Tunable Electronic Excitations in Hybrid Organic-Inorganic Materials: Ground-State and Many-Body Perturbation Approaches

Abstract

<p>Three-dimensional (3D) Hybrid Organic-Inorganic Perovskites (HOIPs) have been investigated intensively for application in photovoltaics in the last decade due to their extraordinary properties, including ease of fabrication, suitable band gap, large absorption, high charge carrier mobility, etc. However, the structure and properties of their two-dimensional (2D) counterparts, especially those with complex organic components, are not understood as deeply as the 3D HOIPs. Due to the easing of spatial constraints for the organic cations, 2D HOIPs potentially have more structural flexibility and thus higher tunability of their electronic properties compared to the 3D HOIPs. Motivated by a desire to demonstrate such flexibility and tunability, a series of 2D HOIPs with oligothiophene derivative as the organic cations and lead halide is investigated in the first part of this work. Initial computational models with variable organic and inorganic components are constructed from the experimental structure of 5,5''-bis(aminoethyl)-2,2':5',2''':5'',2'''-quaterthiophene lead bromide (AE4T\ch{PbBr4}). \textit{Ab initio} first-principles calculations are performed for these materials employing density functional theory with corrections for van der Waals interactions and spin-orbit coupling. The set of 2D HOIPs investigated is found to be understandable within a quantum-well-like model with distinctive localization and nature of the electron and hole carriers. The band alignment types of the inorganic and organic component can be varied by rational variation of the inorganic or organic component. With the computational protocol shown to work for the above series of oligothiophene-based lead halides, a more extensive family of the oligothiophene-based 2D HOIPs is then investigated to demonstrate their structural and electronic tunability. For AE2T\ch{PbI4}, the disorder of the organic cations are investigated systematically in synergy between theoretical techniques and experimental reference data provided by a collaborating group. A staggered arrangement of AE2T cations is revealed to be the most stable packing pattern with the correct band alignment types, in agreement with experiment results from optical spectroscopy. Another representative class of 2D HOIPs based on oligoacene derivatives is investigated to show structural and electronic tunability similar with their oligothiophene based counterparts. In the final part of the thesis, an all-electron implementation of Bethe-Salpeter equation (BSE) approach based on the $GW$ approximation is developed using numeric atom centered orbital basis sets, with the aim of developing first steps to a formal many-body theory treatment of neutral excitations, which goes beyond the independent-particle picture of density functional theory. Benchmarks of this implementation are performed for the low-lying excitation energies of a popular molecular benchmark set (``Thiel's" set) using results obtained using the Gaussian-orbital based MolGW code as reference values. The agreement between the BSE results computed by these two codes when using the same $GW$ quasiparticle energies validate our implementation. The impact of different underlying technical approximations to the $GW$ method is evaluated for the so-called ``two-pole" and ``Pad{\' e}" approximate evaluation techniques of the $GW$ self-energy and resulting quasiparticle energies. To reduce the computational cost in both time and memory, the convergence of the BSE results with respect to basis sets and unoccupied states is examined. An augmented numeric atom centered orbital basis set is proposed to obtain numerical converged results.</p>