Article

Nanosphere Templated Continuous PEDOT:PSS Films with Low Percolation Threshold for Application in Efficient Polymer Solar Cells

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST) , Daejeon 305-701, Republic of Korea.
ACS Nano (Impact Factor: 12.88). 08/2012; 6(9):7902-9. DOI: 10.1021/nn3022926
Source: PubMed
ABSTRACT
Nanometer-sized monodisperse polystyrene nanospheres (PS NS) were designed as an opal template for the formation of three-dimensionally continuous poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. The resultant films were successfully applied as the anode buffer layer (ABL) to produce highly efficient polymer solar cells (PSCs) with enhanced stability. The conductivity of the PS NS-PEDOT:PSS films was maintained up to ϕ(PS) = 0.75-0.80, indicating that the formation of continuous PEDOT:PSS films using PS NS templates was successful. To demonstrate the applicability of the PS NS-PEDOT:PSS film for organic electronics, 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 ϕ(PS) = 0.8. In particular, the power conversion efficiency of the P3HT:OXCBA PSC with a PS NS-PEDOT:PSS ABL (ϕ(PS) = 0.8) was greater than 5% and the air stability of the device was significantly enhanced.

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KANG ET AL. VOL. 6
NO. 9
7902 7909
2012
www.acsnano.org
7902
August 11, 2012
C 2012 American Chemical Society
Nanosphere Templated Continuous
PEDOT:PSS Films with Low
Percolation Threshold for Application
in Ecient 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
C
onducting polymers have attracted a
great deal of attention due to their
potential use in many future applica-
tions in low cost, exible, and printable
electronic devices.
19
In particular, poly-
(3,4-ethylenedioxythiophene):poly(styrene
sulfonate) (PEDOT:PSS) has been used com-
mercially in sol i d electrolyte capacitors, anti-
static coatings, and other applications.
10,11
In
addition, PEDOT:PSS polymers are the most
commonly used anode buer 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.
10,1214
However, the acidity and
hygroscopic nature of PEDOT:PSS can lead
to serious degradation of organic electro-
nics.
15,16
For example, the PEDOT:PSS can
etch the ITO lm and cause interface in-
stability via indium diusion into the active
layer.
17,18
In addition, the relatively high
cost and poor mechanical properties of
PEDOT:PS S-base d ABLs could limit the com-
mercialization of the organic electronics.
19,20
To achieve high conductivity, stability,
and low cost, blending the conducting poly-
mer with a low-cost polymer matrix that
can provide the desired mechanical proper-
ties without interfering with the electrical
properties of the conducting polymer is
a promising solution.
2125
The blending ap-
proach requires the conducting polymer
phase to be continuous, the percolation
threshold to be low, and the length scale
to be controlled.
26
For 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
separated.
27,28
The opal template approach,
which is based on self-assembled colloids
with a uniform size distribution, is a power-
ful method of producing continuous con-
ducting polymer lms from small amounts
of conducting polymers without damaging
the electrical properties.
2933
For 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
orders of magnitude.
29
As another example,
Caruso et al. reported the preparation of an
inverse opal-structured polyaniline and poly-
pyrrole using the self-assembled structure of
* Address correspondence to
bumjoonkim@kaist.ac.kr.
Received for review May 23, 2012
and accepted August 11, 2012.
Published online
10.1021/nn3022926
ABSTRACT Nano-
meter-sized monodis-
perse polystyrene nano-
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) lms. The resultant lms were successfully applied as the anode
buer layer (ABL) to produce highly ecient polymer solar cells (PSCs) with enhanced stability.
