This article is part of the series 12th Trends in NanoTechnology International Conference (TNT2011).

Open Access Nano Express

Different interface orientations of pentacene and PTCDA induce different degrees of disorder

Angela Poschlad12, Velimir Meded12, Robert Maul1 and Wolfgang Wenzel2*

Author Affiliations

1 Steinbuch Centre for Computing, Karlsruhe Institute of Technology (KIT), Karlsruhe, 76131, Germany

2 Institute for Nanotechnology, Karlsruhe Institute of Technology (KIT), Karlsruhe, 76131, Germany

For all author emails, please log on.

Nanoscale Research Letters 2012, 7:248  doi:10.1186/1556-276X-7-248


The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/7/1/248


Received:11 January 2012
Accepted:17 April 2012
Published:14 May 2012

© 2012 Poschlad et al.; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Organic polymers or crystals are commonly used in manufacturing of today‘s electronically functional devices (OLEDs, organic solar cells, etc). Understanding their morphology in general and at the interface in particular is of paramount importance. Proper knowledge of molecular orientation at interfaces is essential for predicting optoelectronic properties such as exciton diffusion length, charge carrier mobility, and molecular quadrupole moments. Two promising candidates are pentacene and 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA). Different orientations of pentacene on PTCDA have been investigated using an atomistic molecular dynamics approach. Here, we show that the degree of disorder at the interface depends largely on the crystal orientation and that more ordered interfaces generally suffer from large vacancy formation.

Keywords:
Organic interfaces; Organic electronic devices; Interface disorder; Molecular dynamics; PTCDA; Pentacene

Background

Organic light emitting diodes (OLEDs), organic solar cells, organic thin films transistors, etc. are made of organic polymers or crystals [1-3]. The effect of the disorder in organic devices on optoelectronic properties was analyzed by Rim et al. [4]. They showed an increased photocurrent generation with improved molecular order. It occurs due to the influence of the stacking on the exciton diffusion length. Hu et al. measured a strong dependence of the conductance across highly oriented pentacene nanocrystals on the packing orientation [5]. The influence of packing on charge transport in organic solids was also analyzed using Monte Carlo methods [6]. Kwiatkowski et al. [6] were able to predict the mobilities of electron and holes for ordered and disordered Alq3. Different functional organic materials were reviewed by Ishii et al. [7]. They highlighted the energy level alignment and electronic structures at organic/inorganic and organic/organic interfaces of, for example, Alq3, 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA) and 1,4,5,8-tetrathiafulvalene (TTF).

In our work, the morphology of interfaces between pentacene [8] and PTCDA [9] was analyzed (Figure 1a). Both molecules form different crystal modifications. Pentacene is known to have a high temperature (HT) and a low temperature (LT) polymorph. Yoneya et al. [8] showed that the LT polymorph is destabilized by substrates and transforms into HT polymorph. Therefore, the HT polymorph was used as the base for simulations. For PTCDA, the αpolymorph [9] was used.

thumbnailFigure 1. Pentacene and PTCDA: Chemical formulas and interface formation. (a) Chemical formulas of PTCDA (top) and pentacene (bottom) are presented. (b) Example of the realistic interface formed by PTCDA (212) on pentacene (100). After full MD relaxation cycle, pentacene molecules have closed the holes coming from the zigzag nature of the PTCDA surface in a disordered fashion. (c) Example of the realistic interface formed by PTCDA (212) on pentacene (001). After full MD relaxation cycle, pentacene molecules remain in crystalline structure forming vacancies coming from the zigzag nature of the PTCDA surface.

Molecular orientation at interfaces is decisive for predicting optoelectronic properties such as exciton diffusion length [10], charge carrier mobility [11], and molecular quadrupole moments [12]. Verlaak et al. analyzed the impact of the molecular quadrupole moments, influenced by e. g., material and crystal orientation on the interface energetics. An insight on models of electronic processes across organic interfaces is given by Beljonne et al. [13], while a review of the corresponding theoretical approaches is presented by Brédas [14].

Our study of organic-organic pentacene/PDCDA interfaces is organized as follows: after a brief introduction presented above, we proceed with the presentation of the methods followed by the results and some conclusive remarks.

