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Nano Res
1
Solution-Processed Copper Nanowire Flexible
Transparent Electrodes with PEDOT:PSS as Binder,
Protector and Oxide-Layer Scavenger for Polymer
Solar Cells
Jianyu Chen,1 Weixin Zhou,1 Jun Chen,1 Yong Fan,1 Ziqiang Zhang,1 Zhendong Huang,1 Xiaomiao
Feng,*1() Baoxiu Mi,1 Yanwen Ma, *1() Wei Huang*1,2()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0583-z
http://www.thenanoresearch.com on September 15, 2014
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Nano Research
DOI 10.1007/s12274-014-0583-z
Template for Preparation of Manuscripts for Nano Research
TABLE OF CONTENTS (TOC)
Solution-Processed Copper Nanowire Flexible
Transparent Electrodes with PEDOT:PSS as
Binder, Protector and Oxide-Layer Scavenger for
Polymer Solar Cells
Jianyu Chen,1 Weixin Zhou,1 Jun Chen,1 Yong Fan,1
Ziqiang Zhang,1 Zhendong Huang,1 Xiaomiao Feng,*1
Baoxiu Mi,1 Yanwen Ma, *1 Wei Huang*1,2
1 Key Laboratory for Organic Electronics & Information
Displays (KLOEID) and Institute of Advanced Materials
(IAM), Nanjing University of Posts &
Telecommunications (NUPT), Nanjing 210046, China
2 Jiangsu-Singapore Joint Research Center for
Organic/Bio-Electronics & Information Displays and
Institute of Advanced Materials, Nanjing Tech University,
Nanjing 211816, China
Copper nanowires were embedded into pre-coated poly-(4,3 ethylene
dioxythiophene):poly(styrensulfonate) on polymer films by solution
processing to form flexible transparent conductive electrodes, which
are used as anodes for bulk heterojunction solar cells, giving a power
conversion efficiency of 1.4%.
Solution-Processed Copper Nanowire Flexible
Transparent Electrodes with PEDOT:PSS as Binder,
Protector and Oxide-Layer Scavenger for Polymer
Solar Cells
Jianyu Chen,1 Weixin Zhou,1 Jun Chen,1 Yong Fan,1 Ziqiang Zhang,1 Zhendong Huang,1 Xiaomiao
Feng,*1() Baoxiu Mi,1 Yanwen Ma, *1() Wei Huang*1,2()
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 2014
KEYWORDS
Copper nanowires, ploy-(4,3-ethylene
dioxythiophene):poly(styrenesulfonate)
films, flexible transparent electrodes,
solution processing, organic
photovoltaics
ABSTRACT
The easy oxidation and surface roughness of Cu nanowire (NW)
films are the main bottlenecks for their usage in transparent
conductive electrodes (TCEs). Herein, we developed a facile and
scaled-up solution route to prepare Cu NW-based TCEs by
embedding Cu NWs into pre-coated smooth ploy-(4,3-ethylene
dioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) films on
poly(ethyleneterephthalate) (PET) substrate. The so obtained Cu
NW-PEDOT:PSS/PET films own low surface roughness (~70 nm in
height), high stability toward oxidation and good flexibility. The
optimal TCEs show a typical sheet resistance of 15 Ω sq-1 at high
transparency (76 % at λ = 550 nm) and have been used successfully
to make polymer (poly(3-hexylthiophene):phenyl-C61-butyric acid
methyl ester ) solar cells, giving an efficiency of 1.4%. The overall
properties of Cu NW-PEDOT:PSS/PET films demonstrate their
potential application to indium tin oxide replacement for flexible
solar cells.