The conductivity of the PS NS-PEDOT:PSS lms was maintained up to φ
PS
= 0.750.80,
indicating that the formation of continuous PEDOT:PSS lms using PS NS templates was
successful. To demonstrate the applicability of the PS NS-PEDOT:PSS lm for organic electronics,
the PS NS-PEDOT:PSS lms were used as ABLs in two dierent PSCs: P3HT:PCBM and P3HT:
OXCBA. The photovoltaic performances of both PSCs were maintained up to φ
PS
= 0.8. In
particular, the power conversion eciency of the P3HT:OXCBA PSC with a PS NS-PEDOT:PSS ABL
(φ
PS
= 0.8) was greater than 5% and the air stability of the device was signicantly enhanced.
KEYWORDS: nanosphere
.
PEDOT:PSS
.
opal template
.
polymer solar cell
.
air stability
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PS colloidal particles.
34,35
We recently reported a meth-
od for producing continuous conducting poly(triphenyl-
amine) lms with low percolation thresholds using
a PS colloidal template and a g old nanoparticle
compatibilizer.
31
However, to the best o f our knowl-
edge, none of the previous work that used the colloid
template method focused o n thin lm (i.e., less than
100 nm) applications. In contrast, many of the elec-
tronic and/or optical applications require the use of
thin lms 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 exible 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 lms and demonstrate their use as ecient 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 lms
were fabricated using PS NS as opal templates. To
investigate the eects of the PS NS on the electrical
properties of the PS NS-templated PEDOT:PSS lms
(PS NS-PEDOT:PSS), the conductivity and morphologi-
cal behavior of the PS NS-PEDOT:PSS lms were carefully
measured as a function of the PS NS volume fraction
(φ
PS
). Surprisingly, the conductivity of PS NS-PEDOT:PSS
lms and the continuity of the PEDOT:PSS phase were
maintained up to φ
PS
=0.750.80. To further demon-
strate the applicability of the PS NS-PEDOT:PSS lm for
organic electronics, the PS NS-PEDOT:PSS lm was used
as an ABL in two dierent PSCs of poly(3-hexylthio-
phene):phenyl-C
61
-butyric acid methyl ester (P3HT:
PCBM) and poly(3-hexylthiophene):o-xylenyl C
60
bis-
adduct (P3HT:OXCBA). The photovoltaic properties of
both PSCs were maintained up to φ
PS
=0.8.Inparticular,
the P3HT:OXCBA device with a PS NS-PEDOT:PSS ABL
(φ
PS
= 0.8) exhibited a power conversion eciency (PCE)
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 lm, PS NSs were
added to an aqueous solution of PEDOT:PSS. There are
two important requirements for the use of PS NS as a
template in thin PEDOT:PSS lms, which are the follow-
ing: (1) PS NSs should be well dispersed and stabilized
in the same solvent (i.e., water) as the PEDOT:PSS
polymers and (2) the PS NSs should possess a diameter
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
and 70 nm.
36
To meet these requirements, monodisperse
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 φ
PS
of
the PS NS-PEDOT:PSS lms. First, various amounts of
monodisperse PS NSs were added to an aqueous
solution of PEDOT:PSS to produce dierent φ
PS
values.
The concentrations of the polymer mixtures (PS NS þ
PEDOT:PSS) in the water were controlled to produce PS
NS-PEDOT:PSS lms with similar thickness of approxi-
mately 60 nm for all dierent φ
PS
values after spin
coating the solution of the blends for 40 s at 2000 rpm
onto ITO substrates. The lms were thermally annealed
at 150 °C for 20 min to remove water. Figure 1 shows
SEM images of PS NS-PEDOT:PSS lms with dierent
φ
PS
values: (a) PS NSs, (b) pristine PEDOT:PSS (φ
PS
= 0),
(c) PS NS-PEDOT:PSS (φ
PS
= 0.3), (d) PS NS-PEDOT:PSS
(φ
PS
= 0.65), and (e) PS NS-PEDOT:PSS (φ
PS
= 0.8).
