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PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure
TSF-34108; No of Pages 6 Thin Solid Films xxx (2015) xxx–xxx
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Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure Myoung-Soo Kim a, Da-Hyeok Lee a, Ki-Bo Kim a, Seok-Heon Jung b, Jin-Kyun Lee b,⁎, Beom-Hoan O a, Seung-Gol Lee a, Se-Geun Park a,⁎⁎ a b
Department of Information Engineering, INHA University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea Department of Polymer Science and Engineering, INHA University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea
a r t i c l e
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Available online xxxx Keywords: Poly(1H,1H,2H,2H-perfluorodecyl methacrylate) Micro-contact printing Silver Nanowire Lift-off process
a b s t r a c t This paper describes a lift-off method of Ag nanowire (Ag NW) patterning using a poly(1H,1H,2H,2Hperfluorodecyl methacrylate) polymer (PFDMA) structure as a mask which is prepared by micro-contact printing. Unlike a conventional photoresist mask, the PFDMA polymer is inert to the dispersion solvent of Ag NW. In addition, the hydrofluoroether solvent used for removing the mask layer of patterned PFDMA films after Ag NW deposition does not chemically affect the polyethylene naphthalate (PEN) substrate or poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coated on Ag NW layer. In this method, Ag NW/ PEDOT:PSS composite films were patterned and the effects of the hot-press method were examined to further improve the electrical and optical properties of the composite films. Moreover, the hot-press method at 110 °C has an advantage of applying low pressure to make Ag NW/PEDOT:PSS embedded into PEN films compared to that of pressing samples without heating. The ratio of resistance change of patterned and hot-pressed composite film was only below 1% after repeated bending test. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently, silver nanowire (Ag NW) has been studied extensively as an alternative to transparent doped metallic oxides, such as indium tin oxide (ITO) [1–20]. Ag NW offers higher electrical conductivity and mechanical flexibility than ITO [1–3]. The patterning method of Ag NW is crucial if they are to be used as a replacement for ITO films in many applications, such as touch screens [4,5], organic solar cells [6,7], organic light emitting diodes [8], or flexible electrodes [9,10]. On the other hand, the patterning of Ag NW has a limitation because Ag NW is dispersed in solution before forming thin films [3]. Generally, Ag NW is dispersed in organic solvents, such as methanol [7,10] or isopropyl alcohol (IPA) [5], and is coated on substrates using wire-wound rod [11], spray [12–14], brush [6,15], or spin coating [7,9,10] methods. For Ag NW for transparent electrode preparation, etching technique is a common patterning method [16,17]. Etch mask patterns are formed on substrates coated with Ag NW films and then Ag NW films in the unmasked region are removed by a selective etching process [16,17]. However, the etching technique has problems such as contamination or undercuts. Ag NW films coated on patterned mold are, in some cases, transferred directly to the substrate by contact printing [18,19] or Ag NW patterns are prepared only on the dopamine-modified region ⁎ Corresponding author. Tel.: +82 32 860 7481. ⁎⁎ Corresponding author. Tel.: +82 32 860 7434. E-mail addresses: [email protected] (J.-K. Lee), [email protected] (S.-G. Park).
of the substrate [20]. Both methods can cause protrusions of Ag NW at the edges of the patterns and have a limitation in pattern resolution. An alternative method for the lift-off technique using a conventional organic photoresist also has a problem because the dispersion solvent of Ag NW can attack chemically the patterned photoresist mask. Another problem might be possible reaction of resist removing solvent with Ag NW films or organic substrates such as polyvinyl pyrrolidone. Therefore, a chemical reaction between the materials should be checked first in a lift-off process. This paper reports a lift-off technique in which Ag NW film patterning can be processed by replacing the resist material and the resist removing solvent with poly(1H,1H,2H,2H-perfluorodecyl methacrylate) polymer (PFDMA) film and hydrofluoroethers, respectively. In order to improve the electrical and optical properties of Ag NW/ poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) films have been added and effects of the spray-coated and hot-pressed composite films have been examined. 2. Experiment The poly(dimethylsiloxane) (PDMS) mold, which was the master of the replica, was prepared by curing its prepolymer (Sylgard 184, Dow Corning) from the patterned photoresist (AZ 7220, Clariant) on a Si wafer. The PDMS mold had line/spacing patterns of 10/10 μm, 15/ 15 μm, 20/20 μm and 25/25 μm, and their line height was 1.6 μm. 1H,1H,2H,2H-perfluorodecyl methacrylate (FDMA) was solved at
http://dx.doi.org/10.1016/j.tsf.2015.02.028 0040-6090/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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The surface morphology of the patterned Ag NW films with various line and space sizes and patterned Ag NW/PEDOT:PSS composite films were examined by field emission scanning electron microscopy (FESEM, S-4300, Hitachi). The topography of the patterned films was analyzed by scanning probe microscopy (LEXT OLS4500, Olympus) in dynamic mode. A four-point probe (CMT-SR2000N, AIT) was used to measure the sheet resistance in auto contact mode. Optical transmittance was measured using a UV–vis spectrometer (Lambda 750, PerkinElmer). The same patterns of 25 μm width and 975 μm pitch and 1 × 5 cm size were prepared on the pressed and hot-pressed films, respectively. Durability of both samples were investigated by a bending test using semicircular polyethylene cylinders with different bending radii of 20 mm and 15 mm. It was a cyclic test with automated bending mode. Fig. 1. SEM image of spray-coated Ag NW films. Some Ag NW films are observed on PFDMA lines.
