Browsing by Subject "Perovskite"
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Item Characterization of Transient Charged Defect Dynamics in Lead-Iodide Perovskite Solar Cells(2018-07) Jacobs, SamuelThe origin of current-voltage hysteresis in perovskite solar cells has been primarily attributed to charged mobile defects in the perovskite photon absorbing layer. The exact nature of these defects and their migrating dynamics under an electric field remain as outstanding questions. As a means to alter the ratio of mobile charge defects in the methyl ammonium lead iodide photon absorbing layer, perovskite was fabricated with varying lead iodide to methyl ammonium iodide ratios, namely stoichiometric, lead iodide excess, methyl ammonium iodide excess and iodide excess (from addition of lithium iodide). The combination of varying excess constituents in perovskite and analyzing the current response to step voltage increases/decreases provides a novel approach to investigate the defect and hysteresis dynamics in perovskite solar cells. The stoichiometric and methyl ammonium iodide excess devices, which displayed severe hysteresis, demonstrated slower current responses to step voltage increases and faster responses for step voltage decreases. Current response times were reduced for iodide and lead iodide excess devices, which displayed minimal current hysteresis during voltage scans. There was less discrepancy between step direction for these conditions. Activation energies extracted from temperature-dependent step voltage measurements align with the theorized activation energy of iodide vacancies from other works. The stoichiometric and methyl ammonium iodide excess devices display the lowest activation energy for step voltage decreases which leads to greater hysteresis by readily formable and mobile defects. The iodide excess device shows the largest activation energy for either step direction, further supporting the possibility that iodide vacancies are dominantly responsible for perovskite solar cell hysteresis. Our results reveal, for the first time, a quantifiable correlation among activation energy, current response time, and net current change due to step voltage input, which suggests careful fabrication and material selection is crucial to well-formed crystals whose defect activation energies are maximized to minimize hysteresis.Item Epitaxial Growth of thin film strontium cobaltite: a feasibility study.(2012-06) Gulden, TobiasIn this work we present a feasibility study of epitaxial growth of thin films of strontium cobaltite, SrCoO3-\delta. The properties of strontium doped lanthanum cobaltite, La1-xSrxCoO3, have been widely studied for dopant concentration x<0.5, but little work has been performed on the x=1 member of the series. The main issue is that this is not a thermodynamically preferable state and close to stochiometric SrCoO3 in polycrystalline samples can only be obtained under high pressures of oxygen (>10kbar) or by electrochemical oxidation. However, theoretical calculations predict a phase change with respect to strain in epitaxially grown samples, from ferromagnetic-metallic behaviour in the bulk state to insulating-ferroelectric-antiferromagnetic behaviour for strongly strained films. This provides strong motivation for epitaxial growth of SrCoO3-d films. In this work we will present a feasibility study by using the methods of high-pressure oxygen sputtering (typically 1.0-4.0mbar) on SrTiO3(001) and LaAlO3(001) substrates. As anticipated, the presence of oxygen vacancies is a severe problem, but also epitaxial stabilization of non-cubic phases, an unexpected issue, arises. These are found to grow in multiple orientations. Overall, the samples exhibit only weak or no ferromagnetism, even though bulk SrCoO3 is known to be a strong ferromagnet. Based on the results, we present an outline for suggested further research on this topic.Item Exploration of Carrier Transport and Novel Devices in Emerging Semiconductors(2022-08) Golani, PraffulWith scaling and performance of silicon-based transistors reaching their fundamental limits, a cross-disciplinary effort has gone into identification of novel material systems and device architectures that can outperform conventional solutions. Two systems that have shown good promise are van der Waals (vdW) semiconductors and semiconducting perovskite oxides. vdW semiconductors have already been used to demonstrate conventional MOSFETs and TFETs because of their atomically smooth surfaces and extremely thin body thicknesses which result in enhanced electrostatic gate control and improved scalability. On the other hand, semiconducting perovskite oxides have a large bandgap, low carrier effective masses and ability to form unique heterostructures making them interesting candidates for high-power high-frequency applications. The purpose of this thesis is to explore the electrical and material characterization results of electronic devices fabricated from Black Arsenic (vdW semiconductor) and SrSnO3 (perovskite oxide), by diving into their fundamental carrier transport studies. Exfoliated flakes of black arsenic (BAs) were used to fabricate MOSFETs which demonstrated ambipolar transport. The fabricated devices showed layer-dependent transport with high on/off ratios, high mobility and low off-current. Low temperature characterization revealed presence of low Schottky barrier height at the Ni/BAs interface