KANG ET AL.
’ NO. 9
August 11, 2012
C2012 American Chemical Society
Nanosphere Templated Continuous
PEDOT:PSS Films with Low
Percolation Threshold for Application
in Efficient Polymer Solar Cells
Dong Jin Kang,†Hyunbum Kang,†Ki-Hyun Kim,†and Bumjoon J. Kim†,*
†Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea
tions in low cost, flexible, and printable
electronic devices.1?9In particular, poly-
sulfonate) (PEDOT:PSS) has been used com-
mercially in solid electrolyte capacitors, anti-
static coatings, and other applications.10,11In
addition, PEDOT:PSS polymers are the most
commonly used anode buffer layers (ABLs)
in solution-processed organic electronics
(e.g., polymer solar cells (PSCs) and organic
light-emitting diodes (OLED) due to their
high electrical conductivity and water
solubility.10,12?14However, the acidity and
hygroscopic nature of PEDOT:PSS can lead
to serious degradation of organic electro-
nics.15,16For example, the PEDOT:PSS can
etch the ITO film and cause interface in-
stability via indium diffusion into the active
layer.17,18In addition, the relatively high
cost and poor mechanical properties of
PEDOT:PSS-based ABLs could limit the com-
mercialization of the organic electronics.19,20
To achieve high conductivity, stability,
mer with a low-cost polymer matrix that
canprovide the desired mechanical proper-
ties without interfering with the electrical
properties of the conducting polymer is
apromising solution.21?25The blending ap-
proach requires the conducting polymer
phase to be continuous, the percolation
threshold to be low, and the length scale
to be controlled.26For instance, simple
blending usually does not allow the degree
of morphological control that ensures a
continuous conducting polymer because
polymer blends tend to be macrophase
which is based on self-assembled colloids
pyrrole usingthe self-assembled structure of
great deal of attention due to their
potential use in many future applica-
with a uniform size distribution, is a power-
ful method of producing continuous con-
ducting polymer films from small amounts
of conducting polymers without damaging
the electrical properties.29?33For example,
Kramer et al. reported a high internal poly-
meric phase emulsion system of a doped
conducting polyaniline-phenolsulfonate poly-
mer driven by self-assembled micrometer-
sized polystyrene (PS) colloids in the pres-
ence of a block copolymer. Compared to
traditional blending procedures, this pro-
cess reduces the percolation threshold for
electrical conductivity by a factor of 10
and increases the conductivity by several
Caruso et al. reported the preparation of an
*Address correspondence to
Received for review May 23, 2012
and accepted August 11, 2012.
spheres (PS NS) were
designed as an opal
template for the for-
mation of three-dimensionally continuous poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate) (PEDOT:PSS) films. The resultant films were successfully applied as the anode
The conductivity of the PS NS-PEDOT:PSS films was maintained up to φPS= 0.75?0.80,
indicating that the formation of continuous PEDOT:PSS films using PS NS templates was
the PS NS-PEDOT:PSS films were used as ABLs in two different PSCs: P3HT:PCBM and P3HT:
OXCBA. The photovoltaic performances of both PSCs were maintained up to φPS= 0.8. In
KEYWORDS: nanosphere.PEDOT:PSS.opal template.polymer solar cell.
KANG ET AL.
’ NO. 9
od for producing continuous conducting poly(triphenyl-
amine) films with low percolation thresholds using
a PS colloidal template and a gold nanoparticle
compatibilizer.31However, to the best of our knowl-
template method focused on thin film (i.e., less than
100 nm) applications. In contrast, many of the elec-
tronic and/or optical applications require the use of
thin films that are less than 100 nm in thickness. In
particular, while conducting PEDOT:PSS polymers are
the most commonly used ABLs in solution-processed
organic electronics and flexible devices, the optimal
thickness of the ABL is typically less than 100 nm.36
In the present study, we develop a facile method for
creating nanosphere-templated continuous PEDOT:
PSS films and demonstrate their use as efficient and
stable ABLs in PSCs (Scheme 1). First, monodisperse,
60-nm sized PS nanospheres (PS NSs) were designed
and synthesized by emulsion polymerization in water.
