Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G39: FirstPrinciples Modeling of ExcitedState Phenomena in Materials V: GW+BSE for Organic MaterialsFocus

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Sponsoring Units: DCOMP DMP DCP Chair: Yan Li, American Physical Society Room: 703 
Tuesday, March 3, 2020 11:15AM  11:51AM 
G39.00001: Reproducibility, precision and accuracy in GW calculations Invited Speaker: Michiel van Setten For many years, computational limits have forced GW calculations to employ approximations without the possibility to systematically evaluate their validity. In such a situation, the search for accuracy and precision can become very challenging, up to the point where one risks sacrificing precision for the sake of accuracy. With increased computational resources, this situation is changing but performing fully converged none approximated GW for solids is still a big challenge. For finite size systems, however, calculations can be more tractable. 
Tuesday, March 3, 2020 11:51AM  12:03PM 
G39.00002: Solving the BetheSalpeter Equation on a Subspace: Approximations and Consequences for BandEdge and CoreLevel Excitons in Quasi Lowdimensional Materials Diana Qiu, Felipe H. da Jornada, Steven Louie It is known that environment can dramatically renormalize the quasiparticle energy gap and exciton binding energies in quasi lowdimensional materials, but the effect of environmental screening on the energy splitting of the spinsinglet and spintriplet exciton states is less explored. The renormalization of the exciton binding energy arises from additional environmental screening of the attractive direct Coulomb term in the kernel of the BetheSalpeter equation (BSE). The repulsive exchange interaction responsible for the singlettriplet slitting is in principle unscreened, though it has been argued that in practical calculations using a subspace of the full Hilbert space the exchange interaction should be modified and effectively screened by states outside of the chosen subspace, the “S” approximation. We explore the accuracy of the S approximation for different systems, including molecules, heterostructures and core level excitations. We show, additionally, that the S approximation is actually exact in the limit of small exciton binding energies provided that a screening consistent with the TammDancoff approximation is employed. 
Tuesday, March 3, 2020 12:03PM  12:15PM 
G39.00003: PyreneStabilized Acenes as Intermolecular Singlet Fission Candidates: Importance of Exciton WaveFunction Convergence Xingyu Alfred Liu, Rithwik Tom, Xiaopeng Wang, Bohdan Schatschneider, Noa Marom Singlet fission (SF) is the conversion of a singlet exciton into two triplet excitons. SF could increase the efficiency of organic solar cells by harvesting two carriers from one photon. Polyacene crystals, such as tetracene and pentacene, have shown outstanding SF performance. However, their instability prevents them from being utilized in SFbased photovoltaic devices. In search of practical SF chromophores, we use manybody perturbation theory with the GW approximation and BetheSalpeter equation to study the excitonic properties of pyrenestabilized acenes. We propose a criterion to determine the convergence of exciton wavefunctions with respect to the fine kpoint grid used in the BerkeleyGW code. An opensource Python code is presented to perform exciton wavefunction convergence checks and streamline the doubleBader analysis of exciton character. We find that the singlet excitons in pyrenestabilized acenes have a higher degree of charge transfer character than in the corresponding acenes. The pyrenefused tetracene and pentacene derivatives exhibit comparable excitation energies to their corresponding acenes, making them potential SF candidates. The pyrenestabilized anthracene derivative is considered as a possible candidate for triplettriplet annihilation (TTA). 
Tuesday, March 3, 2020 12:15PM  12:27PM 
G39.00004: An energetics perspective on why there are so few triplettriplet annihilation emitters Xiaopeng Wang, Rithwik Tom, Xingyu Alfred Liu, Noa Marom The efficiency of organic solar cells may be increased by utilizing photons with energies below the band gap of the absorber. This may be enabled by upconversion of low energy photons into high energy photons via triplettriplet annihilation (TTA). The quantum yield of TTA is often low due to competing processes. The singlet pathway where a highenergy photon is emitted has a 1/9 probability according to the ClebschGordon rules. The quintet pathway is typically too high in energy to be accessible, leaving the triplet pathway as the main competing process. Using manybody perturbation theory in the GW approximation and the Bethe–Salpeter equation, we calculate the energy release in both the singlet and triplet pathways in 61 chromophores of different chemical families. We find that in most cases the triplet pathway is open and has a larger energy release than the singlet pathway. Thus, the energetics perspective explains why there are so few TTA emitters and their low quantum yields. 
