Browsing by Author "Weinberger, Christopher, advisor"

Now showing 1 - 3 of 3

Open Access
A study of oxide/CdTe interfaces for CdTe photovoltaics using atomistic modeling
(Colorado State University. Libraries, 2021) Thiyagarajan, Aanand, author; Sampath, W. S., advisor; Weinberger, Christopher, advisor; Martinez Pozzoni, Umberto, committee member; Sites, James R., committee member; Popat, Ketul, committee member
Solar photovoltaics (PV) has undergone a dramatic transformation over the past few decades and is now a widespread electricity generation source. Among currently existing PV technologies, the thin film sector led by cadmium telluride is the most promising. Cadmium Telluride (CdTe) PV has experienced unprecedented growth and is now a major commercial player. However, the field has a few challenges to overcome until it reaches its full potential. The focus of this study is the interface between the CdTe-based absorber and the front window layer. Traditionally, cadmium sulfide has been used as the window layer in such devices. At the Next Generation Photovoltaics (NGPV) center in Colorado State University, superior devices have been demonstrated using magnesium zinc oxide (MgxZn1-xO or MZO) as the window layer. This is attributed to the larger bandgap of MZO causing a pickup in the current and the open circuit voltage. A magnesium to zinc atomic ratio of 23:77 has shown optimal performance characteristics. Alloying CdTe with Se to form cadmium selenium telluride (CdSexTe1-x or CST) has resulted in further improvements. One way to determine the quality of an interface is to study the electronic band alignment at that interface. Existing band alignment models show only limited features and hence there is a need for a more sophisticated approach to investigate complex characteristics. This study uses atomistic modeling based on Density Functional Theory (DFT) to investigate certain structural and electronic properties of the oxide and the oxide/absorber interface. The technique solves for electronic structures of materials based on electron density and predicts the structural properties of materials to a high degree of accuracy. Electronic characteristics are determined using a semi-empirical method known as DFT-1/2. A mathematical formulation called Green's Function (GF) has been incorporated within the model to simulate device structures. The bulk properties of MZO such as lattice constant, band gap, band edges and electron effective mass are established and compared to experiment. Following this, the band alignment at the MZO/CdTe and MZO/CST interfaces is determined, along with band offsets and interface states. The influence of chlorine in the deposition process is also investigated. This work is the first of its kind to study the oxide-CdTe and oxide/CST interfaces using DFT+GF and provides new insights into the electronic characteristics at the interface. Bulk properties of the MZO match experimental reports. Termination chemistry plays a significant role in the band bending and in the presence of defect states at the oxide/absorber interface. Calculations indicate that a Mg/Zn-Te interface is energetically preferred, with experimental reports pointing to the same. Moreover, varying the magnesium composition in the MZO alloy affects the magnitude of the band offsets. The interface band alignment results are close to those seen experimentally. A small amount of chlorine may help alleviate interface defect states by chemical passivation, possibly due to the removal of dangling bonds.
Open Access
Modeling deformation twinning in BCC transition metals
(Colorado State University. Libraries, 2023) Faisal, Anik H. M., author; Weinberger, Christopher, advisor; Radford, Donald, committee member; Ma, Kaka, committee member; Heyliger, Paul, committee member
Deformation twinning is one of the important deformation mechanisms in body centered cubic (BCC) transition metals, especially under low temperature and high strain rate conditions. Plastic deformation via deformation twinning has been studied for decades both experimentally and computationally however, atomic level insights such as critical nuclei size, their local atomic structures and energetics which are important parameters in modeling twin nucleation has been lacking. In this work, deformation twins in BCC transition metals and their atomic level structures and energetics have been rigorously studied to reveal the full atomic level details of twin nucleation and propagation. As such, critical thickness of deformation twins in BCC transition metals have been a topic of debate with many computational and experimental studies accepting a three-layer twin thickness based on nucleation from a screw dislocation without proof whereas recent in-situ experiments suggest six-layer thick twin nuclei observed via High resolution transmission electron microscopy (HRTEM). In this study, we have determined the critical twin nuclei thickness in these metals using atomistic simulations to examine atomic structure and energetics of deformation twins under both zero and nonzero finite pure shear stresses. Our study reveals that twins in group VB BCC transition metals nucleate as two-layer thick nuclei under stress as opposed to the three-layer thick twin nuclei under zero stress. For group VIB BCC transition metals, for both zero and nonzero stresses, the critical twin nuclei thickness is two layer near reflection. As the twins grow and stress is relieved, twins under finite stresses adopt configurations that are much closer to the zero stress stability predictions. In addition to nucleation, growth of mechanisms of twins are explored and computational insights into the growth of twins in Tungsten bicrystals explaining multi-layer growth as opposed to layer-by-layer growth associated with small barriers. Free-end string simulations were used to investigate energy barrier associated with homogeneous twin nucleation using embedded atom method (EAM) potentials. Since homogeneous twin nucleation occurs near the ideal strengths of the material described by the potentials, energy barrier calculations were not possible for all BCC transition metals as some available potentials break down under large stresses. Moreover, density functional theory (DFT) simulations are known to be more accurate in describing atomic bonding but direct nucleation simulations in bulk crystals is prohibitively expensive. Hence, existing dislocation nucleation models are thoroughly analyzed to examine the behavior of these models near ideal strength of the material because spontaneous nucleation of dislocations occurs at high stresses. From there, a robust homogeneous twin nucleation model that includes elastic interaction among the twinning dislocation loops is developed which is able to replicate energy barrier data from free-end string simulations for multiple interatomic potentials. This model takes atomistic simulation inputs such as the concurrent twinning generalized stacking fault (GSF) energy curves and corresponding burgers vector of the twinning dislocations to compute the energy barriers as a function of applied stress. This model can be useful in modeling homogeneous twin nucleation all BCC transition metals and has the potential advantage of using DFT simulation inputs for accurate description of atomic bonding within the twin nuclei. Finally, nucleation stresses for twinning in bulk crystals have been studied to investigate whether the formation of twinning in experimental studies were initiated by homogeneous nucleation. Upper and lower bounds of stress values required for homogeneous twin nucleation has been computed and a semi-empirical model has been developed to predict homogeneous twin nucleation stresses as a function of temperature and strain rate. This analysis shows that reported critical resolved shear stress (CRSS) values in experimental studies are not associated with homogeneous twin nucleation despite some modeling studies claiming otherwise.