The highly stable and dispersible properties of PS NSs
in water enable the PS NSs to be easily blended with
PEDOT:PSS. Therefore, continuous PEDOT:PSS films
were fabricated using PS NS as opal templates. To
investigate the effects of the PS NS on the electrical
properties of the PS NS-templated PEDOT:PSS films
(PS NS-PEDOT:PSS), the conductivity and morphologi-
measured as a function of the PS NS volume fraction
(φPS). Surprisingly, the conductivity of PS NS-PEDOT:PSS
films and the continuity of the PEDOT:PSS phase were
maintained up to φPS= 0.75?0.80. To further demon-
strate the applicability of the PS NS-PEDOT:PSS film for
as an ABL in two different PSCs of poly(3-hexylthio-
phene):phenyl-C61-butyric acid methyl ester (P3HT:
PCBM) and poly(3-hexylthiophene):o-xylenyl C60bis-
adduct (P3HT:OXCBA). The photovoltaic properties of
the P3HT:OXCBA device with a PS NS-PEDOT:PSS ABL
of greater than 5%, and the air stability of the device
increased by a factor of 3.
RESULTS AND DISCUSSION
To create a continuous conducting PEDOT:PSS
phase at a low concentration in the film, PS NSs were
added to an aqueous solution of PEDOT:PSS. There are
two important requirements for the use of PS NS as a
ing: (1) PS NSs should be well dispersed and stabilized
in the same solvent (i.e., water) as the PEDOT:PSS
less than 100 nm with a very narrow size distribution
because the optimal thickness of a PEDOT:PSS ABL for
PSCs and OLEDs is typically reported to be between 50
PS NSs were carefully designed and synthesized via
emulsion polymerization in water, and the reaction con-
ditions (e.g., stabilizer concentration) were controlled. A
washing procedure (i.e., several cycles of centrifugation in
deionized water) was used to purify the synthesized PS
NSs, which resulted in 60-nm diameter PS NSs with
excellent monodispersity, as shown in Figure 1a.
The morphological behavior of the PS NS-PEDOT:
PSS blends was investigated as a function of the φPSof
the PS NS-PEDOT:PSS films. First, various amounts of
monodisperse PS NSs were added to an aqueous
solution of PEDOT:PSS to produce different φPSvalues.
The concentrations of the polymer mixtures (PS NS þ
NS-PEDOT:PSS films with similar thickness of approxi-
mately 60 nm for all different φPSvalues after spin
coating the solution of the blends for 40 s at 2000 rpm
at 150 ?C for 20 min to remove water. Figure 1 shows
SEM images of PS NS-PEDOT:PSS films with different
φPSvalues: (a) PS NSs, (b) pristine PEDOT:PSS (φPS= 0),
(c) PS NS-PEDOT:PSS (φPS= 0.3), (d) PS NS-PEDOT:PSS
(φPS= 0.65), and (e) PS NS-PEDOT:PSS (φPS= 0.8).
Whereas the image of the pristine PEDOT:PSS film in
d clearly show the presence of PS NSs without macro-
phase separation or aggregation of the PS NSs on the
micrometer scale. Additionally, as the φPSincreased,
Scheme 1. Schematic illustration of the nanosphere template approach used to produce a three-dimensionally continuous
conducting PEDOT:PSS film. The use of the film as an ABL resulted in highly efficient PSCs with enhanced air stability.
KANG ET AL.
’ NO. 9
in the blended film increased. As illustrated in
Figure 1e, at φPSvalues greater than 0.8, the film was
completely covered by a monolayer of self-assembled
PS NSs. In contrast with the films in Figure 1a, void
space was not observed between the PS NSs in
Figure 1e, indicating that the PEDOT:PSS phase infil-
trated the space surrounding the PS NSs.
