSe quantum dots have the highest quantum yield comparing with other shape or crystal phase of the dots. Then, the Mn doped ZnSe quantum dots were deposited onto the BN sheets, which were micron-sized and fabricated by chemical exfoliation, for high resolution imaging. It is the first demonstration of utilizing ultrathin carbon free 2D atomic crystal as support for high resolution imaging. Phase contrast images reveal moiré interference patterns between nanocrystals and BN substrate that are used to determine the relative orientation of the nanocrystals with respect to the BN sheets and interference lattice planes using a newly developed equation method. Double diffraction is observed and has been analyzed using a vector method. As only a few microns sized 2D atomic crystal, like BN, can be fabricated by the chemical exfoliation. Chemical vapour deposition (CVD) is as used as an alternative to fabricate large area graphene. The mechanism and growth dynamics of graphene domains have been investigated using Cu catalyzed atmospheric pressure CVD. Rectangular few layer graphene domains were synthesized for the first time. It only grows on the Cu grains with (111) orientation due to the interplay between atomic structure of Cu lattice and graphene domains. Hexagonal graphene domains can form on nearly all non-(111) Cu surfaces. The few layer hexagonal single crystal graphene domains were aligned in their crystallographic orientation over millimetre scale. In order to improve the alignment and reduce the layer of graphene domains, a novel method is invented to perform the CVD reaction above the melting point of copper (1090 ºC) and using molybdenum or tungsten to prevent the balling of the copper from dewetting. By controlling the amount of hydrogen during the growth, individual single crystal domains of monolayer over 200 µm are produced determined by electron diffraction mapping. Raman mapping shows the monolayer nature of graphene grown by this method. This graphene exhibits a linear dispersion relationship and no sign of doping. The large scale alignment of monolayer hexagonal graphene domains with epitaxial relationship on Cu is the key to get wafer-sized single crystal monolayer graphene films. This paves the way for industry scale production of 2D single crystal graphene.

Organic photovoltaics have attracted considerable research and commercial interest due to their lightness, mechanical flexibility and low production costs. There are two main approaches for the fabrication of organic solar cells – solution and vacuum processing. The former relies on morphology control in polymer-fullerene blends resulting from natural phase separation in these systems. The latter takes advantage of solvent-free processing allowing highly complex multi-junction architectures similar to inorganic solar cells. This work aims to combine the benefits of both by depositing conjugated polymers using vacuum thermal evaporation. By employing this unconventional approach it aims to enhance the efficiency of organic photovoltaics through increased complexity of the thin-film architecture while improving the nanoscale morphology control of the individual active layers. The thesis explores the vacuum thermal deposition of polythiophenes, mainly poly(3-hexylthiophene) (P3HT) and side-group free poly(thiophene) (PTh). A variety of chemical techniques, such as NMR, FT-IR, GPC, DSC and TGA, are used to examine the effect of heating on chemical structure of the polymers. Optimal processing parameters are identified and related to the resulting thin-film morphology and charge transport properties. Efficient photovoltaic devices based on polythiophene donors and fullerene acceptors are fabricated. Materials science techniques AFM, XRD, SEM, TEM and MicroXAM are used to characterize topography and morphology of the thin films, and UV-Vis, EQE, I-V and C-V measurements relate these to the optical and electronic properties. The results of the study show that polymer side groups have a strong influence on molecular packing and charge extraction in vacuum-deposited polymer thin films. Unlike P3HT, evaporated PTh forms highly crystalline films. This leads to enhanced charge transport properties with hole mobility two orders of magnitude higher than that in P3HT. The effect of molecular order is demonstrated on polymer/fullerene planar heterojunction solar cells. PTh-based devices have significantly better current and recombination characteristics, resulting in improved overall power conversion efficiency (PCE) by 70% as compared to P3HT. This confirms that the chemical structure of the molecule is a crucial parameter in deposition of large organic semiconductors. It is also the first-ever example of vacuum-deposited polymer photovoltaic cell. Next, vacuum co-deposited PTh:C60 bulk heterojunctions with different donor-acceptor compositions are fabricated, and the effect of post-production thermal annealing on their photovoltaic performance and morphology is studied. Co-deposition of blended mixtures leads to 60% higher photocurrents than in thickness-optimized PTh/C60 planar heterojunction counterparts. Furthermore, by annealing the devices post-situ the PCE is improved by as much as 80%, achieving performance comparable to previously reported polythiophene and oligothiophene equivalents processed in solution and vacuum, respectively. The enhanced photo-response is a result of favourable morphological development of PTh upon annealing. In contrast to standard vacuum-processed molecular blends, annealing-induced phase separation in PTh:C60 does not lead to the formation of coarse morphology but rather to an incremental improvement of the already established interpenetrated nanoscale network. The morphological response of the evaporated PTh within the blend is further verified to positively differ from that of its small-molecule counterpart sexithiophene. This illustrates the morphological advantage of polymer-fullerene combination over all other vacuum-processable material systems. In conclusion, this processing approach outlines the conceptual path towards the most beneficial combination of solution/polymer- and vacuum-based photovoltaics. It opens up a fabrication method with considerable potential to enhance the efficiency of large-scale organic solar cells production.

