We have investigated the electronic properties of four charge-transfer cocrystals involving 1,5-diaminonaphthalene (DAN) as donor and fluoranil (FA), chloranil (CA), bromanil (BA), and iodanil (IA) as acceptors. While DAN-FA, DAN-CA, and DAN-BA crystallized in a mixed-stack fashion, DAN-IA crystallized with segregated stacks. For the mixed-stack cocrystals, electronic-structure calculations using density functional theory predict large electron–hole couplings with small effective masses, which strongly suggests that these DAN-XA cocrystals are suitable for charge-transport applications. Among the four cocrystals, DAN-CA crystallized in a noncentrosymmetric space group; according to our computational analysis, it is predicted to be weakly ferroelectric with a second-order electrical susceptibility (χ(2)) similar to that of urea. The ionicities (ρ) of the cocrystals calculated using Mulliken population compare well with the experimental results. The couplings between donor and acceptor molecules in DAN-IA are very small, leading to a very small ρ. This is not typical for a system with a segregated-stack packing motif, indicating that hydrogen and halogen bondings can have a strong impact on the structure–property relations in cocrystals.

Recent theoretical studies suggest that the modulation of the electronic couplings (transfer integrals) between adjacent molecules by lattice vibrations, i.e., the so-called nonlocal electron−phonon coupling, plays a key role in the charge-transport properties of molecular organic semiconductors. However, a detailed understanding of this mechanism is still missing. Here, we combine density functional theory calculations and molecular mechanics simulations and use a chemistry-based insight to derive the nonlocal electron−phonon coupling constants due to the interaction of charge carriers with the optical lattice vibrations in the naphthalene crystal. The results point to a very strong coupling to both translational and librational intermolecular vibrational modes as well as to intramolecular modes. Along some crystal directions, the nonlocal interactions are found to be dominated by nontotally symmetric vibrational modes which lead to an alternation (Peierls-type dimerization) pattern. Importantly, we introduce two parameters that can be used: (i) to quantify the total strength of the nonlocal electron−vibration mechanism in the form of a reorganization energy term; and (ii) to define the extent of the thermal fluctuations of the electronic couplings. Interestingly, zero-point fluctuations are seen to be very significant.

Density functional theory (DFT) approaches based on range-separated hybrid functionals are currently methods of choice for the description of the charge-transfer (CT) states in organic donor/acceptor solar cells. However, these calculations are usually performed on small-size donor/acceptor complexes and as result do not account for electronic polarization effects. Here, using a pentacene/C60 complex as a model system, we discuss the ability of long-range corrected (LCR) hybrid functionals in combination with the polarizable continuum model (PCM) to determine the impact of the solid-state environment on the CT states. The CT energies are found to be insensitive to the interactions with the dielectric medium when a conventional time-dependent DFT/PCM (TDDFT/PCM) approach is used. However, a decrease in the energy of the CT state in the framework of LRC functionals can be obtained by using a smaller range-separated parameter when going from an isolated donor/acceptor complex to the solid-state case.

We report a novel synthesis to ultra high purity 7,14-bis((trimethylsilyl)ethynyl)dibenzo[b,def]-chrysene (TMS-DBC) and the use of this material in the growth of single crystals by solution and vapor deposition techniques. We observe that the substrate temperature has a dramatic impact on the crystal growth, producing two distinct polymorphs of TMS-DBC; low temperature (LT) fine red needles and high temperature (HT) large yellow platelets. Single crystal X-ray crystallography confirms packing structures where the LT crystals form a 1D slipped-stack structure, while the HT crystals adopt a 2D brickwork motif. These polymorphs also represent a rare example where both are extremely stable and do not interconvert to the other crystal structure upon solvent or thermal annealing. Single crystal organic field-effect transistors of the LT and HT crystals show that the HT 2D brickwork motif produces hole mobilities as high as 2.1 cm2 V–1 s–1, while the mobility of the 1D structure is significantly lower, at 0.028 cm2 V–1 s–1. Electronic-structure calculations indicate that the superior charge transport in the brickwork polymorph in comparison to the slipped-stack polymorph is due to the presence of an increased dimensionality of the charge migration pathways.

