Electronic Structure based Investigations of Hybrid Perovskites and Their Nanostructures
Ruyi Song, Ph.D. Candidate
Weitao Yang, Ph.D., Advisor
Volker Blum, Ph.D., Advisor
Abstract:Perovskites are a type of semiconductors with outstanding optoelectronic properties. Three-dimensionally connected hybrid perovskites gained an important position as an innovative solar-cell material by including organic cations. Related molecularly engineered materials, for example, two-dimensionally connected layered crystals and nanostructures offer a wide range of compositional, structural, and electronic tunability. Based on theoretical simulations, this dissertation aims to help the understanding of the relationship between the components and structure of hybrid perovskites and their electronic and spintronic properties.
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 "bowing effect" on the bandgap values; 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 energy band spin properties in hybrid perovskites, 1) the author analyzed the frontier bands of the 2D hybrid perovskite (R/S/Racemic-NEA)2PbBr4 and revealed a giant spin-splitting originated from the inorganic moiety; 2) the author (with experimental collaborators) identified an inter-octahedron distortion prototype as the crucial geometric descriptor for spin-splitting in 2D hybrid perovskites; 3) for perovskite nano-crystals with chiral surface ligands, simulations 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 DFT simulations, the application of tensor processing units (designed by Google), to quantum chemistry calculations 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 performance of this new hardware.