Large-Scale Stretchable Semiembedded Copper Nanowire

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Large-Scale Stretchable Semi-Embedded Copper Nanowires Transparent Conductive Films by Electrospinning Template Xia Yang, Xiaotian Hu, Qingxia Wang, Jian Xiong, Hanjun Yang, Xiangchuan Meng, Licheng Tan, Lie Chen, and Yiwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08606 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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ACS Applied Materials & Interfaces

Large-Scale Stretchable Semi-Embedded Copper Nanowires Transparent Conductive Films by Electrospinning Template a,b

Xia Yang , Xiaotian Huc, Qingxia Wanga,b, Jian Xiongd, Hanjun Yanga,b, Xiangchuan Menga,b, Licheng Tan*a,b, Lie Chena,b, Yiwang Chen*a,b a

College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

b

Jiangxi Provincial Key Laboratory of New Energy Chemistry/Institute of Polymers, Nanchang University, Nanchang 330031, China c

Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), 2 Zhongguancun Beiyi Street, Beijing 100190, China

d

Guangxi Key Laboratory of Information Materials, School of Materials Science and Engineering, Guilin University of Electronic Technology, 1 Jinji Road, Guilin 541004, China Corresponding author: Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail: [email protected] (Y. Chen); [email protected] (L. Tan).

Author contributions: X. Yang and X. Hu contributed equally to this work. Abstract With recent emergence of wearable electronic devices, the flexible and stretchable transparent electrodes are the core components to realize innovative devices. Copper nanowires (CuNWs) network is commonly chosen because of its high conductivity and transparency. However, the junction resistances and low aspect ratios still limit its further stretchable performance. Herein, a large-scale stretchable semi-embedded CuNWs transparent conductive film (TCFs) was fabricated by electrolessly depositing Cu on the electrospun poly (4-vinylpyridine) (P4VP) polymer template semiembedded in polydimethylsiloxane (PDMS). Compared with traditional CuNWs which as-coated on the flexible substrate, the semi-embedded CuNWs TCFs showed low sheet resistance (15.6 Ω sq-1 at ∼82% transmittance), as well as outstanding stretchability and mechanical stability. The light-emitting diode (LED) connected the stretchable semi-embedded CuNWs TCFs in the electric circuit still lighted up even after stretching with 25% strain. Moreover, this semi-embedded CuNWs TCFs was successfully applied in the polymer solar cells as stretchable conductive electrode which yielded a power conversion efficiency of 4.6% with 0.1 cm2 effective area. The

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large-scale stretchable CuNWs TCFs show potential for wearable electronic devices development. Keywords: Copper nanowires; Stretchability; Transparent electrodes; Electroless deposition; Polymer solar cells

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Introduction In recent years, wearable electronic devices have attracted considerable attentions, such as wearable touch screen panels1, stretchable solar cells2-5, smart clothing6-8, artificial skins9, transistor10, organic light-emitting diode (OLED)11 and smart windows11. Flexible and stretchable transparent electrodes are key components to realize these innovative devices. The reason is that flexible and stretchable transparent electrodes have intersection of desirable electrical conductivity, optical transparency and mechanical stability. Retaining electrical conduction at high strain deformation is one of the major challenges associated with many of these devices and systems. However, indium tin oxide (ITO)12, the most commonly transparent electrode material, has some crucial drawbacks (complicated technics, high cost and brittleness), which limits its practical applications. The stretchable electrode with inferior performance becomes a bottleneck for application in wearable electronic devices. Recently, many valuable substitutes including conducting polymers13-15, graphene16, metallic nanowires (NWs)10,11,17, carbon nanotubes18 and hybrid materials19-21, have been developed for stretchable transparent electrodes. Among these candidates, metallic NWs show excellent conductivity, transparency, large-scale solution processability, flexibility and stretchability due to their intrinsic properties. For example, copper nanowires (CuNWs) with average diameter of ~28 nm and length range from several hundreds of micrometers to several millimeters were reported as electrodes for green phosphorescent OLED11. Silver nanowires (AgNWs)@iongel composite films were prepared for flexible transparent electrodes21. In general, fully interconnected metallic NWs exhibits high electrical conductivity since they are absent of the issue of high wire-to-wire junction resistance. Post-treatments such as thermal

annealing22,

wetchemical

coating23,24,

plasmonic

welding25

and

electrowelding26 can greatly reduce junction resistance. However, these strategies are usually complicated and not suitable for flexible substrates.

