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- Genre: Doctoral Dissertation
Description
The application of silicon thin films in solar cells has evolved from their use in amorphous silicon solar cells to their use as passivating and carrier-selective contacts in crystalline silicon solar cells. Their use as carrier-selective contacts has enabled crystalline silicon solar cell efficiencies above 26%, just 3% shy of the theoretical efficiency limit. The two cell architectures that have exceeded 26% are the silicon heterojunction and tunnel oxide passivating contact cell. These two cell architectures use two different forms of silicon thin films. In the case of the silicon heterojunction, the crystalline wafer is sandwiched between layers of intrinsic amorphous silicon, which acts as the passivation layer, and doped amorphous silicon, which acts as the carrier-selective layer. On the other hand, the tunnel oxide passivating contact cell uses a thin silicon oxide passivation layer and a doped polycrystalline silicon layer as the carrier-selective layer. Both cell structures have their distinct advantages and disadvantages when it comes to production. The processing of the silicon heterojunction relies on a low thermal budget and leads to high open-circuit voltages, but the cost of high-vacuum processing equipment presents a major hurdle for industrial scale production while the tunnel oxide passivating contact can be easily integrated into current industrial lines, yet it requires a higher thermal budgets and does not produce as high of an open-circuit voltage as the silicon heterojunction. This work focuses on using both forms of silicon thin films applied as passivating and carrier-selective contacts to crystalline silicon thin films.First, a thorough analysis of the series resistivity in silicon heterojunction solar cells is conducted. In particular, variations in the thickness and doping of the individual
ii
contact layers are performed to reveal their effect on the contact resistivity and in turn the total series resistivity of the cell. Second, a tunnel oxide passivated contact is created using a novel deposition method for the silicon oxide layer. A 21% efficient proof-of-concept device is presented demonstrating the potential of this deposition method. Finally, recommendations to further improve the efficiency of these cells is presented.
ContributorsWeigand, William (Author) / Holman, Zachary (Thesis advisor) / Yu, Zhengshan (Committee member) / Bertoni, Mariana (Committee member) / Tongay, Sefaattin (Committee member) / Arizona State University (Publisher)
Created2023
Description
Modulated nanostructures are short-range periodic composition fluctuations observed in metals, semiconductors, and ceramic alloys and have an important effect on the mechanical, electrical, and magnetic properties of the materials. Their presence is often attributed to spinodal decomposition. In the past, such modulations have been analyzed using X-ray diffraction, electron diffraction, and diffraction contrast imaging techniques. The investigations gave useful information about the spatial distribution of the modulations but nothing about the composition fluctuation of the modulated structure. In this study, the composition amplitude of the fluctuations was directly measured using STEM Imaging and atomic resolution spectroscopy in small unit cell Au-Pt and Cu-Ti alloys. With the development of field emission sources, aberration correctors, and ADF imaging for STEM microscopes, measuring the amplitude of the modulation directly and examining the diffused interface is possible.Au-Pt alloys, featuring a nearly symmetrical solid-state miscibility gap, and a Cu-Ti alloy with an asymmetrical metastable miscibility gap were chosen for investigation. Three Au-Pt alloys of different compositions were analyzed at a specific temperature for varying aging times. The study successfully employed atomic resolution energy dispersive X-ray spectroscopy (EDS) for Au-Pt alloys and atomic resolution electron energy loss spectroscopy (EELS) for Cu-Ti alloys to measure composition variation and diffused interfaces across modulations.
In Au-Pt alloys, it was shown that the wavelength, as well as the composition amplitude of the modulations, increases as the alloy is aged for a longer time. Non-uniform distribution of wavelength and composition amplitude of modulated structures was observed across the samples. Results shows competitive growth mechanisms occurring in nanostructures with a range of wavelengths/amplitudes. Composition of Au-rich and Pt-rich regions deviates significantly from equilibrium at the selected temperature, more aging is necessary to observe coherency loss. Similarly, in Cu-Ti alloys, variations in wavelengths and composition amplitude of modulations were observed, along with clustering and ordering reactions. Ordering was specifically noted in Ti-rich regions of the alloy aged for an extended period. Diffused interfaces were observed in both alloy systems rather than chemically sharp interfaces, and the contrast in HAADF STEM images in both systems was predominantly influenced by strain effects. Hence utilizing HAADF STEM for composition measurement of modulations is impractical due to the impact of strain on HAADF intensity, specifically due to surface relaxation effects.
