Metallic Grid Electrode Fabricated via Flow Coating for High

Mar 19, 2015 - Materials Research Center, Samsung Advanced Institute of Technology (SAIT), Suwon 443-803, Republic of Korea. •S Supporting Informati...
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Metallic Grid Electrode Fabricated via Flow Coating for HighPerformance Flexible Piezoelectric Nanogenerators Jae Hoon Park,†,⊥ Dong Yun Lee,∥,⊥ Wanchul Seung,§ Qijun Sun,† Sang-Woo Kim,†,§ and Jeong Ho Cho*,†,‡ †

SKKU Advanced Institute of Nanotechnology (SAINT), ‡School of Chemical Engineering, and §School of Materials Science and Engineering, Sungkyunkwan University, Suwon440-746, Republic of Korea ∥ Materials Research Center, Samsung Advanced Institute of Technology (SAIT), Suwon 443-803, Republic of Korea S Supporting Information *

ABSTRACT: Transparent conducting electrodes (TCEs) based on metallic grid structures have been extensively explored for use in flexible and transparent electronics according to their excellent conductivity and flexibility. Previous fabrication methods have been limited by the complexity and expense of their processes. Here, we have introduced a simple and cost-effective flow-coating method for preparing flexible and transparent metallic grid electrodes using silver nanoparticles (AgNPs). The process comprises only two steps, including patterning and sintering the horizontal AgNPs lines, followed by patterning and sintering the longitudinal AgNPs lines. The grid width could be easily controlled by varying the concentration of the AgNP solution and the grid spacing could be controlled by varying the distance moved by a translation stage between intermittent stops. The optimized Ag grid electrode exhibited an optical transmittance at 550 nm of 86% and a sheet resistance of 174 Ω/sq. The resulting Ag grid electrodes were successfully used to prepare a flexible piezoelectric nanogenerator. This device showed good performance, including an output voltage of 5 V and an output current density of 0.5 μA/cm2.

1. INTRODUCTION Transparent conducting electrodes (TCEs) are essential components in various organic electronic devices, including organic light-emitting diodes (OLEDs), organic solar cells (OSCs), and touch screens.1−8 The material most widely used for preparing TCEs is indium tin oxide (ITO); however, the use of ITO suffers from many problems, including the scarcity of indium resources, the high temperatures required for processing, and the brittleness of ITO.9,10 The need for costeffective fabrication processes and flexibility in electronic devices motivates efforts toward developing novel flexible TCEs with a low electrical resistance and a high optical transmittance. Several types of flexible transparent electrodes have been widely investigated, including electrodes based on conducting polymers, metal nanowires, carbon nanotubes (CNTs), or graphene.11−17 The trade-off between optical transmittance and electrical conductivity in these materials has presented significant challenges to application development. The performance requirements of flexible and transparent electronics may potentially be satisfied by TCEs based on metallic grid structures that retain a high conductivity while permitting 100% optical transmission through “holes” in the grid. The optical transmittance depends on the spatial area covered by the metal grid, which is a function of the grid spacing and line width. The sheet resistance may be effectively tuned by controlling the thickness of the metal grid.18−20 Metallic grid electrodes permit large reversible deformations © 2015 American Chemical Society

under strains applied along certain axes because the grid lines tend to rotate in a manner that transforms the open squares into rhombi under application of a tensile strain at the ends of the structure.21−23 Previous studies have fabricated metal grid electrodes via photolithography, e-beam lithography, imprint lithography, or transfer printing.24−29 These conventional methods have required expensive vacuum systems and complex multisteps processes that are inappropriate for the commercialization of large-area low-cost application. Flow-coating, which is one of the more efficient patterning methods, has been recognized as a simple, cost-effective, nonlithographic method for fabricating line or grid structures.30−33 The grid dimensions, including the line spacing, width, and thickness, can be finely tuned to optimize the device properties. The mechanism underlying flow-coating techniques depends on establishing convective flow in an evaporating solvent droplet such that nonvolatile solutes (e.g., polymers, nanoparticles, nanorods, or DNA, among others) are carried to the pinned droplet edges via an outward flow of solvent from the interior.30−36 This process is known as the “coffee-ring” effect.37,38 In this paper, we fabricated transparent metallic grid electrodes via flow-coating by directly patterning silver nanoparticles (AgNPs) without using lithographic techniques. Received: January 24, 2015 Revised: March 17, 2015 Published: March 19, 2015 7802

DOI: 10.1021/acs.jpcc.5b00771 J. Phys. Chem. C 2015, 119, 7802−7808

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The Journal of Physical Chemistry C

Figure 1. (a) Photographic image of the home-built flow-coating setup. (b) Schematic illustration of the AgNP line formation process. (c) Schematic diagram of the home-built flow-coating setup. (d) The velocity of the linear translation stage as a function of time.

