Nanostructures of n-type organic semiconductor in a p-type matrix via self-assembly of block copolymers

Article abstract of DOI:10.1021/ma0481656

The block copolymerization of 4-vinyltriphenylamine and a perylene bisimide acrylate was analyzed using a nitroxide-mediated living radical polymerization. The functionalized block copolymer consists of one block with a hole transport moiety and second block with an electron transport moiety with light absorption properties. It was observed that the polymers show phase separation with increasing amounts of dye that build up in nanostructures of perylene bisimide in a PvTPA matrix over a large area. The formation of n-type organic semiconductor in a polymer matrix opens up new concept in nanoscience and molecular electronics.

Full text of DOI:10.1021/ma0481656

8832
Macromolecules 2004, 37, 8832-8835
polymerized via nitroxide-mediated controlled radical
Nanostructures of n-Type Organic
Semiconductor in a p-Type Matrix via
Self-Assembly of Block Copolymers
polymerization (Scheme 1). In Figure 1a the linear
dependence between the reaction time and the conver-
sion for conversions up to 50% is shown, which is
consistent with the controlled nature of the polymeri-
zation. The polydispersities are in the range 1.20-1.26.
With longer reaction time the polydispersity increases
to 1.43 for 80% conversion. A small excess (5 mol %) of
the free nitroxide 3 was added to optimize the control
of the polymerization. The free nitroxide is necessary
so that it can act as an artificial persistent radical
functioning similar to the persistent radical effect.15
Initial experiments without the free nitroxide resulted
in poorer control of the reaction and therefore higher
polydispersities. Two macroinitiators, PvTPA1 4A and
PvTPA2 4B, were synthesized using the same tech-
nique. The polymer properties are listed in Table 1.
Even for high molecular weights the polydispersities are
low. These homopolymers are amorphous with glass
transition temperatures of about 144-145 °C and are
thermally stable.
Stefan M. Lindner and Mukundan Thelakkat*
Makromolekulare Chemie I, Universita¨t Bayreuth,
Universita¨tsstrasse 30, 95440 Bayreuth, Germany
Received September 7, 2004
Revised Manuscript Received October 18, 2004
There has been an increasing interest in the morphol-
ogy of organic semiconductors1-3 in film because of its
strong influence on the performance of devices using
these materials. Especially for electrooptical applica-
tions like light-emitting diodes and solar cells, an
increase in the interface area between the electron
(ETM) and the hole transport material (HTM) could
enhance the performance since the interface is the active
area for charge recombination or separation. This
phenomenon was studied experimentally4 and by nu-
merical simulations.5 In photovoltaic devices the light
is absorbed by the dye, and the generated exciton
(electron-hole pair) has to diffuse to the interface
between ETM and HTM where they are separated. The
exciton diffusion length is only on the order of a few
nanometers, which is at least 10 times smaller than the
optical absorption depth. This limits the efficiency for
simple two-layer devices.6 To overcome this problem, the
active organic layer has to be structured on a nanometer
scale. Such microphase separation can be realized using
block copolymers. Synthetic attempts for obtaining
functionalized block copolymers have been successfully
tried out by Hadziioannou et al.7 using p-phenylenevi-
nylene and a partially fullerene functionalized block, by
Zentel et al.8 using triphenylamines and NLO-function-
alized triphenylamines, by Stupp et al.9 using functional
triblock copolymers, and in our group by using metal-
centered bifunctional polymers.10 In this work, we use
poly(vinyltriphenylamine) as HTM, which is known to
be a stable and good hole conductor. The other block is
made up of perylene bisimide11 acrylate, which has high
electron mobility and high light fastness so that it can
also be used as a dye as it strongly absorbs light between
400 and 600 nm. The perylene-3,4:9,10-tetracarboxylic
bisimide monomer 5 (PerAcr) was designed unsym-
metrically with a swallow-tail substituent12 for good
solubility and an acrylate group at the other end for the
polymerization. Thus, in this concept, all the three
functions of light absorption, hole transport, and elec-
tron transport are taken care of in a self-assembling
system with the challenging task of creating nanostruc-
tures with large amount of interface suitable for solar
cell applications.
The perylene bisimide monomer 5 was also homopo-
lymerized to get the polymer PPerAcr 7. This perylene
containing homopolymer 7 has a melting point of 190
°C, but no glass transition temperature could be de-
tected down to -50 °C. For the polymerization of the
perylene bisimide substituted acrylates longer reaction
times were necessary, and the polydispersity of the
homopolymer 7 was higher than those of the homopoly-
mers 4. This may be due to the dilution of the acrylate
group in the monomer 5, which mainly consists of the
substituted perylene core. Similar effects for the dilution
with solvent were previously reported.16 If polymer 7
was used as a macroinitiator for the polymerization of
4-vinyltriphenylamine (2), the polydispersities were
about 2 with a low molecular weight shoulder in the
GPC eluogram, which can be attributed to the macro-
initiator. This result suggests that the solubility of the
macroinitiator may limit this synthetic pathway, and
therefore, the synthetic route starting from PvTPA
macroinitiator was used (Scheme 1).
The macroinitiator PvTPA1 4A was used to synthe-
size the block copolymers PvTPA1-bl-PPerAcr 6A-6C.
All the block copolymerizations were performed in 1,2-
dichlorobenzene with a 5 mol % excess of the free
nitroxide 3. The polymers exhibited polydispersities
below 1.5 for dye contents up to 40 wt %. The composi-
tions of the block copolymers were determined by NMR
spectroscopy. The UV/vis spectra of this series of block
copolymers 6A-6C prepared from the same macroini-
tiator 4A show the expected increase in perylene
absorption proportional to the incorporation of perylene
bisimide block, as seen in Figure 1b. Additionally, the
dye content in 6A-6C was also determined using UV/
vis spectroscopy making use of the extinction coefficient
of PPerAcr 7. The dye contents were 13.0, 40.8, and 73.2
wt % for 6A, 6B, and 6C, respectively. These values are
in close agreement with the compositions calculated
from NMR spectroscopy. The block copolymers 6A-6C
are thermally stable with Tonsets obtained from TGA well
above 300 °C. DSC measurements showed that the glass
transition temperatures and the melting points of the
block copolymers 6A-6C are in the same range as those
of the corresponding homopolymers 4 and 7. These
To get well-defined block copolymers, we used the
nitroxide-mediated living radical polymerization.13 This
controlled radical polymerization technique tolerates a
wide range of functional groups and is a metal-free
method. The initiator 1 is a second-generation initiator
which was first reported by Hawker et al.14 With this
initiator styrene and acrylate derivates could be polym-
erized. As a first step, 4-vinyltriphenylamine (2) was
* Corresponding author:
uni-bayreuth.de.
e-mail mukundan.thelakkat@
10.1021/ma0481656 CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/30/2004

