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Flexible Polymer/Metal/Polymer and Polymer/Metal/Inorganic Trilayer Transparent Conducting Thin Film Heaters with Highly Hydrophobic Surface Tae-Woon Kang,†,∥ Sung Hyun Kim,†,‡,∥ Cheol Hwan Kim,† Sang-Mok Lee,§ Han-Ki Kim,§ Jae Seong Park,† Jae Heung Lee,† Yong Suk Yang,‡ and Sang-Jin Lee*,† †

Chemical Materials Solutions Center, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea Department of Nano Fusion Technology, Pusan National University, Busan 46241, Republic of Korea § Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, 1 Seocheon-dong, Yongin, Gyeonggi-do 17104, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Polymer/metal/polymer and polymer/metal/inorganic trilayer-structured transparent electrodes with fluorocarbon plasma polymer thin film heaters have been proposed. The polymer/metal/polymer and polymer/metal/inorganic transparent conducting thin films fabricated on a large-area flexible polymer substrate using a continuous roll-to-roll sputtering process show excellent electrical properties and visible-light transmittance. They also exhibit water-repelling surfaces to prevent wetting and to remove contamination. In addition, the adoption of a fluorocarbon/metal/ fluorocarbon film permits an outer bending radius as small as 3 mm. These films have a sheet resistance of less than 5 Ω sq−1, sufficient to drive light-emitting diode circuits. The thin film heater with the fluorocarbon/Ag/SiNx structure exhibits excellent heating characteristics, with a temperature reaching 180 °C under the driving voltage of 13 V. Therefore, the proposed polymer/metal/polymer and polymer/metal/inorganic transparent conducting electrodes using polymer thin films can be applied in flexible and rollable displays as well as automobile window heaters and other devices. KEYWORDS: polymer/metal/polymer electrode, Flexible, polymer composite target, thin film heater, roll-to-roll sputtering

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INTRODUCTION

used because it is low in cost, but it has lower transparency than Ag.23−28 OMO transparent electrodes have been reported to have sheet resistances below 10 Ω sq−1 and high transmittances exceeding 80%. In addition, they show good mechanical flexibility compared to conventional transparent conducting oxides, and they are suitable for use in electronic devices. However, the outer bending radii of reported films remain greater than 5 mm, insufficient for use in bendable or rollable electronic devices. In this study, novel flexible transparent electrodes with waterrepellent surfaces, consisting of a polymer/metal/polymer (PMP) and polymer/metal/inorganic (PMI) trilayer structure, are proposed. The electrodes utilize bottom and top layers of plasma polymer nanoscale thin films via continuous roll-to-roll sputtering. With the plasma polymer fluorocarbon thin film, the PMP and PMI transparent electrodes exhibit excellent low sheet resistances and high transmittances, in addition to extreme flexibility, water repulsion, and antireflection property. Furthermore, devices of light-emitting diode (LED) electronic

Many advanced devices, including displays, perovskite and organic solar cells, smart windows, internet-of-things (IoT) sensors, and automotive surface heaters, require flexible, transparent, and electrically conductive electrode materials. In−Sn oxide (ITO), a typical transparent electrode, has been used for large-area flat-panel displays and touch-screen panels, as the most widely used transparent electrode material in recent decades.1,2 However, because of the high production cost of In, lack of mechanical flexibility from the nature of the oxide ceramic, and insufficient visible-light transmittance at low resistances, many alternatives have been extensively studied. Different researchers have proposed Ag nanowires (AgNWs), poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), graphene, carbon nanotubes (CNTs), metal grids, and oxide/metal/oxide (OMO) trilayers as substitutes for ITO.2−11 Of these, OMO trilayers have attracted attention because they exhibit high conductivity, visible-light transmittance, chemical stability, and flexibility, as well as a relatively easy manufacturing process.11−15 ITO, ZnO, Ga-doped ZnO (GZO), Al-doped ZnO (AZO), InZnO, and ZnS are mainly used as oxide layers.11−22 Ag is the most widely applied as the metal layer because it has excellent antireflection properties; Cu is also © 2017 American Chemical Society

