Novel Concept for Fabricating a Flexible Transparent Electrode Using

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A Novel Concept for Fabricating a Flexible Transparent Electrode Using the Simple and Scalable Self-Assembly of Ag Nanowires Young-Tae Kwon, Jong Woon Moon, Yo-Min Choi, Seil Kim, Seung Han Ryu, and Yongho Choa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00148 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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The Journal of Physical Chemistry

A Novel Concept for Fabricating a Flexible Transparent Electrode Using the Simple and Scalable Self-Assembly of Ag Nanowires

Young-Tae Kwon,#,† Jong Woon Moon,#,‡ Yo-Min Choi,§ Seil Kim,† Seung Han Ryu,† and Yong-Ho Choa†, ‡,*

†

Department of Fusion Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Korea

‡

Department of Bionano Technology, Hanyang University, Ansan 426-791, Republic of Korea

§

Material and Components Technology Center, Korea Testing Laboratory, Ansan 426-791, Republic of Korea

ABSTRACT

We introduce a new concept for transparent electrodes via the self-assembly of a silver nanowire (Ag NW) network with a cell shape. A transparent conductive network was achieved by forming an array of Ag NWs around droplets of a solvent with higher vapor pressure in Ag NWs ink. The difference in vapor pressure and viscosity of the solvent causes an Ag NWs network with a cell shape, and the cell size can be easily controlled from 10 µm to 100 µm using the solvent ratio. The cell network of Ag NWs with a high optical transmittance (> 92 %) and low sheet resistance (40 Ω/sq) was simply fabricated on flexible polymer films of large scale using a Meyer rod coating. In addition, we also studied and demonstrated the figure of merit of the transparent electrode between our method and a random Ag NWs network from the general method. The performance of the transparent electrode may be applied to a wide array of optoelectronic devices and can replace transparent conductive oxides such as Al-doped ZnO and indium tin oxide.

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1. INTRODUCTION Transparent electrodes are fundamental components in optoelectronic devices, including displays and solar cells. Recently, many researchers have moved towards the manufacturing of flexible electronics with a low-cost, easy and large-scale process. The sheet resistance (Rs) and optical transmittance (T) are the most important features for transparent electrodes, which require different performances depending on the application. For example, > 1 kΩ/sq is sufficient for the application of antistatic and electromagnetic shielding; touch screens require 400 – 1000 Ω/sq; organic light-emitting diodes (OLED) and solar cells require < 10 Ω/sq.1-3 To date, indium tin oxide (ITO) has been commonly used for this purpose, resulting in a market share reaching 90% for transparent electrode fields due to the high optical transmittance and low sheet resistance.3,4 However, the use of ITO as transparent electrodes has many drawbacks for the next generation of optoelectronic devices with flexibility due to the brittleness of conductive oxide, scarcity of indium, and difficulty in large areas. Therefore, many transparent electrode materials have been considered to replace ITO for future flexible optoelectronics. Several promising candidates include carbon-based materials such as CNT5-7 and graphene,8-10 polymer materials such as PEDOT:PSS,11-13 Ag metal materials such as nanoparticles (NPs)14-18 and nanowires (NWs).19-23 Among these materials, Ag metal materials have attractive performance as transparent electrodes in terms of conductivity and stability compared to carbon-based materials and polymers.15,20 Ag materials are well-known for having a high conductivity and resistance to oxidation compared to different metals.24 Transparent electrodes based on Ag nanomaterials are divided in to two configurations, a regular Ag grid1418,25

and random Ag NWs self-assembly,26-30 which can exhibit optical transparency and sheet resistance superior