11 wt.% in hydrofluoroether solvent (HFE-7500, 3 M). This solution was spin-coated on PDMS molds at 1000 rpm for 30 s and PFDMA films covered the PDMS mold surface after drying at room temperature. Micro-Contact Printing (μCP) was performed and parts of PFDMA films coated on lines of the PDMS mold were transferred onto substrates such as glass (Marienfeld) or polyethylene naphthalate (PEN, Teonex® Q65HA-125, Teijin DuPont Films) without any external pressure at room temperature [21]. 2 ml of Ag NW solution (NANOPYXIS), consisting of Ag nanowires with 25 ± 5 μm length dispersed in isopropyl alcohol (IPA) at 0.06 wt.%, was spray-coated on glass or PEN substrate, whose surface had PFDMA line and space patterns, using an airbrush (KP-HP-TH 0.5 mm, Iwata) at 70 °C. PEDOT:PSS (Sigma-Aldrich) with 1.3 wt.% was diluted in deionized water to have a concentration of 0.01 wt.% or 0.65 wt.%. Each solution was then mixed with IPA with a 1:4 ratio. Some of the samples were coated by this solution of PEDOT:PSS at 70 °C to form Ag nanowire/PEDOT:PSS composite films. The samples were positioned in the hydrofluoroether solvent (HFE-7300, 3 M) for 30 s in order to remove the PFDMA mask layers. The composite films were pressed without heating and hot-pressed at 110 °C for 2 min to 300 kPa, respectively.
3. Results and discussion A previous study reported that PFDMA films could be patterned by μCP on various substrates using a PDMS mold utilizing the very low adhesion force between PFDMA films and PDMS mold [21]. The PFDMA films could be dissolved in hydrofluoroethers without chemically attacking the organic substrates such as poly(methylmethacrylate) and poly(vinyl pyrrolidone), which opens the possibility of the orthogonal processing of diverse materials [21]. In this study, the patterned PFDMA films were used as the mask material for the lift-off process of Ag NW instead of a conventional photoresist. Fig. 1 shows the surface morphology of the Ag NW spraycoated on PEN substrates with patterned PFDMA films. The Ag NW films were formed on an un-patterned surface without damaging the PFDMA films because of the fluoropolymer as the PFDMA films was not dissolved in the dispersion organic solvent. The thickness of the transferred PFDMA films is an important parameter for the lift-off process and it determines the possible thickness range of the Ag NW films. The PFDMA films need to be thick enough to prevent a cross connection between lines. From the PDMS mold with a line height of 1.6 μm, the PFDMA films at 11 wt.% can have a maximum thickness of 480 nm. The highly fluorinated polymer has very limited adhesion to both non-fluorinated organic materials and hydrophilic surfaces [21].
Fig. 2. SEM images of the patterned Ag NW films with a width of (a) 10 μm, (b) 15 μm, (c) 20 μm, and (d) 25 μm, respectively. When PFDMA pattern pitch is less than 20 μm, Ag NW is often cross-connected.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
M.-S. Kim et al. / Thin Solid Films xxx (2015) xxx–xxx
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Fig. 5. SEM images of the surface morphology of the patterned Ag NW/PEDOT:PSS composite films with PEDOT:PSS (0.01 wt.%, 4 ml). The inset shows a cross-sectional SEM image of the composite films.
Therefore, very few Ag NW remain on PFDMA films after coating and most of the Ag NW on PFDMA films was swept away by the air dispensed from the airbrush at a dispensing pressure of 0.5 MPa onto the un-patterned surfaces where PFDMA did not cover. Fig. 2 shows the patterned Ag NW on the glass substrates with various lines and space widths of the patterns after removing the PFDMA films with the hydrofluoroether solvent. In all the figures, finely defined edge lines were observed at the edge of the patterned Ag NW films. The main reason for this was that the Ag NW on the un-patterned regions was swept away to the edge by the dispensed air. The Ag NW films by the spray-coating were formed by overlapping drops of the Ag NW Fig. 3. SEM images of (a) line and space pattern of Ag NW/PEDOT:PSS composite films with PEDOT:PSS (0.65 wt.%, 2 ml), (b) its surface morphology. The inset shows a cross-sectional image of the composite films.
Fig. 4. SEM images of coated Ag NW films (a) without annealing, (b) with annealing at 200 °C for 20 min.