This work investigates the applicability of fast detectors to the technique of microscope-mode imaging mass spectrometry. By ionising analyte from a large area of the sample, and projecting the desorbed ions by the use of ion optics through a time-of-flight mass spectrometer onto a two-dimensional detector, time- (and hence mass-) dependent distributions of ions may be imaged. To date, this method of imaging mass spectrometry has been limited by the ability to image only one mass window of interest per experimental cycle, limiting throughput and processing speed. Thus, the alternative microprobe-mode imaging mass spectrometry is currently the dominant method of analysis, with its superior mass resolution. The application of fast detectors to microscope-mode imaging lifts the restriction of the detection of a single mass window per experimental cycle, potentially decreasing acquisition time by a factor of the number of mass peaks of interest. Additional advantages include the reduction of sample damage by laser ablation, and the potential identification of coincident co-fragments of different masses originating from the same parent molecule. Theoretical calculations and simulations have been performed confirming the suitability of conventional time-of-flight velocity-mapped ion imaging apparatus for imaging mass spectrometry. Only small modifications to the repeller plate and laser beam path, together with the adjustment of the accelerating potential field, were required to convert the apparatus to a wide (7mm diameter) field-of-view ion microscope. Factors affecting the mass and spatial resolution were investigated with these theoretical calculations, with theoretical calculations predicting a spatial resolution of about 26um and m/Dm of 93. Typical experimental data collected from velocity-mapped ion imaging experiments were collected, and characterised in order to provide specifications for a novel time-stamping detector, the Pixel Imaging Mass Spectrometry detector. From these data, the suitability of thresholding and centroiding on the new detector was determined. Initial experiments using desorption/ionisation on silicon and conventional charge-coupled device cameras confirmed the correct spatial-mapping of the apparatus. Matrix-assisted laser desorption/ionisation techniques (MALDI) were used in experiments to determine the spatial and mass resolutions attainable with the apparatus. Experimental spatial resolutions of 14.4um and m/Dm of 60 were found. The better experimental spatial resolution indicates a higher directionality of initial velocities from MALDI desorption than used in the theoretical predictions, while the poorer mass resolution could be attributed to limitations imposed by the use of the phosphor screen. Proof-of-concept experiments using fast-framing cameras and the new time-stamping detectors confirmed the feasibility of multiple mass acquisition in time-of-flight microscope mode ion imaging. Mass-dependent distributions were acquired of different pigment distributions in each experimental cycle. Finally, spatial-mapped images of coronal mouse brain sections were acquired using both conventional and fast detectors. The apparatus was demonstrated to provide accurate spatial distributions with a wide field-of-view, and multiple mass distributions were acquired with each experimental cycle using the new time-stamping detector.