to other materials. However, the Ag grid and random Ag NWs self-assembly methods have several drawbacks. In the case of the Ag grid, although the transparent electrodes with a high repeatability and device stability can be fabricated by controlling the grid width, spacing, and thickness of the grid, the fabrication method is often complicated, costly, and time-consuming because the process can include photolithography, direct printing, or embossing.2,3,25 Especially, the transparent electrode fabrication method using coffee ring effect is timeconsuming due to repetition of dropping one droplet and then drying, and need to additional expensive equipment for formation of the constant droplet. Additionally, the random Ag self-assembly method, which is suitable for solution-based processes, is simple and easy to mass-produce, however the performance of the resultant transparent electrodes is dependent on the length of the nanowire, resulting in poor connectivity and high resistivity.15,25 Therefore, to fabricate the transparent electrode of high performance for simple and inexpensive method, it is important to manufacture the Ag grid pattern using the self-assembly capable of


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solution process. In this work, we successfully manufactured transparent electrodes with a novel concept, and report several new advancements using Ag NWs. First, the Ag NWs network with a cell shape, similar to the Ag grid, was selfassembled on flexible polyethylene terephthalate (PET) films by mixing water-dispersed Ag NWs and a small amount of solvents such as α-terpineol, cyclohexanol, and 1-dodecanol. This cell structured self-assembly would be an easily scalable and fast fabrication method using the existing cheap coating equipment, such as meyer rod, slot die, doctor blade, spin coating, and gravure techniques. Furthermore, we experimentally explained the mechanism by differences in solvent evaporation. In this regard, the self-assembled cell size was easily controlled from 10 µm to 100 µm with respect to the solvents ratio. Transparent electrodes were employed on a Meyer rod technique for scalable coating roll-to-roll of Ag NWs. Finally, the method was compared to the random Ag NWs self-assembly method, resulting in self-assembly into cell shaped Ag NWs clearly showing enhanced performance of the transparent electrodes.

2. EXPERIMENTAL METHODS Preparation of Ag NWs ink. The aqueous solution contained 0.5 wt% Ag NWs and was purchased from NANOPYXIS. The concentration of Ag NWs in the aqueous solution was adjusted from 0.1 to 3.0 wt%. The ink formulation was made by mixing the Ag NWs solution (2.5 g), methanol (6 g, MeOH), α-terpineol solvent (1.5 g, 90 % Sigma Aldrich), cyclohexanol (99 %, Sigma Aldrich), or 1-dodecanol (98 %, Sigma Aldrich). The ink was then briefly sonicated for 1 min to retain the length and diameter of Ag NWs. Fabrication of Ag NWs transparent electrodes. To fabricate the transparent electrode, the PET films were treated using oxygen plasma equipment (ICP-PIE, Standard Asher RIE System, SNTEK) for 10 s to form a hydrophilic surface. The Ag NW ink was dropped on the PET film, and then quickly pulled down using a Meyer rod (R.D. Specialties #16, 40.6 µm wet film thickness), as shown in Figure 1d. The Ag NWs coated PET film was dried at 70 oC. To remove the residual solvents, the Ag NWs electrode was dipped in methanol for 10 min, and heated at 150 oC for annealing the Ag NWs for 1 h. Characterization. The contact angle of DI water was analyzed using a contact angle analyzer (Phoenix 300 plus, SEO) at room temperature to evaluate the wettability of the PET films. The optical transmittance and sheet resistance values of the Ag NWs electrode were measured by a UV/VIS spectrometer (Mega-2100, Scinco) and a four-point probe (CMT-SR2000N, AIT). The reference of the UV/VIS spectrometer was the original PET film. The haze value was analyzed using a haze meter (NDH-2000N, Nippon Denshoku). The sheet resistance of the



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samples was the average of 10 measurements. The cell structure of the Ag NWs was observed using a fieldemission scanning electron microscope (FE-SEM; S-4800, Hitachi). To investigate the mechanical properties of the prepared Ag NWs transparent electrodes, a bending test was performed using the home-made IPC sliding tester.31 The samples (20 mm wide, 150 mm long) were loaded into the bending system consists of lower and upper plates. In the system, the upper plate is fixed, while the lower plate moves laterally with stroke of 180 cycles/min. The resistance changes were measured over the bending test of 10,000 cycles. We also tested their mechanical properties under the extreme condition of folding and crushing the transparent electrodes. The Ag NWs self-assembled on PET film (12 x 12 cm2) was connected with a blue LED and an external voltage of 5 V. The condition of ink component was the Ag NWs solution (2.5 g), methanol (6 g, MeOH), and α-terpineol solvent (1.5 g). After the folding, crushing and then unfolding, the LED test was performed.