Theoretically, longer metallic NWs can decrease junctions, to lower the percolation threshold Nc and reach the contact pad, expressed as

, L is the

length of NWs. It is understandable that for longer NWs, Nc is lower, and the 3



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probability of having a continuous pathway becomes higher, resulting in lower sheet resistance (Rs). Except for spin-coating27 only applied in small areas, other common surface deposition strategies for fabricating NWs film such as printing28,29, magnetron sputtering30 and electron beam evaporation31 with high preparation cost, which limit their further application. Furthermore, CuNWs possess low bulk resistivity and Rs to 1.7×10-6 Ω·cm and 0.19 Ω·sq-1, respectively, which are superior to Ag NWs (bulk resistivity and sheet resistance is 3.08×10-6 Ω·cm and 0.51 Ω·sq-1, respectively)32-37. But poor stretchability is still the key issue for the development of CuNWs transparent conductive films (TCFs). Therefore, developing a simple, low-cost and scalable method for fabricating smooth, large-area and stretchable CuNWs TCFs with high optoelectronic properties has become imperative.

In this work, large-scale and stretchable CuNWs TCFs were fabricated by electrolessly depositing Cu on the semi-embedded electrospun poly (4-vinylpyridine) (P4VP) NWs network. To guarantee high conductivity, transmittance performance and complete metallization coverage, reducing the diameter of P4VP NWs as much as possible, and adjusting the time of activation and electroless deposition were investigated in detail. The ultralong and junctions-free CuNWs TCFs showed good photoelectric properties with transmittance of 82% and sheet resistance of 15.6 Ω/sq. Furthermore, excellent stretchability and mechanical stability were obtained. Compared to the as-coated CuNWs mounted on polydimethylsiloxane (PDMS), there was no obvious increase in R/R0 values for semi-embedded CuNWs in PDMS, as a function of the exposure duration time (in 85% relative humidity and 25 °C), bending 2000 cycles (with radius of 1 cm), uniaxial stretching strains and stretching cycles (at 25% strain), respectively. The light-emitting diode (LED) connected the stretchable CuNWs in the electric circuit still lighted up even after stretching with 25% strain. Moreover, polymer solar cells (effective area of 0.1 cm2) based on this CuNWs electrode exhibited a high average power conversion efficiency of 4.6%, which presenting excellent feasibility for the wearable electronic devices.

Results and Discussion Large-scale stretchable junction-free CuNWs TCFs were fabricated by electrolessly depositing Cu on the semi-embedded electrospun P4VP NWs network, illustrated in 4


