3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and characterization of copper chalcogenide nanoparticles and their use in solution processed photovoltaics(2015) Kalenga, Pierre MubiayiPhotovoltaic cells offer a good alternative to the fossil fuels. Several approaches are being analysed in order to have solar cells that are capable to conquer the energy market all around the world. Quantum dots (QDs) have already proven features that can be taken into account to improve the properties of solar cells. Metal selenide nanoparticles (NPs) possess semiconducting behaviours that can vary with their structural and optical properties evolving from their synthesis. The reaction parameters such as the method, time, solvent and precursors can affect the growth and nucleation of particles and thus impose on the properties of the synthesized materials. The performance of solar cells made of the synthesized metal selenides will then be dependent upon the properties of the NPs used as active layer. Furthermore, the electrical current generation also depends on the structure of the deposited active layer and its interface with other films to be assembled for the device. The binary copper selenide, ternary copper indium selenide (CISe), quaternary copper indium gallium selenide (CIGSe) and quinary copper zinc tin sulphur selenide (CZTSSe) NPs were synthesized via conventional colloidal method (CCM) and microwave assisted method (MAM). The MAM has a particular interest as it is less time consuming and can easily be a large scale synthesis. Photovoltaic devices were fabricated from the synthesized materials as proof of concept for photovoltaic activities. The CCM was used to optimize various parameters for the synthesis of each type of the chalcogenide materials as this is easily controllable than the ones from the sealed vessel from MAM. The dependency of properties of all copper chalcogenide NPs on the time, precursor concentration, temperature and solvent of synthesis have been demonstrated via various characterization techniques including ultraviolet-visible-near infrared spectroscopy, photoluminescence spectroscopy, X-ray diffractometry and transmission electron microscopy. The binary copper selenide was first synthesized and considered as a template for evaluation of the use of copper chalcogenide materials in solar cells. Relatively smaller copper selenide NPs with average sizes of 4.5 and 6.0 nm were obtained from conventional colloidal and microwave assisted methods respectively. The sample yielded from the microwave assisted method possessed less polydispersed NPs. The later had better crystallinity in which prevailed a single cubic Cu2Se phase. To the best of our knowledge this is the first evidence of defined shapes and nearly single phase of small sized copper selenide NPs synthesized by mean of the MAM. The copper selenide particles synthesized via this method were used to fabricate a Schottky device. The conditions of copper selenide synthesis were optimized to 250 oC, 30 min of CCM synthesis using oleylamine (OLA) and a Cu/Se ratio of 1:1. Nearly hexagonal facets with blue-shifted absorption band edge of monodispersed NPs sizing 4-8 nm in diameter were obtained. The synthesized copper selenide showed better crystallinity with a single cubic Cu2Se phase. A Schottky device using MAM synthesized copper selenide NPs as the semiconducting layer was fabricated at room temperature. The diode effect was demonstrated with the electrical parameters such as the ideality factor, barrier height and the series resistances extracted from the experimental current-voltage data using the thermionic theory and Cheung’s modification. The thermionic theory resulted in the ideality factor of 4.35 and the barrier height of 0.895 eV whilst the Cheung’s method resulted in the ideality factor, barrier height and series resistance of 1.04, 2.59 10-3 eV and 0.870 Ω respectively. The ternary copper indium selenide NPs showed that the MAM allowed the formation of copper rich NPs alongside secondary products. The synthesis of the ternary sample via CCM was optimized using uncapped precursors (no TOP was added) in OLA at 220 oC for 30 min. The synthesized CuInSe2 NPs possessed a large blue-shift in their absorption band edges and emission peaks. The nearly stoichiometric CuInSe2 particles with diameter sizes of 5-9 nm were found in tetragonal crystalline orientation. The cyclic voltametry (CV) and the absorption spectra showed a large blue-shifted energy gap, about 0.95 eV, an increase from the bulk, proving the quantum confinement effects of synthesized copper indium selenide quantum dots. The CuInSe2 NPs were thus used as absorbing materials in the quantum dot sensitized solar cell devices (QDSSCs). The QDSSC devices were assembled via treatment of the titanium oxide, quantum dot layers and their interface. This was done by the treatment of copper indium selenide surface with mercapto-propionic acid (MPA) and ethanedithiol (EDT) during the deposition of the quantum dots onto TiO2 films. The MPA treatment did not reveal positive effects on copper indium selenide thin film and the assembled device under our optimized working conditions. However the use of EDT allowed the improvement of electron transport. The short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) obtained from the current-voltage (J-V) curves reached the values of 324 μA cm-2, 487 mV and 43% respectively, indicating that the investigated quantum dots possess electrical properties. For the quaternary copper indium gallium selenide, relatively small sized NPs were synthesized via CCM and MAM. The CCM synthesized CIGSe NPs were less agglomerated with a shorter tailing in absorption than those from MAM. The stoichiometric CuIn0.75Ga0.25Se2 showed less agglomerated and highly crystalline particles with a large blueshifted absorption band edge and a smaller full width at halth maximum (FWHM) of the emission peak compared to CuIn0.5Ga0.5Se2 and CuIn0.25Ga0.75Se2. The use of OLA as solvent of synthesis improved the growth and dispersivity of copper indium gallium selenide NPs. The particles with a large blue-shifted absorption band edge, a lattice of tetragonal phase, more monodispersed CIGSe and possessing an average size of 6.5 nm were obtained from CCM synthesis using OLA. The OLA as-synthesized CIGSe NPs were used in thin film for the assembly of QDSSC. The device exhibited