3. RESULTS AND DISCUSSION The contact angle images were captured using a charge-coupled device (CCD) camera. The contact angle of Ag NWs ink on untreated PET film was 74.22o, which decreased to 34.68o after oxygen plasma treatment (Figure 1a). The introduction of polar groups (-OH, -COOH, and -C=O) to the PET film surface enhances wettability between the Ag NWs ink and PET films, resulting in a uniform Ag NWs film coating without agglomeration.32 As shown in Figure 1b and Figure S1, the average diameter and length of the Ag NWs used was approximately 31.73 ± 4.41 nm and 20.75 ± 3.03 µm, respectively. Figure 1c shows a formulated Ag NW ink composed of an aqueous Ag NWs solution with 0.5 wt% Ag NWs, MeOH, and α-terpineol. The Ag NWs ink was developed for a simple and scalable Meyer rod coating in Figure 1d. Figure 2a, b, and c show the self-assembled Ag NW cell network on PET films fabricated with respect to the α-terpineol volume of 0.5, 1.5, and 2.5 g; because the Ag NWs inks were fixed to a weight of 10 g, the volume of MeOH was adjusted to 7.0, 6.0, and 5.0 g, respectively. As the α-terpineol volume increases, the cell size also increased from 6.71 to 126 µm. The size of the cell composed of Ag NWs was constant, while the network was similar to transparent electrodes of a metallic grid made by a complicated process in terms of repetition of constant spacing and size. However, there are no significant differences in optical transmittance (Figure 2d) and sheet resistance, as listed in Table S1, because the absolute Ag NWs in the ink was the same. Recently, the printed array of rings composed of Ag NPs were reported for a coffee stain effect, where a droplet containing solid particles was dropped, the droplet was dried, and the solid particles were closely packed to the droplet


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rim.14,33 Although the transparent electrodes have a high optical transmittance, it has some intrinsic limitations: various nozzle sizes for forming the droplet would be required for controlling the ring size, and the process is complicated and time-consuming. By tuning the volume of the solvents, various cell sizes of conductive Ag NWs can be fabricated, and the performance was ~50 Ω/sq. We evaluated the performance of the cell-shaped self-assembly and random self-assembly for comparing the two types of Ag NWs in the optical transmittance and sheet resistance. The cell shaped self-assembly was fixed by the solvent volume of 2.5 g. In contrast, random self-assembly was conducted by experiments under the same condition with inks composed of an aqueous Ag NWs solution (2.5 g) and MeOH (7.5 g) without additional solvents. The cell-shaped and random self-assembled transparent electrodes with several concentrations of Ag NWs are shown in Figure 3. First of all, it could be confirmed that the morphology of the Ag NWs is well retained after sonication process, as shown in the random self-assembly images of 0.1 and 0.25 wt% of Figure 3. Different Ag NW coverage densities were obtained with increasing concentration of Ag NW ink. Interestingly, the average cell size of Ag NWs was nearly similar at ~ 30 µm, despite the different densities of the Ag NWs. The volume of α-terpineol was most likely the dominant parameter determining the cell size. Figure S2 is the optical transmittance spectra for the cell shaped and random self-assemblies with different densities. Based on these spectra, the optical transmittance at a wavelength of 550 nm was plotted as a function of sheet. Based on these spectra, the optical transmittance at a wavelength of 550 nm was plotted as a function of sheet resistance in Figure 4. It seems reasonable that the cell shaped self-assembly is readily to build the specific conductive network and transmittance at the same Ag NWs content loading than the random self-assembly. To judge the performance of the silver network as a transparent electrode, we used the figure of merit defined as the ratio of the bulk DC conductivity and optical conductivity (σdc/σop).34-36 Herein, the higher σdc/σop, the better performance of the transparent electrodes. ௓ ఙ೚೛ ିଶ ) ೞ ఙ೏೎

