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Nano Res
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Direct synthesis of highly conductive
PEDOT:PSS/graphene composites and their
applications in energy harvesting systems Dohyuk Yoo1, Jeonghun Kim2, and Jung Hyun Kim1() Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0433-z
http://www.thenanoresearch.com on February 21, 2014
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Direct Synthesis of Highly Conductive
PEDOT:PSS/Graphene Composites and Their
Applications in Energy Harvesting Systems
Dohyuk Yoo1, Jeonghun Kim2, and Jung Hyun Kim1*
1 Yonsei University, Republic of Korea 2 Dongjin Semichem Co., Ltd., Republic of Korea
Highly conductive PEDOT:PSS/graphene composites were
directly synthesized by in situ polymerization. The electrical
conductivity of the composite film was enhanced by 40.6 %,
reaching 637 S cm-1 by introduction of 3 wt% graphene
without any further complex reduction processes of graphene.
2
Direct Synthesis of Highly Conductive PEDOT:PSS/Graphene Composites and Their Applications in Energy Harvesting Systems
Dohyuk Yoo1, Jeonghun Kim2, and Jung Hyun Kim1 () 1 Department of Chemical and Biomolecular Engineering, Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749 (Republic
of Korea) 2 Electronic Materials Division, R&D Center, Dongjin Semichem Co., Ltd. 625-3 Yodang-Ri, Yanggam-Myeon, Hwaseong-Gun,
Gyeonggi-Do, (Republic of Korea)
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT We report for the first time highly conductive PEDOT:PSS/graphene composites by in situ polymerization and
their applications in a thermoelectric device and a platinum (Pt)-free dye-sensitized solar cell (DSSC) as energy
harvesting systems. Graphene was dispersed in a solution of poly(4-styrenesulfonate) (PSS) and
polymerization was directly carried out by the loading of a 3,4-ethylenedioxythiophene (EDOT) monomer to
the dispersion. The content of the graphene was varied and optimized for the highest electrical conductivity.
The composite solution was ready to use without any reduction process because reduced graphene oxide was
used. The fabricated film had a conductivity of 637 S cm-1 with an enhancement of 41 % by the introduction of 3
wt% graphene without any further complicated reduction processes for graphene. The highly conductive
composite films were applied to an organic thermoelectric device, and the device showed a power factor of 45.7
μW m-1K-2 which is 93 % higher than the device based on pristine PEDOT:PSS. In addition, the highly
conductive composite films were used for Pt-free DSSC showing an energy conversion efficiency of 5.4 %,
which is 21 % higher than that of a DSSC with PEDOT:PSS.
KEYWORDS
Direct synthesis, conductive polymer, graphene composite, thermoelectric material, dye-sensitized solar cell
1. Introduction
Highly conductive films using metal
nanostructures [1-4], conductive polymers (CPs) [5],
carbon-based materials [6], and hybrid materials [7],
have attracted a great deal of attention in various
applications such as transparent electrodes [8], solar
Nano Res DOI (automatically inserted by the publisher) Research Article
———————————— Address correspondence to [email protected]
3
cells [9], light-emitting diodes (LEDs) [8], and
thermoelectric devices [5,10,11] for reducing the
processing cost or replacing expensive oxides (e.g.
indium tin oxide) [12]. Especially, conductive
polymers are promising materials for transparent
electrodes and conducting layers due to their
unique optical property, high conductivity, light
weight, low cost, flexibility and excellent
processability in industrial manufacturing. As the
electrical conductivity of CP is determined by core
factors including polymer species [13,14],
polymerization methods [15-17], type and
concentration of dopants [15], and post treatments
[11,18], these factors should be considered for the
desired applications. Among the CPs,
poly(3,4-ethylenedioxythiophene):poly(4-styrenesul
fonate) (PEDOT:PSS) is the most promising material
because it has water-dispersibility, good
conductivity, low material cost, high transparency,
and excellent processability. Therefore, PEDOT:PSS
has been broadly used as host and guest materials
in hybrid systems to enhance electrical conductivity
and performance [19-21]. Carbon-based materials
such as carbon nanotubes (CNTs) [22] and graphene
are good candidates for making conductive
composites with PEDOT:PSS through surface
modification and stabilizing techniques [23].
Especially, graphene is a novel carbon nanomaterial
due to its unique electrical and optical properties,
high transparency, good conductivity, bendability,
and excellent stability [24-31].
Recently, research has focused on the fabrication
of PEDOT:PSS/graphene composites via the
following four main methods and approaches. The
first is the simple mixing of graphene oxide (GO)
and PEDOT:PSS. The GO content in PEDOT:PSS
can be over 10 % due to its high solubility in water.
