Electronic coarse-graining of long conjugated molecules: Case study of non-fullerene acceptors.

By considering only one electronic state per molecule, charge transport models of molecular solids neglect intramolecular charge transfer. This approximation excludes materials with quasi-degenerate spatially separated frontier orbitals, such as non-fullerene acceptors (NFAs) and symmetric thermally activated delayed fluorescence emitters. By analyzing the electronic structure of room-temperature molecular conformers of a prototypical NFA, ITIC-4F, we conclude that the electron is localized on one of the two acceptor blocks with the mean intramolecular transfer integral of 120 meV, which is comparable with intermolecular couplings. Therefore, the minimal basis for acceptor-donor-acceptor (A-D-A) molecules consists of two molecular orbitals localized on the acceptor blocks. This basis is robust even with respect to geometry distortions in an amorphous solid, in contrast to the basis of two lowest unoccupied canonical molecular orbitals withstanding only thermal fluctuations in a crystal. The charge carrier mobility can be underestimated by a factor of two when using single site approximation for A-D-A molecules in their typical crystalline packings.

Hydroxyl Defects in LiFePO4 Cathode Material: DFT+U and an Experimental Study.

Lithium iron phosphate, LiFePO4, a widely used cathode material in commercial Li-ion batteries, unveils a complex defect structure, which is still being deciphered. Using a combined computational and experimental approach comprising density functional theory (DFT)+U and molecular dynamics calculations and X-ray and neutron diffraction, we provide a comprehensive characterization of various OH point defects in LiFePO4, including their formation, dynamics, and localization in the interstitial space and at Li, Fe, and P sites. It is demonstrated that one, two, and four (five) OH groups can effectively stabilize Li, Fe, and P vacancies, respectively. The presence of D (H) at both Li and P sites for hydrothermally synthesized deuterium-enriched LiFePO4 is confirmed by joint X-ray and neutron powder diffraction structure refinement at 5 K that also reveals a strong deficiency of P of 6%. The P occupancy decrease is explained by the formation of hydrogarnet-like P/4H and P/5H defects, which have the lowest formation energies among all considered OH defects. Molecular dynamics simulation shows a rich structural diversity of these defects, with OH groups pointing both inside and outside vacant P tetrahedra creating numerous energetically close conformers, which hinders their explicit localization with diffraction-based methods solely. The discovered conformers include structural water molecules, which are only by 0.04 eV/atom H higher in energy than separate OH defects.

The Role of Semilabile Oxygen Atoms for Intercalation Chemistry of the Metal-Ion Battery Polyanion Cathodes.

Using the orthorhombic layered Na2FePO4F cathode material as a model system we identify the bonding of the alkali metal cations to the semilabile oxygen atoms as an important factor affecting electrochemical activity of alkali cations in polyanion structures. The semilabile oxygens, bonded to the P and alkali cations, but not included into the FeO4F2 octahedra, experience severe undercoordination upon alkali cation deintercalation, causing an energy penalty for removing the alkali cations located in the proximity of such semilabile oxygens. Desodiation of Na2FePO4F proceeds through a two-phase mechanism in the Na-ion cell with a formation of an intermediate monoclinic Na1.55FePO4F phase with coupled Na/vacancy and Fe2+/Fe3+ charge ordering at 50% state of charge. In contrast, desodiation of Na2FePO4F in the Li-ion cell demonstrates a sloping charge profile suggesting a solid solution mechanism without formation of a charge-ordered intermediate phase. A combination of a comprehensive crystallographic study and extensive DFT-based calculations demonstrates that the difference in electrochemical behavior of the alkali cation positions is largely related to the different number of the nearest neighbor semilabile oxygen atoms, influencing their desodiation potential and accessibility for the Na/Li chemical exchange, triggering coupled alkali cation-vacancy ordering and Fe2+/Fe3+ charge ordering, as well as switching between the "solid solution" and "two-phase" charging mechanistic regimes.

Theoretical description of structural and electronic properties of organic photovoltaic materials.

