Journal of Materials Science & Technology, 2020, 48(0): 156-162 DOI: 10.1016/j.jmst.2020.01.058

Research Article

Robust ultrathin and transparent AZO/Ag-SnOx/AZO on polyimide substrate for flexible thin film heater with temperature over 400 °C

Zhaozhao Wanga,b, Jia Li,a,*, Junjun Xua,b, Jinhua Huangb, Ye Yangb, Ruiqin Tanc, Guofei Chenb, Xingzhong Fangb, Yue Zhaoa, Weijie Song,b,d,**

a School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China

b Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

c Faculty of Information and Computer Science, Ningbo University, Ningbo 315211, China

d Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou 213164, China

Corresponding authors: * E-mail addresses:lijia@nimte.ac.cn(J. Li);** Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail addresses:weijiesong@nimte.ac.cn(W. Song).

Received: 2019-10-8   Accepted: 2020-01-28   Online: 2020-07-1

Abstract

We developed flexible and transparent thin film heaters (TFHs) with a structure of AZO/Ag-SnOx/AZO/polyimide (PI) which exhibited superior stability under operational conditions. Ultrathin and robust doped silver (Ag) films were produced by introducing a small amount of SnOx during the sputtering process. The AZO/Ag-SnOx/AZO stacks showed the best figure of merit value of 139.9, which was higher than that of ITO counterpart (25.5) with the same thickness. In addition, it exhibited excellent durability, which showed unaffected optical and electrical properties under the heat treatment of 500 °C for 30 min in air, highly-accelerated temperature and humidity stress test (HAST) at 121 °C and 97%RH for 36 h, 10,000 times of scratching and 1500 times of inner and outer bending test. Furthermore, the TFHs based on AZO/Ag-SnOx/AZO/PI achieved a high temperature of 438.8 °C with response time in several seconds, which outperformed most previous studies. The robust high-temperature TFHs in this research hold promising commercial applications in flexible and high temperature occasions.

Keywords: Ultrathin Ag films ; SnOx Doping ; Durability ; High temperature thin film heater

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Cite this article

Zhaozhao Wang, Jia Li, Junjun Xu, Jinhua Huang, Ye Yang, Ruiqin Tan, Guofei Chen, Xingzhong Fang, Yue Zhao, Weijie Song. Robust ultrathin and transparent AZO/Ag-SnOx/AZO on polyimide substrate for flexible thin film heater with temperature over 400 °C. Journal of Materials Science & Technology[J], 2020, 48(0): 156-162 DOI:10.1016/j.jmst.2020.01.058

1. Introduction

Transparent heaters, which generate heat when applied a few volts while keeping visually transparent, have wide usage in avionics, temperature-controlled liquid crystal displays, smart energy efficient windows for vehicles and buildings, and medical devices [[1], [2], [3], [4], [5]]. Currently, the commonly-used ITO-based heaters are limited by the disadvantages such as slow thermal response, brittleness and indium deficiency, which urges researchers to seek alternative materials for flexible TFHs [6]. Therefore, carbon-based materials (graphene and carbon nanotube), conducting polymers, and metal networks (metal nanowires and grids) have been extensively investigated. Among these alternatives, carbon nanotubes and graphene with the sheet resistance of 100-5000 Ω/sq suffer from their large-scale uniformity [7]. Ag nanowire and its hybrid structures are featured by low sheet resistance and high flexibility while their stability problem leads to a low saturation temperature of less than 250 ℃ [8,9]. In the highest temperature of 600 °C reported up to now, Au network is designed on quartz, while its network structure would lead to inhomogeneous heat distribution and haze issues [10].