ContributorsSawant, Ronit Prasad (Author) / Carpenter, Ray (Thesis advisor) / Treacy, Michael (Committee member) / Crozier, Peter (Committee member) / Liu, Jingyue (Committee member) / Arizona State University (Publisher)
Created2024
Description
Phase-field (PF) models are one of the most powerful tools to simulate microstructural evolution in metallic materials, polymers, and ceramics. However, existing PF approaches rely on rigorous mathematical model development, sophisticated numerical schemes, and high-performance computing for accuracy. Although recently developed surrogate microstructure models employ deep-learning techniques and reconstruction of microstructures from lower-dimensional data, their accuracy is fairly limited as spatio-temporal information is lost in the pursuit of dimensional reduction. Given these limitations, a novel data-driven emulator (DDE) for extrapolation prediction of microstructural evolution is presented, which combines an image-based convolutional and recurrent neural network (CRNN) with tensor decomposition, while leveraging previously obtained PF datasets for training. To assess the robustness of DDE, the emulation sequence and the scaling behavior with phase-field simulations for several noisy initial states are compared. In conclusion, the effectiveness of the microstructure emulation technique is explored in the context of accelerating runtime, along with an emphasis on its trade-off with accuracy.Meanwhile, an interpolation DDE has also been tested, which is based on obtaining
a low-dimensional representation of the microstructures via tensor decomposition and subsequently predicting the microstructure evolution in the low-dimensional space using Gaussian process regression (GPR). Once the microstructure predictions are obtained in the low-dimensional space, a hybrid input-output phase retrieval algorithm will be employed to reconstruct the microstructures. As proof of concept, the results on microstructure prediction for spinodal decomposition are presented, although the method itself is agnostic of the material parameters. Results show that GPR-based DDE model are able to predict microstructure evolution sequences that closely resemble the true microstructures (average normalized mean square of 6.78 × 10−7) at time scales half of that employed in obtaining training data. This data-driven
microstructure emulator opens new avenues to predict the microstructural evolution by leveraging phase-field simulations and physical experimentation where the time resolution is often quite large due to limited resources and physical constraints, such as the phase coarsening experiments previously performed in microgravity.
Future work will also be discussed and demonstrate the intended utilization of these two approaches for 3D microstructure prediction through their combined application.
ContributorsWu, Peichen (Author) / Ankit, Kumar (Thesis advisor) / Iquebal, Ashif (Committee member) / Jiao, Yang (Committee member) / Zhuang, Houlong (Committee member) / Arizona State University (Publisher)
Created2024
Description
This work systematically investigates structure-stability relations in various polymer derived ceramic (PDC) systems and metal organic frameworks (MOFs), both of which are hybrid materials. The investigation of silicon carbides (SiC) confirms thermodynamic stabilization of PDCs with increasing mixed bonding (Si bonded to both C, O and/or N). The study of more complex silicon oxycarbide (SiOC) structures shows stabilization of SiOCs with increasing pyrolysis temperature (between 1200 and1500 oC), and points to dissimilarities in the stabilizing effect of different mixed bonding environments (SiO3C, SiO2C2, or SiOC3) and their relative amounts. Analyses of quaternary silicon oxycarbonitride (SiC(N)(O)) materials suggests increased stabilization with increasing N content, and superior stabilization due to SiNxC4-x compared to SiOxC4-x mixed bonds. Investigation of the energetics of metal filler (Nb, Hf, Ta) incorporation in SiOCs shows that choice of metal filler influences the composition, structural evolution, and thermodynamic stability in PDCs. Ta fillers can stabilize otherwise unstable SiO3C mixed bonds. Independent of metal incorporation or lack thereof, in SiOC systems, higher pyrolysis temperature (1200-1500 oC) forms more stable ceramics. The stabilizing effect of order/disorder of the free carbon phase is system-dependent. The work on (MOFs) highlights stabilization trends obtained from the investigation of zeolitic imidazolate frameworks (ZIFs) and boron imidazolate frameworks (BIFs) based on azolate linkers. Study of the energetics of metal (Co(II), Cu (II), and Zn (II) ) substitution in isostructural ZIFs shows that in MOFs the stabilizing effect of metal is dependent on both framework topology (diamondoid (dia) > sodalite (SOD)) and dimensionality (2D > 3D). Thermodynamic analyses of metal substitution (Ag(I), Cu(I), and Li (I)) in isostructural ii SOD and dia BIF systems confirm increase in density as a general descriptor for increased stabilization in MOFs. The study of energetics of guest-host interactions during CO2 incorporation in azolate frameworks (i.e., ZIF-8) shows strong dependence of energetics of adsorption on choice of linker and metal. Additionally, several energetically favorable reaction pathways for the formation of CO3-ZIF-8 have been identified. Both PDCs and MOFs show a complex energetic landscape, with identifiable system dependent and general structural descriptors for increased thermodynamic stabilization and tunability of the energetics of guest-host interactions.