Received: July 9, 2017 Accepted: September 5, 2017 Published: September 5, 2017 33129

DOI: 10.1021/acsami.7b09837 ACS Appl. Mater. Interfaces 2017, 9, 33129−33136

Research Article

ACS Applied Materials & Interfaces circuits and transparent film heaters were created using the PMP and PMI hydrophobic transparent electrodes. Both types of proposed transparent electrode successfully drove the LED electronic circuits, and the transparent film heaters showed excellent performance with rapid temperature increases to 180 °C. The demonstrated water-repellent and flexible transparent electrodes can be used in a wide variety of applications, such as deicing and defrosting for automobiles, outdoor solar-cell devices, and greenhouses.

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EXPERIMENTAL SECTION

Large-Area PMP and PMI Thin Film Fabrication Using Continuous Roll-to-Roll Sputtering. The roll-to-roll sputtering system (ULVAC, SPW-060) consisted of three modules: the unwinder, winder, and main sputtering modules. The main sputtering module contains four separate compartments with three midrange-frequency (MF) dual cathodes and one direct-current (dc) cathode for depositing multilayer films. In this experiment, we used the three MF cathodes for the deposition of the plasma polymer fluorocarbon (FC) thin film using a CNT/PTFE composite target (composite target, see Figure S1 in the Supporting Information), ITO thin film with an ITO target (In2O3/ SnO2 = 92.5:7.5 w/w, Mitsui Mining & Smelting Corporation), and SiNx thin film with a Si target. For more information on sputtered FC thin films fabricated by using a composite target, refer to our previous study.29 The Ag thin film for the conducting layer was deposited using the dc cathode with a Ag target. Before deposition, the PET substrate film was pretreated using a heater and Ar/O2 ion plasma to remove surface contamination and improve the adhesion between the bottomlayer FC, ITO, and SiNx thin films and the PET substrate in the unwinder chamber. Characterizations of Sputtered PMP and PMI Multilayer Thin Films. The sheet resistances of the PMP and PMI trilayer thin films were measured at room temperature using a standard four-point probe technique (MCP-T610, Mitsubishi Chemical Analytech). The microstructures of the PMI multilayer thin films were examined using fieldemission transmission electron microscopy (FE-TEM, TECNAI G2 F30 S-TWIN, FEI Co.), and the elements in each layer were analyzed by time-of-flight secondary ion mass spectroscopy (TOF-SIMS 5, IONTOF, Münster). The optical transmittances were measured in the wavelength range 300−2400 nm using an optical spectrometer (U-4100, Hitachi). The water contact angle of the top plasma polymer FC film was measured with a contact angle analyzer (PHEONIX 300 touch, Surface Electro Optics). The sheet resistance of trilayers was measured using a standard four-point probe technique (Loresta-GP, Mitsubishi Chemical). The mechanical properties of the PMP and PMI multilayers were evaluated using a specially designed inner/outer bending system. In addition, dynamic fatigue bending tests were performed using a labdesigned cyclic bending machine, operated at a frequency of 0.5 Hz for 10 000 cycles at the fixed outer bending radius of 10 mm. The resistances of the PMP and PMI trilayers were measured during cyclic bending.

Figure 1. (a) Schematic of fabrication of composite targets. (b) Illustrations of the roll-to-roll sputtering process to deposit FC/Ag/FC trilayers on PET substrate. (c) Structures of PMP and PMI trilayers prepared on PET film substrate using a large-area roll-to-roll sputtering system.

