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Photovoltaic devices are receiving growing interest in both industry and research institutions due to the great demand for clean and renewable energy. Among all types of solar cells, cadmium sulfide (CdS) – cadmium telluride (CdTe) and cadmium sulfide (CdS) - copper indium diselenide (CuInSe2 or CIS) heterojunctions based thin film solar cells are of great interest due to their high efficiency and low cost. Further improvement in power conversion efficiency over the traditional device structure can be achieved by tuning the optical and electric properties of the light absorption layer as well as the window layer, utilizing nano template-assisted patterning and fabrication. In this dissertation, simulation and calculation of photocurrent generation in nanowires (NW) based heterojunction structure indicated that an estimated 25% improvement in power conversion efficiency can be expected in nano CdS – CdTe solar cells. Two novel device configurations for CdTe solar cells were developed where the traditional thin film CdS window layer was replaced by nanowires of CdS, embedded in aluminum oxide matrix or free standing. Nanostructured devices of the two designs were fabricated and a power conversion efficiency value of 6.5% was achieved. Porous anodic aluminum oxide (AAO) was used as the template for device fabrication. A technology for removing the residual aluminum oxide barrier layer between indium tin oxide (ITO) substrate and AAO pores was developed. Causes and remedies for the non-uniform barrier layer were investigated, and barrier-free AAO on ITO substrate were obtained. Also, vertically aligned nanowire arrays of CIS of controllable diameter and length were produced by simultaneously electrodepositing Cu, In and Se from an acid bath into the AAO pores formed on top of an aluminum sheet. Ohmic contact to CIS was formed by depositing a 100 nm thick gold layer on top and thus a Schottky diode device of the Au/CIS nanowires/Al configuration was obtained. Material properties of all these nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), absorption measurement. Current-voltage (I-V), capacitance-voltage (C-V) and low-temperature measurements were performed for all types of devices and the results were analyzed to advance the understanding of electron transport in these nano-structured devices.

In this work, organic solar cells of the configuration ITO/Pedot:PSS/CuPc/PTCBI/Al (Indium tin oxide/poly(3,4-ethylenedioxythiophene): polystyrene sulfonic acid/copper phthalocyanine/3,4,9,10-perylenetetracarboxylic bisbenzimidazole/aluminum) were investigated. A high open-circuit voltage (Voc) of 1.15 V was obtained when the PTCBI layer was 7 nm thick. Lower Voc values were observed for the same structure with silver, copper and gold electrodes instead of aluminum. However, short-circuit current density (Jsc) with these electrodes was much higher (4 mA/cm2) than in the case of aluminum (0.12 mA/cm2). Results were interpreted in terms of a modified CuPc/Al Schottky diode for the thin PTCBI case and a CuPc/PTCBI heterojunction for the thick PTCBI case. Also, the formation of a thin, protective aluminum oxide layer under the aluminum electrode was postulated. For devices with silver, copper and gold electrodes, absence of this protective layer was thought to be the cause of a relatively lower Voc and higher Jsc.

Indium tin oxide (ITO) and molybdenum are substrates of choice in the manufacture of the CdS-CIS photovoltaic cell, which is the base for the leading thin-film solar cell technology. Substantial advancement in this technology is expected if these devices can be made in nanoporous alumina (AAO) templates. The first step to this endeavor is to learn to form AAO templates on molybdenum and ITO substrates. This was accomplished, and the results are reported in this thesis. Starting substrates were glass, coated with either a thin molybdenum layer or a thin ITO layer. Aluminum was deposited on top of this conducting substrate. Oxalic acid was used as the electrolyte for anodization. In the case of molybdenum substrates, average pore diameter was 45 nm when an anodization voltage of 40 volts was used for approximately 46 minutes; current density was approximately 23 amps/sq. m. In the case of ITO substrates, pores of 45 nm diameter were obtained for approximately 20 minute anodization at 40 V; current density was 40 amps/sq. m; annealing of aluminum layer prior to anodization, at 550 oC (degrees Centigrade) for 90 minutes was needed to obtain good pores. A one micrometer thick CdS layer was electrodeposited inside the AAO pores on top of the ITO substrate. In preliminary experiments, CdS/Cu2S photovoltaic heterojunctions with an open circuit voltage of 242 mV were formed inside the nanopores.