ContributorsLeonel, Gerson J (Author) / Navrotsky, Alexandra (Thesis advisor) / Dai, Lenore (Committee member) / Thomas, Mary (Committee member) / Singh, Gurpreet (Committee member) / Friščić, Tomislav (Committee member) / Arizona State University (Publisher)
Created2023
Description
The first part of this dissertation focuses on quantum structures with type-II band alignment, which are designed for applications in infrared photodetection and optical nonlinearity. A short- and mid-wavelength infrared dual-band optically-addressed photodetector structure has been designed and fabricated by molecular beam epitaxy, which is used to demonstrate the operational principles of optical address for extended tri-band detection. High-resolution x-ray diffraction and photoluminescence measurement were used to characterize the samples and revealed excellent crystalline quality and optical properties. An analytical model has been developed to address the effects of luminescence coupling and light leakage effects in optically-addressed tri-band photodetectors in terms of the absorber thicknesses and photoluminescence quantum efficiencies.Beyond superlattices, asymmetric quantum wells with type-II band alignment find application in optical nonlinearity enhancement which is the result of increased wavefunction overlap and larger electric dipole moments of the interband transitions compared to the conventional structures with type-I band edge alignment. The novel type-II AQW structure exhibits interband second-order susceptibility tensor elements ranging between 20 pm/V to 1.60×103 pm/V for nearly-resonant optical rectification and difference frequency generation applications at near-infrared and terahertz wavelengths, an improvement of nearly one order of magnitude over the type-I structures and one to three orders of magnitude over natural crystals such as LiNbO3, KTP, or GaAs. A factor of 2-3 further enhancement of the tensor elements is achieved by optimizing the well widths and band offsets of the type-II asymmetric quantum wells.
The second part of the dissertation reports the study of CdSe thin films with mixed zincblende and wurtzite phases grown on lattice-matched InAs(100) substrate using molecular beam epitaxy. These CdSe thin films reveal single-phase zincblende (ZB) structure with high crystalline quality with low defect density. In contrast, CdSe layers grown on lattice-matched InAs(111)B (As-terminated) substrates under different growth temperatures and Cd/Se flux ratios all have their demonstrated mixed ZB and wurtzite phases in coexistence confirmed by high-resolution x-ray diffraction, transmission electron microscopy and photoluminescence measurements. The reason for these properties is due to the small formation energy difference between the ZB and WZ phases of CdSe, which has been confirmed by density functional theory simulations.
ContributorsJu, Zheng (Author) / Zhang, Yong-Hang YHZ (Thesis advisor) / Smith, David DJS (Committee member) / Johnson, Shane SRJ (Committee member) / Ponce, Fernando FAP (Committee member) / Arizona State University (Publisher)
Created2023
Description
Air conditioning is a significant energy consumer in buildings, especially in humid regions where a substantial portion of energy is used to remove moisture rather than cool the air. Traditional dehumidification methods, which cool air to its dew point to condense water vapor, are energy intensive. This process unnecessarily overcools the air, only to reheat it to the desired temperature.This research introduces thermoresponsive materials as efficient desiccants to reduce energy demand for dehumidification. A system using lower critical solution temperature (LCST) type ionic liquids (ILs) as dehumidifiers is presented. Through the Flory-Huggins theory of mixtures, interactions between ionic liquids and water are analyzed. LCST ionic liquids demonstrate superior performance, with a coefficient of performance (COP) four times higher than non-thermoresponsive desiccants under similar conditions. The efficacy of ionic liquids as dehumidifiers is assessed based on properties like LCST temperature and enthalpic interaction parameter.