To provide further evidence that the PEDOT:PSS
polymer infiltrated between the PS NSs and formed a
three-dimensionally continuous PEDOT:PSS phase in
the PS NS-PEDOT:PSS films, the electrical conductivitiy
of PS NS-PEDOT:PSS films with various φPSvalues were
measured on patterned ITO/glass substrates. Figure 2
shows the conductivities of PS NS-PEDOT:PSS films
with various φPSvalues. All of the conductivity values
were obtained from 60-nm thick films with an area of
11 ? 2 mm2, which were prepared under identical
conditions to the samples used in the morphological
studies shown in Figure 1. As a control, the hole
conductivity of a pure PEDOT:PSS film (i.e., without PS
NSs) was measured. The hole conductivity was 4.39
S/cm, which is in good agreement with the previously
of 0.9, the conductivity was very low (∼10?2S/cm). As
the φPSdecreased and the PEDOT:PSS volume fraction
Figure1. Surfacemorphologiesof PSNS-PEDOT:PSS filmswithdifferentφPSvalues,whichweremeasuredbySEM;(a)PS NSs
NS-PEDOT:PSS (φPS= 0.65), and (e) PS NS-PEDOT:PSS (φPS= 0.8). The scale bar represents 1 μm in length.
KANG ET AL.
’ NO. 9
blend increased sharply until the φPEDOT:PSS values
reached 0.2?0.25, which indicated that the φPEDOT:PSS
threshold for the percolation of PEDOT:PSS (0.2?0.25)
resulted in a continuous phase. The observed trend in
the conductivity of the blended films was consistent
with the morphological behavior, which indicated that
PEDOT:PSS infiltrated into the empty spaces between
the densely packed PS NSs and created a continuous
phase that formed an electrical pathway. According to
sphere-packing theory, the maximum volume fraction
for packing monodisperse spheres into a hexagonally
close-packed or face-centered cubic lattice is 0.74.38
Therefore, if the PS NSs form a close-packed structure
continuous PEDOT:PSS phase between the PS NS
template. Evidence of the close-packed structure of
PS NSs can be clearly observed in the SEM image
shown in Figure 1e. In addition, the electrical conduc-
tivity of the PS NS-PEDOT:PSS films increased drama-
tically until φPEDOT:PSS = 0.20?0.25. Therefore, the
experimental value of φPEDOT:PSSobtained for the con-
tinuity of PEDOT:PSS phase matched the predicted
theoretical value of φPEDOT:PSS= 0.26, which is the
minimum value of φPEDOT:PSSthat can maintain a three-
threshold showed reasonably good agreement with
the predicted theoretical value, we would like to dis-
cuss the reason for the measured value (φPEDOT:PSS=
0.20?0.25) being slightly lower than the predicted value
(φPEDOT:PSS= 0.26). First, it could be explained by the
presenceofprotruded PS NSs in the PS NS-PEDOT:PSS
ABL because of the slight mismatch between the
thickness of the ABL film and the size of the PS NSs
(Supporting Information, Figure S1). More importantly,
the morphology of the system can reorganize upon
thermal annealing at the temperature of 150 ?C that
was used in the fabrication of the device. In this case,
the PS NSs can undergo some rearrangement upon
annealing, deforming to polyhedrons wetted by the
PEDOT:PSS phase, and leading to a lower percolation
A deeper insight into the effects caused by the φPS
the PS NS-PEDOT:PSS films can be obtained by exam-
ining the photovoltaic performance of PSCs with PS
shows the J?V curves of the bulk-heterojunction
(BHJ) PSCs as a function of the φPSvalues under AM 1.5
illumination at 100 mW cm?2. The BHJ-type PSCs were
fabricated with an identical ITO/ABL/active layer/LiF/Al
structure but different φPSvalues ranging from 0 to 0.9
in the PS NS-PEDOT:PSS ABL. For the active layer of
the PSCs, P3HT was used as an electron donor, and
OXCBA or PCBM was applied as an electron acceptor.
system,39?42because the lowest unoccupied molecu-
lar orbital (LUMO) of OXCBA is higher than that of
PCBM, P3HT:OXCBA solar cells show higher Vocvalues
and greater PCE of more than 5%.43?45To investigate
the φPSeffects on the performance of the PSCs, the
P3HT:OXCBA devices were prepared under identical
conditions, including the same blend ratio of P3HT to
OXCBA (1:0.6, w/w) and solvent concentration; how-
ever, the φPS values were different for each ABL.