In contrast, another advantageous alternative, oxide-metal-oxide (OMO) multilayer films based on ultrathin metal especially Ag have gained increasing attention due to their good photoelectric properties and superior flexibility [11,12]. Seok et al. reported multi-stacked electrodes with 14 nm AgPdCu intermediate layer, which exhibited very homogeneous temperature distribution at 119.8 °C. However, the usage of precious metal will inevitably increase the cost and process complexity. Normally, during the growth process of ultrathin Ag films, suffering from poor wettability on high surface energy substrate, the Ag atoms tend to have an island growth behavior instead of planar growth at low thickness, leading to a high threshold thickness and an inferior conductivity as well as some stability problems [[13], [14], [15]]. It is reported that the deterioration with white-dot defect appears on OMO multilayer films surface in damp-heating environment, and the agglomeration of metal atoms in intermediate layer occurs at a high temperature [16,17]. The aforementioned APC alloy layer co-doped by metal elements of Pd, Cu or Au was proved to be the most effective strategy to improve the thermal and damp-heat stability of the ZnO/Ag-alloy/ZnO multilayer, in which the white point defect deterioration occurred under the condition of 85 °C and 85% relative humidity (RH) with air postponed from 1 d to more than 5 d [18]. Besides that, some other methods that were originally developed to improve the wettability of Ag atoms are also beneficial for improving the durability. Guo et al. reported an ultrathin and low-loss Al doped Ag film, which exhibited a better thermal stability in a N2 atmosphere while suffering from an inferior electrical property due to the Al induced impurities [19]. A small amount of O2 or N2 doping to form the AgOx and CuNx layer could reduce the percolation threshold thickness of the metal film to about 4 nm, and improve the oxidation resistance in the air of the thin films at the same time [20,21]. It was also reported that 1 nm Ni capping layer could protect the underlying Ag layer over time under 85 °C and 85% RH [22]. Instead of the reported strategies that require the precise control of sputtering parameters, this paper presents a more robust ultrathin Ag film by co-sputtering Ag and SnO2 without reducing the optoelectrical performance of OMO structure.

As we all know, the candidate materials for substrates of many flexible optoelectronic devices, such as polyethylene terephthalate (PET), polycarbonate (PC), poly (propylene glycol adipate) (PPA) etc., may be severely damaged during high-temperature deposition process [23,24]. In comparison, polyimide (PI) is an important class of polymeric materials with good mechanical and chemical properties as well as thermal durability [25,26]. Although many reports have demonstrated the feasibility of many materials for the preparation of TFHs on flexible substrates (including PI), there is no report on the preparation of high-temperature (above 400 °C) TFHs on PI which can withstand very high temperature.

In this paper, we report a low threshold growth of Ag-SnOx thin film on both glass and PI substrates by co-sputtering Ag and SnO2. Results show that the ultrathin Ag-SnOx film and AZO/Ag-SnOx/AZO stacks exhibited outstanding thermal durability, damp-heat stability, scratch resistance and mechanical flexibility. The performance of flexible high-temperature TFHs based on AZO/Ag-SnOx/AZO thin film has also been demonstrated.

2. Experimental

2.1. Preparation of the thin films

Ag-SnOx thin films were deposited by co-sputtering 3-inch Ag and SnO2 targets using magnetron sputtering at room temperature with pure argon (Ar) as the sputter gas. AZO (3 inch, 2 wt% Al2O3 doped ZnO) and ITO (10 wt% SnO2 doped In2O3) targets were used for the fabrication of AZO/Ag-SnOx/Ag stacks and ITO control samples, respectively. Both soda-lime float glass and polyimide films were used as the substrates which were sonicated in specific cleaning solution and deionized water for 20 min in order before deposited. The base pressure was evacuated to 6.8 × 10-4 Pa and maintained at pressure of 0.15-0.20 Pa during deposition. The sputtering power of Ag, SnO2 and AZO targets were fixed at 40 W, 8 W and 100 W, respectively. All the thickness of the films was controlled by deposition time.

2.2. Characterization

The transmittance spectra and thickness of the prepared thin films were measured and fitted by ellipsometry measurements (J.A.Woollam, M2000DI). The sheet resistance was recorded using a four-point probe system (NAPSON, Cresbox). The surface morphologies and roughness were observed using an atomic force microscope (Veeco, Dimension 3100 V) and a fluorescence microscope (Olympus, BX51). The atomic concentrations and chemical states of the elements were obtained using X-ray photoelectron spectroscopy (XPS, Kratos, AXIS ULTRADLD). The crystallinity of the samples were characterized by X-ray diffraction (XRD, Bruker AXS D8 Advance, USA) with CuKα radiation (1.5406 Å). The thermal stability of the thin films was assessed at different temperatures in air by an electrical resistance furnace (YFX12/16Q-YC). The damp-heat stability was measured at a temperature of 121 °C, a RH of 97% with a pressure of 0.1 MPa using HAST (PC4 R8, Hirayama). The scratch resistance was investigated by an architectural paint scrubber (JTX-II GB/T 9266). The mechanical flexibility was demonstrated by a home-made bending machine for bending repeatedly.

