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This thesis describes the work carried out under HYPIX, a collaborative research project aimed at developing hybrid organic/inorganic optoelectronic devices, with particular focus given to imaging systems used in the optical pumping of organic polymer gain media. State-of-the-art generations of inorganic light-emitting diodes are presented, along with an overview of the evolution of such devices over the years. A brief outlook is also given on current advances in organic semiconductor gain materials research, as well as laser cavity design. The challenges in designing an optical system capable of pumping organic materials with highly divergent light sources are highlighted, together with the methods that were used to overcome them. Various attempts at optically pumping several organic materials are detailed, highlighting the importance of combining several factors such as energy density, pump spot size and pump pulse duration all at once in order to achieve lasing in this new type of hybrid device. Possible improvements which could in the future lead to the first indirectly electrically pumped organic semiconductor laser are listed. Secondary experiments are also described, covering the proof-of-concept use of computer- controlled, inorganic micro-scale light sources as a spatially-resolving spectrometer for organic polymer targets. A deviation of under 10 % from absorbance values calculated for the samples using a commercial spectrometer is shown.

Excitons are quasi-particles, which are responsible for energy transport in organic semiconductors. Excitons are therefore instrumental in understanding the photophysics of organic opto-electronic devices. The present work focused on describing the dynamics of spin-forbidden, long-lived triplet excitons in archetypal organic materials such as CBP. Triplet excitons lifetime and diffusion length are here estimated from modelling the results of triplet-triplet photoinduced absorption spectroscopy, steady-state photoluminescence spectroscopy and time-resolved photoluminescence measurements. The last two measurements are performed using a modified time-of-flight method, whereby the investigated material is adjacent to a phosphorescent sensing layer and optically excited from the opposite side. As the thickness of the material is increased, the variations of phosphorescence intensity coming from the sensing layer is correlated to the exciton diffusion parameters. We show that for fluorescent materials such as CBP, the near-field component of this emission couples to guided modes supported by the structure and directly excites the sensitizer - here Ir(ppy)3 doped into CBP - which lead to an overestimation of the diffusion length. In addition, we investigate a strategy to mitigate the effect of guided modes by using an optical quenching layer of C6. This results in an estimated triplet exciton lifetime in the ms range and a diffusion length in excess of 30 nm, based on modelling the steady-state and time-resolved emission of the sensing layer when varying CBP thickness.

Networks-on-Chip (NoCs) have been proposed as a scalable, reliable and power-efficient communication fabric for chip multiprocessors (CMPs) and multiprocessor systems-on-chip (MPSoCs). NoCs determine both the performance and the reliability of such systems, with a significant power demand that is expected to increase due to developments in both technology and architecture. In terms of architecture, an important trend in many-core systems architecture is to increase the number of cores on a chip while reducing their individual complexity. This trend increases communication power relative to computation power. Moreover, technology-wise, power-hungry wires are dominating logic as power consumers as technology scales down. For these reasons, the design of future very large scale integration (VLSI) systems is moving from being computation-centric to communication-centric. On the other hand, chip’s physical parameters integrity, especially power and thermal integrity, is crucial for reliable VLSI systems. However, guaranteeing this integrity is becoming increasingly difficult with the higher scale of integration due to increased power density and operating frequencies that result in continuously increasing temperature and voltage drops in the chip. This is a challenge that may prevent further shrinking of devices. Thus, tackling the challenge of power and thermal integrity of future many-core systems at only one level of abstraction, the chip and package design for example, is no longer sufficient to ensure the integrity of physical parameters. New designtime and run-time strategies may need to work together at different levels of abstraction, such as package, application, network, to provide the required physical parameter integrity for these large systems. This necessitates strategies that work at the level of the on-chip network with its rising power budget. This thesis proposes models, techniques and architectures to improve power and thermal integrity of Network-on-Chip (NoC)-based many-core systems. The thesis is composed of two major parts: i) minimization and modelling of power supply variations to improve power integrity; and ii) dynamic thermal adaptation to improve thermal integrity. This thesis makes four major contributions. The first is a computational model of on-chip power supply variations in NoCs. The proposed model embeds a power delivery model, an NoC activity simulator and a power model. The model is verified with SPICE simulation and employed to analyse power supply variations in synthetic and real NoC workloads. Novel observations regarding power supply noise correlation with different traffic patterns and routing algorithms are found. The second is a new application mapping strategy aiming vii to minimize power supply noise in NoCs. This is achieved by defining a new metric, switching activity density, and employing a force-based objective function that results in minimizing switching density. Significant reductions in power supply noise (PSN) are achieved with a low energy penalty. This reduction in PSN also results in a better link timing accuracy. The third contribution is a new dynamic thermal-adaptive routing strategy to effectively diffuse heat from the NoC-based threedimensional (3D) CMPs, using a dynamic programming (DP)-based distributed control architecture. Moreover, a new approach for efficient extension of two-dimensional (2D) partially-adaptive routing algorithms to 3D is presented. This approach improves three-dimensional networkon- chip (3D NoC) routing adaptivity while ensuring deadlock-freeness. Finally, the proposed thermal-adaptive routing is implemented in field-programmable gate array (FPGA), and implementation challenges, for both thermal sensing and the dynamic control architecture are addressed. The proposed routing implementation is evaluated in terms of both functionality and performance. The methodologies and architectures proposed in this thesis open a new direction for improving the power and thermal integrity of future NoC-based 2D and 3D many-core architectures.

