PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Synthesis of C60-end capped P3HT and its application for high performanceof P3HT/PCBM bulk heterojunction solar cells†

Jea Uk Lee,a Jae Woong Jung,a Todd Emrick,b Thomas P. Russellb and Won Ho Jo*a

Received 12th November 2009, Accepted 3rd February 2010

First published as an Advance Article on the web 5th March 2010

DOI: 10.1039/b923752f

A new C60-end capped poly(3-hexylthiophene) (P3HT-C60) was synthesized via a simple three-step

process, and used as a compatibilizer for P3HT/PCBM composite for the purpose of controlling the

morphology of P3HT/PCBM composite film, and thus improving the long-term thermal stability of

solar cell performance. When a small amount of P3HT-C60 was added to P3HT/PCBM, the

bicontinuous and nanometre-scale film morphology was developed and preserved for 2 h of annealing

at 150 �C. Furthermore, the addition of P3HT-C60 as a compatibilizer suppressed large-scale phase

separation of P3HT/PCBM composite even after prolonged annealing time (8 days), and as a result, the

P3HT/PCBM/P3HT-C60 bulk heterojunction solar cells exhibited the excellent long-term thermal

stability of device performance.

Introduction

For efficient bulk heterojunction (BHJ) solar cells, the three-

dimensional bicontinuous and nanometre-scale morphology of

the active layer is of the utmost importance, because efficient

photo-induced charge generation, transport and collection at

each electrode crucially depend on the morphology of the

composite films.1,2 Over the last decade, many research groups

have systematically analyzed external parameters such as the

relative concentration of the donor and acceptor, the choice of

spin-coating solvent,3,4 thermal annealing,5,6 and solvent

annealing procedures7,8 in order to optimize the BHJ

morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-

C61-butyric acid methyl ester (PCBM) composite films for high-

efficiency solar cells. However, controlling the donor/acceptor

composite morphology and optimizing the device performance

are extremely difficult since the nanometre-scale morphology of

the active layer is not thermodynamically stable and therefore

obtained by kinetically controlling the non-equilibrium state.

Furthermore, exposure to sunlight for long periods will cause

macrophase separation of donor/acceptor blend with micro-

metre-scale, which is much larger than the exciton diffusion

length in conjugated polymers.9 It has been reported that a pro-

longed thermal annealing induces the formation of large aggre-

gation of PCBM, which deteriorates the device performance.10,11

One approach towards controlling the nanometre-scale

morphology and improving the long-term thermal stability of

conjugated polymer-fullerene BHJ solar cells is to use diblock

copolymers, which have two different blocks of conjugated

polymers and fullerene, as a compatibilizer.12–14 Addition of the

diblock copolymer compatibilizers can lower the interfacial

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: Powerconversion efficiencies of the solar cell devices with various loadings ofP3HT-C60. See DOI: 10.1039/b923752f

This journal is ª The Royal Society of Chemistry 2010

tension between the P3HT and PCBM phases, and suppress

coalescence, due to the preferential location of the diblock

copolymer at the interface between two phases, resulting in

reduction of phase sizes of the P3HT/PCBM composite and

improving the long-term thermal stability. However, the

synthesis of diblock copolymer compatibilizer having P3HT and

fullerene blocks is burdensome because of multiple post-poly-

merization steps and low solubility of fullerenes.15 Furthermore,

the presence of a substantial amount of insulating moieties

required for introducing C60 in the second block can deteriorate

the charge carrier transport in the BHJ devices.13

To overcome these limitations of diblock copolymer, we report

here the simple synthesis of a C60-end capped P3HT as a compa-

tibilizer for P3HT/PCBM composite (Scheme 1). Very recently,

Hillmyer and his co-workers16 synthesized a difunctional,

fullerene-terminated P3HT (C60-P3HT-C60), and reported its

optical properties and microstructures. However, they used the

C60-P3HT-C60 as an internal electron accepting-donating-

accepting material, not as a compatibilizer. Furthermore, they did

not provide any data for photovoltaic performances.

