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Sensors and Actuators A 250 (2016) 123–128

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

hotoresist-assisted fabrication of thermally and mechanically stableilver nanowire-based transparent heaters

anseok You a,b, Byeong-Kwon Ju b,∗, Jong-Woong Kim a,∗

Display Materials & Components Research Center, Korea Electronics Technology Institute 68 Yatap-dong, Bundang-gu, Seongnam 463-816, Republic oforeaDisplay and Nanosystem Laboratory, College of Engineering, Korea University, Seoul, 136-713, Republic of Korea

r t i c l e i n f o

rticle history:eceived 6 July 2016eceived in revised form 2 September 2016ccepted 15 September 2016vailable online 17 September 2016

eywords:ilver nanowirelexible transparent electrodehotoresist

a b s t r a c t

Networked structures of percolated silver nanowires (AgNWs) are an important substitute for brittleindium tin oxide (ITO)-based transparent electrodes, owing to their high ductility and tunable opticaland electrical conductivities. Recently, AgNWs have been used in the fabrication of flexible transpar-ent heaters, but only when firmly adhered to the underlying polymer substrate so that the electrodesare reliably flexible. Another requirement is that these electrodes must be passivated from the atmo-sphere, preserving them even when the fabricated heaters are biased at high voltages or exposed toharsh environments. Here, we used conventional photolithography with a coating of commercial pho-toresist, UV exposure and development, in order to make protected AgNW networks. For this, AgNWnetworks preformed on a transparent polymer were used as a photomask layer, so that the photoresist

ransparent heaterhotolithography

could be developed to be selectively present on the AgNWs. As a result of this simple approach, themechanical/thermal stability and heating performance of our AgNWs-based transparent heaters weresuccessfully enhanced. It displays an increase of <2% in Rs when bent to a radius of 500 �m for 10,000cycles, and a biasing voltage to the heater rapidly increased its temperature above 160 ◦C within a very

short time period.

. Introduction

Silver nanowires (AgNWs) are a promising material for flexi-le and transparent electrodes by virtue of their high electricalonductivity, high aspect ratio (resulting in low-density perco-ation), and their intrinsic high ductility [1–10]. A number ofifferent approaches have been suggested for using AgNWs tochieve diverse structural configurations that could be employedn the fabrication of organic light emitting diodes [11,12], organichotovoltaics [13–15], touch sensors [16,17] and pressure sensi-ive devices [18,19]. Recently, AgNW-based structures have alsoeen used to produce transparent heaters, made possible by their

ow surface resistivity and high chemical stability. For instance,im et al. reported that uniformly interconnected AgNW networksould be successfully employed in the fabrication of transparent

lm heaters, and the fabricated heaters simultaneously possessigh transparency and good heating capability; these could beeated above 100 ◦C within 50 s [20]. According to Ji et al., the

∗ Corresponding authors.E-mail addresses: bkju@korea.ac.kr (B.-K. Ju), wyjd@keti.re.kr,

yjd78@gmail.com (J.-W. Kim).

ttp://dx.doi.org/10.1016/j.sna.2016.09.021924-4247/© 2016 Elsevier B.V. All rights reserved.

© 2016 Elsevier B.V. All rights reserved.

thermal response of AgNW-based transparent heaters could beenhanced by hybridization with poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) [21]. Other materials such asgraphene [22] and metal oxides [23] have also been used to enhancethe heating performance of AgNW-based heaters.

Meanwhile, enhancing the mechanical stability of AgNWelectrodes has been extensively studied, focusing primarily onincreasing AgNW adhesion to the underlying polymer [24,25].Using a transparent adhesive between the nanowire network andthe polymer is an effective and intuitive method to firmly attachAgNW networks onto polymers [24,25]. However, increased filmthickness, decreased transparency (due to the addition of an inter-layer), and deteriorated processibility (by the sticky properties ofthe adhesive) are relevant issues with this method. Moreover, mostof the nanowires in this case would be exposed to air, which canpromote melting or oxidation when heated. Inverted layer pro-cessing was developed to fully bury these AgNW networks intothe surface of the transparent polymers [11,26]. Highly enhancedmechanical stability and thermal stability can be achieved by

employing this method, but a peeling-off procedure is requiredafter curing the polymer to flip over the fabricated film. This causesthe surface of the electrode to be exposed such that a device can befabricated or power sources can be connected. However, inverted

