High-Resolution and Large-Area Patterning of Highly Conductive

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Functional Inorganic Materials and Devices

High-Resolution and Large-Area Patterning of Highly Conductive Silver Nanowire Electrodes by Reverse Offset Printing and Intense Pulsed Light Irradiation Kyutae Park, Kyoohee Woo, Jongyoun Kim, Donghwa Lee, Yumi Ahn, Dongha Song, Honggi Kim, Dongho Oh, Sin Kwon, and Youngu Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Materials & Interfaces

High-Resolution and Large-Area Patterning of Highly Conductive Silver Nanowire Electrodes by Reverse Offset Printing and Intense Pulsed Light Irradiation Kyutae Park†, Kyoohee Woo‡, Jongyoun Kim†, Donghwa Lee†, Yumi Ahn†, Dongha Song‡, §, Honggi Kim†, Dongho Oh§, Sin Kwon‡, *, and Youngu Lee†, * †Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333, Techno Jungang Daero, Hyeonpung-Eup, Dalseong-Gun, Daegu, 42988, Republic of Korea ‡Advanced Manufacturing Systems Research Division, Korea Institute of Machinery and Materials (KIMM), 156, Gajeongbukro, Yuseong-Gu, Daejeon, 34103, Republic of Korea §Department of Mechanical Engineering, Chungnam National University, 99, Daehakro, Yuseong-Gu, Daejeon, 34134, Republic of Korea *E-mail: [email protected], [email protected]

KEYWORDS. silver nanowire, high-resolution, patterning, reverse offset printing, intense pulsed light irradiation

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ABSTRACT.

Conventional printing technologies such as inkjet, screen, and gravure printing have been used to fabricate patterns of silver nanowire (AgNW) transparent conducting electrodes (TCEs) for a variety of electronic devices. However, they have critical limitations in achieving micrometerscale fine line width, uniform thickness, sharp line edge, and pattering of various shapes. Moreover, the optical and electrical properties of printed AgNW patterns do not satisfy the performance required by flexible integrated electronic devices. Here, we report a high-resolution and large-area patterning of highly conductive AgNW TCEs by reverse offset printing and intense pulsed light (IPL) irradiation for flexible integrated electronic devices. A conductive AgNW ink for reverse offset printing is prepared by carefully adjusting the composition of AgNW content, solvents, surface energy modifiers, and organic binders for the first time. High-quality and highresolution AgNW micro-patterns with various shapes and line widths are successfully achieved on a large-area plastic substrate (120 x 100 mm2) by optimizing the process parameters of reverse offset printing. The reverse offset printed AgNW micro-patterns exhibit superior fine line widths (up to 6 µm) and excellent pattern quality such as sharp line edge, fine line spacing, effective wire junction connection, and smooth film roughness. They are post-processed with an IPL irradiation, thereby realizing excellent optical, electrical, and mechanical properties. Furthermore, flexible OLEDs and heaters based on reverse offset printed AgNW micro-patterns are successfully fabricated and characterized, demonstrating the potential use of the reverse offset printing for the conductive AgNW ink.


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INTRODUCTION Silver nanowire (AgNW) has received considerable attention as a novel material for flexible transparent conducting electrodes (TCEs) because of its outstanding physical properties such as high optical transmittance, low sheet resistance, and excellent mechanical flexibility.1-9 Therefore, AgNW TCE has been one of the most promising candidates for flexible electronic devices such as touch screen panels,10, 11 sensors,12, 13 solar cells,14-18 and organic light-emitting diodes (OLEDs).1822

However, it is still very difficult to utilize AgNW as an electrode for integrated electronic

devices due to the lack of a suitable micropatterning process. To extend the application of AgNW TCE to numerous electronic applications, new processes must be developed to obtain patterns with desired line width and thickness at the micrometer level.23 So far, a photolithographic process has been used to obtain fine patterns of AgNW TCE on a micrometer scale. The micro-patterned AgNW TCE fabricated using the photolithography process has shown excellent pattern resolution and electrical networking.21-22, 24-25 However, the photolithographic process for AgNW TCE is complicated, requires expensive equipment, and consumes a lot of chemicals, resulting in high manufacturing costs.25 Therefore, new fine patterning technology for AgNW TCE should be developed for emerging optoelectronic devices. Recently, various printing technologies such as inkjet, screen, flexographic, gravure, offset and template assisted transfer printings have received considerable attention as promising patterning techniques because of many advantages, including short process time, low material waste, low cost equipment, and low manufacturing cost.26-37 Moreover, the printing technologies can be combined with a roll-to-roll (R2R) process for large-area and high throughput manufacturing.31 Especially, high-resolution printing technologies are desirable for fabricating integrated electronic devices and improving their performance. Recently, several research groups have attempted to


