Partial disorder is an inherent property of self-assembled organic semiconductors that complicates their rational design, because electronic structure, self-assembling properties, and stability all have to be accounted for simultaneously. Therefore, the understanding of charge transport mechanisms in these systems is still in its infancy. A theoretical study of charge transport in organic semiconductors was performed on self-assembled layers of [1]benzothieno[3,2-b]benzothiophene functionalized with alkyl side chains. Analysis showed that semiclassical dynamics misses static (on time scales of charge transport) disorder while the solution of the master equation combined with the high-temperature limit Marcus theory for charge transfer rates does not take into account molecular dynamic modes relaxing on a time scale of charge hopping. A comparison between predictions based on a perfectly ordered and a realistic crystal structure reveals the strong influence of static and dynamic disorder. The advantage of two-dimensional charge transporting materials over one-dimensional ones is clearly shown. The Marcus theory-based prediction of 0.1 cm2 V−1 s−1 is in good agreement with our FET mobility of 0.22 cm2 V−1 s−1, which is an order of magnitude lower than that reported in the literature [Ebata, H.; et al. J. Am. Chem. Soc. 2007, 129, 15732].

We present a method for evaluating electrostatic and polarization energies of a localized charge, charge transfer state, or exciton embedded in a neutral molecular environment. The approach extends the Ewald summation technique to polarization effects, rigorously accounts for the long-range nature of the charge-quadrupole interactions, and addresses aperiodic embedding of the charged molecular cluster and its polarization cloud in a periodic environment. We illustrate the method by evaluating the density of states and ionization energies in thin films and heterostructures of organic semiconductors. By accounting for long-range mesoscale fields, we obtain the ionization energies in both crystalline and mesoscopically amorphous systems with high accuracy.

Mesoscale modeling of organic semiconductors relies on solving an appropriately parametrized master equation. Essential ingredients of the parametrization are site energies (driving forces), which enter the charge transfer rate between pairs of neighboring molecules. Site energies are often Gaussian-distributed and are spatially correlated. Here, we propose an algorithm that generates these energies with a given Gaussian distribution and spatial correlation function. The method is tested on an amorphous organic semiconductor, DPBIC, illustrating that the accurate description of correlations is essential for the quantitative modeling of charge transport in amorphous mesophases.

Organic solar cells rely on the conversion of a Frenkel exciton into free charges via a charge-transfer state formed on a molecular donor–acceptor pair. These charge-transfer states are strongly bound by Coulomb interactions and yet efficiently converted into charge-separated states. A microscopic understanding of this process, though crucial to the functionality of any solar cell, has not yet been achieved. Here we show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority carrier trapping across a donor–acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices.

Continuous drift–diffusion models are routinely used to optimize organic semiconducting devices. Material properties are incorporated into these models via dependencies of diffusion constants, mobilities, and injection barriers on temperature, charge density, and external field. The respective expressions are often provided by the generic Gaussian disorder models, parametrized on experimental data. We show that this approach is limited by the fixed range of applicability of analytic expressions as well as approximations inherent to lattice models. To overcome these limitations we propose a scheme which first tabulates simulation results performed on small-scale off-lattice models, corrects for finite size effects, and then uses the tabulated mobility values to solve the drift–diffusion equations. The scheme is tested on DPBIC, a state of the art hole conductor for organic light emitting diodes. We find a good agreement between simulated and experimentally measured current–voltage characteristics for different film thicknesses and temperatures.

Simulations of organic semiconducting devices using drift-diffusion equations are vital for the understanding of their functionality as well as for the optimization of their performance. Input parameters for these equations are usually determined from experiments and do not provide a direct link to the chemical structures and material morphology. Here we demonstrate how such a parametrization can be performed by using atomic-scale (microscopic) simulations. To do this, a stochastic network model, parametrized on atomistic simulations, is used to tabulate charge mobility in a wide density range. After accounting for finite-size effects at small charge densities, the data is fitted to the uncorrelated and correlated extended Gaussian disorder models. Surprisingly, the uncorrelated model reproduces the results of microscopic simulations better than the correlated one, compensating for spatial correlations present in a microscopic system by a large lattice constant. The proposed method retains the link to the material morphology and the underlying chemistry and can be used to formulate structure–property relationships or optimize devices prior to compound synthesis.