The research also delves into thermoresponsive solid desiccants, particularly polymers, using the Vrentas-Vrentas model. This model offers a more accurate depiction of their behaviors compared to the Flory-Huggins theory by considering elastic energy stored in the polymers. Moisture absorption in thin film polymers is studied under diverse conditions, producing absorption isotherms for various temperatures and humidities. Using temperature-dependent interaction parameters, the behavior of the widely-used thermoresponsive polymer (TRP) PNIPAAm and hypothetical TRPs is investigated. The parameters from the model are used as input to do a finite element analysis of a thermoresponsive dehumidifier. This model demonstrates the complete absorption-desorption cycle under varied conditions such as polymer absorption temperature, relative humidity, and air speed. Results indicate that a TRP with enhanced absorption capacity and an LCST of 50℃ achieves a peak moisture removal efficiency (MRE) of 0.9 at 75% relative humidity which is comparable to other existing thermoresponsive dehumidification systems. But other TRPs with even greater absorption capacity can produce MRE as high as 3.6. This system also uniquely recovers water in liquid form.
ContributorsRana, Ashish (Author) / Wang, Robert RW (Thesis advisor) / Green, Matthew MG (Committee member) / Milcarek, Ryan RM (Committee member) / Wang, Liping LW (Committee member) / Phelan, Patrick PP (Committee member) / Arizona State University (Publisher)
Created2023
Description
Fuel cells, particularly solid oxide fuel cells (SOFC), are important for the future of greener and more efficient energy sources. Although SOFCs have been in existence for over fifty years, they have not been deployed extensively because they need to be operated at a high temperature (∼1000 °C), are expensive, and have slow response to changes in energy demands. One important need for commercialization of SOFCs is a lowering of their operating temperature, which requires an electrolyte that can operate at lower temperatures. Doped ceria is one such candidate. For this dissertation work I have studied different types of doped ceria to understand the mechanism of oxygen vacancy diffusion through the bulk. Doped ceria is important because they have high ionic conductivities thus making them attractive candidates for the electrolytes of solid oxide fuel cells. In particular, I have studied how the ionic conductivities are improved in these doped materials by studying the oxygen-vacancy formations and migrations. In this dissertation I describe the application of density functional theory (DFT) and Kinetic Lattice Monte Carlo (KLMC) simulations to calculate the vacancy diffusion and ionic conductivities in doped ceria. The dopants used are praseodymium (Pr), gadolinium (Gd), and neodymium (Nd), all belonging to the lanthanide series. The activation energies for vacancy migration between different nearest neighbor (relative to the dopant) positions were calculated using the commercial DFT code VASP (Vienna Ab-initio Simulation Package). These activation energies were then used as inputs to the KLMC code that I co-developed. The KLMC code was run for different temperatures (673 K to 1073 K) and for different dopant concentrations (0 to 40%). These simulations have resulted in the prediction of dopant concentrations for maximum ionic conductivity at a given temperature.
ContributorsAnwar, Shahriar (Author) / Adams, James B (Thesis advisor) / Crozier, Peter (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2011
Description
With the increasing focus on developing environmentally benign electronic packages, lead-free solder alloys have received a great deal of attention. Mishandling of packages, during manufacture, assembly, or by the user may cause failure of solder joint. A fundamental understanding of the behavior of lead-free solders under mechanical shock conditions is lacking. Reliable experimental and numerical analysis of lead-free solder joints in the intermediate strain rate regime need to be investigated. This dissertation mainly focuses on exploring the mechanical shock behavior of lead-free tin-rich solder alloys via multiscale modeling and numerical simulations. First, the macroscopic stress/strain behaviors of three bulk lead-free tin-rich solders were tested over a range of strain rates from 0.001/s to 30/s. Finite element analysis was conducted to determine appropriate specimen geometry that could reach a homogeneous stress/strain field and a relatively high strain rate. A novel self-consistent true stress correction method is developed to compensate the inaccuracy caused by the triaxial stress state at the post-necking stage. Then the material property of micron-scale intermetallic was examined by micro-compression test. The accuracy of this measure is systematically validated by finite element analysis, and empirical adjustments are provided. Moreover, the interfacial property of the solder/intermetallic interface is investigated, and a continuum traction-separation law of this interface is developed from an atomistic-based cohesive element method. The macroscopic stress/strain relation and microstructural properties are combined together to form a multiscale material behavior via a stochastic approach for both solder and intermetallic. As a result, solder is modeled by porous plasticity with random voids, and intermetallic is characterized as brittle material with random vulnerable region. Thereafter, the porous plasticity fracture of the solders and the brittle fracture of the intermetallics are coupled together in one finite element model. Finally, this study yields a multiscale model to understand and predict the mechanical shock behavior of lead-free tin-rich solder joints. Different fracture patterns are observed for various strain rates and/or intermetallic thicknesses. The predictions have a good agreement with the theory and experiments.
