Highly uniaxial orientation in oligo(p-phenylenevinylene) films induced during wet-coating process

Full text of DOI:10.1021/ja810123b

Published on Web 01/29/2009
Highly Uniaxial Orientation in Oligo(p-phenylenevinylene) Films Induced
During Wet-Coating Process
Takeshi Nishizawa,† Hady Kesuma Lim,† Keisuke Tajima,*,† and Kazuhito Hashimoto*,†,‡
Department of Applied Chemistry, Graduate School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan, and HASHIMOTO Light Energy ConVersion Project, Exploratory Research for
AdVanced Technology (ERATO), Japan Science Technology Agency (JST)
Received December 29, 2008; E-mail: k-tajima@light.t.u-tokyo.ac.jp; hashimoto@light.t.u-tokyo.ac.jp
The molecular orientation of π-conjugated polymers and oligo-
mers in thin solid films is of great importance for achieving high
performance in organic electronic devices such as field effect
transistors,1 light emitting diodes,2 and photovoltaic devices,3 given
that the molecular shapes and the electronic properties of the organic
materials are highly anisotropic. Various techniques have been used
to align molecules in films including rubbing,4 zone-casting,5
friction transfer,6 solution shearing,7 and irradiation with polarized
light.8 Among them, a rubbing method is one of the most widely
used techniques for achieving uniaxial alignment of molecules. In
Figure 1. (a) Molecular structures of oligo(p-phenylenevinylene)s, 1 (n )
this method, polymer thin films such as polyimides are mechanically
rubbed and used as alignment layers. The molecules in contact with
the alignment layers interact with the surface to orient their easy
axes along the rubbing direction, and the alignment is transferred
throughout the films. In conventional rubbing method, however,
the driving force to transfer the alignment induced at the interface
is self-organization of thermotropic liquid crystals (LCs)9 or the
van der Waals interactions of the crystalline molecules during
vacuum evaporations.10 Thus, induction of the LC phase transition
by thermal annealing or compatibility with a dry deposition
technique is a prerequisite for this alignment method, which could
limit molecular designs and device applications. If weak intermo-
lecular interactions in solution such as π-π or polar/nonpolar
interactions can be utilized in dynamic situations such as a spin-
coating process to transfer the molecular orientation induced at the
interface, it would be a useful approach for controlling the alignment
of molecules in thin solid films. Moreover, such spontaneous
alignment of molecules without any special treatment such as
induction of thermotropic LC phases would be easily applicable to
the organic optoelectronic devices.
6), 2 (n ) 12), and 3 (n ) 1). (b) Absorption spectra of the as-cast film of
1 (n ) 6) on a rubbed PEDOT:PSS layer with polarized incident light
parallel (solid line) or perpendicular (dashed line) to the rubbing direction.
layer (alignment layer) (see SI). The film thicknesses as measured
by a profilometer (Dektak 6M, ULVAC) were about 96, 75, and
60 nm for 1, 2, and 3, respectively. Note that the films were
completely dried after the spin-coating and no thermal treatment
or induction of any LC phases was conducted on the OPV films.
Figure 1b shows the absorption spectra of the as-cast film of 1
on a rubbed PEDOT:PSS film with the incident light linearly
polarized parallel or perpendicular to the rubbing direction. Strong
absorption was observed when the light was parallel, while much
weaker absorption was observed when it was perpendicular,
resulting in a high dichroic ratio (A///A ) of 41.0 at the maximum
absorption of 414 nm. The order parameter defined by S ) (A// -
A )/(A// + 2A ) was calculated as 0.93, which is among the highest
values reported for uniaxially aligned films with conventional
conjugated LC molecules.11 We confirmed that there was no
anisotropic absorption without the rubbing procedure on the
PEDOT:PSS films. These results indicate that 1 is highly uniaxially
aligned with the long axis of the molecule parallel to the rubbing
direction. Considering the fact that the sample is simply an as-cast
film spin-coated from a dilute solution, this highly anisotropic
absorption or alignment of the molecules throughout the film with
the thickness of approximately 100 nm is quite surprising.
To further investigate the uniaxial alignment of the molecules,
in-plane XRD measurement of the film was conducted. Figure 2a
shows the XRD patterns of the as-cast film of 1 with the incident
X-ray beam parallel or perpendicular to the rubbing direction. When
the incident X-ray beam was parallel, two peaks were observed at
2θ of 19.8° and 23.4° (d-spacings of 4.5 and 3.8 Å, respectively),
which could be assigned to the π-π stacking of the oligo(phe-
nylenevinylene) groups.12 In contrast, when the incident X-ray beam
was perpendicular to the rubbing direction, no diffraction peak was
observed in this region. These results indicate that the long axis of
the molecules is uniaxially aligned parallel to the rubbing direction,
which coincides with the results observed in the polarized absorption
spectra. The presence of the diffraction peak from the π-π stacking
In this study, we report the synthesis of novel oligo(p-phenyle-
nevinylene)s (OPVs) and their highly uniaxial alignment in the as-
cast films spin-coated from solution onto rubbed polymer alignment
layers such as poly(3,4-ethylenedioxythiophene)/poly(styrene sul-
fonic acid) (PEDOT:PSS) film. The chemical structures of the OPVs
are shown in Figure 1a. By changing the alkyl side chain lengths,
the π-π interactions between the oligo(p-phenylenevinylene)
groups and the polar/nonpolar interactions of the molecules are
tuned to realize the alignment of the molecules during the spin-
coating process. The anisotropic structures of the OPVs in the thin
films were investigated by measuring polarized absorption spectra
and in-plane X-ray diffractions (XRD). Application to an electronic
device was also demonstrated.