The major market barriers to the use of photovoltaic solar cells are high cost and long payback time of conventional technologies, based largely on the silicon material. In order to overcome the environmental problem resulting from the consumption of fossil fuels, all western countries are required to impose heavy subsidies to encourage the use of solar cells in the reduction of carbon consumption; thereby making them highly unsustainable. Therefore, it is necessary to develop solar cells based on low-cost metal oxides with large natural resources. The objective of this program is to investigate the effects of doping on the structural, optical and electrical properties of low-cost metal oxides, such as doped ZnO and copper oxides (CuO and Cu4O3). These are synthesised via sputter deposition and thermal oxidation method in air. Al doped ZnO is an n-type direct semiconductor with a band gap of around 3.5eV. Its crystalline structure is wurtzite, which is deposited widely by the RF reactive magnetron sputtering technology. In my work, the Al doped ZnO thin films were deposited by sputter with metal and ceramic targets. On the one hand, the influence of RF power on the structural, electrical and optical properties of Al doped ZnO thin films were investigated when they were deposited with metal targets. Conversely, the influence of O2 flow rate on the structural, electrical and optical properties of Al doped ZnO thin films was examined when they were deposited with ceramic targets. CuO is a p-type indirect semiconductor with a narrow band gap of 1.0-1.4eV. Its crystalline structure is monoclinic crystal system. CuO nanowires (NWs) were fabricated by the thermal oxidation method in air. It was found that CuO NWs not only grows on Cu sheets, but also on the Si, FTO, Al doped ZnO and glass substrates. For the growth of CuO NWs, the expanding parameters should meet the following requirements: growing temperature: >390°C and growing duration: ≥6hrs. The peeling-off of the CuO NWs on Cu sheets resulted from the formation of Cu8O and Cu64O between the Cu sheets and Cu2O layer. The electrical properties of a single CuO NW were measured using a nano probe station. The contact behaviour between a CuO NW and metal electrodes (Au and W) was schottky. The electrical resistivity of a CuO NW depended on the diameter of the NW. The contact behaviour between CuO NWs on Cu sheets with silver paste top electrodes was schottky as well. A simple PV cell based on CuO NWs-PCBM p-n heterojunction was fabricated, and the short circuit current, open voltage and fill factor of the PV cell was also measured. It indicated that CuO NWs can be utilized to fabricate diodes and PV cells. Copper oxides thin films were deposited by RF reactive magnetron sputtering technology. The phase structure of copper oxides thin films depended on the sputtering parameters. When the thin film was deposited without a bias power, only CuO was detected in the copper oxide thin films. The electrical properties of CuO thin films depended on the O2 fraction during the sputter process. The current-voltage (I-V) characteristics of CuO thin films with Cu electrodes demonstrated that it was influenced by the O2 fraction during the sputter process. Moreover, Cu4O3 is a p-type indirect semiconductor with narrow band gap of 1.0-1.4eV and its crystalline structure is tetragonal crystal system. When the copper oxide thin films were deposited with a bias power, only Cu4O3 phase was detected. Its structural, optical and electrical properties were studied. The optical band gap of Cu4O3 thin film was 1.37eV. Hall properties of Cu4O3 thin films were 1020cm-3, 10-2cm2·V-1·s-1 and 10-1Ω·cm. The Cu4O3-Al Abstract III doped ZnO p-n heterojunction demonstrated excellent rectifying performance, indicating that Cu4O3 is a good candidate for fabricating diodes and PV cells. In addition, Cu4O3 thin films were annealed at different temperatures in air. Furthermore, I studied the influence of annealing temperature on the structural, optical and electrical properties of Cu4O3 thin films.

