In the global context of increasing oil prices and public concern regarding the safety of nuclear plants, renewable forms of energy are called upon to play a major role in tomorrow’s energy market. Among the various forms of renewable energies, solar power holds the greatest potential for development. Despite the constant improvement of photovoltaic technologies over the past few year, these technologies are rapidly approaching the theoretic performance limits. New ideas and materials are required to overcome this bottleneck and to take full advantage of solar power. With a band-gap energy spanning the full solar spectrum, and an absorption coefficient ten times higher than competing materials, indium gallium nitride alloys are amongst the most promising solar-cell materials. Nevertheless, fundamental issues related to the fabrication and doping of InGaN alloys still hamper the development of InGaN-based photovoltaics. In the present thesis, conducted within the framework of the ANR project NewPVonGlass, the growth of InGaN alloys suitable for photovoltaics using metalorganic vapor-phase epi- taxy (MOVPE) is studied. A combination of several cutting-edge characterization tools is employed to determine the fundamental mechanism that govern the growth of InGaN. Based on the results of this study, an innovative procedure that allows the growth of hig-quality InGaN epitaxial layers is demonstrated and is used for the fabrication of InGaN-based solar cells.

Octahedral, six co-ordinate ruthenium complexes containing acid substituted polypyridyl ligands have proved particularly successful as dyes for Dye-Sensitised Solar Cells (DSSCs); thus there have been hundreds, if not thousands of these types of complexes synthesised and studied. Ruthenium dyes are now incorporated into commercial DSSCs; yet there is limited understanding of the interactions between the dye and the liquid I-/I3- electrolyte, which can both facilitate and hinder the generation of current and voltage within the cell. Monodentate –NCS ligands incorporated into ruthenium dyes appear to interact strongly with I- and I2 within the electrolyte. The analogous –NCO containing dye has therefore been synthesised to study the effect of the chalcogen atom on these interactions. Substituting –NCS for -NCO resulted in a significant change in the spectroscopic and electronic properties of the molecular dye, with destabilisation of the HOMO of the dye causing a red shift in dye absorption. Possibly due to this change in properties, the nature of the chalcogen atom was shown to have a significant impact on the performance of the dye in a DSSC. The effect of the nature of substituents on ancillary ligands of Ru(H2-dcbpy)(4,4`-Y2- bpy)(NCS)2 dyes on recombination across the TiO2 – electrolyte interface within the cell has been proven significant due to changes in the strength of binding of I2 to the substituent groups. However, few substituent groups have been investigated; therefore a series of halogenated dyes, where Y = Cl, Br, were synthesised. The effect of the nature of the halogen on dye recombination was significant, although the trends observed were not consistent with the reported data for iodine binding. Similar trends were observed for the analogous series, Ru(H2-dcbpy)(5,5’-Y2- bpy)(NCS)2 where Y = F, Cl, Br. By comparison of this series of dyes with the 4,4` dyes, it was discovered that the position of the substituent had a significant effect on the rate of recombination within the solar cell, as well as the electrochemical and spectroscopic properties of the dyes themselves. Such isomeric effects have not been previously reported. In the synthesis of these dyes, and in attempting the synthesis of five other ruthenium dyes, many barriers to efficient dye synthesis were discovered. Therefore, an investigation into the synthesis of ruthenium dyes has been conducted. By analysis of the breakdown products formed a number of avoidable side reactions, including decarboxylation and ruthenium catalysed nucleophilic substitution of the bpy ligands, were shown to occur. Problems associated with the high lability of the ruthenium centre at high temperatures have also been explored, and the use of UV/vis monitoring to aid optimisation of the reaction conditions was implemented. Thus, the development of two novel synthetic procedures allowed the synthesis of the dyes investigated during the course of this thesis.

