title: Structural and optoelectronic properties of heterogeneous metal-halide perovskites from first principles
Creators: name: Biega, Raisa-Ioana
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: First Principles Calculations
subject: Density Functional Theory
subject: Many-Body Perturbation Theory
subject: Bethe-Salpeter Equation
subject: Nudged Elastic Band
subject: Metal-Halide Perovskites
subject: Electronic Structure
subject: Optoelectronic Properties
subject: Excitonic Properties
subject: Dimensional Reduction
subject: Vacancy Migration
subject: 530

description: The development of high-performance light harvesting devices has been one of the main focuses of the photovoltaics community and materials with high crystallinity and low defect concentrations have been thought to be the perfect candidates. In the past decade metal-halide perovskites have challenged this conventional understanding of semiconducting behavior. They have successfully been used in optoelectronic applications with unexpected and remarkably high efficiencies, despite the significant inhomogeneities in local chemistry, defect density, and lattice structure introduced through the facile synthesis methods typically used to grow perovskite films for photovoltaic applications. Furthermore, the full material class of halide perovskites exhibits tremendous chemical and structural heterogeneity leading to a great diversity of optoelectronic properties. This thesis contributes to the atomistic understanding of the effect of different forms of heterogeneity on structural and optoelectronic properties of metal-halide perovskites through an in-depth theoretical study using state-of-the-art first principles calculations. Double metal-halide perovskites are an emerging class of materials with promising semiconducting properties. In these materials, chemical heterogeneity is introduced through the alternating mono- and trivalent metal cations that build the inorganic crystalline lattice. We use density functional theory and ab initio many-body perturbation theory within the G0W0 approximation and the Bethe-Salpeter equation approach to study the effect of this chemical heterogeneity on the electronic and excited state structure of halide double perovskites. We find that the magnitude of exciton binding energies and the significance of local field effects in these materials are highly dependent on their band edge orbital character and thus provide atomistic insight into light-matter interactions in this technologically relevant class of materials. By lowering the dimensionality of both single and double metal-halide perovskites via incorporation of large organic molecules, additional structural heterogeneity is introduced because of symmetry breaking along one direction. We perform first principles density functional theory calculations to investigate the effect of macroscopic structural heterogeneity on the structural and electronic properties of such quasi-2D single and double perovskites. We disentangle the effects of chemical substitution, atomic structure, and dimensionality on the electronic structure of these systems and find significant steric effects and band gap changes in both single and double quasi-2D materials in stark difference to their 3D counterparts. Finally, we study local structural microscale heterogeneities in the all-inorganic halide perovskite CsPbBr3 introduced through intrinsic defects, focusing specifically on halogen-mediated vacancy migration, which has been demonstrated to contribute the most to phase segregation and material degradation. We perform first principles density functional theory calculations to compute energy barriers and minimum-energy pathways of bromine vacancy migration in the bulk and at the surface of cubic CsPbBr3. Moreover, we analyze the effect of macroscopic structural heterogeneities at the surface by passivation of the perovskite surface with alkali-halide monolayers. Our results show that the undesirable substantially lower migration barrier at the surface can be mitigated through passivation with suitable alkali-halide monolayers.