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A new route to a well-defined block copolymer with alternating PEO-solubilising groups and fluorinated distyrylbenzene units was established. The Horner Wittig reaction was used as the polycondensation reaction. The non-fluorinated analogue of this block copolymer was prepared via the Wittig reaction. Both polymers were soluble in chloroform and free-standing films could be cast from solution. The position of the HOMO and LUMO energy levels of these two materials were determined by a combination of cyclic voltammetry, UV photoelectron spectroscopy and UV/Vis absorption spectroscopy. The presence of fluorine substituents on the distyrylbenzene unit had no influence on the HOMO-LUMO band-gap (3.0 eV). However, the position of these two energy levels relative to the vacuum level was shifted to higher energies (0.85 eV shift) in the case of the fluorinated block copolymer. The photoluminescence quantum efficiency of the fluorinated block copolymer was 17%, that of the non-fluorinated block copolymer was 34%. The former was used as the electron conducting layer in a light emitting diode with poly(p-phenylene vinylene) as the emissive layer. The latter was used as the emissive layer in light emitting diodes. Luminances over 2000 cd/m(^2) were observed for devices based on the non-fluorinated block copolymer using indium tin oxide as the anode and aluminium as the cathode. The luminescence efficiency of such devices was as high as 0.5 cd/A, corresponding to an internal quantum efficiency of 1.1%.Furthermore, an oligo(p-phenylene vinylene) was synthesised that contained two terminal fluorinated benzene rings and two central non-fluorinated benzene rings, all connected by vinylene bridges. This material aggregated in a 'brickwall' motif, where each molecule overlaps with two halves of molecules in the row above and below. The structure of this J aggregate is due to aryl-fluoroaryl-interactions and was demonstrated by X-ray crystal structure analysis.

In this study, silicon nanocrystals (NC) were synthesized in silicon dioxide matrix by ion implantation followed by high temperature annealing. Annealing temperature and duration were varied to study their effect on the nanocrystal formation and optical properties. Implantation of silicon ions was performed with different energy and dose depending on the oxide thickness on the silicon substrate. Before device fabrication, photoluminescence (PL) measurement was performed for each sample. From PL measurement it was observed that, PL emission depends on nanocrystal size determined by the parameters of implantation and annealing process. The peak position of PL emission was found to shifts toward higher wavelength when the dose of implanted Si increased. Two PL emission bands were observed in most cases. PL emission around 800 nm originated from Si NC in oxide matrix. Other emissions can be attributed to the luminescent defects in oxide or oxide/NC interface. In order to see electroluminescence properties Light Emitting Devices (LED) were fabricated by using metal oxide semiconductor structure, current-voltage (I-V) and electroluminescence (EL) measurements were conducted. I-V results revealed that, current passing through device depends on both implanted Si dose and annealing parameters. Current increases with increasing dose as one might expect due to the increased amount of defects in the matrix. The current however decreases with increasing annealing temperature and duration, which imply that, NC in oxide behave like a well controlled trap level for charge transport. From EL measurements, few differences were observed between EL and PL results. These differences can be attributed to the different excitation and emission mechanisms in PL and EL process. Upon comparision, EL emission was found to be inefficient due to the asymmetric charge injection from substrate and top contact. Peak position of EL emission was blue shifted with respect to PL one, and approached towards PL peak position as applied voltage increased. From the results of the EL measurements, EL emission mechanisms was attributed to tunneling of electron hole pairs from top contact and substrate to NC via oxide barrier.