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Figure 1. It is well known that template-directed synthesis has been recognized as the most straightforward strategy to guide the growth of Cu nanocrystals into nanowires38, and electrospinning is a highly effective technique to fabricate continuous and ultralong nanofibers39. Moreover, electroless deposition is a wellestablished method for depositing metal films with low cost and large scale40. In this work, P4VP NWs was used as the template for electroless deposition, due to effective adsorbtion of metal ions and nanoparticles ascribed from strong affinity of pyridyl groups40. The key issue is how to selectively deposit Cu on the P4VP NWs instead of the substrate and prepare junction-free CuNWs to guarantee excellent conductivity and transmittance performance. In general, the surface roughness of nanowires based TCFs can strongly affect the optoelectronic performance41. The inferior mechanical durability was attributed to the weak adhesion between commercial (or modified) metal NWs network and the target substrate, further limited the application in flexible and stretchable devices42.43. Therefore, the electrospun P4VP NWs network was semiembedded into hydrophobic elastomer PDMS substrate. We fabricated large-scale stretchable junction-free CuNWs TCFs by three main steps. First, the viscous liquid PDMS pre-polymers were spin-coated and pre-cured on polyethylene terephthalate (PET) substrate. Then P4VP NWs were electrospun onto PDMS film, following curing PDMS to form semi-embedded NWs, which could reduce the surface roughness and enhance the mechanical stability and oxidation resistance. Second, to form a Pd catalyst seed layer and nucleation sites, the semi-embedded NWs were activated by immersion into PdCl2/HCl solution. Finally, stretchable CuNWs TCFs were successfully fabricated by electrolessly depositing Cu onto the semi-embedded NWs. The photographs of stretchable CuNWs TCFs are shown in Figure 1. During this process, Pd2+ was reduced by formaldehyde (HCHO) and a Pd seed layer was formed on the surface, which provided nucleation sites and functionalized as catalysts for Cu electroless deposition during subsequent particles enlargement. The detail chemical equations, mechanism of activation and electroless deposition are illustrated in Figure 1.

The diameter and morphology of P4VP NWs as template directly affected the electroless deposition process and the surface roughness of the corresponding CuNWs TCFs. It could be controlled by adjusting the polymer solution viscosity and the 5



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electrospining voltage41. Scanning electron microscopy (SEM) images of electrospun P4VP NWs prepared under different conditions and statistics of the specific morphologies (diameter and beads) as function of viscosity of electrospinning solution versus voltage are shown in Figure 2. Generally, the P4VP NWs used as template for preparing TCFs needed to be less than 200 nm in diameter and without beads, which were electrospun by regulating viscosity and voltage as 95.8 mPa·s and 9 KV, respectively. In this work, the electrospun P4VP NWs with diameter of ∼90 nm (Figure S1) were used to further electroless deposition. X-ray photoelectron spectroscopy (XPS) spectra (Figure 2) were performed to characterize P4VP NWs with activation and electroless deposition process. After activation, the Pd 3d5/2 peak was deconvoluted and fitted as two subpeaks at 337.7 eV and 338.8 eV, which corresponded to Pd2+ and Pd-N bond, respectively. After electroless deposition, the existence of Pd0 peak with binding energy of 335.4 eV as indicated by the Pd 3d5/2 peak, suggested that Pd2+ had been reduced to Pd0, and Pd seeds should appear and bind to the NW surface which were favorable for catalyzing subsequent Cu electroless deposition. The XPS results agrees very well with the previous report44, 45. Furthermore, Cu 2p XPS spectrum, X-ray diffraction (XRD) pattern and the energydispersive spectroscopy (EDS) mapping from the corresponding SEM images further verify that Cu precursors have been reduced onto the P4VP NWs46,47, as shown in Figure 2. The green color in EDS represents CuK characteristic radiation, which indicating Cu has well grown on the surface of the semi-embedded polymer fibers. Apparently, the substrate without P4VP NWs template was free of metal deposition, whereas Pd seed catalysts binding on template successfully accelerated metallization on P4VP NWs.

To guarantee complete metallization coverage, high transmittance performance and conductivity, reducing the diameter of P4VP NWs as much as possible and adjusting the time of activation and electroless deposition were necessary to investigate. The diameter of the electrolessly deposited CuNWs is ∼140 nm from the SEM image shown in Figure 3a. To achieve junction-free CuNWs with low contact resistance, we used a “fuses” method by immersing semi-embedded P4VP NWs into the activation solution, which not only formed Pd catalyst seeds and nucleation sites, but also conglutinated the junctions at the NWs intersections. The coverage degree of the 6