We investigate the relationship between molecular order and charge-transport parameters of the crystalline conjugated polymer poly(3-hexylthiophene) (P3HT), with a particular emphasis on its different polymorphic structures and regioregularity. To this end, atomistic molecular dynamics is employed to study an irreversible transition of the metastable (form I′) to the stable (form I) P3HT polymorph, caused by side-chain melting at around 350 K. The predicted side-chain and backbone–backbone arrangements in unit cells of these polymorphs are compared to the existing structural models, based on X-ray, electron diffraction, and solid-state NMR measurements. Molecular ordering is further characterized by the paracrystalline, dynamic, and static nematic order parameters. The temperature-induced changes of these parameters are linked to the dynamics and distributions of electronic coupling elements and site energies. The simulated hole mobilities are in excellent agreement with experimental values obtained for P3HT nanofibers. We demonstrate that a small concentration of defects in side-chain attachment (90% regioregular P3HT) leads to a significant (factor of 10) decrease in charge-carrier mobility. This reduction is due to an increase of the intermolecular part of the energetic disorder and can be traced back to the amplified fluctuations in backbone–backbone distances, i.e., paracrystallinity. Furthermore, by comparing to poly(bithiophene-alt-thienothiophene) (PBTTT) with its higher hole mobility, we illustrate how transport in P3HT is disorder limited as a result of its side-chain structure.

We establish a link between the microscopic ordering and the charge-transport parameters for a highly crystalline polymeric organic semiconductor, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). We find that the nematic and dynamic order parameters of the conjugated backbones, as well as their separation, evolve linearly with temperature, while the side-chain dynamic order parameter and backbone paracrystallinity change abruptly upon the (also experimentally observed) melting of the side chains around 400 K. The distribution of site energies follows the behavior of the backbone paracrystallinity and can be treated as static on the time scale of a single-charge transfer reaction. On the contrary, the electronic couplings between adjacent backbones are insensitive to side-chain melting and vary on a much faster time scale. The hole mobility, calculated after time-averaging of the electronic couplings, reproduces well the value measured in a short-channel thin-film transistor. The results underline that to secure efficient charge transport in lamellar arrangements of conjugated polymers: (i) the electronic couplings should present high average values and fast dynamics, and (ii) the energetic disorder (paracrystallinity) should be small.

Surfaces facilitate chemical reactions occurring in biological and synthetic systems with wide-ranging applications from energy conversion to catalysis and sensing. Microscopic understanding of the structure and dynamics that underpin these reactions is keenly pursued with novel experimental techniques such as sum frequency generation and laser-assisted photoemission spectroscopy. Herein, we demonstrate a method for interpreting the time-resolved observation of the Stark effect to provide an in situ optical probe of the charge dynamics during an interfacial reaction. The analysis holds broad potential for investigating charge migration in surface-bound catalysts and sensors, as well as photocenter and retinal proteins, even when the Stark parameters of the material are unknown. We demonstrate the analysis with respect to the energy conversion reaction in solid-state dye-sensitized solar cells.

By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes, we find that the internal acceptor–donor–acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields, substantial energetic disorder, and pronounced Poole–Frenkel behavior, which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder, which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition, the presence of a well-defined π-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping.

The use of blue phosphorescent emitters in organic light-emitting diodes (OLEDs) imposes demanding requirements on a host material. Among these are large triplet energies, the alignment of levels with respect to the emitter, the ability to form and sustain amorphous order, material processability, and an adequate charge carrier mobility. A possible design strategy is to choose a π-conjugated core with a high triplet level and to fulfill the other requirements by using suitable substituents. Bulky substituents, however, induce large spatial separations between conjugated cores, can substantially reduce intermolecular electronic couplings, and decrease the charge mobility of the host. In this work we analyze charge transport in amorphous 2,8-bis(triphenylsilyl)dibenzofuran, an electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies, especially for electrons, while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a high electron mobility and a small Poole–Frenkel slope, supporting our conclusions. By linking electronic structure, molecular packing, and mobility, we provide a pathway to the rational design of hosts with high charge mobilities.

We outline the objectives of microscopic simulations of charge and energy transport processes in amorphous organic semiconductors, describe the current status of techniques used to achieve them, and list the challenges such methods face when aiming at quantitative predictions.

By analyzing electrostatic and polarization effects in amorphous dicyanovinyl-substituted oligothiophenes, we conclude that local molecular dipole moments result in a large, spatially correlated, energetic disorder. This disorder increases with the number of thiophene units in the oligomer and leads to an unexpected reduction of charge carrier mobility in a more ordered (smectic) mesophase, observed for the longest of the studied oligomers (hexamers). This reduction in mobilities contradicts the common belief that more ordered phases of organic semiconductors have a better charge carrier mobility. In this particular case, the amorphousness leads to a better-connected charge percolating network, helping to bypass deep energetic traps. By comparing mobilities of amorphous and crystalline mesophases we conclude that vacuum deposited thin organic films have well ordered polycrystalline morphologies.

