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PAPER www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and photophysical property of well-defineddonor–acceptor diblock copolymer based on regioregularpoly(3-hexylthiophene) and fullerene†
Jea Uk Lee,a Ali Cirpan,b Todd Emrick,b Thomas P. Russellb and Won Ho Jo*a
Received 1st August 2008, Accepted 3rd December 2008
First published as an Advance Article on the web 28th January 2009
DOI: 10.1039/b813368a
A new, well-defined diblock copolymer (P3HT-b-C60) based on regioregular poly(3-hexylthiophene)
(P3HT) and fullerene was synthesized. First, regioregular P3HT was synthesized through Grignard
metathesis polymerization, and then methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate
(HEMA) were copolymerized by using an end-functionalized P3HT as a macroinitiator for the atom
transfer radical polymerization to yield a diblock copolymer (P3HT-b-P(MMA-r-HEMA)). A fullerene
derivative functionalized with carboxylic acid, [6,6]-phenyl-C61-butyric acid (PCBA), was then
chemically linked to the HEMA unit in the second block (P(MMA-r-HEMA)) to produce a diblock
copolymer with the second block containing fullerenes. Annealing thin films of the copolymer revealed
nanometer-scale phase separation, a more suitable morphology for enabling excitons generated in the
P3HT domain to more efficiently reach the donor–acceptor interface, relative to simple blends of P3HT
and C60. As a result, photoluminescence of the P3HT-b-C60 diblock copolymer in the films showed
a complete quenching of photoluminescence of P3HT, which is indicative of charge transfer between
P3HT and fullerene.
Introduction
The need to develop inexpensive renewable energy resources
stimulates scientific research for the production of efficient and
low-cost photovoltaic devices. Polymer solar cells based on
conjugated polymer and fullerenes have opened a new avenue to
develop economically renewable energy resources, because they
can be fabricated to extend over a large area by means of low-
cost printing and coating techniques that can pattern photoactive
organic materials on lightweight flexible substrates.1–3 Recently,
it was reported that thin-film bulk heterojunction solar cells
fabricated by simple blending of regioregular poly(3-hexylth-
iophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) have improved significantly the power conversion effi-
ciency.4–9 By optimizing device structure and manufacturing
conditions, i.e., the choice of spin-coating solvent,10,11 slow
drying of spin-coated films,5,12 thermal annealing13,14 and solvent
annealing15,16 of blends, bulk heterojunction solar cells have
reached power conversion efficiencies as high as 4–5% under AM
1.5 (AM ¼ air mass) illumination.
Despite remarkable recent progress, bulk heterojunction solar
cells still have several problems for achieving the 10% power
conversion efficiency level desired for commercialization. First,
since the conjugated polymers and electron acceptors in bulk
heterojunction solar cells have been randomly interspersed
aDepartment of Materials Science and Engineering, Seoul NationalUniversity, Seoul 151-742, Korea. E-mail: whjpoly@snu.ac.krbDepartment of Polymer Science and Engineering, University ofMassachusetts, Amherst, MA 01002, USA
† Electronic supplementary information (ESI) available: 1H NMR, 13CNMR spectra, TEM images, UV-Visible spectra, DSC thermogram,and elemental analyses. See DOI: 10.1039/b813368a
This journal is ª The Royal Society of Chemistry 2009
throughout the film, the electron donor (conjugated polymer)
and/or electron acceptor (such as fullerene or its derivatives) may
form isolated domains in which electrons and/or holes are trap-
ped. Second, blends of conjugated polymer and fullerene deriv-
atives usually result in macrophase separation, limiting charge
separation and thereby lower power conversion efficiency in the
photovoltaic device, because the phase-separated domain size
(�100 nm) is much larger than the exciton diffusion length
(�10 nm) in conjugated polymers.17,18
To overcome these limitations, synthetic attempts to synthe-
size donor–acceptor block-like copolymers have been made,
because block copolymers may self-assemble into well-defined
nanostructured morphologies (lamellae or cylinder), which can
provide excitons with large interface for charge separation. Rod–
coil diblock copolymers based on p-phenylenevinylene and
partially fullerene-functionalized polystyrene have been
proposed by Hadziioannou and his co-workers,19–21 and diblock
copolymers consisting of poly(vinyltriphenylamines) and poly-
acrylate bearing perylene diimide have been synthesized by
Lindner et al.22 However, the device performance of these
diblock copolymers was sub-optimal, partly because the block
copolymers do not contain the P3HT–fullerene pair, which has
currently been known as the best pair for polymer solar cells.