Open Access
Multiscale study of the pearlitic microstructure in carbon steels: atomistic investigation and continuum modeling of iron and iron-carbide interfaces
(Colorado State University. Libraries, 2018) Guziewski, Matthew, author; Weinberger, Christopher, advisor; Heyliger, Paul, committee member; Kota, Arun, committee member; Ma, Kaka, committee member
While the behavior of carbon steel has been studied extensively for decades, there are still many questions regarding its microstructures. As such, classical atomistics is utilized to obtain further insight into the energetics, structure, and mechanical response of the various interfaces between iron and iron-carbides. Simulations were constructed for the commonly reported orientation relationships between ferrite and cementite within pearlite: the Bagaryatskii, the Isaichev, and the Pitsch-Petch, as well as their associated near orientations. Dislocation arrays are found to form for all orientation relationships, with their spacing and direction a function of lattice mismatch. Within each orientation relationship, different interfacial chemistries are found to produce identical dislocation spacings and line directions, but differing interfacial energies. This chemistry component to the interfacial energy is characterized and it is determined that in addition to the lattice mismatch, there are two structural factors within the cementite terminating plane that affect the energetics: the presence of like site iron pairs and proximity of carbon atoms to the interface. Additionally, an alternate method for determining the interfacial energy of systems in which there are multiple chemical potentials for a single element is developed and implemented, an approach which is likely valid for other similar systems. Atomistics finds the Isaichev orientation relationship to be the most favorable, while the "near" orientation relationships are found to be at least as energetically favorable as their parent orientation relationships. A continuum model based on O-lattice theory and anisotropic continuum theory is also applied to the atomistic results, yielding interfacial energy approximations that match well with those from atomistics and allowing for the characterization of the Burgers vectors, which are found to lie in high symmetry directions of the ferrite on the interface plane. The continuum model also allowed for the analysis of the system with changing lattice and elastic constants. This revealed that while most of the orientations had relatively small variation in their energetics with these changes, the Isaichev orientation was in fact very sensitive to variations in the lattice constants. The use of temperature dependent values for lattice and elastic constants suggested that while the Isaichev is most favorable at low tempertaures, other orientations may become more favorable at high temperatures. This combined atomistic/continuum approach was also applied to the austenite-cementite system and used to compare the proposed habit planes of both the Pitsch and Thompson-Howell orientation relationships. This analysis found the two orientation relationships to be unique, a point of previous contention, with the Pitsch the more favorable. Atomistic modeling was further used to investigate the mechanical response to compressive and tensile straining of the pearlitic orientation relationships. A range of interlamellar spacings and ferrite to cementite ratios are considered, and values for important mechanical properties including elastic modulus, yield stress, flow stress, and ductility are determined. Mechanical properties are shown to be largely dependent on only the volume ratios of the cementite and ferrite, with the interlamellar spacing having an increasing role as it reaches smaller values. Slip systems and Schmid factors are determined for a variety of loading states in both the transverse and longitudinal directions and were used to fit to simple elasto-plastic models. Transverse loading is observed to follow simple 1-D composite theory, while longitudinal loading requires the consideration of the strain compatibility of the interface. Orientation, and specifically the alignment of slip planes in the ferrite and cementite, was also determined to play a role in the mechanical response. Alignment of favorable slip planes in the cementite, notably the {100}θ and {110}θ, with high symmetry directions in the ferrite was found to greatly enhance the ductility of the system in longitudinal loading, as well as allow for lower flow stresses in transverse loading.