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Title:	First-principles density functional theory studies on perovskite materials
Author:	Lamichhane, Aneer
View Online:	njit-etd2021-010 (xiii, 144 pages ~ 14.6 MB pdf)
Department:	Department of Physics
Degree:	Doctor of Philosophy
Program:	Materials Science and Engineering
Document Type:	Dissertation
Advisory Committee:	Ravindra, N. M. (Committee chair) Fiory, Anthony (Committee member) Dias, Cristiano L. (Committee member) Ahn, Ken Keunhyuk (Committee member) Gokce, Oktay H. (Committee member) Lamsal, Chiranjivi (Committee member)
Date:	2021-05
Keywords:	Density functional theory Elastic properties Electronic properties Energy gap - refractive index Optical properties Perovskites
Availability:	Unrestricted
Abstract:	Perovskites are a family of materials with a diverse combination of different elements. As a consequence, they exhibit numerous functionalities such as pyroelectric, piezoelectric, ferroelectric, and ferromagnetic with applications in photovoltaic cells, LEDs, superconductivity, colossal magneto-resistance, and topological insulators. After 2009, perovskites have gained notoriety as suitable materials for solar cells and alternative candidates to silicon-based conventional solar cells. Generally, oxide perovskites exhibit good dielectric properties, halide perovskites display good photonic qualities, and chalcogenide perovskites are used in applications in solid-state lighting, sensing, and energy harvesting. In this dissertation, various types of perovskites ranging from oxide to halide are investigated along with their structural, elastic, electronic, and optical properties. The mode of study is the first-principles calculations performed with density functional theory, implemented in the VASP (The Vienna Ab initio Simulation Package) codes. Energy gap and refractive index are two critical properties, whose prior knowledge is required for designing optoelectronic devices. A model is developed by which these two quantities are correlated in both oxide and halide perovskites. This model is consistent with other well-established models and predicts the refractive index with greater accuracy. A comprehensive study of alkaline earth metal zirconate perovskites CaZrO3, SrZrO3 and BaZrO3 is performed. The effect of the cation size on their overall properties is analyzed using both standard and hybrid functionals. Moreover, this study also shows the comparative efficacy between standard and hybrid functionals. The bond strengths in these zirconate perovskites are studied using the concept of the charge flow, which arises due to the coupling between lattice displacement and electrostatic field. In performing the study of optical properties, the value of the dimensionless constant in the Penn model is determined to be 0.86. Based on the study of phonons, it is noted that these zirconate perovskites are suitable for thermal coating materials. Likewise, the structural phase study in halide perovskite, CsPbBr3, is predicted the transition temperature as well as the order of phase, in accord with the experimental results. Its 2D counterparts, Cs2PbBr4 and CsPb2Br5, are also studied in parallel with CsPbBr3. Their higher exciton binding energy is explained based on the screening factor. One of the major problems in perovskite-based solar cells is their unpredictability in behavior due to external conditions such as moisture, temperature, etc. In order to address such an issue, it is better to maintain the system under isosymmetric stress. A detailed concept of imposing isosymmetric stress with hydrostatic compression is discussed. This notion is applied to various cubic halide perovskites, ABX3 (A=K, Rb, Cs; B=Ge, Sn, Pb and X=Cl, Br, I). The influence of cation-anion exchange, together with the effect of spin-orbit coupling, is studied.