Macromolecules, Vol. 37, No. 24, 2004
Communications to the Editor 8833
Scheme 1. Living Radical Polymerization and Block Copolymerization of vTPA and PerAcr Monomers via
Nitroxide-Mediated Polymerization (NMP)
Table 1. Overview of Molecular Weight and Thermal Properties of the Homopolymers and Block Copolymers
Mn [g/mol]a
PDIa
wt % of PerAcrb
Tg [°C]c
Tm [°C]c
TGA-5% [°C]
4A: PvTPA1
15 830
23 210
17 610
24 170
37 710
19 900
1.22
1.19
1.37
1.47
1.97
1.65
143.6
145.3
141.9
137.7
149.5
d
378
376
380
391
396
404
4B: PvTPA2
6A: PvTPA1-bl-PperAcr1
6B: PvTPA1-bl-PperAcr2
6C: PvTPA1-bl-PperAcr3
7: PPerAcr
13.7
40.3
78.9
100
181.3
187.7
198.0
189.8
a Measured against polystyrene standards via GPC. b Calculated from 1H NMR spectra. c Heating rate 10 K/min; values taken from
second heating curve. d Tg could not be observed down to -50 °C.
thermal properties and the fact that all block copolymers
give optically clear films indicate a microphase separa-
tion. The final proof for microphase separation was
achieved with transmission electron microscopy. TEM

