Renewable energy has become the preferred source to replace the ever-depleting conventional energy sources. Photovoltaics, commonly known as solar cells, have been an attractive option in this regard, but their high cost and relatively low efficiency compared to conventional power generation techniques has prevented widespread application. Silicon has been the most widely used photovoltaic material for decades. Thin film solar cells have become an attractive alternative for the conventional bulk crystalline solar cells since they are cheaper to produce. However, the thin film counterpart of crystalline silicon, known as hydrogenated amorphous silicon (a-Si: H), has its drawbacks in photovoltaic applications. The use of excimer lasers for crystallisation of a-Si: H has been widely investigated for microelectronic applications. This thesis is concerned with excimer laser crystallisation of comparatively thick a-Si: H films for photovoltaic applications. Films thicker than 300 nm, which are necessary for adequate light harvesting, undergo partial melting upon excimer laser irradiation up to energy densities of 300 mJcm 2. Partially melted and solidified films result in stratified structures with different grain sized layers with an unconverted amorphous layer at the bottom. These stratified films are employed as light absorbers in different device configurations. Also, the resulting nanocrystalline films are considered to be less susceptible to light degradation due to fractional contributions from the amorphous phase. Surface morphology resulting from excimer laser crystallisation is identified as a low cost light trapping technique. Hydrogenation is found to increase conductivities of excimer laser crystallised silicon, which can be used to further improve the material. For 100 nm thick films an energy window is identified which results is nanopolycrystalline silicon with enhanced band gaps. This phenomenon is proposed to be due to a combination of a critical composition of hydrogen in the films along with confinement effects due to smaller crystallites. Simple Schottky barrier devices and p-i-n structures were utilised to investigate the behaviour of stratified absorber layers. Schottky barrier solar cells show promising photon conversion efficiencies, however, at low photon densities. Optimisation of device fabrication methodology is necessary in order to further improve the p-i-n solar cells which are affected by high series resistances and shunting problems

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Solar energy is a readily available alternative energy source that can be utilised to help reduce carbon emission; typically created by traditional, yet depleting, fossil fuel sources such as coal, gas and oil. For this reason, there has been a rapid increase in the installation and development of grid-connected Photovoltaic (PV) systems. Typically, grid-connected PV systems require a power electronic inverter to interface with the distribution grid. This thesis is concerned with the power quality interactions which arise between the distribution grid and the connected inverter. In particular, it considers the effect of grid operating conditions on the harmonic performance of the system. In the first part of the thesis, the mechanism of the interaction between the distribution grid and the grid-connected inverter system is investigated. Specific attention is given to researching the impact this interaction has on the current controller of the grid-connected inverter system. Initially, a simulation model is developed to investigate the effect that variation in the grid operating conditions has on the harmonic performance of the PV inverter current controller. Simulation results demonstrate that the magnitude and relative phase angle of the background grid voltage harmonics (with respect to the fundamental) has a direct impact on the inverter system output current harmonics. The second part of the thesis focuses on the experimental validation of these simulation results. An experimental grid-connected inverter system is described, and the harmonic performance is fully analysed and assessed. Results are shown to compare favourably with theoretical and simulation studies. During the experimental work, the inverter output low pass filter is found to interact with the grid impedance; creating a low impedance resonance whose frequency varies over time with respect to the grid impedance. For this reason, the final part of the thesis concentrates on the suppression of this resonance. A novel adaptive PI control scheme is devised, which is tuned via feedback from a resonance detection bandpass. The proposed control scheme is experimentally compared to a conventional PI control scheme and shown to offer superior harmonic and resonant characteristics.

