Nano-sized Ag inserted into ITO films prepared by continuous roll-to-roll sputtering for high-performance, flexible, transparent film heaters

Abstract

We demonstrate high-performance, flexible, transparent film heaters fabricated on a conductive Ag layer inserted into ITO films prepared by pilot-scale roll-to-roll (RTR) sputtering. The RTR sputtered ITO/Ag/ITO (IAI) multilayer on a 700 mm wide PET substrate at room temperature exhibits a high optical transmittance of 88.2% and a very low sheet resistance of 3.0 ohm per square, which are ideal for a low-voltage transparent film heater. The time–temperature profiles and heat distribution analysis demonstrate that the performance of the transparent film heater with the IAI multilayer is superior to that of a film heater with graphene, carbon nanotubes, amorphous ITO, Ag nanowires, or Ag networks, due to its very low sheet resistance (3–17 ohm per square). In particular, the thickness of the Ag interlayer in the IAI multilayer critically impedes the voltage for attaining 100 °C in the transparent film heater because the Ag interlayer provides a main conduction path in the IAI multilayer. In addition, the bending and cycling fatigue tests demonstrate comparable flexibility of the IAI multilayer to other transparent electrodes, indicating the potential of the IAI multilayer as an electrode for a highly flexible film heater. Therefore, the RTR sputtered IAI multilayer is the best substitute for conventional metallic Ag nanowire or ITO film heaters in order to realize a large-area, transparent, flexible film heater that is applicable to flexible and transparent defogging/deicing and heating systems.

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1 Introduction

Transparent and flexible film heaters (TFHs) have attracted much interest as defogging/de-icing windows and heating sources for automobiles, displays, sensors, reactions cells, microchips, and vinyl greenhouses.^1–3 In general, a TFH operates through the resistance heating of a transparent conductive layer coated onto a rigid or flexible substrate. Therefore, to obtain a high-performance TFH, the development of highly transparent, low sheet resistance, chemically stable, mechanically flexible, and large-area scalable electrodes is very important. This is because thermal response time, steady heating temperature, operating voltage, temperature uniformity, and cycling stability are critically affected by the quality of these transparent and flexible electrodes. In addition, with respect to the fabrication cost of TFHs and application of the covered TFHs, the large-area and cost-effective roll-to-roll (RTR) coating process of transparent electrodes should be considered. To date, the most commonly used transparent electrode in TFHs is indium-tin-oxide (ITO), which is prepared through a sputtering process due to its low resistivity, high transparency and matured mass production. Although amorphous ITO (a-ITO) films could easily be coated onto a flexible substrate using conventional DC sputtering at room temperature, the scarcity and high cost of indium is one of the critical disadvantages of ITO films.^4–10 In addition, the brittleness of thick a-ITO films prevents their use as flexible and transparent heaters. As a replacement for a-ITO films, various transparent electrode materials, including carbon nanotubes (CNTs), graphene, Ag nanowire, Ag grids, Au wire network, poly(3,4-ethylene dioxylene thiophene):poly(styrene sulfonic acid) (PEDOT:PSS), and Ga-doped ZnO film, have been suggested for transparent heaters.^10–21 However, carbon-based transparent electrodes, such as CNT and graphene sheets still show fairly high sheet resistance without additional doping, surface treatment, or hybridization with other conductive materials. Yoon et al. reported a transparent film heater on a single-wall CNT electrode with a fairly high sheet resistance of 356 ohm per square.²² Kang et al. also reported a TFH fabricated on a graphene electrode with a sheet resistance of 43 ohm per square.²³ Recently, Ilanchezhiyan et al., suggested a CNT functionalized cotton fabric-based heater for wearable heating applications.²⁴ However, to realize a large-area TFH, a high-quality transparent electrode with much lower sheet resistance than those of CNT and graphene sheet is necessary. Recently, a Ag nanowire network-based TFH was extensively investigated due to its low sheet resistance.^9,23,25 However, the Ag nanowire network exhibited critical drawbacks such as poor adhesion, non-uniform topography, easy degradation under ambient conditions, and instability against static electricity. In particular, the current constriction on specific Ag nanowires, which are the current conduction path, led to melting of the Ag nanowire or degradation of the flexible substrate during the heating process. Although PEDOT:PSS films have also been employed as transparent and flexible electrodes for TFH due to their superior flexibility and simple coating process, the instability and high sheet resistance remain critical problems.^6,12,26–28 Recently, a RTR-sputtered oxide-metal-oxide (OMO) multilayers have emerged as promising transparent electrodes for flexible optoelectronic devices due to its very low resistivity, high transmittance and good flexibility.^29,30 In our previous work, we demonstrated the high potential of RTR sputtered OMO multilayer electrode for replacement of the conventional ITO electrode in organic solar cells, organic light-emitting diodes, touch screen panels, transparent thin film transistors, and transparent memory devices.^29–32 However, there have been no reports on the use of RTR-sputtered OMO multilayer film as transparent, flexible electrodes for high-performance TFHs, which is a main component of defogging/de-iciding systems.