Introduction
Organic photovoltaics (OPVs) stand for a
revolutionary, new direction of future solar cells with
advantages of thinness, light weight, flexibility and
low cost. They have become commercially feasible
over recent years since the power conversion
efficiencies (PCE) over 9% were achieved by several
Address correspondence to Wei Huang, [email protected]; Yanwen Ma, [email protected]; Xiaomiao Feng, [email protected]
Nano Research
DOI (automatically inserted by the publisher)
Research Article
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2 Nano Res.
groups [1-5]. Followed by these progresses, another
critical demand for integrating devices on flexible
electrodes with high electrical conductivity and high
optical transparency is brought out in order to meet
the production technologies such as roll-to-roll
manufacturing and novel applications. Although
current state-of-the-art electrode material indium tin
oxide (ITO) can be sputtered on polymer substrates,
the intrinsic bottlenecks of ITO, such as brittleness
and supply scarcity still exist in ITO/polymer
substrates and their degraded conductivity is not
suitable for applications in OPV devices [6]. Hence
researchers have for years sought high-performance
ITO alternatives from conductive polymers, carbon
nanomaterials and metal nanowires [7-9].
Recent studies reported that metal nanowires,
typically Ag nanowires (NWs) [10-14], have many
advantages in the application to transparent
conductive electrodes because of their excellent
mechanical, optical and especially conductive
properties. Ag NWs can form percolation network
that owns much lower wire-wire resistance (<50 Ω)
[15] than carbon nanotubes (>1000 Ω) [16]. In
addition to resistance merit, they also have
solution-processing convenience as compared with
chemical-vapor-deposition grown graphene, another
possible ITO alternative [7, 8]. Over recent years,
several research groups have verified that Ag
nanowire film can serve as electrodes in OPV devices
in replacement for ITO electrodes. Brabec et al. [10]
made an efficiency of 2.5% with
poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric
acid methyl ester (PCBM) cells on high quality Ag
NW network films with transparency (T) of over 90%
and sheet resistance (R) as low as 9 Ω sq-1. The
devices’ efficiency reported by de Mello et al. [11] was
2% when Ag NW electrodes were used with R = ~29
Ω sq-1 and T = ~95%, while the efficiency reached up
to 3.5% in revised devices with the assistance of TiO2
buffer layers. You and Wiley et al. [12] extended
P3HT active layer to those polymers synthesized by
themselves on Ag NW electrodes with R = 33 Ω sq-1
and T = 84% and increased the efficiency from 1.1%
(P3HT) to 2.8%. They also constructed flexible cells
on Ag NW/ poly(ethylene terephthalate) (PET)
substrate that demonstrated recoverable efficiency of
2.5% after large deformation to 120o. To depress the
surface roughness of Ag NW film and reduce the
shorts between electrodes and organic active layers,
Peumans et al. [13] embedded Ag NWs into
conductive ploy-(4,3-ethylene dioxythiophene):
poly(styrene-sulfonate) (PEDOT:PSS) by lamination
to form smooth electrodes with R = 12 Ω sq-1 and T =
86%, enhancing OPV devices to 4.2% efficiency. Pei et
al. [14] developed Ag NW-polymer composite
electrodes by overcoating polymethacrylate on Ag
NW films and obtained a series of electrodes by
varying the wires’ length and content. The optimal
electrode is the mixed short and long nanowires that
combines high surface coverage of the short ones and
high transmittance of the long ones. On the
electrodes with R = 10 Ω sq-1 and T = 80%, their cells
showed a PCE of 3.28%.
The above progresses definitely exhibit that Ag
NWs are qualified to be ITO alternative in OPV
devices. However, the limited resources and high
price of silver need to be considered when Ag NWs
are used in large scale. Since copper ranks second
only to silver as highly conducive metals, and it is
much more abundant and cheaper than silver, Cu
NWs are of great interest in replacement of Ag NWs
[17-22]. After several synthesis routes to high quality
Cu NWs have been developed, recent studies have
paid much attention to Cu NW transparent
conductive films and their application in devices.