Fabrication of the Flexible Piezoelectric Nanogenerator. Poly(vinylidenefluoride-co-trifluoroethylene) [P(VDFTrFE)] was purchased from PIEZOTECH. The P(VDFTrFE) copolymer (20 wt %) was dissolved in DMF, and the solution was stirred for more than 24 h. A 5.5 μm thick P(VDFTrFE) layer was spin-coated onto the Ag metallic grid/singlelayer graphene (SLG) electrodes. SLG was synthesized by thermal CVD and was transferred onto Ag metallic grid electrodes using a poly(methyl methacrylate) (PMMA) layer.39−41 The spin-coated P(VDF-TrFE) layer was crystallized by annealing at 140 °C for 3 h. The 100 nm thick Ag top electrodes were thermally evaporated through a shadow mask under 10−7 Torr. Electrical poling was applied to align dipoles and enhance the crystallinity of the P(VDF-TrFE) β phase by applying an electric field of 100 MVm−1 for 30 min. Characterization. The optical transmittance of the Ag metallic grid electrodes was characterized by UV−vis spectrophotometry (Agilent 8453), and the sheet resistance was measured by applying the four-point probe technique using Keithley 2182A and 6221 units. The output voltage and output current density of the piezoelectric NGs based on Ag metallic grid electrodes were measured using a nanovoltmeter (Keithley 2182A) and a picoammeter (Keithley 6475), respectively.

The line width and spacing were systematically controlled by varying, respectively, the concentration of AgNPs in solution and the moving distance between intermittent stops. These factors affected both the optical transmittance and the sheet resistance of the electrode. The optimized TCE exhibited an optical transmittance at 550 nm of 86% and a sheet resistance of 174 Ω/sq. Our solution-processed metallic grid electrodes were successfully used as the electrode in a flexible piezoelectric nanogenerator (NG). The resulting devices exhibited a high output voltage (∼5 V) and high output current densities (∼0.5 μA/cm2) that were much higher than the values obtained from an NG prepared using a pristine graphene electrode. This metallic grid fabrication technique provides a simple and costeffective method for preparing TCEs compared with traditional TCE fabrication processes.

2. EXPERIMENTAL SECTION Fabrication of a Silver Grid Electrode. Silver nanoparticle (AgNP) solutions with various concentrations (1−10 mg/mL) were prepared by dissolving AgNPs (Nanopaste, NPS-J, Harima Chemicals Group, Inc., Japan) in toluene. The particle size was around 10 nm. The solution was stirred for 24 h to form a homogeneous dispersion of AgNPs. A polyethylene naphthalate (PEN) substrate was sequentially cleaned with acetone, 2-propanol, and deionized water, followed by drying with nitrogen gas. The flow-coating setup consisted of an angled polymer blade attached to a vertical translation stage and a linear translation stage attached to a piezo nanopositioner (Physik Instrumente (PI) GmbH & Co., KG), as shown in Figure 1a.35 A 75 μm thick polyethylene terephthalate (PET) blade was scored 1.2 mm from the edge to make a hinge. The PET blade was then fixed rigidly at a 40° angle relative to the vertical translation stage and brought into contact with the substrate. The 6 μL AgNPs solutions with various concentrations (1−10 mg/mL) were injected between the PET blade and the substrate and were trapped by capillary force under the blade. The linear translation stage was moved at a constant speed of 1.5 mm/s, with an intermittent stop time of 1 s and a programed moving distance (50−250 μm). The AgNP line patterns were formed on the PEN substrate and then thermally sintered in a chamber at 150 °C for 1 h under a nitrogen atmosphere. The stage was rotated by 90°, and a second set of AgNP lines was patterned, followed by thermal sintering of the AgNPs.