In order to minimize the amount of insulating moieties, the C60-

end capped P3HT (P3HT-C60) was synthesized via controlled

polymerization followed by directly linking a fullerene derivative at

the end of the regioregular P3HT chain. The chemical structure of

P3HT-C60 was identified by nuclear magnetic resonance (NMR),

Fourier transform infrared (FT-IR) spectroscopy, gel permeation

chromatography (GPC), matrix-assisted laser desorption ioniza-

tion time-of-flight mass spectroscopy (MALDI-TOF MS), and

thermogravimetric analysis (TGA). We examined the effect of

P3HT-C60 compatibilizer on the morphology of P3HT/PCBM

composite and investigated the relation between the morphology

change and the long-term stability of solar cell performance.

Results and discussion

Synthesis of P3HT-C60

A new P3HT-C60 compatibilizer was synthesized through the use

of controlled Grignard metathesis (GRIM) polymerization

J. Mater. Chem., 2010, 20, 3287–3294 | 3287

Scheme 1 Synthesis scheme of P3HT-C60. 9-BBN: 9-borabicyclo[3.3.1]nonane, DCC: 1,3-dicyclohexylcarbodiimide, DMAP: dimethylaminopyridine.

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method and subsequent post-polymerization functionalization

(Scheme 1). Hydroxypropyl-terminated P3HT (P3HT-OH) was

synthesized according to the procedure reported in the litera-

ture.17 Synthesis of regioregular polythiophenes and subsequent

end-group functionalization of these polymers have been devel-

oped by McCullough and his co-workers,18,19 and many other

research groups have used the method to synthesize various

kinds of polythiophene derivatives and polythiophene diblock

copolymers.20–24 In this study, end-functionalized P3HT with

relatively low molecular weight (Mn ¼ 6000, Mw/Mn ¼ 1.12 by

GPC) was synthesized, because lower molecular weight polymer

could be more effective to locate at the interface between P3HT

and PCBM phases and thus to act as a compatibilizer.

As shown in Scheme 1, carboxylate-functionalized fullerene,

[6,6]-phenyl-C61-butyric acid (PCBA), was linked covalently to

the end of the P3HT-OH to yield the final product, P3HT-C60. At

first, we used thionyl chloride to convert the carboxylic acid of

the PCBA to acyl chloride. Although the esterification of the

hydroxyl of P3HT-OH with the acyl chloride of the functional-

ized fullerene was successful, photovoltaic properties of the

obtained product were not satisfactory. Since the thionyl chlo-

ride is a strong oxidizing agent for carbon materials,25 it is

expected that PCBA undergoes p-type doping when treated with

thionyl chloride, and as a result the obtained product is easily

oxidized. To avoid this problem, we employed Steglich (DCC/

DMAP) esterification of the hydroxyl of P3HT-OH with the

carboxylic acid of PCBA. The Steglich esterification is one of the

most convenient methods for the formation of ester bond since

the reaction condition is mild but the conversion is very high due

to the favourable catalytic action of DMAP.26 Consequently,

P3HT-OH was successfully converted to the desired product,

P3HT-C60, which was very stable in air.

The esterification of P3HT-OH with PCBA was identified by1H NMR and FTIR. When the 1H NMR spectrum of P3HT-C60

in CDCl3 is compared with that of P3HT-OH (Fig. 1), methylene

proton signal adjacent to hydroxyl group of P3HT-OH is

3288 | J. Mater. Chem., 2010, 20, 3287–3294

down-field shifted from 3.75 to 4.2 ppm after esterification with

PCBA, confirming that the hydroxyl group of P3HT-OH is

esterified.