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24 B. You et al. / Sensors and A

ayer processing can sometimes be time-consuming and result inefects, such as extruded AgNWs from the surface of the polymer.urthermore, the procedure is typically not compatible with theork flow of commonly used fabrication procedures (deposition of

lectrode materials on a transparent polymer and subsequent pat-erning). This implies that a simpler and well-established methods still needed to fabricate mechanically and thermally stable elec-rodes that can be used as flexible transparent heaters, withoutmploying any additional materials or peeling-off procedures.

Herein, we used a photoresist (PR)-assisted approach toake mechanically and thermally stable AgNW-based transparent

eaters. A positive-type PR was employed to selectively preservehe AgNWs coated on a transparent polymer. A regular photolithog-aphy (PL) procedure was employed to develop the PR layer andreserve the nanowire network. The amount of PR coverage onhe nanowires and their adhesion to the underlying polymer wereetermined, as were the effects of PR coverage on the heating per-

ormance and optical transparency.

. Materials & methods

A schematic of the surface modification with AgNWs is shownn Fig. 1. Here, a glass substrate was first cleaned sequentially withetergent, de-ionized water and isopropanol, and then a varnishf colorless polyimide (cPI: Kolon Industries INC., Korea) was spinoated onto it. The sample was annealed at 200 ◦C for 1 h to form

cPI film 20 �m in thickness on the glass. Several drops (0.3 mL)f an AgNW-containing ink (Nanopyxis Ltd., Korea) (average wireiameter and length of 30 nm and 20 �m, respectively) were thenpplied, and a #8 Mayer rod (R.D. Specialties, Inc., USA) was imme-iately rolled over the surface to evenly spread the ink across thelass surface. A commercial positive-type PR (AZ GXR-601, AZ-EM,SA) was spin coated onto the AgNW electrode at a rotation speedf 3000 rpm for 30 s, followed by baking at 90 ◦C for 3 min. The sam-le was exposed to UV light (MA6/BA6, SUSS MicroTec, Germany)

rom the glass side as described in the 5th sequence of Fig. 1, andhen developed by dipping in a positive-type developer (DPD-200,ongjin Semichem, Korea). Once the cover material (developedR) was selectively formed on the AgNW network, the sample wasoaked in water (25 ◦C) to help safely peel the film from the sup-orting glass through hygroscopic swelling of the cPI film.

The optical transmittance of the films was measured using aV–vis spectrophotometer (Jasco V-560, Japan), while the sheet

esistance (Rs) was measured with a non-contact measurementystem (EC-80P, Napson Corp., Japan). The surface morphology waseasured by atomic force microscopy (AFM; XE-100TM, Park Sys-

ems, USA). A field-emission scanning electron microscope (FESEM;SM6700F, JEOL Ltd., Japan) was used to investigate the microstruc-ures of the AgNW networks. The mechanical stability of the filmas evaluated using an automatic bend-testing machine (Bend-

ng tester, Jaeil Optical Systems, Korea), whereby bending radiusesf 0.5 mm and 0.1 mm were used to induce ∼2% and ∼10% strain,espectively. The films were bent at a cycle rate of 0.3 Hz, with theiresistance being measured during the outward bending cycles. Forpplying voltage to the fabricated heaters, a source meter (KEITH-EY, 2430 1KW Pulse Source Meter, USA) was used. A thermocouplencorporated in a multimeter (KEITHLEY, 7700 20 Chan Multiplex,SA) and IR camera (FLIR, T335, USA) was employed to measure

he temperature of the heaters during voltage application.

