http://repository.aust.edu.ng/xmlui/handle/123456789/458 This collection contains theses of Theoretical Physics Students from 2016-2025 Mon, 27 Apr 2026 12:34:46 GMT 2026-04-27T12:34:46Z http://repository.aust.edu.ng/xmlui/handle/123456789/5177 Dynamics of Bose-Einstein Condensates in Optical Lattice Ratchet Potential Systems Kabir, Salihu Suraj This thesis aims to explore intriguing phenomena observed in Bose-Einstein condensates (BEC) subjected to asymmetric optical potentials. The phenomena of interest here is directed transport, wherein the BEC is made to travel in a specific direction without applying a net force. This phenomenon becomes possible in systems out of equilibrium when certain symmetries are broken. Three different scenarios have been analyzed: (i) Bose-Einstein Condensates with time-dependent interactions subjected to a kicked ratchet potential, (ii) Non-interacting Bose-Einstein Condensates in a non-Hermitian kicked ratchet potential and (iii) Non-interacting Bose-Einstein Condensates in a kicked ratchet potential whose phase is spatially modulated. In the first scenario, the role of atom-atom interaction on the resulting directed current is studied. A notable correlation between the kicking strength K and the interaction parameter ̃g has been uncovered. A critical boundary within the (K, g ̃) space distinguishes between quasi-periodicity and complete chaos, indicating that strong interactions lead to full chaos. Within the realm of full chaos, significant currents and current reversals, emerge, disrupting the symmetry of the current spectrum. Beyond the stability range where |g ̃|≤ 1, directed transport is no longer ensured. In the second scenario, the impact of non-Hermitian kicking on a cold atom exposed to an asymmetric ratchet potential was explored. This non-Hermiticity stems from dissipative interactions influenced by the environment, leading to either atom-gain or loss effects on the atom. It was realized in this study that non-Hermiticity can either impede or enhance the atom’s transport. Additionally, substantial atom-gain may induce reversals in the current direction. Quantum resonance notably emerges as a pivotal factor in dictating these outcomes. Finally, we considered the effect of a spatial modulation on the transport of this system. Before illustrating the influence of this phase, we observed that within the regimes where current reversals can occur, the probability of negative current peaks is consistently lower than that of positive peaks. Furthermore, both probabilities are symmetric around the value 0.5, an intriguing observation that warrants further investigation. With the inclusion of the phase, our calculations of the transport current revealed that transport can be optimized for faster dynamics. Higher current values were achieved compared to the zero-phase scenario. Notably, starting from a zero-phase regime where current reversals are present, such as the regime of full chaos, the phase not only enhances and rectifies the transport current but also allows for its complete suppression (current blockade). From the current landscapes plotted as functions of the spatial phase θ and the potential strength P, regions of optimal currents were identified at critical values of θ and P. Tue, 20 May 2025 00:00:00 GMT http://repository.aust.edu.ng/xmlui/handle/123456789/5177 2025-05-20T00:00:00Z http://repository.aust.edu.ng/xmlui/handle/123456789/5109 Effects of Pressure, Bending and Annealing Temperature on the Mechanical and Optoelectronic Properties of Perovskite Solar Cells Oyelade, Omolara Victoria The increasing needs for clean and sustainable energy stimulate the growing interest in photovoltaic (PV) technology using organic-inorganic hybrid perovskite materials. However, perovskite solar cells (PSCs) are faced with stability problem due to the presence of cracks or defects within the perovskite absorber and along the interfaces of the multilayered PSC structures. It is therefore important to improve on our understanding of their degradation pathways and mechanical stabilities. In this thesis, the effects of pressure, bending and processing annealing temperature on the mechanical and optoelectronic properties of perovskite solar cells are studied. First, the effects of pressure on photoconversion efficiencies of perovskite solar cells (PSCs) are studied using a combined experimental and analytical/computational technique. The results show that crystallization, absorbance, and the power conversion efficiencies of PSCs can be significantly improved by the application of pressure. This leads to the closing-up of voids and the corresponding increase in the interfacial surface contact lengths, which increase with increasing pressure. The observed improvement in the power conversion efficiencies (9.84 to 13.67%) was observed with increased pressure between 0 and 7 MPa, attributed largely to the effects of increased surface contact and the compaction and infiltration of the TiO2 layers with perovskite during the application of pressure. At higher pressure values (> 7 Mpa), the damage due to sink of the perovskite layers into the mesoporous layers results in reductions in the photoconversion efficiencies of PSCs. The understanding of the variations in the mechanical properties of organic-inorganic hybrid perovskites structures that are processed at different annealing conditions is then studied for ultimate device performance and robustness. We show that the temperature at which perovskite film is annealed affects the mechanical properties of the devices fabricated. The size dependence ii of hardness is due to the increase in the density of geometrically necessary dislocations (GNDs) with decreasing indentation size. The indentation size effects are characterized between the micron- and nanoscales by a bi-linear strain gradient plasticity (SGP) framework with source limited and established dislocation substructures. The measured microstructural length scales decrease with increasing annealing temperature to 130oC, after which it began to increase, causing films annealed beyond 130oC to have reduced strengths because the larger microstructural length scales correspond to larger dislocation spacings and weaker dislocation interactions. Perovskite solar devices annealed at temperatures