Recently, various structures composed of polythiophene and
fullerenes, i.e., graft-block type copolymer,23 oligothiophene–
fullerene dyad,24 and double-cable polythiophene with C60,25
have been synthesized and their film morphologies were investi-
gated. Although these copolymers have revealed some interesting
morphologies, well-defined structures have not been observed,
and as a result they did not show satisfactory photovoltaic
properties. Therefore, it is reasonable to assume that an ideal
molecular structure for polymer photovoltaics would be a
J. Mater. Chem., 2009, 19, 1483–1489 | 1483
well-defined electron donor–acceptor type diblock copolymer
based on P3HT (electron donor) and fullerene (electron acceptor).
As an approach to materialize this ideal molecular structure
for an active layer material of polymer solar cells, in this paper,
a new well-defined diblock copolymer based on P3HT and
fullerene has been synthesized (Scheme 1). Very recently, Had-
ziioannou and his co-workers26 reported a similar work to ours
about a fullerene-grafted P3HT-based rod–coil block copolymer.
However, the molecular weight of P3HT in their block copol-
ymer was not large enough to be suitable for the electron donor
material, and moreover the microphase-separated morphology
of the copolymer, which is very important for photovoltaic
performances, was not presented in their paper.
To synthesize the well-defined diblock copolymer in this study,
the Grignard metathesis (GRIM) polymerization (as described
by McCullough and co-workers27) and the atom transfer radical
polymerization (ATRP) were used for P3HT block and fullerene-
containing block, respectively. The chemical structure and self-
assembly behavior of the diblock copolymers were identified by
nuclear magnetic resonance (NMR), Fourier transform infrared
(FT-IR) spectroscopy, elemental analysis, gel permeation chro-
matography (GPC), X-ray diffraction (XRD), and transmission
electron microscopy (TEM). The photophysical property of the
diblock copolymer in solid state was also investigated by UV-
Visible and fluorescence spectroscopy.
Results and discussion
A new P3HT-b-C60 diblock copolymer was prepared by first
synthesizing bromoester-terminated P3HT, which was used as
a macroinitiator for ATRP (Scheme 1). Regioregular P3HT and
the P3HT-macroinitiator from P3HT were synthesized according
to the procedure reported in the literature.27,28 The P3HT-mac-
roinitiator synthesized in this study exhibited reasonable
molecular weight with narrow molecular weight distributions
(Mn ¼ 10 500, Mw/Mn ¼ 1.2 by GPC). The degree of regior-
egularity of the P3HT-macroinitiator is 98% when measured by1H NMR spectroscopy (see ESI†).
Scheme 1 Synthesis scheme of P3HT-b-C60 diblock copolymer.
PMDETA: N,N,N0,N0,N00-pentamethyldiethylenetriamine; DCC: 1,3-
dicylohexylcarbodiimide; DMAP: dimethylaminopyridine.
1484 | J. Mater. Chem., 2009, 19, 1483–1489
The end-functionalized P3HT was used as a macroinitiator for
the copolymerization (by ATRP) of methyl methacrylate
(MMA) and 2-hydroxyethyl methacrylate (HEMA) to form the
second block of the desired structure. The hydroxyl groups of
HEMA provide handles for attaching fullerene derivatives to the
polymer. It is noteworthy that the solubility of the diblock
copolymer decreases with increasing HEMA content, as poly
(HEMA) is soluble only in polar solvents such as DMF, water,
Fig. 1 1H NMR spectra of (a) P3HT-b-P(MMA-r-HEMA) and (b)
P3HT-b-C60 diblock copolymer in CDCl3.
This journal is ª The Royal Society of Chemistry 2009
Fig. 2 FT-IR spectra of (a) P3HT-macroinitiator, (b) P3HT-b-P(MMA-
r-HEMA), and (c) P3HT-b-C60 diblock copolymer.
