Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K29: Firstprinciples Modeling of ExcitedState Phenomena in Materials VII: Organic and Hybrid MaterialsFocus

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Sponsoring Units: DCOMP DMP DCMP DCP Chair: Jefferson Bates, Appalachian State Univ Room: LACC 406A 
Wednesday, March 7, 2018 8:00AM  8:36AM 
K29.00001: Embedded manybody perturbation theory for organic and hybrid disordered systems Invited Speaker: Xavier Blase We present the merging of the GW and BetheSalpeter equation (BSE) formalisms with continuous or discrete polarisable models [1,2] allowing the study of the electronic and optical properties of molecular systems embedded in complex electrostatic and dielectric environments. We show in particular that the absolute position with respect to the vacuum level of the band edges of organic semiconductors can be obtained with accuracy both in the bulk and at the surface. We further demonstrate that the combination of BSE with polarisable models allows to account simultaneously for "state specific" and "linear response" effects, solving a long standing problem faced by timedependent DFT calculations. As a first application, we discuss the mechanisms allowing to understand the doping mechanisms in organic semiconductors where donor/acceptor levels are in general very deep. [3] 
Wednesday, March 7, 2018 8:36AM  8:48AM 
K29.00002: The predictive power of “oneshot” GW calculations for halide perovskites Linn Leppert, Tonatiuh Rangel Gordillo, Jonah Haber, Jeffrey Neaton Halide perovskites like the hybrid organicinorganic APbX_{3} (A=CH_{3}NH_{3}^{+}, CH(NH_{2})_{2}^{+},... and X=I, Br, Cl) or the allinorganic double perovskite Cs_{2}BiAgBr_{6} are a class of compounds showing great promise as photovoltaic absorber materials. Accurately predicting their band gaps is challenging for first principles electronic structure methods because of the intricate coupling of structural and electronic degrees of freedom [1] as well as strong spinorbit interactions. We use ab initio manybody perturbation theory, employing a “oneshot” G_{0}W_{0} approach, to calculate the fundamental band gaps of experimentally characterized halide perovskites. We investigate the impact of the density functional theory starting point, pseudopotentials, and eigenvalue selfconsistency on the calculated quasiparticle band gap. With our calculations, we assess whether there might be a “one size fits all” oneshot GW approach for this material class [2]. 
Wednesday, March 7, 2018 8:48AM  9:00AM 
K29.00003: ManyBody Perturbation Theory for molecules: Physicists' approach versus Chemists' approach Fabien Bruneval First devised in the 50's, ManyBody Perturbation Theory proposes to capture the electronic correlation by performing expansions. Along with the years, physicists, mostly focused on infinite solids, and chemists, mostly interested in molecules, have developed different approximations or, in other words, have selected different Feynman diagrams. Some consider all the diagrams up to a given order [1], others select a family of diagrams and perform infinite summation on this class [2]. 
Wednesday, March 7, 2018 9:00AM  9:12AM 
K29.00004: Understanding The Structurefunction Relation In Natural Dyes MARIAMI RUSISHVILI, Sara Laporte, Luca Grisanti, Alessandra Magistrato, Stefano Baroni Understanding the relation between the structure of a molecule and the function it may express is one of the main goals of chemical physics. Natural dyes, which differ by tiny chemical details or which operate in slightly different conditions may express a broad range of colors. The question arises as to which chemical, structural, and environmental features determine the color expressed by a specific molecule in specific environments. In order to answer this questions, we have developed a multiscale protocol, based on timedependent density functional theory, ab initio molecular dynamics [1], and advanced sampling techniques [2], to study the optical properties of complex molecular species in realistic conditions. We apply this protocol to the most abundant natural dyes, anthocyanins, and we show that the various colors observed at different pH conditions result from a balance of chemical and dynamical effects that can be exploited, together with copigmentation, to identify natural variants of the same molecule with customdesigned chromatic properties. 