Wiley et al. [17-19] prepared flexible Cu NW films
exhibiting R = 30 Ω sq-1 and T = 85%. Other groups
also reported promising results in which the
performance on glass or plastics up to 51 Ω sq-1 at
93% and 30 Ω sq-1 at 85% [20-22]. Simonato and
co-workers [20] applied glacial acid acetic solution or
PEDOT:PSS cover layer to cleaning up the nanowires,
which is an important post-treatment procedure
because of the always existing pre-formed oxide
layer on Cu NWs, and then prepared flexible films
with R = 55 Ω sq-1 at 94%. Based on such high-quality
films, they realized the potential of Cu NW
electrodes in capacitive touch sensors. Recently, Leo
and Sachse et al. [21] pioneered small-molecule OPV
on Cu NWs/glass transparent electrodes (R = 24 Ω
sq-1 and T = 82%) giving a PCE of 3%. Compared with
Ag NWs successfully used in prototype devices of
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3 Nano Res.
polymer solar cells, however, Cu NW based cell is
seldom reported to date due to the oxidation and
storage issues as well as compatibility with active
layers that need to be resolved [23]. Here we report
the preparation of Cu NWs/PEDOT:PSS composite
flexible electrodes via solution processing and their
application in polymer solar cells. PEDOT:PSS
conducting polymer plays multi-function including
oxide-layer scavenger, binder and protector,
guaranteeing high-performance composite films with
smooth surface, low resistance, high stability and
flexibility obtained. The resulting polymer solar cells
exhibit acceptable PCE of 1.4%, indicating that Cu
NWs are a promising ITO alternative for OPVs.
Results and discussion
Cu NWs were synthesized by the route developed by
Zeng and Wiley et al. [17-19, 24]. The NWs have a
length of 20-30 μm and uniform diameter of 50±5 nm
(Figure S1 in Supporting Information). Cu NWs were
sprayed onto PET substrates and following passed
mechanical presses to form Cu NW/PET films, whose
morphology is well depicted by the scanning electron
microscopy (SEM) images in Figure 1a. The
well-connecting percolation network of Cu NWs is
beneficial for high conductivity. Cu NW-PEDOT:PSS
/PET composite films were prepared by rising and
pressing Cu NWs onto the pre-coated PEDOT:PSS
films at their non-solidified (semi-solid) state as
illuminated in Figure 1c. Unlike the totally exposed
Cu NWs on the Cu NW/PET films, these NWs on Cu
NW-PEDOT:PSS/PET films were nearly completely
buried by polymers, producing a smooth surface
(Figure 1b). The low surface roughness is beneficial
to reduce short in the OPV devices. As reflected by
the enlargement image (inset in Figure 1b), the
neighboring Cu NWs were tightly bonded by
PEDOT:PSS at the nanowire junction. Such a
nanosoldering will enhance the conductivity and
decrease NW-NW contact resistance by increasing
the contact surface area under the assistance of the
conducting polymer [25]. To further reveal the
structure of Cu NWs and PEDOT:PSS composite,
they were scratched off from the film for
transmission electron microscopy (TEM) observation.
The TEM image in Figure 1d shows that two
nanowires are coated and jointed by polymer.
According to the study by Ko and Lee [25], et al.,
PEDOT:PSS solution tended to attach metal NWs and
especially accumulate at NW junction during solvent
evaporation because of strong capillary force. The so
formed nanosoldering also significantly improved
the mechanical stability and adhesion of the NW
network to the substrate. Bare Cu NWs can be easily
detached from the surface of PET by scotch tape,
whereas those bonded by PEDOT:PSS did not give
any change under naked-eye observation even after
tens of runs as shown in Figure 1e, indicating their
strong mechanical adhesion to the plastic substrate.
We also carried out a different approach to prepare
Cu NWs and PEDOT:PSS composite films by
depositing PEDOT:PSS onto Cu NW/PET films and
denoted them as PEDOT:PSS-c-Cu NW/PET. The
surface roughness of this type films, however, was
much higher than that of Cu NW-PEDOT:PSS/PET
films (Figure S2 in Supporting Information). Hence,
embedding metal nanowires into preformed polymer
films is an efficient approach to fabricate smooth
composite films [13, 14].