Thermal annealing was performed to optimize the
device performance. Table 1a summarizes the device
characteristics of P3HT:OXCBA BHJ PSCs with different
φPSvalues. For the control ABL composed of pure
PEDOT:PSS (φPS= 0), the device exhibited a PCE of
5.22%, an open circuit voltage (VOC) of 0.85 V, a short-
factor (FF) of 0.60, which is consistent with the pre-
viously reported values.43As the φPSincreased (i.e., the
φPEDOT:PSSdecreased), the PSC performance remained
greater than 5%. Notably, even at φPS= 0.8, the PCE of
the P3HT:OXCBA device was greater than 5%, and the
solar cell parameters including the VOC, JSC, and FF
values (VOC, 0.84 V; JSC, 10.49 mA cm?2; and FF, 0.58)
were not different from that of the control sample.
sharply to 1.05%. The observed trends in the photo-
voltaic performance in terms of φPSvalues were con-
sistent with those of the conductivity and film
morphology of PS NS-PEDOT:PSS films. When φPSis
less than 0.8, PEDOT:PSS can completely fill the empty
spaces between the densely packed PS NSs, creating a
continuous phase that forms a conducting pathway;
therefore, the PS NS-PEDOT:PSS film can successfully
function as an ABL in the PSCs. In contrast, φPSvalues
PSS that was insufficient for the formation of a con-
tinuous pathway for hole conduction.46To further con-
firm the trends observed for photovoltaic performance
as a function of the φPSvalue, P3HT:PCBM-based PSCs
(ITO/ABL/P3HT:PCBM/LiF/Al) were fabricated using PS
Figure 2. Conductivities of the PS NS-PEDOT:PSS films as a
function of φPEDOT:PSS(= 1 ? φPS). The scheme illustrates a
conducting pathway in the PS NS-PEDOT:PSS film.
KANG ET AL.
’ NO. 9
NS-PEDOT:PSSs with various φPS values as the ABL.
Figure 3b and Table 1 b compare the device character-
with various φPSvalues. All of the devices with φPSvalues
between 0 and 0.8 showed similar performance (PCE =
3.4%). This trend was in good agreement with that
obtained for the P3HT:OXCBA system.
To gain a deeper insight into the performance of PS
NS-PEDOT:PSS ABLs in the PSCs, the space charge-
limited current (SCLC) hole mobility was measured
for devices containing ABLs with various φPSvalues
(Supporting Information, Figure S2). The devices with
an ITO/ABL/P3HT:OXCBA/Au structure were fabri-
cated, resulting in a hole-only device.47SCLC devices
containing PS NS-PEDOT:PSS ABLs with φPSvalues
ranging from 0 to 0.8 had similar hole mobility values
10?4cm2V?1s?1; at φPS= 0.8, hole mobility = 1.4 ?
10?4cm2V?1s?1). All of the hole mobility values were
in good agreement with the values reported for a
P3HT:OXCBA device that used pure PEDOT:PSS as the
ABL.39,40,47?49In contrast, the SCLC device with φPS=
0.9 had a greatly reduced hole mobility value of 8.4 ?