Fig. 6. SEM images of (a) pressed Ag NW/PEDOT:PSS composite films without heating and (b) hot-pressed Ag NW/PEDOT:PSS composite patterns at 110 °C for 2 min at 300 kPa. The inset shows SEM images of its surface morphology. Yellow and red circle indicate a good and poor electrical junction, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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solution. Another explanation for these observations was the coffeering effect in that when a coffee drop dries on a surface, it leaves a ring-like deposit along the edge [22]. The formable minimum pattern width is determined by the length of the Ag NW. In the solution, the length of the Ag NW was 25 ± 5 μm. If the pitch of line and space was less than the length of the Ag NW, lines were connected to each other, as shown in Fig. 2(a) and (b). Consequently, higher resolution can be obtained with shorter Ag NW length. In case of Ag NW as transparent electrodes, composite films consisting Ag NW with other materials such as carbon nanotubes [23, 24], graphene [25], or conducting polymers [5,7,9,14,17], are reported to be more effective in forming a smooth film surface and improving the electrical conductivity by making the electrical junctions among the Ag NW. Patterning the composite films by etching, etch selectivity between the conducting materials and substrates should be considered. On the other hand, the main merit of the lift-off process using a fluoropolymer of this study over other methods is that it can offer the wide selectivity of materials to form patterned composite films. PEDOT:PSS does not react with the patterned PFDMA films. Therefore, patterns of the Ag NW/PEDOT:PSS composite films after coating of PEDOT:PSS with 0.65 wt.% and 2 ml on glass substrate can be formed easily, as shown in Fig. 3(a). Fig. 3(b) and the inserted image show fully passivated films with PEDOT:PSS. The fully passivated films showed a smooth surface compared to the films with Ag NW only as shown in Fig. 2. Fig. 4 shows SEM images of the non-annealed and annealed Ag NW films on glass substrates at 200 °C for 20 min. The annealing process can improve the electrical conductivity of the films by making junctions among the Ag NW as shown in Fig. 4(b). Therefore, the composite films were all annealed at 200 °C for 20 min. After annealing the
composite films, the mean sheet-resistance decreased from 12.3 Ω/sq to 11.5 Ω/sq. On the other hand, in the case of using organic substrates, annealing of Ag NW/PEDOT:PSS to make a junction cannot be applied because of thermal damage to the organic substrates at high temperature. There are many nanowire bonding methods such as welding, soldering, and mechanical bonding [26]. Among the methods, the mechanical bonding such as press or hot-press method has an advantage for large-area fabrication [26–29]. In order to make a smooth surface, the press method without heating needs a high pressure up to GPa [27] or MPa [28] range. In contrast, the hot-press method has an advantage of low pressure to make a smooth surface by embedding Ag NW into PET films [29]. For the mechanical bonding method, the Ag NW does not need to be passivated fully with the PEDOT:PSS films, as shown in Fig. 5. The Ag NW films just coated with PEDOT:PSS of 0.01 wt.% and 4 ml still have a non-uniform surface, as shown in the inserted image of Fig. 5. After the PEDOT:PSS coating on the Ag NW, the films were pressed without heating and hot-pressed at 110 °C for 2 min to 300 kPa, respectively. Fig. 6(a) shows that the pressure was not enough to embed Ag NW into PEN substrate. In addition, electrical junctions between Ag NW were not made completely as shown in red circles of the inset image. However, Fig. 6(b) shows that the patterned Ag NW/PEDOT:PSS films can be embedded into the PEN substrate by the hot pressing method. This process can improve both the electrical conductivity and make a smooth surface. Fig. 7 shows the images of bare PEN films, patterned Ag NW films, fully passivated films with PEDOT:PSS, and hot-pressed Ag NW/ PEDOT:PSS films on a PEN substrate, respectively. The bare PEN films have the high value of a ten point height (Sz) of 270.676 nm due to the presence of natural roughness as shown in the inserted image in
Fig. 7. Scanning probe microscopy images of (a) bare PEN films (b) patterned Ag NW films, (c) fully passivated films with PEDOT:PSS (0.65 wt.%, 2 ml), and (d) hot-pressed Ag NW/ PEDOT:PSS films on a PEN substrate.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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Fig. 6. The roughness average (Sa), root mean square parameter (Sq), and ten point height (Sz) of the hot-pressed films were significantly smaller than the other samples. This smooth surface of the hotpressed films causes a slight increase in transmittance compared to the PEDOT:PSS-coated Ag NW films. Fig. 8 shows the sheet resistance measured at 10 positions of the Ag NW spray-coated films, fully passivated Ag NW films with PEDOT:PSS, PEDOT:PSS coated Ag NW films, and hot-pressed Ag NW/PEDOT:PSS films on a PEN substrate. Ag NW spray-coated films without any treatment showed large variations in sheet resistance, as shown in Fig. 8(a). This suggests that the Ag NW spray-coated films may not be bonded close to each other and just sit on the network structure of the Ag NW as shown in Fig. 4(a). Therefore, the degree of junctions of the Ag NW can be changed easily by an external force, which could be the contact force between the Ag NW and the probe head in this study. The coating of PEDOT:PSS reduces the variations of the sheet resistance by making stable electrical connections among the Ag NW. On the other hand, PEDOT:PSS is not good enough to bind fully each Ag NW as shown in Fig. 8(b) and (c). The embedded composite films are more effective in binding each Ag NW, which leads to an improvement in the sheet resistance compared to other samples, as shown in Fig. 8(d). Fig. 9 shows the transmittance spectra of the bare PEN films and fabricated films with the line and space pattern widths of 25 μm. The mean transmittance of the hot-pressed Ag NW/PEDOT:PSS films was 78.37% in the visible wavelength range between 400 and 800 nm. The mean transmittance of the spray-coated Ag NW films and PEDOT:PSS coated Ag NW films was 77.62% and 77.53% of the average transmittance, respectively. The coating of PEDOT:PSS induced high transmittance above 520 nm compared to the spray-coated Ag NW films. The transmittance of glass substrates with Ag NW/PEDOT:PSS bilayer patterns, which has 25 μm line and space patterns, showed 81.15% transmittance in the visible wavelength range between 400 and 700 nm. The transmittance increased with increasing pitch as shown in the inserted image. The pressed and hot-pressed films with the patterns of 25 μm width and 975 μm pitch and 1 × 5 cm sizes were prepared, respectively and tested bending test on semicircular cylinders and cyclic bending test. Fig. 10(a) shows that the pressed film has a large resistance change by bending on semicircular cylinders with bending radii of 15 mm because the connection of Ag NW was detached by bending. In contrast, the resistance of hot-pressed films was slightly changed as 1.8% under the same condition. Fig. 10(b) shows the result after cyclic bending test of the hot-pressed electrode. The film retains its conductivity even under bending and recovers to close to its initial resistance once the film is released after 1000 cycle test. This result indicates that the embedded Ag NW/PEDOT:PSS into PEN films was effective in sustaining the electrical properties due to their good mechanical reliability.
