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A Novel Patterning Method for Nanomaterials and Its Application to Flexible Organic Light-Emitting Diodes Jin-Hoon Kim, and Jin-Woo Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19173 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

A Novel Patterning Method for Nanomaterials and Its Application to Flexible Organic Light-Emitting Diodes Jin-Hoon Kim and Jin-Woo Park* Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, Korea Keywords: Patterning, Capillary force, Spinning, Ag nanowires, Organic light-emitting diodes

ABSTRACT We present a simple, low-cost, and scalable method to form various patterns of nanomaterials with different dimensions and shapes using capillary and centrifugal forces. The desired patterns were formed on the surfaces of polydimethylsiloxane (PDMS) stamps, and the PDMS stamps were conformally contacted with the surfaces of flexible polymer substrates. Solutions of nanomaterials, such as metal nanowires and nanoparticles, were then drop-casted at one open end of the microchannels formed at the interface of the polymer substrate and PDMS stamp. The nanomaterial solutions penetrated the microchannels due to capillary force interactions between the surfaces and the fluid. The solvents of the nanomaterial solutions exfiltrated from the entrance of microchannels due to the coffee ring effect. Then, solvent remaining in the microchannels was discharged by applying a centrifugal force by spinning the polymer

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substrate/PDMS stamp system. Due to the synergistic effect of the capillary force, coffee ring effect, and centrifugal force, uniform patterns of the nanomaterials with clearly defined edges were formed for a variety of pattern shapes and substrates. Furthermore, the direct patterning approach resulted in a significant reduction in the amount of wasted materials. Finally, flexible organic light-emitting diodes (f-OLED) were successfully fabricated on the finely patterned nanowire electrodes.

1. INTRODUCTION

Conventional electronic devices, such as those applied in energy harvesting, displays, and medicine, have become wearable and even bio-implantable, as electronic devices have evolved from being bulky and rigid to mobile, flexible, and foldable.1-2 As the applications for wearable devices have expanded, the demand for highly flexible, optically transparent, and electrically conductive materials have also increased.3 Up to now, various flexible, transparent, and conductive materials have been developed, including conductive polymers, graphene, carbon nanotubes (CNT), metal nanowires, and their composites.4-7 Among them, silver nanowires (AgNWs) are one of the most promising candidates as an electrode material due to their superior transparency, conductivity, and mechanical flexibility.5, 8 Almost all practical applications using AgNWs require patterned electrodes instead of fully coated AgNWs (f-AgNWs).9-10 However, as AgNWs are typically formed using solution-based coating processes, such as spin-coating, bar coating, and spray coating, the patterning of AgNWs into shapes with micrometer dimensions is challenging.11 Conventional patterning techniques, such as shadow masking10 and photolithography,9,

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have been exploited for patterning


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AgNWs. Among them, shadow masking is a simple and cost-effective process but cannot pattern AgNWs down to the micrometer scale. In contrast, photolithography can pattern AgNWs in micrometer dimensions; however, this method requires multiple processing steps and expensive equipment. Recently, many novel methods for patterning AgNWs have been proposed.11,

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Several

groups have suggested patterning AgNWs by utilizing the varying wettability of solvent on substrate surfaces.14-15 Yang et al.14 deposited a hydrophobic fluoropolymer ‘cytop’ on glass substrates using photolithography, thereby allowing the AgNW solution to selectively wet the cytop-free regions of the hydrophilic glass surface.14 Consequently, the AgNWs were aligned and patterned on the glass substrate after solvent evaporation. However, this method requires complicated photolithographic patterning processes. Moreover, the residual cytop coating on the substrate must be removed prior to the fabrication of devices atop the patterned AgNW (pAgNW) electrode. Chou et al.15 reported the patterning of AgNWs on a hydrophobic PDMS substrate by using surface activation processes such as oxygen plasma or UV ozone surface treatment.11 The oxygen or UV ozone plasma-treated PDMS substrate was covered with a mask outlining the desired AgNW pattern. AgNW solutions were then selectively coated on the regions of the PDMS surface that became hydrophilic as a result of the oxygen or UV ozone plasma treatment. However, this plasma-assisted patterning method can only be used on hydrophobic substrates. Hence, the p-AgNWs must be transferred to the desired substrate. Moreover, the pattern line width (w) achieved using the plasma-assisted technique was limited to a minimum of 100 µm.15 Other methods for patterning AgNWs utilize etching processes. Park et al.16 patterned AgNWs using capillary-force-based soft lithography (CFL).16-17 They selectively coated and cured a


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hydrogel solution atop the f-AgNWs using a PDMS stamp. The desired p-AgNW shape was prepared by rinsing off the UV light-cured hydrogel patterns covering the unwanted regions of the f-AgNWs.16 However, based on this patterning method, w was limited to a minimum of 30 µm.17 Yoon et al.18 used micro-contact printing of a siloxane oligomer to pattern AgNWs with a w of 25 µm.18 They printed the siloxane oligomer on the f-AgNWs by using a patterned PDMS stamp, and then, the siloxane oligomer was cured after exposure to UV radiation. The siloxanefree AgNWs were then etched by a H2O2 solution. Even though this method could easily pattern AgNWs down to a w of 25 µm, additional etching processes were needed to remove the cured siloxane layer and expose the underlying p-AgNWs. In this work, we report a simple, residue-free, clean, and low-cost method for patterning AgNWs and nanoparticles (as shown in Figure 1) without the complications of multiple steps, expensive equipment, substrate limitations, or residual materials. At the initial stage of our novel patterning process, a nanomaterial solution filled the microchannels formed by the PDMS stamp and substrate due the capillary force experienced by the solution. Afterwards, the solvent was discharged from the microchannels due to the coffee ring effect. Subsequent spinning helped accelerate the removal of the solvent from the microchannels. Microscopic analyses, including optical microscopy (OM) and field-emission scanning electron microscopy (FE-SEM), were used to verify the exact patterning mechanism of the novel patterning process and to evaluate the uniformity and edge clarity of the p-AgNWs. The electrical, optical, and mechanical properties of the f-AgNWs and p-AgNWs were measured and compared using a two-point probe multimeter, UV-visible spectroscopy, and cyclic and static bending tests, respectively. f-OLEDs were then fabricated to verify the application of p-AgNWs as electrodes for optoelectronic devices.