Figure 1b shows the sequential process for fabricating the PMP multilayer thin film, in which the bottom FC layer is deposited at an MF power of 2 kW in the forward direction. With a change in direction, the Ag interlayer is deposited using a direct-current (dc) cathode, and then, the continuous top FC layer is finally deposited in the forward direction. For the fabrication of the PMI multilayer, the bottom layer of ITO or SiNx is deposited using the MF cathode in the forward direction, and the metal and polymer are fabricated in the same manner as those in the PMP structure. Consequently, large-area PMP and PMI transparent conducting electrode films are successfully fabricated by continuous roll-toroll sputtering, as shown in Figure 1c. Sheet resistances and optical transmittances at a wavelength of 550 nm of single layers are measured. The sheet resistances of single-layer 60 nm FC, 40 nm ITO, and 40 nm SiNx thin films deposited on PET film substrate are out of range, 247 Ω sq−1, and out of range, respectively. Additionally, optical transmittances at 550 nm wavelength are 91.05%, 81.55%, and 82.12%, respectively. Figure 2a shows the optical transmittance properties of the PMP multilayer film in the wavelength range 300−2400 nm as a function of Ag thickness, ranging from 6 to 12 nm. In general, sputtered FC thin films show high optical transparency because they have amorphous structures and relatively low optical constants.30 By ellipsometry analysis, the optical constant of the sputtered FC thin film is n ∼ 1.38 when an FC composite target (CNT/PTFE 5:95 w/w) is used. On the basis of previous experiments (Figure S2), the thickness of the bottom and top FC thin films was symmetrically set to 60 nm. The PMP film with a 6 nm thick Ag interlayer shows a different trend with low optical transmittance, which is attributed to the Ag layer formation in islands, instead of a uniform thin film.31 However, as the Ag interlayer thickness is increased from 8 to 10 nm, the PMP film shows an increase in optical transmittance because of the antireflection effect, caused by internal interference within the multilayer structure. The large-area PMP films fabricated by continuous roll-to-roll sputtering have good optical trans-

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RESULTS AND DISCUSSION Figure 1a shows the schematic procedure of the composite target fabrication for the deposition of the plasma polymer FC thin film. To impart electrical conductivity to the polymer target for MF sputtering, we mixed 5 wt % CNT powder (HANOS CM-280, Hanwha Chemical) into PTFE powder (A7-J, DuPont Mitsui). The conductive composite target (5:95 w/w CNT/PTFE) was shaped into a 960 × 150 × 6 mm3 rectangular strip using hightemperature compression molding for continuous roll-to-roll sputtering. Large-area PMP and PMI trilayer thin films were fabricated on a 100 μm thick, 520 mm wide polyethylene terephthalate (PET) film (V7610, SKC) using a roll-to-roll sputtering system at room temperature. In this experiment, an FC thin film was used as the polymer, Ag as metal, and ITO and SiNx thin films as inorganic layers. 33130


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ACS Applied Materials & Interfaces

Figure 2. Optical transmittances and sheet resistances of PMP and PMI transparent conducting electrodes: (a) optical transmittance and (b) sheet resistance of the FC/Ag/FC trilayer with increasing Ag thickness. Optical transmittance of (c) FC/Ag/ITO and (d) FC/Ag/SiNx trilayers as a function of bottom layer thickness. (e) Photographs of PMP and PMI films placed on (upper row) a backlight module and (lower row) white paper. (f) Photograph of a bent FC/Ag/SiNx trilayer.

property by the fluorocarbon thin film on the top layer could be implemented as it is, and a flexible hydrophobic transparent electrode could be maintained. Figure 2c,d shows the transmittance spectra of the PMI films in which ITO and SiNx thin films are adopted as the bottom inorganic layers, respectively, to increase the optical transmittance in the visible range. Normally, materials with higher refractive indices provide better antireflectance in OMO multilayered structures.21 When SiNx is used for the bottom layer, the transmittance reaches 78% with a sheet resistance of 5 Ω sq−1 (resistivity of 5.5 × 10−5 Ω cm). The photographs included in Figure 2e realistically show the degrees of transparency of the PMP and PMI transparent conducting films when placed on backlight modules and pieces of white paper. Both PMP and PMI films exhibit excellent transparency under the conditions of transmission and reflection. Figure 2f shows a photograph of a flexible and transparent PMI (I = SiNx) film. The PMP and PMI trilayers tend to show a significant decrease in transmittance in the near-infrared (IR) region, and a transmittance finally reached less than 10% over 2000 nm wavelength. Because of these IR blocking properties, the PMP and PMI trilayer electrodes are expected to exhibit a lowemissivity performance that conserves energy by blocking IR in the sunlight.

