<p>When the amorphous silicon (a-Si) dangling bonds are bonded to hydrogen the concentration of the dangling bond is decreased. The resulting film is called hydrogenated amorphous silicon (a-Si:H). The reduction in the dangling bonds concentration improves the optoelectrical properties of the film. The improved properties of a-Si:H makes it possible to manufacture electronic devices including a solar cell. A solar cell device based on the hydrogenated amorphous silicon (a-Si:H) was fabricated using the Hot-Wire Chemical Vapour Deposition (HWCVD). When an n-i-p solar cell configuration is grown, the norm is that the p-doped layer is deposited from a mixture of silane (SiH4) gas with diborane (B2H6). The boron atoms from diborane bonds to the silicon atoms and because of the number of the valance electrons, the grown film becomes a p-type film. Aluminium is a group 3B element and has the same valence electrons as boron, hence it will also produce a p-type film when it bonds with silicon. In this study the p-doped layer is grown from the co-deposition of a-Si:H from SiH4 with aluminium evaporation resulting in a crystallized, p-doped thin film. When this thin film is used in the n-i-p cell configuration, the device shows photo-voltaic activity. The intrinsic layer and the n-type layers for the solar cell were grown from SiH4 gas and Phosphine (PH3) gas diluted in SiH4 respectively. The individual layers of the solar cell device were characterized for both their optical and electrical properties. This was done using a variety of experimental techniques. The analyzed results from the characterization techniques showed the films to be of device quality standard. The analysed results of the ptype layer grown from aluminium showed the film to be successfully crystallized and doped. A fully functional solar cell was fabricated from these layers and the cell showed photovoltaic activity.<br /> &nbsp / </p>

It has been debated that among all the renewable energy alternatives, only solar energy offers sufficient resources to meet energy demands. Silicon thin film solar cells are at the frontier of commercial solar technology. Hot wire chemical vapour deposition (HWCVD) is the technique of choice for silicon thin film deposition due to the absence of ion bombardment and its independence toward geometry or electromagnetic properties of the substrate, as seen by plasma enhanced chemical vapour deposition (PECVD). With the implementation of nanostructures in a multi-band gap tandem solar cell, considerable improvement has been achieved over the single junction solar cells. Defect assisted tunnelling processes at the junctions between individual solar cells in a tandem structure solar cell largely affect the efficiency of these solar cells. In this contribution, the investigation toward the improvement of silicon thin films for tandem solar cell application is initiated. This study reports on the effects of hydrogen dilution and deposition time on six silicon thin films deposited at six specific deposition regimes. The thin film properties are investigated via X-Ray diffraction analysis, Raman spectroscopy, Fourier transform infra-red spectroscopy, elastic recoil detection analysis, scanning and transmission electron microscopy and UV-visible spectroscopy. This investigation revealed the dominating etching effect of atomic hydrogen with the increase in hydrogen dilution and a bonded hydrogen content (CH) exceeding 10 at.% for each of the six thin films. The optically determined void volume fraction and static refractive index remain constant, for each thin film, with the change in CH. A new deposition procedure, utilising the deposition conditions of the previously investigated thin films, is performed by HWCVD to deposit two silicon thin films. This deposition procedure involved either increasing (protocol 1) or decreasing (protocol 2) hydrogen dilution during deposition. Structural and optical variation with depth was observed for the dynamically deposited silicon thin films, with nano-voids existing across the entire cross section and bond angle variations which are indicative of good structural order. The optical absorption curves differ for the two silicon thin films whereas the optical density remains constant for both. / >Magister Scientiae - MSc

