Highly Transparent, Conductive, and Bendable Ag Nanowire

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Highly Transparent, Conductive and Bendable Ag Nanowire Electrode with Enhanced Mechanical Stability based on Polyelectrolyte Adhesive Layer Tieqiang Wang, Chengsheng Luo, Fuchun Liu, Linlin Li, Xuemin Zhang, Yunong Li, En-Hou Han, Yu Fu, and Yonghua Jiao Langmuir, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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Highly Transparent, Conductive and Bendable Ag Nanowire Electrode with Enhanced Mechanical Stability based on Polyelectrolyte Adhesive Layer Tieqiang Wang,†, * Chengsheng Luo,‡ FuChun Liu,§ Linlin Li,† Xuemin Zhang,† Yunong Li,† Enhou Han, § Yu Fu†, * and Yonghua Jiao‡, * † College of Sciences, Northeastern University, Shenyang 110819, P. R. China; ‡ College of Life and Health Sciences, Northeastern University, Shenyang 110819, P. R. China. § State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, P. R. China ∗

Corresponding author: E-mail: [email protected], [email protected], and [email protected].

Abstract In this paper, a highly transparent, conductive and bendable Ag nanowire based electrode with excellent mechanical and air-stability was prepared through introducing adhesive polyelectrolyte multilayer between the Ag nanowire networks and polyethylene terephthalate (PET) substrate. The introduction of the adhesive layer was performed based a peel-assembly-transfer procedure, and the adhesive polyelectrolyte greatly improved the mechanical of the Ag nanowire transparent conductive films (TCFs) without obviously attenuating the morphology and optoelectrical properties of the Ag nanowire networks. The as-prepared Ag nanowire

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TCFs simultaneously possess high optical transparency, good conductivity, excellent flexibility, as well as remarkable mechanical stability. It is believed that the proposed strategy would pave a new way in preparing flexible transparent electrodes with long-term stability, which is significant in the development and practical application of flexible transparent electronic device operated in severe environment.

Introduction Transparent conductive films (TCFs), which provide both good transmittance and electrical conductivity, are an indispensable component of optoelectronic device, such as touch screens, photovoltaic cells, liquid crystal displays, and light emitting diodes.1-7 Nowadays, indium tin oxide (ITO) has dominated the TCFs industry due to its excellent optoelectrical performance and technological maturity.8 However, ITO has some inherent shortcomings, including brittleness and expensiveness, facing the rapid growth of next-generation flexible electronics and optoelectronic, which requires light-transmitting electrodes possessing both mechanical flexibility and environmental stability, in addition to good optical transparency and electrical conductivity.9,

10

Aiming at substitutes to the traditional ITO, many materials,

including carbon nanotubes,11-13 metallic nanowires,9, 14-16 graphene,17-19 conductive polymer,20-22 and metal grids23 have been investigated. Among them, Ag nanowire (AgNW) TCFs constructed from random networks of NWs have garnered intensive interest owing to the excellent intrinsically electrical conductivity of AgNWs. Up to now, many flexible AgNW TCFs have been achieved using various techniques including transfer printing, air-spraying, and bar coating.24-30 Despite these exciting


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achievement, several challenging issues remain, especially the poor adhesion forces between the AgNWs and the flexible substrates, which greatly hinder the stability of AgNW TCFs. Therefore, it is of paramount importance for practical application to develop effective strategies for the improvement of the mechanical stability of AgNW TCFs without compromising their optoelectronic properties. At present, various enhancing strategies, mostly using an over-coating layer either on the AgNWs or on the whole substrate, have been proposed to improve the mechanical stability of AgNW TCFs.31-36 Although such strategies have shown improvement of their mechanical stability to some extent, the over-coating layer may also unavoidably affect the electrical contact between the AgNW TCFs and the subsequent functional layers on the TCFs, thus, affecting the performance of the flexible electronic devices, such as solar cells and OLEDs. Recently, we have proposed a peel-assembly-transfer (PAT) strategy, which introduces adhesive polyelectrolyte multilayer between the metallic microstructures and the substrate, to improve the mechanical stability of the metallic microstructures.37 The proposed PAT strategy exhibits significant improvement of the mechanical stability, meanwhile, the optical and electrical properties of the metallic microstructures are well preserved owing to the exposure of the microstructures to the external atmosphere. Here in, AgNW networks were transferred and stabilized onto flexible polyethylene terephthalate (PET) substrates based on the PAT strategy to prepare AgNW TCFs with highly mechanical stability and excellent bendable character for flexible electronic device applications. The morphology of the AgNW networks were well preserved


