Ph.D. / The two-step growth process, involving the selenization and sulfurization of sputter deposited CuInGa alloys has been identified as a commercially viable method to produce large area Cu(In1-xGax)(Se1-ySy)2 absorber films for solar cell application. The success of this method is however limited by insufficient control over the lattice parameters and band gap of the compound due to phase segregation associated with non-uniform Ga and S incorporation. This study provides an approach to overcome this limitation by investigating the influence of process parameters on the structural features of the Cu(In1-xGax)(Se1-ySy)2 films. In this approach, films were partially selenized in optimum H2Se/Ar flow to produce composite alloys comprising of a mixture of binary selenides (InSe, CuSe and GaSe) and at least one group I-III-VI ternary alloy. The subsequent reaction step in H2S/Ar produced homogeneous Cu(In1-xGax)(Se1-ySy)2 films. The lattice constants of the resulting films varied linearly with an increase in the S/(S+Se) ratio in accordance with Vegard’s law. The Raman spectra of the films were characterized by the presence of the A1-Se mode near 180 cm-1 and a low intensity, A1-S mode around 290 cm-1. With an increase in the S/(S+Se) ratio of the films, the FWHM of the A1-Se mode increased and its frequency shifted linearly towards that of A1-S mode. A corresponding increase in the value of the Urbach energy, attributed to an increase in chalcopyrite crystal alloy disorder, was observed from the analysis of the transmission and reflectance data. 0.45 cm2 area devices with conversion efficiencies between 12% and 15%, were fabricated from absorber layers with the (112) x-ray diffraction peak position between 27.1°and 27.2°, corresponding to the S/(S+Se) ratio of about 0.18 to 0.20. The process scale up was demonstrated by the fabrication of large area, (30 x 40) cm2 modules, with conversion efficiencies of 10%.

M.Sc. / CuInSe2 (CIS) is considered to be one of the most promising candidates for high efficiency thin film solar cells. The reaction of metallic alloys to a reactive selenium atmosphere (H2Se/Ar or elemental Se vapour) is a promising growth technique to produce CIS thin films of high crystalline quality. However, up to now, the control of the final film quality has been critically influenced by the loss of material and subsequent formation of detrimental binary phases during the high temperature selenization stages. In this study, it is shown that this phenomenon is strongly related to the selenization temperature and, in particular, the ramping procedure followed to the final selenization temperature. Metallic alloys which were selenized in H2Se/Ar at 400°C or slowly heated in 20 minutes to temperatures around 400°C were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) to have nonuniform surface morphologies, highly defected 0.8-2 !lm sized grains and to contain Cuselenide binary phases. Energy dispersive X-ray spectroscopy (EDS) analysis confirmed the generally reported sharp increase in the Cu/In atomic ratio for these classes of samples. In contrast, rapid heating (in 2 minutes) of identical metallic alloys to temperatures above 400°C, resulted in uniform, dense films with low defect density 1 !lm sized grains void of any evidence of secondary phases. X-ray fluorescence (XRF) Kal,2 measurements of metallic alloys at different stages of selenization revealed no evidence of material losses. XRF depth profiles, however, explained this discrepancy by revealing a pronounced segregation of In towards the Mo back contact when the samples were selenized at 400°C, or slowly heated to temperatures around 400°C. This segregation was dramatically reduced in films rapidly heated and selenized at temperatures above 400°C. For the purpose of comparison, metallic alloys were also reacted to elemental Se vapour. The structural features (grain size and preferred orientation) ofthese films differed significantly from those selenized under similar conditions in H2Se/Ar. The results from this study, including photoluminescence (PL) measurements obtained from these films, were used to affect the fabrication of CIS absorbers with excellent material properties and solar cell devices with moderate conversion efficiencies.