above 130oC have poor performance. The results show that perovskite solar cell devices annealed at 130oC exhibit optimal performance and attractive combinations of mechanical properties. Finally, the underlying failure mechanisms associated with flexible perovskite solar cells (FPSCs) are elucidated for deformation and cracking under monotonic and cyclic bending. The mechanical robustness of the inverted flexible PSCs is increased with increasing fraction of polyethylene oxide (PEO) in the double-cation perovskite precursor, which promotes the grain size and passivates the defects of the film. The associated changes in the optical transmittance of the perovskite-PEO absorber and the PCEs of the multilayered FPSCs structures are elucidated under monotonic and cyclic bending. The failure mechanisms of the perovskite films for different radii of bending were observed using a scanning electron microscope before computing the interfacial fracture energies in the multilayer devices using finite element simulations. The failure mechanisms are then used to explain the degradation of the optoelectronic properties of flexible perovskite solar cells. Main Theses Wed, 05 Jan 2022 00:00:00 GMT http://repository.aust.edu.ng/xmlui/handle/123456789/5109 2022-01-05T00:00:00Z http://repository.aust.edu.ng/xmlui/handle/123456789/5108 New Frontiers for Solar Cells and Light Emitting Devices Asare, Joseph Solar energy, with its abundance and availability, would ultimately replace dwindling fossil fuel reserves in this new era of cleaner and more efficient energy as the world surges on to new technologies and horizons. This research investigates new frontiers for solar cells and light emitting devices: from organic solar cells and light emitting devices to policy. The effects of bending on the electrical, optical, structural and mechanical properties of flexible organic photovoltaic (OPV) cells were explored first. Bulk heterojunction organic solar cells were fabricated on Polyethylene terephthalate (PET) substrates using Poly-3-hexylthiophene: [6, 6]-phenyl-C61-butyric acid methyl ester (P3HT: PCBM) as the active layer and Poly (3, 4- ethylenedioxythiophene) Polystyrenesulfonate (PEDOT: PSS) as the hole injection layer. All the organic layers were deposited by the method of spin coating while the Al cathode was vacuum thermally evaporated. Electrical, optical and deformation characteristics were measured as layers were deposited. The relationship between the optoelectronic performance of the various device layers and the applied mechanical strains were analyzed. The effects of stress and strain on the current-voltage characteristics of the device and its failure were modeled using finite element analysis. With this knowledge that bending strains affect the optoelectronic and failure mechanisms in bendable/ flexible OPVs, a year-long survey assessment was conducted on a rural off-grid community in central Kenya to determine the different factors that affected the adoption of solar lanterns in the community. Impact on the people’s socio-economic, health, and education levels were also assessed. The lanterns were shown to have a 96% adoption rate in the sample community and this resulted in a 14.7% drop in annual lighting-related expenditures. Main Thesis Sun, 05 Feb 2017 00:00:00 GMT http://repository.aust.edu.ng/xmlui/handle/123456789/5108 2017-02-05T00:00:00Z http://repository.aust.edu.ng/xmlui/handle/123456789/5044 Optimization of Fabrication Processes of Non Halide Pb Precursor Based Perovskite Solar Cells Dahiru, Muhammad Sanni The organic-inorganic halide perovskite solar cells have continued to attract serious research interest since the first introduction as solar light absorber by Miyasaka and co-workers with a power conversion efficiency (PCE) of 3.8% in 2009 to the current value of over 25.2%. This outcome is due to the tremendous advantages of the perovskite materials ( e.g., as the tunable bandgap, direct bandgap, large optical absorption coefficient, high charge carrier mobilities, low exciton binding energy, and cost-effective solution process-able). Despite the successes recorded so far on lead halide perovskite solar cells the problem of toxicity and stability is still unresolved. In this thesis, novel approaches were used to optimised the fabrication processes of perovskite solar cells obtained from dehydrated lead acetate as source materials in other to improve efficiency and stability. The impact of solution concentration on the photovoltaic and the material properties of perovskite solar cells (PSCs) obtained from dehydrated Pb-acetate precursors was investigated. The optimal solution concentration was found to be 1.0 M which provide the best efficiency. Reproducibility is also an important problem in perovskite solar cells fabrication processes. In this study, we reported a low-temperature, one-step solution process to fabricate perovskite solar cells. These perovskite films were aged at 200 sec before thermal annealing at 90 oC for 5 min. Uniform perovskite films with fewer pinholes were obtained by this technique. The inverted planar (n-i-p) perovskite solar cell device resulted in a power conversion efficiency of 13%. A substantial finding was that the devices demonstrated high reproducibility. We also investigated the effect of annealing temperature on the optical and structural properties of the films and the photovoltaic performances of the fabricated solar cell devices. For the aforementioned process, a low-temperature, one-step solution process, the optimal temperature was achieved at 90 oC. In another study, the role of hafnium acetylacetonate buffer on the performance and stability of perovskite solar cells were investigated. The optimum concentration of the Hfaca was found to be 1.0 mg/ml resulting in power conversion efficiency (PCE) of 12.23% corresponding to over 30% improvement when compared to the control device (PCBM/Ag) without Hfaca which has a PCE of 8.89%. Hfaca as a buffer layer leads to superior stability retaining about 90% of its original values of the PCE after 15 days of storage in the glove box compared to the control device which retains 70% of the initial PCE value under the same storage. Fri, 12 Mar 2021 00:00:00 GMT http://repository.aust.edu.ng/xmlui/handle/123456789/5044 2021-03-12T00:00:00Z