2.3. Fabrication and measurements of TFHs

To evaluate the performance of the AZO/Ag-SnOx/AZO thin films as a transparent flexible high temperature TFHs, the thickness of the bottom and top layer was set as 40 nm with the middle layer was fixed at 10 nm. The size of the AZO/Ag-SnOx/AZO-based TFHs was set as 30 mm × 35 mm with copper foil attached at both edges of the device to act as contact electrode. For comparison, the same thickness of ITO thin film with a transmittance of 75.7% (including substrate) and sheet resistance of 49.5 Ω/sq was also fabricated at room temperature as a control sample (as shown in Fig. 1). The DC voltage was applied by a power supply (UTP3704S, UNI-T) and the temperature of the TFHs was measured by a noncontact IR thermal imager (E85, FLIR).

Fig. 1.

Fig. 1.   Schematic diagram of the fabrication process of TFHs based on AZO/Ag-SnOx/AZO and ITO thin films.


3. Results and discussion

3.1. Growth and durability of Ag-SnOx thin film

For the single metal layer, the percolation threshold thickness of Ag-SnOx thin films was decreased to less than 6 nm with a sheet resistance of 27.9 Ω/sq, which was better than that of pure Ag thin films (as shown in Fig. S1). Herein, underlying and upper layers of AZO with the fixed thickness of 40 nm were chosen and the performance of AZO/Ag-SnOx/AZO stacks on PI substrate was explored. Fig. 2(a) shows the transmittance of AZO/Ag-SnOx/AZO thin films with various intermediate layer thicknesses. The AZO/Ag-SnOx/AZO stacks showed comparable transmittance at 6-10 nm, while suffered from an inferior transmittance when thickness increased to 12 nm. Taking into account optical and electrical factors, the figure of merit (FOM) is usually considered as a valid parameter and defined as the radio of σOP to σDC [27]:

$\text{FOM}={}^{{{\sigma }_{\text{DC}}}}/{}_{{{\sigma }_{\text{OP}}}}={}^{188.5}/{}_{({}^{1}/{}_{\sqrt{T}}-1){{R}_{\text{s}}}}$

where T is the transmittance of the thin film at 550 nm and Rs is the sheet resistance of the thin film. Fig. 2(b) shows that the AZO/Ag-SnOx/AZO thin film with an intermediate layer thickness of 10 nm had the best FOM. The FOM value of the ITO control sample was also calculated. Sputtered at room temperature with the thickness less than 100 nm, ITO thin film on PI exhibited inferior optical and electrical properties with an FOM value of 25.5. The transmittance and sheet resistance of the AZO/Ag-SnOx/AZO and ITO thin film were 79.1%, 75.7% (including substrate) and 10.8 Ω/sq, 49.5 Ω/sq, respectively. Fig. 2(c) and (d) exhibits the AFM surface morphology images and roughness of the flexible PI and PI/AZO/Ag-SnOx/AZO thin films, respectively. Compared with rigid glass substrate, the flexible PI substrate showed a rough surface with some protuberances, leading to a Ra roughness of 3.8 nm, which was a challenge to the growth of ultrathin conductive film. The PI/AZO/Ag-SnOx/AZO stack showed a more uniform and smoother surface morphology, as shown in Fig. 2(d). The result in Fig. 2(d) manifests the feasibility of high-quality growth as well as the ideal optical and electrical performance.

Fig. 2.

Fig. 2.   (a) Transmittance spectra, (b) sheet resistance and FOM of PI/AZO/Ag-SnOx/AZO with different Ag thicknesses; AFM images of (c) pure PI and (d) PI/AZO/Ag-SnOx/AZO thin film. XPS profiles of (e) overall spectra, (f) Ag 3d and (g) Sn 3d of AZO/Ag (40 nm/10 nm) and AZO/Ag-SnOx (40 nm/10 nm) thin films.