mittances in the visible region. However, deterioration of optical properties is observed when the thickness of the Ag interlayer exceeds 12 nm because the characteristics of the Ag metal increase in prominence as the Ag layer thickness increases, thus reducing the transmittance of the PMP film.32 As a result, the FC 60 nm/Ag 10 nm/FC 60 nm structure possesses the maximum optical transmittance of 68% in the visible range. Figure 2b shows the sheet resistance values of the Ag-inserted PMP trilayers as a function of Ag thickness. For the film of 6 nm Ag thickness, the sheet resistance is not measured because a conductive layer does not form. For 10 and 12 nm thicknesses of the Ag layer, the PMP multilayers show extremely low sheet resistances of less than 5 Ω sq−1. Consequently, we prepared a PMP multilayer film with a sheet resistance of 4.5 Ω sq−1 (resistivity of 5.9 × 10−5 Ω cm) and a transparency of 68%. The PMP trilayer exhibits excellent sheet resistance as an electrode, but shows a low visible-light transmittance which is insufficient for use as a transparent electrode. Therefore, for an increase in the transmittance in the visible-light region through the internal interference effect, the PMI structure is suggested by changing the bottom polymer thin film to an inorganic thin film such as ITO (n ∼ 2.0) and SiNx (n ∼ 2.2) having a high refractive index. In this case, as the bottom layer is changed in the trilayer structure, the water-repellent 33131


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Figure 3. (a) Water contact angles of FC/Ag/ITO trilayer thin films as a function of FC thickness. (b) Sheet resistance and optical transmittance of optimized FC/Ag/FC, FC/Ag/ITO, and FC/Ag/SiNx trilayer thin films. (c) SIMS depth profile of FC 60 nm/Ag 10 nm/ITO 40 nm trilayer film. (d) Cross-sectional TEM image of FC 60 nm/Ag 10 nm/ITO 40 nm trilayer film.

Figure 4. (a) Mechanical bending test results of FC/Ag/FC, FC/Ag/ITO, and FC/Ag/SiNx trilayer thin films. Cyclic fatigue test results of (b) FC/Ag/ FC, (c) FC/Ag/ITO, and (d) FC/Ag/SiNx trilayer thin films.

Along with a touch screen or solar cell, the film can be used as a low-resistance transparent electrode that resists contamination. Figure 3b compares the sheet resistance and the transmittance graphs of the thickness-optimized PMP and PMI trilayer films. The optimized PMP trilayer film has extremely low sheet resistance, while the PMI film has better optical characteristics

Figure 3a shows the measured water contact angle as a function of the top FC layer thickness. The surface of the uncoated Ag/ITO film is hydrophilic, with a water contact angle of 64°, whereas the coating of FC is highly hydrophobic, with a water contact angle exceeding 100°. Therefore, when the FC thin film is used as the top layer, outside contamination is prevented. 33132