10?6cm2V?1s?1. The amount of PEDOT:PSS with φPS=
0.9 in the PS NS-PEDOT:PSS film was insufficient for the
formation of a continuous network of PEDOT phase
within the film, which resulted in lower SCLC hole
mobility and hole-injection ability in the ABL and a
decrease in the performance of the PSCs. And, the
the trends observed for the electrical conductivity and
Although the air stability of PSCs is of great impor-
tance for commercialization, the acidity and hygro-
scopic nature of PEDOT:PSS can negatively affect the
stability of organic electronics.15,16Therefore, the ef-
fects of the φPSvalue on the stability of P3HT:OXCBA
devices were investigated because each PS NS-PEDOT:
PSS ABL film contained a different amount of PEDOT:
PSS. To compare the stabilities of PSC devices with
different φPSvalues, variations in device performance
ambient conditions, as shown in Figure 4. The unen-
capsulated devices were exposed to an atmosphere of
air for over 7000 min at room temperature. A clear
Figure 3. Current density?voltage (J?V) characteristics of
PSCs with various φPSvalues for PS NS-PEDOT:PSS ABLs
(φPS= 0, 0.3, 0.4, 0.5, 0.65, 0.8, and 0.9): (a) P3HT:OXCBA
and (b) P3HT:PCBM BHJ PSCs under AM 1.5 illumination at
100 mW cm?2.
TABLE 1. Device characteristics of (a) P3HT:OXCBA- and
(b) P3HT:PCBM-based BHJ-type PSCs using PEDOT:PSS
and PS NS-PEDOT:PSS with various OPSvalues as the ABL
FF PCE (%)
Figure 4. Air stability tests of P3HT:OXCBA PSCs with
various φPSvalues in the ABL under ambient conditions.
KANG ET AL.
’ NO. 9
trend can be observed in Figure 4, which shows an
enhancement in the air stability of the PSCs as the φPS
values increased. For example, the PCE of the control
to air for 7000 min without encapsulation. In contrast,
the PSC with PS NS-PEDOT:PSS (φPS= 0.8) exhibited
improved stability wasattributed mainly to the PS NSs,
nature caused by the PEDOT:PSS. In addition, it is ex-
pected that, after proper surfactants are introduced into
the ABL, lower volume fractions (φPEDOT:PSS< 0.26) of
PEDOT:PSS for the percolation threshold of the contin-
uous conducting domain and further improvement in
the stability of the PSCs can be achieved.30,31,50?52
We have successfully developed a simple and effi-
cient method for the fabrication of semiconducting
polymer blend thin filmsthat combined the properties
of electrical conductivity and enhanced stability. PS
NS-PEDOT:PSS films were designed and produced by
forcing the conducting PEDOT:PSS polymers into a
three-dimensionally continuous minor phase via the
self-assembly of a monolayer of 60-nm PS NSs. The
resultant films were successfully applied as efficient
and stable ABLs in PSCs. The effects of the addition of
PS NSs with various φPSvalues on the properties of the
PS NS-PEDOT:PSS layer were investigated, including
the conductivity and morphological behavior. The
conductivity of the PS NS-PEDOT:PSS film and the
continuity of the PEDOT:PSS phase in the film were
maintained, even at a low φPEDOT:PSSof 0.2?0.25. This
number was in good agreement with the void space
fraction between hexagonally close packed spheres,
which indicated that a continuous PEDOT:PSS film was
successfully formed using the PS NS template. The
trend of the photovoltaic performance in terms of the
φPSvalues was consistent with those of the conductiv-
ity and film morphology of the PS NS-PEDOT:PSS films.
P3HT:OXCBA devices with a PS NS-PEDOT:PSS as the
ABL (φPS= 0.8) exhibited PCEs of over 5% with dramat-
ically enhanced air stability. Therefore, this approach is
a promising route for the production of semiconduct-
and low cost.
Materials. Commercial PEDOT:PSS in water (PH 500, BASF),
P3HT (P200, BASF) and PCBM (Nano-C) were used without
further purification. The o-xylenyl C60bis-adduct (OXCBA) was
synthesized and purified as described in our previous work.43
Synthesis ofthe PSNSs. PSNSsweresynthesizedviaanemulsion
Styrene monomer was purified with an aluminum oxide column.
which included poly(vinylpyrrolidone) (PVP) (Mw= 55000 g/mol),
and 2,2-azobis-(isobutyronitrile) (AIBN). The PVP and styrene
monomer were dissolved in deionized water in a 500-mL, three-
neck flask and the mixture was stirred at 40 ?C. After 30 min of
was gradually increased to 70 ?C. After being heated for 24 h at
PS NSs were filtered and repeatedly washed by being centri-
fuged in deionized water to remove the residual styrene and
a Hitachi S-4800 scanning electron microscope.
