DOI: 10.1002/10.1002/admi.201500452
Article type: Full Paper in Advanced Materials Interfaces
Effects of Molecular Orientation in Acceptor-Donor Interfaces between Pentacene and C60 and Diels-Alder
Adduct Formation at the Molecular Interface
Tobias Breuer*, Andrea Karthäuser, and Gregor Witte
Fachbereich Physik, Philips-Universität Marburg, D-35032 Marburg, Germany
E-mail: [email protected]
Keywords: organic heterostructures, acceptor/donor interfaces, pentacene, fullerenes, NEXAFS
Abstract
Interfaces between pentacene and Buckminster-Fullerene (C60) have attracted interest due to their application as oligomeric model system
for organic solar cells. As the actual device characteristics in such implementations are crucially controlled by the interface structure, detailed
investigations of this interface on a molecular level are mandatory. In this study we analyze the influence of the orientation of the pentacene
molecules in highly-ordered crystalline bottom layers on the characteristics of such internal interfaces. We show that the interface structure
is driven by temperature-controlled diffusion of C60 molecules to the pentacene step-edges in the case of uprightly-oriented pentacene. For
lying pentacene in the bottom layer, no step-edge decoration is observed while the wetting of the pentacene layer is enhanced. Furthermore,
the stability of the interface against intercalation and re-orientation has been analyzed by means of NEXAFS spectroscopy, showing that the
orientation of the pentacene molecules at the interface remains unchanged. Instead, we observe strong indication for chemical modification
of the molecular entities by the formation of Diels-Alder adducts between C60 and pentacene. Finally, we show that C60 forms crystalline
islands in thicker films only on top of uprightly-oriented pentacene while rather amorphous films are formed on lying pentacene.
1. Introduction
The study of organic semiconductors (OSC) has gained
significant interest in the past decade due to their possible
application in organic electronic devices like light-emitting
diodes (OLED),[1,2] organic field-effect transistors (OFET)[3-5] or
organic photovoltaics (OPV).[6-8] Though first devices have been
realized, their utilization on a broader scale is hampered by an
insufficient fundamental understanding of their electronic
properties and the interplay between structure formation and
electronic characteristics. This is especially true for organic
heterostructures, such as donor/acceptor blends utilized in
OPVs,[7] which have to provide sufficient light absorption and
allow for efficient charge separation as well as loss-free charge
transport to the electrodes. These characteristics are frequently
not determined by the intrinsic electronic properties of the
utilized compounds, but are instead mainly controlled by the
interface morphology and electronic interface coupling.[9,10]
Theoretical studies have predicted that also the relative
molecular arrangement between acceptor and donor affects the
charge-transfer exciton energies and their dissociation
dynamics.[11,12] Since actual devices mostly consist of polymeric
blends with rather complex, disordered internal interfaces,
structural characterizations on a microscopic level are not
1
possible, so that this projection has not yet been proven
experimentally.
One strategy to gain the required knowledge in this respect is
preparing well-defined model systems of organic
heterostructures consisting of oligomeric units and studying their
characteristics as function of the interface geometry. For OPVs,
the combination of the OSCs pentacene (C22H14, PEN) and
Buckminster-Fullerene (C60) as donor/acceptor pair constitutes
such a prototypical model system.[13-16] Its oligomeric nature
allows in particular the preparation of well-defined interfaces
with controllable relative molecular orientation in stacked
heterostructures (cf. visualization in the top of Figure 1) so that
precise structure-property relationships can be derived. Though
this material combination has been used to fabricate OPVs[16-19]
and ambipolar OFETs,[20-22] the influence of different relative
molecular orientations at the interface has not been addressed
experimentally. This is particular unfortunate as pronounced
differences of electronic and structural properties have been
theoretically projected for different relative arrangements of PEN
and C60.[11,12,23-26]
In the present work we have prepared well-defined PEN bottom
layers with different exclusive molecular orientation and studied
their influence on the interface formation with subsequently
deposited C60 films. These interfaces were studied with respect to
their morphology, molecular orientation and stability against a
theoretically proposed mutual intercalation[26] as well as chemical
modification. We note that the latter aspects are not directly
accessible by application of standard laboratory techniques. For
example, scanning tunneling microscopy (STM) cannot be used
for such an analysis, since it is only sensitive to the surface of the
heterostructures but does not provide information on their
internal interface. Also X-ray diffraction (XRD) cannot be
utilized in this context since it is not sufficiently surface-sensitive
and does not allow the characterization of amorphous PEN films
that are proposed for the intercalation process. To still gain
insights into the interface structure, near edge X-ray absorption
fine structure spectroscopy (NEXAFS) was employed to
monitor the molecular orientation in the outermost PEN layers
before and after deposition of small amounts of C60 on top of the
pentacene films. Due to its low probe depth (~2 nm, cf. Figure
2b), this technique allows to exclusively determine the
orientation in the top layers. Furthermore, it is not restricted to
crystalline regions and therefore enables observing possible
reorientation of molecules into amorphous configurations.[27]
Additional analyses were performed by application of atomic
force microscopy (AFM) to analyze the morphology of the
interfaces and XRD to determine the crystalline order in thicker
films.