Solid-state dye-sensitized solar cells (ssDSCs) offer the possibility of high power conversion efficiencies (PCEs) of over 20%. However, after more than a decade of research, devices still barely reach over 7% PCEs. In this thesis, limitations to device performance are studied in detail, and solutions for future advancement are put forward. In the first part of the thesis, factors limiting charge generation are explored by studying the crystallization environment of mesoporous TiO2 self-assembled through block copolymers. It was found that the density and distribution of sub band gap states are a function of the synthesis conditions and critically affect the performance characteristics of the self-assembled titania used in ssDSCs. As a result, the self-assembled mesoporous oxide system presented in this thesis outperforms for the first time the conventional nanoparticle based electrodes fabricated and tested under the same conditions, with demonstrated PCEs of over 5%. In chapters 6, 7, and 8, the factors limiting the diffusion length and hence, the thickness of the fabricated devices, are carefully examined. Previous literature points towards insufficient pore-filling of the hole transporting material (HTM) as the main limiting factor. In chapter 6, a pore-filling study is shown where a new technique to evaluate the pore-filling fraction of the HTM in the conventional mesoporous metal oxide electrode is also presented and conclude that sufficient pore-filling of thick films can easily be achieved. Another usual strategy to extend the electron lifetime in the devices and thus, the charge diffusion length, involving thin film coatings of insulating metal oxides is examined in chapter 7, with satisfactory results for SnO2-based ssDSCs. The diffusion length can also be extended if the factors limiting the diffusion of charges through the device are identified and removed, as presented in chapter 8. Finally, a study on the stability of the ssDSC is presented in chapter 9. The developments achieved enable long term stability to be effectively targeted, and represent a key milestone towards commercial realization of ssDSCs.

We study correlated two-level quantum impurity models coupled to a metallic conduction band in the hope of gaining insight into the physics of nanoscale quantum dot systems. We focus on the possibility of formation of a spin-1 impurity local moment which, on coupling to the band, generates an underscreened (USC) singular Fermi liquid state. By employing physical arguments and the numerical renormalization group (NRG) technique, we analyse such systems in detail examining in particular both the thermodynamic and dynamic properties, including the differential conductance. The quantum phase transitions occurring between the USC phase and a more ordinary Fermi liquid (FL) phase are analysed in detail. They are generically found to be of Kosterlitz-Thouless type; exceptions occur along lines of high symmetry where first-order transitions are found. A `Friedel-Luttinger sum rule' is derived and, together with a generalization of Luttinger's theorem to the USC phase, is used to obtain general results for the $T=0$ zero-bias conductance --- it is expressed solely in terms of the number of electrons present on the impurity and applicable in both the USC and FL phases. Relatedly, dynamical signatures of the quantum phase transition show two broad classes of behaviour corresponding to the collapse of either a resonance or antiresonance in the single-particle density of states. Evidence of both of these behaviours is seen in experimental devices. We study also the effect of a local magnetic field on both single- and two-level quantum impurities. In the former case we attempt to resolve some points of contention that remain in the literature. Specifically we show that the position of the maximum in the spin resolved density of states (and related peaks in the differential conductance) is not linear in the applied field, showing a more complicated form than a simple `Zeeman splitting'. The analytic result for the low-field asymptote is recovered. For two-level impurities we illustrate the manner in which the USC state is destroyed: due to two cancelling effects an abrupt change in the zero-bias conductance does not occur as one might expect. Comparison with experiment is made in both cases and used to interpret experimental findings in a manner contrary to previous suggestions. We find that experiments are very rarely in the limit of strong impurity-host coupling. Further, features in the differential conductance as a function of bias voltage should not be simply interpreted in terms of isolated quantum dot states. The many-body nature of such systems is crucially important to their observed properties.

This thesis reports the study of single photon sources that emit one infrared wavelength photon at a time, creating cavity quantum electrodynamical effects for applications such as quantum information processing. This work considers two major single photon sources: a) InAs single quantum dots and b) single carbon nanotubes, which both emit in the infrared range. Photonic crystal slabs and photonic crystal waveguides are served as distinctive passive devices with manipulated photonic band-gaps to control the propagating light. A simulation of leaky modes of two-dimensional photonic crystal slabs is introduced to constrain model parameters in the device design. Fullerenes are used as fluorescent material to achieve resonance of a leaky mode with excitation 1492 nm and emission at 1519 nm and to see enhancement of the PL. We include novel characterization techniques and PL measurements to show sharp emission peaks from single quantum dots and successfully couple them to micro-cavities. The strong coupling effect is observed and is amongst the best examples of cavity-dot structures achieved to date. Single-walled carbon nanotubes have shown anti-bunched light emission, thus we systematically study them as another possible candidate of single photon sources. PLE spectra show clear evidence of the existence of excited states, and time evolution measurements reveal the disorder induced diffusion, which separate the tubes into a series of quantum dots. These strongly confined states are concluded as the origin of the possibility that single-walled carbon nanotubes are single photon sources.