Research Article

ACS Applied Materials & Interfaces than the PMP film. Figure 3c depicts the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth profile, showing the changes in the mass intensity of CF−, Ag−, and InO− anions in the course of sputtering of the FC/Ag/ITO trilayer film. The species are detected in the order of CF−, Ag−, and InO− over the sputtering time, confirming that the multilayer FC/Ag/ITO structure is deposited correctly. Figure 3d is a cross-sectional TEM image of the FC/Ag/ITO trilayer film. Pt was coated with a protective layer to prevent damage during focused ion beam milling. The TEM image depicts the three layers of FC, Ag, and ITO in agreement with the SIMS results, verifying that each layer is uniformly deposited with thicknesses of approximately 60, 10, and 40 nm for the FC, Ag, and ITO layers, respectively. The thick dark layer on top of the FC is Pt, deposited via focused ion beam to process the film. In the conventional OMO structure, when the FC plasma polymer is used instead of an oxide layer, the PMP and PMI trilayer conducting electrodes have not only hydrophobicity but also improved mechanical flexibility relative to the single-layer oxide-based transparent electrode thin film. We investigated the effect of bending on the electrical resistance of the PMP and PMI transparent conducting films with the layer structures of FC/Ag/ FC, FC/Ag/ITO, and FC/Ag/SiNx, as shown in Figure 4. Figure 4a shows the measured cracking characteristics of the PMP and PMI trilayer transparent conductive films after outer bending tests. The FC/Ag/FC trilayer films show the best bending properties, with cracks generating at the bending radius of 3 mm. The PMI films, using either SiNx or ITO as the inorganic thin film bottom layers, crack at the radius of 5 mm. It was reported that OMO films with oxide layers withstood bending to a radius of 6 mm.22,23,28 However, trends in technological development require flexibility in transparent electrodes to bending radii of 5 mm or smaller to realize bendable and rollable devices. Thus, both the PMP and PMI thin films are suitable for next-generation flexible electronic devices. The mechanical robustness and flexibility of the films were investigated by repeatedly bending them 10 000 times to a fixed 10 mm bending radius.33−36 After 10 000 bending cycles, neither the FC/Ag/FC nor the FC/Ag/ITO films display visible changes in morphology (Figure S3) or resistance, as shown in Figure 4b− d. However, for the FC/Ag/SiNx trilayer, a slight resistance change is observed during the fatigue test. This is due to the slight cracking of the SiNx film during bending, although the fracture does not progress because of the flexible Ag and polymer films. Therefore, the mechanical properties of the FC/Ag/SiNx trilayer thin film are expected to stabilize after the initial cracking. This outstanding performance may be attributed to the robustness and flexibility of the PMP and PMI films. To test the electrical properties of the flexible transparent conducting electrodes containing water-repellent polymer thin films, we performed the LED on/off test after constructing a simple circuit, as shown in Figure 5. Figure 5a shows the KRICT logo on the LED module when the PMP transparent electrode film is placed on the plane, and the electrodes are connected to the circuit. We also performed tests to evaluate the electrical properties of the films in the bending state. Figure 5b shows that the LED circuit works well even when the film is bent. It is expected that the PMP and PMI trilayer films can be applied as high-performance transparent electrodes with excellent flexibility, high water repellency, and good conductivity. To demonstrate another application of these electrodes as hydrophobic transparent heaters, we fabricated PMP and PMI film heaters by continuous roll-to-roll sputtering on PET

Figure 5. Photographs of LED circuit-driving test of water-repellent transparent PMI electrode. LED on/off tests (a) with water dropped on the electrode surface, and (b) in a bent state.