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after the PAT procedure, resulting in nearly intact preservation of the highly transparent and conductive properties of the AgNW networks. Meanwhile, owing to the flexibility of PET substrate and AgNWs, the as-prepared AgNW TCFs is also bendable. What's more, the as-prepared AgNW TCFs exhibit excellent stability against ultrasonic and adhesion treatment due to the strong adhesive interaction introduced by the inserted polyelectrolyte multilayer. Together with all these excellent features desired for flexible electrode materials, it is believed that the as-prepared AgNW TCFs would show great potential in flexible transparent electronic devices, and pave the way for their further practical application, especially in severe environment.

Experimental Section Materials. Glass and PET substrate were treated in a plasma cleaner for 10 min to create a hydrophilic surface and then rinsed repeatedly with Milli-Q water (18.2 MΩ cm) and ethanol. The substrates were dried in nitrogen gas before used. AgNWs (50-nm diameter, dissolved in ethanol) were purchased from Nanjing XFNANO Materials TECH Co., Ltd. Polyetherimide (PEI, Mw = 70 000, 50 wt%) were purchased from Alfa Aesar. Poly(acrylic acid) (PAA, Mw = 100 000, 35 wt%) were purchased from Aldrich. The PEI and PAA were diluted to 1.0 mg/mL before LBL assembling. Polylactic acid (PLA, Mw = 300 000) was purchased from Shandong Institute of Medical Instruments. Sulfuric acid, hydrogen peroxide, dichloromethane, sodium lauryl sulfate, and acetone were all used as received. Preparation. The AgNW networks were fabricated through simply spin-coating


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AgNW solution on the glass substrates. To peel the AgNW networks off from the glass substrate, acetone solution of PLA (100 mg/mL) was firstly spin-coated onto AgNW network to form a layer of the PLA film, after thermal treatment at 60 °C for 1 hour, the PLA film with AgNWs embedded in the film was immersed in Milli-Q water and peeled off from the glass substrate under water. After peeling off the AgNW networks, layer-by-layer (LBL) assembling of the PEI/PAA adhesion multilayer was performed on the AgNWs by immersing the PLA film with AgNWs in PEI and PAA solutions alternatively. Finally, the AgNW network with the PEI/PAA adhesion layer was compressed onto the target PET substrate, and AgNW TCFs with the inserted polymer adhesion underlayer were left on the PET substrate after dissolving the PLA film in CH2Cl2. Characterization. The scanning electron microscopy (SEM) images were taken using a Hitachi S-3400N electron microscope. Before imaging, samples were sputter-coated with a 10 nm Au layer. The transmission spectra of the AgNW TCFs were recorded by a PERSEE TU-1810 spectroscope using PET substrate as blank sample. The sheet resistance of the AgNW TCFs were measured using four probe method. As for the transmittance at 550 nm and sheet resistance of the AgNW TCFs, at least five measurements were performed and averaged for all of the data reported here. The stability of the AgNW TCFs against ultrasonic treatment was evaluated using a commercial ultrasonic cleaner (rated power = 100 w). The stability test against adhesion were performed through adhering the whole sample with 3M tape for several times under a certain pressure (~6.0 kPa).