This work reports on a highly flexible and transparent ITO/Ag/ITO (IAI) multilayer with a very low sheet resistance prepared through pilot-scale large-area RTR sputtering at room temperature for a transparent electrode to be used in high performance TFHs. By inserting a highly conductive Ag interlayer between ITO layers, the IAI multilayer achieved a sheet resistance of 3.0 Ω per square, a resistivity of 2.5 × 10⁻⁵ Ω cm, a transmittance of 88.2%, and a bending radius of 6 mm. Furthermore, the performance of TFHs with the IAI multilayer electrodes were examined and compared to that of TFHs fabricated on amorphous ITO, single-wall CNT, graphene, Ag nanowire, and Ag network electrodes, demonstrating the outstanding properties of IAI multilayer films as a transparent electrode for high-performance TFHs.

2 Experimental

2.1 Continuous roll-to-roll sputtering of IAI multilayer

IAI multilayer films were continuously sputtered at room temperature on a PET substrate with a width of 700 mm and length of 200 m using a pilot-scale RTR sputtering system (Ulvac SPW-060). Prior to the sputtering of the bottom ITO layer, the PET substrate was passed through a 300 °C heater in a vacuum chamber to remove moisture. Then, the surface of the PET substrate was pretreated by an Ar/O₂ ion beam operated at a DC pulsed power of 300 W. After surface treatment of the PET substrate, the 40 nm thick bottom ITO layer was sputtered onto the PET substrate using rectangular ITO dual targets (950 mm × 127 mm); the operating conditions were a mid-range frequency (MF) power of 2.2 kW, working pressure of 3 mTorr, Ar/O₂ flow rate of 400/4 sccm, and rolling speed of 1.0 m min⁻¹ (ESI in Fig. S1†). After coating the bottom ITO layer, the Ag interlayer was continuously sputtered onto the bottom ITO layer as a function of Ag thickness at a working pressure of 3 mTorr, Ar flow rate of 400 sccm, and rolling speed of 1.0 m min⁻¹. The thickness of the Ag layer was controlled by DC power supplied to the Ag target, ranging from 0.2 to 0.6 kW. After deposition of the Ag interlayer, a 40 nm-thick top ITO layer was sputtered on the Ag interlayer. The sputtering conditions of the top ITO layer were the same as with the bottom ITO layer in order to form a symmetric IAI structure.