Optoelectronic characterisation techniques are assessed in their application to three solar cell systems. Charge injection barriers are found in PbS/ZnO colloidal quantum dot solar cells through the use of temperature dependent current-voltage and capacitance-voltage measurements. The injection barriers are shown to complicate the Mott-Schottky capacitance analysis which determines built-in bias and doping density. A model that incorporates depletion capacitance and a constant capacitance arising from the injection barriers is given to explain the Mott-Schottky plots. The junction mechanism at the PbS/ZnO interface is found to transition from excitonic to p-n behaviour based on the amount of UV photodoping the cell has received. External quantum efficiency analysis at different photodoping times reveals a growing charge collection region within the material, demonstrating the shift to p-n behaviour. This is further supported by the observance of depletion capacitance behaviour after, but not before, UV photodoping. Defects within GaAs cells containing InAs quantum dots are found to enhance the sub-bandgap performance of the cell using external quantum efficiency analysis. This is verified by illuminated current-voltage analysis using a 1000 nm high pass optical filter to block photons of larger energy than the bandgap. Using capacitance-voltage analysis, high temperature rapid thermal annealing is shown to induce defects in dilute nitride cells, which explains the drop in open circuit voltage compared to lower temperature annealed cells. The doping level of polymer solar cells exposed to air is found to increase with continued exposure using Mott-Schottky capacitance analysis. Current-voltage measurements show the formation of an Al2O3 barrier layer at the polymer/aluminium interface. The usefulness of capacitance-voltage measurements to probe the polymer/fullerene interface is investigated in thermally evaporated thiophene/C60 cells.

This thesis is concerned with studying the fascinating quantum properties of real-world nanostructures embedded in a noisy condensed matter environment. The interaction with light is used for controlling and manipulating the quantum state of the systems considered here. In some instances, laser pulses also provide a way of actively probing and controlling environmental interactions. The first two research chapters assess two different ways of performing all-optical spin qubit gates in self-assembled quantum dots. The principal conclusion is that an `adiabatic' control technique holds the promise of achieving a high fidelity when all primary sources of decoherence are taken into account. In the next chapter, it is shown that an optically driven quantum dot exciton interacting with the phonons of the surrounding lattice acts as a heat pump. Further, a model is developed which predicts the temperature-dependent damping of Rabi oscillations caused by bulk phonons, finding an excellent agreement with experimental data. A different system is studied in the following chapter: two electron spin qubits with no direct interaction, yet both exchange-coupled to an optically active mediator spin. The results of this study show that these general assumptions are sufficient for generating controlled electron spin entanglement over a wide range of parameters, even in the presence of noise. Finally, the Radical Pair model of the avian compass is investigated in the light of recent experimental results, leading to the surprising prediction that the electron spin coherence time in this molecular system seems to approach the millisecond timescale.

We investigated the threshold current, Ith, and its temperature dependence, To(Ith), for 1.3μm and 1.5μm InGaAsP) based lasers for 77K < T < 400K and considered the temperature variation of the radiative current, To(lRad). Below a ‘breakpoint’ temperature, TB, both Ith and To(Ith) are dominated by radiative-recombination where Ithcn2 (n is the carrier density) and To(Ith)=To(lRad)=T. Above TB, To(Ith) decreases strongly and by room temperature To(Ith)=50-60K while To(IRad) remains ~T (2/3T for bulk devices). Furthermore, Ithn3 consistent with a dominant non-radiative Auger-recombination process, amounting to 80%Ith in 1.5 μm devices and up to 50%Ith in 1.3μm devices. These results are consistent with the pressure dependence of Ith which decreases with increasing pressure due to the reduction of Auger-recombination with increasing band gap. 1.5μm devices show a stronger decrease than 1.3μm devices. We conclude that a direct Auger-recombination process producing ‘hot-holes’ (d-CHSH) is chiefly responsible for the high temperature sensitivity of these devices. Above room temperature, we observe a further decrease in To(Ith) which, from measurements of the temperature variation of the differential quantum efficiency, we attribute to an increase in optical loss, ai. For the 1.3μm devices, we also find that the n dependence of Ith increases above 3. This, we suggest, is due to spill-over of holes resulting in both non-radiative mono-molecular recombination and inter-valence band absorption in the barrier/SCH regions. This explains why ai is relatively high in the 1.3μm devices. AlGalnAs-based 1.3μm lasers show a lower temperature dependence than InGaAsP-based devices. At room temperature To(Ith)~100K where non-radiative current accounts for only -25% Ith. We additionally observe a high TB=220K and for the first time, measure an increase in Ith with pressure for a 1.3μm laser. These results are consistent with a dominant radiative current at room temperature and explain the reduced temperature sensitivity of AlGalnAs-based 1.3μm devices compared to conventional InGaAsP-based devices.

