Tunable Electronic Excitations in Hybrid Organic-Inorganic Materials: Ground-State and Many-Body Perturbation Approaches
Chi Liu, Ph.D. candidate
Volker Blum, advisor
Wednesday, June 19, 2019 - 1:00pm to 3:00pm
Location: French Family Science Center 3232
Avery, Meg

The 3D Hybrid Organic-Inorganic Perovskites (HOIPs) has been investigated intensively for application in photovoltaics in the last decade due to their extraordinary properties, including easy fabrication, suitable band gap, large absorption, high charge carrier mobility, etc. However, the structure and properties of their 2D counterparts, especially those with complex organic components, are not understood as deeply as the 3D HOIPs. Due to the easing of spatial constraint for the organic cations, the 2D HOIPs potentially has more structural flexibility and thus higher tunability in electronic properties compared with the 3D HOIPs. With the motivation to demonstrate such flexibility and tunability, a series of 2D HOIPs is selected with oligothiophene derivative as the organic cations, and lead halide as the inorganic framework. Initial computational models with varying organic or inorganic component are constructed from the experimental structure of 5,5''-bis(aminoethyl)-2,2':5',2''':5'',2'''-quaterthiophene lead bromide (AE4TPbBr4). Ab initio first-principle calculations are performed for these materials employing density functional theory with corrections for van Der Waals interaction and spin orbit coupling. These 2D HOIPs are found to be understandable by 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 switched between Type I and Type II by rational variation of the inorganic or organic component. With the computational protocol shown to work for the above series of oligothiophene based lead halide, a more complete family of the oligothiophene based 2D HOIPs is then investigated to demonstrate their structural and electronic tunibility. In the study of AE2TPbI4, the disorder of the organic cations are investigated systematically as a synergy of theoretical and experimental techniques. The staggered packing of AE2T cations is revealed to be the most stable packing pattern with the correct band alignment types in agreement with experiment optical results. Another representative class of 2D HOIPs based on oligoacene derivatives is investigated to show structural and electronic tunability similar with their oligothiophene based counterparts. To describe correctly the neutral excitations which is missing in the hybrid density functional theory relied on, an all-electron implementation of Bethe-Salpeter equation (BSE) approach is developed using numeric atom centered orbital basis sets in the FHI-aims package. Benchmarks of this implementation is performed for the low-lying excitation energies of a popular molecular benchmark set (the "Thiel's" set) using the MolGW results as reference values. The agreement between the BSE results computed by these two codes provided the same GW quasiparticle energie validate our implementation. The impact of different underlying GW approximations is evaluated for the "Two-pole" and "Pade's" approximations in FHI-aims. To reduce the computational cost in both timings and memory, the convergence of the BSE results with respect to basis sets and unoccupied states is conducted. An augmented numeric atom centered orbital basis set is proposed to obtain numerical converged results using the aug-cc-pV5Z results as the reference values with reduced computational effort. This non-periodic BSE implementation, together with computational tools based on the hybrid density functional theory that has been demonstrated to work for large 2D HOIPs, serve as a foundation for the calculation and prediction of structural, electronic and optical properties of HOIPs and complex hybrid materials in general.