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Figure 1. Schematic description of the novel patterning process.

2. RESULTS AND DISCUSSION

Figure 1 schematically describes the novel patterning process. As shown in Figure 1, microchannels were formed at the interface of the PDMS stamp and substrate due to their conformal contact. AgNW solutions were dropped at one open end of the microchannels (denoted ‘front’ in Figures 1 and 2). Then, the AgNW solution infiltrated the microchannels from the front to the other end (denoted ‘back’ in Figures 1 and 2), as shown in Figure 1 and in the OM images in Figure 2a, due to the capillary force between the fluid and the surface of the microchannels. The microchannels were completely filled with the solution a few seconds after dropping the solution at the front of the microchannels, as shown in Figure 1 and Figure 2a. The capillary pressure (Pca) inside the microchannel is given as:19 Pca = γ LV (

2 cos θ cos θ + cos θ ` + ) h w

(1)


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where h and w are the height and width of the microchannels (or patterns), respectively; γLV is the liquid-vapor surface energy between the solvent and air; and θ and θ` are the contact angles of the solution with the PDMS stamp and substrate (polyimide (PI) was used as the substrate in this work), respectively. Based on Equation (1), Pca increases as w and h decrease.


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Figure 2. (a) OM images of the injected fluid flow at the back side of the channel, (b) OM images of the fluid flow exfiltrated from the back side of the channel, and (c) FE-SEM images of the AgNWs at the location where the solution was dropped. (d) Schematic description of the novel patterning mechanism.

According to Figure 1, Figure 2b, and Movie S1 (Supporting Information), a few minutes after the capillaries were filled with the solution, the solvent inside the channels started to retract towards the front, and the AgNWs remained inside the channels, which can be considered to be a drying process. Samples of the residual solvent during and after the drying process were analyzed by FE-SEM to understand the mechanism of solvent exfiltration from the microchannels, as shown in Figure 1 and Figure 2b. Figure 2c shows the FE-SEM images of the AgNWs near the front of the PDMS stamp. In this region, the AgNWs formed an arch-like shape that looks similar to a coffee ring stain, which is strong evidence that the coffee ring effect is a driving force for expelling solvent from the capillaries.20 The coffee ring effect is a physical phenomenon induced by the surface tension of a drying or evaporating liquid with dispersed particles. When a drop of particle-containing liquid dries on a solid surface, the particles are deposited in a ring-like shape on the solid surface.20 This is because during the drying process, the circular edge of the liquid drop becomes pinned to the substrate by the surface tension, and the capillary flow from the center of the droplet to its outward circular edge moves the dispersed particles to the edge of the droplet as liquid evaporation proceeds.21 After complete evaporation of the liquid, the dispersed particles are deposited at the circular edge of the droplet in high concentrations.21


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Figure 2d schematically describes solvent exfiltration from the microchannels induced by the coffee ring effect: as the solvent evaporates from the residual solution at the front of the microchannels, the fluid retracts from the capillaries to keep the edges of the solution droplet on the substrate.22 This natural drying process, however, has drawbacks: 1) it takes a very long time (e.g., over 10 min to remove the solvent from 1.5 cm-long microchannels) and 2) some channels were clogged by residual solvent. Thermal annealing may be an easy way to improve the drying process.23 Solutions of long AgNWs (approximately 150 µm in length, denoted l_AgNWs) were infiltrated into 100 µm-wide channels and annealed at 80 °C for 10 min.23 According to Figure S1 (Supporting Information), the thermal annealing process failed to make the patterns have clearly defined edges, and uniform AgNW density along the length of the patterns, which is most probably to be due to the thermal deformation or distortion of the PDMS stamp upon heating. Instead, here, we spun the substrate/PDMS stamp system to accelerate solvent removal from the residual solution and, finally, from the microchannels, as described in Figure 1. Samples were prepared with both l_AgNW and short AgNW (approximately 25 µm in length, denoted s_AgNW) solutions. Both l_AgNW and s_AgNW solutions were infiltrated into 100, 50, and 25 µm-wide channels, and spinning was applied using a spin-coater (a complete description of the patterning process is found in the experimental section). Complete removal of the solvent from the channels was achieved in a few minutes, and clogging did not occur. OM images of the patterned long AgNWs (p-l_AgNWs) and patterned short AgNWs (ps_AgNWs) are shown in Figures 3 and S2 (Supporting Information). According to Figure 3a and 3b, and S2a (Supporting Information), p-l_AgNWs are aligned along the channel direction and have clearly defined edges, except for the p-l_AgNWs with a w of 25 µm. In p-l_AgNWs with a


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w of 25 µm, some AgNWs protrude from the edges of the microchannels of the PDMS stamp, as shown in Figure S2a (Supporting Information). The protrusion of l_AgNWs was due to the w of the microchannels being much smaller than the length of l_AgNWs. To the best of our knowledge, there has been no report of patterning the l_AgNWs used in this study. Most works concerning AgNW patterning have used s_AgNWs.10-11, 14, 18


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Figure 3. OM images of the p-l_AgNWs with a w of (a) 100, and (b) 50 µm together with the ps_AgNWs with a w of (c) 25 and (d) 10 µm. (e) Photograph of large-area p-l_AgNWs with the area having the dimensions of 3.5 cm in length and 6 cm in width.