Excited states of dicyanovinyl-substituted oligothiophenes are studied using many-body Green’s functions theory within the GW approximation and the Bethe-Salpeter equation. By varying the number of oligomer repeat units, we investigate the effects of resonant–antiresonant transition coupling, dynamical screening, and molecular conformations on calculated excitations. We find that the full dynamically screened Bethe-Salpeter equation yields absorption and emission energies in good agreement with experimental data. The effect of resonant–antiresonant coupling on the first singlet π → π* excitation monotonically decreases with increasing size of the molecule, while dynamical screening effects uniformly lower the excitation energies.

Discotic mesophases are known for their ability to self-assemble into columnar structures which serve as semiconducting molecular wires. Charge-carrier mobility along these wires strongly depends on molecular packing which is controlled by intermolecular interactions. Using solid-state NMR and molecular dynamics simulations we relate how conformations of alkyl and glycol side chains affect helical pitch and angular distribution of molecules within the columnar structures of perylenediimide derivatives. Using the high-temperature limit of Marcus theory we then establish a link between the secondary structure and charge-carrier mobility. Simulation results are compared to pulse-radiolysis time-resolved microwave conductivity measurements. We conclude that for achieving high charge-carrier mobilities in discotics, side chains with specific interactions are required in order to minimize the translational and orientational molecular disorder in the columns.

Charge carrier dynamics in an organic semiconductor can often be described in terms of charge hopping between localized states. The hopping rates depend on electronic coupling elements, reorganization energies, and driving forces, which vary as a function of position and orientation of the molecules. The exact evaluation of these contributions in a molecular assembly is computationally prohibitive. Various, often semiempirical, approximations are employed instead. In this work, we review some of these approaches and introduce a software toolkit which implements them. The purpose of the toolkit is to simplify the workflow for charge transport simulations, provide a uniform error control for the methods and a flexible platform for their development, and eventually allow in silico prescreening of organic semiconductors for specific applications. All implemented methods are illustrated by studying charge transport in amorphous films of tris-(8-hydroxyquinoline)aluminum, a common organic semiconductor.

Quantitative structure–property relationships (QSPRs) have been developed and assessed for predicting the reorganization energy of polycyclic aromatic hydrocarbons (PAHs). Preliminary QSPR models, based on a combination of molecular signature and electronic eigenvalue difference descriptors, have been trained using more than 200 PAHs. Monte Carlo cross-validation systematically improves the performance of the models through progressive reduction of the training set and selection of best performing training subsets. The final biased QSPR model yields correlation coefficients q2 and r2 of 0.7 and 0.8, respectively, and an estimated error in predicting reorganization energy of ±0.014 eV.

We analyze the relationship among the molecular structure, morphology, percolation network, and charge carrier mobility in four organic crystals: rubrene, indolo[2,3-b]carbazole with CH3 side chains, and benzo[1,2-b:4,5-b′]bis[b]benzothiophene derivatives with and without C4H9 side chains. Morphologies are generated using an all-atom force field, while charge dynamics is simulated within the framework of high-temperature nonadiabatic Marcus theory or using semiclassical dynamics. We conclude that, on the length scales reachable by molecular dynamics simulations, the charge transport in bulk molecular crystals is mostly limited by the dynamic disorder, while in self-assembled monolayers the static disorder, which is due to the slow motion of the side chains, enhances charge localization and influences the transport dynamics. We find that the presence of disorder can either reduce or increase charge carrier mobility, depending on the dimensionality of the charge percolation network. The advantages of charge transporting materials with two- or three-dimensional networks are clearly shown.

We study dilute solutions of poly(2,3-diphenyl phenylene vinylene) with hexyl (DP6-PPV) and decyl (DP10-PPV) side chains in two solvents, chloroform and toluene. For this purpose, atomistic and coarse-grained models are parametrized using quantum-chemical calculations and structure-based coarse-graining, respectively. Our simulations indicate that the difference in the aggregation behavior of two derivatives can not be rationalized just in terms of the greater steric hindrance imposed by the longer side chains of DP10-PPV. The coarse-grained model describes qualitatively the DP10-PPV derivative in chloroform, where aggregation does not occur. Although the computed structure factors for this system qualitatively agree with experiments for low concentrations, the calculated persistence length is bigger than the one experimentally reported, hinting at the presence of defects in polymer chains.

