Magister Scientiae - MSc (Physics)

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Charge transfer efficiency and optical properties of P3HT/PCBM spin coated thin films
(UWC, 2009) Van Heerden, Brian Abraham; Arendse, Christopher J.; Malgas, G.F.
The efficiency of organic photovoltaics is influenced by the structure of the polymer, the morphology of the film, the interfaces between the layers, the choice of electron acceptor material and the ratio between the electron acceptor material and the polymer. In this project, we have used regioregular poly (3-hexylthiophene) (rr-P3HT) as the electron donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a derivative of the C60 fullerene, as the acceptor material. Different weight-ratios of rr-P3HT to PCBM were prepared by stirring for several hours in a chloroform solution and subsequently spin coated onto crystalline silicon and transparent conductive oxide/glass substrates. The effect of the PCBM concentration on the photo-induced optical emission and absorption properties of rr-P3HT was investigated by photoluminescence and ultraviolet-visible spectroscopy, respectively. Changes in the structural properties, as a function of the weight-ratio, were probed by high-resolution transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. Results show that the structural integrity and crystallinity of rr-P3HT is compromised with the addition of excessive amounts of PCBM, which has a negative impact on the optical absorption of rr-P3HT and the photo-induced charge transfer mechanism between the rr-P3HT and PCBM. This work illustrates that blending rr-P3HT with an equal weight of PCBM results in an optimum configuration for improved performance in organic photovoltaic devices.
Deposition of silicon nanostructures by thermal chemical vapour deposition
(University of the Western Cape, 2015) Khanyile, Sfiso Zwelisha; Arendse, Christopher J.; Muller, T.F.G.
In this thesis we report on the deposition of silicon nanostructures using a 3-zone thermal chemical vapour deposition process at atmospheric pressure. Nickel and gold thin films, deposited by DC sputtering on crystalline silicon substrates, were used as the catalyst material required for vapour-solid-liquid growth mechanism of the Si nanostructures. The core of this work is centred around the effect of catalyst type, substrate temperature and the source-to-substrate distance on the structural and optical properties of the resultant Si nanostructures, using argon as the carrier gas and Si powder as the source. The morphology and internal structure of the Si nanostructures was probed by using high resolution scanning and transmission electron microscopy, respectively. The crystallinity was measured by x-ray diffraction and the high resolution transmission electron microscopy. For composition and elemental analysis, Fourier transform infrared spectroscopy was used to quantify the bonding configuration, while electron energy-loss spectroscopy in conjunction with electron dispersion spectroscopy reveals the composition. Photoluminescence and UV-visible spectroscopy was used to extract the emission and reflection properties of the synthesized nanostructures.
Dynamic variation of hydrogen dilution during hot-wire chemical vapour deposition of silicon thin films
(University of the Western Cape, 2013) Towfie, Nazley; Arendse, Christopher J.; Muller, T.F.G.; Malgas, G.
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
Hydrogenisation of metals
(University of the Western Cape, 2013) Ngwanakgagane, Sentsho Zelda; Topic, M.; Arendse, Christopher J.
Transition metals are a group of metals which are light in weight and have high hydrogen solubility. Their interaction with hydrogen is exorthermic and this phenomenon makes them “ideal” candidates for various applications of hydrogen storage systems. This explains why the phenomenon of hydrogen storage in Pd is used as a model for hydrogen storage systems because of the nature of absorption associated with it (like a sponge even at low temperatures). The hydrogenation process can be conducted at either room or high temperatures in a furnace under low pressure-low hydrogen gas concentration-short hydrogenation time (LP-LC-ST) and in intelligent gravimetric analyser under high pressurehigh hydrogen gas concentration-long hydrogenation time conditions. Most of the research on hydrogen storage sytems is based on gravimetric analysis of absorbed and desorbed hydrogen concentration. In this work, a comparison study of the hydrogen content in pure Pd, Pd-Pt coated systems, Pd-Pt alloys, commercially pure Ti and Ti-6Al-4V alloy determined by gravimetric methods and elastic recoil detection analysis (which is based on the detection of recoiled hydrogen after interaction with He+ ions) technique was investigated. The changes in the structural properties and the hydrogen content of the materials when exposed to a hydrogen gas environment for different durations at various system temperatures and pressures will be reported. These changes have an effect on the microstructure of CP-Ti and Ti-6Al-4V alloy and structural properties of all the hydrogenated materials. The results obtained from optical microscopy, scanning electron microscopy, x-ray diffraction, intelligent gravimetric analyser, digital balance, elastic recoil detection analysis and Vickers hardness test, show the following: it is found that hydrogenation of Pd at elevated temperatures (550 ˚C and 650 ˚C) does not yield hydrides under LP-LC-ST conditions. However, at room temperature the absorption of hydrogen occurred faster at the beginning of the process. Furthermore, the absorption of hydrogen increased with pressure where optimum absorption (0.67 wt. % hydrogen concentration) occurred under a system pressure of 2000 mbar. After pressure release, the remaining hydrogen content in the Pd sample was 0.6 wt. %. The Pd-Pt coated system provide hydrogen mobility at 550 and 650 ˚C where hydrides were formed under LP-LC-ST conditions. In addition to the decrease of hydrogen solubility in Pd-Pt alloys with an increase in Pt content, the probability of the alloys to achieve full saturation also decreases with an increase in Pt content under HP-HC-LT conditions. CP-Ti and Ti-6Al-4V alloy absorb substantial amount of hydrogen in the first hour of room temperature hydrogenation under LP-LC-ST conditions but hydrides were not formed. Therefore, under LP-LC-ST conditions at room temperature, Pd is able to store hydrogen in the form of hydrides whereas Ti and Ti-6Al-4V alloy could not. The 550 ˚C is the optimum temperature for hydrogenation of CP-Ti under LP-LC-ST conditions. The Ti- 6Al-4V alloy absorb optimum hydrogen at 650 ˚C under LP-LC-ST conditions. Consequently, the change of microhardness of CP-Ti and Ti-6Al-4V alloy was found to depend on hydrogenation temperature.