published_or_final_version / abstract / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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It is generally accepted nowadays that a significant cost reduction in terrestrial solar cell application could be brought about by investigating alternative fabrication techniques for solar cells. It is believed that screen printing (or the so called thick film technique) is one such technique which promises a potentially low cost method for fabricating flexible, large area solar energy conversion cells. The active research on this technique started in 1976 in Japan. However, it was not until 1983, that wide interest developed when the Matsushita group in Japan reported an efficiency of 12.8% for their entirely screen printed CdS/CdTe solar cells. This was the highest reported efficiency for any thin film solar cell. However, the details of the fabrication processes of these cells were not reported and several scientific groups in the world started to explore this technique. The first published report was in 1985. In the last few years these groups have reported results on various aspects of this technique. Nevertheless there are still major parameters to be investigated. This thesis represents a concise reference for the application of the screen printing technique to solar cells. In the course of this study many new investigations have been made which supplement the previous work by other groups. Starting with a pure CdS powder with suitable grain size and distribution is a prerequisite for achieving the best morphological and electrical behaviour of screen printed layers of CdS. Careful paste mixing is of uppermost importance which can override any other parameters involved in the fabrication processes. It is essential to impose restricted sintering conditions for adequate utilization of the doping and fluxing function of the CdCl(_2) material. Standardization of the printing, preparation and sintering conditions involved in the fabrication processes were necessary to ensure reproducible CdS layers. Good quality screen printed layers were fabricated on soda lime substrates. The significance of other substrate materials for CdS preparation was also investigated and optimum substrate choice is suggested. The properties of the CdS screen printed layers were investigated by forming simple Schottky devices and more complicated heterojunction solar cells. Good rectification behaviour of the Schottky diodes was achieved. The CdS/CdTe solar cells revealed a wide spectral response. However, the photovoltaic behaviour was relatively poor largely due to the high resistivity of the CdTe part of the cell structure.

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Ruthenium (II) complexes are used extensively in photoelectrochemical and photophysical devices, such as Dye-Sensitized Solar Cells (DSSCs). The use of Cu(I) as a possible replacement for Ru(II) has to date had limited exploration, but has obvious advantages in terms of low cost and high abundance. However, Cu(I) typically undergoes conformational change from tetrahedral towards square planar upon oxidation or MLCT excitation, often leading to reduced stability, reduced electron transfer rates and reduced excited state lifetime, thus impairing useful function. Typically, steric constraints are used to prevent this; however these can often be synthetically intensive, involving multi-step and low yielding synthetic pathways. In this work, we explore “blocking” functionality using two different ligands combined with a range of bipyridyl ligands with varying substituent groups. The study has looked into the synthesis of heteroleptic Cu(I) complexes of the general formula: [Cu(POP)(bipyridyl)][BF4], where POP = bis[2-(diphenylphosphanyl)phenyl] ether, and [Cu(pmppE)(bipyridyl)], where pmppE = hydrazono pyrazol-5-thiones(one). The work presented in this thesis focuses on the synthesis, and subsequent photoelectrochemical and photophysical characterisation of Cu(I) complexes, yielding results that open new avenues for design of functional Cu(I) systems. Solar cell testing also revealed photovoltages comparable to those of existing Cu(I) DSSC sensitisers. An extensive spectroscopic study of [Cu(POP)(dmbpy)]+ and [Cu(POP)(tmbpy)]+ has revealed the latter to have the significantly larger quantum yield: 65 % and 4% respectively in PMMA at 300 K. A complimentary computational investigation was carried out in order to gain a better understanding of how structural rigidity affects emission properties.

Approved for public release; distribution is unlimited / Exploring semiconductor lifetime, reliability and performance is a never-ending science for today's modern electronics. One significant problem that affects all of these areas is radiation-induced damage. Making calculations to determine how semiconductor devices will hold up in radiation-harsh environments has to be achieved in order to determine system lifetime once placed in their operational capacity. Today's high-technology investments in such areas as satellite design, medical advances, military and commercial hardware, demand thorough understanding in radiation damage. Modeling semiconductor devices with computer-based simulation will provide a cost and time savings over a repetitive design and testing sequence. This thesis models and simulates an industry standard solar cell and a light emitting diode (LED), using the SILVACO ATLASTM computer-based program. Using this software, these simulations are generated based on known radiation-induced defects on gallium arsenide (GaAs) semiconductive devices derived from Deep Level Transient Spectroscopy (DLTS) studies. A comparison is then made with another radiation-induced damage prediction method, known as Non-Ionizing Energy Loss (NIEL), to see if the SILVACO ATLASTM models can be used as an alternative. / Lieutenant, United States Navy