However, because the conductivity of GO is poor, a
further reduction process of GO after film
formation is essential to exhibit the conductivity of
the graphene [32]. In addition, homogeneous
dispersion of graphene in PEDOT:PSS is limited
because graphene is foldable and PEDOT:PSS
nanoparticles of 30-50 nm are in colloidal state. The
second approach is to mix graphene oxide and PSS,
and then polymerize the EDOT monomer. However,
this approach has an additional reduction process
with hydrazine (N2H4) before polymerization and
the resulting powder product should be redispersed
in solvent and showed just a 7 S cm-1 increase. In
this synthesis, a low molecular weight PSS was
used and PEDOT:PSS showed a low electrical
conductivity [33]. The third method is to use a
small stabilizing molecule for the exfoliation and
dispersion of GO, and then after the reduction
process with hydrazine, the dispersed graphene
was mixed with commercially available PEDOT:PSS.
Mixing ionic additives with PEDOT:PSS can induce
the deterioration of solution stability such as a high
viscosity and gelation of the solution [8]. In these
methods, the ‘reduction’ is the unavoidable
essential process using the very toxic hydrazine
chemical for GO. Furthermore, importantly,
hydrazine is very critical to doped CPs because
hydrazine is used as a strong reductant in CP
synthesis and CP could be dedoped and lose
conductivity. The last method is to directly mix a
reduced graphene oxide (RGO) with commercial
PEDOT:PSS products [23]. Direct mixing of RGO
and PEDOT:PSS has a limitation to add a large
amount of RGO, and is difficult to disperse RGO
sheets well in the PEDOT:PSS. Indeed, a low cost,
highly conductive, facile synthetic, and mass
production method for a well-dispersed
CP/graphene composite without a toxic treatment
process is highly desirable.
Herein, we report for the first time the liter-scale
direct synthesis of a highly conductive
PEDOT:PSS/graphene composites by a careful in
situ polymerization of PEDOT with a presence of
reduced graphene oxide (RGO) and high molecular
weight PSS. PSS acts as both a dispersant of RGO
4
Figure 1. Schematic synthesis flow of PEDOT:PSS/graphene composites. a) Dispersion of graphene with PSSA by ultrasonication
in double-jacketed reactor. i) N2 inert gas purging for 20 min and then injection of EDOT monomer. ii) Initiation of polymerization
by adding oxidants at closed system. Magnified area shows polymerized PEDOT:PSS on graphene sheets. b) Chemical structures of
PSS and EDOT. c) Photographs of a dispersion of graphene oxide(GO), reduced graphene oxide (RGO), PSS, RGO/PSS in water
after sonication, and final PEDOT:PSS/graphene composite after oxidative polymerization.
and dopant of PEDOT. We found that the
electrical conductivity of the composites increased
from 453 to 637 S cm-1 when composite had 3 wt%
of graphene without any further complex reducing
steps. We have analyzed the chemical structures,
morphology, and optical and electrical properties of
the PEDOT:PSS/graphene composites. We also
demonstrate applications of highly conductive
composite films in energy harvesting systems
high-performance organic thermoelectric devices
and Pt-free dye-sensitized solar cells (DSSCs).
2. Results and Discussion
The dispersion of graphene is of pivotal
importance in polymer/graphene composite
development because improper dispersion often
leads to aggregation of particles and, thus, to
fabrication of unclear films. Therefore, the proper
choice of dispersant for graphene and
functionalization of graphene with polyelectrolyte
are critical [34,35] Ionic materials such as ionic
liquid, small molecules, and polyelectrolytes can be
hybridized with GO and RGO for stabilization
[8,33,36-41]. Here we report a simple and good
method to improve the conductivity of PEDOT:PSS
by using PSS as both a dispersant for RGO and a
good dopant for PEDOT.
As shown in Figure 1c, GO was dispersed in
water while RGO was not without the aid of a
stabilizer. Interestingly, RGO was dispersed in
water after adding PSS polyelectrolyte and
sonication treatment, and this dispersed solution
5
was stable for over 30 days. The desired amount of
graphene was added to PSS dissolved distilled
water and dispersed by ultrasonication (Figure 1a).
After the degassing process, EDOT was added to
the graphene- dispersed solution, and then iron (III)
sulfate and sodium persulfate were added. The
oxidative polymerization was carried out at low
temperature for a slow reaction rate. During the
polymerization, the inner reactor was maintained
with a flow of nitrogen gas to prevent the
overoxidation of PEDOT which could cause a
decrease in conductivity. Finally, a well-dispersed
PEDOT:PSS/graphene composite without
aggregation and gelation was made with a
purification method using an ion exchange resin for
the removal of residual ions. The key point of our
synthetic method is to prepare highly conductive
PEDOT:PSS/graphene without any further
reduction processes using toxic chemicals or
unfavorably high temperature. As a result, our
method has the following advantages. First, the
RGO showed a good dispersion property using a
polyelectrolyte with higher molecular weight rather
than small molecules in an aqueous solution. In
comparison, RGO was not dispersed in water and
was still floating on the surface after 3 hr without
PSS (Figure 1c). Second, a large amount of graphene
can be used for the mass production of composite.