Figure 2a and b show the visible spectra of the
Cu NW/PET films before and after PEDOT:PSS
wrapping. The surface coverage for each film was
tuned by varying spray time. The previous studies by
other researchers have well revealed that Cu
NW-based films keep nearly constant transmittance
over the entire ultraviolet-visible-near infrared
(UV-Vis-NIR) region [18, 20]. This advantage of Cu
NW films provides their wide application fields in
optoelectronics ranging from UV to NIR devices. The
transmittances of the as-prepared Cu NW/PET films
decrease with increasing surface coverage of Cu NWs
(Figure 2a) and further show a certain reduction of
4-9% after being incorporated with polymer (Figure
2b). From these transmittance reduction degree, we
can estimate that the thickness of PEDOT:PSS layer is
about 100 nm [26]. To further acquire the effects of
polymer coating, the T as a function of R for each
film is plotted in Figure 2c. At low NW coverage, Cu
NW/PET film can achieve R = 1200 Ω sq-1 at T = 93%
while the corresponding PEDOT:PSS coated sample
shows R = 760 Ω sq-1 at T = 87%. The R of Cu
NW/PET film can be significantly reduced by
elevating Cu NW coverage, which reaches 100 Ω sq-1
at T = 76% and 50 Ω sq-1 at T = 62%. The most
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interesting change is that after PEDOT:PSS coating,
the R continually decreases with a little lose of T. For
example, we can prepare a Cu NW-PEDOT:PSS/PET
film owing R = 15 Ω sq-1 at T = 76%, whose
conductivity is high enough for OPV studies though
transparence has much improved space. The role
played by PEDOT:PSS in the conductive
improvement of metal NW network has been
explained as nanosoldering to bind NWs and
scavenger to removal surface oxide layer [20, 25]. The
former is easily understood as shown in SEM and
TEM images in Figure 1. The later, however, is much
difficult to analyst because the oxide layer is too thin
to be measured, especially coated by a layer of
polymer. To reveal this, we immersed heavily
oxidized Cu NWs into PEDOT:PSS ethanol solution.
The oxidized species were effectively dissolved and
simultaneously the rough surface became smooth
(Figure S3 in Supporting Information). The removal
of insulating oxide layer is very important for
preparing high conductive Cu NW network because
only in this condition, ohmic contact could be formed
among NWs. Hence PEDOT:PSS is a remarkable
oxide-layer scavenger in liquid state. As shown in
Figure 2d, the bare Cu NWs were sensitive to oxygen
and moisture to form oxide species and the resistance
increased by a factor of 4 after 30 days, whereas the
Cu NW-PEDOT:PSS/PET film were very stable and
remained fairly constant conductivity. The long-term
stability, on the other hand, indicates that the erosion
capability of solid-state PEDOT:PSS to Cu NWs is
dramatically depressed and becomes insignificant.
These results clearly indicate that the major challenge
encountered by Cu NW electrodes, i.e., instability to
oxidation, could be well resolved by embedding in
PEDOT:PSS, a wide used and well incorporated
materials in organic optoelectronics. Metal NW-based
electrodes have an outstanding advantage in
flexibility compared with ITO. This merit definitely is
maintained by Cu NW-PEDOT:PSS composite films
because of flexible polymer combined (Figure S4 in
Supporting Information).