Photo-physical properties of lead-tin binary Perovskite thin films
(University of Western Cape, 2021) Mabiala, Floyd Lionel; Arendse, Christopher J.; Maaza, Malik
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.
Post-deposition doping of silicon nanowires
(University of the Western Cape, 2018) Slinger, Jane Bronwyn; Arendse, Christopher J.
Silicon nanowires (Si NWs) continue to demonstrate superior properties to their bulk counterparts, with respect to their morphological and electrical transport properties for the use in photovoltaic (PV) applications. The two most common and simplest approaches for Si NW fabrication are the bottom-up approach, namely, vapour-liquidsolid (VLS) growth and the top-down approach, namely, the metal-assisted chemical etching (MaCE) fabrication technique. Thermal diffusion of phosphorus (P) in Si is at present the primary method for emitter formation in Si solar cell processing. Most work done in the literature that is based on the diffusion doping of Si NWs has been carried out by means of VLS-grown Si NWs. Therefore, there is a lack of the understanding of the particular diffusion mechanism of applying the phosphorus dopant source to the MaCE-grown Si NWs.
Vertically aligned silicon nanowires synthesised by metal assisted chemical etching for photovoltaic applications
(University of the Western Cape, 2015) Ngqoloda, Siphelo; Cummings, F.R.; Arendse, Christopher J.; Motaung, D.E.
One-dimensional silicon nanowires (SiNWs) are promising building blocks for solar cells as they provide a controlled, vectorial transport route for photo-generated charge carriers in the device as well as providing anti-reflection for incoming light. Two major approaches are followed to synthesise SiNWs, namely the bottom-up approach during vapour-liquid-solid mechanism which employs chemical vapour deposition techniques. The other method is the top-down approach via metal assisted chemical etching (MaCE). MaCE provides a simple, inexpensive and repeatable process that yields radially and vertically aligned SiNWs in which the structure is easily controlled by changing the etching time or chemical concentrations. During MaCE synthesis, a crystalline silicon (c-Si) substrate covered with metal nanoparticles (catalyst) is etched in a diluted hydrofluoric acid solution containing oxidising agents. Since the first report on SiNWs synthesised via MaCE, various publications have described the growth during the MaCE process. However lingering questions around the role of the catalyst during formation, dispersion and the eventual diameter of the nanowires remain. In addition, very little information pertaining to the changes in crystallinity and atomic bonding properties of the nanowires post synthesis is known. As such, this study investigates the evolution of vertical SiNWs from deposited silver nanoparticles by means of in-depth electron microscopy analyses. Changes in crystallinity during synthesis of the nanowires are probed using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Deviations in the optical properties are quantified using optical reflectivity measurements by employing ultraviolet-visible (UV-Vis) spectroscopy, whereas the bonding configurations of the nanowires are probed by Raman and Fourier transforms infrared spectroscopy. Diameters of 50 – 200 nm vertical SiNWs were obtained from scanning electron micrographs and nanowires lengths linearly increased with etching time duration from about 130 nm after 30 seconds to over 15 μm after 80 minutes. No diameter modulations along nanowires axial direction and rough nanowires apexes were observed for nanowires obtained at longer etching times. These SiNWs remained crystalline as their bulk single crystalline Si wafers but had a thin amorphous layer on the surface, findings confirmed by TEM, XRD and Raman analysis. Nanowires were found to be partially passivated with oxygen with small traces of hydrogen termination, confirmed with infrared absorption studies. Finally, low optical reflection of less than 10% over visible range compared to an average of 30% for bulk Si were measured depicting an antireflective ability required in silicon solar cells.

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