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Fig. 9. Transmittance of a bare PEN films, spray-coated Ag NW films, Ag NW and PEDOT:PSS (0.01 wt.%, 4 ml) coated films, fully passivated films with PEDOT:PSS (0.65 wt.%, 2 ml), hot-pressed films, with a line and space width of 25 μm, respectively. The inset shows the transmittance of a bare glass substrate, the patterned composite films with line and space width of 25 μm, with width of 25 μm and pitch of 75 μm, with width of 25 μm and pitch of 200 μm, respectively.
offers wide flexibility to use nano-materials in a dispersion solvent and can be applied to prepare electrically conducting patterns on transparent and flexible substrates. In the case of using organic substrates, the
4. Conclusion This paper described a patterning method of Ag nanowire/PEDOT:PSS composite bilayer films using highly hydrophobic fluoropolymer patterns that can avoid solvent orthogonality problems. This patterning method
Fig. 8. Sheet resistance of (a) Ag NW spray-coated films, (b) fully passivated films with PEDOT:PSS (0.65 wt.%, 2 ml), (c) Ag NW and PEDOT:PSS (0.01 wt.%, 4 ml) coated films, and (d) hot-pressed films. Measurement was done at 10 positions.
Fig. 10. The ratio of resistance change of (a) the pressed and hot-pressed electrode on semicircular polyethylene cylinders with different bending radii of curvature of 20 mm and 15 mm and (b) the hot-pressed film as the number of bending cycles. The test performed by placing the film between the two platforms and bending the films by reducing the distance between the platforms. Each sample was tested 10 times and 5 times, respectively.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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electrical properties of the composite films can be improved by hotpressing at lower temperature than annealing treatment. In addition, the hot-press method can achieve a smooth surface and mechanical stability of the films by embedding Ag nanowire/PEDOT:PSS into organic films at lower pressures than press method. The simplicity of this technique can offer wide flexibility and potential for many applications, such as nano-material-based devices and the lift-off deposition of nanomaterials. Acknowledgement This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Education, Science and Technology (NRF-2013R1A1A2-00-5260) and partly by Inha University through its Basic Research Promotion Program. References [1] D. Langley, G. Giusti, C. Mayousse, C. Celle, D. Bellet, J. Simonato, Flexible transparent conductive materials based on silver nanowire networks: a review, Nanotechnology 24 (2013) 452001. [2] L. Hu, H. Wu, Y. Cui, Metal nanogrids, nanowires, and nanofibers for transparent electrodes, MRS Bull. 36 (2011) 760. [3] D.S. Hecht, R.B. Kaner, Solution-processed transparent electrodes, MRS Bull. 36 (2011) 749. [4] J. Lee, P. Lee, H. Lee, D. Lee, S.S. Lee, S.H. Ko, Very long Ag nanowire synthesis and its application in a highly transparent, conductive and flexible metal electrode touch panel, Nanoscale 4 (2012) 6408. [5] J. Lee, P. Lee, H.B. Lee, S. Hong, I. Lee, J. Yeo, S.S. Lee, T.S. Kim, D. Lee, S.H. Ko, Roomtemperature nanosoldering of a very long metal nanowire network by conductingpolymer-assisted joining for a flexible touch-panel application, Adv. Funct. Mater. 23 (2013) 4171. [6] S.B. Kang, Y.J. Noh, S.I. Na, H.K. Kim, Brush-painted flexible organic solar cells using highly transparent and flexible Ag nanowire network electrodes, Sol. Energy Mater. Sol. C 122 (2014) 152. [7] W. Gaynor, G.F. Burkhard, M.D. McGehee, P. Peumans, Smooth nanowire/polymer composite transparent electrodes, Adv. Mater. 23 (2011) 2905. [8] X.Y. Zeng, Q.K. Zhang, R.M. Yu, C.Z. Lu, A new transparent conductor: silver nanowire film buried at the surface of a transparent polymer, Adv. Mater. 22 (2010) 4484. [9] L. Yang, T. Zhang, H. Zhou, S.C. Price, B.J. Wiley, W. You, Solution-processed flexible polymer solar cells with silver nanowire electrodes, ACS Appl. Mater. Interfaces 3 (2011) 4075. [10] Z. Yu, Q. Zhang, L. Li, Q. Chen, X. Niu, J. Liu, Q. Pei, Highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes, Adv. Mater. 23 (2011) 664.
