Poschlad et al. Nanoscale Research Letters 2012, 7:248
http://www.nanoscalereslett.com/content/7/1/248
NANO EXPRESS
Open Access
Different interface orientations of pentacene
and PTCDA induce different degrees of
disorder
Angela Poschlad1,2 , Velimir Meded1,2 , Robert Maul1 and Wolfgang Wenzel2*
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,
*Correspondence: WW:[email protected]
2 Institute for Nanotechnology, Karlsruhe Institute of Technology (KIT),
Karlsruhe, 76131, Germany
Full list of author information is available at the end of the article
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 α polymorph [9]
was used.
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].
© 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.
Poschlad et al. Nanoscale Research Letters 2012, 7:248
http://www.nanoscalereslett.com/content/7/1/248
Page 2 of 5
Figure 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.
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.
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].
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
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,
Table 1 Comparison of calculated and experimental relevant parameters
Bonds in pentacene
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C7
C4-C21
C6-C19
C-H
Ab initio
1.43
1.38
1.44
1.4
1.42
1.43
1.46
1.46
1.1
Exp
1.441
1.358
1.428
1.381
1.409
1.396
1.453
1.464
na
MD
1.397
1.394
1.395
1.399
1.395
1.395
1.403
1.403
1.088
C-C-C
C-C-C
C-C-C
C-C-C
C-C-C
C-C-C
C-C-C
C-C-C
C-C-C
C-C-C
Exp
121
123
124
124
123
119
119
118
118
118
MD
120.2
120.1
120.1
120.3
120.2
119.8
119.8
119.9
119.8
119.8
Angles in pentacene
A representative selection of bonds and angles of pentacene is presented. A comparison of measured [24] and ab initio [25] parameters with results from the MD
calculations is displayed showing a generally good agreement between the methods.
Poschlad et al. Nanoscale Research Letters 2012, 7:248
http://www.nanoscalereslett.com/content/7/1/248
Page 3 of 5
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.
Figure 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.
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.
Results and discussion
In order to quantify the disorder at the pentacene/PTCDA
interface, we used distribution of φ, defined as the angle
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 π −π 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.
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,
Figure 3 Definition of the angle defining the molecular orientation along with a distribution at one interface. (a) φ is 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.
Poschlad et al. Nanoscale Research Letters 2012, 7:248
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Figure 4 Angular distribution of angle φ as function of distance from the interface. The 2d-histograms (a–i) show the angle φ depending on
the distance to the interface at z = 0, where φ is 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.
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.
Author details
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.
Received: 11 January 2012 Accepted: 17 April 2012
Published: 14 May 2012
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.
2. Roncaliu J: Molecular bulk heterojunctions: an emerging approach
to organic solar cells. Acc Chem Res 2009, 42:1719–1730.
3. Reese Bao: Organic single-crystal field-effect transistors. Mat Today
2007, 10:20–27.
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.
Poschlad et al. Nanoscale Research Letters 2012, 7:248
http://www.nanoscalereslett.com/content/7/1/248
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Brédas J, Norton J, Cornil J, Coropceanu V: Molecular understanding of
organic solar cells: the challenges. Acc Chem Res 2009, 42:1691–1699.
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.
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.
Singh UC, Kollman PA: An approach to computing electrostatic
charges for molecules. J Comp Chem 1983, 5:129–145.
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.
Sousa da Silva A, Vranken W, Laue E: ACPYPE - Antechamber python
parser interface. [http://code.google.com/p/acpype/]
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.
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.
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.
ACPYPE Wiki: Testing ACPYPE amb2gmx function. [http://code.
google.com/p/acpype/wiki/TestingAcpypeAmb2gmx]
Campbell RB, Robertson MJ, Trotter J: The crystal and molecular
structure of pentacene. Acta Cryst 1961, 14:705.
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.
Page 5 of 5
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.
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.
doi:10.1186/1556-276X-7-248
Cite this article as: Poschlad et al.: Different interface orientations of pentacene and PTCDA induce different degrees of disorder. Nanoscale Research
Letters 2012 7:248.
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