To ensure that photovoltaics contributes significantly to future world energy production, the cost per watt of producing solar cells needs to be drastically reduced. The use of n-type silicon wafers in conjunction with industrial print technology has the potential to lower the cost per watt of solar cells. The use of n-type silicon is expected to allow the use of cheaper Cz substrates, without a corresponding loss in device efficiency. Printed metallisation is well utilised by the PV industry due to its low cost, yet there are few examples of its application to n-type solar cells. This thesis explores the use of n-type Cz silicon with printed metallisation and diffusion from printed sources in creating industrially applicable solar cell structures. The thesis begins with an overview of existing n-type solar cell structures, previous printed thick film metallisation research and previous research into printed dopant sources. A study of printed thick-film metallisation for n-type solar cells is then presented, which details the fabrication of boron doped p-type emitters followed by a survey of thick film Ag, Al, and Ag/Al inks for making contact to a p-emitter layer. Drawbacks of the various inks include high contact resistance, low metal conductivity or both. A cofire regime for front and rear contacts is established and an optimal emitter selected. A study of printed dopant pastes is presented, with an objective to achieve selective, heavily doped regions under metal contacts without significantly compromising minority carrier lifetime in solar cells. It is found that heavily doped regions are achievable with both boron and phosphorus, but that only phosphorus paste was capable of post-processing lifetime compatible with good efficiencies. The effect of belt furnace processing on n-type silicon wafers is explored, with large losses in implied voltage observed due to contamination of Si wafers from transition metals present in the belt furnace. Due to exposure to chromium in the belt furnace, no significant advantage in using n-type wafers instead of p-type is observed during the belt furnace processing step. Finally, working solar cells with efficiencies up to 16.1% are fabricated utilising knowledge acquired in the earlier chapters. The solar cells are characterised using several new photoluminescence techniques, including photoluminescence with current extraction to measure the quality of metal contacts. The work in this thesis indicates that n-type printed silicon solar cell technology shows potential for good performance at low cost.

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Magister Scientiae - MSc (Chemistry) / The increasing cost of fossil fuel energy production and its implication in environmental pollution and climate change created high demand for alternative and renewable sources of energy. This has led to great interest in research in the field of photovoltaic or solar cells Due to the abundance of sunlight, the technology is sustainable, non-polluting and can be implemented at places where power demand is needed, for example in rural areas. Solar cell devices that have been commercialized are currently based on silicon technology, involving the use of monocrystalline, polycrystalline and amorphous silicon. Although they produce highly efficient solar cells, the cost of Si solar cells is too high. Second generation solar cell materials such as cadmium telluride and third generation materials such as perovskites and organic polymers have been receiving much attention recently. However, they lack the efficiency of Si solar cells. This research proposes the development of high energy conservation photovoltaic cells from novel low-cost organosilicon polymers. The aim was to develop novel highly branched organosilane polymers such as poly(3-hexythiophene), polydi(thien-2-yl)dimethylsilane, poly(3-hexyl- [2,2'] bithiophenyl-5-yl)-dimethyl-thiophen-2yl-silane) as electron donors along with zinc oxide nanorod as the electron acceptor which were able to bring the efficiency of the resultant photovoltaic cell close to that of current Si solar cell. / 2021-08-31