EEV buried channel charge coupled devices (BC CDs) with technological variations have been studied with respect to their response to 70kVp X-rays. Process variations considered are the conventional BCCD, scintillator coated BCCDs (Gadox(Eu) and Csl(Tl)) and the inversion mode device. The work was made necessary by the use of these CCDs for dental X-ray imaging. Effects investigated include changes in device operating voltages and dark current. The dark current buildup has been characterised in terms of a prompt component seen immediately following irradiation, and a time dependent component which occurs gradually. A major part of this work was the determination of the location and concentration of the energy states responsible for this dark current buildup. Also a novel aspect of the work was the derivation of an expression describing the time dependent component as a function of time and temperature. Effects associated with the bias dependence of the BCCD have also been considered, with particular regard to the effect of a negative substrate bias, and the theoretical explanation has been developed. The findings of this work have demonstrated the suitability of these devices for the commercial application of imaging X-rays for dentistry.

All semiconductor devices need electrical accessibility and thus some form of metal contact is required. This contact may be rectifying or ohmic, but the metals used in the chip construction are often unsuitable for making connections to the outside world. Invariably, layered metallizations are used to make the top metallic layer suitable for wire or tape ultrasonic-thermocompression bonding, usually involving aluminium or gold, or soldering using lead-tin alloys. When a semiconductor device is fabricated, it goes through a number of processes at the end of which it is metallized, passivated, encapsulated, and packaged, or any combination of these. The device engineer knows the structure of the device which will include a number of semiconductor-metal, insulator-metal, and metal-metal interfaces. In order to ascertain the operational reliability of the device, accelerated life-tests and predelivery burn-in or screening (or both) based on life tests are often carried out. This stressing involves operating the device either at high temperatures or at high current densities in normal atmospheric, corrosive or highly irradiated environments, or in environments consisting of combinations of these. The different interfaces in the device may change their characteristics through materials transferred by various means at the different stages mentioned above. The interfacial changes and any resultant alterations in the bulk of the constituting materials invariably alters the electrical or mechanical performances (or both) of the device which is said to have degraded. More importantly, operation of the device at high power levels or at high stressing causes thermal runaway and device failure. The work carried out and described in this thesis focuses on failures which occur in the connective paths, known as lines between individual device internal transistors. Firstly, the operation of the VLSI device in adverse elevated thermal and elevated current density conditions will be described. Whilst the process failure mechanisms under these conditions are well documented and mathematically defined, a new technique involving mechanical stress modelling has been developed to predict failures locations in the Al-Si 1% lines. Secondly, a new procedure has been developed to artificially 'age' VLSI devices in order to observe any degradation which may occur in the connecting paths. Several factors have been identified which can contribute to line degradation. These are high current density, high temperature and high mechanical stress. These factors together give rise to electromigration, resulting in the physical movement of line material, which eventually results in catastrophic line failure. It has been previously thought that only temperature and current density were the controlling factors, but this investigation has shown that mechanical stress has a major influence. A new model to predict electromigration phenomena locations in conductive paths has thus been developed based on mechanical stress, in addition to current density and surrounding temperature.