We review recent progress in the modeling of organic solar cells and photovoltaic materials, as well as discuss the underlying theoretical methods with an emphasis on dynamical electronic processes occurring in organic semiconductors. The key feature of the latter is a strong electron-phonon interaction, making the evolution of electronic and structural degrees of freedom inseparable. We discuss commonly used approaches for first-principles modeling of this evolution, focusing on a multiscale framework based on the Holstein-Peierls Hamiltonian solved via polaron transformation. A challenge for both theoretical and experimental investigations of organic solar cells is the complex multiscale morphology of these devices. Nevertheless, predictive modeling of photovoltaic materials and devices is attainable and is rapidly developing, as reviewed here.

Benchmark Data Set of Crystalline Organic Semiconductors.

This work reports a Benchmark Data set of Crystalline Organic Semiconductors to test calculations of the structural and electronic properties of these materials in the solid state. The data set contains 67 crystals consisting of mostly rigid molecules with a single dominant conformer, covering the majority of known structural types. The experimental crystal structure is available for the entire data set, whereas zero-temperature unit cell volume can be reliably estimated for a subset of 28 crystals. Using this subset, we benchmark r2SCAN-D3 and PBE-D3 density functionals. Then, for the entire data set, we benchmark approximate density functional theory (DFT) methods, including GFN1-xTB and DFTB3(3ob-3-1), with various dispersion corrections against r2SCAN-D3. Our results show that r2SCAN-D3 geometries are accurate within a few percent, which is comparable to the statistical uncertainty of experimental data at a fixed temperature, but the unit cell volume is systematically underestimated by 2% on average. The several times faster PBE-D3 provides an unbiased estimate of the volume for all systems except for molecules with highly polar bonds, for which the volume is substantially overestimated in correlation with the underestimation of atomic charges. Considered approximate DFT methods are orders of magnitude faster and provide qualitatively correct but overcompressed crystal structures unless the dispersion corrections are fitted by unit cell volume.

Charge storage mechanisms of a π-d conjugated polymer for advanced alkali-ion battery anodes.

The demand for fast-charging metal-ion batteries underlines the importance of anodes that work at high currents with no risk of dendrite formation. NiBTA, a one-dimensional Ni-based polymer derived from benzenetetramine (BTA), is a recently proposed promising material for safe fast-charging batteries. However, its charge-discharge mechanisms remained unclear and controversial. Here we solve the controversies by providing the first rigorous study using a combination of advanced theoretical and experimental techniques, including operando and ex situ X-ray diffraction, operando Raman spectroscopy and ex situ X-ray absorption near-edge spectroscopy (XANES). In safe potential ranges (0.5-2.0 V vs. M+/M, M = Li, Na or K), NiBTA offers high capacities, fast charge-discharge kinetics, high cycling stability and compatibility with various cations (Li+, Na+, K+). In the Na- and K-based cells, fast bulk faradaic processes are manifested for partially reduced states. Atomistic simulations explain the fast kinetics by facile rotations and displacements of the macromolecules in the crystal, opening channels for fast ion insertion. The material undergoes distinct crystal structure rearrangements in the Li-, Na- and K-based systems, which explains different electrochemical features. At the molecular level, the charge storage mechanism involves reversible two-electron reduction of the repeating units accompanied by a change of the absorption bandgap. The reversible reduction involves filling of the orbitals localized at the ligand moieties. No reduction of NiBTA beyond two electrons per repeating unit is observed at potentials down to 0 V vs. M+/M.

Small Polarons in Two-Dimensional Pnictogens: A First-Principles Study.

We report the first-principles study of small polarons in the most stable two-dimensional pnictogen allotropes: blue and black phosphorene and arsenene. While both cations and anions of small hydrogen-passivated clusters show charge localization and local lattice distortions, only the hole polaron in the blue allotrope is stable in the infinite size cluster limit. The adiabatic polaron relaxation energy is found to be 0.1 eV for phosphorene and 0.15 eV for arsenene. The polaron is localized on lone-pair orbitals with half of the extra charge distributed among 13 atoms. In the blue phosphorene, these orbitals form the valence band's top with a relatively flat band dispersion. However, in the black phosphorene, lone-pair orbitals hybridize with bonding orbitals, which explains the difference in hole localization strength between the two topologically equivalent allotropes. The polaron's adiabatic barriers for motion are small compared to the most strongly coupled phonon frequency, implying the polaron barrierless motion.