Methods

The molecular dynamic (MD) simulations of the interfaces between PTCDA and pentacene have been performed with the atomistic molecular dynamics package GROMACS (Stockholm Center for Biomembrane Research, Stockholm, Sweden and Biomedical Centre, Uppsala, Sweden) [15] using the generalized amber force field (GAFF) parameterization [16] for organic molecules, having Yoneya et al.’s work [8] in mind, and ESP charges [17] calculated with the semi-empirical quantum chemistry package MOPAC (Stewart Computational Chemistry, Colorado Springs, CO, USA) [18]. The parameter conversion from amber to GROMACS was done with the help of Antechamber python parser interface (ACPYPE) [19], the recommended tool for using GAFF with GROMACS, cf [8,20-22]. After simulation, a check of basic molecule parameters was done and the results for the example of pentacene are presented in Table 1. A more detailed report on relative errors in energy, dehidrals, etc can be found in the ACPYPE wiki [23].

Table 1. Comparison of calculated and experimental relevant parameters

The systems were simulated with a step size of 0.5 fs for more than 3 ns at a temperature of 300 K using a Berendsen thermostat [26] for temperature control. The van der Waals cut-off was set to 1.2 nm, the Coulomb cut-off to 5 nm and the relative permittivity was set to four which was taken from Wang et al. [27]. No periodic boundary conditions were used owing to the different crystal lattices.

Three surfaces were chosen and combined. For pentacene the surfaces (100), (010), and (001) were used and for PTCDA the surfaces used are (102), (-221), and (212) as defined by Miller indices. The combination of these surfaces led to nine different interface facets, e. g., (212) on (010) and (-221) on (001), as depicted in Figure 1b,c showing their relaxed structures, leaving rotation and translation as degrees of freedom. An optimal relative orientation within each of these nine facets was found by performing four simulations each with relative orientations from being twisted against each other. After a total energy comparison, the structure with the lowest mean energy per molecule of the fully relaxed systems was chosen. As an example, the energy-evolution for the interface facet (-221)//(100) is shown in Figure 2. The set of simulations were done on systems arranged to fill a 10 × 10 × 10 nm3 cube with each crystal type, filling half the space.

thumbnailFigure 2. Time-evolution of mean energy per molecule for the four interfaces of (-221) PTCDA and (100) pentacene. The triangles mark the mean energy at subsequent time steps where each relative orientation is represented by a different color. After few hundred picoseconds, equilibrium is reached and the energy, driven by the given temperature, fluctuates around an average value. The dashed lines represent the average energy between 1.5 ns and 3 ns of simulation time.

Results and discussion

In order to quantify the disorder at the pentacene/PTCDA interface, we used distribution of ϕ, defined as the angle between the molecular and the interface plane (or rather their respective normals) as shown in Figure 3. Owing to the fact that the molecules will start to relax, they will start to deviate from the bulk values. The more molecules have different ϕ, the more disordered is the structure.

thumbnailFigure 3. Definition of the angle defining the molecular orientation along with a distribution at one interface. (a) ϕis the angle between the normal of the molecule plane and the normal of the interface plane (z-axis), i. e. the angle between the molecule plane and the interface plane. (b) Distribution of the angle ϕ(blue area) for the relaxed pentacene molecules at the interface of (-221)//(100) (pictogram in the left upper corner). Contributing pentacene molecules have a PTCDA neighbor with maximal atom-atom distance of 0.4 nm. The red dashed lines at 145.5 degree and 157 degree mark the values for the ideal morphology.

In the histograms of Figure 4, the y-axis was defined as distance in Å from the (ideal) interface in z-direction, while the x-axis shows the angle distribution. Light blue regions mark the disordered regions. Two clear patterns can be observed: 1) size of the disordered region can vary from 2 to 16 Å, and 2) the disorder seems to spread asymmetrically from the ideal interface, clearly preferring pentacene-rich regions. The first pattern can be explained as having two competing effects at the interface, one being the optimization of the intermolecular distance/interaction and the other being the conservation of bulk properties. The second pattern can be understood in the light of much stronger ΠΠ stacking of the PTCDA molecules, leading to a stronger intermolecular interactions, and greater energies are required to disrupt these molecules from their bulk positions when compared to pentacene bulk.