2.2 Characterization of the RTR-sputtered IAI multilayer

The electrical and optical properties of the RTR-sputtered IAI multilayer were examined using Hall measurements (HL5500PC, Accent Optical Technology) and a UV/visible spectrometer (UV 540, Unicam) as a function of the thickness of the Ag interlayer. The microstructures and interfacial structures of the optimized IAI multilayer mesh electrodes were examined using high-resolution electron microscopy (HREM). Fast Fourier Transform (FFT) images were obtained from a cross-sectional HREM specimen prepared by focus ion beam (FIB) milling. The mechanical properties of the IAI multilayers were evaluated using a specially designed inner/outer bending system. The outer bending test induced tensile stress on the film, whereas the inner bending test induced compressive stress. In addition, dynamic fatigue bending tests were performed using a lab-designed cyclic bending test machine, operated at a frequency of 0.5 Hz for 10 000 cycles. The resistance of the IAI multilayer was measured throughout cyclic bending.

2.3 Fabrication and evaluations of the TFHs

To demonstrate the feasibility of the IAI multilayer as a transparent electrode for TFHs, conventional film heaters (50 × 50 mm²) with two-terminal side contacts were fabricated on the IAI multilayer electrode (Fig. S2†). For comparison, a-ITO, CNT, graphene, Ag NW, and Ag network films were also used as transparent electrodes. In addition, a large-scale IAI-based TFHs (200 × 290 mm²) was fabricated to show feasibility of the IAI electrode for large-area TFHs. After wet cleaning of the IAI multilayer, a 200 nm-thick Ag side contact electrode was sputtered onto the IAI multilayer. The DC voltage was supplied by a power supply (OPS 3010, ODA technologies) to the IAI-based TFHs through an Ag contact electrode at the film edge. The temperature of TFHs was measured using a thermocouple mounted on the surfaces of the TFHs and an IR thermal imager (A35sc, FLIR) (Fig. S3†). For the defrost test, the IAI multilayer was place in a refrigerator for 60 min to form frost on the surface of the IAI multilayer.

3 Results and discussion

Fig. 1a shows the schematic RTR sputtering process and image of an IAI multilayer grown on a 700 mm wide PET substrate using pilot-scale RTR sputtering at room temperature. The IAI film consisted of three layers. The bottom ITO layer acts as an adhesion layer between the IAI multilayer and PET substrate, while the top ITO layer acts as a symmetric oxide layer to realize the antireflection effect in the OMO structure. The nano-sized Ag layer provides the main conduction path in the IAI multilayer and mechanical flexibility due to its high strain failure.³³ The picture in Fig. 1a shows the IAI coated on the 700 mm wide PET substrate that was rolled on a rewind roller during the continuous RTR sputtering process. Fig. 1b shows the cross-sectional TEM image of the IAI (40/12/40 nm) multilayer with enlarged images of the PET substrate, bottom ITO, Ag interlayer, and top ITO layer. In the as-sputtered IAI multilayer, the Ag interlayer exists as a continuously connected layer between the top and bottom ITO layers. The enlarged cross-sectional image clearly demonstrates that the as-deposited IAI multilayer has a symmetric structure consisting of crystalline bottom and top ITO layers. The fast Fourier transform (FFT) pattern of the bottom and top ITO in the inset shows strong spots which is a feature of a polycrystalline structure. It was previously reported that the microstructure of a direct current (DC)-sputtered ITO film is amorphous or nano-crystalline embedded amorphous, due to its low process temperature. Therefore, an additional crystallization process is necessary to improve the electrical and optical properties of ITO films. However, the microstructure of the mid-range frequency (MF)-sputtered ITO film in our pilot-scale RTR sputtering set-up is well-developed polycrystalline with a bixbyite structure. Because the MF power could effectively deliver an alternating positive and negative bias to the dual-cathodes, which are well suited for use in a mass production line, high power can be supplied to the dual ITO targets.^34–36 Therefore, the ITO films grown by a MF power showed a well-developed polycrystalline structure even though it was prepared at room temperature. The enlarged image of the bottom (A and B) and top ITO layers (D) of Fig. 1b clearly showed a polycrystalline ITO film, as confirmed by X-ray diffraction results (ESI Fig. S4†). The XRD plot of the IAI film in Fig. S4† shows the strong crystalline ITO peaks at 2θ = 29.9° (222), 36.0° (400), 43.2° (431) and crystalline Ag peak at 2θ = 38.2° (111) as well as PET substrate peaks. In addition, the enlarged interface (C) of Fig. 1b between crystalline ITO and the Ag interlayer is very clear and smooth without interfacial reactions because each ITO and Ag layer was continuously sputtered without breaking the vacuum during the RTR sputtering process.