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Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure Myoung-Soo Kim a, Da-Hyeok Lee a, Ki-Bo Kim a, Seok-Heon Jung b, Jin-Kyun Lee b,⁎, Beom-Hoan O a, Seung-Gol Lee a, Se-Geun Park a,⁎⁎ a b
Department of Information Engineering, INHA University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea Department of Polymer Science and Engineering, INHA University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Available online xxxx Keywords: Poly(1H,1H,2H,2H-perfluorodecyl methacrylate) Micro-contact printing Silver Nanowire Lift-off process
a b s t r a c t This paper describes a lift-off method of Ag nanowire (Ag NW) patterning using a poly(1H,1H,2H,2Hperfluorodecyl methacrylate) polymer (PFDMA) structure as a mask which is prepared by micro-contact printing. Unlike a conventional photoresist mask, the PFDMA polymer is inert to the dispersion solvent of Ag NW. In addition, the hydrofluoroether solvent used for removing the mask layer of patterned PFDMA films after Ag NW deposition does not chemically affect the polyethylene naphthalate (PEN) substrate or poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coated on Ag NW layer. In this method, Ag NW/ PEDOT:PSS composite films were patterned and the effects of the hot-press method were examined to further improve the electrical and optical properties of the composite films. Moreover, the hot-press method at 110 °C has an advantage of applying low pressure to make Ag NW/PEDOT:PSS embedded into PEN films compared to that of pressing samples without heating. The ratio of resistance change of patterned and hot-pressed composite film was only below 1% after repeated bending test. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently, silver nanowire (Ag NW) has been studied extensively as an alternative to transparent doped metallic oxides, such as indium tin oxide (ITO) [1–20]. Ag NW offers higher electrical conductivity and mechanical flexibility than ITO [1–3]. The patterning method of Ag NW is crucial if they are to be used as a replacement for ITO films in many applications, such as touch screens [4,5], organic solar cells [6,7], organic light emitting diodes [8], or flexible electrodes [9,10]. On the other hand, the patterning of Ag NW has a limitation because Ag NW is dispersed in solution before forming thin films [3]. Generally, Ag NW is dispersed in organic solvents, such as methanol [7,10] or isopropyl alcohol (IPA) [5], and is coated on substrates using wire-wound rod [11], spray [12–14], brush [6,15], or spin coating [7,9,10] methods. For Ag NW for transparent electrode preparation, etching technique is a common patterning method [16,17]. Etch mask patterns are formed on substrates coated with Ag NW films and then Ag NW films in the unmasked region are removed by a selective etching process [16,17]. However, the etching technique has problems such as contamination or undercuts. Ag NW films coated on patterned mold are, in some cases, transferred directly to the substrate by contact printing [18,19] or Ag NW patterns are prepared only on the dopamine-modified region ⁎ Corresponding author. Tel.: +82 32 860 7481. ⁎⁎ Corresponding author. Tel.: +82 32 860 7434. E-mail addresses: [email protected] (J.-K. Lee), [email protected] (S.-G. Park).
of the substrate [20]. Both methods can cause protrusions of Ag NW at the edges of the patterns and have a limitation in pattern resolution. An alternative method for the lift-off technique using a conventional organic photoresist also has a problem because the dispersion solvent of Ag NW can attack chemically the patterned photoresist mask. Another problem might be possible reaction of resist removing solvent with Ag NW films or organic substrates such as polyvinyl pyrrolidone. Therefore, a chemical reaction between the materials should be checked first in a lift-off process. This paper reports a lift-off technique in which Ag NW film patterning can be processed by replacing the resist material and the resist removing solvent with poly(1H,1H,2H,2H-perfluorodecyl methacrylate) polymer (PFDMA) film and hydrofluoroethers, respectively. In order to improve the electrical and optical properties of Ag NW/ poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) films have been added and effects of the spray-coated and hot-pressed composite films have been examined. 2. Experiment The poly(dimethylsiloxane) (PDMS) mold, which was the master of the replica, was prepared by curing its prepolymer (Sylgard 184, Dow Corning) from the patterned photoresist (AZ 7220, Clariant) on a Si wafer. The PDMS mold had line/spacing patterns of 10/10 μm, 15/ 15 μm, 20/20 μm and 25/25 μm, and their line height was 1.6 μm. 1H,1H,2H,2H-perfluorodecyl methacrylate (FDMA) was solved at
http://dx.doi.org/10.1016/j.tsf.2015.02.028 0040-6090/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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The surface morphology of the patterned Ag NW films with various line and space sizes and patterned Ag NW/PEDOT:PSS composite films were examined by field emission scanning electron microscopy (FESEM, S-4300, Hitachi). The topography of the patterned films was analyzed by scanning probe microscopy (LEXT OLS4500, Olympus) in dynamic mode. A four-point probe (CMT-SR2000N, AIT) was used to measure the sheet resistance in auto contact mode. Optical transmittance was measured using a UV–vis spectrometer (Lambda 750, PerkinElmer). The same patterns of 25 μm width and 975 μm pitch and 1 × 5 cm size were prepared on the pressed and hot-pressed films, respectively. Durability of both samples were investigated by a bending test using semicircular polyethylene cylinders with different bending radii of 20 mm and 15 mm. It was a cyclic test with automated bending mode. Fig. 1. SEM image of spray-coated Ag NW films. Some Ag NW films are observed on PFDMA lines.