Whereas the image of the pristine PEDOT:PSS lm in
Figure 1b shows a homogeneous lm with no contrast,
SEM images of PS NS-PEDOT:PSS lms in Figure 1 c and
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 φ
PS
increased,
the area occupied by the monolayered PS NS assembly
Scheme 1. Schematic illustration of the nanosphere template approach used to produce a three-dimensionally continuous
conducting PEDOT:PSS lm. The use of the lm as an ABL resulted in highly ecient PSCs with enhanced air stability.
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in the blended lm increased. As illustrated in
Figure 1e, at φ
PS
values greater than 0.8, the lm was
completely covered by a monolayer of self-assembled
PS NSs. In contrast with the lms in Figure 1a, void
space was not observed between the PS NSs in
Figure 1e, indicating that the PEDOT:PSS phase inl-
trated the space surrounding the PS NSs.
To provide further evidence that the PEDOT:PSS
polymer inltrated between the PS NSs and formed a
three-dimensionally continuous PEDOT:PSS phase in
the PS NS-PEDOT:PSS lms, the electrical conductivitiy
of PS NS-PEDOT:PSS lms with various φ
PS
values were
measured on patterned ITO/glass substrates. Figure 2
shows the conductivities of PS NS-PEDOT:PSS lms
with various φ
PS
values. All of the conductivity values
were obtained from 60-nm thick lms with an area of
11 2mm
2
, 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 lm (i.e ., without PS
NSs) was measured. The hole conductivity was 4.39
S/cm, which is in good agreement with the previously
reported value of pristine PEDOT:PSS.
10,37
At a high φ
PS
of 0.9, the conductivity was very low (10
2
S/cm). As
the φ
PS
decreased and the PEDOT:PSS volume fraction
(φ
PEDOT:PSS
=1 φ
PS
) increased, the conductivity of the
Figure 1. Surface morphologies of PS NS-PEDOT:PSS lms with dierent φ
PS
values, which were measured by SEM; (a) PS NSs
and the corresponding histogram (inset) of the size distribution, (b) pristine PEDOT:PSS, (c) PS NS-PEDOT:PSS (φ
PS
= 0.3), (d) PS
NS-PEDOT:PSS (φ
PS
= 0.65), and (e) PS NS-PEDOT:PSS (φ
PS
= 0.8). The scale bar represents 1 μm in length.
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blend increased sharply until the φ
PEDOT:PSS
values
reached 0.20.25, which indicated that the φ
PEDOT:PSS
threshold for the percolation of PEDOT:PSS (0.20.25)
resulted in a continuous phase. The observed trend in
the conductivity of the blended lms was consistent
with the morphological behavior, which indicated that
PEDOT:PSS in ltrated 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
and function as a template, the remaining void volume
fraction of 0.26 should be lled by PEDOT:PSS to form a
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 lms increased drama-
tically until φ
PEDOT:PSS
= 0.200.25. Therefore, the
experimental value of φ
PEDOT:PSS
obtained for the con-
tinuity of PEDOT:PSS phase matched the predicted
theoretical value of φ
PEDOT:PSS
= 0.26, which is the
minimum value of φ
PEDOT:PS S
that can maintain a three-
dimensional, continuous minor phase within the mono-
dispersed spheres. Although our measured percolation
threshold showed reasonably good agreement with
the predicted theoretical value, we would like to dis-
cuss the reason for the measured value (φ
PEDOT:PSS
=
0.200.25) being slightly lower than the predicted value
(φ
PEDOT:PSS
= 0.26). First, it could be explained by the
presence of protruded PS NSs in the PS NS-PEDOT:PSS
ABL because of the slight mismatch between the
thickness of the ABL lm 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 °Cthat
wasusedinthefabricationofthedevice.Inthiscase,
the PS NSs can undergo some rearrangement upon
annealing, deforming to polyhedrons wetted by the
PEDOT:PSS phase, and leading to a lower percolation
threshold.