Fig. 1 (a) Schematic of the RTR sputtering process and structure of the IAI multilayer prepared on a PET substrate using large area RTR sputtering system. The picture shows a large-area IAI multilayer coated onto 700 mm wide PET substrate and loaded in the RTR sputter system. (b) Cross-sectional TEM image of the RTR-sputtered IAI (40/12/40 nm) multilayer on the PET substrate with enlarged images (A–D) of each layers. The inset of the enlarged image is a fast Fourier transform pattern obtained from bottom ITO, Ag interlayer and top ITO films.

Fig. 2a shows the optical transmittance values of the RTR-sputtered IAI multilayer films as a function of Ag thickness from 6 to 12 nm. The insets show the transparency and color of the RTR-sputtered IAI multilayer films with different Ag interlayer thicknesses. The IAI multilayer film with 6 nm-thick Ag interlayers showed a slightly low optical transmittance, because very thin Ag forms disconnected Ag islands, resulting in light scattering.³⁷ The blueish color in the inset indicates the light scattering on the Ag islands in the IAI multilayer films. However, with a Ag interlayer thickness greater than 8 nm, the IAI multilayer showed an increase in optical transmittance. Due to the anti-reflection effect of the OMO triple layer, the RTR-sputtered IAI multilayer exhibited a high optical transmittance in the visible wavelength region, even with the opaque metal Ag interlayer.³² In particular, between 8 and 12 nm thicknesses of the Ag interlayer, the IAI multilayer showed a high optical transmittance and clear transparency. Fig. 2b shows the sheet resistance and resistivity of the RTR-sputtered IAI multilayer as a function of Ag interlayer thickness. In general, the electrical properties of the OMO films were primarily affected by the metal interlayer thickness because the metal layer provides a main conduction path. Compared to the 70 nm-thick amorphous ITO (a-ITO) film grown at room temperature with a resistivity of 1.6 × 10⁻³ ohm cm and sheet resistance of 241.1 ohm per square, the IAI multilayer showed a significantly reduced sheet resistance and resistivity due to the highly conductive Ag interlayer. Without post-annealing or in situ substrate heating, the 6 nm-thick Ag-inserted IAI multilayer demonstrated a sheet resistance of 17.4 ohm per square and a resistivity of 13.2 × 10⁻⁵ ohm cm. With increasing Ag interlayer thickness, the sheet resistance and resistivity of the IAI multilayer significantly decreased. The IAI multilayer with a 12 nm thick Ag interlayer showed the lowest sheet resistance of 3.0 ohm per square and a resistivity of 2.5 × 10⁻⁵ ohm cm, which is much lower than those of conventional a-ITO or c-ITO films. Fig. 2c compares the optical transmittance of several flexible and transparent electrodes, such as transferred graphene, a-ITO, CNTs, commercial Ag NWs, and an Ag network, as well as the IAI multilayer electrode. The optimized IAI multilayer showed greater optical transmittance than that of several flexible, transparent electrodes at a 550 nm wavelength. This is due to the anti-reflection effect of the OMO multilayer, indicating the potential of the RTR-sputtered IAI multilayer as a highly transparent electrode for TFHs. Fig. 2d also compares the sheet resistance of several flexible and transparent electrodes with an IAI multilayer electrode. Compared to the other flexible and transparent electrodes, the RTR-sputtered IAI multilayer showed a much lower sheet resistance (3.0 ohm per square), comparable to that of a metallic conductor. In the operation of TFHs, a low sheet resistance is very important because the total power consumption (P) and saturated heater temperature are primarily affected by the sheet resistance of the transparent electrodes.