substrates. In order to measure the electrothermal characteristics, we cut the film heaters to 100 × 100 mm2 in size, and an electrical potential was applied across the two electrodes through a Cu tape contact from a dc power supply (OPS-1005, ODA). The surface temperature was monitored using an infrared thermal imager (C2, FLIR), as illustrated in Figure S4. To the best of our knowledge, this is the first report of hydrophobic transparent thin film heaters. Figure 6a shows photographs of the fabricated PMPand PMI-based film heaters and the corresponding infrared images, which show uniform heat distributions over the films at their steady-state temperatures. When the input voltage is increased to 10 V, all film heaters reach temperatures above 100 °C, confirming operability at low voltages. The PMP film heater reaches a temperature of 144 °C under the applied voltage of 13 V. Notably, the FC/Ag/SiNx film heater reaches 180 °C at only 13 V. We could have achieved higher temperatures by using a polymer substrate other than a PET film that could withstand high temperatures. These PMP and PMI film heaters would be suitable for quickly removing dew and frost from outdoor windows. The temperature distributions of the PMP and PMI films under the applied voltages are homogeneous. This uniformity is attributed to the excellent thermal and electrical conductivities of the materials, as well as the uniform surfaces of the PMP and PMI trilayers. Figure 6b shows the heat and uniformity measurement of the FC/Ag/ITO trilayer film in the bent state. Under the applied voltage of 11 V, the temperature increases quickly, as in the planar film, and a uniform temperature distribution is exhibited even in the film’s bent state. Figure 6c displays the experimentally measured electrothermal performance as a function of applied voltage. The input voltage is modulated from 0 to 14 V, causing a significant increase in the slope of the temperature. Figure S5 shows the generated current characteristics as a function of the voltage applied to the film heater. When a voltage of 13−14 V is applied, a current of 2.5− 2.6 A is generated, and 32.5−36.4 W of power is consumed. Both PMP and PMI film heaters show similar electrothermal properties, but the PMI heaters exhibit higher reaction rates and temperatures. The power dissipated in a resistive conductor can be described by Joule’s first equation: 33133


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Figure 6. Evaluation of heat from FC/Ag/FC, FC/Ag/ITO, and FC/Ag/SiNx multilayer thin films. (a) Surface thermal images of FC/Ag/FC, FC/Ag/ ITO, and FC/Ag/SiNx films as a function of increasing applied voltage. (b) Top, thermal image of the bent-state FC/Ag/ITO film heater under 11 V; bottom, photograph of the bent FC/Ag/ITO film heater mounted on a heater module. (c) Graph of temperature characteristics of PMP and PMI film heaters as a function of applied voltage. (d) Graph of temperature characteristics of PMP and PMI film heaters over time under a voltage of 10 V.

P = V 2/R

potential application of a highly hydrophobic transparent PMP or PMI heater is a vehicle window defroster, since moisture and frost must be removed quickly from vehicle windows to avoid affecting the driver’s visibility.

(1)

where V is the applied voltage and R the total resistance. It is apparent from eq 1 that the temperature T increases with the applied voltage V and decreases with the film heater’s sheet resistance R for a fixed sample geometry. The PMP and PMI film heaters exhibit similar surface resistance characteristics; however, the FC/Ag/SiNx film displays higher-temperature characteristics than either the FC/Ag/FC or FC/Ag/ITO films. This is attributed to the bottom SiNx layer acting as a thermal barrier, containing the heat generated in the Ag thin film layer. That is, the film heater has a “closed” configuration to reduce heat loss and thus improve thermal efficiency.31,32 Figure 6d shows the development of the sample temperature over time as the PMP and PMI films on PET substrates are heated. The power source is directly connected to the electrode plates and maintained at an output voltage of 10 V. Regardless of the bottom layer material, the PMP and PMI heaters show fast heating responses to reach steady-state temperatures in 60 s. Under the application of the same input voltage (10 V), the maximum steady-state temperature reaches 110 °C, indicating that the PMP and PMI multilayers with resistances below 5 Ω sq−1 are very suitable as flexible film heaters. It is possible to combine hydrophobicity, flexibility, transparency, and high-heating performance at low voltages, typically below 12 V, which is of interest for many applications. One

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CONCLUSION We proposed novel hydrophobic transparent heaters with PMP and PMI trilayer structures prepared using plasma polymer FC thin films and suitable for continuous fabrication by large-area roll-to-roll sputtering. The PMP electrode with Ag as a middle interlayer and FC on both sides showed a sheet resistance of 4.5 Ω sq−1 and a transmittance of 68% in the visible range for the optimized structure. The outer bending test showed remarkable mechanical flexibility characteristics with a minimum bending radius of 3 mm and excellent cyclic fatigue resistance. With the use of a highrefractive-index inorganic film as the bottom layer in the trilayer structure, the visible-light transmittance reached 78% while maintaining a low sheet resistance. Successful demonstrations of an LED circuit and a transparent film heater were given, using the PMP and PMI trilayer thin films. The LED circuit confirmed that the on/off characteristics of the transparent electrode are excellent in both the planar and the bent states. Both PMP and PMI film heaters exhibited excellent behavior, reaching high temperatures exceeding 100 °C within approximately 60 s and showing uniform temperature distribu33134