FT-IR spectra of the P3HT-OH and P3HT-C60 are compared

in Fig. 2. The characteristic bands of P3HT-OH are observed at

3100–3600 cm�1 (OH stretch), 2950–2850 cm�1 (aliphatic C–H

stretch), 1510–1450 cm�1 (ring stretch), 1375 cm�1 (methyl

deformation), 1100 cm�1 (C–O stretch), and 820 cm�1 (aromatic

C–H out-of-plain vibrations), while the FT-IR spectrum of

P3HT-C60 shows the complete disappearance of OH stretching

vibration and the appearance of C]O stretching vibration at

1735 cm�1. The appearance of sharp peak at 528 cm�1, which is

characteristic for fullerene, also confirms that PCBA is linked

covalently to the end of the P3HT chain.

Fig. 3 shows the GPC traces of P3HT-OH and P3HT-C60.

Both samples show a unimodal, symmetric peak with narrow

molecular weight distribution (Mn ¼ 6000, Mw/Mn ¼ 1.12 for

P3HT-OH and Mn ¼ 6300, 1.18 for P3HT-C60 by GPC). For

exact determination of molecular weight, we performed

a MALDI-TOF MS experiment, which reveals that the number

average molecular weights of P3HT-OH and P3HT-C60 are 4305

and 5160 g mol�1, respectively, as shown in Fig. 4. The molecular

weight difference (855 g mol�1) between P3HT-OH and P3HT-

C60 clearly indicates the attachment of C60 to the P3HT chains.

TGA was used to quantify the exact content of C60 in P3HT-

C60. Fig. 5 shows the TGA thermograms of PCBA, P3HT-OH,

and P3HT-C60, where PCBA shows minor decomposition at

around 200 and 370 �C, due to the loss of a short alkyl chain and

a benzene ring attached to C60, leaving a C60 adduct which results

in the char yield of 82% at 800 �C. Since the char yields of P3HT-

C60 and P3HT-OH are 37% and 29%, respectively, the weight

fraction of C60 in P3HT-C60 is estimated to be 0.13, indicating

that most of P3HT-OH ($90%) was functionalized by fullerene

derivative.

To investigate the effect of the attachment of C60 on the

crystallization of P3HT, differential scanning calorimetry (DSC)

This journal is ª The Royal Society of Chemistry 2010

Fig. 1 1H NMR spectra of (a) P3HT-OH and (b) P3HT-C60 in CDCl3.

Fig. 2 FT-IR spectra of P3HT-OH and P3HT-C60.

Fig. 3 GPC traces of P3HT-OH (dashed line) and P3HT-C60 (solid line).

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was used. As can be seen in Fig. 6, both melting (Tm) and crys-

tallization (Tc) temperatures of P3HT are decreased as the

fullerene is linked to the end of P3HT chain. This is because the

bulky end group of the polymer chain acts as an impurity in

P3HT crystallite that reduces the size of P3HT crystallite or

changes molecular interaction.27 The crystallinity (Xc) of P3HT-

OH and P3HT-C60 can be calculated by integrating the area

under the melting peak, normalizing the values by the amount of

This journal is ª The Royal Society of Chemistry 2010

P3HT present in the sample, and comparing this with the liter-

ature value for the enthalpy of melting of the theoretical 100%

crystal, DHm,P3HT ¼ 99 J g�1.28 The Xc values calculated from

DSC peaks were 9% and 4% for P3HT-OH and P3HT-C60,

respectively.

Morphology control and device optimization

For the preparation of stable polymer solar cells, chemical and

physical degradation mechanisms should be considered.29

Chemical degradation of solar cells is mainly caused by the

diffusion of oxygen and water into the device, photo-oxidation of

conjugated polymers, and chemical reactions between electrode

material and the active polymer. In bulk heterojunction solar

cells, however, prevention of physical degradation such as large-

scale phase separation of active layer morphology is also very

important for the stability of the device performance. Hence, it is

necessary to monitor the morphology change with the annealing

time and to correlate this with the solar cell performance for the

device optimization and evaluation of the long-term stability of

polymer solar cells.