In this study, graphene of more than 0.3 g could be
dispersed in 1000 mL of water with PSS and it was
shown that large scale above 1 liter can be possible
(Figure S1). Additionally, synthesized solutions
were coated on glass for their absorbance spectra
and all films showed a near-IR (NIR) absorption
which is a clear proof of the doped state of PEDOT
through the successful synthesis in the composite
(Figure S3). Third, the final composite solution was
stable for more than six months as shown in Figure
1c. In comparison, a mixture of pre-synthesized
PEDOT:PSS and graphene exhibited a poor
dispersion property with aggregated graphene
particles in our study as shown in Figure S2a. The
Figure 2. FT-IR spectra of the pristine graphene and the
synthesized PEDOT:PSS/G3 composites.
Figure 3. XRD patterns of graphene, PEDOT:PSS, and
PEDOT:PSS/G3.
results indicate that the graphene is well-dispersed
and incorporated with PEDOT:PSS nanoparticles by
direct synthesis. Therefore, this in situ synthetic
method is a promising method for the production
of a stable PEDOT:PSS/graphene composite.
Successful oxidative polymerization of EDOT in the
presence of PSS/graphene by oxidants was confirmed
by Fourier-transform infrared spectroscopy (FT-IR).
As shown in Figure 2, the absorption peaks at 3435
and 1168 cm-1 are attributed to the remaining –OH
group and C-O-C bonds on the surface of the
graphene, which are typical peaks for reduced
graphene oxide [37]. The peaks in the spectrum at 1521,
6
1312, and 1196 cm-1 are attributed to the C=C and C-C
bonds of the thiophene ring [7,15] and the sulfonic
acid group of the PSS [37], respectively. The FT-IR
results clearly confirmed that the graphene has polar
groups for the dispersion and that the
polymerization of PEDOT:PSS was occurred in the
presence of graphene. To confirm the formation of
the composite, X-ray diffraction (XRD) of graphene,
PEDOT:PSS, and PEDOT:PSS/graphene was
obtained as shown in Figure 3. Pristine graphene
showed a broad peak at 21.5 ° and after dispersion
with PSS and in situ polymerization with EDOT,
the peaks of PEDOT:PSS at 17.5 ° and 25.8 °
appeared as main peak. This indicates that
diffraction peak of graphene disappeared and the
peak of the nanocomposite was almost identical to
PEDOT:PSS. This result clearly shows that the
reduced graphene oxide was well-dispersed by the
polyelectrolyte, PSS. In addition, the layered and
stacked graphene structure was separated and
mixed in the PEDOT:PSS matrix through the in situ
polymerization of PEDOT:PSS from PSS dispersed
with graphene in an aqueous solution (see the last
illustration in Figure 1a) [42].
As further evidence, transmission electron
microscopy (TEM) images in Figure 4 show that the
PEDOT:PSS was polymerized on the surface of
graphene sheet and covered the graphene. This
indicates that the water-soluble PSS was inserted into
the layered graphene sheets and stabilized on the
surface of the graphene sheet. PEDOT:PSS was
connected from the surface of the graphene to the
outer side of the graphene; the graphene was located
in PEDOT:PSS matrix, which means that the
PEDOT:PSS and graphene were clearly used as host
and guest materials, respectively, in the conductive
composite.
X-ray photoelectron spectroscopy (XPS)
measurement was carried out to investigate the
oxidation of the graphene in the presence of a large
amount of the strong oxidant, sodium persulfate
which is a core material for oxidative polymerization
of the heterocyclic EDOT monomer (Figure 5 and
Figure 4. TEM images of a) PEDOT:PSS/graphene films (scale
bar: 500 nm) and b) magnified image (scale bar: 50 nm).
Figure S6). Typical carbon bonds in the reduced
graphene oxide, PEDOT:PSS and PEDOT:PSS/
graphene were observed at ~284.5 (C-C), ~285.9
(C-O) and ~287.3 eV (C=O), respectively [8,32].