3. RESULTS AND DISCUSSION The line patterns formed during the flow-coating process via the well-known “coffee-ring effect”. The ring stains were generated by a capillary flow during drying of the solution droplet. The three-phase (liquid−solid−gas) contact line of the solution droplet tended to remain stationary as the volume of the droplet decreased due to solvent evaporation. The volume of droplet was reduced by evaporation from the edge of the droplet and was replenished from the interior so that the edgeward flow carried the nonvolatile elements in the solution to the droplet edge. As the contact angle of the solution droplet fell below a critical angle, the solution was pulled away from the contact line and a new contact line was pinned. The contact line continually switched between a pinned state and a depinned state. The flow-coating method adopted in this work (Figure 1a) achieved control over the contact line at the liquid−solid−gas interface using a polymer blade attached to a vertical translation stage. This approach enabled systematic control over the formation of multiple periodic lines. The AgNP solution was 7803

DOI: 10.1021/acs.jpcc.5b00771 J. Phys. Chem. C 2015, 119, 7802−7808

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Figure 2. Optical microscopy (OM) images of the AgNP line patterns prepared by flow-coating (a) before and (b) after a thermal sintering process. (c) AgNP line widths as a function of the AgNP solution concentration. (d) Resistivity of the sintered single AgNP lines as a function of the AgNP solution concentration.

Figure 3. (a) Schematic illustration of the procedure used to fabricate the Ag metallic grid electrodes using the flow-coating method. (b) Optical microscopy (OM) image of the Ag metallic grid electrodes. The inset shows a photographic image of the Ag metallic grid electrodes. (c) Optical transmittance of the Ag metallic grid electrodes fabricated using various concentrations of AgNP in solution. (d) Sheet resistance and optical transmittance as a function of the grid width.

varying the solution concentration and the moving distance, respectively. The line width of the AgNP pattern was systematically controlled by varying the concentration of the AgNPs in solution (1−10 mg/mL). Figure 2a shows optical microscopy images of the AgNP line patterns, revealing that the line width increased with the AgNP concentration. For example, 1 and 10 mg/mL AgNP concentrations produced line widths of 2.7 and 13.1 μm, respectively. The line width is plotted as a function of the AgNP concentration in Figure 2c. This trend could be understood qualitatively in view of the fact that higher concentrations drove more AgNPs toward the edge of the meniscus, resulting in wider lines. Note that the thickness (or height) of the Ag line remained constant (60 nm) due to the fixed height induced by the strong capillary forces between the scored polymer blade and the substrate (Figure S1, Supporting Information). Higher AgNP concentrations exceeding 7 mg/ mL yielded AgNP clusters in the region between the lines,

injected between the polymer blade and the substrate and was trapped by capillary forces. The substrate attached to the linear translation stage remained stationary during the stopping time; therefore, the nonvolatile AgNPs in the toluene solution migrated to the contact line during toluene evaporation, and a line of AgNPs was deposited, as shown in Figure 1b. The polymer blade was then translated a certain distance, which stretched the meniscus until the contact angle fell below the critical receding angle. The capillary force exceeded the pinning force, and the contact line moved to a new position. As a result, the contact angle was restored to its initial value, leaving behind a new AgNP line. These steps were repeated, as mentioned above, to yield parallel lines of AgNPs (Figure 1c). The distance through which the linear translation stage moved between intermittent stop times (1 s) was varied from 50 to 250 μm, and its moving velocity was 1.5 mm/s (Figure 1d). In this study, the line width and spacing were controlled by 7804

DOI: 10.1021/acs.jpcc.5b00771 J. Phys. Chem. C 2015, 119, 7802−7808

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Figure 4. (a) Schematic illustration of the charge generation process in the flexible piezoelectric NGs based on the Ag grid electrodes before (left) and after (right) application of the strain, respectively. (b) Output voltage and (c) output current density of the piezoelectric NGs based on singlelayer graphene (SLG), horizontal Ag lines/SLG, vertical Ag lines/SLG, and Ag grid/SLG under a strain of 0.18%. The inset shows OM images of the bottom electrodes.

