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(2023)ThesisDue to the unique photovoltaic properties and ease of fabrication, organic-inorganic halide perovskites have generated considerable research interest. The perovskite solar cell can be applied to many applications, by tuning the bandgap. Inter of Things (IoT) devices and tandem solar cell applications, in particular, have been required for the wide bandgap perovskite solar cells. However, wide bandgap perovskite solar cells have band alignment mismatch problems, leading to charge recombination at the interface of perovskite, resulting in encouraging low device performance and decrease device stability. The first part of this thesis includes the study of the structure and working mechanism of perovskite solar cells. In addition, the defect of the perovskite was explained about how the majority of defects formed. This is caused by shallow defect energies within the bandgap, low density of deep traps, and low trap-charge interaction cross-sections which are occurred during the interaction between traps and charges. After that, the explanation of the reason how wide bandgap is applied for the indoor application. There is previous work on the tuning of the band alignment between perovskite and hole transfer layer which improved the efficiency of hole transfer, resulting in high device performance under the low light intensity condition. Lastly, the experiment of the thesis is focused on the address of the band alignment mismatch by adding two dimensional (2D) BA2PbBr4 perovskite layer for the tunnelling effect between the electron transport layer (ETL) and perovskite layer. The tunnelling layer of 2D perovskite improved the 3D perovskite crystal quality and charge transport from the 3D perovskite to ETL. As a result, the power conversion efficiency under the 200 lux white light emitting diodes (LED) light for the IoT devices was 43.70% with around 1 V of open circuit voltage and improved the device stability under the 1000 lux of white LED up to 1200 hrs.
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(2022)ThesisMultijunction solar cells based on silicon are predicted to achieve an efficiency of 40-45% for a top cell with a band gap of 1.6-1.9 eV. However, there are currently no known materials with suitable band gaps able to deliver high efficiencies. Two classes of materials that have been proposed for top cells are alloys of CuGaSe2 and alloyed oxide perovskites. CuGaSe2 has a suitable band gap (1.68 eV) for a top cell on silicon, but the maximum efficiency achieved is only 11%, while that of the closely-related CuInGaSe2 (band gap 1.14 eV) is 23.35%. The low efficiency of CuGaSe2 has been attributed to anti-site defects. Therefore, suppressing this defect formation is critical to achieving higher efficiencies. On the other hand, most oxide perovskites have band gaps that are too high (>2 eV) to be used as top cells on silicon, hence strategies such as alloying are required to lower their band gaps. In this work, the effects of alloying CuGaSe2 with Ag, Na, K, Al, In, La and S were investigated using Density Functional Theory (DFT) calculations. The band gaps of the alloyed compounds and formation energies of anti-site defects were calculated to find alloying elements that can increase the defect formation energy but maintain the band gap. CuGaSe2 alloyed with Al at 50at% showed the highest increase (compared to unalloyed CuGaSe2) in the defect formation energy (by ~0.20 eV) followed by Na (~0.15 eV) and S (~0.10 eV), both at 50at%. However, the band gap of the Al alloy (~2.15 eV) is too high for a top cell, while those of Na (~1.95 eV) and S (~1.91 eV) are slightly above the upper limit. Thus, alloying with these elements is not an ideal route towards significantly increasing the formation energy of anti-site defects while maintaining the band gap of CuGaSe2. However, some of the factors that influence the defect formation energy are identified, potentially leading to design rules for future work. Defect formation energies were found to be higher in structures with more positively charged Ga and negatively charged Se atoms. Analysis of bond lengths revealed a positive correlation between shorter Ga and Se bonds and higher defect formation energies. Band gaps of various alloyed oxide perovskites were calculated using DFT. BiFeO3 was alloyed with Y and Sb; LaFeO3 with Cr and Sb and YFeO3 with Bi and Sb. YFeO3 alloyed with Sb at 50at%, was found to have a band gap of 1.4-2.1 eV (depending on the basis set used) which is in the range for a top cell.
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(2023)ThesisHollow fibre (HF) membrane modules implemented in submerged membrane bioreactors (MBR) and pressurised applications have been widely accepted for both wastewater treatment and polishing wastewater treatment plant (WWTP) effluents. Further innovations in membrane technologies and wastewater treatment market competitiveness, however, are restricted by high manufacturing and operational costs, where a trade-off exists between membrane system design and filtration performance. In the current work, the effects of HF lengths, physical characteristics and system fouling mitigation techniques were investigated to further optimize filtration performance. The following experimental approaches were considered, (1) small-scale filtration experiments with various HF membrane lengths and fibre dimensions, (2) the development of theoretical filtration models and the assessment of filtration simulations, and (3) pilot-scale filtration performance of prototype large-scale membrane modules in wastewater. Two mathematical models for constant TMP filtration using dead-end HF membranes were developed using firstly the Darcy friction factor, and secondly, the Hagen–Poiseuille model. The models allowed for the overall theoretical lumen pressure drop values, local flux distributions and overall filtration performance to be extensively studied. Laboratory-scale filtration experiments using HF membranes of different lengths (0.5 – 2.0 m) were undertaken with the objective of demonstrating the influence of lumen pressure drop in overall filtration performance. Though greater permeate volumes were obtained when using modules prepared with longer HF membranes, such systems experienced greater lumen pressure loss. These losses reduced the operating TMPs effectiveness, resulting in greater non-uniformity in local fluxes across the length of the HF membranes. The magnitude of losses and degree of non-uniformity in such longer systems were extensively studied, allowing for the identification of effective loss reduction techniques, such as the incorporation of HF membranes with larger inner diameters (ID) in the membrane modules. Pilot scale investigations were undertaken to evaluate the influence of HF length on overall performance in real wastewater feeds. Prototype full-scale modules were prepared with HF membrane of different lengths (1.6 – 2.0 m) and ID. Longer modules demonstrated greater filtration performance as the influence of increased lumen pressure drop due to longer fibre lengths was effectively offset by the enhanced fibre dimensions. Overall, the results presented in this study reveal that a significant interplay exists between module design (including length, packing density, slack, and fibre size), filtration process design (feedwater quality, biomass concentration, aeration rate, aeration/shear efficiency) and the critical flux (of threshold flux) conditions. In conclusion, the incorporation of longer length HF membranes in pressurised and submerged MBR modules has been proven to be a promising innovation which offers enhanced filtration capabilities.