Fig. 3 GPC traces of P3HT-macroinitiator (dotted line), P3HT-b-
P(MMA-r-HEMA) (dashed line), and P3HT-b-C60 diblock copolymer
(solid line) in THF with refractive index detection. Inset shows a GPC
trace of P3HT-b-C60 decomposed into two components, P3HT-b-C60
diblock copolymer and P3HT-macroinitiator.
and methanol, all poor solvents for P3HT. The molecular weight
of the second block (P(MMA-r-HEMA)) was ca. 5000, and the
mole fraction of HEMA in the second block was about 0.3, from
which the number of MMA and HEMA units in the second
block is estimated to be 30 and 12, respectively, when those are
calculated from 1H NMR peak integrals, as shown in Fig. 1a.
These calculated data were well matched with the data obtained
from elemental analysis (see ESI†). The molecular weight (Mn)
and the polydispersity (Mw/Mn) of P3HT-b-P(MMA-r-HEMA)
are 14 500 and 1.4, respectively, when measured by GPC.
Carboxylate-functionalized fullerenes were linked covalently
to HEMA in the second block to form the P3HT-b-C60 diblock
copolymer. For this we used carbodiimide (DCC/DMAP)
esterification of the HEMA hydroxyls with the carboxylic acids
of the fullerene derivative, [6,6]-phenyl-C61-butyric acid (PCBA)
(Scheme 1). Since the solubility of PCBA is very low (limited to
CS2, pyridine, 1,2-dichlorobenzene and their mixtures), the
esterification reaction between P3HT-b-P(MMA-r-HEMA) and
PCBA was carried out in the mixed solvent of 1,2-dichloroben-
zene/CS2 (1 : 1, v/v). The crude product was dissolved in THF,
a good solvent for P3HT-b-P(MMA-r-HEMA) but a poor
solvent for PCBA, filtered, and then centrifuged several times to
remove unreacted PCBA. 1H NMR analysis unambiguously
shows the a down-field shift of the methylene protons (d ¼ 4.3
ppm) upon HEMA esterification (Fig. 1b). Both the appearance
of the aromatic proton peaks due to the phenyl group of PCBA
in the 1H NMR spectra and the presence of typical C60 peaks
(d z 130–150 ppm)29,30 in the 13C NMR spectrum (see ESI†)
provide strong evidence that C60 is linked covalently to the
diblock copolymer. On average, nine PCBAs were grafted onto
each diblock copolymer, as calculated by integration of the 1H
NMR signals, which corresponds to about 75% HEMA units
linked with PCBA. The amount of PCBAs estimated from NMR
is slightly higher than the amount (8.2 PCBAs) estimated from
elemental analysis. This difference may arise from incomplete
combustion of fullerenes at elemental analysis (see ESI). The
final product, P3HT-b-C60 diblock copolymer, is soluble in
various organic solvents such as THF, toluene, chloroform, and
1,2-dichlorobenzene.
The chemical structures of the P3HT-macroinitiator,
P3HT-b-P(MMA-r-HEMA), and P3HT-b-C60 were also identi-
fied by FT-IR, as shown in Fig. 2. When the spectrum of the
P3HT-macroinitiator is compared with that of the P3HT-b-
P(MMA-r-HEMA) spectrum, it reveals that three characteristic
bands corresponding to OH stretching, C]O stretching, and
C–O stretching vibrations appear at 3100–3600 cm�1, 1735 cm�1,
and 1150 cm�1, respectively, in the P3HT-b-P(MMA-r-HEMA)
spectrum, indicating that MMA and HEMA are successfully
introduced by ATRP. After introduction of PCBMs, the signif-
icant decrease of the OH stretching band and the increase of the
C]O stretching band in the P3HT-b-C60 diblock copolymer
spectrum identify the formation of covalent bonding between the
hydroxyl of HEMA and the carboxylic acid of PCBA.