The first comprehensive discovery of carbon nanotubes (CNTs) by S. Iijima in 1991 sparked a huge scientific interest in investigating its unique structure and attractive properties. A multitude of potential applications of CNTs in modern science and technology has been envisaged very early after their discovery. While a few applications are realized on a commercial scale, many are still constrained to laboratory investigations for a constant improvement to meet the service needs. Moreover, some studies are still aimed at further understanding the very growth mechanism. The work reported in this thesis deals with two main topics: The first part of the thesis was aimed at investigating the influence of various supported catalyst precursors on the growth morphology of multiwalled CNTs (MWCNTs) by low-temperature thermal CVD (chemical vapour deposition). The results were explained with the help of thermodynamic calculations of equilibrium phases formed during the reduction reactions inside the CVD reactor. Striking an equilibrium between the respective oxide phase and the metallic phase of the active catalyst species forms the basis for a vertically aligned growth of CNTs. A new class of supported catalysts based on manganese oxide (MnO) was developed. It has been shown that such a method of thermodynamic analysis paves the way for a theoretical assessment of CNT growth morphology. Second part of the thesis is devoted to the growth and field emission characterization of large-array MWCNTs on diverse substrate materials. One of the burgeoning areas of research involves the application of CNTs as electron field emitters in x-ray computed tomography or display technologies. Although several research groups investigated the field emission behaviour of CNTs on different substrate materials, those studies carry at least two important drawbacks: Firstly, a vast majority of the publications report the emission characteristics of individual CNT or an individual vertically aligned CNT (VACNT) bundle. By measuring so, the electric field shielding effects between various CNTs in an array would not be accounted for. Therefore, in this work, large-area emitters grown on stainless steel, copper, molybdenum and silicon substrates were subjected to emission measurements under similar pulsed operation mode, so that a direct comparison would be possible. Entangled CNTs on stainless steel showed a poor emission current density, but a long-term stable emission of 10 mA for more than 96 hours (4 days). The emission current density of CNTs on Cu and Mo was further low, but the threshold field (ETh) on the former was desirably low (~2 V µm-1). Secondly, the existing literature concerning emission characteristics of large-area CNT emitters reports either a high emission current density (Jmax) or a good long-term stability, but fails to demonstrate both simultaneously. It was shown in this work that VACNTs grown on a specific patterned Si substrate displayed an excellent combination of emission current density (5.78 A cm-2) along with a long-term stable emission of 40 mA current for ~730 hours at 10% duty cycle (effective emission time: 73 hours). Based on these results, a hypothesis emphasizing a new parameter, the ratio of the cumulative area of the CNTs to that of the substrate (ACNTs/Asubstrate), was put forth to explain the emission efficiency of large-area emitters. This hypothesis needs further verification by means of simulations. / Iijimas Publikation über Kohlenstoffnanoröhren (CNT) im Jahre 1991 löste ein großes wissenschaftliches Interesse daran aus, die einzigartige Struktur von CNTs und deren attraktive Eigenschaften zu untersuchen. Schon kurz nach der Entdeckung von CNTs wurde das große Potential von CNTs für die moderne Naturwissenschaft und vielfältige Anwendungen erkannt. Einige solcher Anwendungen wurden bereits verwirklicht, viele andere sind gegenwärtig noch im Entwicklungstadium. Auch die Wachstumsmechanismen von CNTs werden momentan weiter untersucht. Die hier vorgelegte Doktorarbeit behandelt zwei Hauptthemen: Der erste Teil widmet sich der Untersuchung des Wachstums von mehrwandigen Kohlenstoffnanoröhren (MWCNTs) durch thermische chemische Gasphasenabscheidung (CVD) bei niedrigen Temperaturen, wobei besonders der Einfluss verschiedener Katalysatormaterialien auf die Nanoröhren-Morphologie im Mittelpunkt steht. Die Ergebnisse können erklärt werden mit Hilfe von thermodynamischen Berechnungen der Gleichgewichtsphasen, die sich während der Reduktionsreaktionen im CVD-Reaktor bilden. Ein Wachstum von senkrecht ausgerichteten CNTs hängt ab von einem Gleichgewicht zwischen der Oxidphase und der metallischen Phase der aktiven Katalysatorkomponenten. Im Rahmen dieser Arbeit wurde eine neue Klasse von Zweikomponenten-Katalysatoren auf der Grundlage von Manganoxid (MnO) entwickelt. Es kann gezeigt werden, dass eine thermodynamische Analyse als Grundlage für eine theoretische Beurteilung des CNT-Wachstumsmechanismus dienen kann. Der zweite Teil der Doktorarbeit ist dem Wachstum von ausgedehnten MWCNT-Anordnungen sowie der Untersuchung der Feldemissionscharakteristik dieser Proben gewidmet, wobei verschiedene Substratmaterialien berücksichtigt wurden. Die Anwendung von CNTs als Elektronen-Feldemitter für Computertomographie und für Bildschirme ist ein attraktives und wachsendes Forschungsgebiet. Zwar wurde das Feldemissionsverhalten von CNTs auf verschiedenen Substraten bereits von mehreren Forschergruppen untersucht, jedoch sind mit diesen Studien Unzulänglichkeiten verbunden: Erstens behandelt die Mehrzahl der Publikationen die Emissionscharakteristik von individuellen CNTs oder von individuellen senkrecht ausgerichteten CNT-Bündeln. Dabei wurden allerdings elektrostatische Abschirmeffekte durch benachbarte CNTs nicht berücksichtigt. Daher wurden im Rahmen dieser Arbeit großflächige Emitter auf Edelstahl-, Kupfer-, Molybdän- und Siliziumsubstraten hergestellt und hinsichtlich ihrer Emissionscharakteristik im gepulsten Regime untersucht, so dass ein direkter Vergleich zwischen den Proben auf verschiedenen Substraten möglich ist. Gegenseitig umschlungene CNTs auf Edelstahl zeigten eine geringe Emissionsstromdichte, dafür war die Emission jedoch langzeitstabil mit 10 mA über mehr als 96 Stunden (vier Tage). Die Emissionsstromdichte von CNTs auf Cu und Mo war ebenfalls niedrig, allerdings im Falle von Cu-Substraten verbunden mit einem vorteilhaft niedrigen Feldschwellwert (ETh) von etwa 2 V µm-1. Zweitens berichtet die vorhandene Literatur über großflächige CNT-Emitter mit einer hohen Emissionsstromdichte (Jmax) oder einer guten Langzeitstabilität, beides gleichzeitig wird allerdings in diesen Arbeiten nicht gezeigt. In der vorliegenden Arbeit werden senkrecht ausgerichtete CNTs auf speziellen strukturierten Si-Substraten vorgestellt, die eine ausgezeichnete Kombination von Emissionsstromdichte (5,78 A/cm2) und einem über 730 Stunden langzeitstabilen Emissionsstrom von 40 mA aufweist, wobei die Arbeitsphase 10 % und damit die effektive Emissionszeit 73 Stunden beträgt. Auf Grundlage dieser Ergebnisse kann ein neuer Erklärungsansatz vorgestellt werden: Das Verhältnis von aufsummierter CNT-Fläche zur Substratfläche (ACNTs/Asubstrate) wird als neuer Parameter eingeführt und zur Erklärung der Emissionseffizienz von großflächigen Emittern verwendet. Diese Arbeitshypothese sollte durch Simulationsrechnungen verifiziert werden.