. Results and discussion

Here, we selected cPI as a substrate for flexible transparenteaters because of its high glass transition temperature (>350 ◦C),igh modulus of elasticity, and high transparency [11]. First, we

ors A 250 (2016) 123–128

investigated the effects of the PL on the microstructure of theAgNW-based electrodes formed on the cPI film. Fig. 2a and crespectively shows FESEM and AFM images of AgNWs depositedon cPI, while those after PL application are shown in Fig. 2b and d.The pristine AgNWs in Fig. 2a demonstrate that the edges of theas-deposited AgNWs are quite clear, and there is no discerniblematerial covering the AgNWs. In this case, the individual nanowiresare irregularly stacked, resulting in a porous networked structureas can be seen in Fig. 2c as well. High peak to valley roughness(Rpv) indicates that the contact areas and adhesion between thenanowires and cPI are largely limited, originating primarily fromthe low surface energy of the cPI. The poor adhesion makes it dif-ficult to achieve high mechanical stability, implying that a simpleand post-processible method is necessary at this stage for furtherenhancement. The AgNW network preformed on a transparent sub-strate could be used as a photomask layer to shield from UV lightwhen irradiated from the substrate side [27]. After development,the non-irradiated PR selectively remains on the AgNWs as shownin Fig. 2b, while the irradiated PR was removed from the vacantareas between the nanowires. Fig. 2d shows that the roughnessvalues were increased by this procedure. This revealed that the self-masking concept was successful, in that the PR residues elevatedonly the highest areas (top of the PR on AgNWs) without increasingthe lowest areas (valleys on the surface of cPI).

A powerful benefit that could be obtained by this approach isthat the transmittance is not noticeably affected by whole layerdeposition. In Fig. 3, the transmittance and haziness of the cPI,AgNWs/cPI, PR/AgNWs/cPI and developed PR/AgNWs/cPI are com-pared. Over a very broad spectral range of 420–750 nm, all samplesexcept the PR/AgNWs/cPI exhibit high transmittance ( > 80%). Asteep decrease in transmittance near 400 nm originates from thehigh light absorption of aromatic compounds in cPI and the for-mation of charge transfer complexes in their highly conjugatedmolecular structures. A deposition of PR onto the AgNWs/cPIresulted in a large decrease in transmittance and increase in hazi-ness due to the poor transparency of the PR. Of note, excess PRon the vacant sites could be successfully removed by subsequentdevelopment, and the overall optical performance of the electroderecovered to that of the AgNWs/cPI. Considering that the Rs ofthe fabricated electrode was approximately 35 ohm/sq, this per-formance is comparable to that of commercially available ITO films(Rs: 100 ohm/sq and transmittance: 88%). Rs was not affected bythe PL procedures.

In order to investigate the heating performance of the fabricatedtransparent heaters (30 mm by 30 mm in dimension), input volt-ages were supplied to the heating films through two-terminal sideAg electrodes, forming stable contacts with the heaters along oppo-site edges. Fig. 4a and b respectively shows the heating profilesof the AgNWs/cPI and the developed PR/AgNWs/cPI heaters as afunction of time. The temperatures of both heaters were exponen-tially increased from room temperature to 80% of their steady-statetemperatures in less than 50 s. The high lateral conductivity of thepercolated AgNW networks caused the rapid and efficient heat-ing performances of these heaters. The increase in the bias voltagefrom 3 V to 8 V elevated the steady-state temperature from 27 ◦Cto 73 ◦C. Applying a bias voltage of 9 V damaged the heaters beforethey could reach a steady-state temperature. This indicates thatpure AgNWs deposited on a polymer film are not suitable forhigh-performance heaters, since the fully exposed nanowires arenot resistant to biasing. When biased with higher voltages, theunstable nanowire junctions exert a detrimental effect on the net-work stability, resulting in oxidation, melting or breaking of the

AgNWs. Surprisingly, the developed PR selectively formed on theAgNW networks enhanced the thermal stability of the heaters asshown in Fig. 4b. The heaters with developed PR could sustainmuch higher voltages, reaching 14 V without showing any signs of

B. You et al. / Sensors and Actuators A 250 (2016) 123–128 125

iption

ehphctte

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Fig. 1. Schematic descr

lectrical breakdown. Under an applied bias voltage of 15 V, theeater suffered breakdown before reaching a steady-state tem-erature. By applying a bias voltage of 14 V, the heater could beeated above 160 ◦C mainly due to the increased current-carryingapacity. Recalling that the electrical and optical performances ofhe AgNWs/cPI and the developed PR/AgNWs/cPI are nearly iden-ical, the enhancement is mainly caused by the passivation of thexposed nanowires.