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junction-free nanowires after activation versus additive acid volume in activation solution and activation time is shown in Figure S2. It was found that immersion time for 75 s in activation solution (with 70 µL HCl) could achieve junction-free coverage up to 100%, which avoiding the typical high junction resistance for solutionprocessed metallic NWs directly. The atomic force microscopy (AFM) image in Figure 3b obviously shows the fused CuNWs-CuNWs junctions. The optical transmittance and conductivity of CuNWs TCFs have been conducted by optimizing the electroless deposition time. Seen from Figure S3, the reduction in transmittance (T) and sheet resistance (Rs) existed with increasing the deposition time. Rs-T relationships of the electrolessly deposited Cu NWs reflect in Figure 3c. A high transmittance approaching 82% (at 550 nm) with relative superior Rs value (15.6 Ω·sq-1) was comparable to other flexible TCFs reported in the literature, such as those based on metallic nanowires11,48, carbons16,18,49, conductive polymers13,14,50, architectures51-53 and hybrids19,20 (Figure 3d). Ultralong and junctions-free CuNWs with high conductivity and transparency produced by the combination of electrospinning polymer template and electroless deposition would have real potential in photoelectric devices, because the percolation threshold and continuous charge transport pathway can be easily achieved.

In general, the surface roughness of electrolessly deposited nanowires is higher than those of ordinary solution-processed NWs. Additional post-treatment or complicated process need to deal with the roughness issue. In our work, the semi-embedded CuNWs displayed relative low surface roughness. Figure 4a shows the cross-section SEM image of semi-embedded CuNWs. A root-mean-square surface roughness (RMS) of 10.5 nm and maximum peak-to-valley (Rpv) range of 70 nm are noted from AFM image (Figure 4b). Moreover, the semi-embedded CuNWs exhibited outstanding mechanical durability and stretchability due to the strong adhesion between electrolessly deposited NWs and PDMS substrate. Figure 4c shows relative resistance changes (R/R0) of the semi-embedded CuNWs film in our work and ascoated CuNWs film after exposure in a harsh condition (i.e., 25 °C and 85% relative humidity). The semi-embedded CuNWs shows a negligible resistance change after exposure 10 h, indicating its extraordinary chemical stability against oxidation in humid environment, while the reference sample exhibits significant increase in 7



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resistance value. Furthermore, R/R0 values of the as-coated and semi-embedded CuNWs, as a function of bending cycles (with radius of 1 cm) has been evaluated (Figure 4d). R/R0 value of the as-coated CuNWs increases 64 times after 800 cycles, while that of semi-embedded CuNWs increases only by 0.3% after bending up to 2000 cycles, suggesting its superior bending durability. Importantly, the excellent stretchability has be investigated by analyzing the R/R0 values versus different uniaxial stretching strains (Figure 4e) and stretching cycles at 25% strain (Figure 4f), respectively. In contrast to severe resistance change (increasing 245% after 800 cycles) for as-coated CuNWs, the semi-embedded NWs shows no obvious R/R0 change after > 2000 stretching cycles at strain of 25%. From SEM images of the semi-embedded CuNWs at different stretching strain shown in Figure 4f, no apparent difference on the morphologies can be observed. High bending and stretching stability of CuNWs is attributed to the soft P4VP polymer nanowires template and semiembedded technique with elastic PDMS.

To determine the practicability of stretchable semi-embedded CuNWs TCFs, the applications in the light-emitting diode (LED) circuit and polymer solar cells (PSCs) were conducted. Photograph of the illuminated LED biased at 3 V using large-scale (3×3 cm) Cu NWs TCFs is shown in Figure 5a. Seen from the Video S1 in Support Information, there was no obvious brightness change on different location of the electric circuit. In particular, LED lamp still lighted up even after stretching with 25% strain. And the polymer solar cell (PSCs) was fabricated with the structure of stretchable semi-embedded Cu NWs transparent electrode/PEDOT: PSS/PTB7-th: PC71BM/Ca/Al (Figure 5b). The photograph of PSCs is revealed in Figure 5c. The current density-voltage (J-V) characteristics of the PSCs were measured under one standard sun using a solar simulator with an Air Mass 1.5 global (AM 1.5G) and an irradiation intensity of 100 mW cm2. The flexible PSCs based on stretchable CuNWs transparent electrode yielded a power conversion efficiency (PCE) of 4.6% with open circuit voltage (Voc) of 0.75 V, short current density (Jsc) of 12.02 mA cm-2 and fill factor (FF) of 50.8% at 0.1 cm2 area (Figure 5d).