Semiconductor devices can be fabricated by growing III-V heteroepitaxial layers which are coherently strained to a III-V substrate. The relaxation of layer lattice strain through the nucleation of misfit dislocations near the interface causes a drop in performance for these devices. This thesis uses two non destructive x-ray techniques to examine relaxation in III-V epitaxial layers; double crystal diflractometry and x- ray topography. The dynamical and kinematical theories of x-ray diffraction are discussed in chapter 2. The apparatus used for double crystal diffractometry and x-ray topography and the theory of operation of these techniques is discussed in chapter 3. The properties of misfit dislocations in III-V epitaxial layers and the critical layer thickness at which relaxation occurs are discussed in chapter 4.Double crystal diffractometry and x-ray topography have been used to examine relaxation in epitaxial layers of AlAs on GaAs, InGaAs on GaAs, GaAsSb on GaAs, InGaAs on InP and an InGaAs superlattice on InP. All layers were deposited on 001 orientated substrates. Asymmetric double crystal rocking curves have been analysed using a novel technique which allows deduction of the position of an hhl layer reflection in reciprocal space. The layer unit cell parameters in the [110] and iTO] directions are determined from this. Individual misfit dislocation lines can be resolved by topography for dislocation line densities less than 0.2 μm(^-1)In each of these samples the layer relaxation was found to be asymmetric about the (110) directions. The sensitivity of diffractometry and topography to the detection of layer relaxation has been compared for samples with different thicknesses and dislocation line densities. The resolution of these techniques to the determination of layer relaxation has been shown to meet for a 1 μm layer of AlAs on GaAs. Tilt between the epitaxial layer lattice and the substrate has been measured for coherently strained and partially relaxed epitaxial layers grown on 001 orientated substrates. The lattice tilt in (110) directions was found to increase with misfit dislocation line density in these directions. Two theoretical models have been developed describing the relationship between lattice tilt and misfit dislocation line density and the tilts predicted by these compared with experiment. At high dislocation densities measurements of layer relaxation by diffractometry indicate that the images recorded by topography represent bundles of misfit dislocations and not individual dislocation fines. The number of dislocation lines per bundle was found to decrease with decreasing layer relaxation. Bunching of misfit dislocations into dislocation bundles is also observed on topographs from a low dislocation density sample where the individual dislocation hues are resolved. Screw dislocations in a strained layer and an interaction between two 60 dislocations to form a mixed dislocation have been characterised using Burgers vector analysis. Interference fringes have been observed on 004 double crystal rocking curves recorded from an ultra thin In GaAs layer sandwiched between a GaAs substrate and a GaAs cap. The position and intensity of these fringes was found to be sensitive to the composition and thickness of the In GaAs layer. Comparison between simulated and experimental rocking curve data allowed determination of the layer thickness to within a single monolayer and layer composition to within 0.5%. Topography of this sample showed that the dislocation line density varied from zero to 0.12 μm(^-1)across the wafer. The critical layer thickness and Indium concentration at which the first few misfit dislocation fines were observed was measured as 162 ± 2 A and 17 ± 0.5 %.

The electrical characteristics of the Metal-Insulator-Semiconductor - Switch (MISS) device with Silicon-Rich-Oxide (SRO) as the semi-insulating material have been comprehensively studied at room temperature in an exploratory way. The SRO films were deposited by atmospheric pressure chemical vapour deposition (APCVD) at 650ºC with SiH(_4) and N(_2)O reactant gases and N(_2) carrier. The react ant gas phase ratio R(_o) varying from 0.09 to 0.25 and the deposition time varying from 0.6 to 2 min. Some preliminary investigations on SRO-MIS devices were also carried out in order to understand the electronic process in the structure. Various parameters which governed the switching behaviour of an MISS were investigated. In general the switching characteristics are similar to those of the tunnel oxide MISS. The geometrical dependence of the switching behaviour in the tunnel oxide MISS has been extended to the present device by looking at the effects of electrode area, junction area, electrode perimeters and of a metal guard ring. Other effects, such as SRO deposition time, work function difference, gold doping, heat treatment, light illumination and film ageing were also observed. The dynamic characteristic of the device was studied using a double pulse technique. The characteristics of the three-terminal SRO-MISS were studied in both forward and reverse bias. The former exhibited a thyristor-like characteristic and the latter a transistor-Hke characteristic. A preliminary study on the MIS-emitter transistor was carried out with different emitter areas. In general the characteristics are the same as for the equivalent tunnel oxide devices. However it was also found that if the n-type epilayer is very thin the transistor characteristics exhibits an N-type negative resistance. The negative resistance region of the two-terminal MISS has been shown to be stable and the stability has been analysed in terms of equivalent circuit elements. The reason for the stability is that the device also has an negative capacitance. This has been proved experimentally and it is a new property of the MISS structure which never been reported before. The negative capacitance has been measured as a function of electrode area, SRO type and light illumination. An important circuit application for the negative capacitance has also been suggested and demonstrated