http://ir.bdu.edu.et/handle/123456789/1840 2026-10-12T21:03:09Z 2026-10-12T21:03:09Z Tadesse, Baye http://ir.bdu.edu.et/handle/123456789/17335 2026-09-07T08:18:27Z 2026-06-29T00:00:00Z

A Comparative Analysis of A Three-Stage Cmos Op-Amp and A Two-Stage Cmos Op-Amp With Current Buffer Tadesse, Baye Over the past decades CMOS technology has been continuously scaling and resulted in sustained improvement in transistor speeds. However, achieving higher DC open-loop gain from the op-amps became challenging. Also, the transistor threshold voltages did not decrease at the same rate as the supply voltage. This thesis work presents a detailed comparative study on the DC open loop gain and voltage swing from two different CMOS topologies: A three-stage CMOS op-amp and a two-stage CMOS op amp with current buffer circuit. We have designed, simulated, and analyzed the two circuit topologies using industry standard Cadence OrCAD and Allegro SPB v17.4 software. The three-stage CMOS op-amp exhibited 156.55 dB, 428.67 MHz, and 670 as DC gain, GBW, and PM respectively. Whereas, the two-stage CMOS op-amp with current buffer circuit show 133.33 dB, 6.87 MHz, and 73.60 results of DC gain, GBW, and PM

2026-06-29T00:00:00Z Yekitwork, Abebe Temitmie http://ir.bdu.edu.et/handle/123456789/17103 2026-08-31T09:09:15Z 2026-01-01T00:00:00Z

Interface and Crystallization Engineering For Efficient Pb–Sn Mixed Perovskite Solar Cells Yekitwork, Abebe Temitmie Lead-tin (Pb-Sn) mixed perovskite solar cells (PSCs) enable bandgap tuning (1.1–1.4 eV), reduce toxicity, and offer great potential for efficient all-perovskite tandem devices. Although they exhibit promising optoelectronic properties, Pb–Sn mixed perovskites still face significant challenges that limit their overall device performance. One critical issue is the loss of open-circuit voltage (VOC) due to energy-level misalignment and defects arising from the coexistence of Pb and Sn within the perovskite structure. This energy-level mismatch hinders efficient charge extraction at the interfaces. At the same time, the mixed-metal-induced defects create local variations in the conduction and valence bands, leading to band tailing, bandgap fluctuations, and enhanced non-radiative recombination. Another issue is the rapid formation of the perovskite film, especially the Sn-rich phase, leading to tiny gaps where Pb and Sn atoms should be, thereby creating vacancies. Consequently, the resulting films exhibit poor interface between the perovskite and adjacent transport layers, further promoting recombination. Moreover, uncontrolled and poor crystallization of the perovskite film can create tiny gaps, pinholes, or grain boundaries, which act as shunt pathways. These key factors lead to leakage currents and a reduction in VOC, consequently decreasing overall device efficiency. This thesis aims to address these limitations by using interfacial engineering and controlling crystallization kinetics. In the first project, we improved the charge transport dynamics of the Pb-Sn perovskite (FASnI₃)₀.₆(MAPbI₃)₀.₄ using an interface engineering strategy by inserting an ultra-thin layer of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) between the commonly used hole transport layer (HTL) poly(3,4 ethylenedioxythiophene) polystyrene sulfonic acid (PEDOT: PSS) and the Pb-Sn mixed perovskite layer. This selective interfacial material effectively passivates surface defects, aligns more closely with the perovskite’s valence band, and thereby enhances charge extraction. As a result, the VOC was substantially improved, reaching 0.82 V, thereby increasing the power conversion efficiency (PCE) to 20.3%. ix In the second project, the HTL–perovskite interface was improved by optimizing the PTAA interlayer concentration in single-junction narrow-bandgap (NBG) perovskite solar cells. This work built upon the advancements of the first project to enhance the compatibility of NBG cells within all-perovskite tandem architectures. In these configurations, the NBG sub-cell functions as the bottom cell, where efficient charge extraction and suppressed interfacial recombination are critical for maximizing device performance. By integrating this high-performance NBG cell with a wide-bandgap (WBG) top cell, a monolithic all-perovskite tandem solar cell was realized. Specifically, Cs₀.₃FA₀.₆MA₀.₁Pb(I₀.₇Br₀.₃)₃, which has a bandgap of approximately 1.70 eV—the system was successfully translated into an all-perovskite tandem structure, achieving a VOC exceeding 2.0 V and a PCE of 25.1%. In the third project, a gas quenching (GQ) approach is introduced as an alternative to conventional anti-solvent (AS) quenching to achieve slower crystallization, smoother perovskite films with fewer pinholes, enhanced VOC, and improved PSC efficiency. The GQ approach resulted in slower crystallization and modified film quality. Devices fabricated using the GQ approach also demonstrated better performance with higher VOC values. Specifically, devices prepared with the AS method achieved a champion PCE of 18.3%, whereas those prepared with the GQ method achieved a higher PCE of 19.1%. This improvement in efficiency, combined with enhanced reproducibility and the simplicity and environmental friendliness of the GQ approach, underscores its advantages for processing NBG Pb-Sn mixed perovskite films. Therefore, the GQ method holds significant potential for application in the fabrication of NBG Pb-Sn PSCs.