Microcrystal Electron Diffraction for Molecular Design of Functional Non-Fullerene Acceptor Structures.

Understanding the relationship between molecular structure and solid-state arrangement informs about the design of new organic semiconductor (OSC) materials with improved optoelectronic properties. However, determining their atomic structure remains challenging. Here, we report the lattice organization of two non-fullerene acceptors (NFAs) determined using microcrystal electron diffraction (MicroED) from crystals not traceable by X-ray crystallography. The MicroED structure of o-IDTBR was determined from a powder without crystallization, and a new polymorph of ITIC-Th is identified with the most distorted backbone of any NFA. Electronic structure calculations elucidate the relationships between molecular structures, lattice arrangements, and charge-transport properties for a number of NFA lattices. The high dimensionality of the connectivity of the 3D wire mesh topology is the best for robust charge transport within NFA crystals. However, some examples suffer from uneven electronic coupling. MicroED combined with advanced electronic structure modeling is a powerful new approach for structure determination, exploring polymorphism and guiding the design of new OSCs and NFAs.

Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. I. The formation of the ppσ-network.

Semiconductor glasses exhibit many unique optical and electronic anomalies. We have put forth a semiphenomenological scenario [A. Zhugayevych and V. Lubchenko, J. Chem. Phys. 133, 234504 (2010)] in which several of these anomalies arise from deep midgap electronic states residing on high-strain regions intrinsic to the activated transport above the glass transition. Here we demonstrate at the molecular level how this scenario is realized in an important class of semiconductor glasses, namely chalcogen and pnictogen containing alloys. Both the glass itself and the intrinsic electronic midgap states emerge as a result of the formation of a network composed of σ-bonded atomic p-orbitals that are only weakly hybridized. Despite a large number of weak bonds, these ppσ-networks are stable with respect to competing types of bonding, while exhibiting a high degree of structural degeneracy. The stability is rationalized with the help of a hereby proposed structural model, by which ppσ-networks are symmetry-broken and distorted versions of a high symmetry structure. The latter structure exhibits exact octahedral coordination and is fully covalently bonded. The present approach provides a microscopic route to a fully consistent description of the electronic and structural excitations in vitreous semiconductors.

Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. II. The intrinsic electronic midgap states.

We propose a structural model that treats in a unified fashion both the atomic motions and electronic excitations in quenched melts of pnictide and chalcogenide semiconductors. In Part I [A. Zhugayevych and V. Lubchenko, J. Chem. Phys. 133, 234503 (2010)], we argued these quenched melts represent aperiodic ppσ-networks that are highly stable and, at the same time, structurally degenerate. These networks are characterized by a continuous range of coordination. Here we present a systematic way to classify these types of coordination in terms of discrete coordination defects in a parent structure defined on a simple cubic lattice. We identify the lowest energy coordination defects with the intrinsic midgap electronic states in semiconductor glasses, which were argued earlier to cause many of the unique optoelectronic anomalies in these materials. In addition, these coordination defects are mobile and correspond to the transition state configurations during the activated transport above the glass transition. The presence of the coordination defects may account for the puzzling discrepancy between the kinetic and thermodynamic fragility in chalcogenides. Finally, the proposed model recovers as limiting cases several popular types of bonding patterns proposed earlier including: valence-alternation pairs, hypervalent configurations, and homopolar bonds in heteropolar compounds.

An intrinsic formation mechanism for midgap electronic states in semiconductor glasses.

We argue that semiconducting quenched liquids and frozen glasses may exhibit a set of peculiar electronic states of topological origin. These states reside at strained regions arising during structural reconfigurations between distinct aperiodic states intrinsic to quenched melts. The strained regions are domain walls separating the distinct aperiodic states; their number is about 10(20) cm(-3) in all glassformers owing to the universal dynamics of deeply supercooled melts. Even though located near the middle of the forbidden gap, the topological states are rather extended in one direction while being centered at under- and overcoordinated atoms. The states exhibit the reverse charge-spin relation, the majority of states being diamagnetic and charged. The topological states may be sufficient to account for a number of irradiation-induced phenomena in amorphous semiconductors, including electron spin resonance signal, midgap absorption, photoluminescence, and the fatigue of photoluminescence. We propose experiments to test the present microscopic picture.