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Results and Disscussions The schematic procedure for the transfer and stabilization of the AgNW network onto PET substrate is illustrated in Figure 1. In brief, AgNW solution was firstly spin-coated onto the glass substrate to prepare a transparent and conductive AgNW network. Then, a thin film of polylactic acid (PLA, ~300 µm thick) was spin-coated onto glass substrate with the AgNWs. After the substrate with PLA is thermal treated in a vacuum oven at 60 °C for ~1 hour to embed the AgNWs into the PLA film, PLA film with the embedded AgNWs was peeled off from the glass substrate. In order to facilitate the peeling process, the peeling process was conducted under water, since water will permeate into the space between the PLA film and the substrate. After the peeling process, there was almost no AgNWs left on the original glass substrate. Next, layer by layer (LBL) assembly of PEI/PAA polyelectrolyte multilayer (~50 nm) was conducted on the surface of AgNW network through alternatively immersing the PLA film in PEI and PAA solutions. Finally, the PLA film with PEI/PAA adhesive multilayer was compressed onto a flexible and transparent PET substrate with a thin layer of the PEI/PAA multilayer. After dissolving the PLA film in dichloromethane, the AgNWs are firmly adhered onto the PET substrate owing to the adhesion between the PEI/PAA multilayer. In order to obtain highly transparent and conductive electrode, the amount of the AgNWs on the surface of the substrate, which is highly related to the transmittance (T) and sheet resistance (Rs) of the TCF, must be optimized according to the work reported previously. Therefore, the concentration of the AgNW solution used to


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prepare a transparent and conductive Ag nanowire network was optimized first of all, and the result is shown in Table S1. In our experiment, the concentration of the AgNW solution was set as the 9.2 mg/mL, since the as-prepared AgNW network possesses both high transparency (T550nm = 91.0 ± 1.5%) and good conductivity (Rs = 34.6 ± 3.0 Ω sq−1). Figure 2(a) shows the SEM image of the as-prepared AgNW network. From the SEM image, it can be seen that the aspect ratio of the AgNWs is extremely high (>100), and the AgNWs intercross with each other, which guarantee the good conductivity of the AgNW network. After the PAT process, the AgNW network was transferred and adhered on the PET substrate, and the SEM image of the AgNW network on PET was also measured as shown in Figure 2(b). Through comparing the SEM images, it can be clearly concluded that morphology of the AgNW networks maintains well during the PAT process. Both the large aspect ratio of each Ag nanowire and the inter-crossing structure of the AgNW networks are preserved thanks to the protection of the PLA film. Meanwhile, AFM measurements were also performed to investigate the topology transition during the PAT process. As shown in AFM images, the vertical distance between the top of AgNWs and the substrate surface is about 45 nm before the PAT process (Figure 2(c)), while the vertical distance decreases slightly to 39 nm after the PAT process (Figure 2(d)). According to the AFM measurements, it could be concluded that most part of the Ag NWs network is exposed without protection of over-coating layers, only little part (~10%) of the AgNW is embedded in the assembled PEI/PAA multilayer, which corresponds well with our previous work.37 Furthermore, the XPS spectrum of the


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AgNWs were also measured to prove the partly embedding topology after the PAT process as shown in Figure 2(e). the relatively high intensity of the Ag3d peak also indicates that most part of the AgNW network is exposed to the atmosphere without over-coating layers. Since the morphology of the AgNW network is preserved well during the PAT stabilization procedure, it is believed that the optical and electrical property of the AgNW network would also maintains after the AgNW network is stabilized on the PET substrate. Figure 3(a) shows the transmittance spectra of the AgNW network before and after the PAT stabilization process. The transmittance spectra clearly show that the PAT process shows little influence on the transmittance of the AgNW network, this is attributed to the highly transparence of the adhesive PEI/PAA polyelectrolyte layer. Meanwhile, the high conductivity of the AgNW network (Rs = ~34.0 Ω sq−1) is also preserved during the PAT stabilization process. Since the PEI/PAA multilayers is the key point for the PAT stabilization process, the effect of number of the PEI/PAA multilayers is further investigated. Figure 3(b) and 3(c) show the transmittance at 550 nm and the sheet resistance of the AgNW networks before and after the PAT stabilized with PEI/PAA multilayers of different bilayers. Thanks to the transparence of the adhesive PEI/PAA polyelectrolyte layer, the existence of PEI/PAA layer between PET substrate and AgNWs hardly influence the transmittance of the AgNW TCFs, the as-stabilized AgNW TCFs with different numbers of PEI/PAA bilayer all kept high optical transparence (T > 90%) as shown in Figure 3(a). In addition, owing to the dielectricity of the PEI/PAA multilayer, the sheet resistance of the AgNW networks


