The performance of different GW methods is assessed for a set of 24 organic acceptors. Errors are evaluated with respect to coupled cluster singles, doubles, and perturbative triples [CCSD(T)] reference data for the vertical ionization potentials (IPs) and electron affinities (EAs), extrapolated to the complete basis set limit. Additional comparisons are made to experimental data, where available. We consider fully self-consistent GW (scGW), partial self-consistency in the Green's function (scGW0), non-self-consistent G0W0 based on several mean-field starting points, and a \"beyond GW\" second-order screened exchange (SOSEX) correction to G0W0. We also describe the implementation of the self-consistent Coulomb hole with screened exchange method (COHSEX), which serves as one of the mean-field starting points. The best performers overall are G0W0+SOSEX and G0W0 based on an IP-tuned long-range corrected hybrid functional with the former being more accurate for EAs and the latter for IPs. Both provide a balanced treatment of localized vs delocalized states and valence spectra in good agreement with photoemission spectroscopy (PES) experiments.

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Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules: A Benchmark of GW Methods

P.B.A. was supported in part by US DOE Grant No. DE-FG02-08ER46550. D.A.C. thanks NSF for a summer REU scholarship administered by Stony Brook University (NSF Grant No. PHYS-0851594). J.M.S. was supported by Grants No. FIS2009-12721 and No. FIS2012-37549

/pdf/recovering-hidden-bloch-character-unfolding-electrons-5a7k3kvtyl.pdf

Recovering hidden Bloch character: Unfolding Electrons, Phonons, and Slabs

Due to the strong exciton binding energy (Eb) of organic materials, the energy offset between donor (D) and acceptor (A) materials is essential to promote charge generation in organic solar cells (OSCs). Yet an efficient exciton dissociation from non-fullerene acceptors (NFAs) began to be observed in D/A blends even at very low driving force for hole transfer (ΔHh). The mechanism behind this efficient photoinduced hole transfer (PHT) remains unclear since current estimates from calculations of isolated molecules indicate that Eb > ΔHh. Here we rationalize these discrepancies using density functional theory (DFT), the total Gibbs free energy method and the extended Huckel theory (EHT). First, we employed DFT to calculate Eb for NFAs of three representative groups (perylene diimide derivatives, indacenodithiophene and subphthalocyanines) as well as for fullerene acceptors (FAs). Considering isolated molecules in the calculations, we verified that Eb for NFAs is lower than for FAs but still higher than the experimental ΔHh in which efficient PHT has been observed. Finding the molecular geometry of the excited state, we also obtain that the structural relaxation after photoexcitation tends to further decrease (increase) Eb for NFAs (FAs). This effect helps explain the delayed charge generation measured in some NFA systems. However, this effect is still not large enough for a significant decrease in Eb. We then applied EHT to quantify the decrease of Eb induced by energy levels coupling between stacked molecules in a model aggregate. We then estimated the number of stacked molecules so that Eb approaches ΔHh's. We found that small NFA aggregates, involving around 5 molecules, are already large enough to explain the experiments. Our results are justified by the low energy barrier to the generation of delocalized states in these systems (especially for the hole delocalization). Therefore, they indicate that molecular systems with certain characteristics can achieve efficient molecular orbital delocalization, which is a key factor to allow an efficient exciton dissociation in low-driving-force systems. These theoretical findings provide a sound explanation to very recent observations in OSCs.

Molecular origin of efficient hole transfer from non-fullerene acceptors: insights from first-principles calculations

We investigate some foundational issues in the quantum theory of spin transport, in the general case when the unperturbed Hamiltonian operator $$H_0$$
 does not commute with the spin operator in view of Rashba interactions, as in the typical models for the quantum spin Hall effect. A gapped periodic one-particle Hamiltonian $$H_0$$
 is perturbed by adding a constant electric field of intensity $$\varepsilon \ll 1$$
 in the j-th direction, and the linear response in terms of a S-current in the i-th direction is computed, where S is a generalized spin operator. We derive a general formula for the spin conductivity that covers both the choice of the conventional and of the proper spin current operator. We investigate the independence of the spin conductivity from the choice of the fundamental cell (unit cell consistency), and we isolate a subclass of discrete periodic models where the conventional and the proper S-conductivity agree, thus showing that the controversy about the choice of the spin current operator is immaterial as far as models in this class are concerned. As a consequence of the general theory, we obtain that whenever the spin is (almost) conserved, the spin conductivity is (approximately) equal to the spin-Chern number. The method relies on the characterization of a non-equilibrium almost-stationary state (NEASS), which well approximates the physical state of the system (in the sense of space-adiabatic perturbation theory) and allows moreover to compute the response of the adiabatic S-current as the trace per unit volume of the S-current operator times the NEASS. This technique can be applied in a general framework, which includes both discrete and continuum models.