2931
A deeper insight into the eects caused by the φ
PS
value on the morphological and electrical properties of
the PS NS-PEDOT:PSS lms can be obtained by exam-
ining the photovoltaic performance of PSCs with PS
NS-PEDOT:PSS ABL lms with various φ
PS
values. Figure 3
shows the JV curves of the bulk-heterojunction
(BHJ)PSCsasafunctionoftheφ
PS
values under AM 1.5
illumination at 100 mW cm
2
. The BHJ-type PSCs were
fabricated with an identical ITO/ABL/a ctive layer/LiF/Al
structure but dierent φ
PS
values 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.
Although the P3HT:PCBM blend is the most studied PSC
system,
3942
because the lowest unoccupied molecu-
lar orbital (LUMO) of OXCBA is higher than that of
PCBM, P3HT:OXCBA solar cells show higher V
oc
values
and greater PCE of more than 5%.
4345
To investigate
the φ
PS
eects 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 φ
PS
values were dierent for each ABL.
Thermal annealing was performed to optimize the
device performance. Table 1a summarizes the device
characteristics of P3HT:OXCBA BHJ PSCs with dierent
φ
PS
values. For the control ABL composed of pure
PEDOT:PSS (φ
PS
= 0), the device exhibited a PCE of
5.22%, an open circuit voltage (V
OC
) of 0.85 V, a short-
circuit current density (J
SC
) of 10.30 mA cm
2
, and a ll
factor (FF) of 0.60, which is consistent with the pre-
viously reported values.
43
As the φ
PS
increased (i.e., the
φ
PEDOT:PSS
decreased), the PSC performance remained
greater than 5%. Notably, even at φ
PS
= 0.8, the PCE of
the P3HT:OXCBA device was greater than 5%, and the
solar cell parameters including the V
OC
, J
SC
, and FF
values (V
OC
, 0.84 V; J
SC
, 10.49 mA cm
2
; and FF, 0.58)
were not dierent from that of the control sample.
Finally, as φ
PS
increased to 0.9, the PCE value decreased
sharply to 1.05%. The observed trends in the photo-
voltaic performance in terms of φ
PS
values were con-
sistent with those of the conductivity and lm
morphology of PS NS-PEDOT:PSS lms. When φ
PS
is
less than 0.8, PEDOT:PSS can completely ll the empty
spaces between the densely packed PS NSs, creating a
continuous phase that forms a conducting pathway;
therefore, the PS NS-PEDOT:PSS lm can successfully
function as an ABL in the PSCs. In contrast, φ
PS
values
greater than 0.8 resulted in a volume fraction of PEDOT:
PSS that was insucient for the formation of a con-
tinuous pathway for hole conduction.
46
To further con-
rm the trends observed for photovoltaic performance
as a function of the φ
PS
value, P3HT:PCBM-based PSCs
(ITO/ABL/P3HT:PCBM/LiF/Al) were fabricated using PS
Figure 2. Conductivities of the PS NS-PEDOT:PSS lms as a
function of φ
PEDOT:PSS
(= 1 φ
PS
). The scheme illustrates a
conducting pathway in the PS NS-PEDOT:PSS lm.
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NS-PEDOT:PSSs with various φ
PS
values as the ABL.
Figure 3b and Table 1 b compare the device character-
istics of P3HT:PCBM BHJ PSCs with PS NS-PEDOT:P SS ABL s
with various φ
PS
values. All of the devices with φ
PS
values
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 φ
PS
values
(Supporting Information, Figure S2). The devices with
an ITO/ABL/P3HT:OXCBA/Au structure were fabri-
cated, resulting in a hole-only device.
47
SCLC devices
containing PS NS-PEDOT:PSS ABLs with φ
PS
values
ranging from 0 to 0.8 had similar hole mobility values
of 10
4
cm
2
V
1
s
1
(i.e., at φ
PS
= 0, hole mobility = 2.8
10
4
cm
2
V
1
s
1
;atφ
PS
= 0.8, hole mobility = 1.4
10
4
cm
2
V
1
s
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.