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apply printing technology to the patterning of AgNW TCE.26-33 For example, Coleman and coworkers have reported a AgNW pattern fabricated using inkjet printing.26 However, due to low accuracy, residual material, and limitation of nozzle size, it is impossible to print a AgNW pattern less than 100 µm using inkjet printing. In addition, electrical conductivity of the AgNW pattern is relatively low because only a short AgNW should be used due to nozzle clogging. Pei and coworkers have developed a water-based AgNW ink for screen printing and obtained a line pattern with a resolution of 50 μm.29 However, to improve the conductivity of the AgNW patterns, complicated post-treatment is required to remove additives and fuse the AgNW junctions. Subramanian and co-workers have reported a AgNW pattern using gravure printing.31 The minimum pattern size of the gravure printed AgNW pattern is approximately a few tens of micrometers. In addition, indium zinc oxide (IZO) has to be used as a matrix material to obtain the electrical and mechanical properties of the gravure printed AgNW TCE. Accordingly, conventional contact printing technologies, such as screen and gravure printing, have critical limitations in achieving micrometer-scale fine line width, fine line spacing, sharp line edge and uniform thickness.29-31, 33 Therefore, new printing technology should be developed to achieve highquality and high-resolution micropatterning of AgNW TCE. Reverse offset printing is a direct printing technique that allows patterning of very fine lines and various shapes with uniform thickness. Several studies have demonstrated the usefulness of reverse offset printing for the micropatterning of conductive inks based on metal nanoparticles.34-36 However, until now, there has been no report on the use of reverse offset printing to achieve the micropatterning of AgNW TCE due to the difficulty of preparing a conductive AgNW ink for reverse offset printing. Therefore, it is required to develop new ink formulation strategy and printing conditions for conductive AgNW ink that can be applied to reverse offset printing.


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In this work, we successfully demonstrate a high-resolution and large-area patterning of highly conductive AgNW TCEs by reverse offset printing and IPL irradiation for flexible integrated electronic devices for the first time. A conductive AgNW ink for reverse offset printing is prepared by carefully adjusting the composition of AgNW content, solvents, surface energy modifiers, and organic binders. Moreover, we systemically investigate effects of various ink compositions and printing process parameters on printing characteristics. High-resolution AgNW micro-patterns with various shapes and line widths are easily printed on large-area substrates (120 x 100 mm2) such as glass and plastic. The reverse offset printed AgNW micro-patterns are obtained from a fine line width of 6 µm to a wide pattern of 500 μm or more. They also exhibit outstanding pattern qualities such as fine line widths, sharp line edge, fine line spacing, effective wire junction connection, and smooth film roughness. In addition, they are treated with an intense pulsed light (IPL) irradiation, thereby realizing superior optical, electrical, and mechanical properties. Furthermore, flexible OLEDs and heaters based on reverse offset printed AgNW micro-patterns are successfully fabricated and characterized, demonstrating the potential use of the reverse offset printing for the conductive AgNW ink.

RESULTS AND DISCUSSION Figure 1 illustrates the process of reverse offset printing using a conductive AgNW ink. (Movie S1-S3, Supporting Information) The first step in reverse offset printing is the coating process. The AgNW ink is continuously coated onto a polydimethylsiloxane (PDMS) based blanket rolled on a metal cylinder through a slot die coater at a speed of 3 mm s-1. When a constant amount of AgNW ink is discharged from a slot die coater, it is coated on the blanket without ink shrinkage or spillage while the metal cylinder rotates. At the same time, the solvent in the AgNW ink can be evaporated