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A New Approach to Transport Coefficients in the Quantum Spin Hall Effect

Effect of vacancy defects on electronic properties and wettability of coal surface

Electron affinity (EA) is the energy released when an additional electron is attached to an atom or a molecule. EA is a fundamental thermochemical property, and it is closely pertinent to other imp...

Accurate density functional prediction of molecular electron affinity with the scaling corrected Kohn-Sham frontier orbital energies

Quasi‐2D semiconductors have garnered immense research interest for next‐generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi‐2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi‐2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron–phonon coupling strength mediated by phonons with purely out‐of‐plane vibrational vectors. This is demonstrated in ZrBeSi‐type quasi‐2D systems, where the representative sample Ba1.01AgSb shows a high room‐temperature hole mobility of 344 cm2 V−1 S−1, a record value among quasi‐2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p‐type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi‐type compounds. This work uncovers the relation between electron–phonon coupling and crystal symmetry in quasi‐2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.

Symmetry‐Guaranteed High Carrier Mobility in Quasi‐2D Thermoelectric Semiconductors

https://iopscience.iop.org/article/10.1088/2516-1075/ab5e5a/pdf

Unit cell consistency of maximally localized Wannier functions

Molecular quasiparticle and excitation energies determine essentially the spectral characteristics measured in various spectroscopic experiments. Accurate prediction of these energies has been rather challenging for ground-state density functional methods, because the commonly adopted density function approximations suffer from delocalization error. In this work, by presuming a quantitative correspondence between the quasiparticle energies and the generalized Kohn-Sham orbital energies, and employing a previously developed global scaling correction approach, we achieve substantially improved prediction of molecular quasiparticle and excitation energies. In addition, we also extend our previous study on temporary anions in resonant states, which are associated with negative molecular electron affinities. The proposed approach does not require any explicit self-consistent field calculation on the excited-state species, and is thus highly efficient and convenient for practical purposes.

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Density Functional Prediction of Quasiparticle, Excitation, and Resonance Energies of Molecules With a Global Scaling Correction Approach.

Calcium metavanadate (CaV2O6) and magnesium metavanadate (MgV2O6) have received considerable attention due to their great potential for many practical applications; however, a fundamental understanding of their intrinsic physical properties is still not well established. Here, we present a comprehensive experimental and theoretical study of the optical, electronic, and lattice dynamic properties of CaV2O6 and MgV2O6. We find that both compounds are semiconductors with indirect band gaps and exhibit visible light-emitting properties, which are thus expected to be the ideal candidates for phosphors or optoelectronic devices. Through density functional theory (DFT) calculations, we further show that CaV2O6 exhibits negative thermal expansion (NTE) below 80 K due to the flexible or intense coupled rocking of the CaO6 octahedra. Moreover, using the dual-phonon model, we disclose the hierarchical thermal transport features of these two compounds, in which diffusive channels make a significant contribution to the thermal conductivity κ, resulting in the weak temperature dependence of κ deviating from the typical κ ∼T-1 given by the phonon gas model. With the consideration of normal and diffusive phonons, we predict the ultralow and large anisotropic thermal conductivities of CaV2O6 and MgV2O6. This work provides a fundamental understanding of the optical, electronic, and thermal properties of metal-ion-intercalated layered vanadium oxides, which may promote the functional applications of metavanadates.

Xiaolong Yang

Papers

Accurate density functional prediction of molecular electron affinity with the scaling corrected Kohn-Sham frontier orbital energies

Symmetry‐Guaranteed High Carrier Mobility in Quasi‐2D Thermoelectric Semiconductors

Unit cell consistency of maximally localized Wannier functions

Density Functional Prediction of Quasiparticle, Excitation, and Resonance Energies of Molecules With a Global Scaling Correction Approach.

Comprehensive Study of Electronic, Optical, and Thermophysical Properties of Metavanadates CaV2O6 and MgV2O6.