A method for determination of silver has been developed based on chemical vapour generation atomic absorption spectrometry (CVGAAS). Volatile species of silver in acidified medium were generated by the reduction of sodium tetrahydroborate / these species were sent to a flame-heated quartz tube atomizer (QTA) following isolation by using a gas-liquid separator. Flow injection (FI) was used for sample introduction. Optimization of parameters such as / concentrations of acid and NaBH4 concentration, flow rates of solutions and carrier gas were made. The influences of the well-known chemical modifier, Pd, and the effect of diethyldithiocarbomate (DDTC) were also examined. Interference study has been carried out for Ni(II), Co(II), Cu(II), Fe(III), Au(III), As(III), Pb(II), Se(IV) and Sn(II) . A detection limit of 7.5 ng mL-1 (n=11) was obtained with a 0.2 mL sample volume. With the FI-CVGAAS system 5.6 times sensitivity enhancement was achieved over flame atomic absorption spectrometry (FAAS).

>Magister Scientiae - MSc / This study reports on the effects of hydrogen dilution and deposition time on six silicon thin films deposited at six specific deposition regimes. The thin film properties are investigated via X-Ray diffraction analysis, raman spectroscopy, fourier transform infra-red spectroscopy, elastic recoil detection analysis, scanning and transmission electron microscopy and UV-visible spectroscopy. This investigation revealed the dominating etching effect of atomic hydrogen with the increase in hydrogen dilution and a bonded hydrogen content (CH) exceeding 10 at.% for each of the six thin films. The optically determined void volume fraction and static refractive index remain constant, for each thin film, with the change in CH

Indium nitride has many applications as a semiconductor. High quality films of indium nitride can be grown using Chemical Vapour Deposition (CVD) and Atomic Layer Deposition (ALD), but the availability of precursors and knowledge of the underlaying chemical reactions is limited. In this study the gas phase decomposition of a new indium precursor, N,N-dimethyl-N',N''-diisopropylguanidinate, has been investigated by quantum chemical methods for use in both CVD and ALD of indium nitride. The computations showed significant decomposition at around 250°C, 3 mbar indicating that the precursor is unstable at ALD conditions. A computational study of the surface chemical mechanism of the adsorption of trimethylindium and ammonia on indium nitride was also performed as a method development for other precursor surface mechanism studies. The results show, in accordance with experimental data, that the low reactivity of ammonia is a limiting factor in thermal ALD growth of indium nitride with trimethylindium and ammonia.