To elucidate the chemical state and composition of Ag, Sn and O in the intermediate Ag layer after co-sputtering with SnOx, the XPS spectra of the AZO/Ag-SnOx and AZO/Ag double-layer were measured and shown in Fig. 2(e-g). The XPS survey spectrum in Fig. 2(e) indicated the existence of the elements Ag, Sn, O, C and Zn, while no other impurity peaks were detected. The high-resolution XPS spectra of the Ag 3d and Sn 3d regions are shown in Fig. 2(f, g), respectively. The Ag 3d5/2 peak was located at 368.1 eV, which was corresponded to metallic Ag [28]. No oxidation state peaks could be detected for both pure Ag and Ag-SnOx thin film samples. The Sn 3d5/2 peak was centered at 486.5 eV (Fig. 2(g)), which was assigned to an oxidized state, namely SnOx [29]. Besides, there was a peak located at 499.2 eV, which corresponded to the signal of Zn LMMa coming from the AZO underlying layer. The atomic concentration of Ag to Sn was calculated using the integrated intensities of the Ag 3d and Sn 3d peaks and their sensitivity factors. The atomic fraction of Ag to Sn was determined to be 97:3. Combined with the uniform surface morphology and the chemical state of Ag and Sn atom in Fig. 2, it could be deduced that the Sn atoms existed as SnOx dispersed phase in Ag thin films, which suppressed the surface migration and the diffusion of Ag atoms, leading to a favorable wetting conditions for low-threshold and smooth growth of Ag-SnOx thin films.

For the practical applications of ultra-thin Ag conductive films to optoelectronic devices especially in TFHs, the long-term durability in air, damp and heat environment must be considered. It has been reported that pure Ag thin films became discontinuous with a sharp increase of sheet resistance after annealing due to the inferior wetting behavior of Ag thin film [30]. Herein, the thermal durability of single-layer Ag-SnOx under various annealing temperatures on glass substrate was investigated and the results are shown in Fig. 3(a). Agglomeration began at temperature less than 200 °C for the pure Ag thin films, which was not shown here. Noticeably, the single-layer Ag-SnOx exhibited a significant improvement in the heat stability with the sheet resistance changed only slightly up to a high temperature of 450 °C. Furthermore, after sandwiched by AZO thin films on both sides, the heat stability of the AZO/Ag-SnOx/AZO multilayer was further improved. Fig. 3(b) shows the images of AZO/Ag-SnOx/AZO thin films right after deposition and after heated at 300 °C for 30 min. Both films showed uniform transparency with no degradation after high temperature annealing. In brief, the adding of SnOx dispersed phase could greatly improve the thermal durability in Ag-SnOx single layer and multilayer form, which promised its feasibility under high-temperature conditions.

Fig. 3.

Fig. 3.   (a) Change in sheet resistance of Ag-SnOx and AZO/Ag-SnOx/AZO thin films; (b) Images of AZO/Ag-SnOx/AZO thin film, upper: as deposited, and lower: after heated at 300 ℃ for 30 min; (c-f) Fluorescence microscope images of AZO/Ag-SnOx/AZO thin films for 12-36 h HAST, and AZO/Ag/AZO thin film for 24 h HAST; (g) Sheet resistance and (h) transmittance spectra of AZO/Ag-SnOx/AZO thin film with different hours for HAST.


The damp-heat durability of AZO/Ag-SnOx/AZO thin films was evaluated and compared with that of AZO/Ag/AZO thin films under HAST, in which the penetration of moisture through the films was accelerated. Normally, a 24-h HAST (121 °C/97% RH) is consider to be equal to a 1000 h conventional 85 °C/85% RH test. Fig. 3(c-f) shows the fluorescence microscope images of the AZO/Ag-SnOx/AZO thin films ((c), (d) and (e) for 12 h, 24 h and 36 h, respectively) and AZO/Ag/AZO thin films ((f) for 24 h) after HAST. It is evidently that the surface morphology had no change for the AZO/Ag-SnOx/AZO thin films even after 36 h of treatment, while white-dot defect appeared apparently on the surface of AZO/Ag/AZO thin films. It has been reported that, in the ZnO/Ag/ZnO system, oxygen and water vapor penetrate through the grain boundaries easily in ZnO thin films [31], leading to the moisture-induced silver migration. The reduction of the top-layer/Ag adhesion force triggered the peeling [32], resulting in the white-dot defect deterioration. Here, attributing to the compact surface and fine grains of the Ag-SnOx thin film after SnOx doping, the multilayer films presented an excellent resistance to moisture and oxidation. Fig. 3(g) and (h) plots the change of transmittance and the resistance of the AZO/Ag-SnOx/AZO thin film with various treatment time in HAST. The very stable optical and electrical properties of the AZO/Ag-SnOx/AZO thin film exhibited outstanding damp-heat durability.