39,40,4749
In contrast, the SCLC device with φ
PS
=
0.9 had a greatly reduced hole mobility value of 8.4
10
6
cm
2
V
1
s
1
. The amount of PEDOT:PSS with φ
PS
=
0.9 in the PS NS-PEDOT:PSS lm was insucient for the
formation of a continuous network of PEDOT phase
within the lm, 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
observed trend for the SCLC mobility was consistent with
the trends observed for the electrical conductivity and
PSC performance.
Although the air stability of PSCs is of great impor-
tance for commercialization, the acidity and hygro-
scopic nature of PEDOT:PSS can negatively aect the
stability of organic electronics.
15,16
Therefore, the ef-
fects of the φ
PS
value on the stability of P3HT:OXCBA
devices were investigated because each PS NS-PEDOT:
PSS ABL lm contained a dierent amount of PEDOT:
PSS. To compare the stabilities of PSC devices with
dierent φ
PS
values, variations in device performance
were measured as a function of the storage time under
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 densityvoltage (JV) characteristics of
PSCs with various φ
PS
values for PS NS-PEDOT:PSS ABLs
(φ
PS
= 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 O
PS
values as the ABL
under AM1.5 G-simulated solar illumination (100 mW cm
2
)
active layer φ
PS
V
OC
J
SC
FF PCE (%)
(a)
P3HT:OXCBA 0 0.85 10.30 0.60 5.22
P3HT:OXCBA 0.3 0.85 10.36 0.60 5.30
P3HT:OXCBA 0.4 0.83 10.44 0.58 5.06
P3HT:OXCBA 0.5 0.84 10.27 0.60 5.12
P3HT:OXCBA 0.65 0.84 10.32 0.60 5.19
P3HT:OXCBA 0.8 0.84 10.49 0.58 5.12
P3HT:OXCBA 0.9 0.65 7.31 0.22 1.05
(b)
P3HT:PCBM 0 0.65 8.42 0.61 3.35
P3HT:PCBM 0.3 0.65 8.56 0.63 3.52
P3HT:PCBM 0.4 0.64 8.18 0.66 3.46
P3HT:PCBM 0.5 0.65 8.40 0.64 3.48
P3HT:PCBM 0.65 0.64 8.48 0.64 3.46
P3HT:PCBM 0.8 0.64 8.29 0.63 3.35
Figure 4. Air stability tests of P3HT:OXCBA PSCs with
various φ
PS
values in the ABL under ambient conditions.
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trend can be observed in Figure 4, which shows an
enhancement in the air stability of the PSCs as the φ
PS
values increased. For example, the PCE of the control
device (φ
PS
= 0) decayed by 45% after being exposed
to air for 7000 min without encapsulation. In contrast,
the PSC with PS NS-PEDOT:PSS (φ
PS
= 0.8) exhibited
much greater air stability and 86% of the initial PCE was
retained under the same measurement conditions. The
improved stability was attributed mainly to the PS NSs,
which reduced the degradation of the photoactive and
ITO layers by the acidic conditions and the hygroscopic
nature caused by the PEDOT:PSS. In addition, it is ex-
pected that, after proper surfactants are introduced into
the ABL, lower volume fractions (φ
PEDOT: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.
30,31,5052
CONCLUSIONS
We have successfully developed a simple and e-
cient method for the fabrication of semiconducting
polymer blend thin lms that combined the properties
of electrical conductivity and enhanced stability. PS
NS-PEDOT:PSS lms 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 lms were successfully applied as ecient
and stable ABLs in PSCs. The eects of the addition of
PS NSs with various φ
PS
values 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 lm and the
continuity of the PEDOT:PSS phase in the lm were
maintained, even at a low φ
PEDOT:PSS
of 0.20.25. This
number was in good agreement with the void space
fraction between hexagonally close packed spheres,
which indicated that a continuous PEDOT:PSS lm was
successfully formed using the PS NS template. The
trend of the photovoltaic performance in terms of the
φ
PS
values was consistent with those of the conductiv-
ity and lm morphology of the PS NS-PEDOT:PSS lms.