Sample Preparation and Conductivity Measurements. Solutions of
PS NSs and PEDOT:PSS blends with different φPSvalues were
with various ratios. The volume fraction of polymers (PS NSs þ
PEDOT:PSS) in the solution was controlled to produce approxi-
mately 60-nm-thick films after the solutions were spin coated
onto clean glass substrate. Each solution was sonicated for
at 150 ?C for 20 min to remove water. To assess the conductivity,
using an adhesive mask. The measurements were performed
using a two probe method at room temperature, and the
voltage was supplied by a Keithley 2400 Source Meter, which
ranged from ?10 to 10 V across the sample. The conductivity
long and 2 mm wide).
ties of the newly designed ABLs that consisted of PS NSs and
PEDOT:PSS, BHJ photovoltaic cells using an ITO/ABL/P3HT:
electron acceptor/LiF/Al structure were fabricated. The P3HT
was used as an electron donor, while the PCBM and OXCBA
were used as electron acceptors. The ITO-coated glass sub-
2% Helmanex soap in water. After the substrates were rinsed
extensively with deionized water, they were ultrasonicated in
deionized water followed by isopropyl alcohol. Subsequently,
the substrates were dried for several hours in an oven at 80 ?C.
Aqueous solutions of PS NSs and PEDOT:PSS blends were
prepared with various φPSvalues that ranged from 0 to 0.9.
Each solution was spin coated onto a clean ITO substrate to
produce approximately 60 nm-thick films. The films were
annealed at 150 ?C for 20 min to remove water. After the PS
NS-PEDOT:PSS layer was applied, all of the subsequent proce-
dures were performed in a glovebox under an N2atmosphere.
Solutions of P3HT, PCBM, and OXCBA were prepared in
o-dichlorobenzene and stirred at 100 ?C overnight to ensure
complete dissolution of the materials. Immediately prior to
deposition, the solutions were passed through a 0.2-μm poly-
tetrafluoroethylene syringe filter. For the P3HT:OXCBA and
P3HT:PCBM devices, the solutions of P3HT:OXCBA (1:0.6 w/w)
and P3HT:PCBM (1:0.7 w/w) blends were stirred at room tem-
perature for 1 h, and then spin coated onto the ITO/PS NS-
PEDOT:PSS ABL substrates at 900 rpm. The spincoating step of
the P3HT/acceptor film was conducted immediately after cast-
ing the P3HT/acceptor solution in o-dichlorobenzene onto the
ITO/PS NS-PEDOT:PSS ABL substrate. The surfactants (PVP)
benzene at the device-fabrication conditions (Supporting
Information, Figure S3). The P3HT/acceptor film was dried and
the thicknesses of films were determined to be approximately
∼100 nm. Then, the substrates were placed in an evaporation
chamber and held under high vacuum (i.e., less than 10?6Torr)
for more than 1 h before a layer of LiF (approximately 0.7 nm)
and Al (100 nm) were deposited on the substrate. The config-
uration of the shadow mask afforded four independent devices
on each substrate. After the devices were fabricated, thermal
annealing was performed for 5 min at 150 ?C to optimize the
KANG ET AL.
’ NO. 9
morphology of the BHJ active layer and improve the device
performance by promoting polymer self-organization.
FE-SEM and TEM studies were performed to observe the
morphology of the ABL samples using a Hitachi S-4800 and a
JEOL 2000FX, respectively. The photovoltaic performances of
the solar simulator was carefully calibrated using an AIST-
certified silicon photodiode. The current?voltage behavior
was measured using a Keithley 2400 SMU. The active area of
the fabricated devices was 0.10 cm2.
Conflict of Interest: The authors declare no competing
Acknowledgment. This researchwassupported by theKorea
Research Foundation Grant, funded by the Korean Government
Supporting Information Available: Additional SEM/AFM
images and SCLC mobility curves. This material is available free
of charge via the Internet at http://pubs.acs.org.
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