Since the performance of BHJ solar cells strongly depends

on the various processing conditions such as film thickness,

J. Mater. Chem., 2010, 20, 3287–3294 | 3289

Fig. 4 MALDI-TOF MS spectra of P3HT-OH and P3HT-C60.

Fig. 5 TGA curves of PCBA, P3HT-C60, and P3HT-OH.

Fig. 6 DSC thermograms of P3HT-OH (----) and P3HT-C60 (——),

measured at (a) cooling and (b) heating rate of 10 �C min�1.

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donor/acceptor mixing ratio, and thermal annealing time and

temperature, all devices for this study were fabricated by using

the optimized condition for the standard P3HT/PCBM device:

the film thickness, 200 nm; the mixing ratio of P3HT/PCBM, 1/1

by weight; the thermal annealing temperature, 150 �C. To

investigate the effect of P3HT-C60 addition on the film

morphology and the device performance, 2.5 wt% of P3HT-C60

was added to P3HT/PCBM blend, where 2.5 wt% of P3HT-C60

has been optimized by a separate experiment (see the ESI†).

Fig. 7 shows the dependence of device performance on the

annealing time. When the standard P3HT/PCBM is annealed at

3290 | J. Mater. Chem., 2010, 20, 3287–3294

150 �C for 15 min, the power conversion efficiency (PCE) value

reaches as high as 3.88%, which is the optimized value for the

standard device in our experiments, while the standard device

shows a PCE of 0.83% before annealing. As the annealing time is

further increased, however, the PCE of the standard device

rapidly drops below 3% after 1 h annealing. This thermal insta-

bility of P3HT/PCBM BHJ solar cells have already been

observed by several research groups.9,12,15 However, when

2.5 wt% P3HT-C60 is added to P3HT/PCBM composite, the

maximum efficiency of 3.76% has been achieved at 10 min

annealing. More importantly, the maximum PCE value of the

devices remains nearly unchanged (3.6% after 2 h annealing)

when 2.5 wt% P3HT-C60 is added, as the annealing time is

further increased, as shown in Fig. 7c. It should be mentioned

here that the best efficiency of P3HT/PCBM/P3HT-C60 device is

slightly lower than that of the standard P3HT/PCBM device.

This is probably because we have used in this study the device

fabrication condition that has been optimized only for the

standard P3HT/PCBM device.

To investigate the origin of such long-term stability of P3HT/

PCBM/P3HT-C60 BHJ device, the morphology change was

monitored as a function of annealing time by using TEM. All

composite films show phase-separated structure of bright P3HT

rich phase and dark PCBM rich phase, as shown in Fig. 8. The

TEM image of the as-cast P3HT/PCBM film does not show the

morphology with bicontinuous network structure, whereas the

nanometre-scale phase morphology with network structure is

well developed after 15 min of thermal annealing. This

morphology is attributed to yield the optimum efficiency. When

the annealing time is further increased, a large aggregation of

PCBM is observed. Unlike the standard P3HT/PCBM, however,

the as-cast P3HT/PCBM composite film with 2.5 wt% P3HT-C60

shows distinguishable phase separation between P3HT and

PCBM without thermal annealing, and the domain size is

dramatically decreased down to tens of nanometres after 15 min

of annealing, as shown in Fig. 8b. Furthermore, it is notable that

the phase size of P3HT/PCBM does not change significantly after

This journal is ª The Royal Society of Chemistry 2010

Fig. 7 Typical J–V curves of (a) the standard P3HT/PCBM and (b) P3HT/PCBM/P3HT-C60 BHJ device with the addition of 2.5 wt% P3HT-C60 and

(c) the power conversion efficiency of each device versus annealing time at 150 �C.

Fig. 8 TEM images of (a) the standard P3HT/PCBM and (b) P3HT/PCBM/P3HT-C60 composite film with the addition of 2.5 wt% P3HT-C60 as

a function of annealing time at 150 �C.