P3HT:OXCBA devices with a PS NS-PEDOT:PSS as the
ABL (φ
PS
= 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-
ing polymer blends with high conductivity and stability
and low cost.
METHODS
Materials. Commercial PEDOT:PSS in water (PH 500, BASF),
P3HT (P200, BASF) and PCBM (Nano-C) were used without
further purification. The o-xylenyl C
60
bis-adduct (OXCBA) was
synthesized and purified as described in our previous work.
43
Synthesis of the PS NSs. PS NSs were synthesized via an emulsion
polymerization using a modified procedure reported previously.
53,54
Styrene monomer was purified with an aluminum oxide column.
The other organic reagents were used without further purification,
which included poly(vinylpyrrolidone) (PVP) (M
w
= 55 000 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
stirring, AIBN was added to the solution. The reaction temperature
was gradually increased to 70 °C. After being heated for 24 h at
70 °C, the mixture was cooled to room temperature. The resultant
PS NSs were filtered and repeatedly washed by being centri-
fuged in deionized water to remove the residual styrene and
PVP. The samples were dried in a vacuum oven at 50 °C for 12 h.
The size and size distribution of the PS NSs were measured using
a Hitachi S-4800 scanning electron microscope.
Sample Preparation and Conductivity Measurements. Solutions of
PS NSs and PEDOT:PSS blends with different φ
PS
values were
prepared by mixing aqueous solutions of PS NSs and PEDOT:PSS
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 soluti on was sonicated for
30 min at 25 °C prior to being spin coated. The films wereannealed
at 150 °C for 20 min to remove water. To assess the conductivity,
two parallel electrodes of carbon paste were applied to the films
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
calculations accounted for the geometry of the samples (11 mm
long and 2 mm wide).
Device Fabrication and Measurements. To investigate the proper-
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-
strates were subjected to ultrasonication in acetone followed by
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 φ
PS
values 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 N
2
atmosphere.
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)
stabilized the PS NSs during short-term exposures to o-dichloro-
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
6
Torr)
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
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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 devices were characterized using a solar simulator (Newport
Oriel Solar Simulators) with air mass 1.5 G lters. The intensity of
the solar simulator was carefully calibrated using an AIST-
certied silicon photodiode. The currentvoltage behavior
was measured using a Keithley 2400 SMU. The active area of
the fabricated devices was 0.10 cm
2
.
Conflict of Interest: The authors declare no competing
nancial interest.
Acknowledgment. This research was supported by the Korea
Research Foundation Grant, funded by the Korean Government
(2011-0017943).
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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ARTICLE
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  • Source
    [Show abstract] [Hide abstract] ABSTRACT: The randomly nanotextured back electrode provides a simple and efficient route for enhancing photocurrent in polymer solar cells (PSCs) by light trapping, which can increase light absorption within a finite thickness of the active layer. In this study, we incorporated mono-disperse 60 nm polystyrene nanoparticles (PS NPs) into a 50 nm thick poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) anode buffer layer (ABL) to create a randomly nanotextured back electrode with 10 nm height variations in inverted-type PSCs. The roughened interface between the PS NP-PEDOT:PSS ABL and the Ag electrode scatters light in the visible range, leading to efficient light trapping within the device and enhanced light absorption in the active layer. Inverted PSCs with randomly nanotextured electrodes (ϕ(NP) = 0.31) showed short-circuit current density (J(SC)) and power conversion efficiency (PCE) values that were 15% higher than those of control devices with flat electrodes. External quantum efficiency, reflectance, and optical light scattering as a function of ϕ(NP) were examined to determine the origin of the enhancement in J(SC) and PCE.