When s_AgNWs were used, p-s_AgNWs with clean edges were obtained, even when the microchannel w of the PDMS stamp was decreased to 10 µm, as shown in Figure 3c, 3d, S2b and S2c (Supporting Information). The p-s_AgNWs with a w of 10 µm in Figure 3d are one of the thinnest AgNW patterns ever reported.14, 18 According to these results, we confirmed that in our novel patterning method using spinning, solution injection into the microchannels of the PDMS stamp was induced by capillary force, while subsequent solvent removal was driven by the coffee ring effect and accelerated by centrifugal force induced during spinning. According to our experimental results, the quality of the patterns was not significantly affected by the type of substrate. Furthermore, as shown in Figure 3e for the case of p-l_AgNWs as a demonstration, the novel patterning method can be used to the pattern AgNWs over a large surface area of at least 3.5 cm in length and 6 cm in width. The large-area-type p-1_AgNWs was then showed to be a stable electrode to light-emitting diodes (LEDs, as shown in Figure S3 in Supporting Information) even while being bent. Also, various patterns can be produced by our novel method. Figure S4a and S4b (Supporting Information) show p-AgNWs with mesh and serpentine shapes. The density of AgNWs was uniform along the patterns, and the edges of the patterns were clearly defined. These structures could be used in various applications, such as transparent thin-film heaters, solar cells, touch panels, and stretchable conductors.11, 24-25 Another advantage of this novel patterning method is that it could pattern other types of nanomaterials, such as nanoparticles, polymers, and their composite or hybrid nanomaterials. Figures S4c and


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S4d (Supporting Information) show the line patterns of TiO2 and ZnO nanoparticles with a w of 50 µm, respectively. Figures S4e and S4f (Supporting Information) demonstrate the patterns of hybrid nanomaterials based on AgNWs and TiO2 and ZnO nanoparticles, respectively. Patterned hybrid nanomaterials could be easily obtained by successive injection of two different solutions. For example, a AgNW solution was first injected into the microchannel of the PDMS stamp, and the substrate and stamp were spun to dry the solvent. After the first spinning step, a solution containing nanoparticles was injected, and drying was achieved by the coffee ring effect and spinning. As shown in Figures S4e and S4f (Supporting Information), the hybrid nanomaterials were successfully patterned with uniform density of the two materials and clean edges. These hybrid nanomaterials can be applied as electrodes to various optoelectronic devices, as TiO2, and ZnO are used as charge-transport layers in OLEDs and solar cells.26-28 The patterning of multilayers composed of various nanomaterials should also be possible with this novel method. FE-SEM analysis was performed to analyze the microstructure of the p-AgNWs. As shown in Figure 4a, the top, middle, and bottom regions of a 1.4 cm-long and 100 µm-wide pattern of pl_AgNWs were analyzed using FE-SEM. First, the area density of the AgNWs was analyzed. As shown in Figure 4b, the area density of the AgNWs was similar at all locations in the line pattern, which indicates that very uniform p-AgNWs could be obtained by the novel patterning method. Furthermore, according to the optical surface profile and atomic force microscopy (AFM) images shown in Figure S5 (Supporting Information), it was also verified that very uniform and clean patterns of AgNWs were obtained using the novel patterning method. According to Figure 4c, 4d, and 4e, the AgNWs are almost aligned along the capillaries. The alignment angles of the individual AgNWs were measured from the FE-SEM images to


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quantitatively analyze the degree of alignment of the AgNWs in relation to the direction of the microchannels. As shown in Figure 4c, 4d, and 4e, almost 60% of the AgNWs were aligned to the solvent flow direction (between -20° and 20° offset from the solvent flow direction), regardless of their location in the line pattern. For further analysis, Gaussian fitting was applied to the data in Figure 4 to obtain the center angle (θc) and full width at half maximum (FWHM) of the AgNW alignment distribution. The FWHM shows the deviation in the orientations of the AgNWs relative to the line pattern length and represents the degree of alignment. If AgNWs are perfectly aligned, the FWHM should be 0°, and the FWHM would be 180° when AgNWs are randomly oriented. As shown in Figure 4, the FWHM of the p-l_AgNWs with a w of 100 µm was 23.6°, 23.4°, and 32.3° at the bottom, center, and top of the line pattern, respectively. These values are comparable to the previously reported results using other alignment techniques.14, 29-30 As shown in Figure S6a (Supporting Information), fully coated long AgNWs (f-l_AgNWs) showed a random orientation with a FWHM of 180°, which indicates that the novel patterning method described in this study can both pattern and align the AgNWs. Furthermore, as shown in Figure S6b and S6c (Supporting Information), as w decreases, the FWHM also decreases, which indicates that the degree of alignment increases with decreasing w. The AgNW alignment resulted from the physical guidance of the PDMS stamp microchannels and the shear force induced by the flowing solvent. The capillary force ensures that the solution flows through the microchannels of the PDMS stamp. This solution flow then induces a shear force on the AgNWs, which results in the alignment of the AgNWs to the solution flow direction.29 When the length of the AgNWs was larger than the w of the microchannels, the walls of the PDMS stamp guided the alignment of the AgNWs along the microchannel.