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experiences almost no changes after the PAT stabilization procedure with different numbers of PEI/PAA bilayer (Figure 3(c)). According to our previous work, to guarantee the mechanical stability of the metallic microstructures, the total number of adhesive PEI/PAA bilayers must be no less than 4.5.37 Therefore, in subsequent discussion, the as-prepared AgNW TCFs are all prepared with 5.5 bilayers of inserted PEI/PAA bilayer (2.0 bilayers on PET substrate and 3.5 bilayers on the AgNW networks). Except for the well preserved high transparence and conductivity, the as-prepared AgNW TCFs are highly bendable owing to the excellent flexibility of both the PET substrate and the AgNWs. Figure 4 shows the photograph of the as-prepared AgNW TCF on PET substrate, The AgNW TCF shows high transparency (top) and flexibility (bottom), the sheet resistance of the AgNW TCF keeps unchanged even with a bending angle higher than 180°. In addition, the as-prepared AgNW TCF also exhibits the capability to be bent repeatedly. Quantitatively, we measured the sheet resistance of the AgNW TCF in more than 600 bending cycles as shown in Table 1. The sheet resistance of the AgNW TCF experienced little fluctuation (no more than 20%) during 600 bending cycles, which can be attributed to the excellent flexibility of both the PET substrate and the AgNWs. According to the work reported by Jiang,34 the sheet resistance of a commercial ITO electrode on PET would elevate dramatically when the bending angle increases to 30° owing to the brittle nature of ITO, and the sheet resistance continues increasing during the bending experiment. The well preservation of the conductivity during bending test clearly demonstrates that the as-prepared


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AgNW TCFs are highly bendable and might be a good candidate for electrode of flexible electronic devices. At present, although many flexible electrode materials with similar transparency, conductivity and bendability have been prepared based on metallic or carbon nanowires, the poor mechanical stability of the nanowires on the substrate is still a big challenge greatly restricting their application in flexible electronic devices. According to our previous work, the PEI/PAA adhesive layer will greatly reinforce the interaction between the metallic microstructures and substrate, therefore, it is believed that the as-prepared AgNW TCF will show greatly improved mechanical stability owing to the inserted polyelectrolyte adhesive layer between the AgNWs and PET substrate. In order to evaluate the improved mechanical stability, the as-prepared AgNW TCF was immersed in a ultrasonic bath (operating power 100 w), and the sheet resistance of the sample was measured after a certain treating time as listed in Table 2. The sheet resistance of the as-prepared AgNW TCF shows only a little variation (less than 15%) during long-duration ultrasonic treatment. The strong interaction between the PEI layer and the AgNWs prevents the falling off of the AgNWs from PET substrate. Thus, the AgNW TCF is well preserved against the ultrasonic treatment. In contrast, the control sample (prepared by simply spin-coating AgNWs solutions on PET substrate) shows little tolerance against ultrasonic treatment, the AgNWs fell off from the substrate completely after only 5-min ultrasonic treatment. Such results clearly demonstrate that the existence of the PEI/PAA adhesive layers has greatly improved the mechanical stability of the AgNW TCFs.