thumbnailFigure 4. Angular distribution of angleϕas function of distance from the interface. The 2d-histograms (a–i) show the angle ϕdepending on the distance to the interface at z=0, where ϕis the angle between the molecule plane and the interface plane (see Figure 3a). The distance to the interface is given as the z coordinate of the molecule center in Å. Each histogram represents the results for one of the interface facets configurations given in Miller indices (001), (010), and (100) for pentacene (as marked on the right side of the histograms) and as (102), (212) and (-221) for PTCDA (as marked above the histograms). The box size is proportional to the number of occurrence. The interface location is emphasized by a dashed line with PTCDA located above and pentacene below it. The region of disorder is marked in light blue. Outside the light blue area the crystals are in their bulk phase. The corresponding relaxed crystal morphology is represented by the inset molecular structure.

Conclusions

Analysis of PTCDA/pentacene interfaces was performed with two emerging messages: there seems to be two competing effects, one coming from intermolecular interaction, which leads to disordered interfaces, while the other coming from the preservation of bulk properties results in large interfacial vacancies. Both of the effects would lead to dramatically diminished transport properties. Namely, increased disorder would cause greater energy disorder of the interfacial hopping sites, while interfacial vacancies would lead to diminished intermolecular overlaps, or hopping matrix elements. Whether which of the competing effects is influencing more the hopping transport properties is the focus of our ongoing research. Our second observation is that pentacene seems to be, in general, a more flexible material, which can be observed from the fact that the disordered regions are predominantly pentacene-rich.

Competing interests

The authors declare that they have no competing interests.

Author’s contributions

AP carried out the molecular dynamics calculations, the setup of the initial system and helped in drafting of the manuscript, and revisions. VM helped in analysis and interpretation of data, and drafted the manuscript and revisions. RM provided the calculation of the partial charges. WW participated in the design of the study, formulated the original scientific question and helped in analysis and interpretation of data. All authors read and approved the final manuscript.

Authors’ information

AP is Ph.D. student, VM and RM have Ph.D. degree in physics, and WW is an associate professor at Karlsruhe Institute of Technology.

Acknowledgements

This work was supported by bwGRiD resources and the FP7 project MINOTOR. bwGRiD is the central collection of computing power within the state of Baden-Wuerttemberg operated by eight universities, providing access for local researchers. Further thanks go to Ivan Kondov from SCC/KIT.

References

  1. Yun C, Cho H, Kang H, Lee Y: Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes.

    Appl Phys Lett 2009, 95:053301. Publisher Full Text OpenURL

  2. Roncaliu J: Molecular bulk heterojunctions: an emerging approach to organic solar cells.

    Acc Chem Res 2009, 42:1719-1730. PubMed Abstract | Publisher Full Text OpenURL

  3. Reese Bao: Organic single-crystal field-effect transistors.

    Mat Today 2007, 10:20-27. OpenURL

  4. Rim S, Fink R, Schöneboom J, Erk P, Peumans P: Effect of molecular packing on the exciton diffusion length in organic solar cells.

    Appl Phys Lett 2007, 91:173504. Publisher Full Text OpenURL

  5. Hu W-S, Tao Y-T, Chen Y-F, Chang C-S: Orientation-dependent conductance study of pentacene nanocrystals by conductive atomic force microscopy.

    Appl Phys Lett 2008, 93:053304. Publisher Full Text OpenURL

  6. Kwiatkowski J, Nelson J, Li J, Brédas J, Wenzel W, Lennartz C: Simulating charge transport in tris (8-hydroxyquinoline) aluminium (Alq3).

    Phys Chem Chem Phys 2008, 10:1852-1858. PubMed Abstract | Publisher Full Text OpenURL

  7. Ishii H, Sugiyama K, Ito E, Seki K: Energy level alignment and interfacial electronic structures at organic/metal and organic/organic Interfaces.

    Adv Mater 1999, 11:8. OpenURL

  8. Yoneya M, Kawasaki M, Ando M: Molecular dynamics simulations of pentacene thin films: The effect of surface on polymorph selection.

    J Mater Chem 2010, 20:10397-10402. Publisher Full Text OpenURL

  9. Ogawa T, Kuwamoto K, Isoda S, Kobayashi T, Karl N: 3,4:9,10-Perylenetetracarboxylic dianhydride (PTCDA) by electron crystallography.