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Figure 4. (a) Schematic description of the p-l_AgNWs for FE-SEM analysis, (b) area density of the p-l_AgNWs at the top, middle, and bottom of the channel, and FE-SEM images of the p-


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l_AgNWs on a PI substrate (the left column) and distribution of the degree of alignment of the l_AgNW pattern at the (c) top, (d) middle, and (e) bottom of the channel.

The alignment angle for the p-s_AgNWs with different w was analyzed to further verify the mechanism of AgNW alignment. As shown in Figure S7 (Supporting Information), when the w (between 25 and 50 µm) was larger than the length of the AgNWs, the FWHM was approximately 55°. Such high FWHM is similar to that of the bar-coated AgNWs, which indicates that only the solution flow contributed to the alignment of the AgNWs when the AgNW length was smaller than the microchannel w of the PDMS stamp.14 When the microchannel w (approximately 10 µm) was smaller than the AgNW length, as shown in Figure S7c (Supporting Information), the FWHM was similar to that of the aligned AgNWs, as shown in Figures 4 and S7 (Supporting Information) for p-l_AgNWs. These analyses verified that both guidance by the microchannel wall and solvent flow affected the AgNW alignment during the patterning process.


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Figure 5. (a) RL values of the p-l_AgNWs and p-s_AgNWs with various line width. The inset image shows a schematic image of how the p-AgNWs was prepared for measuring the RL. (b) T values of p-l_AgNWs and p-s_AgNWs. The electrical line resistance (RL) of p-AgNWs and f-AgNWs was measured using a two-point probe multimeter. As schematically shown in the image inset in Figure 5a, the area over which the RL measurements were taken was 1 cm in length and 1.4 cm in width, and two silver-paste contacts were applied at the edges of the electrodes. In this work, varying the wire length, number of coating cycles and concentration of the AgNW solution influence the RL of the pAgNWs, and RL was minimized at certain w. The results are summarized in Figure 5a and Table S1 (Supporting Information). As shown in Figure 5a and Table S1 (Supporting Information), the RL of p-AgNWs increased with decreasing w. However, the RL of p-AgNWs became constant when w was larger than 100 µm. Compared to the RL of p-s_AgNWs, the RL of p-l_AgNWs was more affected by w, as shown in Figure 5a. Among the patterns in Figure 5a and Table S1, the p-l_AgNWs with a line w of 100 µm have the lowest RL of 11 Ω with a corresponding sheet resistance (Rs) of about 25 Ω sq-1. Other patterning methods, especially those using etching processes, could not produce p-AgNWs with an RL as low as 50 Ω at a w as small as 25 µm, similar to in this work (Figure 7a and Table S1).13, 15, 17-18 Liu et al. fabricated p-AgNWs with a w of 150 µm and RL of 400 Ω by a surface activation process.15 Ahn et al. patterned AgNWs into p-AgNWs with a w of 100 or 20 µm but with an RL of 740 or 3350 Ω, respectively, using a photolithographic process.13 Kim et al. used the CFL method to prepare p-AgNWs with a w of 30 µm and RL of 285 Ω.17 Figure 5b compares the optical transmittance (T) of p-l_AgNWs and p-s_AgNWs (with a w of 100 µm and a 100 µm line spacing) with similar RL. The p-l_AgNWs showed a similar T to the


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p-s_AgNWs, which indicates that the AgNW alignment has little effect on T. The p-l_AgNWs with an RL of 23.5 Ω exhibited a T of 95.1% for light with a 550 nm wavelength, and the ps_AgNWs with an RL of 30 Ω exhibited a T of 95.8% for light with a 550 nm wavelength. All samples in Table S1 showed T values over 95% for light with a 550 nm wavelength. The figure of merit (FoM) of transparent electrodes can be then determined based on the Rs and T values using the following equation,31 ܶሺλሻ = 1 +

ଵ଼଼.ହ ఙೀು ோೞ

ఙವ಴

(2)

The calculated FoM values of p-l_AgNW with a w of 100 µm, f-l_AgNWs, and other AgNWbased transparent electrodes reported in the literature were summarized in Figure S8 (Supporting Information).32-37 The FoM value of p-l_AgNWs of about 240 was higher than f-l_AgNW of about 140 and other reported AgNW-based transparent electrodes with the best value of 180, which indicates that p-l_AgNW is an excellent candidate as the transparent electrode for optoelectronic devices


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Figure 6. (a) Schematic images of the bending tests for p-AgNWs and f-AgNWs. (b) Cyclic and (c) static bending test results of the p-AgNWs and f-AgNWs on a PI substrate. FE-SEM images after the cyclic bending tests of (d) f-l_AgNWs and p-l_AgNWs (with a w of 100 µm) bent (e) parallel and (f) perpendicular to the line patterns. The red circles in the FE-SEM images indicate the locations of fractures in the AgNWs.

