Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B20: First-principles Modeling of Excited-state Phenomena in Materials II: Many-body Perturbation Theory (Techniques and Applications)Focus
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Sponsoring Units: DCOMP DMP Chair: Serdar Ogut, University of Illinois at Chicago Room: BCEC 157A |
Monday, March 4, 2019 11:15AM - 11:51AM |
B20.00001: Correlated multi-particle excitations: Green’s function formalism for trions and biexcitons Invited Speaker: Felipe Da Jornada With the experimental isolation of atomically thin one- and two-dimensional materials, it is now possible to measure a variety of charged and neutral multiparticle excitations (trions, biexcitons, etc.) in these systems, many of which display large binding energies. On the theory side, however, while quasiparticle and neutral optical excitations have been successfully treated in many materials with the first-principles GW and GW plus Bethe-Salpeter equation (GW-BSE) approaches, respectively, similar many-body and parameter-free approaches are not available to compute and understand correlated multi-particle excitations. Accordingly, past theoretical studies were often limited to treatments based on model Hamiltonians. In this talk, we present results from a new ab initio approach based on the interacting 3- and 4-particle Green’s function formalism to compute multiparticle excitations [1]. Our new diagrammatic approach that makes use of appropriate screened Coulomb interactions, combined with a high-performance computing implementation, allows us to predict without adjustable parameters that trions and biexcitons in carbon nanotubes are stable at room temperature, and also to reveal in details electronic correlation in these multiparticle excitations. We will also comment on how this formalism can be employed to investigate other high-order excited-state phenomena in the bulk and at the nanoscale. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B20.00002: Exciton-phonon coupling in bulk hexagonal boron nitride from first principles: phonon-assisted UV absorption and emission spectra Fulvio Paleari, Henrique Miranda, Alejandro Molina-Sanchez, Ludger Wirtz We present an ab initio method to calculate phonon-assisted absorption and emission spectra when strong excitonic effects are present. We apply the method to bulk hexagonal BN, a material with indirect band gap which exhibits strong luminescence in the UV range. We first calculate the excitons at the wave vector qi of the indirect gap with the Bethe-Salpeter equation. The coupling of these excitons with the various phonon modes at qi is investigated with group theory and then expressed in terms of a product of the mean square displacement of the atoms and the second derivative of the optical response function with respect to atomic displacement along the phonon eigenvectors. The derivatives are calculated numerically with a finite difference scheme in a supercell commensurate with qi. We use detailed balance arguments to obtain the intensity ratio between emission and absorption processes. Our results[1] explain recent luminescence experiments[2] and reveal the exciton-phonon coupling channels responsible for the emission lines. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B20.00003: Dynamical electronic screening and exciton binding in the Bethe-Salpeter approach Xiao Zhang, Andre Schleife The Bethe-Salpeter equation (BSE) is a theoretical spectroscopy technique that accurately describes optical absorption by considering the screened electron-hole Coulomb interaction. In practice, the frequency dependence of screening is oftentimes neglected to reduce computational cost. This static approximation is often valid due to small exciton-binding energies in bulk inorganic semiconductors, compared to plasmon frequencies. However, for materials with large exciton-binding energies, such as finite and low-dimensional systems, dynamical screening can become important. To explore this, we incorporated dynamical electronic screening into the BSE approach and quantify its impact from first-principles calculations. We study the optical absorption of linear oligoacene crystals and show that the exciton binding energy is on the order of 1 eV. Furthermore, we compute corrections due to dynamic screening, and show that these can be of comparable size, i.e. an order of magnitude larger than in inorganic semiconductors. We also show that including this effect significantly improves agreement of exciton binding energies with experimental results. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B20.00004: Understanding Excited-States of a Perylene Diimide Nanowire from First Principles Theory Tianlun Huang, Sahar Sharifzadeh Perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) are promising optoelectronic materials; they possess strong optical absorption and emission properties, along with a propensity for self-assembly and excellent stability. We utilize density functional theory and many-body perturbation theory within the GW/BSE approximation to investigate the optoelectronic properties of a periodic assembly of PTCDI DNA base surrogates. We predict a bandstructure with significant bandwidth (~0.8 eV) and several spatially delocalizated low-energy optically excited-states. By incorporating electron-phonon interactions, we determine that at finite temperature, the bandwidth is reduced and exciton binding increased, leading to localization of excited-states. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B20.00005: Structural Effects on Excitonic Phenomena in Organic Crystals Sivan Refaely-Abramson, Jonah Haber, Felipe Da Jornada, Steven G. Louie, Jeffrey B Neaton We study structural effects on single- and multi-exciton processes in organic molecular crystals. We use many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to calculate and compare the quasiparticle and excitonic band structures of different polymorphs of solid pentacene, and explore their implications for excited-state processes. We find that structural differences between known phases can lead to large effects on exciton-exciton interactions; and in particular, we predict very different singlet fission rates, revealing a strong sensitivity between singlet fission efficiency and crystal phase. We explore the effect of phonons on this picture, and discuss the implications of our findings on design principles and pathways to optimize photogeneration processes in organic crystals. This work supported by the Department of Energy; computational resources provided by NERSC. |
Monday, March 4, 2019 12:39PM - 1:15PM |
