Organic-inorganic hybrid lead halide perovskites are promising for next-generation solar cells and light-emitting diodes, and it is of high demand to solve their critical issues and to understand their working mechanisms. In this dissertation, using first-principles calculations,

we focus on design of stable and non-toxic alternatives to this class of materials, as well as to understand and optimize their structural, energetic, electronic, and ferroelectric properties for optoelectronic applications.

In the first project, we designed novel optoelectronic materials based on 24 perovskite-related prototype structures by high-throughput computing and data mining. Out of 4507 hybrid halide compounds calculated, we selected 29 compounds adopting five prototype structures for light-emitting diodes and solar energy conversion. All these candidates show appropriate electronic properties and robust stability. The approach of exploring a large variety of prototype structures is transformative to computational design of other functional materials.

In the second project, we further investigated stability diagrams, defect tolerance, and optical absorption of the 29 hybrid halide compounds by high-throughput first-principles calculations. We calculated 2160 neutral and about 5000 charged defect structures to determine defect formation energies and transition energy levels for all possible point defects. Out of the 29 compounds, 15 candidates show high defect tolerance. This work provides detailed guidance on experimental investigation of these novel lead-free optoelectronic materials.

In the third project, we studied ferroelectric dipole ordering in hybrid perovskites. We found that organic cations’ rotational energy barrier is dependent on the cell aspect ratio, and that spontaneous ferroelectric dipole ordering exists with small energy advantage. More importantly, we found that by increasing the cell aspect ratio, strain and doping can enhance the dipole ordering, which could boost electron-hole separations for photovoltaic applications.

In the fourth project, we studied strained epitaxial growth of halide perovskites. Our calculations demonstrate the epitaxial stabilization by calculating detailed thermodynamic terms in the epitaxial nucleation process. We also show that strains control the crystal structure, the bandgap, and the hole effective mass.

Materials design is a cornerstone of every device. Historically, the materials selection process was characterized by a time consuming, expensive, Edisonian approach. In recent years however, rapid advancements in computational power and materials simulation software has spawned the field of computational materials science. Computational materials science opens a new avenue to materials discovery called high-throughput materials design. This approach allows for rapid prototyping of materials in a large, complex chemical space. In this work, the scope of highthroughput materials design approach is used in the analysis of several topics: magnetic full-heuslers, hybrid perovskites, and grain boundary structures. Using high-throughput density functional theory (DFT), we study the surface energy of 68 magnetic full heuslers to guide the synthesis of magnetic tunnel junctions for applications in memory storage devices. We employ a high-throughput machine learning approach to explore the chemical space of single and double perovskite materials for applications in stable, high-performance solar cells. We also look deeper into hybrid perovskite materials in a literature review of two-dimensional hybrid perovskites, which demonstrate greater stability and tunable band gaps with simple fabrication routes. In addition, their strong binding energies lead to strong light emitting properties, with potential applications in light emitting diode devices. We also examine the configurational entropy of yttria-stabilized zirconia grain boundaries and provide example usage and applications of AIMSGB, an open-source python library for grain boundary structure generation.

Defects and interfaces are ubiquitous in polycrystalline metal oxide materials, and their presence gives rise to a host of beneficial and harmful effects. In this thesis we focused on substitutional impurities, heterointerfaces, oxygen vacancies, and grain boundaries, with the goal of enhancing material properties, using first-principles density functional theory calculations. In the first project we studied the energetic and electronic properties of pentavalent-cation-doped SnO2 transparent conducting oxide (TCOs) materials. We found that P-doped SnO2 (PTO), a non-toxic low-cost material, compares favorably with well-known TCOs Sb-doped SnO2 (ATO) and Ta-doped SnO2 (TTO). In addition, we showed that the theoretical charge carrier density (ne) of PTO, ATO, and TTO is nearly identical, indicating that the order-of-magnitude experimental variations in n e for the latter two materials may instead arise from differences in experimental conditions during synthesis. In the second project we explored the possibility of enhancing the properties of the two-dimensional electron gas (2DEG) which forms at the interface of the LaAlO3/SrTiO3 (LAO/STO) heterostructure through n-type layer doping with transition metal cations. We found that Ta(Nb) doping at the interfacial Ti site is energetically favorable, and can significantly enhance the charge carrier density and magnetism of the interfacial 2DEG in LAO/STO. In addition, we found that Ta(Nb) doping at the interfacial Al site can improve 2DEG charge confinement to only two TiO2 atomic layers of the STO substrate.