2026-01-01T00:00:00Z Muhabaw, Assefa http://ir.bdu.edu.et/handle/123456789/17078 2026-08-26T08:10:14Z 2024-10-01T00:00:00Z

Optical Properties of Copper Nanoparticles: A Systematic Review Muhabaw, Assefa Scientific interest in nano particles of various shapes and morphology is growing proportionally. Besides the distinct size characteristic, properties of nano particles vary widely with alteration of structural and morphological features. Metallic nano particles’ characteristics are mostly determined by their shapes and sizes. These shape-dependent nano particles hold an impressive potential for to be used in numerous scientific technologies with promising applications. Due to their small size, metallic nano particles have special optical, magnetic, and electrical, strength, surface area, sensitivity, and stability features. Surprisingly, the phase change occurs when bulk materials are converted into nano particles, which means that materials that were previously non-magnetic become magnetic at the nanoscale. Because of their unique features, nanoscale matter is a separate form of matter from the solid, liquid, gaseous, and plasma states.Copper nano particles have drawn tremendous attention over other metals due to their unique and fascinating property known as Localized Surface Plasmon Resonance (LSPR). Mie theory can describe the optical properties of copper nano particles when the size of the nano particle is not too small.In this study, a critical overview the optical properties of copper nano particles from a theoretical approach (Mie scattering theory) and experimental investigations were intensively reviewed.Additionally, classifications, synthesis techniques, characterization methods of nano particles, and applications of copper nano particles in various fields are offered.

2024-10-01T00:00:00Z Mulugeta, Aregay http://ir.bdu.edu.et/handle/123456789/17063 2026-08-21T06:55:33Z 2026-01-01T00:00:00Z

Climatic Extremes under Climate Change Scenario Over Ethiopian River Catchments Mulugeta, Aregay This doctoral research presents a comprehensive analysis of hydroclimatic extremes across Ethiopia’s eight major river basins, addressing critical gaps in understanding compound climate risks in tropical highland regions. The study employs an innovative tripartite framework integrating ground-based observations from 17 WMO-certified meteorological stations and 28 hydrological gauging stations, high-resolution satellite data from TerraClimate (4 km resolution), and ECMWF ERA5 reanalysis data (0.25° resolution) spanning the period 1986-2023. Through the application of advanced statistical methodologies including standardized drought indices (SPI, SPEI, SSI), Mann-Kendall trend analysis, Sen’s slope estimator, and adapted crossing theory for heatwave characterization, the research reveals profound transformations in Ethiopia’s climate system. The investigation demonstrates a statistically significant intensification of meteorological, agricultural, and hydrological droughts across all basins, with frequency increases of 40-60% since the 1980s. The Abay, Awash, andWabi Shebelle basins emerge as particular hotspots, showing the most pronounced drying trends. Large-scale climate teleconnections, particularly ENSO and Indian Ocean Dipole phases, explain 68-85% of interannual drought variability, with basin-specific sensitivity patterns reflecting Ethiopia’s complex physio-graphic diversity. Flood analysis reveals increasing frequency and magnitude of extreme precipitation events, with flood magnitude increasing by 25-40% since the 1980s. The research documents accelerating drought-to-flood transitions, with soil moisture memory effects reducing transition times by 2.3 days per decade, fundamentally challenging conventional water management approaches that treat these phenomena discretely. Thermal extremes analysis identifies unprecedented warming trends, with heatwave frequency increasing by 150% since the 1990s. The research reveals an ”elevation paradox” whereby Ethiopian highlands are warming faster than lowlands, contradicting global patterns and climate model projections. This anomalous warming is driven by reduced cloud cover (Δalbedo = -0.11), expanding foehn winds, and decreasing atmospheric humidity at higher elevations. Climate projections using statistical downscaling of CMIP6 models under SSP scenarios indicate substantial intensification of all climate extremes through 2050, with meteorological and hydrological droughts projected to increase by 50-200%, flood magnitudes by 30-60%, and heatwave duration by 3-5 days per decade. The research particularly highlights growing risks of compound events, including concurrent drought-heatwave conditions and rapid transitions between hydrological extremes. Theoretical contributions include the development of an integrated framework for analyzing compound climate extremes, the identification of elevation-dependent climate responses that challenge conventional climate theory, and documentation of non-stationarity in teleconnection relationships. Methodological advances feature the novel adaptation of crossing theory for tropical highland heatwave analysis and implementation of a hybrid statistical downscaling approach optimized for compound extreme projection. Practical applications include specific recommendations for updating reservoir operation rules, adjusting agricultural calendars, implementing heat health warning systems, and revising infrastructure design standards. The basin-specific characterization of climate risks provides a scientific foundation for targeted adaptation strategies, with the research already informing improved early warning systems in the Awash Basin pilot project achieving 92% accuracy in drought onset detection. This research provides the most comprehensive assessment to date of hydroclimatic extremes in Ethiopia’s river basins, offering both scientific insights for climate resilience planning and methodological frameworks for similar assessments in other tropical highland regions. The findings underscore the urgent need for transformative adaptation approaches that address the interconnected nature of climate risks in one of the world’s most vulnerable regions.

2026-01-01T00:00:00Z