Fig. 2 (a) Optical transmittance spectra, (b) sheet resistance, and resistivity of the RTR-sputtered IAI multilayers as a function of Ag interlayer thickness. The inset shows the clearance and color of the IAI multilayer films with increasing Ag thickness. (c and d) Comparison of the optical transmittance and sheet resistance of the IAI multilayer with those of other flexible and transparent electrodes such as graphene, amorphous ITO, carbon nanotubes, Ag nanowire, and a Ag network.

The mechanical flexibility of the RTR-sputtered IAI multilayer was evaluated through inner and outer bending and cyclic fatigue tests using a lab-made bending test system. Fig. 3a shows the results of the outer/inner bending tests of several transparent and flexible electrodes with decreasing outer/inner bending radii. The change in resistance of the electrode due to substrate bending can be expressed as (ΔR = R − R₀)/R₀, where R₀ is the initial measured resistance, and R is the resistance measured under substrate bending. The outer bending test results (upper panel) shown in Fig. 3a exhibits that the IAI multilayer had constant resistance until the bending radius reached 6 mm. The following equation can be used to calculate the peak strain for a curved IAI multilayer film with decreasing bending radius.³⁸

here, d_IAI and d_PET are the thicknesses of the IAI multilayer and the PET substrate, respectively. Bending IAI film (40 nm/12 nm/40 nm) on a 100 μm-thick PET substrate to a bending radius of 6 mm resulted in a peak strain of 0.83%. Further decreasing the outer bending radius rapidly increased the resistance change due to crack formation and propagation in the top crystalline-ITO layer. Although the IAI multilayer showed a slightly higher critical outer bending radius than other flexible electrodes, the critical radius of the RTR sputtered IAI film is acceptable for fabrication of TFHs. In the inner bending tests, the measured resistance of the IAI multilayer film was constant until the sample was bent to an inner bending radius of 2 mm (the bending limit), similar to other flexible and transparent electrodes. Even though the IAI films delaminated from the PET substrate under high bending conditions and cracking within the films, the change in resistance was very small. Under compressive stress, the flexible IAI film remained functional despite local delamination of the layer or crack formation, due to overlapping of cracked or delaminated layers. Fig. 3b shows the dynamic outer and inner bending fatigue test results of the optimized IAI multilayer with increasing bending cycles at a fixed inner bending radius of 10 mm. The upper panel of Fig. 3b shows pictures of the dynamic outer/inner bending test steps with decreasing bending radius. Both dynamic outer and inner bending fatigue tests showed no change in resistance (ΔR) after 10 000 bending cycles, demonstrating the superior flexibility of the IAI multilayer. This superior flexibility can be attributed to the high strain failure of the metallic Ag interlayer between the ITO layers.³⁹

Fig. 3 (a) Outer and inner bending test results of several flexible and transparent electrodes with decreasing bending radius. (b) Cyclic outer and inner fatigue test results of the RTR-sputtered IAI multilayer with increasing bending cycles. Upper panels exhibit the outer bending steps during fatigue testing.