11 wt.% in hydrofluoroether solvent (HFE-7500, 3 M). This solution was spin-coated on PDMS molds at 1000 rpm for 30 s and PFDMA films covered the PDMS mold surface after drying at room temperature. Micro-Contact Printing (μCP) was performed and parts of PFDMA films coated on lines of the PDMS mold were transferred onto substrates such as glass (Marienfeld) or polyethylene naphthalate (PEN, Teonex® Q65HA-125, Teijin DuPont Films) without any external pressure at room temperature [21]. 2 ml of Ag NW solution (NANOPYXIS), consisting of Ag nanowires with 25 ± 5 μm length dispersed in isopropyl alcohol (IPA) at 0.06 wt.%, was spray-coated on glass or PEN substrate, whose surface had PFDMA line and space patterns, using an airbrush (KP-HP-TH 0.5 mm, Iwata) at 70 °C. PEDOT:PSS (Sigma-Aldrich) with 1.3 wt.% was diluted in deionized water to have a concentration of 0.01 wt.% or 0.65 wt.%. Each solution was then mixed with IPA with a 1:4 ratio. Some of the samples were coated by this solution of PEDOT:PSS at 70 °C to form Ag nanowire/PEDOT:PSS composite films. The samples were positioned in the hydrofluoroether solvent (HFE-7300, 3 M) for 30 s in order to remove the PFDMA mask layers. The composite films were pressed without heating and hot-pressed at 110 °C for 2 min to 300 kPa, respectively.
3. Results and discussion A previous study reported that PFDMA films could be patterned by μCP on various substrates using a PDMS mold utilizing the very low adhesion force between PFDMA films and PDMS mold [21]. The PFDMA films could be dissolved in hydrofluoroethers without chemically attacking the organic substrates such as poly(methylmethacrylate) and poly(vinyl pyrrolidone), which opens the possibility of the orthogonal processing of diverse materials [21]. In this study, the patterned PFDMA films were used as the mask material for the lift-off process of Ag NW instead of a conventional photoresist. Fig. 1 shows the surface morphology of the Ag NW spraycoated on PEN substrates with patterned PFDMA films. The Ag NW films were formed on an un-patterned surface without damaging the PFDMA films because of the fluoropolymer as the PFDMA films was not dissolved in the dispersion organic solvent. The thickness of the transferred PFDMA films is an important parameter for the lift-off process and it determines the possible thickness range of the Ag NW films. The PFDMA films need to be thick enough to prevent a cross connection between lines. From the PDMS mold with a line height of 1.6 μm, the PFDMA films at 11 wt.% can have a maximum thickness of 480 nm. The highly fluorinated polymer has very limited adhesion to both non-fluorinated organic materials and hydrophilic surfaces [21].
Fig. 2. SEM images of the patterned Ag NW films with a width of (a) 10 μm, (b) 15 μm, (c) 20 μm, and (d) 25 μm, respectively. When PFDMA pattern pitch is less than 20 μm, Ag NW is often cross-connected.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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Fig. 5. SEM images of the surface morphology of the patterned Ag NW/PEDOT:PSS composite films with PEDOT:PSS (0.01 wt.%, 4 ml). The inset shows a cross-sectional SEM image of the composite films.
Therefore, very few Ag NW remain on PFDMA films after coating and most of the Ag NW on PFDMA films was swept away by the air dispensed from the airbrush at a dispensing pressure of 0.5 MPa onto the un-patterned surfaces where PFDMA did not cover. Fig. 2 shows the patterned Ag NW on the glass substrates with various lines and space widths of the patterns after removing the PFDMA films with the hydrofluoroether solvent. In all the figures, finely defined edge lines were observed at the edge of the patterned Ag NW films. The main reason for this was that the Ag NW on the un-patterned regions was swept away to the edge by the dispensed air. The Ag NW films by the spray-coating were formed by overlapping drops of the Ag NW Fig. 3. SEM images of (a) line and space pattern of Ag NW/PEDOT:PSS composite films with PEDOT:PSS (0.65 wt.%, 2 ml), (b) its surface morphology. The inset shows a cross-sectional image of the composite films.
Fig. 4. SEM images of coated Ag NW films (a) without annealing, (b) with annealing at 200 °C for 20 min.