Tapping mode atomic force microscopy (AFM)
and conductive-AFM (C-AFM) were used to further
characterize Cu NW/PET and Cu
NW-PEDOT:PSS/PET films. A strip of Al electrode
was deposited onto one side of the film and thus
formed a circuit with AFM tip (Figure 3a). This
enables us to image both the topography and the
conductivity of the surface at the same time. The
embedding of Cu NWs in PEDOT:PSS reduces the
their surface roughness from ~180 nm (Figure 3b) in
height to ~70 nm (Figure 3d), consistent with the
above SEM result (Figure 1a and b). The current map
in Figure 3c clearly shows the percolation conductive
network of Cu NWs. Some NWs (marked in the
circles in Figure 3b) disappear in the current map
because they lose connection with the whole network.
The total nonconductive (zero current) regions
existed in Cu NW/PET film is estimated about 50%,
which will be a major bottleneck if used in OPV
devices. In contrast, the Cu NW-PEDOT:PSS film
exhibits an even conductive feature on the whole
surface (Figure 3e). In spite of the high resistance of
PEDOT:PSS, they can assist collect charge carriers
and submit them to the embedded Cu NW network
for high-speed transportation. Hence the synergetic
effect of Cu NW and PEDOT:PSS qualifies their
composite films to be one of the most promising
alterative for ITO in future aplications.
To further evaluate the quality of the
as-prepared transparent electrodes, bulk
heterojunction photovoltaic cells using P3HT:PCBM
were fabricated on Cu NW-PEDOT:PSS/PET (R = 15
Ω sq-1, T = 76%) and ITO/glass (R = 15 Ω sq-1, T = 87%)
according to the procedure described in the
Experimental Section. Here PEDOT:PSS layer (40 nm)
used as the hole-transporting layer was covered onto
both ITO and Cu NW-PEDOT:PSS films (Figure 4a).
In addition, PEDOT:PSS layer could definitely tune
the work function of the anodes as well as smooth
their surface, especially for Cu NW-PEDOT:PSS film.
It is worthy to notice that before using this composite
film, we tried to directly fabricate cells on bare Cu
NW films. Though the easy-oxidation problem can
be resolved by processing in a glove box, it was really
difficult to coat PEDOT:PSS film on Cu NWs
smoothly and even the NWs were detached during
the polymer solution spinning. For Cu
NW-PEDOT:PSS film, its compatibility to
PEDOT:PSS layer does not exist. Moreover, the work
function of Cu NWs should be adjusted closely to
that of PEDOT:PSS matrix (~5.2 eV) after such
two-step covering (Figure 4b). The typical current
density-voltage (J-V) curves of NW electrode based
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5 Nano Res.
solar cell under both illumination and dark are
shown in Figure 4c. The device has a short-circuit
current desnity (JSC) of 6.05 mA cm-2, an open-circuit
voltage (VOC) of 0.58 V, fill factor (FF) of 40% and PCE
of 1.4%. On ITO/glass, the JSC is 8.78 mA cm-2, the VOC
is 0.58 V, the FF is 53%, and the PCE is 2.8%.
Obviously, the performance of NW-based device is
lower than that of ITO-based one. We ascribe the
reduced JSC and FF to the 11% decreased
transparency of Cu NW-PEDOT:PSS/PET film with
respect to ITO/glass, which will reduce the amount of
photon-generated carrier and thus increase the serial
resistance. Hence better performance could be
expected for Cu NW-based OPV devices through
some improvement on the transparency and
conductivity of Cu NW-PEDOT:PSS/PET electrodes.
In addition, surface roughness and interlayer contact
are also necessary factors needed to be improved
since the diode-like behavior of Cu NW-based device
is a little worse than ITO-based devices as reflected in
the dark current curves. The performance devices
assembled on PEDOT:PSS-c-Cu NW/PET films (R =30
Ω sq-1, T = 76%) with a low PCE of 0.6% further
support the importance of the surface flatness (Figure
S5 in Supporting Information). The absorbance and
incident photon to current efficiency (IPCE) of the Cu
NW- and ITO-based deveices are shown in Figure 4d.