In addition to the damp and heat durability, scratch resistance and mechanical flexibility are also important. As illustrated in Fig. 4(a), the multilayer samples were placed in an architectural paint scrubber and scraped repeatedly by a stiff brush with ethanol added continuously. The scratch resistance was evaluated by calculating the ratio of the area peeled off to the total area of the film. Even if the number of scratches reached 10,000 times, there was no peeling off for the AZO/Ag-SnOx/AZO thin film. Comparatively, the AZO/Ag/AZO thin film began to fall off at 1000 times, and almost fell out at 6000 times. Furthermore, to demonstrate the flexibility of the AZO/Ag-SnOx/AZO multilayer, the outer and inner bending test was conducted with increasing bending cycles with a bending radius fixed at 4 mm, and the results are shown in Fig. 4(b). The inset of Fig. 4(b) displays the photograph and the steps of the dynamic bending test. R/R0 was used to evaluate the change in the sheet resistance values, where R0 and R are the resistances before and after bending, respectively. It has been reported that ITO thin films could not stand the repeated bending with resistance value increasing rapidly when bended less than 100 times [33]. By contrast, the AZO/Ag-SnOx/AZO thin films were very robust with constant sheet resistance after 1500 bending cycles, no matter under inner bending or outer bending, exhibiting an excellent mechanical durability.

Fig. 4.

Fig. 4.   (a) AZO/Ag/AZO and AZO/Ag-SnOx/AZO thin films’ shedding area as a function of number of scratches. Inset picture is schematic diagrams of architectural paint scrubber. (b) Change in sheet resistance of AZO/Ag-SnOx/AZO thin film as a function of bending cycles.


3.2. TFHs performance

Fig. 5 shows the performance of the AZO/Ag-SnOx/AZO multilayer as TFHs on flexible high-temperature PI substrate. The time-dependent temperature profiles with different input voltage in Fig. 5(a) indicated that the TFHs devices could reach a steady temperature within only a few seconds, exhibiting a quick thermal response. At an applied voltage of 16 V, the highest temperature of 438.8 °C was achieved, beyond that the PI substrate started to deform. The AZO/Ag-SnOx/AZO-based TFHs exhibited fast response and good cycle stability in repeated heating-cooling tests, as shown in Fig. 5(b). Here, the AZO/Ag-SnOx/AZO-based TFHs were compared with the ITO counterpart sputtered at room temperature with the same thickness and size (Fig. 5(c)). With the applied voltage of 10 V, the AZO/Ag-SnOx/AZO-based TFH reached a saturation temperature of 255.4 °C in just 2.5 s, while the ITO-based TFH slowly heated up after 10 s, reaching a peak temperature of only 115 °C. The difference was attributed to the much lower resistance value of the highly conductive AZO/Ag-SnOx/AZO multilayer compared with ITO layer. Fig. 5(d) shows the stable temperature these two TFHs could obtain at different voltages. As can be seen from the figure, the AZO/Ag-SnOx/AZO-based TFHs showed better performance over ITO-based TFHs on flexible substrate.

Fig. 5.

Fig. 5.   (a) Temperature profile of AZO/Ag-SnOx/AZO-based TFH under operation at different input voltages. The inset is an infrared thermal image of TFH at 16 V. (b) Heating cycles at 10 V. (c) Temperature profiles of AZO/Ag-SnOx/AZO-based TFH and ITO-based TFH at the same DC input power of 10 V. (d) Peak temperature as a function of voltage for AZO/Ag-SnOx/AZO-based and ITO-based TFHs.


The FOM of transparent electrode calculated by the ratio of optical and DC conductivities (σDC/σOP) might not be applicable to transparent heaters [34]. In a recent work by Sorel et al. [35], the thermal figure of merit (TFOM) was defined to evaluate the performance of a transparent heater. The TFOM is defined by:

$\text{TFOM}=\text{FOM}{\cdot}^{{{T}_{\text{m}}}-{{T}_{0}}}/{\cdot }_{P}A$

where P is the power supplied to the TFHs and Tm is the peak temperature corresponding to power, T0 is room temperature and A is the size of the TFHs. Using these parameters, we calculated the TFOM of flexible TFHs both for our AZO/Ag-SnOx/AZO multilayers, ITO control samples, and the data reported in previous research for other various transparent heaters such as multi-walled carbon nanotube (MWCNT) [36], graphene [37], silver nanowires (AgNWs) [38], Ag grid [39], and some hybrid materials. Fig. 6 shows the scatter plots of TFOM and response time with the maximum temperature under the unified standard, the larger the TFOM, the shorter the response time, which indicated the superior performance of the TFHs to some extent together with the higher corresponding maximum temperature. As summarized in the diagram, the AZO/Ag-SnOx/AZO-based TFH had the shortest response time and the highest peak temperature with a good TFOM, which outperformed most previous research. Therefore, with the development of colorless transparent polyimide with high temperature thermal stability, the AZO/Ag-SnOx/AZO multilayers based TFHs might have more commercial applications to flexible and high temperature occasions.

Fig. 6.

Fig. 6.   Maximum temperature obtained as a function of TFOM and response times for AZO/Ag-SnOx/AZO-based and ITO-based TFHs compared with other work.


4. Conclusion

In conclusion, we investigated the characteristics of a SnOx-doped ultrathin Ag-SnOx film and AZO/Ag-SnOx/AZO multilayer stacks on both glass and PI substrates prepared by magnetron sputtering. The best FOM of 139.9 was exhibited for AZO/Ag-SnOx/AZO multilayer stacks, which was higher than that of ITO counterpart (25.5). It was noteworthy that the AZO/Ag-SnOx/AZO thin film showed outstanding thermal durability after annealing in air at a high temperature up to 500 °C, good damp-heat stability under 36 h HAST (121 °C, 97% RH, and excellent scratch resistance and mechanical flexibility with no deterioration for 10,000 times of scratching and 1500 times of bending, respectively. In addition, compared with traditional ITO-based TFHs, the AZO/Ag-SnOx/AZO-based flexible high temperature TFH exhibited excellent response time, good cycle performance, and a very high saturating temperature of approximately 450 °C. In short, this SnOx doping strategy greatly improved the growth of ultrathin Ag film and showed great potentials in transparent flexible TFHs, especially for harsh environments and high-temperature conditions.

Acknowledgments

This work was supported financially by the National Key R&D Program of China (No. 2018YFA0209200), the National Natural Science Foundation of China (No. 61774160), the Zhejiang Natural Science Foundation (No. LY19F040003), the Ningbo Natural Science Foundation (Nos. 2018A610142 and 2017A610063), the International Cooperation Program of Ningbo (No. 2017D10005) and the Program for Ningbo Municipal Science and Technology Innovative Research Team (No. 2016B10005).

Appendix A. Supplementary data

Supplementary material related to this article can be found, inthe online version, at doi:https://doi.org/10.1016/j.jmst.2020.01.058.

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A flexible, transparent heater was manufactured by spinning multi-walled carbon nanotubes (MWCNTs) MWCNT sheets were produced by continuously pulling on well-aligned MWCNTs, and films were produced by directly coating a flexible film Single and double sheets were produced with sheet resistances of similar to 699 Omega/sq and similar to 349 Omega/sq and transmittances of 81% to 85% and 67% to 72%, respectively Both films were heated by applying a direct current, and showed rapid thermal responses and uniform temperature distribution The rate of temperature change was found to depend on the power applied Frost on the surface of these films was rapidly removed so that they may have applications as automatic defrosters in vehicles of goggles Crown Copyright (C) 2010 Published by Elsevier Ltd All rights reserved

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We demonstrate a new concept for the fabrication of flexible transparent thin film heaters based on silver nanowires. Thanks to the intrinsic properties of random networks of metallic nanowires, it is possible to combine bendability, transparency and high heating performances at low voltage, typically below 12 V which is of interest for many applications. This is currently not possible with transparent conductive oxide technologies, and it compares well with similar devices fabricated with carbon nanotubes or graphene. We present experiments on glass and poly(ethylene naphthalate) (PEN) substrates (with thicknesses of 125 mu m and extremely thin 1.3 mu m) with excellent heating performances. We point out that the amount of silver necessary to realize the transparent heaters is very low and we also present preliminary results showing that this material can be efficiently used to fabricate photochromic displays. To our knowledge, this is the first report of metallic nanowire-based transparent thin film heaters. We think these results could be a useful approach for the engineering of highly flexible and transparent heaters which are not attainable by existing processes.

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