Solution-Processed Copper Nanowire Flexible Transparent ... · Solution-Processed Copper Nanowire...

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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DOI 10.1007/s12274-014-0583-z

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TABLE OF CONTENTS (TOC)


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%.




Solar Cells



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

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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

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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


4 Nano Res.

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


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


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 .


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).


Nano Res.

Electronic Supplementary Material




Solar Cells



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]


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).


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.


Nano Res.

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.


Nano Res.

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.

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