Except that Cu NW-based device has a stronger
adsorption due to its lower transparency, both
devices own similar absorption characteristic and
IPCE curve shape, suggesting that Cu
NW-PEDOT:PSS composite electrodes are compatible
to the whole device structure. Based on these results,
we anticipate that Cu NWs can be used as ITO
substitute in high performance, low-cost, roll-to-roll
processed flexible OPV cells.
Conclusions
In conclusion, we demonstrate a facile and scaled-up
solution route to fabricate Cu NW-based transparent
conductive electrodes by embedding Cu NWs into
pre-coated PEDOT:PSS films. The incorporation of
PEDOT:PSS polymer brings many advantages: 1)
protecting the easily oxidized Cu NWs, 2) bonding
NWs themselves and NWs-substrate tightly, 3)
removing the oxide layer on Cu NWs, 4) smoothing
the surface and 5) adjusting the comparability of
NW-based electrode with its upper organic/polymer
layer. On the basis of the optimal Cu
NW-PEDOT:PSS/PET films, flexible bulk
heterojunction type OPV cells were fabricated with
efficiency of 1.4%. Further optimization of Cu
NW-PEDOT:PSS/PET film’s transparency and
conductivity, e.g., by using small-diameter and
high-aspect-ratio Cu NWs, may lead to enhance the
cell performance. The strategy developed in this
work is of benefit to promote the application of Cu
NWs in the field of transparent electrodes.
Method
Synthesis of Cu NWs:
In a typical reaction system, 2.65 g copper nitride was
dissolved into 800 mL of high concentrated NaOH
solution (15 mol L-1) containing 12 mL
ethylenediamine (EDA) and 1 mL hydrazine (35
wt%). Here EDA acts as capping agent and hydrazine
is reductant. The above mixed solution was prepared
under ice bath condition. The reaction was started at
75 °C and maintained for 60 min without stirring.
Preparation of Cu NW films:
The as-prepared Cu nanowires were dispersed in the
solution of acetone and ethanol. Then PET film was
placed on 140 °C and then Cu NWs were sprayed
onto it by using an airbrush. Cu NW/PET films were
pressed between two mirror surface plastic mold
steels at 20 MPa for 10 s.
Preparation of CuNW-PEDOT:PSS films:
PEDOT and PSS were dissolved in ethanol solution
and spin-coated on PET film. The thickness of
polymer film is about 100 nm. The as-prepared films
were shortly dried at ambient temperature for 20 s
and acted as substrates for Cu NW deposition by
spray coating. Then the composite films were
annealed at 140 °C for 3 min and pressed at 20 MPa
for 10 s after they were cooled down to room
temperature.
Characterizations:
Cu NWs and their derived films were characterized
by scanning electron microscopy (SEM, Hitachi
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6 Nano Res.
S-4800), transmission electron microscopy (TEM,
Hitachi 7700 at 120 kV), atomic force microscopy
(AFM, Bruker Dimension Icon). The transparency of
each film was measured by UV-Vis
spectrophotometer (Shimadzu, UV-3600) and the
sheet resistance (R) was evaluated by four-point
probe measurement (Keithley 2400 semiconductor
parameter) at room temperature.
Preparation of polymer solar cells:
Polymer solar cells were constructed on
CuNW/PEDOT:PSS/PET (R = 15 Ω sq-1, T = 76%) and
ITO/glass electrodes (R = 15 Ω sq-1, T = 87%).
CuNW-PEDOT:PSS/PET was directly used without
any treatment while ITO/glass was exposed in
oxygen plasma for 50 s before following coating.
PEDOT:PSS with a thickness of approximately 40 nm
was spin-coated onto the two type of electrodes.
P3HT and PC60BM with 1:1 weight ratio dissolved in
1, 2-dichlorobenzene were used as active materials
and spin-coated on the PEDOT:PSS film at 800 rpm
for 20 s. The thickness of the active layer is about 90
nm. Al metal cathode was deposited on the top by
thermal evaporation. The photovoltaic performance
was measured under an air mass of a 1.5 solar
illumination at 100 mW cm−2 (1 sun). Incident photon
to current efficiency (IPCE) tests were carried out on
a QE/IPCE test system (CROWNTECH CTTH-150W).