In the third project we examined polarization and the effect of externally-applied strain on 2DEG properties in LAO/STO. We found that the LAO film polarization (P LAO ) decreases with film thickness, and that there is a critical P LAO value above which 2DEG cannot form. We resolved the long-standing discrepancy between the experimental and theoretical 2DEG charge carrier density through the use of an appropriate slab model. In addition, we showed that [100] uniaxial tensile strain applied on the STO substrate can reduce P LAO and thereby enhance the charge carrier density, electron mobility, and interfacial charge concentration of the LAO/STO 2DEG. In the fourth project we studied termination stability and oxygen vacancy formation in a Σ5 [001] twist grain boundary (GB) structure of SrTiO3 (STO). We found that of the three possible GB terminations, only the SrO/SrO (S/S) and SrO/TiO2 (S/T) can form in the chemical potential range necessary to guarantee bulk STO stability, while the TiO2 /TiO2 (T/T) termination cannot form. We showed that oxygen vacancies tend to segregate adjacent to the GB layer in the S/S system, but at the GB layer in the S/T system. In addition, we demonstrated that oxygen vacancies form more easily in the S/T GB system than the S/S GB system, leaving the door open for possible GB engineering applications. In the final project we considered substitutional impurity segregation to Σ5 (310)/[001] tilt grain boundary structures of ZrO2 , HfO2 , and yttria-stabilized ZrO2 (YSZ). We discovered a fundamental difference in the characteristic segregation profiles of aliovalent and isovalent impurities which is explained by differences in the local electrostatic potential energy. We successfully generated and structurally-optimized a YSZ GB structure bearing high concentrations of yttrium dopants and oxygen vacancies. Finally, we found that Si, Mg, and Ca are strong segregants in YSZ while Al, Ti, and Y are not.

The Heusler/MgO-based magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have attracted extensive interest because of their potential utilization in spin-transfer-torque magnetic random-access memory (STT-MRAM) with long retention time and low switching current. As a large family of intermetallic compounds, Heusler alloys offer fascinating magnetic properties due to their wide variety of element compositions. Nevertheless, the selection of promising Heusler compounds candidates for the application of p-MTJs with high stability and low energy consumption becomes a grand challenge. In this dissertation, we provided a systematic high-throughput computational design of Heusler/MgO heterostructures to search for promising structures with robust materials stability and large perpendicular magnetic anisotropy by employing a series descriptors such as formation energy, convex hull distance, magnetic ordering, lattice misfit, magnetic anisotropy constant, interfacial cleavage energy, spin polarization, and tunnel magnetoresistance (TMR).

In the first project, we focused on the full Heusler (X2YZ) and half Heusler (XYZ) compounds. By using a comprehensive screening over 40, 000 ternary Heusler compounds, 363 full Heusler compounds, and 134 half Heusler compounds were confirmed thermodynamically stable, where five full Heusler compounds and two half Heusler compounds were found promising for designing p-MTJs.

In the second project, we studied the origin of the large interfacial PMA in the Co2FeAl/MgO structure, by analyzing the layer-resolved and atomic-orbital-resolved Ki distributions. Later, the influences of the 26 capping layers on the Co2FeAl/MgO structure were further investigated. Our calculations indicated that adding Fe- and W-capping layers can significantly increase the Ki of the system.

In the third project, we performed a systematic high-throughput screening in selecting the quaternary Heusler/MgO heterostructures. 7 out of 3094 stable quaternary Heusler compounds were found feasible for future applications in the p-MTJs. Their phase stability was further confirmed by using the swapping method.

In the fourth project, we proposed a swapping method to predict the disordering effects in the quaternary Heusler (XX′YZ) compounds. By using the swapping method, we successfully verified the ordered structure of CoFeCrGe, the L21disordering in the CoMnCrAl, and the DO3 disordering in the CoFeMnGe, demonstrating the efficiency of our method.

Two-dimensional electron gas (2DEG) formed at the interface between two insulating perovskite oxides has provided a versatile playground to explore emergent interfacial electronic and magnetic properties. In this thesis our efforts centered on studying the electronic and structural properties of different 2DEG heterostructures (HS), with the goal of designing novel 2DEG HS using first-principles methods.