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(6) Sui, D.; Huang, Y.; Huang, L.; Liang, J.; Ma, Y.; Chen, Y. Flexible and Transparent Electrothermal Film Heaters Based on Graphene Materials. Small 2011, 7, 3186−1392. (7) Kim, D.; Lee, H.-C.; Woo, J. Y.; Han, C.-S. Thermal Behavior of Transparent Film Heaters Made of Single-Walled Carbon Nanotubes. J. Phys. Chem. C 2010, 114, 5817−5821. (8) Kim, D.; Zhu, L.; Jeong, D.-J.; Chun, K.; Bang, Y.-Y; Kim, S.-R.; Kim, J.-H.; Oh, S.-K. Transparent Flexible Heater Based on Hybrid of Carbon Nanotubes and Silver Nanowires. Carbon 2013, 63, 530−536. (9) Li, J.; Liang, J.; Jian, X.; Hu, W.; Li, J.; Pei, Q. Thermally Tunable, Self-Healing Composites for Soft Robotic Applications. Macromol. Mater. Eng. 2014, 299, 1403−1409. (10) Celle, C.; Mayousse, C.; Moreau, E.; Basti, H.; Carella, A.; Simonato, J.-P. Highly Flexible Transparent Film Heaters Based on Random Networks of Silver Nanowires. Nano Res. 2012, 5, 427−433. (11) Guillén, C.; Herrero, J. TCO/metal/TCO Structures for Energy and Flexible Electronics. Thin Solid Films 2011, 520, 1−17. (12) Park, S.-H.; Lee, S.-J.; Lee, J. H.; Kal, J.; Han, J.; Kim, H.-K. Highly Efficient Flexible Quantum-Dot Light Emitting Diodes with an ITO/ Ag/ITO Cathode. Org. Electron. 2016, 30, 112−121. (13) Jeong, J.-A.; Kim, H.-K. Low Resistance and Highly Transparent ITO−Ag−ITO Multilayer Electrode Using Surface Plasmon Resonance of Ag Layer for Bulk-Heterojunction Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2009, 93, 1801−1809. (14) Klöppel, A.; Kriegseis, W.; Meyer, B. K.; Scharmann, A.; Daube, C.; Stollenwerk, J.; Trube, J. Dependence of the Electrical and Optical Behaviour of ITO-Silver-ITO Multilayers on the Silver Properties. Thin Solid Films 2000, 365, 139−146. (15) Bender, M.; Seelig, W.; Daube, C.; Frankenberger, H.; Ocker, B.; Stollenwerk, J. Dependence of Film Composition and Thicknesses on Optical and Electrical Properties of ITO−Metal−ITO Multilayers. Thin Solid Films 1998, 326, 67−71. (16) Park, H.-K.; Kang, J.-W.; Na, S.-I; Kim, D.-Y.; Kim, H.-K. Characteristics of Indium-free GZO/Ag/GZO and AZO/Ag/AZO Multilayer Electrode Grown by Dual Target DC Sputtering at Room Temperature for Low-cost Organic Photovoltaics. Sol. Energy Mater. Sol. Cells 2009, 93, 1994−2002. (17) Park, Y.-S.; Kim, H.-K. Flexible Indium Zinc Oxide/Ag/Indium Zinc Oxide Multilayer Electrode Grown on Polyethersulfone Substrate by Cost-efficient Roll-to-Roll Sputtering for Flexible Organic Photovoltaics. J. Vac. Sci. Technol., A 2010, 28, 41−47. (18) Dimopoulos, T.; Radnoczi, G. Z.; Pécz, B.; Brückl, H. Characterization of ZnO:Al/Au/ZnO:Al Trilayers for High Performance Transparent Conducting Electrodes. Thin Solid Films 2010, 519, 1470−1474. (19) Jeong, J.-A.; Park, Y.-S.; Kim, H.-K. Comparison of Electrical, Optical, Structural, and Interface Properties of IZO-Ag-IZO and IZOAu-IZO Multilayer Electrodes for Organic Photovoltaics. J. Appl. Phys. 2010, 107, 023111. (20) Sahu, D. R.; Lin, S.-Y.; Huang, J.-L. ZnO/Ag/ZnO Multilayer Films for the Application of a Very Low Resistance Transparent Electrode. Appl. Surf. Sci. 2006, 252, 7509−7514. (21) Leftheriotis, G.; Yianoulis, P.; Patrikios, D. Deposition and Optical Properties of Optimised ZnS/Ag/ZnS Thin Films for Energy Saving Applications. Thin Solid Films 1997, 306, 92−99. (22) Choa, S.-H.; Cho, C.-K.; Hwang, W.-J.; Eun, K. T.; Kim, H.-K. Mechanical Integrity of Flexible InZnO/Ag/InZnO Multilayer Electrodes Grown by Continuous Roll-to-Roll Sputtering. Sol. Energy Mater. Sol. Cells 2011, 95, 3442−3449. (23) Ko, E.-H.; Kim, H.-J.; Lee, S.-J.; Lee, J. H.; Kim, H.-K. Nano-sized Ag Inserted into ITO Films Prepared by Continuous Roll-to-Roll Sputtering for High Performance, Flexible, Transparent Film Heaters. RSC Adv. 2016, 6, 46634−46642. (24) Song, S.; Yang, T.; Lv, M.; Li, Y.; Xin, Y.; Jiang, L.; Wu, Z.; Han, S. Effect of Cu Layer Thickness on the Structural, Optical and Electrical Properties of AZO/Cu/AZO Tri-layer Films. Vacuum 2010, 85, 39−44. (25) Park, Y.-S.; Park, H.-K.; Jeong, J.-A.; Kim, H.-K.; Choi, K.-H.; Na, S.-I.; Kim, D.-Y. Comparative Investigation of Transparent ITO/Ag/ ITO and ITO/Cu/ITO Electrodes Grown by Dual-Target DC