Fig. 3 shows the GPC traces of the P3HT-macroinitiator,
P3HT-b-P(MMA-r-HEMA), and P3HT-b-C60 diblock copol-
ymer. The GPC trace of P3HT-b-P(MMA-r-HEMA) shows
a broad peak with a large shoulder where the high molecular
weight shoulder corresponds to P3HT-b-P(MMA-r-HEMA),
while the peak maximum at a later elution time corresponds to
This journal is ª The Royal Society of Chemistry 2009
unreacted P3HT. After introduction of C60, the high molecular
weight shoulder of P3HT-b-P(MMA-r-HEMA) shifted to
a higher molecular weight region (P3HT-b-C60) due to incorpo-
ration of C60 while the unreacted P3HT peak remained
unchanged. Therefore, it is believed that fullerene derivatives
(PCBA) are successfully introduced into P3HT-b-P(MMA-r-
HEMA) to form the P3HT-b-C60 diblock copolymer while
unreacted P3HT homopolymers are mixed in the reaction
product. When the GPC trace of P3HT-b-C60 was decomposed
into two components using a Gaussian function and then the
areas of two components were compared to determine the frac-
tion of diblock copolymer in the mixture (see the inset of Fig. 3),
it revealed that the higher molecular weight part (P3HT-b-C60
diblock copolymer) occupied the majority of the reaction
product ($72%).
Thermogravimetric analysis (TGA) was used to quantify the
C60 content in the P3HT-b-C60 diblock copolymer. Fig. 4 shows
the TGA thermograms of PCBA, P3HT homopolymer, and
P3HT-b-C60 diblock copolymer. PCBA shows minor decompo-
sition at around 200 and 370 �C, due to the loss of a benzene ring
J. Mater. Chem., 2009, 19, 1483–1489 | 1485
Fig. 4 TGA curves of PCBA, P3HT-b-C60 diblock copolymer, and
P3HT homopolymer.
and a short alkyl chain attached to C60, leaving a C60 adduct
which shows the char yield of about 82%. Since both the P3HT-
b-C60 diblock copolymer and P3HT homopolymer exhibit
significant weight loss in the ca. 400–500 �C range and the char
yields of the diblock copolymer and homopolymer are about
51% and 29%, respectively, the weight fraction of C60 in the
P3HT-b-C60 diblock copolymer was estimated to be 0.4.
To examine the effect of PCBA on the crystallization of P3HT
in the diblock copolymer, the XRD patterns were obtained from
powders of the pristine PCBA, P3HT and PCBA blend (P3HT/
PCBA, 1 : 1 (w/w)), P3HT homopolymer and P3HT-b-C60
diblock copolymer (Fig. 5). The P3HT homopolymer clearly
shows a (100) reflection peak at 2q¼ 5.4 �C corresponding to the
chain–chain interlayer distance as well as the (010) reflection
peak (2q ¼ 23.4 �C) caused by the stacking of the chains within
the main chain layers.31 The P3HT/PCBA blend shows several
Fig. 5 Powder X-ray diffractograms of pristine PCBA, P3HT/PCBA
blend, P3HT, and P3HT-b-C60 diblock copolymer after thermal
annealing at 150 �C for 24 h.
1486 | J. Mater. Chem., 2009, 19, 1483–1489
sharp and broad peaks, whose position and shape are very close
to those found on pure PCBA and the P3HT powder sample. In
the diffraction pattern of the P3HT-b-C60 diblock copolymer
after thermal annealing at 150 �C for 24 h, however, diffraction
peaks from PCBA crystallites were not detected while (100) and
(010) peaks from P3HT units in the diblock copolymer were
clearly observed. Consequently, PCBA is not crystallized in the
block copolymer while the P3HT units readily crystallize in the
P3HT-b-C60 diblock copolymer. This seems to be contradictory
to the report of Barrau et al.,32 who observed the formation of
fullerene nanocrystals in fullerene-grafted poly[(2,5-di(20-ethyl)-
hexyloxy)-1,4-phenylenevinylene]-b-poly[butyl acrylate-stat-(4-
methyl)styrene] (DEH-PPV-b-P(BA-stat-C60 MS)) by XRD.
They reported that the formation of these crystals is likely to
impede microphase separation of the fullerene-grafted PPV-
based diblock copolymer, and thus the resulting thin-film
morphology is far from the target nanostructure and leads to
photovoltaic devices with poor performances. The reason for the
difference between their result and ours is believed that the
PCBAs in the P3HT-b-C60 diblock copolymer of our system do
not aggregate to form fullerene nanocrystals due to a bulky
substituent of PCBA while C60 in DEH-PPV-b-P(BA-stat-
C60MS) of their system easily crystallizes because the pristine C60
was directly attached to the block copolymer.