Este trabalho teve como objetivo obter nanocompósitos de nanotubos de carbono de paredes múltiplas (NTCPMs) com TiO2, e caracterizá-los quanto a sua estrutura, características ópticas e atividade fotocatalítica. Os nanocompósitos foram obtidos a partir de NTCPMs comerciais (Baytubes®), e dois diferentes TiO2: um comercial (P25) e um obtido na síntese de TiO2 tendo tetra propóxido de titânio (TTP) como precursor. Foram utilizados dois diferentes sistemas líquidos para a obtenção dos nanocompósitos NTCPM-TiO2: um, em pH ácido e outro, em pH alcalino. Os nanocompósitos obtidos a partir do TTP foram posteriormente tratados termicamente a 400 °C, 500 °C, 600 °C e 700 °C para formação de fases cristalinas de TiO2. Os nanocompósitos foram investigados quanto a sua atividade fotocatalítica, empregando-os como catalisadores na degradação do corante orgânico alaranjado de metila, em solução aquosa, sob radiação ultravioleta. Os resultados foram associados a características da estrutura dos nanocompósitos, utilizando técnicas como difração de raios X, microscopia eletrônica de varredura, microscopia eletrônica de transmissão, espectroscopia Raman e espectroscopia por infravermelho e área superficial específica. A caracterização óptica foi obtida por espectroscopia fotoluminescente e espectroscopia por refletância difusa. A análise térmica foi empregada para quantificar a presença de NTCPMs no nanocompósito empregado como catalisador. O desempenho fotocatalítico dos nanocompósitos foi correlacionado com o efeito do pH dos sistemas líquidos empregados na sua obtenção, natureza da interação (química e/ou física) entre nanotubo de carbono e TiO2, fases presentes no TiO2, energia do gap óptico e presença de defeitos estruturais no TiO2. A maior eficiência na fotocatálise foi observada nos nanocompósitos NTCPMs-TiO2 obtidos a partir do TiO2 comercial, e nos obtidos a partir do precursor TTP tratado termicamente a 500 °C, ambos em meio ácido. Estes resultados puderam ser associados às menores energias de transição e nível de defeitos no TiO2 nesses nanocompósitos, quando comparados aos demais. / This study aimed to obtain nanocomposites from multi-walled carbon nanotubes (MWCNTs) with TiO2, and characterize them according to their structure, optical properties and photocatalytic activity. The nanocomposites were obtained from commercial MWCNTs (Baytubes®) and two different types of TiO2: a commercial one (P25) and one obtained by synthesizing TiO2 with titanium tetra propoxide (TTP) as a precursor. Two different fluid systems were used for obtaining the MWCNT-TiO2 nanocomposites: one with acid pH and the other with alkaline pH. The nanocomposites obtained from TTP were subsequently heat treated at 400 °C, 500 °C, 600 °C and 700 °C to form crystalline phases of TiO2. The nanocomposites were investigated for their photocatalytic activity, employing them as catalysts in the degradation of organic methyl orange dye in an aqueous solution under ultraviolet radiation. The results were associated with the characteristics of the nanocomposites’ structure, using techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, infrared spectroscopy, and specific surface area. Optical characterization was obtained by photoluminescence spectroscopy and diffuse reflectance spectroscopy. Thermal analysis was used to quantify the presence of MWCNTs in the nanocomposite employed as catalyst. The photocatalytic performance of the nanocomposites were correlated with the effect of the pH of the liquid systems employed for obtaining them, the nature of the interaction (chemical and/or physical) between the carbon nanotube and TiO2, the phases present in the TiO2, the optical energy gap and the presence of structural defects in TiO2. The highest photocatalytic efficiency was observed in the MWCNT-TiO2 nanocomposites obtained from commercial TiO2, and in those obtained from the TTP precursor heat treated at 500 °C, both in an acid medium. These results could be associated with the lower transition energy and level of defects in the TiO2 of these nanocomposites when compared to the other samples.

Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro / SILVEIRA JUNIOR, Raimundo Nonato. Propriedades fotoluminescentes da fase Sr3Al2O6 dopada com íons de Cr3+. 2008. 45f. Dissertação (Mestrado em Física) Instituto de Física Armando Dias Tavares, Universidade do Estado do Rio de Janeiro, 2008. Amostras foram preparadas pelo método de difusão a partir dos reagentes químicos SrCO3, Al2O3 e Cr2O3 em proporções estequiométricas. Medidas por difração de raios X mostraram que as amostras possuem 3 fases: Sr3Al2O6, Sr4Al2O7 e uma residual não identificada. Neste trabalho apresentamos medidas de fotoluminescência, excitação da fotoluminescência e absorção fotoacústica, a temperatura ambiente e a 4 K, da amostra Sr3Al2O6 dopada com 1% de íons de Cr3+ em substituição dos íons de Al3+. Os espectros de emissão apresentam três bandas largas com seus baricentros em torno de 760 nm, 841 nm e 675 nm. Estas três bandas de emissão observadas são oriundas de três centros diferentes de íons trivalentes