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What's more, due to the enhanced interaction between AgNWs and the PET substrate, the as-prepared AgNW TCFs also exhibit excellent mechanical stability against physical adhesion. Figure 5(a) shows the relative sheet resistance of the AgNW TCFs after adhered with a commercial 3M adhesive tape repeatedly. The plot data clearly shows that the sheet resistance of the AgNW TCFs changed just a little (~10%) after more than 30-cycle adhesion treatments thanks to the highly enhanced interaction introduced by the inserted PEI/PAA adhesive layer. Further increasing the treatment times would lead to the destruction of the AgNW networks, thus, inducing the increase of the sheet resistance of the AgNW TCFs. Meanwhile, the high transparence character still maintains well (increases from 91.3% to 94.0%) during the adhesion treatment since the falling of AgNW network from the substrate shows little effect on the transmittance of the AgNW TCFs (Figure 5(b)). Moreover, the adhesion treatment of the control sample is also performed to compare with the as-prepared TCFs, the high conductivity of the sample vanishes after only one-cycle adhesion treatment, indicating that the mechanical stability against adhesion is attributed to the existence of the PEI/PAA adhesive multilayer. From the results shown in the ultrasonic and adhesion treatment, we could conclude that the as-prepared AgNW TCFs show remarkable mechanical stability owing to the strong adhesion offered by the PEI/PAA multilayer beneath the AgNW network. Even if the AgNW network are expose to the atmosphere without protection of any over-coating layer, the mechanical stability are still greatly improved. What's more, the lack of over-coating layer on the AgNW would extremely benefit the electrical contact


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between the AgNW electrode and the subsequent functional layers during the fabrication of flexible electronic devices (such as solar cells and OLEDs), thus, greatly reducing the influence of the over-coating layer and guaranteeing the performance of the devices. Finally, we compared the as-stabilized AgNW TCF with other previously reported flexible TCFs based on AgNWs. As shown in Figure 6, the AgNW TCF prepared through the proposed PAT stabilization strategy exhibits a relatively higher optical transparency and a comparable or even better conductivity in comparison with that reported in previously works. Meanwhile, together with the greatly reinforced mechanical stability, it is believed that the as-prepared AgNW TCFs are quite suitable as electrodes for flexible transparent electronic devices, especially that operated in severe environment. Besides of all these advantages, there is also one flaw of the proposed stabilizing strategy. Since there is no over-coating layer above the AgNWs for the as-prepared AgNW TCF, thus, the as-stabilized AgNW TCF show similar air tolerance as control sample. However, when TCFs are used to prepare flexible transparent electronic device (such as solar cell and OLEDs), there will always be several other subsequent layers coated onto the top of the TCFs, (such as the donor and acceptor layer), therefore, protecting the AgNW TCFs from oxidation by external air. Considering the greatly improved mechanical stability of the as-prepared AgNW TCFs, the proposed stabilizing strategy is still of great scientific significance and application prospects.

Conclusion


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In summary, through inserting PEI/PAA polyelectrolyte multilayer between the AgNW networks and PET substrate, we have successfully prepared highly transparent, conductive, bendable and stable AgNW TCFs for flexible electronic device applications. The introduction of PEI/PAA adhesive layer can greatly improve the mechanical stability of the AgNW TCFs without obviously attenuating the optical and electrical properties of the AgNW networks. This method can be easily extended to enhance the stability of other metal-based electrodes, such as copper, aluminum, and their alloys. It is believed that the proposed PAT strategy paves a new way in preparing flexible transparent electrodes with long-term stability, which would be significant in the development of flexible transparent electronic device, especially that operated in severe environment.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 21404021, 21174024, 21503037), the Doctoral Scientific Research Foundation of Liaoning Province (201501149), Fundamental Research Funds for the Central Universities (N150504004, N150504005, N140502001, N140503001, N160504002) and the Open Project of the State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201705).

Supporting Information Available: Supporting Information includes the transmittance and sheet resistance of the AgNW TCFs prepared using AgNWs solution with different concentration. This material is available free of charge via the Internet at http://pubs.acs.org.