Thin-film morphologies of the P3HT-b-C60 diblock copoly-
mers were characterized by TEM. Thin films were prepared by
spin-coating onto silicon wafers, containing a native surface
oxide layer, from 1 wt% CHCl3/pyridine solution (90 : 10 v/v),
followed by heating to 230 �C (above Tm of P3HT-b-C60) under
Fig. 6 TEM images of the self-assembled structure of P3HT-b-C60
diblock copolymer (a) before and (b) after thermal annealing at 150 �C
for 24 h. Inset shows a higher magnification image.
This journal is ª The Royal Society of Chemistry 2009
high vacuum (10�6 Torr) for 5 h, slowly cooling to 150 �C (the
melt crystallization temperature of P3HT-b-C60) and finally
annealing at 150 �C for 24 h. Fig. 6 shows that some aggregation
of PCBA (dark regions) is observed in the thin film of the P3HT-
b-C60 diblock copolymer before annealing (Fig. 6a), while the
diblock copolymer exhibits a well-defined phase morphology
with formation of bicontinuous structure after annealing
(Fig. 6b). To confirm this annealing effect, we prepared
additional thin films from different solvent mixtures, such as
CHCl3/pyridine (95 : 5 v/v). As the volume fraction of pyridine in
the mixed solvent is decreased, it is expected that the aggregation
size of PCBA in the thin film is increased because the solubility of
PCBA in the solvent (CHCl3/pyridine (95 : 5 v/v)) becomes
poorer. However, the thin film also shows a well-defined bicon-
tinuous nanostructure without significant PCBA aggregation,
when it was annealed at 150 �C after complete melting of the
sample at 230 �C (see ESI†). This result suggests that the
bicontinuous nanostructure is the thermodynamic equilibrium
state of the P3H-b-C60 diblock copolymer.
It should be noted that the bicontinuous morphology provides
a large interfacial area between P3HT and PCBA and that the
characteristic length scale of the P3HT-b-C60 block copolymer
microdomain (10�20 nm) is nearly matched with the exciton
diffusion length (�10 nm). Hence it is expected that the excitons
generated in the P3HT domain reach the donor–acceptor inter-
face more efficiently.
UV-Visible spectra of thin films of the P3HT homopolymer
and P3HT-b-C60 diblock copolymer are shown in Fig. 7, where
the spectrum of P3HT/PCBM blend (1 : 1 (w/w)) is also shown
for comparison. The films of the P3HT-b-C60 diblock copolymer
and P3HT/PCBM blend show p–p* absorption of poly-
thiophene around 500 nm and 480 nm, respectively, indicating
Fig. 7 UV-Visible spectra of thin films of P3HT (solid line), P3HT/
PCBM blend before (dashed-dotted line) and after (dashed-dotted-dotted
line) thermal annealing, and P3HT-b-C60 diblock copolymer before
(dotted line) and after (dashed line) thermal annealing. Films were
prepared by spin-coating from CHCl3 solutions (1 wt%) onto a glass
substrate, and the films were annealed at 150 �C for 24 h.
This journal is ª The Royal Society of Chemistry 2009
that both are red-shifted as compared to those in CHCl3 solution
(450 nm, see ESI†) due to the p–p stacking of the thiophene unit
in the solid state. The difference in the extent of red-shift between
the P3HT-b-C60 diblock copolymer and the P3HT/PCBM blend
may arise from the fact that the bulky characteristic of the
fullerene unit changes the aggregation behavior of P3HT units in
the diblock copolymer. After thermal annealing, the absorption
of both films increases and three vibronic absorption shoulders
become clear, indicating an ordering of P3HT chains to form
crystallites. Significant red-shift also appears in both films
because crystalline order involves an enhanced conjugation
length and hence a shift of the absorption spectrum to lower
energies.5
In order to examine qualitatively the efficiency of electron–
hole separation at the donor–acceptor interface, we measured the
photoluminescence quenching. The exciton generated in the
P3HT domain can either reach the donor–acceptor interface
where an exciton is separated into an electron and a hole and
thus lose photoluminescence, or recombine before it reaches the
interface to emit photoluminescence. When the photo-
luminescence spectrum of P3HT-b-C60 is compared with that of
the P3HT/PCBM blend, it reveals that the P3HT-b-C60 in the
film state shows almost complete quenching of photo-
luminescence while the P3HT/PCBM blend film still exhibits
some photoluminescence emission, as shown in Fig. 8. Since the
photoluminescence quenching is a measure of the charge transfer
between P3HT and fullerene, this result suggests that the charge
transfer takes place more efficiently in the P3HT-b-C60 diblock
copolymer film than in the P3HT/PCBM blend film. This can be
explained by the fact that the P3HT-b-C60 diblock copolymer has
a larger interfacial area between P3HT and fullerene than the
Fig. 8 Photoluminescence spectra of thin films of P3HT (solid line),
P3HT/PCBM blend (dashed-dotted-dotted line), and P3HT-b-C60
diblock copolymer (dashed line). The excitation wavelength was 500 nm.