tions. For PMI structures using SiNx as the bottom layer, the temperature reached 180 °C at a low voltage of 13 V because of the thermal barrier effect. Considering their superior hydrophobicity, chemical stability, flexibility, transparency, and large-area scalability, the demonstrated PMP and PMI trilayer conducting electrode films could be applied in bendable and rollable displays; flexible touch-screen panels; heaters for the front, back, and side windows of automobiles; and other applications requiring transparent heating films for either defrosting or temperature-maintenance purposes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09837. Additional figures of the fabrication process, optical transmittance, SEM results, the heating characteristic measurement device, and current characteristics as a function of applied voltage (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sang-Jin Lee: 0000-0001-7554-1037 Author Contributions ∥

T.-W.K. and S.H.K. contributed equally to this work. S.-J.L. designed the study and the experiments. T.-W.K., S.H.K., and C.H.K. fabricated the fluorocarbon thin films using a test sputter system and a pilot-scale roll-to-roll sputter. S.H.K., C.H.K., S.M.L., H.-K.K., J.S.P., and Y.S.Y. analyzed the properties of the fluorocarbon thin films. J.H.L. and S.-J.L. wrote the manuscript. All of the authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Project of Energy Technology Development funded by the Ministry of Trade, Industry and Energy (20173030014330) and by the Core Research Project at Korea Research Institute of Chemical Technology (KRICT) (KK-1706-C00) funded by the Ministry of Science and ICT.

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REFERENCES

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