electrical properties with the Jsc, Voc and FF of 168 μA cm-2, 162 mV and 33% respectively. The overall device performance was poor but may further be improved for further photovoltaic application. The quinary CZTSSe NPs possessed large blue-shifted absorption band edges of 450-460 nm than the bulk material (827 nm). The emission peak at 532 nm and similar FWHM of less than 50 nm were observed in samples from both CCM and MAM. More monodispersed crystals were obtained with both methods whilst the average particle sizes of 10 and 9 nm were yielded from MAM and CCM respectively. The nanoparticles crystallized in tetragonal lattices between copper zinc tin sulphide and copper zinc tin selenide crystals. However, the MAM gave more crystalline phases. The CV and the absorption spectra showed a blue shifted energy gap, about 0.21 eV increase from the buk which is located at 1.51 eV. This is indicative of the quantum confinement effects of synthesized NPs. The evidence of electrical properties was also shown in the QDSSCs fabricated using the MAM synthesized quinary QDs. This was done following the same treatments as for copper indium selenide devices. The Jsc, Voc and FF were found at the maxima of 258 μA cm-2, 395 mV and 38% respectively. The MPA and EDT treatments did not improve the device performance under our working conditions. Nevertheless, the electrical properties observed in the assembled device were indicative of promising efficient solar cells from synthesized CZTSSe NPs.Item Determination and characterization of sulphur in South African coal(2011-06-23) Kalenga, Pierre MubiayiSulphur determination is of priority in coal investigations due to its associated environmental pollution. There are generally various forms of sulphur in coal such as organic and inorganic forms both of which need to be characterized. However, it is the organic sulphur present in coal that is not well understood and studied. In this work, total and various forms of sulphur have been identified in raw coal samples from Camden, Majuba, Lethabo, Kriel, Duvha and Thuthuka power stations in South Africa. Organic sulphur compounds (2- methyl thiophene, 3-methyl thiophene, 2-ethyl thiophene and dibenzothiophene) were characterized from these coal samples in order to determine the quality of South African coal. Organic sulphur compounds were extracted from coal samples by means of ultrasonic bath. The extraction of organic sulphur compounds was first optimised by using various organic solvents such as dichloromethane, toluene and hexane followed by quantification using gas chromatography with a flame ionisation detector. The extraction time was also optimized from 0 to 75 minutes while spiked standard concentrations were varied from 1700 to 17000 mg kg-1 to control the process. Ultrasonic extracts were analyzed by GC equipped with SPB-1 Sulphur column and flame ionisation detector. Microwave assisted extraction was used to extract various forms of sulphur in coal. During sequential extraction, sulphate and pyrite sulphur were extracted from coal samples using hydrochloric acid and nitric acid respectively. Total organic or total sulphur and selected metals in coal were extracted by means of a mixture of concentrated hydrofluoric acid, hydrochloric acid and boric acid. Inductively coupled plasma-optical emission spectroscopy allowed the analysis of all total and other inorganic sulphur forms as well as selected metals from Microwave assisted extraction system after calibration set with standards. Total sulphur compounds and other sulphur forms were also determined using elemental analyzer for carbon, hydrogen, nitrogen and sulphur (CHNS). The standard calibration for CHNS determination was assessed by the use of Sulfamethazine, a certified reference material. 2-methyl thiophene, 3-methyl thiophene, 2-ethyl thiophene and dibenzothiophene were identified in coal samples. The organic sulphur compounds found in most coal samples were methyl thiophenes. Extraction time effect on the recovery of organic sulphur compounds showed that the recovery of organic sulphur investigated was up to 88% with 75 minutes ultrasonic bath extraction. Spiked concentration effect on the recovery of the target compounds was observed at 75 minutes of extraction and the recovery of extraction was generally in the range of 64.42-70.11%. However, an average of 66% recovery of target compounds was obtained from the highest spiked concentration. Duvha raw coal samples were found with more target organic sulphur compounds compared to other samples. Dibenzothiophene was found in highest concentration (15.5 mg kg-1) among the compounds identified. Products from preliminary pyrolysis of coal samples were identified by coupling pyrolysis furnace with GC-FID. The chromatograms showed four peaks identified in pyrolysis products from blank and spike coal samples. Those compounds proved to be the fragmentation products of target organic sulphur compounds. CHNS results showed that carbon, hydrogen, nitrogen and sulphur contents in coal had average values of 66.20%, 2.98%, 1.15% and 0.92% respectively. Those values agreed with figures reported by certified coal samples and confirmed the rank of the coal samples which is of the bituminous type. Microwave assisted extraction followed by Inductively coupled plasma-optical emission spectroscopy results showed that the total sulphur in coal samples had an average value of 8758 mg kg-1 and each sample showed that the total sulphur content is less than 20000 mg kg-1 (2%). Results also showed that the average value of sulphate content (600 mg kg-1) is lower than pyrite (4500 mg kg-1) and organic (3600 mg kg-1) forms. However organic and pyrite forms competed for their presence in coal samples. Microwave assisted extraction followed by Inductively coupled plasma-optical emission spectroscopy results of the various sulphur forms were in the range of values reported in certified material. Coal sulphur content is considered low from 0 to 20000 mg kg-1 and high when it is beyond 20000 mg kg-1. This means that South African coal generally has low sulphur content as referred to by the results obtained in this work. Sulphur compounds were found in coal samples as sulphate, pyrite and organic forms. Thiopene compounds found within the organic forms, moisture, ash and CHNS contents from this study showed that South African coal is generally bituminous type.