Tuesday, March 3, 2020 12:27PM  12:39PM 
G39.00005: Analysis of Excitons in Stacked Perylene Diimide Derivatives Kasidet Jing Trerayapiwat, Aliya Mukazhanova, Sahar Sharifzadeh πstacked organic molecules are tunable lightabsorbing materials that are promising for many optoelectronic applications; thus, it is necessary to understand how both inter and intramolecular interactions influence optical excitations. We use firstprinciples timedependent density functional theory (TDDFT) to study the nature of these interactions in functionalized peryelene diimide oligomers stacked on a DNAlike backbone. Taking a FranckCondon HerzburgTeller approach to vibronic coupling, and sampling of ground state vibrations via molecular dynamics, we obtain an ensemble of excitedstates that may occur. By analyzing ~100 dimer and trimer structures, we elucidate the parameters that correlate with the nature of the excitedstate. Charge transfer (CT) character analysis shows that the degree of CTlike character in a bright state correlates with the strength of electronic transiton and with its intermolecular vibrations. Furthermore, we determine that displacement and rotation between monomers leads to reordering of direct excitations. Such a finding is important for technologies such as singlet fission photovoltaics where the CT excitation character is directly related to performance. 
Tuesday, March 3, 2020 12:39PM  12:51PM 
G39.00006: Firstprinciples G_{0}W_{0} calculations for ferrocene, anthracene and porphyrin Masoud Mansouri, Peter Koval, Daniel SanchezPortal Hedin's GW approximation is wellknown as a powerful method in the material science community because of its high quality and relatively low computational cost. In this study, we evaluate the performance of the G_{0}W_{0} approximation for the charged quasiparticle excitation energies of three gasphase molecules of interest for organic electronics, including ferrocene, porphyrin and anthracene. Moreover, we assess the undesired startingpoint dependence in this method by benchmarking the ionization energies obtained from G_{0}W_{0} on top of the spinresolved HartreeFock and KohnSham methods. The merit of dynamic effective GW selfenergy leads to significant improvement of ionization energies compared to meanfield calculations and deviations less than 0.4 eV as compared to experiment. The calculations are carried out by the use of numerical atomic orbitals and contour deformation technique along with an iterative procedure to deal with the energy dependency of GW selfenergy and full matrix elements of the dynamically screened Coulomb interaction. 
Tuesday, March 3, 2020 12:51PM  1:03PM 
G39.00007: Exciton temperature dependence dictated by localization in organic semiconductors Antonios Alvertis, Richard H. Friend, Akshay Rao, Alex W. Chin, Bartomeu Monserrat The response of organic semiconductors in optoelectronic devices is dominated by room temperature excitons. However, exciton calculations of organic crystals at finite temperature can be challenging, due to the need to combine the effects of excitonphonon coupling and thermal expansion. Here we overcome this challenge by employing a combination of Green's function methods for the electronic structure, and nonperturbative finite difference methods for the interaction with the lattice. We apply our methodology to the acene series of molecular crystals and find that different localization ranges of excitons lead to different responses to excitonphonon coupling and thermal expansion. As a result, the energies of singlet and triplet excitons have different temperature dependencies, as do the singlet energies between the different materials. We expect that the combination of excitonphonon coupling and thermal expansion, together with their interplay with the exciton localization range, provides a general mechanism for understanding the temperature dependence of exciton energies in molecular crystals. 
Tuesday, March 3, 2020 1:03PM  1:15PM 
G39.00008: Probing energy transfer design principles in photosynthetic light harvesting systems with predictive electronic structure calculations Caroline McKeon, Samia M Hamed, Chenchen Song, Jeffrey B Neaton A thorough theoretical understanding of photosynthetic lightharvesting systems is critical to our ability to mimic their unparalleled energy transduction efficiency in synthetic contexts. Here, we present a workflow for calculating optical absorption spectra of chromophore monomers and dimers within the FranckCondon approximation, using classical molecular dynamics simulations, and timedependent density functional theory. We apply our approach to Sulforhodamine B (SRB) and AlexaFluor488 chromophore monomers and dimers and compare our results with complementary experimental absorption spectra. We discuss how conformation, the presence of counterions, solvation models, and choice of functional influence the predicted absorption spectra. We discuss progress towards identifying key factors in the high quantum efficiencies found exclusively in natural photosynthetic systems. This work is supported by the Department of Energy. 
Tuesday, March 3, 2020 1:15PM  1:27PM 
G39.00009: Exciton coherence times and diffusion constants in molecular crystals from excitonphonon coupling with an ab initio GWBSE approach Jonah Haber, Felipe Da Jornada, Sivan RefaelyAbramson, Gabriel Antonius, Steven Louie, Jeffrey B Neaton Predictive theories of exciton dynamics are of growing importance as increasingly complex materials, with strong electronhole interactions, are used in device physics applications. For instance, in organic photovoltaics, an important part of energy conversion processes involves the diffusion of a photoexcited exciton to donoracceptor interfaces where charge separation can occur. To quantitatively understand exciton dynamics, a microscopic theory of excitonphonon interactions is required. Here, we describe an ab initio framework for computing excitonphonon matrix elements, using density functional perturbation theory in conjunction with manybody perturbation theory within the GW plus BetheSalpeter equation (BSE) approach. We apply this formalism to crystalline tetracene, a prototypical organic semiconductor with strong electronhole interactions. We compare and contrast how lowlying spin singlet and triplet excitons couple to the phonon field. We perturbatively compute phononlimited exciton coherence times throughout the Brillioun zone and report exciton diffusion constants, evaluated using the relaxation time approximation. In all cases, we compare with experimental measurements, where available. 