Given that the goal of this study is to produce a mechanicallyexible heater, the passivated nanowires must be able to resisthe external energy used to detach them from the surface of the

olymer. To test this, the fabricated heater was first immersed in aater bath with sonication for up to 300 s. Fig. 5a shows the effects

f varying the sonication time on the Rs of the samples, with the

Fig. 2. FESEM image of (a) AgNWs/cPI and (b) developed PR/AgNWs/cPI. A

for sample fabrication.

measured resistance increasing from 20 s onwards due to a lossof network rigidity for the case of bare AgNWs/cPI. However, theRs of the developed PR/AgNWs/cPI did not significantly vary withsonication for up to 300 s, revealing that the adhesion betweenthe electrodes and polymer could be easily enhanced using a PR-assisted approach. A typical tape test was also employed to evaluatethe adhesion (Fig. 5b). Distinct from that of bare AgNWs/cPI, thedeveloped PR cover AgNWs were not detached from the polymerby repeated tape testing of (up to 15 repetitions). A more interestingbenefit that could be obtained by this method is shown in Fig. 5c;passivation using PR delayed chemical dissolution of the AgNWs,

even when the heater was immersed in an Ag etchant. In the caseof the AgNWs/cPI, the Rs instantly increased once the sample wasdipped into the etchant, but the Rs of the developed PR/AgNWs/cPI

FM topography of (c) AgNWs/cPI and (d) developed PR/AgNWs/cPI.

126 B. You et al. / Sensors and Actuators A 250 (2016) 123–128

Fig. 3. (a) Transmittance and (b) haziness spectra of the cPI, AgNWs/cPI, PR/AgNWs/cPI, and developed PR/AgNWs/cPI.

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Fig. 4. Measured temperature as a function of tim

id not vary for 30 s and then only began to increase slowly. Thisinor increase is possibly due to leaks formed on the PR surface.verall, the physical and chemical stability of the AgNWs/cPI heateras largely enhanced using this simple PL-based approach.

The heating stability of the developed PR/AgNWs/cPI was alsovaluated using a long-term biasing for 7000 s with a constant volt-ge of 12 V, as shown in Fig. 6a. The temperature distribution waslso given in an inset figure. The temperature of the heater rapidlypproached 125 ◦C, and then remained stable without any large

uctuations or decreases during the remainder of the test period.

t is well known that AgNWs are easily oxidized when exposed toir, especially at high temperatures [28]. If the nanowires are oxi-ized, the resistance of the single nanowires and their junctions

Fig. 5. Resistance changes of the fabricated heaters with (a) sonication time in

(a) AgNWs/cPI and (b) developed PR/AgNWs/cPI.

would increase due to the formation of silver oxides at the surface,resulting in decreased heating efficiency [29]. The stable heatingshown in Fig. 6a indicates that the passivation by the PL was veryeffective for suppressing nanowire oxidation. This implies that theheater is practical for many real applications such as in anti-foggingtransparent films for flexible displays or skin-like transparent patchheaters.

In order to achieve transparent heaters that are mechanicallystable under continuous or repeated mechanical deformations, we

also performed mechanical tests on all heaters fabricated in thisstudy. The samples were subjected to long-term cyclic bendingtests to outward radii of 500 �m. The developed PR-assisted heatersdisplay excellent bending fatigue strength; Fig. 6b demonstrates

water bath, (b) number of tape tests, and (c) dipping time in Ag etchant.