Figure 1. AFM micrographs depicting the morphology of C60 /
PEN interfaces on HOPG (a-c) and SiO2 surfaces (d-f). a,d)
pure PEN bottom layer, b-f) after deposition of additional 0.5
nm C60 at b,e) 190 K, c,f) 300 K. Note that the vertical scale bars
in the topographic linescans allow for the comparison of the
height levels of the individual scans.
growth. Films deposited onto SiO2 consist of pyramidal islands
with molecules in upright configuration (cf. Figure 1d)
crystallizing in (001)-orientation in the thin-film phase (cf.
diffraction pattern presented in section 2.4).[28] By contrast, on
pristine graphite surfaces mesa-like islands are formed (Figure
1a), in which the molecules adopt a lying arrangement,
corresponding to the Siegrist-phase (011)-orientation.[29]
First, the morphology of the interface between small amounts of
C60 deposited at 190 K onto thick pentacene bottom layers
(d=30 nm) is investigated. Distinctly different interface
2. Results and Discussion
2.1. Interface Morphology
The morphology and, more importantly, the molecular
orientation in pentacene bottom layers can be precisely
controlled by utilizing different supporting substrates for film
2
morphologies are found for both molecular orientations in the
PEN bottom layers: on uprightly oriented PEN, the C60
molecules preferentially form clusters at the PEN step edges,
leaving the terraces mainly uncovered (cf. Figure 1e). As
presented in Figure 1f), the clusters strongly increase in size and
exhibit enhanced lateral separation at elevated substrate
temperature during C60 deposition, in perfect agreement with
prior findings.[13] Upon deposition on pentacene in lying
molecular orientation on graphite substrates, no indication for
such step edge decoration is found. Although the molecules again
form clusters, of which size increases with rising substrate
temperature, they are rather homogeneously distributed on the
PEN islands (Figure 1b,c). This allows concluding that either the
diffusivity of C60 on the recumbently oriented PEN molecules is
strongly reduced compared to the template with upright
molecular orientation or that the energetic difference between
nucleation at step edges and on terraces is either negligible or
even adverse for the step edges. The latter interpretation is
perfectly in line with theoretical studies, where enhanced
interaction between C60 and the “open” faces of pentacene (like
the (011) and (100) faces[30]) has been predicted, while the
“closed” (001) face was calculated to be energetically
unfavorable for C60.[26] This is the reason, why the C60 molecules
tend to avoid the PEN terraces on SiO2, which consist of (001)
faces, and instead diffuse towards step edges, composited by
more open faces, hence allowing for the respective energetic gain.
On graphite surfaces in contrast, the terraces are formed by the
more open (011) faces, thus redundantizing diffusion to step
edges. Interestingly, we do not observe the formation of onedimensional nanorods of C60 as they have been observed upon
deposition onto pentacene monolayers which were prepared on
silver surfaces.[14,15] This observed different behavior is ascribed
to significant chemical interaction of the PEN layers with the
metallic substrate in that case and structural differences between
such monolayers and the present multilayer films prepared on
graphite.[31,32]
2.2. Molecular Orientation at the Interface and Intercalation
In a recent theoretical work by Fu et al.[26] it has been predicted
that upon deposition the fullerenes intercalate into the PEN film
if the acene molecules adopt a lying configuration and thereby
destroy the crystalline order of the outermost PEN layers (cf.