T = (1 + ଶோబ

(1)

In Eq. (1), Z0 is the impedance of free space (377 Ω). The fits of the data following Eq. (1) result in figures of merit of σdc/σop = 218.6 and σdc/σop = 146.0, as shown in the inset data, with good linearity. The values of the Ag NWs networks were superior to the industry standard of σdc/σop = 35 required in the transparent electrode market, especially the cell shaped self-assembly presenting a better performance than the random self-assembly. In addition, we measured the haze of cell shaped self-assembly with the different concentration of Ag nanowire, in Figure S3. As the concentration of self-assembled Ag NWs inks increased, the haze values were increased from



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0.48, 0.64, 3.11, 5.59 to 10.27 %. The haze value of 0.5 % concentration with the transmittance and sheet resistance of 91.22 % and 54.17 Ω/□ meet the industrial standard for transparent electrode is the optical haze of 2-4 %. Figure 5a shows a schematic diagram of the cell network mechanism that was divided into 3 steps. First, the high vapor pressure MeOH was first evaporated in the Ag NW inks. Second, because the α-terpineol is insoluble in DI water, the low viscosity DI water spreads around the droplet of α-terpineol. The Ag NWs dispersed in DI water were then self-assembled around the droplet of α-terpineol. Finally, all solvents were evaporated, resulting in the formation of Ag NWs cell structures. The process of Ag NWs self-assembly was confirmed in the Supplementary Movie S1. For a proof of mechanism, we tested other solvents, such as cyclohexanol and 1dodecanol, which are commonly insoluble in DI water, soluble in MeOH, and have a high viscosity (DI water : 0.89 cP, MeOH : 0.54 cP, α-terpineol : 36.5 cP, cyclohexanol : 41.07 cP, 1-dodecanol : 18.8 cP). The ink formulation composed of cyclohexanol and 1-dodecanol is as same as the Ag NWs of α-terpineol. The Ag NWs inks with cyclohexanol and 1-dodecanol self-assembled into a cell network, as shown in Figure 5b and c. The cell size, optical transmittance, and sheet resistance are summarized in Table S1. Figure S4 is the optical transmittance spectra with different solvents. In the case of 1-dodecanol, the cell size was larger than the Ag NWs inks of α-terpineol or cyclohexanol due to the different viscosity values. The viscosity is representative of fluid resistance to flow. Therefore, although the volume of 1-dodecanol was as same as α-terpineol, 1-dodecanol which has a lower viscosity than α-terpineol or cyclohexanol is more spread on the films, resulting in the formation of a larger cell size. This mechanism can also explain the relation between α-terpineol volume and cell size in Figure 2. As the volume of α-terpineol determining cell size is increased, the cell size of Ag NWs is increased because the remaining α-terpineol is increased after the evaporation of MeOH. The mechanical stability of transparent electrodes is one of the most important characteristics for flexible electronics. The samples for the bending test were prepared by cutting the Ag NWs self-assembled on PET film as shown in Figure 6b and c. The transparent electrode was loaded into the bending system halfway, and the resistance change was measured in both ends of samples. Figure 6 shows a repeated fatigue bending test with respect to the bending radius of 5, 10, and 15 mm. The resistance of transparent electrodes did not change by within 2 % deviation during 10,000 cycles of bending (5, 10, and 15 mm of bending radius = 1.28, 1.29, and 1.73 % increase). The cell structured Ag NWs transparent electrode demonstrated an excellent mechanical performance compared to the previously reported Ag NWs transparent electrodes.15,37,38 Additionally, a blue light emitting diode (LED) connected to the transparent electrodes were successfully fabricated on a flexible