Solar concentrating systems are often used to decrease the cost of solar energy by redirecting the incident sunlight from a relatively large area onto a photovoltaic cell of smaller area. In addition to the convergence characteristics of the concentrator, indices of refraction and reflectivities which are functions of wavelength can result in an illumination which varies both spatially and spectrally on the solar cell. Nonuniformity can also be induced by concentrator tracking error. The effects of such nonuniform illumination on solar cell performance are of interest.In this investigation, a model of a concentrator solar cell consisting of a network of preexisting one-dimensional models has been developed. This model is analyzed for three sample grid configurations for both spot-focusing and line-focusing concentrator applications.Ada computer programs have been created which, together with a few other pieces of readily available software, are capable of simulating the model. Sample simulations have been performed for line-focusing concentrator applications. These results are presented and discussed. / Department of Physics and Astronomy

Over the past decade, intensive academic and commercial interests have been paid on compounds possessing photochemical properties, namely for their preparation, chemical properties, high efficiency and potential low-cost. Compounds having intense photochemical properties gained great interest due to wide range of potential applications. The sensitizers are one of the key components for high power-conversion efficiency in the dye sensitized solar cells (DSSCs). They are the core components in the organic light-emitting devices (OLEDs) due to their ability to emit light with the wavelengths largely red- shifted from their absorption wavelength. Ruthenium based sensitizers have been tagged “molecular light switches” because, although the fluorescence of these complexes in aqueous solutions is negligible, it increases of greater than 10000 fold in the presence of DNA. Many polypyridyl and dipyrido phenazine ruthenium complexes have achieved high power conversion efficiencies and therefore are of practical interest. Several research groups stated that the dipyrido phenazine ligand may be thought of as comprising two components: a bipyridyl unit and a phenazine unit. These two subunits behave essentially separately, with many molecular orbitals being localised over only one subunit and a redox properties of central phenazine moiety in the dipyrido phenazine ligand are important for the photochemical applications. Therefore a phenazine ligand was selected as a model for the present investigation. The chemistry of phenazine ligand is mostly limited to the late transition metal and f - element complexes. Our laboratory has a rich backgroung in the aluminum and early transition metal chemistry. The aluminum chemistry and early transition metal chemistry are of great interest since aluminum and early transition metal complexes are environmentally friendlier and cheaper than the late transition metal compounds. Another drawback of the ruthenium-based sensitizers is the lack of absorption in the red region of the visible spectrum, and also low molar extinction coefficients. An essential requirement for efficient conversion of solar energy is the good spectral match of the sensitizer absorption to the emission spectrum of solar radiation. In this regard, the ruthenium sensitizers’ spectral response in the lower energy regions is not sufficient. The current project has three parts. In the first part we collected and reviewed known literature regarding the certain classes of non-innocent ligands containing the six-membered carbon- nitrogen heterocycles and regarding the ligands potentially important for the photochemical applications. We also reviewed all available to the data information about the complexes supported by the phenazine ligand. In the second part we have investigated interaction of alkylaluminum compounds and phenazine and observed reduction of phenazine accompanied by formation of dialuminum cage type compounds containing two formally mononegative phenazine ligand. The derivatization of phenazine has been also observed. It resulted in formation of compounds having a stable organic radical. In a third part of our project we have explored interaction of phenazine or thiophenazine with the alkylaluminum compounds and chromium dichloride. The reaction in the three component system resulted in reduction of phenazine ligand and lead to the heterometallic Cr(II) - aluminum complexes containing a formally dinegative phenazine or thiophenazine ligands. When a large excess of triethylaluminum was taken, reduction of phenazine and chromium has been observed leading to the heterometallic multinuclear Cr(I) - aluminum complex containing a formally dinegative phenazine ligands and two chromium atoms in one complex in the rare oxidation state one.

Organic molecules based photovoltaic cells were fabricated in an open laboratory conditions without the use of glove box or clean room. Conducting polymers such as P3HT and PCBM were used as a photo-active layer of the devices. We found significant difference in the performance of the devices by employing two laboratory conditions of the polymer solutions. Enhanced current density has been observed from P3HT/PCBM bulkheterojunction solar cell after diluting a well sonicated polymers solution with fresh chloroform solvent. As the result of such current surge in the devices the efficiency rose to more than double compared to those devices without dilution of the P3HT/PCBM solution. An average power conversion efficiency of 4.5% was then recorded from the new preparation condition. This is an encouraging development toward achieving low cost organic photovoltaic devices. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.