AgNWs were patterned on the flexible PI substrate, and cyclic and static bending tests were performed to evaluate the flexibility of the p-AgNWs. Figure 6a shows the schematic images of the bending test. p-l_AgNWs with a w of 100 µm were compared with f-l_AgNWs. Cyclic bending tests were performed with a 2.5 mm bending radius (rb). In the static bending tests, the


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rb was decreased from 5 to 1 mm. For the p-l_AgNWs, tensile stress was induced during the bending tests in directions both parallel and perpendicular to the line patterns, as shown in Figure 6a. As shown in Figure 6b, the p-l_AgNWs showed a larger increase in RL than the f-l_AgNWs when the cyclic tensile stress was induced parallel to the line patterns. In contrast, the RL of pl_AgNWs showed little changed when the cyclic tensile stress was imposed perpendicular to the line patterns. The RL of p-l_AgNWs increased by only 6% after cyclic bending tests at 2.5 mm rb for 5000 cycles when the bending stress was induced perpendicular to the line patterns, whereas the RL of p-l_AgNWs increased by 40% when the bending stress was induced parallel to the line patterns. The RL of f-l_AgNWs increased by approximately 36% after the cyclic bending tests. As shown in Figure 6c, if the bending stress was imposed perpendicular to the line patterns, the RL of p-l_AgNWs did not increase until the rb was decreased to 1 mm. However, the RL of fl_AgNWs increased by 32% at 1 mm rb. The increase in RL at 1 mm rb for p-l_AgNWs was 36% after the static bending tests when the bending stress was induced parallel to the line patterns. The cyclic and static bending test results indicated that p-l_AgNWs showed anisotropic bending stability. This anisotropic bending stability of p-l_AgNWs was due to the high degree of alignment of the AgNWs in p-l_AgNWs (Figure 4a to 4c). When the direction of the bending strain was parallel to the aligned AgNWs, more stress was induced on the AgNWs. Hence, more AgNWs were expected to suffer catastrophic failure. The microstructure of the AgNWs was investigated after the cyclic bending tests. As shown in Figure 6d, many AgNWs in f-l_AgNWs parallel to the stress direction showed catastrophic failure, whereas fewer AgNWs perpendicular to the stress direction were fractured. When the bending stress was parallel to the pattern direction of p-l_AgNWs, as shown in Figure 6e, more


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AgNWs showed catastrophic failure than in f-l_AgNWs, because more AgNWs were aligned along the strain direction in p-l_AgNWs. In contrast, as shown in Figure 6f, when bent along the perpendicular direction, fewer AgNWs were broken in p-l_AgNWs because most of the AgNWs were perpendicular to the stress direction. The broken AgNWs were critical to the increased RL of the p-AgNWs. Similar results to the aligned p-l_AgNWs were reported in electrospun nanofibers.38 The maximum elongation strain of the random nanofiber networks was found to be higher than that of the aligned nanofiber networks via uniaxial tensile testing.38 Nanofibers aligned along the direction of applied tensile stress were shown to have higher stress values than other nanofibers that were not aligned along the direction of the applied tensile stress.38 f-OLEDs (schematically shown in Figure 7a were fabricated with an anode composed of pl_AgNWs (w of 100 µm) on a PI substrate to verify the applicability of the p-AgNWs as flexible, transparent electrodes. Electrodes with patterned geometries are required to fabricate displays or pixelated sensors.39-41 Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), Super Yellow (SY), doped polyethyleneimine ethoxylated (d-PEIE), and Al were used as the hole-transport layer (HTL), emission layer (EML), electron-transport layer (ETL), and cathode of the f-OLEDs, respectively. All layers, except for Al, were solution processed. d-PEIE is composed of PEIE doped with Cs2CO3, which was shown to improve the performance and lifetime of the OLEDs in our previous study.42 Figure 7b shows the OM image of light emission from the f-OLED based on p-l_AgNWs. The light emission was only observed in the patterned lines of the AgNWs, which indicates that the AgNWs were uniformly patterned, and there was no leakage current between the electrode lines.


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Figure 7. (a) Schematic image of the fabricated f-OLED. (b) OM image of the f-OLED based on p-l_AgNWs under operation. (c) J-V and (d) L-V curves of the f-OLED based on p-l_AgNWs.


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Figure 8. Schematic images of the various f-OLED designs (left column) and the J-L-V curves of the f-OLEDs (right column). The designs of the f-OLEDs are (a) three by two arrays, (b) strips, and (c) large-area pixels.

As shown in Figure 7c and 7d, f-OLEDs based on p-l_AgNWs were successfully fabricated. To the best of our knowledge, the maximum luminance (Lmax) of the f-OLEDs in Figure 7d is the highest among AgNW-based f-OLEDs on PI substrates using solution-based fabrication processes.29 Various designs of f-OLEDs based on p-l_AgNWs were also fabricated. Schematic


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images of three f-OLEDs designs are shown in the left column of Figure 8. The f-OLED designs included three-by-two array, strip, and large-area f-OLED configurations. As shown in the right columns of Figure 8, the performances of the f-OLEDs with various designs were similar to that of the small, single-cell f-OLED in Figure 7. As shown in the images inset in the right column of Figure 8, the light emission was very uniform regardless of the f-OLED shape. These results indicate that AgNWs with a very uniform and homogeneous pattern could be obtained using our novel patterning method. For the AgNWbased OLEDs, the uniformity of the AgNW coating is important because the surface roughness of the AgNWs, which can cause electrical shorts, is high when the AgNWs are not uniformly coated.43 Hence, by showing the successful fabrication of f-OLEDs, the AgNWs were verified to be uniformly coated when the novel patterning method was used.