Interestingly, the directly synthesized PEDOT:PSS
with 3 wt% of graphene (here after denoted as
PEDOT:PSS/G3) and a mixture of pre-synthesized
PEDOT and 3 wt% of graphene (denoted as
PEDOT:PSS/mix-G3) have a similar ratio of peaks,
indicating that the graphene was stable in the
presence of persulfates during oxidative
polymerization (Figure 5b, Figure S6f and i). In
other words, as the graphene surface was in contact
with a large amount of the strong oxidant without
the protection of PSS, RGO was partially
re-oxidized. New carbon bond in the re-oxidized
7
Figure 5. C1s X-ray photoelectron spectroscopy (XPS) spectra
of (a) graphene and re-oxidized graphene, (b) PEDOT:PSS,
PEDOT:PSS/mix-G3 and PEDOT:PSS/G3.
graphene was showed at ~ 288.7 (O-C=O) eV in the
Figure 5a. As the polymerization was carried out in
this state that the PSS was located close to the
surface of the graphene due to the hydrogen
bonding and stacking interaction between PSS and
graphene, PSS on the surface of the graphene could
have inhibited the attack of persulfate and protect
the graphene from further oxidation. Moreover,
binding energy of PEDOT:PSS and PEDOT:PSS/G3
at C-C bonding peak was slightly shifted from
284.46 to 284.51 eV. Because reduced graphene
oxide reserving the bountiful π-electrons achieves a
strong π-π interaction with aromatic structure of
PEDOT and PSS components [23].
The protection effect of PSS and electron-stacking
Figure 6. Raman spectra of (a) pristine graphene and
re-oxidized graphene, (b) PEDOT:PSS and PEDOT:PSS/G3
composites.
interactions between graphene and PEDOT:PSS are
confirmed by Raman spectroscopy of the
PEDOT:PSS and PEDOT:PSS/graphene composites
(Figure 6a-b). As shown in Figure 6a, both graphene
and re-oxidized graphene spectra indicated the D
and G bands. The G band peak of the graphene
sample is identified at 1583.3 cm-1 and that of
re-oxidized graphene without PSS protection is
shown a shift to 1592.9 cm-1. The D band, which
allows to detect a degree of functionalization and
surface faults, represented at 1335.8 cm-1 in
graphene, whereas in the re-oxidized graphene the
peak also shifted to 1348.2 cm-1. The intensity of D
band increases in the re-oxidized graphene,
8
Figure 7. Microscopic images of conductive polymer films after annealing for 20 min at 150 °C. SEM images of a) PEDOT:PSS and
b) PEDOT:PSS/G3 films on the silicon wafer (scale bar: 20 μm). Insets show cross-sectional images of the films (scale bar: 2 μm).
AFM images of c) PEDOT:PSS and d) PEDOT:PSS/G3 films on a silicon wafer. (scale bar: 1 μm)
suggesting a higher surface fault density and a
higher degree of functionalization than its pristine
form due to the oxidation [43]. For the PEDOT:PSS
and PEDOT:PSS/G3 in Figure 6b, the five typical
bands were indicated as a C=C anti-symmetric
stretching (1570 cm-1), C=C asymmetrical
stretching (1501 cm-1), C=C symmetrical
stretching (1440 cm-1), single C-C stretching (1365
cm-1), and C-C inter-ring stretching (1262 cm-1)
[44]. In comparison with the spectrum of
PEDOT:PSS, the peaks of PEDOT:PSS/G3 are
slightly shifted (for example, from 1437.1 cm-1 to
1441.5 cm-1 in case of C=C symmetrical stretching)
according to the strong π-π interaction of aromatic
structures of PEDOT:PSS and electron-rich
graphenes [45].
The synthesized PEDOT:PSS/graphene composite
was spin-coated onto a silicon wafer and dried at
150 °C to study the morphology. As shown in Figure
7a-b, scanning electron microscopy (SEM) images
illustrate that the PEDOT:PSS film has a smoother
surface than that of the PEDOT:PSS/G3 film and the
PEDOT:PSS/G3 film shows that the graphene
particles are covered with PEDOT:PSS on the film
surface. A cross-sectional SEM image of the
PEDOT:PSS shows a typical cross-cut polymer film,
and the PEDOT:PSS/G3 has a layered, and lamellar
structure similar to a well-organized polymer/
graphene composite in thin films [8]. The presence of
well-dispersed and organized graphene nanosheets
might have affected the PEDOT:PSS molecules to
form morphologically favorable structures for charge
transport, resulting in a better conducting channel in
the composites. However, PEDOT:PSS/mix-G3 film
showed the large individual graphene particles on
9
Figure 8. a) Photographs of PEDOT:PSS (left top),
PEDOT:PSS/G3 (right top), and PEDOT:PSS/G5 (left bottom)
film coated on a glass substrate and PEDOT:PSS/G3 (right
bottom) coated on PET film. b) Transmittance spectra of
PEDOT:PSS and PEDOT:PSS/graphene composite films with
various graphene contents. Glass substrate was used as a
background. (Thicknesses of films were shown 100 nm
approximately.)
the surface and a disordered structure was observed
in the cross-sectional image as shown in Figure S2b.