The synthesis of morphologically good thin chalcogenide films via the two-stage route is a chemical challenge. The reactivity towards the chalcogen-bearing reactants of the different metals within the precursor film is a trade-off between thermodynamic driving force and kinetics of binary sulfide formation. In this work, CuSbS2 and Cu3BiS3 thin films were produced by conversion of stacked and co-electroplated metal precursor layers in the presence of elemental sulfur vapour. Ex-situ XRD and SEM/EDS analyses of RTP treated samples were employed to study the reaction sequence and create ‘‘Time-Temperature-Reaction’’ diagrams for the description of the reaction kinetics. Modified Pilling–Bedworth coefficients were introduced for the interpretation of the experimental results. The chalcogenizing conditions have a strong influence on the following aspects: (1) Extent of intermediate phase segregation and/or crystallite size (2) Thermodynamic (de)stabilization at low temperature (3) Thermodynamic (de)stabilization at high temperature The design of a successful synthetic route needs consideration of all these points, so that tailored choices of precursor film configuration and profiles of temperature and reactant partial pressure are made. The synthesis of single crystals of the system Cu-Zn-Sn-S via the Chemical Vapour Transport (CVT) with iodine was investigated. Current knowledge of CVT in multinary systems is limited. A computation of the thermochemistry of the system was performed with the intent to estimate the risk for compositionally incongruent mass transport. Experimental studies reveal no meaningful effect of the iodine pressure employed on the composition of the CZTS products. However, samples obtained under different I2 pressure showed different morphology and had slightly different unit cell sizes. Longitudinally isothermal treatments were carried out with the intent to form large size crystals. Under the investigated conditions, the experiments resulted in the formation of crystals with 2D predominance. A possible explanation for this phenomenon is proposed, based on considerations of the vessel’s thermal conductivity and decomposition/crystallization rate at the steady-state equilibrium.

Composite solar cells (CSCs) composed of a titania layer 'and a conjugated polymer thin film sandwiched between anode and cathode have attracted considerable interest as cost- effective alternatives to silicon-based photovoltaic (PV) devices. The titania layer not only affects the dissociation of excitons photogenerated in the conjugated polymer but also plays a crucial role in collecting injected electrons at the anode. As a result, the morphology and electron transport properties of the titania thin films have a great impact on the PV performance of the CSCs. In this project, I first investigated the microstructure of titania thin films fabricated . by doctor blading a colloidal paste (DB-HT), spin coating sol-gel solutions (SC-SG or SC-PP) and glancing angle deposition (GLAD). Random nanocrystalline Ti02 thin film were obtained by the DB-HT, SC-SG and SC-PP techniques while the pore size, porosity and surface roughness all decrease in this order. The power efficiencies of CSCs based on DB- HT Ti02 were reproducibly greater than 0.5% at 80mW/cm2, outperforming similar devices by other groups. Under the same illumination conditions, CSCs based on SC-SG and SC-PP Ti02 gave efficiencies of only 0.10% and 0.007%. These studies suggested that the titania surface roughness was a critical factor controlling Jsc in bi-Iayer CSCs. For the SC-PP titania layer, observations include that anatase nanocrystals with size 5~ 1 Onm exist in as-deposited layers, and that these can work as a blocking layer and have no detrimental effect on their PV performance. Titania layers with controllable columnar structures were fabricated also by glancing angle deposition. The effects of post-deposition annealing and deposition parameters on the microstructures and optical properties of these textured titania thin films were investigated. After preliminary optimization of annealing conditions and deposition angle, CSCs based on GLAD titania achieved an efficiency of 0.13%. A limiting factor for the inferior performance could be limited polymer penetration into the GLAD titania layer. In the second part of the project, photoconductivity (PC) in nanocrystalline Ti02 thin films was studied to understand the electron transport. It was found that PC evolution is heavily influenced by ambient atmosphere, sample temperature, and illumination conditions. These phenomena were interpreted in terms of electron generation, electron trapping and detrapping, hole trapping and detrapping and electron scavenging. I discovered that the 1 initial normalized decay rate increased with reduced light intensity. The experimental data was fitted using a model developed by a colleague to estimate electron mobility and concentration. Finally, it was found that electron transport in the Ti02 thin films at least in the initial stage of illumination controls the performance of Ti02-based CSCs, and that the quasi steady-state PC in Ti02 film appears to be a good indicator of Jsc value in the devices. !.