de cromo, cada um deles associados a uma fase presente na amostra e todas as bandas correspondem a transição eletrônica 4T2(4F) → 4A2(4F). Além disso, observa-se a linha de zero-fônon 2E(2G) → 4A2(4F) situada em 745 nm para o caso de íons de Cr3+ na fase do Sr3Al2O6. A partir dos espectros de excitação e absorção fotoacústica, podemos determinar os parâmetros de campo cristalino (Dq) e Racah (B e C) para as três fases presentes na amostra. A razão Dq/B para os íons de Cr3+ na fase Sr3Al2O6, indica que a ação do campo cristalino em torno dos íons de Cr3+ é relativamente forte, como previsto na literatura para matrizes óxidas. Em todos os casos, os resultados mostram que os íons de Cr3+ ocupam sítios octaédricos nas três fases. / The mixtures were prepared by the conventional solid-state reaction with high purity SrCO3, Al2O3 and Cr2O3 powders. Measurements by X-ray diffraction show that the samples have three different phases: Sr3Al2O6, Sr4Al2O7 and a unknown residual one. In this work it is presented for the first time, the optical investigation of the Sr3Al2O6 doped with 1% of Cr3+ ions. This investigation is crowed out by emission, photoluminescence excitation and photoacoustic absorption spectroscopy at 4 K and room temperature. The emission spectra have three overlapped broad bands with their barycenters around 760 nm, 841 nm and 675 nm. These three bands are originated from three different emission centers of trivalent chromium ions, each one associated with a distinguished phase in this sample. Moreover, all bands are assigned to the phonon-arrested 4T2(4F) → 4A2(4F) electronic transition. In addition, for the case of Cr3+ ions in Sr3Al2O6 there is a R-line located at 745 nm and attributed to the 2E(2G) → 4A2(4F) zero-phonon transition. From the excitation and photoacoustic absorption spectra, we can determine the crystal-field (Dq) and Racah (B and C) parameters for the three existing phases in the sample. The ratio Dq/B for Cr3+ ions in the Sr3Al2O6 indicates that the action of the crystal-field on to Cr3+ ions is relatively strong.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Amostras policristalinas de Sr(Ga1-xCrx)2O4 com x = 0,01 foram estequiometricamente preparadas pela mistura dos materiais em pó SrCO3, Ga2O3 e Cr2O3. A estrutura cristalina da amostra dopada foi analisada pelas medidas de difração de raios-X. O padrão de difração revelou uma única fase relacionada a fase monoclínica do SrGa2O4. Os dados foram ajustados usando o Método de Rietveld para refinamento de estruturas e os parâmetros da rede foram determinados. A luminescência do íon de Cr3+ na rede do SrGa2O4 foi investigada pelas espectroscopias de excitação e emissão a temperatura ambiente, através das quais verificamos que os íons de Cr3+ estão localizados em dois sítios diferentes. Os espectros de emissão apresentam bandas largas associadas à transição eletrônica 4T2(4F) → 4A2(4F) para ambos os sítios. Estes resultados são analisados pela teoria de campo cristalino e o parâmetro de campo cristalino Dq e os parâmetros de Racah B e C são determinados pelas posições das bandas de excitação. A partir destes parâmetros determinamos um campo cristalino forte para ambos os sítios. Além disto, foram realizadas medidas de espectroscopia fotoacústica que confirmaram as transições identificadas e estimadas nos espectros de excitação. / Polycrystalline samples of Sr(Ga1-xCrx)2O4, with x = 0.01, were stoichiometrically prepared by mixing powder materials SrCO3, Ga2O3 and Cr2O3. The crystalline structure of the doped sample was analyzed by X-ray diffraction measurements. The diffraction patterns showed a single phase, which corresponds to the SrGa2O4 monoclinic phase. The data were fitted by using the Rietveld Method for structural refinements and the lattice parameters were determined. The luminescence of Cr3+ ions in the lattice of SrGa2O4 was investigated by excitation and emission spectroscopies at room temperature and the results show that the emission of Cr3+ ions comes from two different sites. In the emission spectra, we can observe that they present broadbands associated with the electronic transition 4T2(4F) → 4A2(4F) for both sites. These results are analyzed by crystal field theory and the crystal-field parameter Dq and the Racah parameters B and C are determined by the excitation band positions. From these parameters we determined a strong crystal field for both sites. Moreover, photoacoustic spectroscopy measurements were performed and they confirmed the transitions identified and estimated in the excitation spectra.

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