OPVs 1-3 were synthesized by following the procedures in
Supporting Information (SI). The thin films of OPVs were prepared
by spin-coating the chloroform solutions on a rubbed PEDOT:PSS
† The University of Tokyo.
‡ Japan Science Technology Agency.
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2464 J. AM. CHEM. SOC. 2009, 131, 2464–2465
10.1021/ja810123b CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
polyimide and poly(vinyl alcohol) with dichroic ratios of 21.7 (S
) 0.87) and 40.5 (S ) 0.93), respectively. The difference in the
order parameters between the materials might suggest different
interaction strengths between the surfaces and the OPVs, possibly
due to polar interaction with the hexa(ethylene glycol) chains.
Further investigation is necessary to elucidate the mechanism of
the alignment.
The molecular alignment is simply achieved by spin-coating
without any further treatment, and thus can be easily applicable to
thin film electronic devices. To demonstrate this, photovoltaic
devices were fabricated by successively depositing fullerene (1 nm),
bathocuproine (10 nm), and aluminum (40 nm) in vacuo onto the
layer of OPV 1 which was prepared in the same manner as
the films for the absorption spectra (see SI). Figure 2b shows the
photosensitivity of the device under the irradiation of polarized
monochromatic light. Photocurrent generation was observed when
the incident light was parallel to the rubbing direction while it was
almost silent when the polarization was perpendicular, resulting in
a high anisotropic photosensitivity at 440 nm.14 This result shows
the compatibility of this molecular alignment method to the organic
thin film electronic devices.
Figure 2. (a) In-plane XRD patterns of the as-cast film of 1 (n ) 6) on a
rubbed PEDOT:PSS layer with polarized incident light parallel (solid line)
or perpendicular (dashed line) to the rubbing direction. (b) Photosensitivity
of photovoltaic device with 1 (n ) 6) under the irradiation of monochromatic
polarized light parallel (squares) or perpendicular (circles) to the rubbing
direction.
distance in the in-plane XRD measurement also indicates that the
orientation of the conjugated plane of 1 is not parallel to the
substrate (face-on) but is perpendicular to the substrate (edge-on)
or tilted.
The dispersion of the alignment axes was also evaluated by
rotating the substrate horizontally and monitoring the diffraction
intensities at 23.4°. The rotation angle was set as 0° when the
incident light was perpendicular to the rubbing direction. As a result
(see SI), two diffraction peaks were observed at approximately 90°
and 270°. The half-width-half-maximum value of the peak was
estimated to be around 7°, which indicates a narrow distribution
of the alignment axis in the film.
The mechanism of the high molecular alignment during the spin-
coating is not clear at this point. When the concentration of OPV
increases in the solution during the solvent evaporation in the
coating process, OPVs might start to crystallize from the surface
and the orientation could be transferred to the whole organic film
as the film is dried.13 We can expect that the interfacial interactions
between OPV in solution and the alignment layer first induce the
orientation and that the intermolecular interactions between OPV
molecules then induce the alignment throughout the film. Thus,
these two interactions during the spin-coating process are of high
importance for achieving such high alignment of the molecules in
the films.
To investigate this notion, OPVs with different alkyl chain
lengths (2 and 3 in Figure 1) were compared. The thin films of the
two also showed anisotropic absorptions with dichroic ratios of 18.8
(S ) 0.86) at 415 nm for 2 and 3.76 (S ) 0.48) at 442 nm for 3
(see SI). These values are lower than those of 1, indicating that the
molecular alignment is less ordered in the films of 2 and 3. This
might be attributed to differences in the crystallization behavior in
solution during the spin-coating process. The longer alkyl chains
of OPV 2 could provide better solubility than 1 and therefore the
crystallization of 2 could occur at a higher concentration in the
liquid films. This might make the crystallization faster and thus
lower the order of the packing of 2 in the films. In the case of
OPV 3, lower solubility and stronger intermolecular interactions
could cause crystallization in solution with little influence from the
interface at the alignment layer during the spin-coating, which
hinders the transfer of the alignment from the substrate and thus
lowers the order.
In summary, highly uniaxial alignment of oligo(p-phenylenevi-
nylene)s was demonstrated in films during the spin-coating process
by using polymer alignment layers. This simple and easy approach
can be applied to various organic electronic/optoelectronic devices
to control the carrier flow thereby improving the device performance.
Acknowledgment. This work was financially supported in part
by the Global COE Program “Chemistry Innovation through
Cooperation of Science and Engineering”, MEXT, Japan. We thank
Rigaku Co., Ltd. for XRD measurements. T.N. thanks JSPS for
financial support.
Supporting Information Available: Synthesis, absorption spectra,
XRD patterns, preparation of the thin films and devices. This material
is available free of charge via the Internet at http://pubs.acs.org.
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