In fact, the directly synthesized PEDOT:PSS/G3
exhibited a higher increase in electrical conductivity
than that of the simply mixed PEDOT:PSS/mix-G3
with a considerable difference (Table 1). Therefore,
the dispersion of graphene in functional materials is
very important in the preparation of composites and
the direct synthesis of PEDOT:PSS/graphene has an
advantage in composite morphology related to the
conductivity. In addition, an atomic force microscopy
study (AFM) revealed that, while the pure
PEDOT:PSS film had a smooth surface with an
average roughness (Ra) of 6.83 nm, the
PEDOT:PSS/G3 film (Ra = 9.02 nm) had a rougher
surface of PEDOT:PSS with graphene structures as
shown in Figure 7c-d.
We prepared the conductive composite films with
~100 nm thickness with varying graphene contents
by spin-coating onto 75 × 75 mm2 glass substrates.
The composite solution was mixed with 5 % DMSO,
then coated and dried at 150 °C for 30 min. Electrical
conductivity was measured with the 4-probe method
and is summarized in Table 1. According to the
increasing graphene content up to 3 %, the
conductivity was increased, but after loading of 4
wt% graphene, the value decreased a little and
became saturated at ~560 S cm-1. The decrease in
conductivity may be induced by the dispersion from
the over-loading of graphene. As shown in Figure 8a
and Figure S5, all solutions showed a good coating
property without aggregation on the glass and PET
substrates, and the coated films with ~100 nm
thickness showed high transparency.
The PEDOT:PSS/G3 films showed a high
transparency of 92.1 % (background: glass) at 550
nm. The transparency was decreased according to
the graphene content, and most importantly,
PEDOT:PSS/G5 exhibited a transparency of 90.8 %
with a 2.2 % decrease compared to that of
PEDOT:PSS at 550 nm. This result indicates that an
optimized small amount of graphene by direct
synthesis can enhance the conductivity.
The synthesized PEDOT:PSS/graphene composite
with good conductivity showed great potential to
be used as an organic thermoelectric material and a
cathodic material of DSSCs. Because our composites
can be used without any harsh reducing steps for
graphene, the underlying layers such as dyes,
electrodes, and substrates of the devices will
maintain their functions. Thermoelectric materials
have received great attention because they can
directly and effectively convert heat to electricity
10
Table 1. Thermoelectric properties of various conducting polymer/carbon material composites.
Sample Composite
method
σ
(S cm-1) a
S
(μV K-1)
S2·σ
(μW m-1 K-2) Ref.
PEDOT:PSS b - 453 23.100 24.173 present work
Pristine reduced graphene oxide (rGO) - 4.73 0.5200 0.00013
PEDOT:PSS/G1 In-situ 528 24.375 31.371
PEDOT:PSS/G2 In-situ 548 24.750 33.568
PEDOT:PSS/G3 In-situ 637 26.778 45.677
PEDOT:PSS/mix-G3c Mixing 482 23.250 26.055
PEDOT:PSS/G4 In-situ 556 24.715 33.962
PEDOT:PSS/G5 In-situ 559 21.750 26.444
PEDOT:PSS - 0.74 165.82 2.03 23
PEDOT:PSS/Graphene (98:2 weight ratio) Mixing 32.13 58.77 11.09
PEDOT - 23.0 12.9 0.39 48
PEDOT/Graphene (35 wt.%) In-situ 50.8 31.8 5.20
PANI - 4.0 13.5 0.1 49
PANI/Graphene (30 wt.%) In-situ 39.0 27.0 2.6
PANI/Graphene (4:1 weight ratio) Mixing 14.76 20.8 0.64 50
PANI/Graphene (1:1 weight ratio) Mixing 58.89 31.0 5.60
PEDOT:PSS/SWCNT (35 wt.%) Mixing 400 25.0 25.0 44
PANI - 12.5 11.0 0.2 43
PANI/SWCNT (41.4 wt.%) In-situ 125 40.0 20.0
a Measured by four-probe method. b Pristine PEDOT:PSS synthesized without graphene. c Just mixed PEDOT:PSS and
graphene (3 wt% ) composites by ultrasonication.