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2 h of annealing when 2.5 wt% of P3HT-C60 is added, and the

well-defined bicontinuous network morphology also remains

nearly unchanged. This observation leads us to conclude that the

P3HT-C60 is located at the interface between P3HT and PCBM,

and as a result the P3HT-C60 acts as a compatibilizer effectively

to suppress macrophase separation.

Fig. 9 The power conversion efficiencies of the standard P3HT/PCBM

and P3HT/PCBM/P3HT-C60(2.5 wt%) BHJ devices as a function of

annealing time at 150 �C.

Long-term thermal stability

To examine the effect of P3HT-C60 on the long-term stability of

device performance, the PCEs of P3HT/PCBM/P3HT-C60

devices were measured as a function of annealing time at 150 �C.

When the PCEs are plotted against the annealing time, as shown

in Fig. 9, it is obvious that the PCE of the standard P3HT/PCBM

device exhibits the maximum efficiency at 15 min annealing, and

then rapidly decreases below 1% of PCE after 24 h at 150 �C,

which is consistent with the value reported in the literature.15

However, the device with the addition of P3HT-C60 shows very

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Table 1 Power conversion efficiencies of P3HT/PCBM and P3HT/PCBM/P3HT-C60(2.5 wt%) BHJ devices after thermal annealing at 150�C for 24 and 48 h. The devices were annealed before (pre-annealing) andafter (post-annealing) deposition of aluminium electrode

DeviceAnnealingprocedure

Annealingtime/h PCE (%)

P3HT/PCBM Pre-annealing 24 0.7348 0.62

Post-annealing 24 1.1648 0.96

P3HT/PCBM/P3HT-C60(2.5 wt%) Pre-annealing 24 2.7148 2.41

Post-annealing 24 2.8148 2.59

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stable device performance even after 48 h annealing at 150 �C.

The PCE of the device with 2.5 wt% P3HT-C60 slowly decreases

and maintains 3% PCE. To the best of our knowledge, the P3HT-

C60 is more effective for improving the long-term thermal

stability of P3HT/PCBM BHJ solar cells than any other com-

patibilizers ever reported.

TEM observation was used to investigate the morphology

change of blend films after prolonged annealing time at 150 �C.

The standard P3HT/PCBM composite film shows large and

dense aggregates of PCBM after annealing for 48 h (Fig. 10a),

while the P3HT/PCBM/P3HT-C60(2.5 wt%) composite film

exhibits a homogeneous film without any PCBM aggregates even

after 48 h annealing (Fig. 10b). Moreover, when magnified

images of homogeneous parts of both films are compared, it

reveals that the domain size of P3HT/PCBM/P3HT-C60

composite is much smaller than that of P3HT/PCBM composite.

This observation leads us to conclude that the addition of P3HT-

C60 suppresses large-scale phase separation of P3HT/PCBM

blend even after the prolonged annealing time, and consequently

improves the long-term thermal stability of solar cell perfor-

mance.

Some theoretical and experimental studies have reported that

the aluminium electrode can interact with the active layer

materials in the presence of moisture, which may deteriorate the

device performance after thermal treatment.30–32 To investigate

the possibility that the deterioration of device performance after

a long period of thermal annealing is caused by the interface

instability between the aluminium electrode and the active layer,

the devices (pre-annealed devices) were fabricated by depositing

the aluminium electrode on the active layer after thermal

Fig. 10 Comparison of TEM images of (a) the standard P3HT/PCBM

and (b) P3HT/PCBM/P3HT-C60 (2.5 wt%) composite films after thermal

annealing at 150 �C for 48 h. The right-hand images show higher

magnification.

3292 | J. Mater. Chem., 2010, 20, 3287–3294

treatment at 150 �C and their performances were compared with

those of post-annealed devices. When the PCEs of the pre-

annealed devices were compared with those of the post-annealed

devices, as can be seen in Table 1, the pre-annealed P3HT/PCBM

devices exhibit lower efficiencies than the standard post-anneal-

ed devices. This observation leads us to conclude that the inter-

face between the active layer and the aluminium electrode is not

adversely affected by the long time thermal annealing inside the

glove box, and therefore the deterioration of the device perfor-

mance is mainly caused by the large-scale phase separation of

active layer morphology.