8834 Communications to the Editor
Macromolecules, Vol. 37, No. 24, 2004
π-π interaction and crystallization of perylene moieties
are the driving forces for the buildup of nanowires in
block copolymer 6C with high perylene dye content. The
situation here may be very similar to that in rod-coil
polymers in which the stiff rodlike segments cause the
nanostructures.17
Evidence for photoinduced charge transfer is provided
by quenching of photoluminescence (see Supporting
Information). After excitation at a wavelength of 492
nm, the homopolymer 7 shows an intense red fluores-
cence at 632 nm arising from perylene core. Using the
same excitation wavelength, block copolymers 6A-6C
exhibit a quenching of this fluorescence with an ef-
ficiency of more than 95%, which can be ascribed to an
electron transfer from the excited state of perylene to
PvTPA domains. The chance of energy transfer from
perylene to TPA moieties is almost negligible due to the
unfavorable HOMO/LUMO energy levels of the two
species, TPA having a higher band gap energy than the
perylene unit. Further studies of phase separation in
thin films (<500 nm) and the application of such a
nanostructured self-assembled film for photoinduced
charge separation are underway, and the results will
be published later.
In summary, we have shown for the first time that
the block copolymerization of 4-vinyltriphenylamine and
a perylene bisimide acrylate could be achieved by
nitroxide-mediated living radical polymerization with
control of molecular weight and low PDI. These fully
functionalized block copolymers consist of one block with
a hole transport moiety and a second block with an
electron transport moiety having light absorption prop-
erties. In films these polymers show phase separation
on a nanometer scale, and with increasing amounts of
dye they build up nanostructures of perylene bisimide
in a PvTPA matrix over a large area. Initial experiments
show that charge transfer between the domains occurs,
which is essential for the use in photovoltaic devices.
The formation of oriented nanowires of an n-type
organic semiconductor in a polymer matrix opens up
new concepts not only in the field of existing electroop-
tics but also in nanoscience and molecular electronics.
Figure 1. (a) Evolution of molecular weight and conversion
with time for nitroxide-mediated living polymerization of vTPA
to PvTPA. Composition: 6 mmol of 2, 0.03 mmol of 1, 0.0015
mmol of 3, 600 µL of anisole; T ) 125 °C. (b) UV-vis spectra
of 6A-6C measured in THF (concentration: 0.02 mg/mL).
Acknowledgment. The financial support for this
research work from German Research Council (DFG/
SFB 481-Project B4) is gratefully acknowledged.
Supporting Information Available: Experimental pro-
cedures, characterization of materials, TEM cross sections of
6A and 6B, and photoluminescence data. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 2. TEM of cross section of 6C from relatively thick
films (5-6 µm) obtained by melting down the polymers onto a
glass substrate and subsequently tempering them at 200 °C
for 1 h.
References and Notes
(1) Shaheen, S. E.; Brabec, C. J.; Padinger, F.; Fromherz, T.;
Hummelen, J. C.; Sariciftci, N. S. Appl. Phys. Lett. 2001,
78, 841-843.
(2) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia,
E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature
(London) 1995, 376, 498-500.
(3) Granstro¨m, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Ander-
sson, M. R.; Friend, R. H. Nature (London) 1998, 395, 257-
260.
measurements were carried out on thin cross sections
of relatively thick films (5-6 µm) obtained by melting
down the polymer onto a glass substrate and subse-
quently tempering them at 200 °C for 1 h. The TEM of
6A-6C show phase separation on nanometer scale. As
the amount of dye block in copolymer increases, the
perylene moieties in 6C aggregate together to form
nanowires, which are embedded into the PvTPA matrix
(Figure 2). The nanostructures are about 40-60 nm
thick and 1-2 µm long, thus exhibiting high aspect
ratios which favor a better percolation of charge trans-
port. Moreover, the tempering process favors a vertical
orientation of these nanowires on glass substrates, as
can be seen from Figure 2. We assume that the strong
(4) Van Duren, J. K. J.; Yang, X.; Loos, J.; Bulle-Lieuwma, C.
W. T.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J. Adv.
Funct. Mater. 2004, 14, 425-434.
(5) Sylvester-Hvid, K. O.; Rettrup, S.; Ratner, M. A. J. Phys.
Chem. B 2004, 108, 4296-4307.
(6) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183-185.
(7) Stalmbach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.;
Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 5464-5472.

Macromolecules, Vol. 37, No. 24, 2004
Communications to the Editor 8835
(8) Behl, M.; Hattemer, E.; Brehmer, M.; Zentel, R. Macromol.
Chem. Phys. 2002, 203, 503-510.
(14) Benoit, D.; Chaplinski, V.; Braslau, R. Hawker, C. J. J. Am.
Chem. Soc. 1999, 121, 3904-3920.
(15) Fischer, H. Macromolecules 1997, 20, 5666-5672.
(9) Tew, G. N.; Pralle, M. U.; Stupp, S. I. Angew. Chem. 2000,
112, 527-531.
(16) Hawker, C. J.; Barclay, G. G.; Orellana, A.; Dao, J.;
(10) Peter, K.; Thelakkat, M. Macromolecules 2003, 36, 1779-
1785.
Devonport, W. Macromolecules 1996, 29, 5245-5254.
(17) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. Rev. 2001, 101,
3869-3892.
(11) Langhals, H. Heterocycles 1995, 40, 477-500.
(12) Langhals, H.; Demmig, S.; Potrawa, T. J. Prakt. Chem. 1991,
333, 733-748.
(13) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001,
101, 3661-3688.
MA0481656