Conclusions

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Large-scale and stretchable CuNWs TCFs were successfully prepared by electrolessly depositing Cu on the electrospun P4VP polymer template which was semi-embedded in PDMS. The substrate without P4VP NWs template was free of metal deposition, whereas the additions of Pd seed catalysts successfully accelerated metallization on P4VP NWs. Reducing the diameter of P4VP NWs as much as possible and adjusting the time of activation and electroless deposition, were important to guarantee complete metallization coverage, high conductivity and transmittance performance. In particular, a “fuses” method by immersing semi-embedded P4VP NWs into the activation solution could effectively conglutinate the junctions at the NWs intersections, which was favor for achieving junction-free CuNWs with low contact resistance. The ultralong and junctions-free CuNWs semi-embedded in PDMS substrate dramatically enhanced the mechanical stability during bending and stretching, which provided a novel approach to realizing next-generation flexible and stretchable optoelectronic devices.

Experimental Section Materials. Poly (4-vinylpyridine) (P4VP) with molecular weight of 160 000 g/mol was purchased from Sigma-Aldrich. Palladium (II) chloride (PdCl2, 99.9%, 59.8% Pd) and potassium sodium tartrate tetrahydrate (C4H4KNaO6·4H2O, 99%) were purchased from J&K Scientific. Copper sulfate pentahydrate (CuSO4·5H2O, 99.99% metals basis) and ethanol were purchased from Aladdin. Formaldehyde (37 wt% in H2O) and dimethylformamide (DMF, 99.8%) were purchased from Sigma-Aldrich. hydrochloric acid (HCl,12 M) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polydimethylsiloxane (PDMS, Sylgard 184) was purchased from Dow Corning. Poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was from Heraeus CLEVIOS™ PH4083. PTB7-th was purchased from 1-material, Chemscitech Inc., QC, Canada. PC71BM was purchased from Solarmer Materials Inc., China. Chlorobenzene (CB) and 1,8-diiodoctane (DIO) were obtained from Sigma-Aldrich. All the available chemical reagents were used as received without any further purification. Commercial CuNWs (50-200 nm) were bought from Nanjing XFNANO Materials Co., Ltd. for comparison. Light-emitting diode (LED) bulb was bought from Jingcheng Electronic Mall.

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Preparation of semi-embedded P4VP nanowires. First, the PDMS elastomer substrate with thickness of 40 µm was prepared by mixing sylgard 184 “base” and “curing agent” with a weight ratio of 10:1, spin-coating the mixture solution with a speed of 500 rpm for 10 s and 1000 rpm for 30 s on the PET substrate, and pre-curing at 60 °C for 15 min. Second, electrospun P4VP NWs were prepared semi-embedded into the pre-curing PDMS film. 15 wt% of P4VP in dimethylformamide:ethanol solution (1:1, v/v) was loaded in a 10 mL syringe with a needle tip, which was connected to an electrospining machine (QZNT-E04, Foshan Lepton Precision Measurement and Control Technology Co., Ltd). The applied potential onto the needle was 8.1-9.7 kV. The collector was grounded tin-foil, and the PDMS substrate was placed on the foil to collect P4VP NWs. The distance between the syringe needle tip and the collector was 15 cm, and the pump rate was 0.6 mL/h. High electrical field and surface charges pulled P4VP NWs out of the droplet, and the NWs were attracted onto the collector due to electrical force. Finally, the pre-curing PDMS film with semi-embedded P4VP NWs was placed at 80°C for curing 1 h.

Preparation of as-coated P4VP nanowires. First, the PDMS elastomer substrate with thickness of 40 µm was prepared by mixing sylgard 184 “base” and “curing agent” with a weight ratio of 10:1, spin-coating the mixture solution with a speed of 500 rpm for 10 s and 1000 rpm for 30 s on the PET substrate, pre-curing at 60 °C for 15 min, and curing at 80°C for 1 h. Second, spin-coating the commercial Cu NWs with a speed of 1000 rpm for 30 s on the flexible substrate, and thermal annealing in 80℃ for 20 min.