from generated or wasted heat sources. In the early
stage, thermoelectric properties of semiconductors
and metal alloys, including BiTe, CoSb, SiGe, MgSi,
and BiSb, have been studied, but they have
limitations of easy decomposition and oxidation in
an air atmosphere. Alternative thermoelectric
materials such as organic polymers and composites
have recently been found to be attractive due to their
low cost, easy synthesis, processability, and
flexibility [5,10,11]. Conductive polymers are
promising thermoelectric materials because they
have the advantages of mass production, flexibility,
low cost, and a high value of dimensionless figure of
merit, ZT= S2·σ·T/к (σ, S, T, and к are electrical
conductivity, Seebeck coefficient, absolute
temperature, and thermal conductivity, respectively),
based on their high electrical conductivity and low
thermal conductivity. The conductive composite film
was fabricated on a glass substrate and the device
was prepared for measurement as shown in Figure
9a. The power factor (S2·σ) of the
PEDOT:PSS/graphene composite was increased from
24.17 to 45.68 μW m-1K-2 owing to the enhanced
electrical conductivity (Figure 9b-c). This enhanced
power factor was higher than that of previous works
using a simply mixed PEDOT:PSS and graphene (2
wt%) composite with a power factor of 11.09 μW
m-1K-2 [23], the polyaniline/single-walled carbon
nanotube (40 wt%) composite with a power factor of
20 μW m-1K-2 [46], and the PEDOT:PSS/single-walled
carbon nanotube (35 wt%) composite with a power
factor of 25 μW m-1K-2 [47]. The enhanced electrical
conductivity was the main factor for the increased
power factor. It is quite remarkable that, in our study,
the graphene content in the composites was only 3
wt% to obtain the maximum power factor.
Interestingly, the simply mixed composite
(PEDOT:PSS/mix-G3) showed a lower power factor
of 26.06 μW m-1K-2 than the directly synthesized
PEDOT:PSS/G3.
11
Figure 9. Energy harvesting systems including thermoelectric devices and Pt-free DSSCs based on the PEDOT:PSS/graphene
composites. a) Schematic diagram of the PEDOT:PSS/graphene composite film and the Seebeck coefficient measurement setup. b)
Conductivity and Seebeck coefficient and c) power factor of composite films according to the graphene contents. d) Schematic
representation of DSSC with a composite film for replacing Pt layer on cathode. e) J-V curves of DSSCs from PEDOT:PSS,
PEDOT:PSS/G3, and PEDOT:PSS/mix-G3 coated cathodes at 100 mW cm-2.
12
Table 2. Pt-free DSSC performances of conducting polymer/graphene composite counter electrodes with various conducting polymer
composites.
Sample Composite method Jsc (mA cm-2) Voc (V) FF (%) η (%) Ref.
PEDOT:PSS - 11.8 0.72 0.52 4.47 present work
PEDOT:PSS/G3 In-situ 14.1 0.73 0.51 5.41
PEDOT:PSS/mix-G3 a Mixing 12.9 0.74 0.52 5.03
PEDOT:PSS - 10.99 0.72 0.68 2.30 53
PEDOT:PSS/Graphene Mixing 12.96 0.72 0.48 4.50
PEDOT - 12.60 0.77 0.58 5.62 54
PEDOT/Graphene Layer by layer (LBL) 12.60 0.77 0.63 6.26
PANI - 12.86 0.683 0.54 4.78 55
PANI/Graphene In-situ 13.28 0.685 0.67 6.09 a Just mixed PEDOT:PSS and graphene (3 wt% ) composites by ultrasonication.
These results demonstrate that the
PEDOT:PSS/graphene composite has a specific
concentration for high performance [23], and our in
situ direct synthesis offers a predominant method for
enhancing the electrical conductivity. Table 1
summarizes the thermoelectric properties of our
results and previously studied conducting
polymer/carbon material hybrid composites
prepared though different combinations and
preparation methods [23,43,44,48-50]. Typically, the
counter electrodes in dye-sensitized solar cells
(DSSCs) have been prepared by platinum (Pt)
vacuum deposition or thermal annealing of the
precursor solution at high temperature. A Pt layer
plays the role of a catalyzer in redox electrolytes
[51,52], however, Pt is expensive and the coating
process is relatively difficult. Therefore, the progress
of finding alternative materials for the development
of Pt-free DSSCs is expected to reduce the
production cost for DSSCs. PEDOT:PSS is one of the
promising conductive polymer for an alternative to
Pt [47] and has an easier coating process than that of
typical conductive polymers [51]. The synthesized
PEDOT:PSS/graphene composite with the highest
conductivity was applied to the cathode of the DSSC,
and the solar cell performance was characterized as
shown in Figure 9d-e. The cell coated with
PEDOT:PSS/G3 showed Jsc of 14.1 mA cm-2 and a cell
efficiency (η) of 5.41 % which were higher than those
of the pristine PEDOT:PSS (η = 4.47 %) and
PEDOT:PSS/mix-G3 (η = 5.03 %) (Table 2). In
addition, our composite films showed a higher
efficiency than the 4.5 % of the PEDOT:PSS/graphene
composite using pyrenebutyrate-functionalized
graphene [53]. Table 2 compares the performances of
Pt-free DSSCs based on conducting
polymer/graphene composite counter electrodes
with various conducting polymer composites of our
results and in the literature [53-55]. This indicates
that the enhanced conductivity [56] and strong
catalytic activity of the graphene [23,24,57-59]
improved the cell efficiency. Thus, the optimized
PEDOT:PSS/graphene composite with high
conductivity by direct synthesis can be an excellent
candidate functional material for an energy
harvesting systems.