To investigate its feasibility as a flexible and transparent electrode for TFHs, IAI multilayer-based TFHs were fabricated with a size of 50 × 50 mm² using a two-terminal metal contact configuration. Fig. 4a shows the schematic structure and image of TFHs fabricated on RTR-sputtered IAI multilayer electrodes. The DC voltage was applied to the TFHs by a power supply through Ag contact electrodes at the film edge, and the temperature profile was measured by a thermocouple placed on the surface of the TFH (ESI, Fig. S3†). In addition, an infrared (IR) thermal image was obtained using an IR thermometer. Fig. 4b shows the temperature profiles of the IAI multilayer based-TFHs, plotted with respect to input voltage as a function of Ag interlayer thickness from 6 to 12 nm. When the DC voltage was supplied to the IAI multilayer-based TFHs, the temperature of the TFHs gradually increased and reached a maximum. As the input voltage was increased, the temperature of the TFHs increased as shown in all of the temperature profiles. It is noteworthy that the increase in Ag interlayer thickness of the IAI multilayer decreased the DC voltage in order to achieve a temperature of 100 °C. In the case of the IAI multilayer-based TFHs with an Ag interlayer thickness of 6 nm, 11 V of input DC voltage was necessary. However, an increase in Ag interlayer thickness of the IAI multilayer up to 12 nm led to a decrease in input DC voltage to 5 V in order to produce a temperature greater than 100 °C. The lower input voltage of the IAI-based (Ag interlayer) TFHs needed for reaching a temperature 100 °C implies efficient transduction of electric energy into Joule heating in IAI multilayer-based TFHs. More efficient transduction of electric energy in the TFHs was attributed to the lower sheet resistance of the IAI multilayer with the thicker Ag interlayer. Based on Joule's law, we can correlate the sheet resistance of transparent electrodes for TFHs and the generated temperature.^25,40 The power (P) applied to the TFHs during the heating time (t) generated heat (ΔQ_g),

where V is the input DC voltage between contact electrodes, R is the resistance of the TFHs, Q_cond is the heat loss due to conduction in the substrate, Q_conv is the heat loss due to convection in air, and Q_rad is the heat loss due to radiation. In our TFH samples, heat loss due to conduction was negligible because the sample was not placed on a good thermal conductor. In addition, heat loss due to radiation was negligible below 100 °C due to the very low emissivity of the materials. Therefore, air convection is the main path of heat dissipation in IAI multilayer-based TFHs.in eqn (3) and (4), h_conv is a convective heat transfer coefficient, A_conv is the surface area, and T_s and T_i are the saturation and initial temperature, respectively. Based on eqn (3), it is apparent that the saturation temperature of TFHs increases with increasing input DC voltage (V) and with decreasing resistance (R). Therefore, a lower sheet resistance of a flexible and transparent electrode is imperative for fabrication of high-performance TFHs with a lower DC input voltage to achieve a temperature of 100 °C. The lower DC input voltage (5 V) of TFHs with a 12 nm thick Ag interlayer compared to that (11 V) of TFHs with 6 nm-thick Ag interlayer confirms that the lower sheet resistance of the flexible and transparent electrode is critical to obtain high-performance TFHs with a lower input DC voltage. To demonstrate the feasibility of IAI multilayer-based TFHs, a water droplet test was performed on the heated TFHs.

Fig. 4 (a) Schematic structure and image of TFHs fabricated on RTR-sputtered IAI multilayer electrodes. (b) Temperature profile of the IAI multilayer-based TFHs as a function of Ag interlayer thickness under operation at different input voltages.

Fig. 5a shows the water droplet test pictures and IR images of the IAI-based TFHs with a saturation temperature of 110 °C. As soon as a DC input voltage of 5 V was supplied to the IAI-based TFHs, a saturation temperature of 110.8 °C was achieved when Joule heating and convection reached a dynamic balance. Therefore, the water droplet disappeared almost immediately due to the high temperature of the IAI multilayer-based TFHs. Fig. 5b illustrates the defrost test of the IAI-based TFHs before and after frost formation. To ensure uniform frost formation on the surface of the IAI electrode, the samples were placed in a refrigerator for 60 min. At the operating voltage of 5 V, the frost on the surface of the IAI electrode completely disappeared. Effective removal of the frost allows visualization of the Kyung Hee University symbol to appear in the background due to the high transparency of IAI multilayer. This indicated that the RTR-sputtered IAI multilayer is a promising flexible and transparent electrode for a transparent defroster.

Fig. 5 (a) Images of the water droplet test on heated TFHs with an IAI multilayer electrode. (b) Defrosting test results of the IAI multilayer-based TFHs before and after frost formation.