Conductive CuNWs fabricated by activation and electroless deposition. Semiembedded and as-coated P4VP NWs were immersed into the aqueous solution which contained 1g/L PdCl2 and 0.036 M HCl to deposite Pd seed layer for 2 min. The sample was then thoroughly rinsed with deionized water. The Pd seed/P4VP NWs were immersed into the Cu electroless deposition solution, which contained 0.0416 M CuSO4·5H2O, 0.0507 M C4H4KNaO6·4H2O and 0.4375 M NaOH. Different thickness of CuNWs could be achieved by changing the immersion time.

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Fabrication of PSCs. The PSCs were fabricated with the device structure of stretchable semi-embedded CuNWs TCFs/PEDOT: PSS/PTB7-Th: PC71BM/Ca/Al. PEDOT: PSS solution was spin-coated on the stretchable semi-embedded CuNWs transparent electrode with a speed of 4000 rpm for 1 min and then transferred to a glove box to spin-coating the PTB7-th: PC71BM solution with a speed of 1200 rpm for 40 s. Finally, 0.7 nm Ca and 90 nm Al layers were deposited by thermal evaporation under a vacuum of 6×10-4 Torr to accomplish the device fabrication.

Characterization. All the scanning electron microscope (SEM) images were taken by FEI XL30 Sirion SEM equipped with an energy dispersive spectrometer (EDS). The samples were sputtered with ~5 nm gold to prevent electrons charging. X-ray photoelectron (XPS) spectra were collected by SSI SProbe XPS spectrometer with Al Kα source. X-ray diffraction (XRD) pattern was acquired using a Bruker D8 Advance X-ray diffractometer operating with Cu Kα radiation (λ=1.5406 Å). Atomic force microscopy (AFM) image was taken with a Veeco D-3100 atomic force microscope. The optical transmittance of films was measured using UV-Vis-NIR spectroscopy (Cary 5000 UV-Vis-NIR, Agilent) with the transmittance of the air as a baseline. The sheet resistances of the conductive films were measured using a digital Keithley 4200-SCS semiconductor parametric analyzer with a Pro-4 Lucas Lab four-point probe to eliminate contact resistance. Ten randomly selected points on each 3.0 × 3.0 cm sample were measured. Mechanical characterization (bending and stretching) was conducted by high precision stretching machine. The samples were bent to a mandrel with radius of 1 cm and stretch to 25% up to 2000 times. Their sheet resistances were recorded after certain cycles to track the performance degradation. Stretching-induced strain = L1/L0 × 100 (L1: length after stretching, L0: length before stretching). The J-V curves were measured with a Keithley 2400 source meter (Abet Solar Simulator Sun2000) under AM 1.5G light source (100 mW cm-2). The effective area of one cell was 0.1 cm2.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Scanning electron microscopy (SEM) image of the electrospun P4VP 11



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nanowires. The coverage degree of the junction-free nanowires after activation versus additive acid volume in activation solution and activation time. The optical transmittance (T) and sheet resistance (Rs) existed with increasing the deposition time. Video S1 displaying that there was no obvious brightness change on different location of the electric circuit. In particular, LED lamp still lighted up even after stretching with 25% strain. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail: [email protected] (Y. Chen), [email protected] (L. Tan). Author Contributions X. Y. and X. H. contributed equally to this work. Notes Competing financial interests. The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y. C. thanks for support from the National Natural Science Foundation of China (NSFC) (51673091) and National Science Fund for Distinguished Young Scholars (51425304). L. T. thanks for the support from the National Natural Science Foundation of China (NSFC) (51672121). We also thank for support from Natural Science Foundation of Jiangxi Province (20161BBH80044, 20161BCB24004 and 20161ACB20020), and Graduate Innovation Fund Projects of Nanchang University (cx2016057).