ContributorsFei, Huiyang (Author) / Jiang, Hanqing (Thesis advisor) / Chawla, Nikhilesh (Thesis advisor) / Tasooji, Amaneh (Committee member) / Mobasher, Barzin (Committee member) / Rajan, Subramaniam D. (Committee member) / Arizona State University (Publisher)
Created2011
Description
Silicon nanowires were grown epitaxially on Si (100) and (111) surfaces using the Vapor-Liquid-Solid (VLS) mechanism under both thermal and plasma enhanced growth conditions. Nanowire morphology was investigated as a function of temperature, time, disilane partial pressure and substrate preparation. Silicon nanowires synthesized in low temperature plasma typically curved compared to the linear nanowires grown under simple thermal conditions. The nanowires tended bend more with increasing disilane partial gas pressure up to 25 x10-3 mTorr. The nanowire curvature measured geometrically is correlated with the shift of the main silicon peak obtained in Raman spectroscopy. A mechanistic hypothesis was proposed to explain the bending during plasma activated growth. Additional driving forces related to electrostatic and Van der Waals forces were also discussed. Deduced from a systematic variation of a three-step experimental protocol, the mechanism for bending was associated with asymmetric deposition rate along the outer and inner wall of nanowire. The conditions leading to nanowire branching were also examined using a two-step growth process. Branching morphologies were examined as a function of plasma powers between 1.5 W and 3.5 W. Post-annealing thermal and plasma-assisted treatments in hydrogen were compared to understand the influences in the absence of an external silicon source (otherwise supplied by disilane). Longer and thicker nanowires were associated with longer annealing times due to an Ostwald-like ripening effect. The roles of surface diffusion, gas diffusion, etching and deposition rates were examined.
ContributorsJoun, Hee-Joung (Author) / Petuskey, William T. (Thesis advisor) / Drucker, Jeff (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2011
Description
A relatively simple subset of nanotechnology - nanofluids - can be obtained by adding nanoparticles to conventional base fluids. The promise of these fluids stems from the fact that relatively low particle loadings (typically <1% volume fractions) can significantly change the properties of the base fluid. This research explores how low volume fraction nanofluids, composed of common base-fluids, interact with light energy. Comparative experimentation and modeling reveals that absorbing light volumetrically (i.e. in the depth of the fluid) is fundamentally different from surface-based absorption. Depending on the particle material, size, shape, and volume fraction, a fluid can be changed from being mostly transparent to sunlight (in the case of water, alcohols, oils, and glycols) to being a very efficient volumetric absorber of sunlight. This research also visualizes, under high levels of irradiation, how nanofluids undergo interesting, localized phase change phenomena. For this, images were taken of bubble formation and boiling in aqueous nanofluids heated by a hot wire and by a laser. Infrared thermography was also used to quantify this phenomenon. Overall, though, this research reveals the possibility for novel solar collectors in which the working fluid directly absorbs light energy and undergoes phase change in a single step. Modeling results indicate that these improvements can increase a solar thermal receiver's efficiency by up to 10%.
ContributorsTaylor, Robert (Author) / Phelan, Patrick E (Thesis advisor) / Adrian, Ronald (Committee member) / Trimble, Steve (Committee member) / Posner, Jonathan (Committee member) / Maracas, George (Committee member) / Arizona State University (Publisher)
Created2011