3. Conclusions
In summary, we report the direct synthesis of
highly conductive PEDOT:PSS/graphene composites
on a liter-scale by effective dispersion using PSS,
content optimization, in situ polymerization, and a
non-reduction process. We also demonstrate the
applications of the composites in an energy
harvesting system including a thermoelectric device
and a platinum (Pt)-free dye-sensitized solar cell. The
fabricated film showed a conductivity enhancement
of 40.6 % with 3 wt% graphene without any further
complicated reduction processes for the graphene.
The thermoelectric device and the DSSC had better
13
performances when using the optimized
PEDOT:PSS/G3 than that of other composites and
simply mixed composite. In the developments of
highly conductive polymers, our method will help
the synthesis of conductive polymer/carbon
nanomaterial composites. Furthermore, taking
advantage of the mass production, easy process, and
environment-friendly method, our direct synthesis of
PEDOT:PSS/graphene shows expected potential in
low-cost and high-performance optoelectronic
devices. Further optimization of composites by
carbon materials including different kinds of
graphene and CNT and controlled synthesis for high
conductivity and improved performance in devices
will be studied and published in the future.
4. Experimental section
4.1 Materials. Titanium vis(ethyl acetoacetate),
n-butanol, iron (III) sulfate (Fe2(SO4)3, 97 %), sodium
persulfate (Na2S2O8, ≥ 99.0 %),
1-methyl-3-propyl-imidazolium iodide (MPII),
3,4-ethylenedioxythiophene (EDOT), cation
exchange resin, anion exchange resin, and
dimethyl sulfoxide (DMSO) were purchased from
Aldrich Chemicals and used without any further
purification. The pristine nano graphene platelets
(pristine graphene powder, N002-PDR;
conductivity=4.73 S cm-1 measured from a pelletized
film) were purchased from Angstron Materials
(USA). Poly(4-styrene sulfonate) (PSS, Mw: 400,000)
was purchased from AkzoNobel Corporation (USA),
and the TiO2 paste (Ti-Nanoxide T), dye (N719),
electrolyte (Iodolyte AN-50) were purchased from
Solaronix (Switzerland). The FTO (fluorine-doped
tin oxide) glass electrode was purchased from
Pilkington. Co. Ltd. All solvents and chemicals
were used as received.
4.2 Direct Synthesis of PEDOT:PSS/Graphene
Composites. The poly(4-styrene sulfonate) (PSS)
was used as a dispersant and dopant for graphene
and PEDOT, simultaneously. The desired amount
of graphene nanoplatelet powder was directly
mixed with an aqueous PSS solution before the
polymerization of EDOT to prepare the
PEDOT:PSS/graphene composites with the
graphene contents ranging from 1 wt% to 5 wt%.
For the synthesis of the PEDOT:PSS/graphene
composite with 3 wt% of graphene to total solid
content of PSS, the graphene (0.17 g) was added to
the mixed solution of distilled water (1000 g) and
PSS (5.85 g), and then the mixture was stirred for 30
min and ultrasonicated with a tip sonicator in a
10 °C bath for 30 min to prepare a well-dispersed
solution. After the sonication process, the solution
was bubbled using nitrogen gas (99.999 %) for 60
min at a rate of 3 L min-1 to prevent oxidation from
the dissolved oxygen in the water. To this solution,
the EDOT monomer (7.31 g) was added and stirred
by a mechanical stirrer for 30 min. The direct
synthesis of the PEDOT:PSS/graphene composites
was carried out by a Fe3+-catalyzed oxidative
polymerization process. The oxidizing reagents of
iron (III) sulfate (0.21 g, 5.2×10-4 mol) and sodium
persulfate (8.81 g, 3.7×10-2 mol) were dissolved in 30
mL of distilled water by sonication bath,
respectively, and added to the reaction solution.
The polymerization was performed for 24 hr at
10 °C with bubbling nitrogen gas. After the
polymerization of the PEDOT:PSS/graphene, the
product was mixed with 400 mL of a mixture of
cation and anion ion exchange resin for 1 hr and
filtered with 30-μm mesh filter. For comparison, the
pristine PEDOT:PSS solution was synthesized as
described above without the addition of graphene.