References (1) Im, H.-G.; An, B. W.; Jin, J.; Jang, J.; Park, Y.-G.; Park, J.-U.; Bae, B.-S. A highPerformance, Flexible and Robust Metal Nanotrough-Embedded Transparent Conducting Film for Wearable Touch Screen Panels. Nanoscale 2016, 8, 39163922. (2) Lipomi, D. J.; Tee, B. C. K.; Vosgueritchian, M.; Bao, Z. Stretchable Organic Solar Cells. Adv. Mater. 2011, 23, 1771-1775. 12


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(3) Lee, J.; Wu, J.; Shi, M.; Yoon, J.; Park, S. I.; Li, M.; Liu, Z.; Huang, Y.; Rogers, J. A. Stretchable GaAs Photovoltaics with Designs that Enable High Areal Coverage. Adv. Mater. 2011, 23, 986-991. (4) Kaltenbrunner, M.; White, M. S.; Głowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S. Ultrathin and Lightweight Organic Solar Cells with High Flexibility. Nat. Commun. 2012, 3, 770. (5) Hwang, H.; Kim, A.; Zhong, Z.; Kwon, H.-C.; Jeong, S.; Moon, J. ReducibleShell-Derived

Pure-Copper-Nanowire

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Application

to

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Figure 1. (a) Schematic illustration of fabricating stretchable semi-embedded copper nanowires (CuNWs) transparent conductive films (TCFs) based on electroless deposition copper on the electrospun poly(4-vinylpyridine) (P4VP) NWs template. The inset shows the chemical equations and mechanism of activation and electroless deposition. (b, c) The photographs of stretchable CuNWs films (15×15 cm).

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Figure 2. (a) Statistics of the specific morphologies (diameter and beads) of electrospun P4VP NWs prepared under different conditions (viscosity of electrospinning solution versus voltage) and (b) representative scanning electron microscopy (SEM) images. (c) X-ray photoelectron spectroscopy (XPS) spectra of nanowires after activation and electroless deposition, referenced to C 1s at 284.5 eV. (d) XPS core-level spectrum and (e) X-ray diffraction (XRD) pattern of electroless deposition CuNWs. (f) Energy-dispersive spectroscopy (EDS) mapping images of CuNWs. Note that there is no conductive coating for imaging, so the substrates are insulating and cause charging effect between the NWs.

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Figure 3. (a, b) SEM and atomic force microscopy (AFM) images of electroless deposition CuNWs by electrospinning P4VP template. The electroless deposition covers and fuses the entire junction. (c) The transmittance spectra of CuNWs TCFs at the wavelength between 300 and 800 nm. (d) Sheet resistance versus optical transmittance (550 nm) of different TCFs from commercial source and literatures. Metallic nanowires11,48, carbons16,18,49, conductive polymers13,14,50, architectures51-53 and hybrids19,20 are shown for comparison.

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Figure 4. (a) Cross-section SEM image and (b) tapping mode AFM image with line scan as marked in the image and height line profiles of semi-embedded CuNWs. R/R0 values of the as-coated CuNWs mounted on PDMS and semi-embedded CuNWs in PDMS, as a function of (c) the exposure duration time (in 85% relative humidity and 25 °C), (d) bending cycles (with radius of 1 cm), (e) uniaxial stretching strains and (f) stretching cycles (at 25% strain), respectively. (g) SEM images of the semi-embedded CuNWs with different stretching strain.

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Figure 5. (a) Demonstration of the stretchable semi-embedded CuNWs TCFs in the light-emitting diode (LED) circuit. (b) Device structure and (c) photograph of the flexible polymer solar cells based on the stretchable CuNWs as electrode and PTB7-th:PC71BM as active layer. (d) The current density-voltage (J-V) characteristics of the corresponding flexible device under AM 1.5 illumination of 100 mW/cm2. The effective area is 0.1 cm2.

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Graphical abstract

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Large-Scale Stretchable Semiembedded Copper Nanowire

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