This thesis concerns the use of time-resolved optical techniques to study the efficiency-limiting processes in high open-circuit voltage solar cells. Solution-processed large-area organic solar cells offer the potential for inexpensive and scalable production, but suffer from low efficiency. Currently the quantum efficiency in organic solar cells (the number of electrons extracted per incident photon) is only high when a significant amount of the energy from an absorbed photon is lost during the formation of the photo-induced charge-transfer state. This limits the voltage obtainable from state-of-the-art organic solar cells, and thus their overall efficiency. We investigate organic solar cells that retain more of the photon energy in the charge-transfer state, and hence allow for higher operating voltages to be reached. However, in these systems quantum efficiency is low and severely limits their overall power conversion efficiency. This work identifies that the dominant loss mechanism in such a system is terminal recombination into a triplet exciton. This mechanism for recombination will be energetically accessible in all high open-circuit voltage solar cells made from organic materials, and therefore must be considered in the design and optimisation of these devices. Due to the efficiency of this recombination mechanism, concurrent high densities of charges, singlet excitons, and triplet excitons are observed. This allows the interactions between these excited states to be determined. We measure the annihilation rates salient to optoelectronic device operation, allowing for detailed modelling of second-order kinetic effects to be undertaken. Finally, we investigate a polymer:small molecule solar cell and observe how, in this case, charge-transfer is aided by an initial transient of fast charge motion. However, after this faster motion subsides, the charges recapture one another and recombine quickly. This bimolecular recombination explains the observed efficiency of the polymer:small molecule solar cell, and also the dependence of this efficiency on incident light intensity.

Experiments have been undertaken to measure calorimetrically losses in small coils of superconducting multifilamentary composite conductor subjected to small alternating. fields between 5 and 50 Hz superimposed upon a constant bias. It is found that the currents induced in the outer layers screen the wire and the loss is mainly due to these currents. The variation of the loss with the bias, hitherto a somewhat neglected subject, is found to be considerable, with losses increasing greatly as the bias is increased, together in one case with increasing losses when bias field strength is reduced below 1 Tesla. Comparison with losses calculated using the best available theoretical models show considerable agreement; with the exception of the increased losses at low bias fields agreement generally to within 20% of measured results has been obtained. It is found that the magnetoresistance of the matrix material is the most significant factor in determining the dependence of losses upon the bias, leading at high fields to losses more than twice as great as would otherwise occur.

The work presented in this thesis focuses on the subject of photovoltaic devices based on phase-separated blends of conjugated polymers. The advantage of the blend system used in this work is that both materials are optically and electrically active, and their morphologies and relative arrangement impact this activity so that the optimisation of phase separation can be directly applied to optoelectronic devices. A systematic study on device structure is presented, including the use of polymeric electrodes and the influence of the microstructure of thin films on device performance. Conventional fluorescence, fluorescence scanning near-field optical microscopy and atomic force microscopy have been combined to relate film morphology with photovoltaic and photoluminescence efficiencies as a function of surface treatment, concentration, and preparation conditions. The results presented in this dissertation are in agreement with the thermodynamics theory of phase separation developed for non-conjugated polymer composites. The phase-separated structures observed under different solvent evaporation conditions for the composite system study here are consistent with a spinodal-type decomposition mechanism. The composition of the phases was found to be of fundamental importance for device performance, assisting exciton dissociation in blends phase separated on scales larger than the exciton diffusion length. Vertical segregation of blend components was induced through specially developed solution-coating processes to produce a structure optimised for separated charge transport and collection. This self-organisation process, in conjunction with the phase compositions, has resulted in the demonstration of highest photovoltaic efficiencies reported for the polyfluorene materials. It is directly shown how the control of phase separation of a polymer binary blend system in the intermolecular, lateral, and vertical regime can be used to enhance and optimise device performance.

Power source supplies energy for a given application. It is one of the most important factors for the successful design of an engineering application. However, no systematic selection method is available for the selection of power sources. In the first part of this thesis, a rational method based on matching the physical and performance characteristics of power sources to the requirements of the given task is developed. Using power source database and performance characteristics charts, the method is described and demonstrated with a case study. To date, the development of micro-power sources is still at an early stage. It has become one of the great constraints for the autonomous microsystems. In this thesis, an innovative design of a zinc-air microbattery is presented. The proposed structure of the microbattery contains a 3D electrode with a high density of zinc posts that will provide high porosity and reaction area. The simplified cell construction and improved features of the proposed microbattery make it suitable for easy fabrication, and general and practical use in microsystems. A model for the microbattery is built and the internal behaviours and performance predictions are computer-simulated with MatLab. The fabrication process for the microbattery is developed and improved. The electrical test shows that the microbatteries fabricated have a high current output density as predicted, and the output performance of the microbattery is in excellent agreement with the theoretical prediction by modelling. This study has demonstrated the feasibility of the proposed zinc-air microbattery that is suitable for the applications in autonomous microsystems.

This thesis presents a comprehensive analysis of the IGBTs, antiparallel diodes and IGBT power modules in resonant switching conditions using experimental results, PSpice simulation results, Medici mixed mode simulation results and mathematical models. A simplified sinusoidal analysis of the series and parallel resonant converters is carried out with particular attention given to power switch behaviour. Resonant converter behaviour under various operating conditions and parameters is investigated in order to develop a complete understanding before power switch characterisation. Furthermore stresses on the power switch and the resonant tank under extreme conditions are determined. Three identically rated IGBTs representing the complete range of IGBT variants available today are selected for the ZCS and ZVS characterisation. Experimental results are obtained from two application circuits which produce practical resonant conditions. Test instrumcnlation includes a highly accurate Lecroy power measure system. PSpice results are obtained using an accurate IGBT model based on the Kraus model while 2-D mixed mode simulations are used to study the internal carrier dynamics under resonant switching. A comprehensive analysis and modelling of the IGBT conduction and switching losses in ZCS and ZVS is carried out. Mathematical models are developed giving careful consideration to all the significant features and dependant parameters, namely collector current, switching frequency and case temperature. Model refinement and validation is carried out using the device characterisation results obtained above. The refined model can be made available in a manufacturer's datasheet either as characteristic equations or characteristic curves. A detailed study of the parasitic effects on IGBT power modules in hard switching and resonant switching conditions is presented. For resonant switching characterisation of the IGBT power module a test rig accommodating four different resonant inverter topologies is designed and built. Parasitic inductance effects due to each IGBT terminal is analysed individually in order to identify the effects clearly. Current sharing imbalances in paralleled IGBTs within power modules are comprehensively analysed. The findings and tools resulting from this research will be useful for various sectors in the power electronics industry. The IGBT and power diode characterisations as well as the power loss models will assist the power device designers in the development of optimised devices for future applications. Power device selection for resonant applications will be made accurate and easier by including the characteristic curves or equations within published datasheets. Furthermore optimisation of the parasitics in the power modules will be made possible for specific applications. This work can ultimately lead to the availability of application specific IGBTs and IGBT power modules for resonant switching applications.

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