For quantitative comparison, the three films were prepared such that the
sample contains the same amount of P3HT: 0.5 wt% of P3HT homo-
polymer, 1 wt% of P3HT/PCBM blend, and 0.8 wt% of P3HT-b-C60
diblock copolymer in CHCl3.
J. Mater. Chem., 2009, 19, 1483–1489 | 1487
P3HT/PCBM blend, arising from microphase-separated
domains of the diblock copolymer.
Conclusions
We have successfully synthesized a new, well-defined diblock
copolymer based on a P3HT and fullerene derivative via two
controlled polymerization steps (GRIM polymerization and
ATRP) followed by linking a fullerene derivate (PCBA) to the
second block. When thin films of the diblock copolymer (P3HT-
b-C60) are thermally annealed, P3HT-b-C60 showed phase sepa-
ration on a nanometer scale, which allows the excitons generated
in the P3HT domain to reach the donor–acceptor interface more
efficiently. When the photoluminescence spectrum of the P3HT-
b-C60 film is compared with that of the P3HT/PCBM simple
blend film, it reveals that P3HT-b-C60 shows complete quenching
of the photoluminescence while the P3HT/PCBM blend exhibits
some photoluminescence, indicating that the charge transfer
between P3HT and C60 takes place more effectively in the P3HT-
b-C60 diblock copolymer than in the P3HT/PCBM blend.
Experimental
Materials
A Buckminsterfullerene derivative, [6,6]-phenyl-C61-butyric acid
(PCBA), was purchased from Materials Technologies Research
(Cleveland, OH). Dimethylaminopyridine (DMAP) (98%) was
recrystallized from toluene. Tetrahydrofuran (THF) was dried
over sodium/benzophenone under nitrogen and freshly distilled
before use. Toluene was dried over calcium hydride under
nitrogen and freshly distilled before use. Methyl methacrylate
(MMA) (99%, from Acros) was passed through a column of
basic alumina before use. 2-Hydroxyethyl methacrylate (HEMA)
(99%, from Aldrich) was dried over 4 �A molecular sieves and
distilled just before use. All other reagents were purchased from
Aldrich Chemicals (Milwaukee, WI) and used as received.
Synthesis of P3HT-b-P(MMA-r-HEMA)
First, the regioregular P3HT-macroinitiator was synthesized by
following the procedure reported in the literature,27,28 and then
P3HT-b-P(MMA-r-HEMA) was synthesized by ATRP using the
P3HT-macroinitiator. 0.1 g of P3HT-macroinitiator (Mn,NMR ¼10 500, Mw/Mn ¼ 1.2 by GPC) was placed in a 25 mL pear-
shaped flask, to which 2 mL of previously degassed toluene was
added by syringe, after the flask with the macroinitiator was
degassed and backfilled with argon gas. The macroinitiator
solution was stirred until it became homogeneous. Monomer
solution was prepared separately: MMA (1 mL, 9.5 mmol),
HEMA (0.25 mL, 2 mmol), N,N,N0,N0,N00-pentam-
ethyldiethylenetriamine (PMDETA) (0.02 mL, 0.1 mmol) and
CuBr (7.2 mg, 0.05 mmol) were added in a 50 mL round-bottom
flask, and the flask was degassed by three freeze–pump–thaw
cycles. The monomer solution was stirred until it became
homogeneous, and then placed in a 90 �C oil bath. When the
macroinitiator solution was added by cannula into the monomer
solution, the mixture solution became homogenous with a dark
orange color. After the solution was allowed to react for 12 h at
90 �C, the resultant polymer solution was diluted with 50 mL of
1488 | J. Mater. Chem., 2009, 19, 1483–1489
THF. The solution was then passed through a column of Al2O3
to remove copper. The polymer solution was concentrated, and
then precipitated into n-hexane. The precipitated polymer was
collected by vacuum filtration and subsequently washed with n-
hexane and methanol, followed by drying under a vacuum to give
0.18 g of desired product corresponding to 14% of conversion.