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In order to investigate the influence of parasitic resistances, saturation current and diode ideality factor on the performance of photovoltaic devices, parameter extraction routines employing the standard iteration (SI) method and the particle swarm optimization (PSO) method were developed to extract the series resistance, shunt resistance, saturation current and ideality factor from the I-V characteristics of solar cells and PV modules. The well-known one- and two-diode models were used to describe the behavior of the I-V curve and the parameters of the models were determined by approximation and iteration techniques. The SI and the PSO extraction programmes were used to assess the suitability of the one- and the two-diode solar cell models in describing the I-V characteristics of mono- and multicrystalline silicon solar cells, CISS- and CIGSS-based solar cells. This exercise revealed that the two-diode model provides more information regarding the different processes involved in solar cell operation. Between the two methods developed, the PSO method is faster, yielded fitted curves with lower standard deviation of residuals and, therefore, was the preferred extraction method. The PSO method was then used to extract the device parameters of CISS-based solar cells with the CISS layer selenized under different selenization process conditions and CIGSS-based solar cells with varying i-ZnO layer thickness. For the CISS-based solar cells, the detrimental effect of parasitic resistances on device performance increased when the temperature and duration of the selenization process was increased. For the CIGSS-based devices, photogeneration improved with increasing i-ZnO layer thickness. At high forward bias, bulk recombination and/or tunneling-assisted recombination were the dominant processes affecting the I-V characteristics of the devices. v Lastly, device and performance parameters of mono-, multicrystalline silicon and CIS modules derived from I-V characteristics obtained under dark and illuminated conditions were analyzed considering the effects of temperature on the performance of the devices. Results showed that the effects of parasitic resistances are greater under illumination and, under outdoor conditions, the values further declined due to increasing temperature. The saturation current and ideality factor also increased under outdoor conditions which suggest increased recombination and, coupled with the adverse effects of parasitic resistances, these factors result in lower FF and lower maximum power point. Analysis performed on crystalline silicon and thin film devices utilized in this study revealed that parameter extraction from I-V characteristics of photovoltaic devices and, in particular, the implementation of PSO in solar cell device parameter extraction developed in this work is a useful characterization technique.

The objective of this study is to characterize and analyse defects in solar cell devices. Materials used to fabricate solar cells are not defects free and therefore, there is a need to investigate defects in cells. To investigate this, a topographical technique was developed and employed which uses a non-destructive methodology to analyse solar cells. A system was built which uses a technique based on a laser beam induced current (LBIC). LBIC technique involves focusing light on to a surface of a solar cell device in order to create a photo-generated current that can be measured in the external circuit for analyses. The advantage of this technique is that it allows parameter extraction. Parameters that can be extracted include short-circuit current, carrier lifetime and also the external and internal quantum efficiency of a solar cell. In this thesis, LBIC measurements in the form of picture maps are used to indicate the distribution of the localized beam induced current within solar cells. Areas with low minority carrier lifetime in solar cells are made visible by LBIC mapping. Surface reflection intensity measurements of cells can also be mapped using the LBIC system developed in this study. The system is also capable of mapping photo-generated current of a cell below and above room temperature. This thesis also presents an assessment procedure capable of assessing the device and performance parameters with reference to I-V measurements. The dark and illuminated I-V characteristics of solar cells were investigated. The illuminated I-V characteristics of solar cells were obtained using a defocused laser beam. Dark I-V measurements were performed by applying voltage across the cell in the dark and measuring a current through it. The device parameters which describe the behaviour of I-V characteristic were extracted from the I-V data using Particle Swarm Optimization (PSO) method based on a one-and two-diode solar cell models. Solar cells of different technologies were analysed, namely, single-crystalline (c-Si) and multicrystalline (mc-Si) silicon, Edge-defined Film-fed Growth Si (EFG-Si) and Cu(In,Ga)(Se,S)2 (CIGSS) thin film based cells. The LBIC results illustrated the effect of surface reflection features and material defects in the solar cell investigated. IQE at a wavelength of 660 nm were measured on these cells and the results in general emphasised the importance of correcting optical losses, i.e. reflection loss, when characterizing different types of defects. The agreement between the IQE measurements and I-V characteristics of a cell showed that the differences in crystal grains influence the performance of a mc-Si cell. The temperature-dependence of I-V characteristics of a CIGSS solar cell was investigated. The results showed that, for this material, the photo response is reduced at elevated temperatures. In addition to LBIC using a laser beam, solar spectral radiation was employed to obtained device performance parameters. The results emphasised the effect of grain boundaries as a recombination centres for photo-generated hole-pairs. Lastly, mesa diode characterizations of solar cells were investigated. Mesa diodes are achieved by etching down a solar cell so that the plateau regions are formed. Mesa diodes expose the p-n junction, and therefore mesa diode analysis provides a better way of determining and revealing the fundamental current conduction mechanism at the junction. Mesa diodes avoid possible edge effects. This study showed that mesa diodes can be used to characterize spatial non-uniformities in solar cells. The results obtained in this study indicate that LBIC is a useful tool for defect characterization in solar cells. Also LBIC complements other characterization techniques such as I-V characterization.