snapshot of MD-simulation presented in Figure 2a). This
scenario has also been addressed in a work by Cantrell et al.,[24]
where furthermore the influence of potential short axis tilts of
pentacene (cf. angle
in Figure 2b) on the efficiency of
intercalation has been discussed. Unfortunately, up to now no
experimental verification of this projection is available. As
discussed before, the experimental determination of the
molecular orientation directly at the interface (denoted by the
red box in Figure 2a) is rather challenging. Enabled by its low
probe depth, limiting the analyzed region to the top layers only
(cf. Figure 2b), and its sensitivity to crystalline as well as
amorphous regions, NEXAFS spectroscopy appears as ideal
method to address this issue.
Figure 2: a) snapshot of MD-simulation predicting severe
intercalation of C60 molecules into the PEN lattice in lying PEN
configurations (reproduced with permission.[26] Copyright 2013,
Adv. Mater., Wiley), b) sketch of probe depth of NEXAFS
experiment, c) C1s-NEXAFS spectra recorded at different angles
of incidence of pentacene films (d=30 nm) deposited on graphite
(d,e) and SiO2 (f,g) with d),f) dichroism analysis and e),g)
dichroism analysis of films after additional deposition of 0.5 nm
C60 at T=300 K. A sketch of the experimental geometry is
presented in the inset in panel c).
3
performed for samples where the C60 adlayer has been prepared
at low temperature to vary the interface homogeneity, again
yielding equivalent results for both cases (full data sets presented
in Supporting Information, Figure S4,5).
In summary, we have not observed any evidence of C60
intercalation into the PEN lattice, regardless of the temperature
during C60 deposition, post-deposition heating and initial
molecular orientation in the PEN layer.
Figure 2c displays a series of C1s-NEXAFS spectra recorded
under different angles of incidence, , for a thick pentacene film
on graphite. Clearly, the spectrum recorded under gracing
incidence ( 30°, blue spectrum) exhibits stronger intensity of
the *-related signals
in the energetic region of 283 eV to 287 eV [33] than that taken
under normal incidence (90°, red spectrum). Such a dichroism
corresponds to molecules in a lying configuration. A moreprecise
analysis yields an effective orientation of the molecular transition
dipole moment (TDM) of =36° ±5° (cf. Figure 2d, full spectra
presented in the Supporting Information, Figure S1). Note that
this value cannot directly be translated into the orientation of the
long molecular axis due to the herringbone packing of molecules
in the pentacene unit cell.[27] Instead, as shown in previous
studies, the lowest possible unit cell averaged TDM orientation
for pentacene multilayer films with respect to their surface
normal amounts to 28°, corresponding to molecules in the
crystalline (011)-orientation. In this geometry, the PEN
molecules are oriented with their long axis perfectly parallel to
the surface but slight tilts of their short axis (as visualized in
Figure 2b). The observed deviation from this ideal geometry by
8° results from local imperfections at the substrate, where
molecules adopt an upright orientation.[29] After deposition of
C60 onto the pentacene bottom layer, the NEXAFS-signals of
pentacene are superimposed by those of C60 (cf. blue spectrum
Figure 3a, full set in Supporting Information, Figure S1). By
appropriate separation of both signals, the orientation of the
pentacene molecules at the interface can be determined from an
analysis of the dichroism (cf. Figure 2e, for further details on
analysis see Supporting Information). This analysis yields an
effective TDM orientation of =37°±5°, which closely resembles
the value found for the pristine pentacene layer. Hence, the
orientation of the PEN molecules in the outermost layers
remains essentially unchanged and they do not exhibit a notable
re-orientation. Since the potential intercalation process might be
thermally activated, we have also heated the sample to 350 K and
370 K, respectively, for 10 min after C60 deposition. Also in these
cases, the orientation remained unchanged, again indicating the
absence of significant intercalation of the C60 molecules into the
PEN layer (dichroism analysis presented in the Supporting
Information, Figure S1). An additional set of experiments has
also been conducted for a sample where C60 was deposited at 190
K onto the PEN layers prepared on graphite to enable more
homogeneous wetting of the pentacene bottom layer by the
fullerenes. Here, the orientation of the pentacene layer again
remained unchanged and no evidence for intercalation was
found, both, at low temperature and after subsequent heating
(spectra presented in the Supporting Information, Figure S3).