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and absorbed to the PDMS based blanket. Then, the AgNW ink on the blanket is allowed to partially dry, leading to a uniform, smooth, and semi-dried AgNW ink. The second step in reverse offset printing is the off process. The blanket roll is pressed against a silicon or glass-based cliché with various micro-patterns and line widths, which is an engraved printing plate. Accordingly, the semi-dried AgNW ink on the blanket roll is transferred on the cliché. Since the semi-dried AgNW ink has stronger adhesion to the cliché than to the PDMS based blanket, the convex patterns of the cliché can take semi-dried AgNW ink from the PDMS based blanket. All the non-patterned parts in the semi-dried AgNW ink can be removed from the PDMS based blanket. As a result, the semidried AgNW ink that remains on the PDMS based blanket creates a pattern which corresponds to the concave pattern of the cliché. The final step in reverse offset printing is the set process. The remaining AgNW patterns on the PDMS based blanket are pressed against a variety of substrates such as glass and plastic, leading to AgNW micro-patterns with various shapes and line widths. Printing parameters such as coating rate, drying time, printing speed, and rolling blanket pressure are also optimized to obtain high-quality and high-resolution AgNW micro-patterns of various line widths, line spacings, and shapes. Figure 1c shows a photograph of reverse offset printed AgNW micro-patterns on a large-area plastic substrate (120 x 100 mm2). The AgNW micro-patterns are uniformly printed over the entire substrate area. Finally, the AgNW micro-patterns printed on the plastic substrate are treated with an IPL irradiation, which is a recently studied method for sintering printed nanowires with minimal damage, 38-40 thereby realizing superior optical, electrical, and mechanical properties.


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Figure 1. (a) Schematic illustration for fabrication process of a micro-patterned AgNW film using reverse offset printing and IPL irradiation. (b) Photograph of a reverse offset printer. (c) Photograph of a reverse offset printed AgNW film on a large-area plastic substrate (120 x 100 mm2). The conductive AgNW ink for reverse offset printing consists of AgNW, additives, and solvents. The composition of additives and solvents is well known to assist in wetting, dispersion stability, and film formation of reverse offset printing inks.33 In addition, the electrical properties and patterning properties of the printed AgNW micro-patterns are highly dependent on the loading density of AgNW, dispersion stability, solvent vaporization, and surface energy between the AgNW ink and PDMS based blanket. Therefore, to develop a conductive AgNW ink for reverse offset printing, it is necessary to study the relationship among various components constituting AgNW ink. The conductive AgNW ink for reverse offset printing is prepared using polyvinylpyrrolidone (PVP) capped AgNW, with an average length of 25 μm and an average diameter of 32 nm dispersed in isopropyl alcohol (IPA). IPA is chosen as a major solvent because


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it can effectively disperse AgNW capped with PVP. Moreover, it can decrease the surface energy of the AgNW ink, making it easier to get wet on the PDMS based blanket than ethyl alcohol and water. (Figure S2) It is crucial to make a AgNW ink with low surface energy for slot die coating because the PDMS based blanket has strong hydrophobicity. Moreover, because IPA has low boiling point (82.6 ℃) as well as high vapor pressure, it can evaporate rapidly to obtain a uniformly semi-dried AgNW film immediately after the AgNW ink is coated onto the PDMS based blanket. However, unfortunately, the AgNW ink with only IPA solvent tends to create a lot of pinholes and craters when coated onto the PDMS based blanket, suggesting that IPA is not sufficient to achieve the hydrophobicity and surface energy of the AgNW ink for complete coating property. Therefore, a small amount of silicone containing surface additive (BYK333) is added to the AgNW ink as a surface energy modifier to improve the coating property. Figure 2a shows the contact angle measurement to investigate the effect of the surface energy modifier on the wetting property of the AgNW ink. As the content of BYK333 increases from 0 to 0.5 %, the contact angle of the AgNW ink on the PDMS based blanket decreases from 30 to 26.4°. As shown in Figure 2b-d and S4, adding BYK333 into the AgNW ink can eliminate pinholes and craters caused by the surface tension differences between the AgNW ink and PDMS based blanket. It also promotes AgNW ink wetting and leveling without creating ink spillage. This result confirms that the coating property of the AgNW ink can be improved by the addition of BYK333. However, since BYK333 acts as an electrical insulator, the electrical property of the AgNW ink decreases as the amount of BYK333 increases as shown in Figure 2a. This result means that the amount of BYK333 needs to be optimized while maintaining the coating property without compromising the electrical property of the AgNW ink. Thus, only 0.25 wt% of BYK333 is added to satisfy both coating and electrical properties of the AgNW ink at the same time.


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Figure 2. (a) Contact angle and sheet resistance as a function of BYK333 concentration in a AgNW ink. Slot die coating property and contact angle measurements (insets) of AgNW inks with different concentrations of BYK333 (b) 0 %, (c) 0.1 % and (d) 0.25 %. (e) Schematic illustration of cohesive and adhesive forces when Fc

High-Resolution and Large-Area Patterning of Highly Conductive

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