The objective of this dissertation is the development of the necessary processing steps needed to manufacture infrared photodiodes on InAs1-xSbx material. Preliminary surface preparation steps were performed on both InAs and InSb material, thus covering both possible extremes of the antimony mole fraction. The first experiments endeavoured to characterise the effect of several possible etchants with regards to etch rate, repeatability, limitations for photolithographic patterning and the resultant surface roughness. The etchants investigated include a lactic acid based etchant, a sulphuric acid based etchant, an acetic acid based etchant, an ammonium based etchant, a hydrochloric acid based etchant as well as an organic rinse procedure. These cleaning and etching steps were evaluated at several temperatures. Measurements were performed on an Alpha Step stylus profiler as well as an atomic force microscope. Metal-insulator-semiconductor capacitor devices were manufactured, on both InAs and InSb material, in order to investigate the effects of the above-mentioned etchants combined with surface passivation techniques in terms of surface state densities. Capacitance-versus-bias voltage measurements were done to determine the resultant surface state densities and to compare these to the surface state density of an untreated reference sample. The surface passivation techniques included KOH, Na2S as well as (NH4)2S anodisation. Auger electron spectroscopy measurements were done on InAs and InSb material in order to examine possible surface contamination due to the etchants as well as combinations of these etching and anodisation procedures. The extent of surface coverage by contaminants as well as by the intrinsic elements was measured. The results of the cleaning and etching as well as the surface passivation studies were used to manufacture photovoltaic infrared diodes on an MOCVD (metal oxide chemical vapour deposition) grown p-InAs0.91Sb0.09/i- InAs0.91Sb0.09/n-GaSb structure. Current-versus-voltage and electro-optical measurements were performed on the these diodes in order to evaluate the effect of sulphuric acid based etching combined with KOH, Na2S or (NH4)2S anodisation on the detector performance. The results of surface passivated structures were compared to those of an unpassivated reference detector.

Thesis (M. Sc. (Physics)) -- University of Limpopo, 2016 / Titania is a cheap and nontoxic polymorphic material of current interest for a variety of technological applications like in gas sensing and photovoltaic cells. Generally, TiO2, with a band gap of 3.2 eV, can only be excited by a small UV fraction of solar light, which accounts for only 3-5% of the solar energy. Various strategies have been pursued including doping with metallic elements (e.g. Fe) or nonmetallic elements (e.g. N) with the aim of shifting the absorption into the visible range. Since the properties and performance of devices, particularly for high-temperature applications, may be affected by the transformation from one phase to another, it is of significant interest to understand the conditions that affect phase transitions. In the present work TiO2 was doped with platinum (Pt), palladium (Pd), silver (Ag) and gold (Au) at doping levels of 5% weight, following the standard sol-gel methods. Structural characterization was carried out using scanning electron microscopy, Raman Spectroscopy and X-ray diffraction. Optical properties were studied using the Diffused reflectance Spectroscopy (DRS). Doping with Pt and Pd resulted in a lower anatase to rutile phase transformation temperature while doping with Au and Ag did not affect the transformation temperature. SEM micrographs show that the surface contains irregular shaped particles which are the aggregation of tiny crystals at lower temperature range, whereas at higher temperatures (900 °C), spheroids are observe.The reflectance spectra of the metal loaded TiO2 reveal substantial strong spectral cut-off starting from roughly 400 nm to the entire visible region (i.e. they show enhanced absorption).