Tuesday, March 3, 2020 1:27PM  1:39PM 
G39.00010: Effects of dynamical lattice screening on excitonic and optical properties of polar compounds Joshua Leveillee, Xiao Zhang, Andre Schleife The BetheSalpeter equation (BSE) framework is very common for predicting optical and excitonic properties in materials. Typically, only highfrequency electronic dielectric screening is considered in the screened electronhole Coulomb interaction. In materials hosting polar phonon modes that induce macroscopic electric fields, there is an additional dynamical lattice screening effect. To describe this contribution, we use the Shindo approximation and the Fröhlich model to predict exciton binding energies in NaCl, MgO, AlN, and GaN. Dynamical lattice screening exerts a small influence on the excitonic properties of the high band gap materials NaCl and MgO. In AlN, the predicted exciton binding energy reduces from 145 to 112 meV. In GaN, dynamical lattice screening drastically reduces the exciton binding energy from 52 to 30 meV, in good agreement with experiments. The optical spectra for all compounds are predicted to be in good agreement with experimental spectra under the inclusion of effective dynamical lattice screening, and we find that it is a critical contributor to the screened electronhole interaction in polar materials with low exciton binding energies and high longitudinal optical phonon frequencies. 
Tuesday, March 3, 2020 1:39PM  1:51PM 
G39.00011: Structural and Electronic Properties of the NAIDMAC Organic Emitting Diode: A Thermally Activated Delayed Fluorescence Compound Tommaso Francese, Francois Gygi, Giulia Galli Thermally activated delayed fluorescence compounds (TADFs) are a class of purely organic light emitting diodes^{[1] }based on reverse intersystem crossing processes. Despite the successful synthesis of blue/green TADF compounds, the search for efficient orangered fluorescent TADFs remains an open problem. NAIDMAC^{[2]} [C_{37}H_{32}N_{2}O_{2}] is one of the few orangered light emitters with an external quantum efficiency of ~ 30%. Here we present a first principle study of NAIDMAC both in the dilute and solid state limit. We carried out FirstPrinciples Molecular Dynamics simulations with the Qbox code (http://qboxcode.org) at different temperatures and computed ground to excited state transitions by employing rangeseparated hybrid functionals^{[3]}, as well as the SCAN functional^{[4]}. We present results for the energy difference between singlet and triplet excited states (ΔE_{ST}), and we discuss how ΔE_{ST} is related to structural differences in the ground and excited states, in particular to the dihedral angle between the donor and acceptor moieties. 
Tuesday, March 3, 2020 1:51PM  2:03PM 
G39.00012: Predictive and Tractable GW Approach for Energy Level Alignment at OrganicInorganic Interfaces with Significant Charge Transfer Nicholas Lin Quan Cheng, Fengyuan Xuan, Su Ying Quek The energy level alignment (ELA) at organicinorganic interfaces is critical for determining charge injection barriers in organic and molecular electronic devices. Manybody perturbation theory in the GW approximation enables the quantitative prediction of ELA in many systems, but can be computationally challenging for large interfaces. We have recently developed an approach [1] to perform GW calculations on large interface systems, which involves the eXpansion of the polarizability (chi) matrix from a unit cell to the supercell, the Addition of chi from the two subsystems, and the use of wavefunctions from the Full interface to compute the selfenergies. This XAFGW method has been shown to work even in the presence of interface hybridization to form bonding and antibonding orbitals. Here, we show that the XAFGW method fails in some cases with significant interface charge transfer. We modify the XAFGW approach to specifically account for charge transfer effects, and obtain good agreement between the modified XAFGW method and a regular full GW calculation for F4TCNQ on bilayer graphene. We further discuss the application of our new approach to other experimentally relevant systems. 
Tuesday, March 3, 2020 2:03PM  2:15PM 
G39.00013: Electronic structure of semiconductor nanoparticles from stochastic evaluation of imaginarytime path integral: nonrelativistic U(1) lattice gauge theory in the KohnSham basis Andrei Kryjevski In the KohnSham orbital basis imaginarytime path integral for electrons in a semiconductor nanoparticle has a mild fermion sign problem and is, therefore, amenable to evaluation by the standard stochastic methods. Utilizing output from the density functional theory simulations with Perdew, Burke and Ernzerhof exchangecorrelation functional we compute imaginarytime electron propagators in several silicon hydrogenpassivated nanocrystals, such as Si_{35}H_{36}, Si_{87}H_{76} and Si_{147}H_{100}, and extract energies of lowlying electron and hole levels. Our quasiparticle gap predictions agree with the results of recent G_{0}W_{0} calculations from M. Govoni, G. Galli, “Large Scale GW Calculations", J. Chem. Theory Comp., 11 (2015). 
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