Figure 9. Photographs of the f-OLEDs with (a) three-by-two arrays, (b) strip, and (c) large-area designs operating under bending.


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Figure 9 shows the photographs of the f-OLEDs operating under mechanical bending. In addition, Movies S2 and S3 (Supporting Information) show videos of the f-OLEDs operating under repeated mechanical bending cycles. The f-OLEDs were highly stable under both multiple compressive and tensile bending. In the static bending tests of f-OLEDs, the OLED cells were placed at the center of the bent PI substrate to induce accurate bending strain, and rb decreased from 5 to 2 mm. The schematic images of the f-OLED designs for the static bending test are shown in Figure S9a (Supporting Information), while Figure 10a shows the normalized luminance (L) change during the static bending test.

Figure 10. The (a) static and (b) cyclic bending test results for various f-OLEDs.

As shown in Figure 10a, the p-l_AgNW-based OLED showed better bending stability than the f-l_AgNW- and indium tin oxide (ITO)-based OLEDs. The p-l_AgNW-based OLED maintained over 80% and 30% of its initial L (L0) even at the rb of 5 and 2 mm, respectively. Using the pl_AgNWs as the anode, in larger bending radius (greater than 3 mm), the f-OLED showed excellent bending stability when bent in the vertical direction, which is similar to the static bending stability results of only the transparent electrode (excluding the OLED). However, when the bending radius was smaller than 3 mm, the f-OLEDs with p-l_AgNWs showed very similar


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bending stabilities when bent either in the parallel or vertical directions. The L of the f-l_AgNWbased OLED decreased to below 40% of its L0 when rb was 5 mm, while the ITO-based OLED did not light up at all when rb was at or below 5 mm. The severe degradation of the ITO-based OLED was due to the crack formation in the ITO anode upon bending at very small rb.44 However, for the cases of p-AgNWs- and f-AgNWsbased OLEDs, other degradation mechanism should be considered not related to the fragmentation of the anodes. In our previous study, we showed that the electron injection barrier between the interface of the ETL and EML increased with the bending of the OLED, which caused the unbalanced injection of electrons and holes into the OLED, and eventually deteriorated the performance of the device.45 Based on those results, one of the main mechanism for degradation of the f-OLEDs during static bending was not by the fragmentation of the anodes, but due to the interface charge transport deterioration between the ETL and EML. The excellent bending stability of the p-AgNW-based OLED was due to the low residual stress in the devices attributed to the geometry of the patterned anode. When electrodes are patterned, the interfacial area between the electrode and the substrate should decrease.46 Hence, the residual stress induced on the electrodes and devices would decrease.46 If less residual stress was induced to the electrodes and devices, devices could tolerate large bending stresses, because the total stress induced in the devices is a combination of the residual stress and the external applied stress (bending stress).47 Hence, p-AgNW-based f-OLEDs require higher bending stress to reach the critical bending stress than f-AgNW- or ITO-based f-OLEDs. The p-l_AgNW was bent multiple times to further determine its stability at the fixed rb of 10 mm for 100 cycles with the results presented in Figure 10b. Interestingly, both the parallel and perpendicular type of p-l_AgNW OLEDs showed very similar poor bending stability even


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though the parallel type p-l_AgNW OLED was expected to be more stable the than the perpendicular type device. One of the reason for the poor performance of the f-OLEDs was due to the unstable nature of the OLEDs upon exposure to moisture and oxygen gas in the ambient air than having poor cyclic bending stabilities. However, any encapsulation of the f-OLED was not applied to the devices for both the cyclic and static bending tests since it would limit the maximum bending stress that can be applied. As shown in Figure S7b (Supporting Information), the f-OLEDs started to degrade after 15 min exposure to the ambient air. In the case of the static bending test, the experiment was conducted within 20 min after exposing the f-OLEDs to the ambient air, thus the deteriorating effects of the moisture and oxygen gas on the f-OLEDs were minimized, and the true bending stability of the f-OLEDs was determined without any external environmental effects. However, for the cyclic bending test, the experiment was conducted for more than an hour due to the periodic application of bending and thus, the effects of the ambient air exposure could not be ignored. Nevertheless, the cyclic bending test of the f-OLEDs without the deteriorating ambient air effects is the subject of our future work by conducting it in an inert condition such as a nitrogen atmosphere. p-AgNW-based f-OLEDs can be applied to flexible displays, such as passive matrix OLED (PMOLED) displays.48 Moreover, p-AgNWs could also be used in various optoelectronic and electronic devices, such as organic and perovskite solar cells, thin-film transistors, and various sensors.49-50 Additionally, flexible optoelectronic and electronic devices based on p-AgNWs would show better mechanical flexibility than devices containing fully coated electrodes.