Photovoltaic (PV) technology using organic solar cells have attracted much attention, as it is a simple and efficient way to convert solar energy into usable electricity. At present, bulk­heterojunction (BHJ) organic solar cells, which are based on conjugated p­type polymers and n­type fullerene derivatives, have been intensively investigated in both academia and industry. Organic and metallated conjugated small molecules (SM) represent an intriguing and promising class of materials. Atomic­thick 2D nanosheets have attracted tremendous attention recently because of their novel electronic structures and physical properties. This thesis describes the synthesis and characterization of some series of organic and metallated aryleneethynylenes and their applications in BHJ solar cells. To begin with, a brief overview on the background of organic solar cells (OSCs) and two­dimensional (2D) nanomaterials was presented in Chapter 1. In Chapter 2, a new series of multichromophoric small molecular systems of ruthenium(II)­bis(aryleneethynylene) compounds containing triphenylamine and/or thiophene as the donor and benzothiadiazole as the acceptor were designed and obtained by straightforward synthesis and purification procedures with reasonable yields. These ruthenium complexes absorb strongly in the visible region which are potentially attractive materials for photovoltaic cell applications. The best power conversion efficiency (PCE) of 0.66% was achieved for D1 with the open­circuit voltage (Voc) of 0.51 V, short­circuit current density (Jsc) of 4.24 mA cm­2 and fill factor (FF) of 0.31 under illumination of an AM 1.5 solar­cell simulator. Furthermore, in Chapter 3, a new series of small molecular systems of platinum­containing organometallic conjugated molecules containing different donating groups (such as thiophene, BDT, carbazole, and bithiazole), benzothiadiazole and/or dimesitylborane as the acceptors were successfully designed and obtained. Among all the BHJ devices based on these platinum complexes, PT5­based device, which we introduced strong donor group carbazole in the molecule, showed the highest PCE of 1.46% with high Voc of 0.70 V, Jsc of 6.17 mA cm­2 and FF of 0.33 at the optimized active layer thickness of 60 nm, which indicates that the photovoltaic performance can be significantly improved by introducing a strong D group in the molecule. Besides, a new series of organic small molecules M1­M16 of D­A­spacer­A­D structure were successfully designed and obtained. Intramolecular charge transfer (ICT) effect could be observed due to the strong electron­withdrawing units (such as benzothiadiazole, DPP, trizaole and dimesitylborane) and strong electron­donating units (such as triphenylamine, thiophene, BDT, carbazole, and bithiazole), and this effect between the acceptor and donor units causes low bandgap. By introducing strong oligothiophene donor group in the molecule M4, which showed the highest PCE of 3.68% among all the devices with high Voc of 0.95 V, Jsc of 7.76 mA cm­2 and FF of 0.44 at the optimized active layer thickness of 75 nm, which also indicates that the photovoltaic performance can be significantly improved by introducing a strong D group in the molecule. In Chapter 5, we designed and synthesized a new series of the “bottom­up” metal complex nanosheets: p­conjugated bis(dipyrrinato) metal complex nanosheets, including monolayer and multilayer nanosheets. AFM, IR, XPS and SEM analyses have been applied to study the morphologies, chemical state and size or nanostructure of the as­prepared nanosheets, and the results indicated that the “bottom­up” method is useful for the construction of photoresponsive and semiconductive nanosheets. This work is going to enlarge the futurity of the “bottom­up” nanosheet as next­generation nanomaterials. Finally, Chapters 6 and 7 present the concluding remarks and the experimental details of the work described in Chapters 2­5.

The proposed hybrid system generates AC power by combining solar and wind energy converted by a doubly-fed induction generator (DFIG). The DFIG, driven by a wind turbine, needs rotor excitation so the stator can supply a load or the grid. In a variable-speed wind energy system, the stator voltage and its frequency vary with wind speed, and in order to keep them constant, variable-voltage and variable-frequency rotor excitation is to be provided. A power conversion unit supplies the rotor, drawing power either from AC mains or from a PV panel depending on their availability. It consists of a multilevel inverter which gives lower harmonic distortion in the stator voltage. Maximum power point tracking techniques have been implemented for both wind and solar power. The complete hybrid renewable energy system is implemented in a PSIM-Simulink interface and the wind energy conversion portion is realized in hardware using dSPACE controller board.