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PET film with a large area, as shown in Figure S5. After the extreme stress of folding and crushing, the LED intensity with cell shaped Ag NWs had no obvious changes and the conductivity remained. Lastly, intense pulsed light (IPL) sintering was employed in the transparent electrodes self-assembled with 0.5 wt% Ag NWs. The IPL sintering has recently known as an effective ways of enhancing the conductivity of metallic materials without the deformation of polymeric substrates, such as PET, PI and PC, caused by thermal energy.39,40 A topand tilt SEM images of the Ag NWs after 8.0 J·cm-2 illumination is shown in Figure S6, and the junctions between wires were enhanced to form tight contacts. The sheet resistance and optical transmittance are 20.53 ± 2.09 Ω/□ and 89.02 %. This figures were much enhanced then thermal annealing which is 50.71 ± 4.75 Ω/□ (92.64 %). Although the results of IPL annealing were fundamental experiment stage, the properties of transparent electrode were improved, and it will be further studied. To the best of our knowledge, the cell shaped self-assembly of Ag NWs on transparent electrodes has not been previously fabricated based on differences in solvent evaporation. This self-assembly method might be an easily scalable coating using slot die or gravure techniques, and may be applied to metal nanowires and other nanowire materials.

4. CONCLUSIONS In conclusion, the Ag NWs networks of new concept were fabricated via self-assembly of Meyer rod on the PET films. The principle of cell network with Ag NWs is demonstrated by difference of vapor pressure and viscosity among the DI water, MeOH and various solvents such as α-terpineol, cyclohexanol, and 1-dodecanol. The Ag NWs cell network have been identified that the performances with a high optical transmittance (>92%) and low sheet resistance (40 Ω/sq) are superior to random network of Ag NWs using figure of merit (σdc/σop). The mechanical testing demonstrated that the transparent electrodes have extremely flexibility under bending fatigue test with resistance change being less than 2% during 10,000 cycles. The novel concept with cell shaped self-assembly of Ag NWs should be made to increasing the transparent electrodes performance if the annealing condition, such as intense pulse light or plasma technique, is optimized, and will be further studied.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett



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Additional figures of the length and diameter of Ag NWs, and optical transmittance spectra; Graph of haze value with self-assembled Ag NWs; Tables detailing the cell size, optical transmittance, and sheet resistance of Ag NWs network; Photograph of transparent electrodes connected with LED; FE-SEM images of IPL sintered Ag NWs (PDF) Video showing a self-assembled process of Ag NWs inks after the Meyer rod coating (AVI)

■ AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected]

ORCID Yong-Ho Choa: 0000-0002-1254-3593

Notes The authors declare no competing financial interest.

Author Contributions #

These authors equally contributed to this work.

The manuscript was written through contributions of all authors.