In the first project we studied the δ-doping effects on the electronic and energetic properties of LaAlO3/SrTiO3 HS with 23 transition-metal (TM) dopants. It has been found that there is a trade-off between achieving small electron effective mass and obtaining an energetically favorable TM-doped LaAlO3/SrTiO3 system. More importantly, in addition to the experimentally confirmed Mn dopant, we proposed that Fe, Co, Ni, Ru, Rh, Pd, Os and Ir elements can also be promising dopants to yield light effective mass bands and good energetic stability.

In the second project we compared the electronic and energetic properties of TiO2/LaAlO3 and LaAlO3/TiO2 HS. We found that TiO2/LaAlO3 is intrinsically metallic and has a larger interfacial charge carrier density, smaller electron effective mass and a stronger interface cohesion than LaAlO3/TiO2, which shows an insulator-to-metal transition at 4 unit cells of LaAlO3.

In the third project we introduced a hitherto unknown 2DEG formed at the interface between spinel MgAl2O4 and SrTiO3. Our integrated approach combining experimental mea- surements and first-principles calculations reveals that an atomic-thin interfacial Ti-Al-O layer with a thickness of about 4 A ̊ is key to the observed metallic transport. The 2DEG observed at spinel/perovskite interface implies the existence of emergent phenomena at the interfaces between spinel group minerals and perovskite oxides.

In the fourth and fifth project we explored the possibility of creating 2DEG in nonpo- lar/nonpolar perovskite oxide HS. We found that the lattice-mismatch-induced compression strain from the substrate leads to a large polarization in the film, which then drives the charge transfer from the film to the substrate and results in a 2DEG at the interface. In addition, by using high-throughput first-principles calculations and a group of combinatory descriptors, we rapidly designed more than 300 novel nonpolar/nonpolar 2DEG HS.

In the final project we introduced Grain Boundary Maker (GBMaker), an efficient and open-source Python library for generating atomic coordinates in periodic grain boundary models. It is designed to construct various grain boundary structures from cubic and non-cubic initial configurations. GBMaker is expected to greatly accelerate the theoretical investigation of grain boundary properties and facilitate the experimental analysis of grain boundary structures as well.

MoS2-based catalysts have been used in the petroleum industry for decades and it is of long-term interests to improve their catalytic efficiency in a hydrodesulfurization (HDS) process. Our focus is centered on studying the structural, thermochemical, and catalytic properties of the single-layer MoS2 nanosheets, with an objective of generating the ideal HDS catalysts using first-principles calculations.

In the first project, we discovered the size and odd-even effects on the formation of sulfur vacancies in the triangular MoS2 nanosheets. The S-terminated edges were found to be energetically more favorable than Mo-terminated edges. Two types of sulfur dimer vacancies at the center (V_S@Cen) and the corner (V_S@Cnr) site of the S-terminated nanosheets revealed odd-even effects in the vacancies formation with respect to size.

In the second project, we introduce a new material called a monolayer Janus MoXY, which is a promising alternative to the MoS2 catalyst. The Janus MoXY nanosheets can also be found in a triangular shape, exhibiting similar structural properties as MoS2. For the enhancement of the catalytic activity, the ideal Gibbs free energy (ΔG_H) of hydrogen adsorption must be close to zero. Interestingly, our calculated ΔG_H also result in the similar odd-even effect in the hydrogen adsorption with respect to size.

In the third project, we report the transition metal (TM) promotion effects on the catalytic activity of S-terminated hexagonal MoS2 nanosheets using 26 TM elements. The HDS activity of TM-promoted MoS2 nanosheets is evaluated by modelling three consequent steps in an HDS process with a dibenzothiophene (DBT) molecule. The reaction energy of each steps were calculated with respect to the descriptor, binding energy (E_b) of TM sulfides. On the basis of the descriptor, several candidates including Co, Fe, and Cr are identified to be the ideal TM promoters.

In the fourth project, we examined the properties of Bi-doped NaTaO3 for an additional application of photocatalytic water splitting . The Bi-doping sites were found to be dependent on the materials preparation conditions of NaTaO3 and our results showed enhanced photocatalytic hydrogen evolution under visible light for Bi doping, simultaneously, at Na and Ta sites (Bi@(Na,Ta)).

In the fifth project, we theoretically investigated the affinity of Na atoms to B- and N-doped graphene substrates for the application of electrochemical energy storage. Compared to the pristine graphene, B- and N-doped graphene were found to possess different binding energies with the Na atoms with different doping forms.