Synthesis of P3HT-b-C60 diblock copolymer
In a 100 mL round-bottom flask, 0.05 g of P3HT-b-P(MMA-r-
HEMA) and PCBA (0.045 g, 0.05 mmol) were added, and the
flask was degassed under vacuum and backfilled with argon gas.
15 mL of dichlorobenzene/CS2 (50 : 50 v/v) and 3.5 mg of DMAP
(0.03 mmol) were added to the reaction mixture, and the resul-
tant solution was sonicated for 30 min to dissolve PCBA
completely. Then, 60 mg of 1,3-dicylohexylcarbodiimide (DCC,
0.3 mmol) dissolved in 5 mL of dichlorobenzene/CS2 (50 : 50 v/v)
was dropwise added using a syringe and the reaction solution was
stirred at 40 �C for 3 days. After removal of the solvent, the crude
product was dissolved in THF and filtered. Unreacted PCBA was
separated by centrifugation (5000 rpm, 30 min), and the super-
natant solution was concentrated and filtered. This centrifuga-
tion and filtration step was repeated 3 times, and then
precipitated into methanol. The precipitated polymer was filtered
and dried under vacuum to give 0.08 g (84% yield) of the desired
product.
Preparation of P3HT-b-C60 thin films for TEM
Thin films for morphological investigation were prepared by
spin-coating (3000 rpm) from 9 : 1 CHCl3/pyridine solutions (1
wt%) onto silicon wafers with a native oxide surface layer. The
sample was heated in a vacuum oven (10�6 Torr) at 230 �C for 5
h, slowly cooled down to 150 �C and then annealed at that
temperature for 24 h. The films were then floated onto water and
placed on a 500-mesh copper TEM grid.
Characterization
NMR spectra were obtained on a Bruker Avance 400 (referenced
to CDCl3: 1H NMR at 400 MHz and 13C NMR at 100 MHz).
FT-IR spectra of all synthesized polymers were obtained on an
IR spectrometer (FT/IR-660 plus, Jasco). Elemental analyses
were performed by EA1112 (CE Instruments, Italy). The GPC
measurements were done in THF (35 �C, 1.0 mL/min) using
a Knauer K-501 pump with a K-2301 refractive index detector
and a column bank consisting of two Polymer Labs PLGel
Mixed D columns and one PLGel 50 �A column (1.5 � 30 cm).
Molecular weights are reported relative to polystyrene standards.
Thermogravimetric analyses were carried out with a DuPont
TGA 2950 at a heating rate of 10 �C/min under a nitrogen
atmosphere. X-Ray diffraction patterns were recorded with an
M18XHF-SRA diffractometer using Cu Ka (l ¼ 0.154 nm)
radiation. TEM was performed on a JEOL 100CX microscope
with an accelerating voltage of 100 kV. UV-Visible absorption
and fluorescence spectra were obtained on a HP 8452A spec-
trometer and a Perkin-Elmer LS-55 fluorimeter, respectively.
Differential scanning calorimetry (DSC) measurements were
carried out on a TA Instruments 2910 Modulated DSC
This journal is ª The Royal Society of Chemistry 2009
(DuPont). Samples were heated and cooled from 20 to 300 �C at
a rate of 10 �C/min.
Acknowledgements
The authors thank the Ministry of Science and Technology
(MOST), Korea for financial support through the Global
Research Laboratory (GRL) program. The authors also thank
Dr Qingling Zhang for his assistance of P3HT synthesis.
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J. Mater. Chem., 2009, 19, 1483–1489 | 1489