The III-nitride semiconductor materials system is used in thin-film-based optoelectronic devices. GaN and InGaN in particular have been used to produce efficient blue and green light emitting diodes. However, III-nitride thin films typically contain very high densities of dislocations, a line defect known to negatively affect device performance and lifetimes, but despite this the performance of III-nitride-based devices is much less affected by dislocations than devices based on other semiconducting systems. These dislocations have been the subject of extensive study but experimental and theoretical reports still present conflicting data. In particular, dislocations do not exist in isolation in III-nitride thin films, but can interact with point defects, dopants and alloying elements, and may induce local compositional segregation in III-nitride alloy epilayers. This thesis presents a study of the structure and properties of dislocations in GaN and InGaN and of their interactions with point defects, dopants and alloying elements. Specifically, a new theoretical analysis of a common extended dislocation core in GaN is presented and the conclusions are verified by experimental data. Random alloy microstructures are predicted and analysed for InGaN quantum wells of different thicknesses, followed by a theoretical and experimental study of the variation in InGaN alloy composition around dislocation cores. Finally, the mechanisms by which dislocation cores can act as preferential diffusion pathways for native point defects are studied, providing insight into possible mechanisms by which compositional segregation or dislocation movement could occur. The methods presented here have a broad relevance beyond the III-nitride materials system and highlight the importance of assessing the interactions of dislocations with other defects, to achieve a better understanding of dislocation properties and their influence on device performance.

The aims of the PhD project were to understand the mechanisms of cellular uptake as well as the intracellular biostability of oxidised functionalised multi-walled carbon nanotubes (f- MWNTs). Firstly, high resolution transmission electron microscopy (HR TEM) imaging and 3D electron tomography were applied to study the pathways of f-MWNTs into non-phagocytic cells, and more specifically, to study the interaction between f-MWNTs and the cell membrane. After exposing epithelial cells to f-MWNTs for 24 hours, two distinct uptake processes by which NH3+ f-MWNTs can enter epithelial cells were observed. Then, the study was complemented using a combination of TEM imaging and coarse-grained molecular dynamics simulations, to provide insight into the interaction of f-MWNTs with cell membranes and the effect of surface charge on this interaction. Secondly, the question of whether f-MWNTs can be degraded by the body’s own defence mechanisms was addressed. HR TEM techniques were used to assess the graphitic structure and morphology of f-MWNTs injected into the murine brain and after exposure to human monocyte-derived macrophages (HMMs), with the aim of understanding the mechanisms underlying the degradation process. F-MWNTs were found to have a reduced biostability in the brain tissue and in the HMMs. Inside the brain tissue, the degradation occurred rapidly with signs of advanced f-MWNT degradation present after 2 days exposure. In the HMMs, f-MWNT walls were found to delaminate from individual f-MWNTs inside lysosomes after 24 hours exposure. Similar events were observed after 14 days exposure in the cell cytoplasm inside the HMMs. Furthermore, a loss of the graphitic structure was observed. By using scanning TEM electron energy loss spectroscopy (STEM EELS) to compare the near-edge structure of the carbon K-edge prior to, and post injection, graphitic f-MWNTs could be distinguished from the graphitic debris and the amorphous cell background. The combination of HR TEM and STEM EELS techniques provided information about the individual steps, leading to the disintegration of the f-MWNTs, and the morphology of the degradation debris. In order to study the effect of the functionalisation on the biostability, the study was repeated using pristine MWNTs and HMMs. No signs of degradation of the pristine MWNTs were observed after 14 days exposure to the HMMs.