Fig. 6. SEM images of (a) pressed Ag NW/PEDOT:PSS composite films without heating and (b) hot-pressed Ag NW/PEDOT:PSS composite patterns at 110 °C for 2 min at 300 kPa. The inset shows SEM images of its surface morphology. Yellow and red circle indicate a good and poor electrical junction, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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solution. Another explanation for these observations was the coffeering effect in that when a coffee drop dries on a surface, it leaves a ring-like deposit along the edge [22]. The formable minimum pattern width is determined by the length of the Ag NW. In the solution, the length of the Ag NW was 25 ± 5 μm. If the pitch of line and space was less than the length of the Ag NW, lines were connected to each other, as shown in Fig. 2(a) and (b). Consequently, higher resolution can be obtained with shorter Ag NW length. In case of Ag NW as transparent electrodes, composite films consisting Ag NW with other materials such as carbon nanotubes [23, 24], graphene [25], or conducting polymers [5,7,9,14,17], are reported to be more effective in forming a smooth film surface and improving the electrical conductivity by making the electrical junctions among the Ag NW. Patterning the composite films by etching, etch selectivity between the conducting materials and substrates should be considered. On the other hand, the main merit of the lift-off process using a fluoropolymer of this study over other methods is that it can offer the wide selectivity of materials to form patterned composite films. PEDOT:PSS does not react with the patterned PFDMA films. Therefore, patterns of the Ag NW/PEDOT:PSS composite films after coating of PEDOT:PSS with 0.65 wt.% and 2 ml on glass substrate can be formed easily, as shown in Fig. 3(a). Fig. 3(b) and the inserted image show fully passivated films with PEDOT:PSS. The fully passivated films showed a smooth surface compared to the films with Ag NW only as shown in Fig. 2. Fig. 4 shows SEM images of the non-annealed and annealed Ag NW films on glass substrates at 200 °C for 20 min. The annealing process can improve the electrical conductivity of the films by making junctions among the Ag NW as shown in Fig. 4(b). Therefore, the composite films were all annealed at 200 °C for 20 min. After annealing the
composite films, the mean sheet-resistance decreased from 12.3 Ω/sq to 11.5 Ω/sq. On the other hand, in the case of using organic substrates, annealing of Ag NW/PEDOT:PSS to make a junction cannot be applied because of thermal damage to the organic substrates at high temperature. There are many nanowire bonding methods such as welding, soldering, and mechanical bonding [26]. Among the methods, the mechanical bonding such as press or hot-press method has an advantage for large-area fabrication [26–29]. In order to make a smooth surface, the press method without heating needs a high pressure up to GPa [27] or MPa [28] range. In contrast, the hot-press method has an advantage of low pressure to make a smooth surface by embedding Ag NW into PET films [29]. For the mechanical bonding method, the Ag NW does not need to be passivated fully with the PEDOT:PSS films, as shown in Fig. 5. The Ag NW films just coated with PEDOT:PSS of 0.01 wt.% and 4 ml still have a non-uniform surface, as shown in the inserted image of Fig. 5. After the PEDOT:PSS coating on the Ag NW, the films were pressed without heating and hot-pressed at 110 °C for 2 min to 300 kPa, respectively. Fig. 6(a) shows that the pressure was not enough to embed Ag NW into PEN substrate. In addition, electrical junctions between Ag NW were not made completely as shown in red circles of the inset image. However, Fig. 6(b) shows that the patterned Ag NW/PEDOT:PSS films can be embedded into the PEN substrate by the hot pressing method. This process can improve both the electrical conductivity and make a smooth surface. Fig. 7 shows the images of bare PEN films, patterned Ag NW films, fully passivated films with PEDOT:PSS, and hot-pressed Ag NW/ PEDOT:PSS films on a PEN substrate, respectively. The bare PEN films have the high value of a ten point height (Sz) of 270.676 nm due to the presence of natural roughness as shown in the inserted image in
Fig. 7. Scanning probe microscopy images of (a) bare PEN films (b) patterned Ag NW films, (c) fully passivated films with PEDOT:PSS (0.65 wt.%, 2 ml), and (d) hot-pressed Ag NW/ PEDOT:PSS films on a PEN substrate.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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Fig. 6. The roughness average (Sa), root mean square parameter (Sq), and ten point height (Sz) of the hot-pressed films were significantly smaller than the other samples. This smooth surface of the hotpressed films causes a slight increase in transmittance compared to the PEDOT:PSS-coated Ag NW films. Fig. 8 shows the sheet resistance measured at 10 positions of the Ag NW spray-coated films, fully passivated Ag NW films with PEDOT:PSS, PEDOT:PSS coated Ag NW films, and hot-pressed Ag NW/PEDOT:PSS films on a PEN substrate. Ag NW spray-coated films without any treatment showed large variations in sheet resistance, as shown in Fig. 8(a). This suggests that the Ag NW spray-coated films may not be bonded close to each other and just sit on the network structure of the Ag NW as shown in Fig. 4(a). Therefore, the degree of junctions of the Ag NW can be changed easily by an external force, which could be the contact force between the Ag NW and the probe head in this study. The coating of PEDOT:PSS reduces the variations of the sheet resistance by making stable electrical connections among the Ag NW. On the other hand, PEDOT:PSS is not good enough to bind fully each Ag NW as shown in Fig. 8(b) and (c). The embedded composite films are more effective in binding each Ag NW, which leads to an improvement in the sheet resistance compared to other samples, as shown in Fig. 8(d). Fig. 9 shows the transmittance spectra of the bare PEN films and fabricated films with the line and space pattern widths of 25 μm. The mean transmittance of the hot-pressed Ag NW/PEDOT:PSS films was 78.37% in the visible wavelength range between 400 and 800 nm. The mean transmittance of the spray-coated Ag NW films and PEDOT:PSS coated Ag NW films was 77.62% and 77.53% of the average transmittance, respectively. The coating of PEDOT:PSS induced high transmittance above 520 nm compared to the spray-coated Ag NW films. The transmittance of glass substrates with Ag NW/PEDOT:PSS bilayer patterns, which has 25 μm line and space patterns, showed 81.15% transmittance in the visible wavelength range between 400 and 700 nm. The transmittance increased with increasing pitch as shown in the inserted image. The pressed and hot-pressed films with the patterns of 25 μm width and 975 μm pitch and 1 × 5 cm sizes were prepared, respectively and tested bending test on semicircular cylinders and cyclic bending test. Fig. 10(a) shows that the pressed film has a large resistance change by bending on semicircular cylinders with bending radii of 15 mm because the connection of Ag NW was detached by bending. In contrast, the resistance of hot-pressed films was slightly changed as 1.8% under the same condition. Fig. 10(b) shows the result after cyclic bending test of the hot-pressed electrode. The film retains its conductivity even under bending and recovers to close to its initial resistance once the film is released after 1000 cycle test. This result indicates that the embedded Ag NW/PEDOT:PSS into PEN films was effective in sustaining the electrical properties due to their good mechanical reliability.