while electron (hole) mobility vs temperature plot showed mobility was phonon limited. To show practical applications, ambipolarity of the devices was used to demonstrate an inverter and a frequency doubler as well. Ni/BAs interface was further explored, which revealed formation of an in-plane metallic contact to the semiconducting channel. Based upon this observation a self-aligned FET with lowered contact resistance is also proposed. Doped SrSnO3 had already been used to demonstrate MESFETs and RF FETs. However, SrSnO3 has low thermal conductivity which can result in degraded performance due to self-heating. An all-electrical method based on pulsed I-V characterization was performed to determine the thermal resistance and quantify the rise in channel temperature of two-terminal devices under electrical bias. TCAD simulations were performed to show that the rise in channel temperature was in close agreement with the experimental values. To further explore the carrier transport, electrical breakdown in undoped films was studied and contact optimization to doped SrSnO3 was also performed.Item Graphene Sensors and Perovskite Solar Cells for Water Detection(2020-08) Kim, JungyoonWater quality test is the first step for cleaning water which is a fundamental element for human health and the environment. The objective of this research is to develop very small, cheap, fast, accurate sensors to detect pollutants including phosphate, nitrate, mercury, and chloride in waters. This is a new testing and analysis technique, which can provide accurate sensing capability to assess the cleanness of waters at a very low cost. The proposed new technology is to manufacture graphene based sensors using the micro-manufacturing. Graphene is a monolayer of carbon atoms with outstanding electrical properties well studied material for a decade by many research groups. Since graphene sensitively responds to molecules in liquids, this property will enable the tiny sensors to detect pollutants in water with very high sensitivity and super short response time to pollutants. Even though graphene responds to the surroundings, it does not have the selectivity to the specific target. In this research, the selective membranes are synthesized and applied to the graphene based sensors to detect the target ions such as nitrate, phosphate, chloride, and mercury. The selective membranes are prepared with two different key materials including molecular imprinted polymer and ionophore. The sensor is characterized by a semiconductor analyzer, and the sensors are tested with several ion solutions to verify their selectivity. The detection limits of the sensor are 0.82, 0.26, 0.87 mg/L and 1.125 µg/L for nitrate, phosphate, chloride and mercury, selectively. In addition, the detection limit of nitrate is enhanced to 0.32 mg/L using the AAO substrate. Here, this research also includes developing perovskite solar cells as the power source of the sensors. Since solar energy is clean and independent, it is one of the important renewable energy resources. Silicon solar cells have already been commercialized and used to generate electricity in various fields because solar cells can directly generate electricity from photons, and they do not cause a problem to our environment as well. Among several types of solar cells, the perovskite solar cells have been studied by many research groups owing to low-cost fabrication, low fabrication temperature and high efficiency. This research includes the preparation of the materials and fabrication of flexible perovskite solar cells. We also characterize the surface morphology of the perovskite to check the grain size by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The efficiency of solar cells is measured by the solar simulator. We study the relationship between the grain size and the CVD process time and successfully demonstrate the performance of devices. The flexible solar cells show the power conversion efficiency of 7.6 % under the AM 1.5 G. As extended research, we have tried to find the proper hole transport layer (HTL) for the device and applied two HTLs, including PEDOT:PSS and PTAA to the devices.Item Solid Source Metal-Organic Molecular Beam Epitaxy Toward All-Epitaxial Ferroelectric Capacitors(2021-07) Nunn, WilliamGreat strides have been made in the area of thin film synthesis of complex materials. Among these, perovskite oxides have been identified as an immensely important multi-functional class due to exhibiting a large variety of materials properties, including ferroelectricity. Much progress has been made in the development of ferroelectric perovskite oxides but, unfortunately, the model electrode materials desired for many devices mostly contain difficult to work with or “stubborn” elements due to their ultra-low vapor pressures, in evaporation techniques, or low oxidation potentials, in general. Despite the construction of ferroelectric-metal heterostructures having a large impact on device fabrication, deposition of these electrode materials with atomic precision remains challenging in techniques like molecular beam epitaxy (MBE) and has not progressed much past using electron-beam evaporation.To deposit metals and metal oxides in a simpler, more cost-effective, and safer manner, a modification of MBE is developed for the first time here in this work and henceforth referred to as solid source metal-organic MBE. The growth of the simple