which dramatically decreased the optical transmittance of the line electrodes (Figure S2). Unwanted AgNP clusters may have formed in solutions beyond a critical concentration because the AgNPs could not shift completely to the subsequent position during movement of the linear translation stage. The AgNP line patterns fabricated by flow-coating were sintered thermally at 150 °C for 1 h, as shown in Figure 2b. The line width was found to decrease slightly after thermal sintering (Figure 2c). The as-patterned AgNP lines included organic ligand molecules among the AgNPs. During thermal sintering, these molecules decomposed and gradually decreased the distances among AgNPs. The AgNPs ultimately formed contacts with other AgNPs after the ligand molecules had been completely removed, thereby decreasing the line width. The resistivity of the single Ag lines after thermal sintering was measured, as shown in Figure 2d. The Ag lines fabricated using AgNP solutions below 5 mg/mL were not conductive due to disconnections between the Ag lines. By contrast, further increases in the AgNP concentration yielded highly conductive Ag lines after thermal sintering. The resistivities of the single Ag lines decreased as the concentration of AgNPs in solution increased. Higher concentrations produced wider line widths, which decreased the resistivities of the Ag lines. In addition, the line spacing could be controlled by varying the translation distance (from 50 to 250 μm) between intermittent stop times of the polymer blade. The AgNP concentration and velocity of the polymer blade were fixed at 7 mg/mL and 1.5 mm/s, respectively. The patterned AgNP line spacings were correlated with the programmed moving distances (Figure S3). A transparent flexible Ag grid electrode was fabricated as shown in Figure 3a. The AgNP lines were first patterned onto PEN substrates through flow-coating and then sintered thermally in a chamber to prevent the dissolution of AgNP lines during the second line patterning process. The stage was then rotated by 90°. The second AgNP lines were patterned and sintered. Figure 3b shows OM and photographic images of the large-area Ag grid electrode with a grid width of 8 μm and a grid spacing of 200 μm, produced by flow-coating onto a PEN substrate. The optical transmittance and sheet resistance were controlled by varying the AgNP concentration. The optical

transmittance over the range 300−1000 nm decreased with increasing solution concentration (Figure 3c). The absorption peak around 407 nm was attributed to the localized surface plasmon resonance of the AgNP clusters (30−40 nm) in the region between lines.42,43 This peak increased abruptly above a 7 mg/mL AgNP concentration. The sheet resistance and optical transmittance (at 550 nm) of the Ag grid electrodes prepared using various AgNP concentrations (6, 7, 8, 9, and 10 mg/mL) are summarized, as shown in Figure 3d. Both the optical transmittance and sheet resistance decreased as the grid width increased. For example, 7.5 and 10.6 μm grid widths produced sheet resistances (optical transmittances) of 332 Ω/ sq (88%) and 132 Ω/sq (70%), respectively. Flexible transparent Ag electrodes were successfully fabricated using the flow-coating method and were employed as electrodes in flexible piezoelectric NG (Figure 4a). For use as the bottom electrode in a piezoelectric NG, the process described above was used to optimize the properties of a Ag grid pattern, formed from a 7 mg/mL AgNP solution: the moving velocity and intermittent stop time of the polymer blade were 1.5 mm/s and 1s, respectively. The grid width, spacing and thickness were 8 μm, 200 μm, and 60 nm, respectively. The resulting Ag grid electrodes exhibited a sheet resistance of 174 Ω/sq and an optical transmittance at 550 nm of 86%. P(VDF-TrFE) was used as the piezoelectric active material in this work. The most stable phase of PVDF under ambient temperatures and pressures is the α-phase, which is nonpolar and paraelectric due to the centrosymmetry of the unit cell; however, copolymerization with trifluoroethylene (TrFE) in a specific molar ratio (the VDF content was 50− 80%) yielded the piezoelectric crystalline β-phase. The introduction of a third fluoride moiety into the TrFE monomer increased the steric hindrance and favored the all-trans conformation, which induced the formation of the piezoelectric β-phase. Prior to spin-coating the P(VDF-TrFE) layer, a single-layer graphene (SLG) film grown by CVD was first transferred onto the patterned Ag electrodes to form an interlayer that uniformly and effectively attracted the charges induced by the NG. The P(VDF-TrFE) was then spin-coated onto the Ag patterns/SLG 7805

DOI: 10.1021/acs.jpcc.5b00771 J. Phys. Chem. C 2015, 119, 7802−7808

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Figure 5. (a) Output voltage and output current density during the application of a bending and unbending cycle. The inset shows photographic images of the piezoelectric NGs under various strains. (b) Output voltage and output current density as a function of the applied strain. (c) Stability of the flexible piezoelectric NG during bending (strain = 0.18%) and unbending. The inset shows the magnified output voltage signals. (d) Photographic image of an LED that was turned on by the piezoelectric NG based on the Ag grid electrode. The inset image shows the LED in the turned-off state.