4.3 Measurement of thermoelectric properties. To
measure the Seebeck coefficient of the
PEDOT:PSS/graphene composite films, an array of
silver paste was deposited onto the surface of the
PEDOT:PSS/graphene film with a distance of 10 mm
for electrical contact (Figure 9a). This sample was
dried in a vacuum oven at 100 °C for 1 hr. The
14
characterization setup consisted of two Peltier
devices attached to an aluminum heat sink using a
thermal paste to protect against thermal disturbances
and to maintain a controlled temperature gradient.
The current was controlled by using a Keithley 2400
source-meter under a temperature gradient of 5
degrees. Two T type thermocouples were used to
measure the temperature gradient across the
samples.
4.4 Solar Cell Fabrication. A compact TiO2 layer
with a thickness of ~200 nm was prepared by spin
coating of a titanium bis(ethyl acetoacetate) solution
(2 wt% in butanol) onto a FTO (8 Ω/sq) glass
substrate at 2000 rpm for 40 s, followed by
calcination at 450 °C for 30 min. The commercial TiO2
paste (Ti-Nanoxide T) was cast onto the
compact-layer-coated FTO and dried at 50 °C for 1 hr,
followed by successive sintering at 450 °C for 30 min
and cooling to 30 °C for 8 hr. Nanocrystalline TiO2
films of ~9 μm thickness were immersed in the N719
dye solution (0.5 mM in ethanol) for 24 hr at room
temperature. The cathode was prepared by
spin-coating of the conductive polymer/graphene
composite with 5 % DMSO onto a FTO glass
substrate and drying for 30 min at 150 °C. The
photoanode was attached to the counter electrode of
the conductive polymer composite coated cathode
with a hot-melt film (Surlyn, 25 μm) to fabricate a
sandwich-type cell and the assembled cell was filled
with an electrolyte solution. The active area was 0.16
cm2.
4.5 Characterization. The UV-Vis transmittance
spectra of PEDOT:PSS/graphene composite films on
a slide glass was obtained by using a double-beam
UV-Vis spectrophotometer (UV-2101, Shimadzu,
Japan). The FT-IR spectra were obtained by using
TENSOR 27 (Bruker). The binding energy of the
PEDOT:PSS/graphene composite materials was
measured by using an X-ray photoelectron
spectroscopy (XPS, K-alpha, Thermo U.K.) equipped
with a monochromatic AI Kα X-ray source (1486.6
eV). Raman shift spectra of the pristine graphene,
oxidized graphene, and PEDOT:PSS/graphene
composite films were obtained by Raman
spectrometer (LabRam Aramis, Horriba Jovin Yvon)
using a ND:Yag laser source with the wavelength of
532 nm. The surface morphology of the pristine
graphene sheet and PEDOT:PSS/graphene composite
films was imaged with an atomic force microscopy
(AFM; Dimension 3100, Digital Instrument Co.) and
a field-emission scanning electron microscope
(FE-SEM; JSM-6701F, JEOL Ltd.). Inner-morphology
of the PEDOT:PSS/graphene composite film was
imaged by using a transmission electron microscope
(TEM; JEM-2010, JEOL Ltd.) operating at 200 kV. The
PEDOT:PSS/graphene composite solution was mixed
with 5 wt% DMSO and stirred for 1 hr at 50 °C. This
solution was coated by spin-coater and bar coater on
glass or PET film and dried on a hot-plate (30 min,
150 °C). The surface resistivity and thickness of the
PEDOT:PSS/graphene composite films were
measured with a four point probe station (RT-7OV,
NAPSON CORPORATION) and a surface profiler
(alpha step IQ, KLA-Tencor). Photoelectrochemical
characteristics were measured by using an
electrochemical workstation (Keithley Model 2400)
and a solar simulator (1000 W xenon lamp, Oriel,
91193). The light was homogeneous across an 8×8 in2
area and was calibrated with a Si solar cell
(Fraunhofer Institute for Solar Energy System,
Mono-Si+KG filter, Certificate No. C-ISE269) to a sun
light intensity of 1 (100 mW cm-2). This calibration
was confirmed with a NREL-calibrated Si solar cell
(PV Measurements Inc.).
Acknowledgements
We acknowledge the financial support of National
Research Foundation of Korea (NRF) grant funded
by the Korea government(MSIP) (No. 2007-0056091)
and support by Nano Material Technology
Development Program through the National
Research Foundation of Korea(NRF) funded by the
Ministry of Education, Science and Technology
(2012-0006227). Also, This research was supported by
Priority Research Centers Program through the
National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and
Technology (2009-0093823)
15
Electronic Supplementary Material: Supplementary
material (please give brief details, e.g. further details
of the annealing and oxidation procedures, STM
measurements, AFM imaging and Raman
spectroscopy measurements) is available in the
online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-*
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