metal Pt, binary oxide RuO2, and complex perovskite oxide SrRuO3 are shown using metal-organic source temperatures less than 100°C. Furthermore, the metals in these solid metal-organic precursors are in a pre-oxidized state, come bonded with an additional source of oxygen, are air stable, non-toxic, and can be used directly in-vacuum instead of requiring complicated external gas inlets. The growth results from this novel technique introduce it as another advancement in the long history of MBE. Additionally, with regards to the ferroelectric material, control over complex oxide stoichiometry has remained one of the largest issues within oxide MBE synthesis. Here, a different but rapidly expanding metal-organic-based MBE approach, hybrid MBE, is employed for the growth of ferroelectric and dielectric perovskite oxides with great control over the cation stoichiometry and, therefore, the structure and properties. The prototypical ferroelectric BaTiO3 is studied as well as the consequence of substituting Sn for Ti in the growth of the complete BaTiO3 – BaSnO3 alloy system for the first time in MBE. Together, these two approaches are utilized and developed for the goal of creating all-epitaxial in-situ-grown ferroelectric capacitors.Item Study of Heat Losses in Crystalline Silicon and Perovskite Solar Cells(2023-08) Tisha, Zakia TamannaEnergy from the sun is plentiful and sustainable, making it an excellent alternative to fossil fuels. Photovoltaic (PV) solar cells can directly convert this solar energy into electricity. However, PV solar cells face challenges in achieving high efficiency as some of the captured energy is lost as heat or through other means, reducing efficiency and performance. Researchers are constantly trying to improve the efficiency of solar cells. Silicon-based solar cells are widely used and have practical efficiency that keeps improving, reaching close to the theoretical limit of around 30%. One approach to increase the output of solar cells is converting the heat losses back into electricity, consequently boosting the overall efficiency of solar conversion. This heat recycling can be achieved by integrating photovoltaic (PV) devices with thermoelectric materials, which capture and recycle wasted heat. This thesis aims to lay the groundwork required for achieving this objective by studying the heat loss mechanisms and conducting evaluations of some of those mechanisms.This research focuses on understanding and categorizing the losses in solar cells, particularly the below bandgap energy and thermalization losses, which are responsible for more than half of the total losses. Two types of solar cells, crystalline silicon (c-Si) and CH3NH3PbI3 perovskite (C-P), are studied to analyze their loss characteristics.Item Thermal Transport In Nanostructured Materials By Ultrafast Pump-Probe Techniques(2019-08) Wu, XuewangThermal transport in nanostructured materials is crucial to nanotechnology-initiated applications such as electronics, solid-state energy conversion, and biomedical applications. At reduced size, the thermal properties of nanostructured materials can differ greatly from their bulk counterparts due to events such as the scattering of heat carriers. New experimental techniques, which can detect the nanoscale thermal properties, are needed to promote further study in this field. Pump-probe optical techniques, which utilize ultrafast laser pulses with a very short duration time and high-power objective lenses to achieve high temporal and spatial resolutions, make it feasible. Our studies were motivated to advance the understandings of thermal transport in nanostructured materials as functions of various structural parameters, utilizing pump-probe optical techniques and numerical/theoretical methods. In this dissertation, I have presented three research projects of thermal transport in different novel nanostructured materials including ultrathin films, nanoparticles, and nanocomposite. First, we extract the glass-like thermal conductivities of single-crystalline La0.5Sr0.5CoO2.9 (LSCO) epitaxial films with “built-in ordered oxygen vacancies”, through Time-domain thermoreflectance (TDTR) and linear extrapolation. Molecular dynamics simulation (MD) and Boltzmann Transport Equation (BTE) are applied to reveal the suppression mechanisms on thermal conductivity of LSCO due to structural parameters including the oxygen vacancies and orderings, film thickness, and substitution. Second, we study thermal transport across cetyltrimethylammonium bromide (CTAB) and polyethylene glycol (PEG) surfactants on gold nanorods (GNRs) in water solution, utilizing transient absorption (TA) technique. We notice a better thermal performance in PEG compared to that in CTAB on GNRs. Through a multiscale thermal modeling with the incorporation of MD simulation, we reveal such better thermal performance in PEG is due to water penetration and strong covalent bonding between GNR and PEG, which are not present in CTAB. Finally, we report thermal conductivities of direct-contact ZnO nanocrystal (NC) networks with infill materials of ZnO and/or Al2O3 by TDTR, as functions of contact radius between adjacent NCs, doping concentration, and the infill composition. A modified effective medium approximation model is applied to validate the experiment results and reveal the influences of various parameters on the thermal conductivity of this nanocomposite sample system.