Another important issue to estimate the long term stability of

solar cell performance is the test temperature. We tested the long

term stability of solar cell performance at 150 �C because the

temperature of 150 �C has been mostly used to optimize the

device performance and control the active layer morphology.

However, it is not simple to estimate the device lifetime under

atmospheric conditions from the test result at 150 �C. Hence, the

PCEs of P3HT/PCBM/P3HT-C60 devices were measured at

100 �C as a function of annealing time, which is closer to the

accelerated testing condition.33,34 The PCE of the standard

P3HT/PCBM device decreases below 1% of PCE after 140 h at

100 �C, whereas the device with the addition of 2.5 wt% P3HT-

C60 exhibits a much smaller decrease in the PCE as compared

with the standard P3HT/PCBM device, as shown in Fig. 11.

Fig. 11 The power conversion efficiencies of the standard P3HT/PCBM

and P3HT/PCBM/P3HT-C60 (2.5 wt%) BHJ devices as a function of

annealing time at 100 �C.

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It should be noted that the device with P3HT-C60 records 2.5%

PCE after 200 h at 100 �C, which corresponds to thousands of

hours at 25 �C according to the Arrhenius model.

As mentioned above, however, there are several other factors

than the morphology control of active layer that should be

considered for the preparation of stable polymer solar cells.

Therefore it is necessary to measure the PCE under continuous

illumination and atmospheric conditions for thousands of

hours.35,36 Hence, the results reported in this paper may be

somewhat far from the realistic conditions and therefore further

experiments are needed for commercialization of polymer solar

cells.

Conclusions

We have successfully synthesized a new C60-end capped P3HT

via GRIM polymerization step followed by directly linking

a fullerene derivative to the chain end of P3HT, and used it as

a compatibilizer for P3HT/PCBM composite for enhancing long-

term thermal stability of solar cell performance. The synthetic

simplicity of P3HT-C60 with clear definition of molecular struc-

ture provides practical advantages over diblock copolymer

compatibilizers.

The addition of a small amount of P3HT-C60 reduces the

domain size of the P3HT/PCBM composite and suppresses the

macrophase separation for prolonged thermal annealing,

resulting in excellent long-term stability of solar cell perfor-

mance. The improvement of long-term thermal stability of

P3HT/PCBM/P3HT-C60 solar cells can be explained by the fact

that the addition of P3HT-C60 reduces the interfacial tension

between P3HT and PCBM phases, suppressing the phase segre-

gation of P3HT/PCBM composite for prolonged annealing.

Experimental

Materials

A buckminsterfullerene derivative, PCBA, was purchased from

Materials Technologies Research (Cleveland, OH). Tetrahydro-

furan (THF) was dried over sodium/benzophenone under

nitrogen and freshly distilled before use. Regioregular P3HT

(55 kDa) was purchased from Rieke Metal Inc. and PCBM

(>99.5%) was obtained from Nano-C. These chemicals were used

as received without further purification. Poly(3,4-ethyl-

enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)

(Baytron P VP AI 4083) was purchased from H. C. Stark and

passed through a 0.45 mm PES syringe filter before spin-coating.

All other reagents were purchased from Aldrich Chemicals

(Milwaukee, WI) and used as received.

