Electronic Structure Based Investigations of Hybrid Perovskites and Their Nanostructures

Perovskites are a category of semiconductors with outstanding optoelectronic properties. Especially in the last decades, three-dimensionally connected (“3D”) hybrid perovskites gained an important position as an innovative solar-cell material by including organic cations. Related molecularly engineered materials, for example, atomic-scale two-dimensionally connected (“2D”) layered crystals and nano-scale structures offer a wide range of compositional, structural, and electronic tunability. Based on quantum chemistry simulations (specifically, density functional theory), this dissertation aims to contribute to the understanding of the relationship between the components and structure of hybrid perovskites and their electronic properties, related to alloying, energy level alignment in quantum wells, impact of chiral organic constituents on the atomic structure of 2D perovskites and resulting spin character of the electronic levels, and on the structure of related perovskite nanostructures.First, to investigate the tunability of 2D hybrid perovskites, 1) the author simulated the Sn/Pb alloying at the central metal site and explained the corresponding “bowing effect” on the bandgap values with different contribution preferences towards the conduction bands versus valence bands from different elements; 2) taking the conjugation length in different oligothiophene cations and the inorganic layer thickness as two independent factors, the author confirmed a gradual change of quantum well types. Second, to gain an in-depth understanding of the spin properties of the energy bands (specifically, the spin-selectivity) in hybrid perovskites, 1) the author analyzed the frontier bands of the 2D hybrid perovskite S-1-(1-naphthyl)ethylammonium lead bromide and revealed a giant spin-splitting originated from the inorganic moiety; 2) the author (together with experimental collaborators) identified a difference in the inter-octahedron Pb-X-Pb (X stands for the halides) distortion angles as the crucial geometric descriptor for spin-splitting in 2D hybrid perovskites by a correlation analysis of 22 experimental and relaxed structures with various chiral or achiral organic cations; 3) for perovskite nano-crystals with chiral surface ligands, simulations by the author helped to attribute the chirality transfer between organic cations and inorganic substrate to the geometric distortions driven by hydrogen bonds. Third, the author investigated 2D hybrid perovskites containing oligoacene organic cations, validated the theoretical method for geometry evaluation and predicted the expected quantum well type, crystal symmetry, and detailed expected spin-splitting properties that determine the potential for spin-selective transport and optoelectronics Finally, driven by the computational needs of large-scale hybrid perovskites DFT simulations, the application of an innovative hardware, tensor processing units (designed by Google), to quantum chemistry calculations (specifically, to solve for the density matrix) was explored. The author removed the code bottleneck to facilitate the largest “end-to-end” O(N^3) DFT simulations ever reported and benchmarked the accuracy and performance of this new hardware with test cases from biomolecular systems to solid-state and nano-scale materials.