    Full-text · Article · Jan 2013 · Nanoscale
  • [Show abstract] [Hide abstract] ABSTRACT: Fullerene tris-adducts have the potential of achieving high open-circuit voltages (VOC) in bulk heterojunction (BHJ) polymer solar cells (PSCs), because their lowest unoccupied molecular orbital (LUMO) level is higher than those of fullerene mono- and bis-adducts. However, no successful examples of the use of fullerene tris-adducts as electron acceptors have been reported. Herein, we developed a ternary-blend approach for the use of fullerene tris-adducts to fully exploit the merit of their high LUMO level. The compound o-xylenyl C60 tris-adduct (OXCTA) was used as a ternary acceptor in the model system of poly(3-hexylthiophene) (P3HT) as the electron donor and the two soluble fullerene acceptors of OXCTA and fullerene mono-adduct (o-xyenyl C60 mono-adduct (OXCMA), phenyl C61-butyric acid methyl ester (PCBM) or indene-C60 mono-adduct (ICMA)). To explore the effect of OXCTA in ternary-blend PSC devices, the photovoltaic behavior of the device was investigated in terms of the weight fraction of OXCTA (WOXCTA). When WOXCTA is small (< 0.3), OXCTA can generate a synergistic bridging effect between P3HT and the fullerene mono-adduct, leading to simultaneous enhancement in both VOC and short-circuit current (JSC). For example, the ternary PSC device of P3HT:(OXCMA:OXCTA) with WOXCTA of 0.1 and 0.3 exhibited power-conversion efficiencies (PCEs) of 3.91% and 3.96%, respectively, which were significantly higher than the 3.61% provided by the P3HT:OXCMA device. Interestingly, for WOXCTA > 0.7, both VOC and PCE of the ternary-blend PSCs exhibited non-linear compositional dependence on WOXCTA. We noted that the non-linear compositional trend of P3HT:(OXCMA:OXCTA) was significantly different from that of P3HT:(OXCMA:o-xyenyl C60 bis-adduct (OXCBA)) ternary-blend PSC devices. The fundamental reasons for the differences between the photovoltaic trends of the two different ternary-blend systems were investigated systemically by comparing their optical, electrical, and morphological properties.
    No preview · Article · Apr 2013 · ACS Applied Materials & Interfaces
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: Hollow microflower arrays (HMFAs) of poly(3,4-ethylenedioxythiophene) (PEDOT) with several two dimensional hollow nanopetals on each microflower are fabricated on a conducting glass by using ZnO microflower arrays as the template. Various charges are applied for the electrodeposition of the film of PEDOT-HMFAs, intending to investigate their effect on the film's morphology. A morphological variation is observed due to the swelling of PEDOT during the removal of the ZnO template. Cyclic voltammetry (CV) is used to optimize the charges for the deposition of the films of both a flat PEDOT and the PEDOT-HMFAs. Long-term stability of the films in an I−/I3− electrolyte is studied by CV. The PEDOT-HMFA film shows a better stability than those of the films of flat PEDOT and sputtered Pt. The PEDOT-HMFA film is employed as the catalytic material on the counter electrode (CE) of a dye-sensitized solar cell (DSSC). A power conversion efficiency of 7.20% is achieved, at 100 mW cm−2 for the DSSC with the film of PEDOT-HMFAs, which is much higher than that of the cell with the flat PEDOT (6.39%) and comparable to that of the cell with a sputtered Pt film on its CE (7.61%). Electrochemical impedance spectroscopy is used to substantiate the photovoltaic parameters. “Hemispherical diffusion of ions” occurs on each PEDOT hollow microflower of the CE with PEDOT-HMFAs in the DSSC, as against the linear diffusion occurring on the CE with flat PEDOT or sputtered Pt. This type of “hemispherical diffusion of ions” is explained to result in a smaller diffusion resistance of ions and thereby in a much higher fill factor for the DSSC using the CE with PEDOT-HMFAs.
    Full-text · Article · Sep 2013 · Journal of Materials Chemistry A
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