Acknowledgements
This work is jointly supported by the Ministry of
Education of China (No. IRT1148), A Project
Funded by the Priority Academic Program
Development of Jiangsu Higher Education
Institutions, Key Projects for International
Cooperation (BZ2010043), Jiangsu Provincial
NSF (BK2011750, BK20141424).
Electronic Supplementary Material:
Supplementary material (Synthesis method and
characterization of Cu NWs, Dissolution test of Cu
oxide species by PEDOT:PSS solution, I-V curves and
resistance change in bending tests) is available in the
online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-*
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Figure 1. SEM images of (a) Cu NW/PET and (b) Cu
NW-PEDOT:PSS/PET films; (c) Peparation procedure for Cu
NW-PEDOT:PSS/PET films; (d) TEM image of
PEDOT:PSS-coated Cu NWs; (e) Photographs of adhesion test
for Cu NW/PET (top, R = 300 Ω sq-1, T = 81%) and Cu
NW-PEDOT:PSS/PET films (bottom, R = 15 Ω sq-1, T = 76%).
Figure 2. Transmittance spectra of (a) Cu NW/PET and (b) Cu
NW-PEDOT:PSS/PET films; (c) Sheet resistance versus
transmittance for all the films; (d) Sheet resistance changes with
time for Cu NW/PET (T = 55%) and Cu NW-PEDOT:PSS/PET
(T = 55%) films .
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8 Nano Res.
Figure 3. (a) Schematic diagram of the experimental setup for
C-AFM test; (b) Topography images and (c) current maps of Cu
NWs/PET; (d) Topography images and (e) current maps of Cu
NW-PEDOT:PSS/PET.
Figure 4. (a) Photograph and cell structure of Cu NW-based
OPV device; (b) Energy level diagram of Cu NW-based device,
where the work function of Cu NWs is modulated by
PEDOT:PSS twice; (c) Current density-voltage (J-V)
characteristics under dark and simulated AM 1.5 solar irradiation
with 100 mW cm-2 intensity for devices with ITO/glass and Cu
NW-PEDOT:PSS/PET electrodes; (d) IPCE and adsorption of
devices corresponding to (c).
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Electronic Supplementary Material
Solution-Processed Copper Nanowire Flexible
Transparent Electrodes with PEDOT:PSS as Binder,
Protector and Oxide-Layer Scavenger for Polymer
Solar Cells
Jianyu Chen,1 Weixin Zhou,1 Jun Chen,1 Yong Fan,1 Ziqiang Zhang,1 Zhendong Huang,1 Xiaomiao
Feng,*1() Baoxiu Mi,1 Yanwen Ma, *1() Wei Huang*1,2()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
INDEX:
SI 1 Synthesis and characterization of Cu NWs
SI 2 Preparation of PEDOT:PSS-c-Cu NW/PET films
SI 3 Dissolution of Cu oxide species by PEDOT:PSS solution
SI 4 I-V curves and resistance change in bending tests
SI 5 Performance of OPV devices with PEDOT:PSS-c-Cu NW/PET electrodes
Address correspondence to Wei Huang, [email protected]; Yanwen Ma, [email protected]; Xiaomiao Feng, [email protected]
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Nano Res.
SI 1 Synthesis and characterization of Cu NWs
Cu NWs were synthesized by stabilizing the precursor solution in ice bath before reaction and providing a
“stable” environment without stirring during growth. The product gives a homogeneous light burgundy
solution as shown in Figure S1a. The typical SEM iamges (Figure S1b and c) show that the uniform wires have
length of 20-30 μm and diameter of 50±5 nm, which is further supported by the TEM image (Figure S1d).