Ground-State Geometry and Vibrations of Polyphenylenevinylene Oligomers.

Conformational space of polyphenylenevinylene oligomers is systematically investigated computationally at energies relevant for room temperature dynamics in a solvent and in a solid state. Our calculations show that optimal oligomer structures are essentially planar. However, lack of a deep minimum at the planar geometry allows for large molecular deformations even at very low temperatures. At larger angles, rotational motion of dihedrals intermix with two orthogonal bending motions of the entire molecule. In a crystalline environment these degrees of freedom intermix with translational and rotational motions, whereas purely intramolecular modes are well separated. The reliability of our calculations is confirmed by an excellent match of the theoretical and experimental Raman spectra of crystalline stilbene in the entire spectral range including the low-frequency part. Obtained results provide important insights into nature of low-frequency vibrations, which play a key role in charge transport in organic semiconductors.

Tuning Optical Properties of Conjugated Molecules by Lewis Acids: Insights from Electronic Structure Modeling.

Understanding and controlling the optoelectronic properties of organic semiconductors at the molecular level remains a challenge due to the complexity of chemical structures and intermolecular interactions. A common strategy to address this challenge is to utilize both experimental and computational approaches. In this contribution, we show that density functional theory (DFT) calculation is a useful tool to provide insights into the bonding, electron population distribution, and optical transitions of adducts between conjugated molecules and Lewis acids (CM-LA). Adduct formation leads to relevant modifications of key properties, including a red shift in optical transitions and an increase in charge carrier density and charge mobility, compared to the parent conjugated molecules. We show that electron density transfer from the CM to the LA, which was hypothesized to cause the experimental red shift in absorption spectra upon LA binding, can be quantified and interpreted by population analysis. Experimental red shifts in optical transitions for all molecular families can also be predicted by time-dependent DFT calculations with different density functionals. These detailed insights help to optimize a priori design guidelines for future applications.

Polymorphism of Crystalline Molecular Donors for Solution-Processed Organic Photovoltaics.

Using ab initio calculations and classical molecular dynamics simulations coupled to complementary experimental characterization, four molecular semiconductors were investigated in vacuum, solution, and crystalline form. Independently, the molecules can be described as nearly isostructural, yet in crystalline form, two distinct crystal systems are observed with characteristic molecular geometries. The minor structural variations provide a platform to investigate the subtlety of simple substitutions, with particular focus on polymorphism and rotational isomerism. Resolved crystal structures offer an exact description of intermolecular ordering in the solid state. This enables evaluation of molecular binding energy in various crystallographic configurations to fully rationalize observed crystal packing on a basis of first-principle calculations of intermolecular interactions.

Tailored electronic structure and optical properties of conjugated systems through aggregates and dipole-dipole interactions.

A series of PPVO (p-phenylene vinylene oligomer) derivatives with functional groups of varying electronegativity were synthesized via the Horner-Wadsworth-Emmons reaction. Subtle changes in the end group functionality significantly impact the molecular electronic and optical properties of the PPVOs, resulting in broadly tunable and efficient UV absorption and photoluminescence spectra. Of particular interest is the NO2-substituted PPVO which exhibits photoluminescence color ranging from the blue to the red, thus encompassing the entire visible spectrum. Our experimental study and electronic structure calculations suggest that the formation of aggregates and strong dipole-dipole solute-solvent interactions are responsible for the observed strong solvatochromism. Experimental and theoretical results for the NH2-, H-, and NO2-substituted PPVOs suggest that the stabilization of ground or excited state dipoles leads to the blue or red shift of the optical spectra. The electroluminescence (EL) spectra of H-, COOH-, and NO2-PPVO have maxima at 487, 518, and 587 nm, respectively, in the OLED device. This trend in the EL spectra is in excellent agreement with the end group-dependent PL spectra of the PPVO thin-films.