B. You et al. / Sensors and Actuators A 250 (2016) 123–128 127

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ig. 6. (a) Measured temperature of a developed PR/AgNWs/cPI heater at a consturvature radius, (c) measured temperature of the heater during cyclic bending to a

hat the resistance increases by <2% even after 10,000 cycles. Whenompared to the flexibility of bare AgNWs/cPI heaters in the sameraph, the flexibility of the AgNW-based heaters was significantlymproved by the PR overcoat. Fig. 6c also shows the stability atncreased temperatures during repetitive severe mechanical defor-

ation induced by cyclic bending to a radius of 100 �m. Theemperature was not largely affected even at the small bendingadii, revealing high flexibility of the fabricated heaters. This high

echanical durability originates from the unique structure of theeveloped PR/AgNWs/cPI, in that fully embedding the nanowireselow the selectively formed PR enlarges the adhesion betweenhe electrode and cPI. The good adhesion of cPI to the developedR contributes to the high mechanical stability and high thermaltability of the heaters. The reproducibility of our heater was alsovaluated: an instance for the temperature measurement with aepeated biasing of 14 V is given in Fig. 6d. It shows that the heat-ng performance could be successfully reproduced, implying thathe approach introduced here has a potential to be employed in

anufacturing commercial products. The improved heating perfor-ance such as power conversion efficiency was still lowered than

hose of the hybrid heaters reported [20–22]. Considering that thexceptionally high mechanical stability was achieved just by a sim-le post processing, however, we believe that our method is highlyaluable to achieve both the mechanical/thermal stability as wells heating performance in a practical manner.

our heaters are one the flexible transparent heaters having both

he mechanical stability even under the severe stressing environ-

ents and reproducible heating performance.

V voltage, (b) resistance change of the heater during cyclic bending to a 500 �mm curvature radius, and (d) measured temperature for repeated biasing of 14 V.

4. Conclusion

A mechanically and thermally stable AgNWs/polymer-basedheater was successfully fabricated by PR-assisted passivation. Aconventional PL procedure involving a PR coating and UV expo-sure and development resulted in the selective deposition of PRonto preformed AgNWs on cPI. The transmission and haziness spec-tra for the AgNWs/cPI and developed PR/AgNWs/cPI revealed thatthe optical performance was not impacted by this method. How-ever, this unique structural configuration provided the fabricatedheaters with very good mechanical stability, displaying an increaseof <2% in Rs when bent to a radius of 500 �m for 10,000 cycles. Rs

was also stable when the samples were dipped in sonicated waterbaths or when tested by tape attach-and-detach for over 10 cycles.The heating performance of the AgNWs-based heaters was also sig-nificantly enhanced by PR passivation, which largely improved theheating capability of the electrodes in terms of both steady-statetemperature and current-carrying capacity. The application of aconstant voltage to this heater rapidly increased its temperatureabove 160 ◦C within a very short time period. Considering that themethod suggested here is very simple and post-processible, thisexample is expected to be a practical guideline for the fabricationof flexible transparent heaters.

Acknowledgements

This work was supported by the New & Renewable Energy CoreTechnology Program of the Korean Institute of Energy TechnologyEvaluation and Planning (KETEP), granted financial resource from

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Biographies

Mr. Banseok You Mr. You is pursuing the Ph.D. inDepartment of Electrical Engineering from Korea Uni-versity under the supervision of Prof. Byeong-Kwon Ju.He received his B.S. degree in Department of Controland Instrumental Engineering in 2011, and M.S. degreein Department of Electronics and Information Engineer-ing from Korea University in 2014. Currently, he is alsoworking in Korea Electronics Technology Institute as aResearcher under a supervision of Dr. Jong-Woong Kim.

Prof. Byeong-Kwon Ju Prof. Ju received the B.S. and M.S. inDepartment of Electronic Engineering from University ofSeoul, Seoul, Republic of Korea, in 1986 and 1988, respec-tively. He received the Ph.D. in Department of ElectronicEngineering from Korea University, Seoul, Republic ofKorea, in 1995. In 2005, he joined Korea University, wherehe is currently pursuing developments of nanotechnolo-gies in order to achieve various advanced electronics.

Dr. Jong-Woong Kim Dr. Kim received the B.S., M.S. andPh.D. in School of Advanced Materials Science and Engi-neering from Sungkyunkwan University, Suwon, Republicof Korea, in 2001, 2004 and 2008, respectively. Hisresearch interests in those period were developmentof novel materials and processes to achieve micro andnano-scale interconnects using metals, polymers and

ogy Institute, where he is currently pursuing developmentof flexible, stretchable, self-healable and transparent elec-trodes that could be widely employed in various devicesfrom flexible displays to bio-compatible wearable sensors.