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Fig. 9. Transmittance of a bare PEN films, spray-coated Ag NW films, Ag NW and PEDOT:PSS (0.01 wt.%, 4 ml) coated films, fully passivated films with PEDOT:PSS (0.65 wt.%, 2 ml), hot-pressed films, with a line and space width of 25 μm, respectively. The inset shows the transmittance of a bare glass substrate, the patterned composite films with line and space width of 25 μm, with width of 25 μm and pitch of 75 μm, with width of 25 μm and pitch of 200 μm, respectively.
offers wide flexibility to use nano-materials in a dispersion solvent and can be applied to prepare electrically conducting patterns on transparent and flexible substrates. In the case of using organic substrates, the
4. Conclusion This paper described a patterning method of Ag nanowire/PEDOT:PSS composite bilayer films using highly hydrophobic fluoropolymer patterns that can avoid solvent orthogonality problems. This patterning method
Fig. 8. Sheet resistance of (a) Ag NW spray-coated films, (b) fully passivated films with PEDOT:PSS (0.65 wt.%, 2 ml), (c) Ag NW and PEDOT:PSS (0.01 wt.%, 4 ml) coated films, and (d) hot-pressed films. Measurement was done at 10 positions.
Fig. 10. The ratio of resistance change of (a) the pressed and hot-pressed electrode on semicircular polyethylene cylinders with different bending radii of curvature of 20 mm and 15 mm and (b) the hot-pressed film as the number of bending cycles. The test performed by placing the film between the two platforms and bending the films by reducing the distance between the platforms. Each sample was tested 10 times and 5 times, respectively.
Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028
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electrical properties of the composite films can be improved by hotpressing at lower temperature than annealing treatment. In addition, the hot-press method can achieve a smooth surface and mechanical stability of the films by embedding Ag nanowire/PEDOT:PSS into organic films at lower pressures than press method. The simplicity of this technique can offer wide flexibility and potential for many applications, such as nano-material-based devices and the lift-off deposition of nanomaterials. Acknowledgement This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Education, Science and Technology (NRF-2013R1A1A2-00-5260) and partly by Inha University through its Basic Research Promotion Program. References [1] D. Langley, G. Giusti, C. Mayousse, C. Celle, D. Bellet, J. Simonato, Flexible transparent conductive materials based on silver nanowire networks: a review, Nanotechnology 24 (2013) 452001. [2] L. Hu, H. Wu, Y. Cui, Metal nanogrids, nanowires, and nanofibers for transparent electrodes, MRS Bull. 36 (2011) 760. [3] D.S. Hecht, R.B. Kaner, Solution-processed transparent electrodes, MRS Bull. 36 (2011) 749. [4] J. Lee, P. Lee, H. Lee, D. Lee, S.S. Lee, S.H. Ko, Very long Ag nanowire synthesis and its application in a highly transparent, conductive and flexible metal electrode touch panel, Nanoscale 4 (2012) 6408. [5] J. Lee, P. Lee, H.B. Lee, S. Hong, I. Lee, J. Yeo, S.S. Lee, T.S. Kim, D. Lee, S.H. Ko, Roomtemperature nanosoldering of a very long metal nanowire network by conductingpolymer-assisted joining for a flexible touch-panel application, Adv. Funct. Mater. 23 (2013) 4171. [6] S.B. Kang, Y.J. Noh, S.I. Na, H.K. Kim, Brush-painted flexible organic solar cells using highly transparent and flexible Ag nanowire network electrodes, Sol. Energy Mater. Sol. C 122 (2014) 152. [7] W. Gaynor, G.F. Burkhard, M.D. McGehee, P. Peumans, Smooth nanowire/polymer composite transparent electrodes, Adv. Mater. 23 (2011) 2905. [8] X.Y. Zeng, Q.K. Zhang, R.M. Yu, C.Z. Lu, A new transparent conductor: silver nanowire film buried at the surface of a transparent polymer, Adv. Mater. 22 (2010) 4484. [9] L. Yang, T. Zhang, H. Zhou, S.C. Price, B.J. Wiley, W. You, Solution-processed flexible polymer solar cells with silver nanowire electrodes, ACS Appl. Mater. Interfaces 3 (2011) 4075. [10] Z. Yu, Q. Zhang, L. Li, Q. Chen, X. Niu, J. Liu, Q. Pei, Highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes, Adv. Mater. 23 (2011) 664.
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Please cite this article as: M.-S. Kim, et al., Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.02.028