Computational modelling of organic semiconductors: from the quantum world to actual devices

Resume : We will discuss the results of theoretical modelling of adsorption of individual organic molecules and molecular aggregates at insulating surfaces motivated by high resolution non-contact AFM imaging of surface structures. We aim at understanding how the molecular structure, the polar end-groups, as well as the substrate lattice constant and ionic species influence the formation of supra-molecular networks on insulating surfaces. As one such example, we discuss differences in the adsorption, binding, and mobility of Co-Salen molecules at NaCl and NiO (001) surfaces and the theoretical methods used in these studies. As the second example, we present the results of modelling the assembly of CDB molecules at several alkali halide surfaces. These, much larger molecules are composed of a central aromatic part surrounded by two lateral decyloxy chains (O-(CH2)9-CH3). The central part consists of three phenyl rings ended by cyano (CN) groups. We are developing multi-scale methodology for modelling the structure and growth of such networks. This includes embedded cluster QM molecular dynamics of molecules at surfaces, which is used for Genetic Algorithm powered development of force-fields for molecule-surface interactions. These force-fields are then used for modelling the atomistic structure and dynamics of larger aggregates of molecules at surfaces. This information is used for modelling AFM images of surface structures and the mechanisms of their assembly, including the effect of surface defects.

Resume : Novel organic materials are at the focus of current experimental and theoretical activities in order to improve the efficiency of their charge transport characteristics. Theory can provide guidelines to realize high mobility materials either through a molecular picture or through simulations of condensed phases. While the molecular point of view can provide some qualitative tendencies, significant improvements however have still to be accomplished to achieve predictive power of transport simulations in real devices. We present here recent work on transport simulations in organic materials by combining high level electronic structure calculations with molecular dynamics simulations to parametrize effective models for charge transport. Especially the influence of static and dynamical disorder in the electronic structure of the organic systems on the charge mobilities will be addressed.

Resume : Interest in the field of organic electronics is largely provoked by the possibility to fine-tune properties of organic semiconductors by varying their chemical structure. Often, compound design is solely guided by chemical intuition, even though material development would benefit from more rigorous structure-property relationships, which link the chemical structure, material morphology and macroscopic properties. To formulate such relationships, an understanding of physical processes occurring on a microscopic level as well as the development of methods capable of scaling these up to macroscopic dimensions are required [1]. The aim of computer simulations is to facilitate this by zooming in on the behavior of electrons and molecules and by bridging micro- and macroscopic worlds. Here, we describe the current status of methods which allow the linking of molecular electronic structure and material morphology to the mesoscopic/microscopic dynamics of charge carriers and excitons. Special attention is paid to the challenges these methods face when aiming at quantitative predictions [2,3]. [1] V. Ruehle, A. Lukyanov, F. May, M. Schrader, T. Vehoff, J. Kirkpatrick, B. Baumeier, D. Andrienko, J. Chem. Theory Comput., 3335-3345, 2011 [2] F. May, B. Baumeier, C. Lennartz, D. Andrienko, Phys. Rev. Lett., 109, 136401, 2012 [3] M. Schrader, R. Fitzner, M. Hein, C. Elschner, B. Baumeier, M. Riede, K. Leo, P. Baeuerle, D. Andrienko, J. Am. Chem. Soc., 134, 6052-6056, 2012

Resume : According to our recent massive DFT simulations, when creating holes in a bilayer graphene, the two layers tend to zip together along the edges. This phenomenon has also been recently observed by transmission electron microscopy (TEM) [1]. If the holes are arranged periodically, the optimized structure turns to be a carbon nanotube superlattice like closed sp2 carbon network. By varying the size and distance of the holes, a very rich configuration space of systems with diverse electronic structures can be created, which includes both conducting and localized states. Utilizing our wave packet dynamical simulation code [2] we studied the local time- and energy dependent dynamics of the system. By comparing the results using a jellium potential and a local pi-band pseudopotential [3] we were able to separate the geometrical and electronic structure effects, which are intermixed in the band structure. [1] G. Algara-Siller, et al., Carbon 65 (2013) 80-86 [2] G. I. Mark, et al., Phys. Rev. B 85(2012) 125443 [3] A. Mayer, Carbon 42, (2004) 2057

Resume : Understanding the alignment of molecular orbitals and corresponding transmission peaks with respect to the Fermi level of the electrodes is a major challenge in the field of molecular electronics. In order to design functional devices, it is of utmost importance to assess whether controlled changes in the electronic structure of isolated compounds are preserved once they are inserted in the molecular junctions. We shed light here on this central issue by performing Density Functional Theory calculations on junctions including diarylethene-based molecules. We demonstrate that the chemical potential equalization principle allows us to rationalize the existence or not of a Fermi level pinning (i.e., same alignment in spite of varying ionization potentials of the isolated compounds) and points to the essential role played by Metal Induced Gap States (MIGS). We also evidence that the degree of level pinning is intimately linked to the degree of orbital polarization when a bias is applied between the two electrodes. C. Van Dyck et al., PhysChemChemPhys

Resume : Investigation of the geometric and electronic structure of the polyacetylene with substituent’s of different strengths of donor and acceptor functional groups was carried out with 3-21G basis sets, by incorporating The rates of this geometrical isomerization and Arrhenius parameters. In the case of unsubstituted polyacetylene as the reference.. The values of the equilibrium constant for the reaction have also been determined at various temperatures between 300 and 500 K and the value of the energies change calculated. The results demonstrate that the isomerization energies are positive and the barrier heights ∆Ebarrier are expected to be sensitive for the magnitude of substituents effects. Keywords: polyacetylene, rate constant, equilibrium constant.

