Session K29: First-principles Modeling of Excited-State Phenomena in Materials VII: Organic and Hybrid Materials

Embedded many-body perturbation theory for organic and hybrid disordered systems

We present the merging of the GW and Bethe-Salpeter 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 time-dependent 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]

[1] I. Duchemin, D. Jacquemin, X. Blase, J. Chem. Phys. 144, 164106 (2016).
[2] J. Li, G. D'Avino, I. Duchemin, D. Beljonne, X. Blase, J. Phys. Chem. Lett. 7, 2814 (2016).
[3] J. Li, G. D'Avino, A. Pershin, D. Jacquemin, I. Duchemin, D. Beljonne, X. Blase, Phys. Rev. Materials 1, 025602 (2017).   

The predictive power of “one-shot” GW calculations for halide perovskites

Halide perovskites like the hybrid organic-inorganic APbX₃ (A=CH₃NH₃⁺, CH(NH₂)₂⁺,... and X=I, Br, Cl) or the all-inorganic double perovskite Cs₂BiAgBr₆ 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 spin-orbit interactions. We use ab initio many-body perturbation theory, employing a “one-shot” G₀W₀ 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 self-consistency on the calculated quasiparticle band gap. With our calculations, we assess whether there might be a “one size fits all” one-shot GW approach for this material class [2].

[1] L. Leppert, S. Reyes-Lillo, J. B. Neaton, J. Phys. Chem. Lett. 7, 3683 (2016). [2] L. Leppert, T. Rangel, J. Haber, J. B. Neaton, in preparation (2017).   

Many-Body Perturbation Theory for molecules: Physicists' approach versus Chemists' approach

First devised in the 50's, Many-Body 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].

Today with the computer code MOLGW [3], both approaches have been implemented and can be compared directly using the same basis sets for molecules. Here we benchmark the importance of different diagrams for the ionization potential of 34 small molecules.
We study the mean-field starting point dependence and the effect of self-consistency schemes. We conclude on the remarkable robustness of the GW approximation.

[1] L.S. Cederbaum and W. Domcke, Adv. Chem. Phys. 36, 205 (1977).
[2] L. Hedin, Phys. Rev 139, A796 (1965).
[3] F. Bruneval et al., Comput. Phys. Commun. 208, 149 (2016).   

Understanding The Structure-function Relation In Natural Dyes

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 multi-scale protocol, based on time-dependent 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 co-pigmentation, to identify natural variants of the same molecule with custom-designed chromatic properties.
[1]. I. Timrov et al. J. Chem. Theory Compit. 2016, 12, 4423-4429.
[2]. M. Rosa et al. J. Chem. Theory Compit. 2016, 12, 4385-4389.
[3]. M. Rusishvili et al. in preparation.   

Tuning Optical Properties of Dibenzochrysenes by Functionalization: A Many-Body Perturbation Theory Study

Using many-body 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.   

Geometry Distortion and Small Polaron Binding Energy Changes with Ionic Substitution in Halide Perovskites

Solution-processed 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 in-depth 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, CsSnI₃, CsPbBr₃, MAPbI₃, FAPbI₃, MAPbBr₃, FAPbBr₃, MASnI₃, and FASnBr₃. 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.   

Phenyl-Substituted Acene Derivatives as Candidates for Intermolecular Singlet Fission

A significant increase in the solar conversion efficiency of organic photovoltaic (OPV) devices could be achieved by harnessing singlet fission (SF), the down-conversion 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 phenyl-substituted tetracene and pentacene derivatives to elucidate the effect of phenyl substitutions. Many-body perturbation theory (MBPT) in the GW approximation and the Bethe-Salpeter 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.
  

On the Possibility of Singlet Fission in Crystalline Quaterrylene

Singlet Fission (SF), the spontaneous down-conversion 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 many-body perturbation theory in the GW approximation and the Bethe-Salpeter 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 wave-function from double-Bader 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.   

Charge transfer and singlet fission quantum dynamics in organic photovoltaics

We will discuss our recent investigations on singlet fission and charge separation quantum dynamics in organic photovoltaic devices. First, we use a real-time path-integral 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 triplet-triplet 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 time-dependent electronic wavefunction on a pre-computed 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 real-time electron and hole dynamics in a realistic large-scale BHJ model system and resolve the mechanistic debates on free charge carrier generations in OPV devices.   

Quasiparticle excitations of molecular chains on graphene

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 F4-TCNQ molecules on graphene in a field-effect transistor setup and demonstrate the ability to control their charge state by means of the gate voltage. In particular, we employ ab initio density-functional 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.   

Many-body perturbation theory analysis of point defects in bulk and monolayer semiconducting materials

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 high-performance materials. We employ first-principles many-body 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 WS₂ containing a single sulfur vacancy, a likely point defect in this TMDC. We determine that the electronic properties relevant to transport in WS₂ are significantly affected by the presence of this defect, with implications for devices fabricated using this material system.   

First-principles Engineering of Charged Defects for Two-dimensional Quantum Technologies

Charged defects in 2D materials have emerging applications in quantum technologies such as
quantum emitters and quantum computation. Advancement of these technologies requires rational design of ideal defect centers, demanding reliable computation methods for quantitatively accurate prediction of defect properties. We present an accurate, parameter-free and efficient procedure to evaluate quasiparticle defect states and thermodynamic charge transition levels of defects in 2D materials. Importantly, we solve critical issues that stem from the strongly anisotropic screening in 2D materials, that have so far precluded accurate prediction of charge transition levels in these materials. Using this procedure, we investigate various defects in monolayer hexagonal boron nitride (h-BN) for their charge transition levels, stable spin states and optical excitations. We identify C_BV_N (nitrogen vacancy adjacent to carbon substitution of boron) to be the most promising defect candidate for scalable quantum bit and emitter applications.

  

Efficiently Capturing Substrate Screening Effects on Level Alignments at Molecule-Metal Interfaces with GW Calculations

Quasiparticle energies and level alignments at molecule-metal 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 self-energy corrections constructed from image-charge 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.