3. CONCLUSION


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In this study, we demonstrated a low-cost and effective patterning method for AgNWs by combining the capillary force, coffee ring effect, and centrifugal force. With this method, AgNW patterns with various w from 100 to 10 µm were obtained. These results include one of the thinnest AgNW patterns ever reported. Moreover, our novel patterning technique could simultaneously pattern and align AgNWs by adjusting the length of the AgNWs and w of the microchannels of the patterning stamp. Various AgNW shapes, such as mesh and serpentine, can be patterned, and other nanomaterials and their hybrids could be easily patterned as well. The RL of the p-AgNWs prepared in this work was lower than those of other previously reported pAgNWs prepared by etching processes due to the significant number of retained nanowirenanowire junctions. The patterned and aligned AgNWs were also shown to have anisotropic bending stability depending on the bending direction. If tensile strain was induced perpendicular to the p-AgNW alignment, the RL did not significantly change. f-OLEDs were successfully fabricated using the patterned electrodes, which indicates that the p-AgNWs are excellent candidates as electrode materials for flexible displays and other flexible electronic devices. Moreover, due to the patterned structure, the p-AgNW-based f-OLEDs showed better bending stability than the fAgNW- and ITO-based f-OLEDs. The novel nanomaterial patterning method presented in this work has a number of advantages over other approaches; for example, our method provides a simple and cost-efficient way to obtain residue-free, scalable, and clear patterns of nanomaterials. This novel patterning process does not require any complicated or expensive equipment, as is required in photolithography and wet- and dry-etching techniques. Because the microchannels of the PDMS stamp were


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spontaneously filled due to capillary forces, a very small amount of the nanomaterial solution is needed to fill the entire microchannel. The PDMS stamp could be reused multiple times, as no damage was incurred on the stamp during this novel patterning process. AgNWs or other types of nanomaterials could be patterned on any type of substrate, such as glass, polyethylene terephthalate (PET), and polyimide (PI). Furthermore, there is no chemical residue left on the patterned nanomaterials after the patterning processes, as etching or photolithography steps are not needed.

4. EXPERIMENTAL SECTION

PDMS stamp fabrication: The PDMS stamp was fabricated using a photolithographic process. A Si wafer was cleaned in a sonication bath with acetone, isopropyl alcohol, and deionized water in sequence. Each sonication step was performed for 10 min. After solvent cleaning and air blow-drying processes, the Si surface was treated with oxygen plasma at 140 W for 90 s. SU-8 photoresist (from Microchem) was then spin-coated onto the Si wafer at 500 rpm for 10 s and 1000 rpm for 30 s in sequence. The coated SU-8 photoresist was soft baked at 65 °C for 3 min and 95 °C for 7 min in sequence. The sample was then exposed to UV light using a contact aligner (MIDAS System) while covered by a photomask with the desired pattern. Consequently, the samples were soft baked at 65 °C for 1 min and 95 °C for 3 min in sequence and then developed using the SU-8 developer. The patterned SU-8 photoresist on the Si wafer was then used as the master mold. The w and shape of the master mold could be modified by changing the pattern of the photomask. PDMS was prepared by mixing the base and cross-linking or curing agent (Sylgard 184 elastomer kit from Dow Corning) in a weight ratio of 10:1. The liquid


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mixture was then poured onto the master mold and cured at room temperature for over 24 h. After the curing process, the PDMS stamp was released from the master mold. Patterning processes: The PDMS stamp was placed on the PI substrate (from Mitsubishi Gas Chemical, with a thickness of 200 µm), forming conformal contact (Figure 1). The edge of the PDMS stamp was cut using a razor blade to open the microchannels. When patterning the s_AgNWs, the PI substrate was treated with oxygen plasma at 140 W for 90 s prior to placement of the PDMS stamp. The AgNW solution (from Novarials for l_AgNW, and Duksan Hi-Metal for s_AgNW) was then drop-casted at an open end of the microchannels. AgNWs was dispersed in ethanol, and the concentration was varied from 0.25 to 0.65 wt% for s_AgNW and 0.32 to 0.63 wt% for l_AgNW. After the AgNW solution filled the microchannels, spinning was applied at 3000 rpm for 240 s. After the spinning step, the PDMS stamp was peeled from the substrate. To apply thermal annealing for solvent evaporation instead of spinning, the samples were annealed at 80 °C for 10 min after the solution filled the microchannels. A 0.5 wt% solution of ZnO or TiO2 nanoparticles (from Sigma-Aldrich) in ethanol was used for the patterning of nanoparticles. f-OLED fabrication: p-AgNWs, f-AgNWs, and ITO thin films were used as anodes. For the fAgNW-based OLEDs, a 0.13 wt% AgNW solution was spin-coated four times onto a PI substrate. The p-AgNWs and f-AgNWs were annealed at 180 °C for 20 min after the coating process. ITO was deposited using a direct current (DC) magnetron sputtering system. The gas flow rates of oxygen and argon were maintained at 0.1 and 70 sccm, respectively, and the substrate temperature was maintained at 75 °C during the deposition process. As the HTL, a 1:1 solution by weight of PEDOT:PSS (AI 4083, from Heraeus Clevios GmbH) and isopropyl alcohol was spin-coated onto the anode after being filtered through a 100 µm Teflon syringe