Resume : The time-of-flight (TOF) and dark-injection space-charge-limited current (DI-SCLC) measurements are both widely used to obtain the charge mobility in organic semiconductors. However, the charge transport in these cases is still not fully understood. In the TOF measurement, when the carrier density cannot be neglected, the current is said to be space-charge-perturbed (SCP). The effect of space-charge perturbation on the extracted charge mobility is unclear. The commonly assumed conclusion that the peak time in the DI-SCLC measurement equals 0.787 times the transit time in the TOF measurement also lacks validation for organic semiconductors. Using multi-particle Monte Carlo simulations, we have studied the transient currents in organic materials, under conditions varying from space-charge-free (SCF) to space-charge-limited (SCL). The results suggest that the SCP effect is shorter peak time and longer transit time. This conclusion has been confirmed by experiment. For transient SCL currents, the peak time is found to be much shorter than previously thought for systems with large energetic disorders. These results can be explained by the enhanced charge transport by higher carrier density and the non-uniform electric field, which is stronger at the carrier packet front and weaker at the injection part.

Resume : In order to unravel the effect of trap energy on carrier transport in organic disorder semiconductors, a comprehensive study was conducted on hole transport in a series of organic molecular hosts with explicit traps at different trap energies. The mobility measured by time-of-flight experiments was found to decrease significantly at shallow trapping, but hardly decreased at deep trapping, where a sharp decrease of carrier density was observed. By analyzing temperature dependence of the mobility, the decreased mobility at shallow trapping were found to originate from increased energetic disorder and activation energy, whereas both energetic disorder and activation energy are changeless at deep trapping. We find it reasonable to cover the different effects of deep trapping and shallow trapping in a universal mechanism based on the Miller-Abrahams hopping model, and carrier out multiple-carrier Monte Carlo simulations to elaborate how, within this universal mechanism, that deep and shallow traps affect energetic disorder, transporting trajectories and carrier density in different ways. The results suggest transporting at shallow trapping always involve in a multiple-trapping-release process owing to frequent thermal reactivation, thus lead to winding transporting trajectory, increased effective energetic disorder and increased activation energy; while deep traps tend to immobilize the carriers and act as ionized scattering centers, thus mainly decrease the carrier density and just elongate the trajectory slightly. Investigating the change of mobility with continuous trap energy suggests a transition region, rather than a strict borderline exists between deep traps and shallow traps, and in this region carrier transport is controlled by both deep traps and thermal reactivation from shallow traps. These results will be of great value for understanding of the mechanisms, as well as designing of the devices in organic electronics. References: The Journal of Physical Chemistry C, Under review. The Journal of Physical Chemistry C 2012, 116, 19748-19754.

Resume : The performance of organic bulk heterojunction solar cells is strongly dependent on the donor/acceptor morphology. The tremendous complexity of this morphology makes it difficult to include morphological effects in numerical device models. In this talk, I will show an extension of 1D approaches to include morphological effects and introduce a fully 3D drift-diffusion model. 1D drift-diffusion approaches treat the blend of acceptor and donor materials as one effective medium. This model can describe the current-voltage characteristics in terms of basic physics and material parameters but is silent on morphology. Inhomogeneities in the film can be included by sub-dividing the active layer into regions that can be treated in 1D separately. In this way, the effect of fullerene clusters in a polymer/fullerene blend can be described by treating the polymer-rich regions and the fullerene clusters separately. The 3D nanoscale morphology of polymer/zinc oxide solar cells was used as a direct input into a fully 3D optoelectronic model. This model includes the effects of exciton diffusion and quenching; space-charge; interfacial charge separation and recombination; and drift and diffusion of charge carriers. Given the experimental morphologies, the corresponding differences in performance could be reproduced with a single set of parameters. Several morphological aspects that determine the efficiency are discussed and compared to other organic solar cells.

Resume : Since polymer-based devices are typically synthesized from solution, the polymer solubility plays a crucial role in determining their performances: for example, in solution processed poly(3-hexylthiophene) transistors the field effect mobility is markedly different depending on the polymer miscibility in solvents[1]. The prediction of the solubility properties via atomistic simulations is an interesting challenge, but only few works exist on the topic. We apply the Flory-Huggins theory (that is typically used to study the miscibility of macromolecules[2]) for the first time to study the solubility of conjugated conducting polymers in a typical organic solvent via atomistic simulations. The properties of the isolated solvent and polymer are correctly reproduced, and the calculated solubilities of the oligo(3-alkylthiophene)s in tetrahydrofuran as a function of their side chains lengths are in agreement with available experimental data. Present investigation shows that the atomistic approach based on molecular dynamics is a powerful tool to study the solubility of alkylthiophenes in molecular solvents[3]. [1] J.-F. Chang et al., Chem. Mater. 2004, 16,4772-4776 [2] P. J. J. Flory, J. Chem. Phys. 1942, 10, 51 [3] C. Caddeo et al., Macromolecules 2013, 46 (19), 8003-8008