We use a combination of molecular dynamics simulations and density functional theory calculations to investigate the energetic disorder in fullerene systems. We show that the energetic disorder evaluated from an ensemble average contains contributions of both static origin (time-independent, due to loose packing) and dynamic origin (time-dependent, due to electron-vibration interactions). In order to differentiate between these two contributions, we compare the results obtained from an ensemble average approach with those derived from a time average approach. It is found that in both amorphous C60 and C70 bulk systems, the degrees of static and dynamic disorder are comparable, while in the amorphous PC61BM and PC71BM systems, static disorder is about twice as large as dynamic disorder.

The characteristics of the electronic excited states and the charge-transfer processes at organic–organic interfaces play an important role in organic electronic devices. However, charge-transfer excitations have proven challenging to describe with conventional density functional theory (DFT) methodologies due to the local nature of the exchange-correlation potentials often employed. Here, we examine the excited states of model pentacene-C60 complexes using time-dependent DFT with, on one hand, one of the most popular standard hybrid functionals (B3LYP) and, on the other hand, several long-range corrected hybrid functionals for which we consider both default and nonempirically tuned range-separation parameters. The DFT results based on the tuned functionals are found to agree well with the available experimental data. The results also underline that the interface geometry of the complex has a strong effect on the energies and ordering of the singlet and triplet charge-transfer states.

Electronic delocalization effects have been proposed to play a key role in photocurrent generation in organic photovoltaic devices. Here, we study the role of charge delocalization on the nature of the charge-transfer (CT) states in the case of model complexes consisting of several pentacene molecules and one fullerene (C60) molecule, which are representative of donor/acceptor heterojunctions. The energies of the CT states are examined by means of time-dependent density functional theory (TD-DFT) using the long-range-corrected functional, ωB97X, with an optimized range-separation parameter, ω. We provide a general description of how the nature of the CT states is impacted by molecular packing (i.e., interfacial donor/acceptor orientations), system size, and intermolecular interactions, features of importance in the understanding of the charge-separation mechanism.

Oligoacenes such as naphthalene, anthracene, tetracene, and pentacene are among the best hole-transport organic semiconductors. An important parameter in the determination of the hole mobility is the coupling between the charge carrier and the vibrational modes. Here, we have evaluated the hole–vibration coupling constants in the radical-cation ground state of these molecules by means of the range-separated LC-ωPBE and ωB97 density functionals, with non-empirical optimization of the range-separation parameter ω. Our results indicate that both ω-tuned functionals yield similar relaxation energies and coupling constants. A comparison of the simulated vibrational structures of the first ionization band to the gas-phase ultraviolet photoelectron spectroscopy data underlines that the hole–vibration coupling constants derived by means of the non-empirically tuned LC-ωPBE and ωB97 functionals are in excellent agreement with experiment and superior to those derived from B3LYP calculations.

The electronic structures of a series of donor–acceptor mixed-stack crystals have been investigated by means of density functional theory calculations. The results highlight that a number of the donor–acceptor crystals under consideration are characterized by wide valence and conduction bands, large hole and electron electronic couplings, and as a result very low hole and electron effective masses. The fact that the effective masses and electronic couplings for holes and electrons are nearly equal along the stacking directions implies that the hole and electron mobilities in these systems are also similar. In addition, in several of these crystals, charge transport has a two-dimensional character. The impact on the charge transport properties of the electronic couplings between donor and acceptor frontier orbitals and of the related energy gaps is also discussed.