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filter. On the ITO anode, PEDOT:PSS was spin-coated at 4000 rpm for 60 s, whereas on the AgNW anode, PEDOT:PSS was spin-coated six times at 1000 rpm for 60 s. For the AgNWbased OLED, thick HTL layers were used to reduce the effects of the surface roughness of the AgNWs. The HTL was thermally annealed at 120 °C for 10 min under ambient air conditions. The samples were then transferred into a nitrogen-filled glove box. On the HTL, 5 mg ml-1 SY (PDY-132, from Merck) dissolved in toluene was spin-coated at 1500 rpm for 30 s to form the EML. The EML was thermally annealed at 100 °C for 5 min. dPEIE was used as the ETL. The d-PEIE solution contained 0.25 wt% PEIE (from Sigma-Aldrich) and Cs2CO3 (from Sigma-Aldrich) co-dissolved in 2-ethoxyethanol (the weight ratio of PEIE to Cs2CO3 was 10:1). The d-PEIE solution was spin-coated onto the EML at 5000 rpm for 30 s. The samples were then annealed at 110 °C for 5 min. The samples were transferred to a thermal evaporation chamber to deposit the 150 nm-thick Al cathode. The HTL and EML solutions were mixed using a magnetic stirring bar at 300 rpm for 6 hr at room temperature, whereas the ETL solution was stirred at the same stirring rate but at 80 °C. Characterization methods: The p-AgNW microstructures were analyzed using OM and FESEM. The area density of the AgNWs was measured using ImageJ software. The RL of the pAgNWs and f-AgNWs were measured using a two-point probe multimeter. The measurement area for the electrical conductivity analysis was 1 cm in length and 1.4 cm in width, and two silver-paste (P-100, CANS) contacts were applied at the edges of the electrode. The Rs of the pAgNWs and f-AgNWs were measured using a four-point probe. Static and cyclic bending tests were performed to evaluate the mechanical reliability of the electrodes. In the static test, the rb was decreased from 5 to 1 mm, and the cyclic bending tests were performed at an rb of 2.5 mm for 5000 bending cycles. The variation in the RL of the electrodes was measured every 500th


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bending cycle. The current density (J), voltage (V), and L of the f-OLEDs were measured using a source meter (Keithley 2400) and a Konica-Minolta CS-200 chromameter. For the f-OLEDs, only the static bending test was performed, and the rb was decreased from 5 to 2 mm with a constant applied V of 6 V. ASSOCIATED CONTENT Supporting Information. Figures, table and movies of additional experimental data are included. The following files are available free of charge. The Supporting Information file contains the OM images of p-AgNWs with various line w, FE-SEM images of the patterned nano-particles, surface profile images of the p-AgNWs, photographs of the large-area specimen of p-AgNWs, FoM values of the AgNWbased electrodes (p-AgNWs, f-AgNWs, and other AgNW-based electrodes), schematic images of the f-OLEDs for mechanical bending tests, lifetime test results of the f-OLEDs, and table for the RL of p-AgNWs with various line w.

Movie 1 : Video of flow of the injected fluid at the back side of the channel (AVI) Movie 2 : Video of the array design of f-OLEDs operating under mechanical bending (AVI) Movie 3 : Video of the strip design of f-OLEDs operating under mechanical bending (AVI)

AUTHOR INFORMATION

Corresponding Author


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*Corresponding author’s contact information: E-mail: [email protected], Phone: +82 2 2123 5834, Fax: +82 2 312 5375 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Education (grant number 2015R1D1A1A01061340) and the Joint Program for Samsung Electronics-Yonsei University.

ABBREVIATIONS

PDMS, polydimethylsiloxane; f-OLEDs, flexible organic light-emitting diodes; CNT, carbon nanotubes; AgNWs, silver nanowires; f-AgNWs, fully coated AgNWs; w, pattern line width; CFL, capillary-force-based soft lithography; OM, optical microscopy; FE-SEM, field-emission scanning electron microscopy; Pca, capillary pressure; PI, polyimide; l_AgNWs, long AgNWs; s_AgNWs, short AgNWs; p-l_AgNWs, patterned long AgNWs; p-s_AgNWs, patterned short AgNWs; θc, center angle; FWHM, full width at half maximum; f-l_AgNWs, fully coated long AgNWs; RL, electrical line resistance; T, sheet resistance; Rs, figure of merit; FoM, optical transmittance;

rb ,

bending

radius;

PEDOT:PSS

,

Poly(3,4-ethylenedioxythiophene)-


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poly(styrenesulfonate); SY, Super Yellow; d-PEIE, doped polyethyleneimine ethoxylated; HTL, hole-transport layer; EML, emission layer; ETL, electron-transport layer; Lmax, maximum luminance; L0, initial L; L, luminance; J, current density; PMOLED, passive matrix OLED; DC, direct current; V, voltage; AFM, atomic force microscopy; LED, light emitting diode;

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uring Processing and Thermal Cycling in Copper Interconnect Lines. Acta Mater. 2000, 48 (12), 3169-3175. (47) Bouten, P. C. P.; Slikkerveer, P. J.; Leterrier, Y., Mechanics of Ito on Plastic Substrates for F lexible Displays. In Flexible Flat Panel Displays, 2005, pp 99-120. (48) Leroy, P., Addressing Oled Microdisplays. In Oled Microdisplays: Technology and Applicat ions, Templier, F., Ed. NJ, USA, 2014; Chapter 5, pp 119-157. (49) Kim, A.; Lee, H.; Kwon, H.-C.; Jung, H. S.; Park, N.-G.; Jeong, S.; Moon, J., Fully Solutio n-Processed Transparent Electrodes Based on Silver Nanowire Composites for Perovskite Solar Cells. Nanoscale 2016, 8 (12), 6308-6316. (50) Zhao, F.; Chen, D.; Chang, S.; Huang, H.; Tong, K.; Xiao, C.; Chou, S.; Zhong, H.; Pei, Q., Highly Flexible Organometal Halide Perovskite Quantum Dot Based Light-Emitting Diodes on a Silver Nanowire-Polymer Composite Electrode. J. Mater. Chem. C 2017, 5 (3), 531-538.


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Novel Patterning Method for Nanomaterials and Its ... - ACS Publications

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