To compare the heating performance of the RTR-sputtered IAI multilayer electrodes, we fabricated TFHs with several transparent electrodes such as a-ITO, CNTs, graphene, Ag NWs, and an Ag network. The size of the TFHs was fixed at 50 × 50 mm². Fig. 6a depicts the time-dependent temperature of the TFHs with different transparent electrodes. At a constant DC voltage of 5 V, the time-dependent temperature of the TFHs varied with electrode sheet resistance. The IAI multilayer-based TFHs showed the best performance with a higher steady state temperature up to 100 °C at low DC voltage, compared to 53.4 °C for the Ag network, 30.9 °C for the Ag NW, 24.0 °C for CNTs, 23.3 °C for a-ITO film, and 23.5 °C for graphene electrodes. Fig. 6b shows IR images of several TFHs with different transparent electrodes taken by an infrared camera while constant DC voltage of 5 V was supplied. The IAI-based TFHs showed uniform temperature distribution due to the very low sheet resistance of the IAI multilayer. The time-dependent temperature profiles and uniform heat distribution of the IAI multilayer-based TFH indicate that the performance of the IAI multilayer-based heater is superior to those of other TFHs with other transparent electrodes.

Fig. 6 (a) The temperature profile of TFHs with different transparent electrodes measured by a thermocouple. (b) Infrared images of several TFHs with different transparent electrodes at constant input DC voltage of 5 V.

To investigate the feasibility of the RTR sputtered IAI film as a transparent electrode for TFHs, we fabricated large-area TFHs. Fig. 7a shows picture of the large-area TFHs with RTR sputtered IAI multilayer (200 × 290 mm²). The large-area TFH was fixed between lab-made power clips. Fig. 7b demonstrates IR images of large-area TFH taken by a portable IR camera with increasing DC power. The circle in the IR image indicates the temperature sensing spot by a portable IR camera. With increasing DC power, the TFHs reached at a saturation temperature of 100 °C when Joule heating and convection reached a dynamic balance. Because the DC power was supplied by end of the power clips, the upper region of the TFH showed a slightly higher temperature. Therefore, we believed that uniform injection of power into the IAI multilayer improved the temperature uniformity of the IAI-based TFHs. Fig. 7c demonstrates promising applications of the RTR sputtered IAI multilayer-based TFHs. The IAI-based TFHs could be employed as a defogging/deicing window for automobiles or goggles for winter sports. In particular, the IAI-based TFHs could be employed in automobiles because a large-area IAI multilayer could be prepared on a PET substrate using the RTR sputtering process. Therefore, a cost-effective defogging/deicing system could be realized on the front or rear windows of an automobile. In addition, they could be used as a transparent heating source for transparent vinyl greenhouses.

Fig. 7 (a) A photograph of the large-area TFHs with RTR sputtered IAI film (200 × 290 mm²). (b) IR image of TFH taken by a portable IR camera with increasing DC power. (c) Promising applications of IAI multilayer-based TFHs such as automobiles, goggles, and transparent vinyl greenhouses.

4 Conclusions

In summary, we developed high-performance TFHs using highly transparent and flexible IAI multilayer electrodes. Using pilot-scale RTR sputtering, we prepared large-area IAI multilayer electrodes on 700 mm wide PET substrates with a low sheet resistance of 3.0 ohm per square, resulting in low-voltage driven TFHs with good flexibility. Due to the existence of a metallic Ag interlayer in the IAI multilayer, the IAI multilayer showed much lower sheet resistance than a-ITO, CNT, graphene, Ag NWs and Ag network electrodes and good flexibility. These are factors that are important for flexible thin film heaters. The time-dependent temperature profile and heat distribution of IAI-based TFHs demonstrated that the RTR-sputtered IAI multilayer is a promising transparent and flexible electrode for large-area, cost-effective TFHs and can be used as a substitute for conventional amorphous ITO or other transparent electrode materials. Consequently, nano-sized Ag inserted into ITO films prepared by RTR sputtering solves the problems of conventional transparent electrode materials and advances transparent electrode technologies for large-area and cost-effective TFHs.

Acknowledgements

The authors are appreciated for the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A01002415).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08704c

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