■ ACKNOWLEDGMENTS This work was supported by the Human Resources Development program No.20154030200680) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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(30) Lee, J.; Lee, P.; Lee, H.; Lee, D.; Lee, S. S.; Ko, S. H. Very long Ag nanowire synthesis and its application in a highly transparent, conductive and flexible metal electrode touch panel. Nanoscale 2012, 4, 6408-6414. (31) Lee, Y.-I.; Kim, S.; Jung, S.-B.; Myung, N. V.; Choa, Y.-H., Enhanced electrical and mechanical properties of silver nanoplatelet-based conductive features direct printed on a flexible substrate. ACS Appl. Mater. Interfaces 2013, 5, 5908-5913. (32) Kwon, Y.-T.; Lee, Y.-I.; Lee, K.-J.; Choi, Y.-M.; Choa, Y.-H. A Novel Method for Fine Patterning by Piezoelectrically Induced Pressure Adjustment of Inkjet Printing. J. Electron. Mater. 2015, 44, 2608-2614. (33) Layani, M.; Gruchko, M.; Milo, O.; Balberg, I.; Azulay, D.; Magdassi, S. Transparent conductive coatings by printing coffee ring arrays obtained at room temperature. ACS nano 2009, 3, 3537-3542. (34) Doherty, E. M.; De, S.; Lyons, P. E.; Shmeliov, A.; Nirmalraj, P. N.; Scardaci, V.; Joimel, J.; Blau, W. J.; Boland, J. J.; Coleman, J. N. The spatial uniformity and electromechanical stability of transparent, conductive films of single walled nanotubes. Carbon 2009, 47, 2466-2473. (35) van de Groep, J.; Spinelli, P.; Polman, A. Transparent conducting silver nanowire networks. Nano Lett. 2012, 12, 3138-3144. (36) Sepulveda-Mora, S. B.; Cloutier, S. G. Figures of merit for high-performance transparent electrodes using dip-coated silver nanowire networks. J. Nanomater. 2012, 2012, 9. (37) He, T.; Xie, A.; Reneker, D. H.; Zhu, Y., A tough and high-performance transparent electrode from a scalable and transfer-free method. ACS Nano 2014, 8, 4782-4789 (38) Qi, L.; Li, J.; Zhu, C.; Yang, Y.; Zhao, S.; Song, W., Realization of a flexible and mechanically robust Ag mesh transparent electrode and its application in a PDLC device. RSC Adv. 2016, 6, 13531-13536 (39) Song, C.-H.; Ok, K.-H.; Lee, C.-J.; Kim, Y.; Kwak, M.-G.; Han, C. J.; Kim, N.; Ju, B.-K.; Kim, J.-W., Intense-pulsed-light irradiation of Ag nanowire-based transparent electrodes for use in flexible organic light emitting diodes. Org. Electron. 2015, 17, 208-215 (40) Jiu, J.; Sugahara, T.; Nogi, M.; Araki, T.; Suganuma, K.; Uchida, H.; Shinozaki, K., High-intensity pulse light sintering of silver nanowire transparent films on polymer substrates: the effect of the thermal properties of substrates on the performance of silver films. Nanoscale 2013, 5, 11820-11828.



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■ Figure Captions

Figure 1. (a) Contact angle image of DI water on oxygen plasma treated PET film. The inset image is contact angle of DI water on bare PET. (b) SEM image of Ag nanowires. (c) Water dispersed Ag nanowire solution with concentration of 0.5 wt%. (d) Schematic of Meyer rod coating process for Ag nanowire coating on the PET film.

Figure 2. (a-c) SEM images of Ag nanowire array with respect to α-terpineol volume of 0.5, 1.5, and 2.5 ml. (d) The optical transmittance spectra of the self-assembled Ag nanowire.

Figure 3. SEM images of Ag nanowire array according to Ag nanowire concentration using the random and cell shaped self-assembly.

Figure 4. Transmittance at 550 nm plotted as a function of film sheet resistance for random and cell shaped selfassembly Ag nanowire. The performance of two methods is shown for comparison. The inset graph represents fits to the transparent versus sheet resistance using Eq. 1.

Figure 5. (a) Schematic illustration of the self-assembly process of Ag nanowires. SEM images of Ag nanowire films with different additives; (b) cyclohexanol, and (c) 1-dodecanol.

Figure 6. Graph showing resistance change of Ag NWs network with a cell shape under the 10,000 bending cycles at the bends radius of 5, 10, 15 mm. (a) The detailed graph for verifying specific changes. (b) Illustration of the resistance fatigue test sample. The samples are prepared for the size of 150 mm x 20 mm in accordance with the image after the Ag NWs coating on PET film of 15 x 15 cm2. The real optical image of (c) sample and (d) bending test machine.


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■ Figure

Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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■ TOC Graphic


Novel Concept for Fabricating a Flexible Transparent Electrode Using

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