B20.00006: Computational discovery of new materials for intermolecular singlet fission in the solid state Invited Speaker: Noa Marom Intermolecular singlet fission (SF) is the conversion of one photogenerated singlet exciton into two triplet excitons localized on different molecules. SF has the potential to significantly enhance the conversion efficiency of organic solar cells by harvesting two charge carriers from one photon. However, few materials are presently known to exhibit intermolecular SF in the solid state. Using many-body perturbation theory in the GW approximation and Bethe-Salpeter equation (BSE), we have elucidated the effect of crystal packing on the excitonic properties of molecular crystals. To assess the likelihood of new materials to exhibit SF, we have proposed a two-dimensional descriptor based on the thermodynamic driving force for SF and the degree of singlet exciton charge transfer character. To evaluate the latter we have developed the double-Bader analysis method for exciton wave-functions from BSE calculations. We have identified several promising candidates for intermolecular SF in the solid state including monoclinic rubrene, quaterrylene, and phenylated pentacene derivatives. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B20.00007: Phenylated Acene Derivatives as Candidates for Intermolecular Singlet Fission Xiaopeng Wang, Xingyu Alfred Liu, Rithwik Tom, Cameron Cook, Bohdan Schatschneider, Noa Marom Singlet fission (SF), the conversion of one singlet exciton into two triplet excitons, may improve the efficiency of organic photovoltaics. Only a few materials have been experimentally observed to undergo intermolecular SF, most of which are acenes and their derivatives. Using many-body perturbation theory in the GW approximation and the Bethe-Salpeter equation (BSE), we systematically investigate the electronic and excitonic properties of tetracene, pentacene, and their phenylated derivatives in the gas phase and solid state. Their potential for SF is evaluated with respect to the thermodynamic driving force and the singlet exciton charge transfer character. In both gas phase and solid state, pentacene and its derivatives are more promising than tetracene and its derivatives. Within a group of phenylated derivatives with the same acene backbone, increasing the number of phenyl side groups is detrimental for SF in the gas phase. However, solid state properties are additionally affected by intermolecular interactions. We find that a higher number of phenyl side groups results in crystal structures with weak π-stacking and slip stacking, which exhibit higher SF driving force in the solid state. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B20.00008: The Spin-Flip BSE approach and applications to simple molecular systems Bradford Barker, David Strubbe The spin-flip (SF) method allows for description of multi-reference states by considering excitations of a single high-spin reference state, and it has been successfully used in time-dependent density-functional theory (SF-TDDFT) and configuration interaction (SF-CI) approaches to describe molecules with unpaired spins. While SF-TDDFT is significantly less computationally expensive than SF-CI, it has difficulty describing double excitations, Rydberg states, and bond-breaking with conventional functionals. Just as the GW/Bethe-Salpeter (BSE) approach, with its ab initio long-ranged and non-local interaction kernel, allows for a systematic improvement of calculated optical absorption spectra over TDDFT, we consider a spin-flip BSE (SF-BSE) approach to improve SF-TDDFT. This method has a more moderate expense than SF-CI and can be used also for extended systems. We present the theory of SF-BSE and apply it to simple molecular examples of bond-breaking and unpaired spins. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B20.00009: Conjugation length dependence on the electronic and optical properties of oligothiophene-F4TCNQ complexes Ana Valencia, Caterina Cocchi To understand how the donor conjugation length affects the doping mechanisms in organic semiconductors [1], we investigate a series of charge transfer (CT) complexes formed by oligothiophene molecules of increasing length doped by the acceptor F4TCNQ. Using hybrid DFT as a starting point, we assess the electronic and optical properties of these systems from many-body perturbation theory (GW and the Bethe-Salpter equation). We find that the frontier orbitals (HOMO and LUMO) have CT character in all complexes, while deeper occupied and higher virtual states depend on the nT length. The first bright excitation is dominated by the HOMO-LUMO transition occurring approximately at the same energy in all systems. At increasing donor length, higher-energy peaks exhibit different character depending on the donor conjugation length. The rationale offered by our results contributes to clarify the excitation processes in organic donor/acceptor complexes. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B20.00010: First-principles prediction of free-electron screening of the electron-hole interaction in hybrid perovskite MAPbI3 Joshua Leveillee, Andre Schleife Hybrid organic-inorganic perovskite MAPbI3 has captivated the solar cell community as a high-efficiency photovoltaic material. Recent reports find high intrinsic charged defect concentrations in MAPbI3 which donate free electrons to the material. Due to these, the Coulomb potential between electrons and holes is further screened and the exciton binding energy decreases. We use first-principles methods based on many-body perturbation theory to determine the influence of free-electron screening on the predicted excitonic properties and the optical spectrum of MAPbI3. Electronic band structures and gaps are predicted using Hedin’s GW approximation, including the spin-orbit interaction. Electron-hole excitation energies and optical spectra are calculated using the Bethe-Salpeter framework. Our calculations show that exciton binding energies span the experimentally observed range of 31 to 2.5 meV as the free-electron concentration varies between 1012 and 1019 cm-3. The corresponding optical spectra are found in good agreement with experimental results, suggesting that free-carrier screening plays an important role in the optical response of MAPbI3. |
Monday, March 4, 2019 2:03PM - 2:15PM |
B20.00011: First-principles Studies of Tl Activated Halide Scintillator Phosphor Materials: Towards an Understanding of the Scintillation Mechanism Andrew Canning, Mauro Del Ben, Edith Bourret Tl doped halide scintillator phosphors are amongst the most commonly used gamma ray detector materials for medical imaging, high energy physics and nuclear materials detection applications (e.g. CsI:Tl, NaI:Tl). Even so the complete scintillation process in these materials is still poorly understood. In particular in recent years there has been great interest in co-doping these materials to try and improve their detection performance. We have performed first-principles studies based on GGA, hybrid functionals and the GW/BSE method in tandem with experiments to understand the scintillation mechanism in these materials and how it could be improved by co-doping. In particular we have looked at the Tl exciton optical emission states and energy transfer mechanisms from the gamma ray to the Tl. Recently there has also been interest in new Tl bulk scintillators such as TLYC (Tl2LiYCl6) which we have also studied. |
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