The origin of the two prominent solvatochromic near-UV/visible/near-IR absorptions observed for donor–(π-bridge)–acceptor chromophores with ferrocene donors has been investigated using TD-DFT methods. Both chromophores with relatively weak (4-nitrophenyl) and strong acceptors (1,3-diethyl-2-thiobarbituric acid and 3-dicyanomethylidene-2,3-dihydrobenzothiophene-1,1-dioxide) were considered, as were ferrocene and octamethylferrocene donors. Computational predictions of optical properties made using the B3PW91 functional were found to be in good agreement with experimental data. The calculations reveal a complex orbital picture that varies from compound to compound, contribution of multiple configurations to some of the important states, and significant contributions from more than one transition to the experimentally observed bands. Natural transition orbitals have been used to gain an understanding of the charge redistribution associated with the transitions. The relatively weak low-energy bands of the ferrocene derivatives were generally found to have both d–d and metal-to-π-bridge/acceptor charge-transfer character. The stronger higher energy bands were found to be associated with charge transfer from cyclopentadienyl rings and the π bridge toward the acceptor group. The experimental spectra of ruthenocene chromophores differ significantly from those of the analogous ferrocene chromophores; however, the calculations reproduce the key differences and indicate a similar origin for the contributing transitions.

We report on electronic-structure calculations for the pentacene and rubrene crystals, based on experimental crystal geometries measured at different temperatures. The results are in very good agreement with angle-resolved photoelectron spectroscopy data that indicate that the widths of the valence and conduction bands in both materials become narrower at higher temperatures. Our findings strongly suggest that the thermal bandwidth narrowing in the pentacene and rubrene crystals is primarily caused by the thermal expansion of the lattice rather than by a renormalization of the transfer integrals induced by a polaron effect. The effect of thermal expansion on the charge-transport properties is also discussed.Keywords: band narrowing; charge transport; electron−phonon coupling; organic semiconductor crystals; polaron effect;

A key feature of organic π-conjugated materials is the strong connection between their electronic and geometric structures. In particular, it has been recently demonstrated that nonlocal electron−vibration (electron−phonon) interactions, which are related to the modulation of the electronic couplings (transfer integrals) between adjacent molecules by lattice vibrations, play an important role in the charge-transport properties of organic semiconductors. Here, we use density functional theory calculations and molecular mechanics simulations to estimate the strength of these nonlocal electron−vibration couplings in oligoacene crystals as a function of molecular size from naphthalene through pentacene. The effect of each optical vibrational mode on the electronic couplings is evaluated quantitatively. The results point to a very strong coupling to both intermolecular vibrational modes and intramolecular (including high-frequency) modes in all studied systems. Importantly, our results underline that the amount of relaxation energy associated with nonlocal electron−phonon coupling decreases as the size of the molecule increases. This work establishes an original relationship between chemical structure and nonlocal vibrational coupling in the description of charge transport in organic semiconductor crystals.

The exciton-dissociation and charge-recombination processes in organic solar cells based on pentacene/C60 heterojunctions are investigated by means of quantum-mechanical calculations. The electronic couplings and the rates of exciton dissociation and charge recombination have been evaluated for several geometrical configurations of the pentacene/C60 complex, which are relevant to bilayer and bulk heterojunctions. The results suggest that, irrespective of the actual pentacene−fullerene orientation, both pentacene-based and C60-based excitons are able to dissociate efficiently. Also, in the case of parallel configurations of the molecules at the pentacene/C60 interface, the decay of the lowest charge-transfer state to the ground state is calculated to be very fast; as a result, it can compete with the dissociation process into mobile charge carriers. Since parallel configurations are expected to be found more frequently in bulk heterojunctions than in bilayer heterojunctions, the performance of pentacene/C60 bulk-heterojunction solar cells is likely to be more affected by charge recombination than that of bilayer devices.

The 1,4-diiodobenzene (DIB) crystal stands out among molecular organic semiconducting crystals because of its remarkable room-temperature hole mobility (>10 cm2/(V s)). Here, on the basis of a density functional theory study, we demonstrate that the high mobility in DIB is primarily associated with the heavy iodine atoms. We find that along specific crystal directions, both electrons and holes are characterized by a very small effective mass of about 0.5 m0. Interestingly, iodine substitution also leads to a significant decrease in the local hole-vibration coupling compared to benzene; as a result, the electronic coupling for holes is calculated to be much larger than the hole-vibration coupling, which is consistent with the observation of large hole mobility. In marked contrast, the polaron binding energy in the case of electrons is found to be significantly higher than the electronic coupling; this implies that electrons in DIB are strongly localized even at room temperature.