Discotic mesophases are known for their ability to self-assemble into columnar structures and can serve as semiconducting molecular wires. Charge carrier mobility along these wires strongly depends on molecular packing, which is controlled by intermolecular interactions. By combining wide-angle X-ray scattering experiments with molecular dynamics simulations, we elucidate packing motifs of a perylene tetracarboxdiimide derivative, a task which is hard to achieve by using a single experimental or theoretical technique. We then relate the charge mobility to the molecular arrangement, both by pulse-radiolysis time-resolved microwave conductivity experiments and simulations based on the non-adiabatic Marcus charge transfer theory. Our results indicate that the helical molecular arrangement with the 45° twist angle between the neighboring molecules favors hole transport in a compound normally considered as an n-type semiconductor. Statistical analysis shows that the transport is strongly suppressed by structural defects. By linking molecular packing and mobility, we eventually provide a pathway to the rational design of perylenediimide derivatives with high charge mobilities.

Coarse-graining is a systematic way of reducing the number of degrees of freedom representing a system of interest. Several coarse-graining techniques have so far been developed, such as iterative Boltzmann inversion, force-matching, and inverse Monte Carlo. However, there is no unified framework that implements these methods and that allows their direct comparison. We present a versatile object-oriented toolkit for coarse-graining applications (VOTCA) that implements these techniques and that provides a flexible modular platform for the further development of coarse-graining techniques. All methods are illustrated and compared by coarse-graining the SPC/E water model, liquid methanol, liquid propane, and a single molecule of hexane.

We outline the objectives of microscopic simulations of charge and energy transport processes in amorphous organic semiconductors, describe the current status of techniques used to achieve them, and list the challenges such methods face when aiming at quantitative predictions.

Continuous drift–diffusion models are routinely used to optimize organic semiconducting devices. Material properties are incorporated into these models via dependencies of diffusion constants, mobilities, and injection barriers on temperature, charge density, and external field. The respective expressions are often provided by the generic Gaussian disorder models, parametrized on experimental data. We show that this approach is limited by the fixed range of applicability of analytic expressions as well as approximations inherent to lattice models. To overcome these limitations we propose a scheme which first tabulates simulation results performed on small-scale off-lattice models, corrects for finite size effects, and then uses the tabulated mobility values to solve the drift–diffusion equations. The scheme is tested on DPBIC, a state of the art hole conductor for organic light emitting diodes. We find a good agreement between simulated and experimentally measured current–voltage characteristics for different film thicknesses and temperatures.

We study dilute solutions of poly(2,3-diphenyl phenylene vinylene) with hexyl (DP6-PPV) and decyl (DP10-PPV) side chains in two solvents, chloroform and toluene. For this purpose, atomistic and coarse-grained models are parametrized using quantum-chemical calculations and structure-based coarse-graining, respectively. Our simulations indicate that the difference in the aggregation behavior of two derivatives can not be rationalized just in terms of the greater steric hindrance imposed by the longer side chains of DP10-PPV. The coarse-grained model describes qualitatively the DP10-PPV derivative in chloroform, where aggregation does not occur. Although the computed structure factors for this system qualitatively agree with experiments for low concentrations, the calculated persistence length is bigger than the one experimentally reported, hinting at the presence of defects in polymer chains.

Discotic mesophases are known for their ability to self-assemble into columnar structures which serve as semiconducting molecular wires. Charge-carrier mobility along these wires strongly depends on molecular packing which is controlled by intermolecular interactions. Using solid-state NMR and molecular dynamics simulations we relate how conformations of alkyl and glycol side chains affect helical pitch and angular distribution of molecules within the columnar structures of perylenediimide derivatives. Using the high-temperature limit of Marcus theory we then establish a link between the secondary structure and charge-carrier mobility. Simulation results are compared to pulse-radiolysis time-resolved microwave conductivity measurements. We conclude that for achieving high charge-carrier mobilities in discotics, side chains with specific interactions are required in order to minimize the translational and orientational molecular disorder in the columns.

By analyzing electrostatic and polarization effects in amorphous dicyanovinyl-substituted oligothiophenes, we conclude that local molecular dipole moments result in a large, spatially correlated, energetic disorder. This disorder increases with the number of thiophene units in the oligomer and leads to an unexpected reduction of charge carrier mobility in a more ordered (smectic) mesophase, observed for the longest of the studied oligomers (hexamers). This reduction in mobilities contradicts the common belief that more ordered phases of organic semiconductors have a better charge carrier mobility. In this particular case, the amorphousness leads to a better-connected charge percolating network, helping to bypass deep energetic traps. By comparing mobilities of amorphous and crystalline mesophases we conclude that vacuum deposited thin organic films have well ordered polycrystalline morphologies.