Figure S1. (a) Photo of the as-prepared Cu NW solution; (b) and (c) SEM images of Cu NWs; (d) TEM image
of Cu NWs.
SI 2 Preparation of PEDOT:PSS-c-Cu NW/PET films
PEDOT:PSS coated Cu NW composite films were prepared by deposing PEDOT:PSS solution onto Cu
NW/PET films (denoted as PEDOT:PSS-c-Cu NW/PET). The SEM and AFM images of PEDOT:PSS-c-Cu
NW/PET films are shown in Figure S2. Compared with the Cu NW-PEDOT:PSS/PET films fabricated by
embedding approach, PEDOT:PSS-c-Cu NW/PET films have a more irregular surface, whose roughness is
over 150 nm according to the AFM topological image (Figure S2b).
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Nano Res.
Figure S2. SEM (a) and AFM images (b) of PEDOT:PSS-c-Cu NW/PET films.
SI 3 Dissolution of Cu oxide species by PEDOT:PSS solution
In order to evaluate the capability of PEDOT: PSS in dissolving Cu oxide species, we mixed oxidized Cu
NWs (exposed in ambient air for one month) with PEDOT:PSS solution. After settling for 24 h, the NWs and
polymer were dried at 100 °C and then powder were obtained. The morphology of oxidized Cu NWs mixed
with PEDOT:PSS at beginning is shown in Figure S3a. It is seen that many nanoparticles exist on the surface
of NWs due to the formation of copper oxides. As known, PEDOT: PSS solution is an acid with pH of 2, in
which H+ protons are enough active to react with Cu oxide species. After the dissolve of the oxide
nanoparticles by PEDOT: PSS solution, the surface of Cu NWs becomes much smoother than that of oxidized
ones (Figure S3b). The XRD patterns in Figure S3c clearly show the composition change of oxidized Cu NWs
before and after PEDOT:PSS treatment. In addition to Cu signal, the diffraction peaks belonging to Cu2O and
CuO also exit in the XRD curve of the oxidized Cu NWs. But for the treated Cu NWs, the signals of Cu
oxides almost vanish, suggesting that they were removed by PEDOT: PSS solution effectively.
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Figure S3. SEM images oxidized Cu NWs mixed with PEDOT:PSS at 0 h (a) and 24 h (b); (c) XRD patterns of
oxidized Cu nanowires before and after PEDOT:PSS treatment.
SI 4 I-V curves and resistance change in bending tests
I-V curves for the film with a T of 69% were carried out at different bending angles are given in Figure S4a.
The slop of I-V curve presents a tiny increase with increasing bending angle. The R will rise from 8.0 to 10.0
Ω sq-1 after compression bending 1000 times at 120o, while recover to 8.5 Ω sq-1 when bend released (Figure S
4b), indicating the high flexibility and stability owned by Cu NW-PEDOT:PSS films.
Figure S4. Flexibility tests of Cu NW-PEDOT:PSS/PET films. (a) I-V curves at different bending angles; (b)
Sheet resistance versus bending times. Inset in (a) shows the measure method for bending angle.
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SI 5 Performance of OPV devices with PEDOT:PSS-c-Cu NW/PET electrodes
The OPV devices using PEDOT:PSS-c-Cu NW/PET films (R =30 Ω sq-1, T =76% ) as anodes presented an optimal
PCE of 0.6%, a short-circuit current desnity (JSC) of 2.40 mA/cm2, an open-circuit voltage (VOC) of 0.56 V, and fill
factor (FF) of 45%, indicating whose performance is worse than the devices assembled on Cu
NW-PEDOT:PSS/PET films. The main reason should be the higher surface roughness of PEDOT:PSS-c-Cu
NW/PET films (Figure S2) in comparison with that of Cu NW-PEDOT:PSS/PET films (Figure 1b and Figure 3b).
Figure S5. Current density-voltage (J-V) curves under dark and illumination for devices with PEDOT:PSS-c-Cu
NW/PET electrodes.