Therefore, a potential influence of the interface homogeneity can
be safely ruled out. Finally, equivalent analyses have been
conducted for PEN films with upright molecular orientation
prepared on SiO2. For this initial PEN orientation, no C60
intercalation into the PEN lattice has been proposed.[26] In this
case, the experimental results perfectly agree with these
predictions, since the molecular orientation remains unchanged
at values of about 78° (cf. Fig. 2 f,g). Like for the heterostructures
prepared on graphite, corresponding analyses have also been
2.3. Chemical Modification at the Interface
Figure 3: C1s-NEXAFS spectra of a) PEN/C60 heterostructure
prepared on graphite at T=300 K after subsequent heating to 300
K, 350 K and 370 K for 10 min showing a rising signal at 285.1
eV (red arrow). b) Heterostructure as before after desorption of
PEN (by heating at 400 K, red) and equivalent heterostructure
prepared on SiO2 (blue) compared to PEN:C60 Diels-Alder
adducts (grey) and a reference spectrum of pure C60 [13] (green).
c) Comparison of NEXAFS signatures of the Diels-Alder adducts
and reference spectra of naphthalene[39] and benzene[40] together
with visualizations of various reported PEN:C60 Diels-Alder
adducts.[34,38]
4
molecules in the bottom layer) as well as SiO2 substrates
(uprightly-oriented PEN). Also the signature observed after
desorption of the PEN film is similar for both substrates, showing
that the initial orientation of the PEN molecules does not
determine whether the observed chemical modification takes
place. However, it was observed that the new spectral feature
exhibits larger intensity for films prepared on graphite compared
to those prepared on SiO2, indicating that films with lying
molecular orientation of the PEN molecules are more susceptible
to this reaction than those with uprightly-oriented molecules
(data presented in Supporting Information, Fig. S7). To verify
that these observations actually correspond to Diels-Alder
adducts of PEN and C60, the NEXAFS signature of such adducts
has also been analyzed. For that purpose, monomeric Diels-Alder
adducts have been prepared by solution-mediated reaction from
heated toluene following the route by Cataldo et al.[36] and their
NEXAFS signature was recorded (grey spectrum in Figure 3b).
Clearly, the spectrum exhibits good congruence with that of the
heterostructures after PEN desorption, also in the higherenergetic region related to σ*-transitions (higher energy region
presented in the Supporting Information, Fig. S8). Interestingly,
the remaining adducts extracted by selective thermal desorption
of non-reacted PEN experience strongly reduced dichroism
compared to the initial films indicating that the Diels-Alderadducts do not form an oriented film but instead exhibit
statistical orientation or are uniformly oriented at an angle of
close to 55° (magic angle [27], corresponding angle-resolved
NEXAFS spectra are presented in the Supporting Information,
In previous works on interfaces of PEN and C60 it has been
assumed that both compounds do not react with each
other.[11,12,23-26] However, for PEN and C60, the formation of
molecular aggregates by Diels-Alder reaction has been reported
upon precipitation from solution.[34-36] Therefore, potential
chemical modification also at the interface of the stacked
heterostructure requires appropriate consideration, especially in
view of the distinct consequences for device applications. A
closer inspection of the isolated NEXAFS spectra of PEN indeed
reveals additional intensity compared to pure PEN occurring at
285.1 eV. This effect becomes even clearer, when the
heterostructure is additionally heated. Clearly, the height of the
signal rises with increasing temperature (cf. red arrow in Figure
3a), indicating that this chemical reaction leading to the novel
feature is thermally activated. Since no spectral changes are
observed for films of the respective pure compounds
(temperature-dependent spectra of the isolated compounds are
presented in the Supporting Information, Fig. S6), this effect can
be directly attributed to an interaction between PEN and C60. To
isolate this feature, the PEN layers were thermally desorbed from
the heterostructures by gentle heating to temperatures of 400 K
for 10 min.[37] Consequently, the NEXAFS signature changes
drastically compared to the lower temperatures since the PENrelated signals are absent. Interestingly, the resulting signature
(red spectrum in Figure 3b) is also significantly different from
that of pure C60 (green dashed line). Particularly, a distinct
resonance at 285.1 eV is observed, which perfectly fits to the
signal observed in the post-heated heterostructure. We note that
the rising spectral feature has been observed for heterostructures
prepared on both, graphite (lying orientation of the PEN
Figure 4: AFM micrographs of C60/PEN heterostructures prepared at T=300 K after desorption of excess PEN molecules on graphite (top)
and SiO2 (bottom). Apparently, the height of the remaining clusters is similar and clearly smaller than that of the desorbed PEN bottom
layers. Note that the vertical scale bars in the topographic linescans allow for the comparison of the height levels of the individual scans.