In this thesis, we develop aspects of a framework for the multi-scale modelling of charge transport in several organic semiconductors relevant for technological application. In the first chapter, we build a coarse-grained model for fullerene multi-adducts which clearly distinguishes between the influence of energetic and structural disorder. The model reveals that charge transport in fullerene multi-adducts is limited by energetic disorder due to different isomers. Solar cells containing single isomers of higher fullerene adducts are expected to achieve higher power conversion efficiencies. Secondly, we create a multi-scale model to investigate the effect of beta-phase conformers on hole transport in poly(9,9)dicotylfluorene. A combination of quantum-chemical calculations and charge transport simulation confirms that beta-phase conformers act as traps and, when torsional disorder of the polymer backbone is included, can explain the experimentally observed mobility reduction due to beta-phase. In the third project, grain boundaries in tri-isopropylsilylethynyl (TIPS) pentacene are characterised. On the basis of two-grain structures with varied mutual orientation generated with atomistic molecular dynamics, we compute energy landscapes and electronic coupling elements. We find that the effect of the grain boundary is relatively weak for long interfaces, but that small contact areas between grains may impede charge transport more strongly due to highly non-uniform energy barrier heights. The final chapter focuses on one-dimensional organic magnetoresistors with alternating acceptor, spacer and donor units. Combining quantum-chemical calculations and kinetic Monte-Carlo simulations allow us to define design rules for high magnetoresistance, among which minimising the offset between HOMO and LUMO energy levels is the most challenging one. Within the range of researched materials, polymers built from dicyanoqyinonediimine (DCNQI) acceptors, fluorene spacers and 3-ethylendioxythiophene (3-EDOT) donors are most promising for magentoresistance application.

The objective of this work is to explore, understand and exploit the properties of solution processable organic semiconductors. The work contained within this thesis has lead to two first author peer-reviewed published papers, several co-author papers and seven thesis chapters: these, augmented with experimental data have evolved to provide a significant contribution to this area of science. Chapter 1 of my thesis gives the reader a detailed introduction to the area of field effect transistors based on solution processable organic semiconductor materials. Chapter 2 is a short overview of some of the experimental techniques used within this body of work. The aim of this chapter, is not only to document the detailed experimental conditions used, but also to provide the experimentalist with practical advice. Chapter 3 identifies a route to achieve a significant material cost reduction by employing a commodity polymer as a low cost diluent. Chapter 4 focuses on the influence of the semiconductor’s side chain substituents on the material’s performance and leads to the provision of material set pathway for improved compatibility of the organic semiconductor (OSC) with its dielectric counterpart. This work could effectively reduce limitations placed on the dielectric, facilitating large scale production. Chapter 5 discusses the benefits and science behind molecular alignment and anisotropy. Chapter 6 demonstrates the benefit of blending a high and low molecular weight semiconductor. Chapter 7 investigates charge injection from the electrodes into the semiconductor layer, thereby optimizing the material’s performance by way of not being limited by contact resistance, which is known to impair the transistor device performance of some OSCs. Each of the chapters is aimed at highlighting the progress that has been made within this thesis and to provoke further research in the field of organic electronics. The majority of this research was carried out at Merck Chemicals Ltd.

This thesis describes the electrical behaviour of solution-processed zinc oxide thin film transistors (ZnO TFTs) fabricated at low temperature. First, the electrical properties of solution-processed ZnO films are reported. Spin-coated ZnO films annealed at 150 °C exhibit significant sensitivity to the ambient environment. However, their stability can be improved by hydrogen plasma treatment. Zinc oxide TFTs (channel width/length = 4000/200 μm) fabricated by chemical spray pyrolysis at the low process temperature of 140 °C are investigated. The resulting transistors exhibit a saturation mobility of 2 cm2/Vs measured in air; this value is reduced to 0.5 cm2/Vs under vacuum. The effect of hydrogen plasma treatment on spin-coated ZnO TFTs is then studied. The electrical characteristics of untreated TFTs exhibit large hysteresis and a positive threshold voltage shift on repeated measurements. These effects are reduced by the hydrogen plasma and an increase in carrier mobility is observed. In a further investigation, a solution-processed silicon dioxide gate insulator for application in the TFTs is used; a perhydropolysilazane (PHPS) precursor is spin-coated with subsequent thermal treatment to form the SiO2 layer. Exposure to oxygen plasma leads to an acceleration of the conversion reaction, resulting in good insulating properties (leakage current density of ~10-7 A/cm2) and TFT performance (channel width/length = 1000/50 μm, carrier mobility of 3.2 cm2/Vs, an on/off ratio of ~107, a threshold voltage of -1.3 V and a subthreshold swing of 0.2 V/decade). Finally, a photolithographic process is introduced for the fabrication of ‘short’ channel solution-processed ZnO TFTs. Optimum processing conditions are established and used for the fabrication of transistors having various channel dimensions. Devices with a minimum channel length of 5 μm possessed a mobility of 1.5 x 10-2 cm2/Vs, on/off ratio of 106 and good contact between the S/D electrodes and the semiconductor. The relatively low mobility could originate from gate insulator roughness caused by the photolithographic processes.