electrodes, and the assembly was thermally annealed at 140 °C for 3 h to improve the orientations of the crystalline βphase.44−46 The surface morphology and cross-section of the P(VDF-TrFE) layer are shown in Figure S4. After the top Ag electrode had been deposited by thermal evaporation, an electrical poling process was applied to develop highly aligned molecular dipoles in the P(VDF-TrFE) layer.47,48 Parts b and c of Figure 4 show the typical output voltages and current densities of a series of flexible piezoelectric NGs fabricated from four different bottom electrodes and submitted to a strain of 0.18%. The SLG-only results (black) are included for comparison with the horizontal Ag lines/SLG (red), the vertical Ag lines/SLG (green), and Ag grid/SLG (blue). The NG prepared with the SLG bottom electrode yielded the lowest output voltage of 1.2 V because the SLG sheet resistance (964 Ω/sq) was higher than the values obtained from the other electrodes. Higher output voltages (2 V) were observed in piezoelectric NGs based on both horizontal Ag lines/SLG and vertical Ag lines/SLG bottom electrodes. Importantly, the NG prepared with the Ag grid/SLG showed a much higher output voltage exceeding 4.8 V, which could be explained as follows. First, the grid/SLG patterns yielded a much lower sheet resistance than the line-patterned electrodes. Second, the contact area between the Ag grid pattern and the P(VDFTrFE) layer was twice the contact area of the line-patterned electrodes and was more efficient at attracting the charges produced by the strain-induced piezoelectric potential. Similarly, the current densities of the NGs prepared with different bottom electrodes followed the same trend as the output voltage (Figure 4c). The mechanism by which the piezoelectric NGs generated power could be described as follows. In the fabricated piezoelectric NG, the aligned negative dipoles (V−) repelled electrons and trapped holes at the bottom Ag patterns/SLG

electrode, whereas the aligned positive dipoles (V+) repelled holes and trapped electrons at the top electrode (lower panel of Figure 4a). As a compressive strain was applied to the NG, the enhanced piezoelectric potential drove the electrons from the bottom Ag patterns/SLG electrode to the top Ag electrode through an external circuit, where they were immediately neutralized by the holes present at the top electrode. A positive output voltage and output current density were generated. The excellent insulating properties of the P(VDF-TrFE) resulted in the accumulation of electrons (holes) at the interface between the top (bottom) electrode and the P(VDF-TrFE). As the strain was released, the piezoelectric potential immediately vanished so that the accumulated electrons flowed back through the external circuit and were neutralized by the holes accumulated at the bottom electrode. This process gave rise to a negative output voltage and output current density. Accordingly, the repeated compression and release procedures produced a periodic alternating output voltage and output current density. The output signals of the flexible piezoelectric NGs prepared using the Ag grid/SLG bottom electrode were investigated as a function of the applied strain. Strains between 0.10% and 0.18% were applied to the NGs, as shown in the inset of Figure 5a. The applied strain (εy) was caculated from from εy = h/2R,47 where h is the thickness of the PEN substrate and R is the radius of curvature (Figure S5). Figure 5a shows the output pulse signals under different strains: 0.10% (black), 0.12% (red), 0.14% (green), 0.16% (blue), and 0.18% (gray). The output voltages gradually increased from 2.7 to 4.8 V as the strain increased, while the current density increased from 0.27 to 0.51 μA/cm2. The statistical output signals under different strains are illustrated in Figure 5b. A larger strain induced a higher piezoelectric potential in the NG, which increased the output pulse signals. The durability of the NG was tested over 7806

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3000 cycles (Figure 5c). Highly stable output voltages with alternating positive and negative biases were measured in the durability tests. Finally, the fabricated NG was successfully shown to be capable of driving a commercial LED, as shown in Figure 5d.

4. CONCLUSIONS In summary, we demonstrated the fabrication of large-area, flexible, transparent metal grid electrodes via a two-step flowcoating method using Ag NPs. The flow-coating method was simple, was low-cost, and did not require lithographic techniques for fabricating a metallic grid structure. The metal grid line width and spacing were precisely controlled by varying the AgNP concentration and programmed moving distance, respectively. The resulting Ag grid electrodes exhibited an optical transmittance at 550 nm of 86% and a sheet resistance of 174 Ω/sq. The patterned metallic grid structure was successfully used in a flexible piezoelectric NG. The flowcoating method described here complements current standard TCE fabrication processes and may play an important role in preparing large-area transparent electrodes for use in future electronic devices.

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ASSOCIATED CONTENT

* Supporting Information S

AFM and OM images of the Ag line pattern, optical transmittance of the Ag metallic line electrodes, SEM images of P(VDF-TrFE) films, and schematic geometry of the bending test. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.H.C.) E-mail: [email protected]. Author Contributions ⊥

These authors contributed equally this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (2013M3A6A5073177) and Basic Science Research Program (2013R1A1A2011897 and 2009-0083540) of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Korea.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b00771 J. Phys. Chem. C 2015, 119, 7802−7808

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DOI: 10.1021/acs.jpcc.5b00771 J. Phys. Chem. C 2015, 119, 7802−7808