Synthesis of C60-end capped P3HT

First, hydroxypropyl-terminated P3HT was synthesized by

following the procedure reported in the literature,17 and then C60-

end capped P3HT was synthesized by Steglich esterification of

hydroxypropyl-terminated P3HT and PCBA. In a 100 mL

round-bottom flask, 0.04 g of hydroxypropyl-terminated

P3HT (Mn ¼ 6000, Mw/Mn ¼ 1.12 by GPC) and PCBA (0.045 g,

0.05 mmol) were added, and the flask was degassed

under vacuum and backfilled with argon gas. 6 mL of

This journal is ª The Royal Society of Chemistry 2010

dichlorobenzene/CHCl3 (50/50 v/v) and 0.5 mg of DMAP

(0.004 mmol) were added to the reaction mixture, and the solu-

tion was sonicated for 30 min to dissolve PCBA completely.

Then, 10 mg of 1,3-dicyclohexylcarbodiimide (DCC, 0.05 mmol)

dissolved in 2 mL of dichlorobenzene/CHCl3 (50/50 v/v) was

added dropwise using a syringe, and the solution was stirred for

reaction at 30 �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 supernatant solution was then concentrated and filtered. The

centrifugation and filtration step was repeated 3 times, and then

precipitated into methanol. The precipitated polymer was filtered

and dried under vacuum to give 0.038 g (81% yield) of desired

product.

Device fabrication

A mixture of P3HT/PCBM (1/1 by weight) was dissolved in 1,2-

dichlorobenzene at a concentration of 2 wt%, and then 2.5 wt%

of P3HT-C60 was added to the solution with respect to the total

weight of P3HT/PCBM. The solutions were stirred for 12 h. The

photovoltaic devices were fabricated according to the following

procedure: ITO-coated glass (15 U/square) was cleaned with

acetone followed by ultrasonication in acetone and isopropyl

alcohol, and then was kept at 200 �C for 30 min. After complete

drying, ITO-coated glass was treated with UV-ozone. After spin-

coating of PEDOT:PSS with 30 nm in thickness, the device was

dried at 120 �C for 30 min under a nitrogen atmosphere. P3HT/

PCBM (or P3HT/PCBM/P3HT-C60) blend solutions were spin-

coated on the top of PEDOT:PSS layer at 700 rpm for 60 s. Al

(100 nm thickness) were thermally evaporated under vacuum

lower than 10�6 Torr on the top of the active layer. The device

was annealed at 150 �C on a digital hot plate under a nitrogen

atmosphere inside a glove box.

Characterization and measurements

The chemical structure was identified by a 500 MHz 1H NMR

spectrometer (Bruker, Avance 500). FT-IR spectra of all

synthesized polymers were obtained on an IR spectrometer (FT/

IR-660 plus, Jasco). Molecular weight and its distribution were

measured by GPC using THF as an eluent. Molecular weights

were reported relative to polystyrene standards. MALDI-TOF

MS (Voyager-DE STR Biospectrometry Workstation, Applied

Biosystems Inc.) was used for estimation of exact molecular

weights of synthesized polymers. Thermogravimetric analyses

were carried out at a heating rate of 10 �C min�1 under a nitrogen

atmosphere using a thermogravimetric analyzer (TA 2050, TA

Instruments). Melting and crystallization temperatures were

measured by heating and cooling the sample from 20 to 250 �C at

a rate of 10 �C min�1 using a differential scanning calorimeter

(TA Instruments, 2920 Modulated DSC).

The specimen for TEM measurement was prepared by floating

the film from the glass substrate on deionized water and trans-

ferring to TEM grid. TEM observation was performed on

a LIBRA 120 microscope with an accelerating voltage of 120 kV.

The photovoltaic performance was measured under nitrogen

atmosphere inside the glove box. The current density-voltage

(J–V) characteristics were measured with a Keithley 4200

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source-meter under AM 1.5 G (100 mW cm�2) simulated by

a Newport-Oriel solar simulator. The light intensity was cali-

brated using a NREL certified photodiode and light source meter

prior to each measurement. The active area was determined at

0.04 cm2 by attaching a shadow mask onto solar cell device.

Acknowledgements

The authors thank the Ministry of Education, Science and

Technology (MEST), Korea for financial support through the

Global Research Laboratory (GRL) program.

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