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