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Anionic Polyelectrolyte for Aqueous PEDOT Dispersions: Toward Printable Transparent Electrodes. Angew. Chem. Int. Ed. 2015, 54, 8506-8510. (23) Tvingstedt, K.; Inganäs, O. Electrode Grids for ITO Free Organic Photovoltaic Devices. Adv. Mater. 2007, 19, 2893− 2897. (24) Gaynor, W.; Burkhard, G. F.; McGehee, M. D.; Peumans, P. Smooth Nanowire/Polymer Composite Transparent Electrodes. Adv. Mater. 2011, 23, 2905−2910. (25) Madaria, A. R; Kumar, A.; Zhou, C. Large Scale, Highly Conductive and Patterned Transparent Films of Silver Nanowires on Arbitrary Substrates and their Application in Touch Screens. Nanotechnology 2011, 22, 245201. (26) Hu, L.; Kim, H. S.; Lee, J-Y; Peumans, P.; Cui, Y. Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes. ACS Nano 2010, 4, 2955−2963. (27) Xu, F.; Durham, J. W., III; Wiley, B. J.; Zhu, Y. Strain-Release Assembly of Nanowires on Stretchable Substrates. ACS Nano 2011, 5, 1556−1563. (28) Scardaci, V.; Coull, R.; Lyons, P. E.; Rickard, D.; Coleman, J. N. Spray Deposition of Highly Transparent, Low-Resistance Networks of Silver Nanowires over Large Areas. Small 2011, 7, 2621−2628. (29)

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Figure 1. The 3D (left) and sectional (right) schematic PAT procedure for the insertion of the PEI/PAA adhesion multilayer between AgNWs and PET substrate. (a) spin-coating PLA; (b) peeling off PLA film; (c) assembling of PEI/PAA multilayer; (d) compressing onto PET; (e) dissolving PLA film.


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Figure 2. The SEM images of the AgNW network before (a) and after (b) the PAT process. The AFM height phase images (left) and line profile (right) of the AgNW network before (c) and after (d) the PAT process. (e) the XPS spectrum of the AgNW network after the PAT process.


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Figure 3. (a) The transmittance spectra of the Ag NW network before and after the PAT process. (b) The transmittance at 550 nm and (c) the sheet resistance of the original AgNW network and as-prepared AgNW TCFs with different numbers of PEI/PAA bilayer.


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Figure 4. The photograph of the as-prepared AgNW TCF on PET substrate indicating that the film is transparent and flexible.


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Figure 5. The plot date of the (a) sheet resistance and (b) transmittance at 550 nm against the times of adhesion treatment with commercial 3M tape. The sheet resistance of the AgNW TCFs changed just a little (~10%) after 30 adhesion treatments and increases after further increasing the treatment times, while the transmittance changed a little (2%~3%) during the adhesion treatment.


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Figure 6. Plot of transmittance at 550 nm vs sheet resistance of the AgNW films in this work and other literatures. The data for the other AgNW films were taken from literature and replotted for comparison.


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Table 1. The sheet resistance and the normalized sheet resistance of the as-prepared AgNW TCF after bending for different times. Number of Bending Times

Rb (Ω sq-1)

Rb/R0 (%)

0 10 30 100 150 210 360 450 600

34.8 38.1 31.9 41.7 36.5 37.8 28.3 38.1 34.9

100 109 91.7 120 105 109 81.3 109 100

R0 is the sheet resistance of the original AgNW TCF; Rb is the sheet resistance after bending;


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Table 2. The sheet resistance and the normalized sheet resistance of the as-prepared AgNW TCF after ultrasonic treated for different time. Ultrasonic Time (min)

Ru (Ω sq-1)

Ru/R0 (%)

0 5 15 30 50 90

32.1 33.6 35.2 28.2 35.1 27.3

100 105 110 87.9 109 85.0

R0 is the sheet resistance of the original AgNW TCF; Ru is the sheet resistance after ultrasonic treatment;


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For Table of Contents Use Only

Highly Transparent, Conductive and Bendable Ag Nanowire Electrode with Enhanced Mechanical Stability based on Polyelectrolyte Adhesive Layer Tieqiang Wang*, Chengsheng Luo, Fuchun Liu, Linlin Li, Xuemin Zhang, Yunong Li, Enhou Han, Yu Fu* and Yonghua Jiao*


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Highly Transparent, Conductive, and Bendable Ag Nanowire

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