Wednesday, March 7, 2018 9:12AM  9:24AM 
K29.00005: Tuning Optical Properties of Dibenzochrysenes by Functionalization: A ManyBody Perturbation Theory Study Nicolas Dardenne, Roberto Cardia, Jing Li, Giuliano Malloci, Giancarlo Cappellini, Xavier Blase, JeanChristophe Charlier, GianMarco Rignanese Using manybody perturbation theory, the optical properties of modified compact and angular dibenzochrysenes are investigated. First, for a series of already existing molecules in that family, the computed absorption spectra are benchmarked against experimental data in order to evaluate the strengths and the limitations of the method. Then, the computed absorption spectra are presented for newly designed dibenzochrysenes. One of the main results is that the addition of thiophenyl groups at specific positions produces an enhancement of the absorption in the visible region. The information provided in this study can be used as a guide to synthesize optimized materials for solar cells. 
Wednesday, March 7, 2018 9:24AM  9:36AM 
K29.00006: Geometry Distortion and Small Polaron Binding Energy Changes with Ionic Substitution in Halide Perovskites Amanda Neukirch, Liujiang Zhou, Iwnetim Abate, Jacky Even, Sergei Tretiak Solutionprocessed organometallic perovskites have demonstrated remarkable performances in optoelectronic devices and applications. Despite the extraordinary progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices, remain open. Here we report the results from an indepth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using isolated structures. Local lattice symmetry, electronic structure, and electron phonon coupling are interrelated in polaron formation in hybrid halide perovskites. To illustrate these aspects, first principles calculations are performed on CsPbI3, CsSnI_{3}, CsPbBr_{3}, MAPbI_{3}, FAPbI_{3}, MAPbBr_{3}, FAPbBr_{3}, MASnI_{3}, and FASnBr_{3}. This study will focus on how ionic substitution changes the polaron binding energy in the material. It is found that in all cases that hole polaron formation is associated with lattice contraction, while electron polaron formation is associated with lattice expansion. 
Wednesday, March 7, 2018 9:36AM  9:48AM 
K29.00007: PhenylSubstituted Acene Derivatives as Candidates for Intermolecular Singlet Fission Xingyu Liu, Xiaopeng Wang, Cameron Cook, Bohdan Schatschneider, Noa Marom A significant increase in the solar conversion efficiency of organic photovoltaic (OPV) devices could be achieved by harnessing singlet fission (SF), the downconversion of one singlet exciton into two triplet excitons. Acene and acene derivatives, such as pentacene, tetracene, and rubrene, exhibit intermolecular SF in the solid state. Here, we systematically examine a family of crystalline phenylsubstituted tetracene and pentacene derivatives to elucidate the effect of phenyl substitutions. Manybody perturbation theory (MBPT) in the GW approximation and the BetheSalpeter equation (BSE) is used to describe the electronic and excitonic properties of isolated molecules and molecular crystals. We show that the properties of acene derivatives may be modified by changing the number of phenyl side groups, which affects the single molecule properties, as well as the crystal packing. This produces a versatile family of acene derivatives with tunable excitonic properties, some of which are attractive candidates for intermolecular SF. 
Wednesday, March 7, 2018 9:48AM  10:00AM 
K29.00008: On the Possibility of Singlet Fission in Crystalline Quaterrylene Xiaopeng Wang, Xingyu Liu, Cameron Cook, Bohdan Schatschneider, Noa Marom Singlet Fission (SF), the spontaneous downconversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improving organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit SF, most of which are acene derivatives. Recently, there has been a growing interest in the rylene family of polycyclic aromatic hydrocarbons (PAHs) as potential SF materials. We use manybody perturbation theory in the GW approximation and the BetheSalpeter equation (BSE) to investigate the possibility of SF in crystalline quaterrylene. A new method is presented for determining the percent charge transfer character (%CT) of an exciton wavefunction from doubleBader analysis. This enables relating exciton probability distributions to crystal packing. Based on comparison to known and predicted SF materials with respect to the energy conservation criterion (E_{S}2E_{T}) and %CT, crystalline quaterrylene is a promising candidate for SF. Furthermore, quaterrylene is attractive for OPV applications thanks to its high stability, narrow optical gap, and lowest lying triplet excitation energy in the optimal range to maximize solar conversion efficiency. 