Note that the vertical scale bars in the topographic linescans allow for the comparison of the height levels of the individual scans.
5
Fig. S9). In previous studies of solution-mediated Diels-Alder
adduct formation, different adducts including not only dimeric
configurations but also combinations of three molecules have
been observed (cf. Figure 3c).[38] The NEXAFS spectra of
complex molecular entities are frequently interpreted as
superposition of the signatures of electronically decoupled units
following the so-called building block approach. This concept is
applicable when the conjugation is lifted and smaller nonconjugated aromatic subunits are formed. Therefore, the present
spectrum of the Diels-Alder adducts should be explicable as
superposition of C60 and naphthalene or phenylene-like
signatures depending on the actual adduct. Indeed, the NEXAFS
signatures of naphthalene[39] and benzene[40] exhibit significant
absorption at the position of the additional resonance in the
adduct (cf. Figure 3d), which thus supports our interpretation.
Furthermore, we have also analyzed the morphology of the
heterostructures after applying the heating steps to enable the
spectral analysis which was discussed before. As presented in
Figure 4, the morphology of the heterostructures prepared on
graphite as well as on SiO2 remains essentially unchanged upon
heating. Possibly, slight tendencies of cluster agglomeration can
be identified for the heterostructures prepared on graphite after
desorption of the PEN bottom layers. However, strong
modifications can be ruled out since both, the cluster heights as
well as their lateral alignment are essentially equivalent before
and after the heating processes. This can be nicely seen for the
heterostructures prepared on SiO2, where even after desorption
of the PEN layer, the initial step edge decoration is still visible.
This analysis also allows to conclude that not only the PEN
molecules in uncovered areas but also those underneath the C60
islands have desorbed from the surface since the total height of
the remaining clusters (~ 5-8 nm) is clearly smaller than the
thickness of the initial PEN film (30 nm). This can furthermore
be perfectly identified for the heterostructures on SiO2 since the
C60 cluster heights after PEN desorption are equivalent, while the
initially observed molecular steps of the PEN bottom layers,
indicated by the dashed lines in the linescans of Figure 4, are no
longer visible after desorption of the PEN layer.
Figure 5: X-ray Diffractograms of C60-films (d=30 nm)
deposited onto pentacene multilayer films (d=30 nm) in a)
upright orientation (on SiO2, top), b) lying orientation (on
graphite, bottom) presented on linear intensity scale
(diffractograms offset for clarity).
growth of C60 while the fullerene molecules form a rather
amorphous film on lying pentacene. Moreover, morphological
analyses by AFM have been performed to find out about
potential correlations between the observed differences in film
crystallinity and the film morphologies (data presented in
Supporting Information, Fig. S10). For PEN bottom layers in
upright molecular orientation (on SiO2), the resulting C60 islands
exhibit increased size and smoothness
compared to those formed on lying PEN molecules (on
graphite), nicely corresponding to their different crystalline
nature. This can be interpreted in two different ways. First, the
step-edge decoration which takes place only on PEN films of
uprightly-oriented molecules might lead to the formation of
stable nuclei which can ripe rather efficiently upon further C60
deposition. The second explanation is a reduced diffusivity of C60
molecules on recumbently-oriented PEN which hinders the
formation of extended C60 islands on the heterostructures
prepared on graphite. This would also be in line with the
observed reduced dewetting tendency on graphite compared to
samples prepared on SiO2 as well as the proposed stronger
interaction of C60 with PEN in lying orientation than with
uprightly-oriented PEN.