Philosophiae Doctor - PhD / When the amorphous silicon (a-Si) dangling bonds are bonded to hydrogen the concentration of the dangling bond is decreased. The resulting film is called hydrogenated amorphous silicon (a-Si:H). The reduction in the dangling bonds concentration improves the optoelectrical properties of the film. The improved properties of a-Si:H makes it possible to manufacture electronic devices including a solar cell. A solar cell device based on the hydrogenated amorphous silicon (a-Si:H) was fabricated using the Hot-Wire Chemical Vapour Deposition (HWCVD). When an n-i-p solar cell configuration is grown, the norm is that the p-doped layer is deposited from a mixture of silane (SiH4) gas with diborane (B2H6). The boron atoms from diborane bonds to the silicon atoms and because of the number of the valance electrons, the grown film becomes a p-type film. Aluminium is a group 3B element and has the same valence electrons as boron, hence it will also produce a p-type film when it bonds with silicon. In this study the p-doped layer is grown from the co-deposition of a-Si:H from SiH4 with aluminium evaporation resulting in a crystallized, p-doped thin film. When this thin film is used in the n-i-p cell configuration, the device shows photo-voltaic activity. The intrinsic layer and the n-type layers for the solar cell were grown from SiH4 gas and Phosphine (PH3) gas diluted in SiH4 respectively. The individual layers of the solar cell device were characterized for both their optical and electrical properties. This was done using a variety of experimental techniques. The analyzed results from the characterization techniques showed the films to be of device quality standard. The analysed results of the ptype layer grown from aluminium showed the film to be successfully crystallized and doped. A fully functional solar cell was fabricated from these layers and the cell showed photovoltaic activity. / South Africa

Philosophiae Doctor - PhD / When the amorphous silicon (a-Si) dangling bonds are bonded to hydrogen the concentration of the dangling bond is decreased. The resulting film is called hydrogenated amorphous silicon (a-Si:H). The reduction in the dangling bonds concentration improves the optoelectrical properties of the film. The improved properties of a-Si:H makes it possible to manufacture electronic devices including a solar cell.A solar cell device based on the hydrogenated amorphous silicon (a-Si:H) was fabricated using the Hot-Wire Chemical Vapour Deposition (HWCVD). When an n-i-p solar cell configuration is grown, the norm is that the p-doped layer is deposited from a mixture of silane (SiH4) gas with diborane (B2H6). The boron atoms from diborane bonds to the silicon atoms and because of the number of the valance electrons, the grown film becomes a p-type film. Aluminium is a group 3B element and has the same valence electrons as boron, hence it will also produce a p-type film when it bonds with silicon.In this study the p-doped layer is grown from the co-deposition of a-Si:H from SiH4 with aluminium evaporation resulting in a crystallized, p-doped thin film. When this thin film is used in the n-i-p cell configuration, the device shows photo-voltaic activity.The intrinsic layer and the n-type layers for the solar cell were grown from SiH4 gas and Phosphine (PH3) gas diluted in SiH4 respectively. The individual layers of the solar cell device were characterized for both their optical and electrical properties. This was done using a variety of experimental techniques. The analyzed results from the characterization techniques showed the films to be of device quality standard. The analysed results of the ptype layer grown from aluminium showed the film to be successfully crystallized and doped.A fully functional solar cell was fabricated from these layers and the cell showed photovoltaic activity.

Magister Scientiae - MSc / In this study SBA-15 mesoporous silica template was synthesized and used as a sacrificial template in the preparation of ordered mesoporous carbon material. A chemical vapour deposition (CVD) technique using LPG or alternatively sucrose, pyrolyzed upon a mesoporous Si matrix were used to produce nanostructured ordered mesoporous carbon (OMC) with graphitic character after removing the Si template. The sucrose method was found to be a suitable route for preparing OMC. The OMC was used as a conductive three dimensional porous support for depositing catalytic nanophase Pt metal. Deposition of Pt nanoparticles on OMC was accomplished using a CVD method with Pt(acac)2 as a precursor. The synthesized nano-composite materials were characterized by several techniques such as, HRTEM, HRSEM, EDS, XRD, BET, TGA, FT-IR and CV. / South Africa

>Magister Scientiae - MSc / Organic-inorganic lead-based perovskite has exhibited great performance in the past few years. However, the lead (Pb) embedded in those compounds is a significant drawback to further progress, due to its environmental toxicity. As an alternative, tin (Sn) based-perovskites have demonstrated promising results in terms of electrical and optical properties for photovoltaic devices, but the oxidation of tin ion- from stannous ion (Sn2+) to stannic ion (Sn4+) presents a problem in terms of performance and stability when exposed to ambient conditions. A more feasible approach may be in a Pb-Sn binary metal perovskite in pursuit of efficient, stable perovskite solar cells (PSCs) with reduced Pb-content, as compared to pure Pb- or Sn-based PSCs. Here, we report on the deposition of a Pb-Sn binary perovskite by sequential chemical vapor deposition.