    Acta Cryst B 1999, 55:123-130. Publisher Full Text OpenURL

  10. Najafov H, Lee B, Zhou Q, Feldman L, Podzorov V: Observation of long-range exciton diffusion in highly ordered organic semiconductors.

    Nat Mater 2010, 9:938-943. PubMed Abstract | Publisher Full Text OpenURL

  11. Vehoff T, Baumeier B, Troisi A, Andrienko D: Charge transport in organic crystals: role of disorder and topological connectivity.

    J Am Chem Soc 2010, 13:11702-11708. OpenURL

  12. Verlaak S, Beljonne D, Cheyns D, Rolin C, Linares M, Castet F, Cornil J, Heremans P: Electronic structure and geminate pair energetics at organic–organic interfaces: the case of pentacene/C60 heterojunctions.

    Adv Func Mat 2009, 19:3809-3814. Publisher Full Text OpenURL

  13. Beljonne D, Cornil J, Muccioli L, Zannoni CJ, Castet F: Electronic processes at organic-organic interfaces: insight from modeling and implications for opto-electronic devices.

    Chem Mater 2011, 23:591-609. Publisher Full Text OpenURL

  14. Brédas J, Norton J, Cornil J, Coropceanu V: Molecular understanding of organic solar cells: the challenges.

    Acc Chem Res 2009, 42:1691-1699. PubMed Abstract | Publisher Full Text OpenURL

  15. Lindahl E, Hess B, van der Spoel D: GROMACS 3.0: a package for molecular simulation and trajectory analysis.

    J of Mol Model 2001, 7:306-317. OpenURL

  16. Wang J, Wolf R, Caldwell J, Kollman P, Case D: Development and testing of a general amber force field.

    J Comput Chem 2004, 25:1157. PubMed Abstract | Publisher Full Text OpenURL

  17. Singh UC, Kollman PA: An approach to computing electrostatic charges for molecules.

    J Comp Chem 1983, 5:129-145. OpenURL

  18. Stewart JJP: Optimization of parameters for semiempirical methods V: modification of NDDO approximations and application to 70 elements.

    J Mol Modeling 2007, 13:1173-1213. Publisher Full Text OpenURL

  19. Sousa da Silva A, Vranken W, Laue E: ACPYPE - Antechamber python parser interface. [ http://code.google.com/p/acpype/ webcite]

  20. Ramalho TC, França TC, Cortopassi WA, Gonçalves AS, da Silva AW, da Cunha EF: Topology and dynamics of the interaction between 5-nitroimidazole radiosensitizers and duplex DNA studied by a combination of docking, molecular dynamic simulations and NMR spectroscopy.

    J Mol Struc 2011, 992:65-71. Publisher Full Text OpenURL

  21. Punkvang A, Saparpakorn P, Hannongbuam S, Wolschann P, Beyer A, Pungpo P: Investigating the structural basis of arylamides to improve potency against M. tuberculosis strain through molecular dynamics simulations.

    Europ J Med Chem 2010, 45:5585-5593. Publisher Full Text OpenURL

  22. Balajee R, Rajan MSD: Molecular docking and simulation studies of farnesyl trasnferase with the potential inhibitor theflavin.

    J Appl Pharm Sci 2011, 8:141-148. OpenURL

  23. ACPYPE Wiki: Testing ACPYPE amb2gmx function [ http://code.google.com/p/acpype/wiki/TestingAcpypeAmb2gmx webcite]

  24. Campbell RB, Robertson MJ, Trotter J: The crystal and molecular structure of pentacene.

    Acta Cryst 1961, 14:705. Publisher Full Text OpenURL

  25. Endres RG, Fong CY, Yang LH, Witte G, Wöll C: Structural and electronic properties of pentacene molecule and molecular pentacene solid.

    Comp Mat Sci 2004, 29:362-370. Publisher Full Text OpenURL

  26. Berendsen HJC, Postma JPM, van Gunsteren W F, DiNola A, Haak JR: Molecular dynamics with coupling to an external bath.

    J Chem Phys 1984, 81:3684. Publisher Full Text OpenURL

  27. Wang Y, Chengb H, Wanga Y, Hub T, Hob J, Leeb C, Leia T, Yeha C: Influence of measuring environment on the electrical characteristics of pentacene-based thin film transistors.

    Thin Solid Films 2004, 467:215-219. Publisher Full Text OpenURL