2.4. Molecular Orientation in Bottom Layer Determines
Crystallinity
Finally, we have also examined the structural order in thicker C60
top layers. For uprightly-oriented pentacene, template-mediated
crystalline growth of the fullerenes has been reported.[16,41] Again,
the orientation of the underlying PEN molecules is expected to
have severe implications for this effect. To enlighten this issue,
we have prepared comparably thick C60 films (nominal thickness
30 nm, Tgrowth=320 K) on top of pentacene bottom layers with
different molecular orientation. As presented in Figure 5, we
observe the (111)C60- and (222)C60-peaks additionally to the
(00n)PEN, TF-peaks on SiO2 exactly in line with the previous
results.[16] For the case of C60 deposited onto lying PEN on
graphite, however, we only observe the (022)PEN-peak and no
signals corresponding to crystalline C60. We therefore conclude
that only uprightly-oriented pentacene induces crystalline
3. Conclusions
The present study has revealed important new findings which
enable an enhanced understanding of the interface in
prototypical organic heterostructures of PEN and C60 and put a
number of previous studies in a new complexion. Investigations
of the interface morphology and the crystalline order in thicker
films clearly show that the molecular orientation of PEN in the
bottom layer directly determines the structural characteristics of
6
C60 top layers and thereby the electronic properties of the entire
heterostructure.[19,41,42] Most importantly, we have found
evidence for the formation of PEN/C60 Diels-Alder adducts at
the interface, challenging the common assumption of this
intermolecular interface being chemically inert. Since both, the
interpretations of device characteristics of PEN/C60-based OPVs
and OFETs as well as theoretical models of this system have not
considered chemical interaction between both compounds,[16-19]
the present findings will allow for a clearly improved
understanding of the fundamental processes in this prototypical
organic donor/acceptor complex. Also the observed absence of
the theoretically proposed intercalation of C60 molecules into the
PEN lattice for PEN molecules in lying configuration[26] can be
explained by this process, since the adducts will sterically hinder
the intercalation process. Therefore, also extended theoretical
approaches with appropriate consideration of chemical
interaction should be applied to gain additional insights from this
perspective.
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4. Experimental Section
All molecular films were grown under high vacuum conditions
onto HOPG (ZYA quality, mosaic spread < 0.4°), graphenecoated Si (Graphenea, Spain) or natively oxidized Si(100)
substrates by means of organic molecular beam deposition
(OMBD). The morphology of the films was characterized by
atomic force microscopy (AFM) using an Agilent SPM 5500 System operated in tapping mode. The crystalline film structure was
investigated by means of X-ray diffraction (XRD) with a Bruker
D8 Discovery diffractometer using monochromatized Cu Kα
radiation ( =1.54056 Å) and a LynxEye silicon strip detector.
NEXAFS measurements were performed at the HE-SGM dipole
beam line of the synchrotron storage ring BESSY II in Berlin
(Germany) in partial electron-yield mode (PEY) using a
channel-plate detector with a retarding field of -150 V.
A more detailed description of the experimental setups and data
evaluation is provided in the Supporting Information.
Supporting Information
Supporting Information is available from the Wiley Online
Library or from the author.
Acknowledgements
We
acknowledge
support
by
the
Deutsche
Forschungsgemeinschaft (Grant SFB 1083, TP A2) and the
Helmholtz-Zentrum Berlin (electron storage ring BESSY II) for
provision of synchrotron radiation at beamline HE-SGM.
Received: 17. August 2015
Revised: 10. November 2015
Published online: 14. December 2015
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Table of Contents Entry
Well-defined interfaces in different relative orientation of the prototypical organic heterostructure pentacene/C60
exhibit strikingly different physical characteristics. While thermally activated diffusion to molecular steps is observed for
pentacene bottom layers in upright molecular orientation, the C60 toplayer covers pentacene films with lying molecular
orientation homogeneously. Furthermore, strong proof for the formation of Diels-Alder adducts, which distinctly
controls the electronic properties at the molecular interface is observed.
Keywords:
organic heterostructures, acceptor/donor interfaces, pentacene, fullerenes, NEXAFS
Dr. T. Breuer*, Andrea Karthäuser, Prof. Dr. G. Witte
Effects of Molecular Orientation in Acceptor-Donor Interfaces between Pentacene and C60
and Diels-Alder Adduct Formation at the Molecular Interface
9