Wednesday, March 7, 2018 10:00AM  10:12AM 
K29.00009: Charge transfer and singlet fission quantum dynamics in organic photovoltaics Pengfei Huo, Sharma Yamijala We will discuss our recent investigations on singlet fission and charge separation quantum dynamics in organic photovoltaic devices. First, we use a realtime pathintegral approach to explore the singlet fission processes in pentacene. We observed destructive interference effects between the two charge transfer (CT) mediated fission pathways due to their opposite electronic couplings with the triplettriplet pair state. Second, we apply an approximated quantum dynamics approach to investigate the competing charge transfer and charge separation pathways in organic photovoltaic (OPV) bulk heterojunction (BHJ). This approach propagates the timedependent electronic wavefunction on a precomputed molecular dynamics trajectory. The electronic wavefunction is expanded with instantaneous Density Functional Tight Binding (DFTB) orbitals along the nuclear trajectory. This approximated approach allows us to characterize the realtime electron and hole dynamics in a realistic largescale BHJ model system and resolve the mechanistic debates on free charge carrier generations in OPV devices. 
Wednesday, March 7, 2018 10:12AM  10:24AM 
K29.00010: Quasiparticle excitations of molecular chains on graphene Johannes Lischner, HsinZon Tsai, Jiong Lu, Arash Omrani, Kenji Watanabe, Takashi Taniguchi, Steven Louie, Alex Zettl, Michael Crommie The ability to understand and control the electronic properties of molecular assemblies at the nanoscale is crucial for future technologies. In this talk, we study quasiparticle excitations of linear chains of F4TCNQ molecules on graphene in a fieldeffect transistor setup and demonstrate the ability to control their charge state by means of the gate voltage. In particular, we employ ab initio densityfunctional theory and GW calculations to parametrize a model Hamiltonian which describes the adsorbed molecules as Anderson model impurities interacting via the screened Coulomb interaction. In agreement with recent scanning tunnelling experiments, our calculations predict a range of gate voltages at which every other molecule is occupied by an extra electron. 
Wednesday, March 7, 2018 10:24AM  10:36AM 
K29.00011: Manybody perturbation theory analysis of point defects in bulk and monolayer semiconducting materials Kirk Lewis, Sahar Sharifzadeh Defects can strongly influence the electronic properties of semiconducting materials. An accurate and detailed knowledge of the influence of defects is central to the design of new highperformance materials. We employ firstprinciples manybody perturbation theory within the GW/BSE approximation to investigate the influence of point defects on the electronic properties of bulk and monolayer semiconducting materials. For a +1 charged nitrogen vacancy within bulk GaN, we develop an approach to systematically identify defects and their energies from GW/BSE calculations. By analysis of the bandstructure and optical absorption spectrum, we predict that this particular defect does not significantly alter the optoelectronic properties of GaN. Furthermore, the same methodology is applied to monolayer WS_{2} containing a single sulfur vacancy, a likely point defect in this TMDC. We determine that the electronic properties relevant to transport in WS_{2} are significantly affected by the presence of this defect, with implications for devices fabricated using this material system. 
Wednesday, March 7, 2018 10:36AM  10:48AM 
K29.00012: Firstprinciples Engineering of Charged Defects for Twodimensional Quantum Technologies Yuan Ping, Ravishankar Sundararaman, Dario Rocca, Feng Wu Charged defects in 2D materials have emerging applications in quantum technologies such as 
Wednesday, March 7, 2018 10:48AM  11:00AM 
K29.00013: Efficiently Capturing Substrate Screening Effects on Level Alignments at MoleculeMetal Interfaces with GW Calculations Zhenfei Liu, Felipe H. da Jornada, Steven Louie, Jeffrey Neaton Quasiparticle energies and level alignments at moleculemetal interfaces can be accurately captured by the ab initio GW approach. However, the computational cost for such GW calculations for typical interfaces is relatively high, given the large system size and chemical complexity of typical interface systems. Approximate selfenergy corrections constructed from imagecharge models have been used to compute level alignments with good accuracy but require the definition of an image plane. In this work, we study a primary bottleneck of GW calculations at interfaces, namely the calculation of the polarizability. We explore models and approximations to compute this quantity more efficiently and accurately, with the aim of computing molecular resonance energies at interfaces and capturing substrate screening effects without the need of defining an image plane. 
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