CERAMICSINTERNATIONAL

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http://dx.doi.org0272-8842/& 20

nCorrespondinE-mail addre

(2015) 7146–7150

Ceramics International 41 www.elsevier.com/locate/ceramint

Highly flexible ZnO/Ag/ZnO conducting electrodefor organic photonic devices

Jun Ho Kima, Jeong Hwan Leeb, Sang-Woo Kimb,c, Young-Zo Yood, Tae-Yeon Seonga,n

aDepartment of Materials Science and Engineering, Korea University, Seoul 136-713, KoreabSchool of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Korea

cSKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University (SKKU), Suwon 440-746,Korea

dDuksan Hi-Metal Co. Ltd., Yeonam-dong, Buk-gu, Ulsan 683-804, Korea

Received 5 December 2014; accepted 5 February 2015Available online 12 February 2015

Abstract

We investigated the electrical, optical and bending characteristics of ZnO (40 nm)/Ag (18.8 nm)/ZnO (40 nm) multilayer film deposited onpolyethylene terephthalate (PET) substrate and compared them with those of indium-tin-oxide (ITO) (100 nm thick). The ITO single and ZnO/Ag/ZnO multilayer films gave maximum transmittance of 92.9% and �95% at �530 nm, respectively. For the ITO single and ZnO/Ag/ZnOmultilayer films, the carrier concentration was measured to be 1.19� 1020 and 6.68� 1021 cm�3, respectively and the mobility was 32.06 and21.06 cm2/V s, respectively. The sheet resistance was 175.99 and 4.98 Ω/sq for the ITO single and ZnO/Ag/ZnO multilayer films, respectively.Haacke's figure of merit (FOM) of the ITO single and ZnO/Ag/ZnO multilayer films was calculated to be 2.36� 10�3 and 104.5� 10�3 Ω�1.The ZnO/Ag/ZnO multilayer films showed dramatically improved mechanical stability when subjected to bending test.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: D. ZnO; Ag; Multilayer; Flexibility; Transparent conducting electrode

1. Introduction

Flexible transparent conductive oxides (TCOs) are technologi-cally very important for their applications in optoelectronic,photovoltaic devices, and displays [1–3]. Sn-doped indium oxide(ITO) is most commonly used because of its superior optical andelectrical properties [4,5]. However, indium is a rare and expensivemetal, which will result in a rapid increase in the fabrication costsin future applications. Thus, a variety of oxides with high trans-mittance, viz. SnO2 [6], ZnO [7], TiO2 [8], and Nb2O5 [9], havebeen widely studied to develop cost-effective TCO. In addition, forflexible organic light emitting diode (OLEDs) applications [10],transparent conducting electrodes (TCEs) must have bendingstability together with reliable electrical and optical properties.The mechanical flexibility is essential for the realization of low cost

/10.1016/j.ceramint.2015.02.03115 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ82 2 3290 3288; fax: þ82 2 928 3584.ss: tyseong@korea.ac.kr (T.-Y. Seong).

roll-to-roll process for organic optoelectronic devices [11]. To meetthe requirements, transparent oxides sandwiching a thin metal film,i.e., dielectric/metal/dielectric (D/M/D) multilayers have beenextensively investigated, including SnO2/Ag/SnO2 [12], Nb2O5/Ag/Nb2O5 [13], Al2O3/Ag/Al2O3 [14], WO3/Ag/WO3 [15], ZnSnO/Ag/ZnSnO [16], MoO3/Ag/MoO3 [17], TiInZnO/Ag/TiInZnO[18], ZrON/Ag/ZrON [19] TiO2/Ag/TiO2 [20], and ZnO/Ag/ZnO[21,22]. Ag is commonly used as the middle layer for D/M/Dmultilayers, since Ag thin films (less than 20 nm thick) show lowresistance and high transmittance in the visible spectrum. Forinstance, Yu et al. [12], investigating the effects of Ag layerthickness and SnO2 layer thickness on the electrical and opticalproperties of SnO2/Ag/SnO2 tri-layer films prepared on quartzglass substrates, reported that the SnO2/Ag/SnO2 multilayer film(50 nm/5 nm/50 nm) exhibited the maximum FOM of 6.0�10�2Ω�1 with a sheet resistance of 9.67 Ω/sq, a resistivity of1.0� 10�4 Ω cm and an average transmittance of 94.8% in thevisible region. In addition, Fan and Bachner [20] showed that a

Fig. 2. XRD patterns obtained from reference 100 nm-thick ITO film andoptimized ZnO/Ag/ZnO (40 nm/18.8 nm/40 nm) multilayer film deposited onPET substrates.

J. Ho Kim et al. / Ceramics International 41 (2015) 7146–7150 7147

TiO2/Ag/TiO2 multilayer (18 nm/18 nm/18 nm) prepared with RFsputtering gave a reflectivity of 98% at 10 μm and a transmissionof �80% in the 500–600 nm region. Sahu et al. [21] investigatedthe optical and electrical properties of ZnO/Ag/ZnO multilayerelectrodes as functions of ZnO and Ag thicknesses and reportedthat the optimum thickness of Ag films was 6 nm for high opticaltransmittance and good electrical conductivity, e.g., a sheetresistance of 3Ω/sq and a transmittance of 90% at 580 nm. Hajjet al. [22], investigating the electrical and optical properties of ZnO/Ag/ZnO multilayer electrodes prepared by ion beam sputtering forflexible optoelectronic devices, showed that the introduction of aAg layer between two ZnO layers decreased the sheet resistanceand widened the transmittance window in the visible region. TheZnO/Ag/ZnO (35 nm/10 nm/20 nm) multilayer electrode had asheet resistance of 6Ω/sq, a transmittance ofZ80% in the visibleregion, and figure of merit (FOM) of 16.5� 10�3Ω�1. In thisstudy, the optical and electrical properties of ZnO/Ag/ZnO multi-layer were also investigated and compared with those of ITO singlefilm, that were deposited with a radio frequency (RF) sputteringsystem at room temperature. FOM was also calculated tocharacterize the performance of the multilayer. The samples weresubjected to bending test to investigate their mechanical flexibility.

2. Experimental procedure

ZnO/Ag/ZnO multilayer thin films were sequentially depositedon polyethylene terephthalate (PET) substrates by an RF magne-tron sputtering system. Ceramic ZnO target (99.99% purity) andpure Ag target (99.99% purity) were used at room temperatureunder a base pressure of less than 1� 10�6 Torr. Before beingloaded into the sputtering chamber, the PET substrates (1.5�1.5 cm2) were cleaned with methanol and deionized water for15 min per cleaning agent in an ultrasonic bath, and finally dried ina N2 stream. Prior to deposition, the ITO, ZnO and Ag targets werepre-sputtered for 30 min to remove contaminants. ZnO and Agwere deposited using RF powers of 90 W and 30 W at Ar flowrates of 30 sccm and 13 sccm, respectively, under a workingpressure of 10 mTorr. During sputtering, the PET substrate wasconstantly rotated at a speed of 12 rpm for ZnO and 23 rpm forAg. The ZnO thickness of 40 nm and Ag thickness of 18.8 wereselected based on the dependence of ZnO thickness on the opticaland electrical properties [23]. For comparison, 100 nm-thick ITO

Fig. 1. A cross-section TEM image of ZnO/Ag/ZnO (40 nm/18.8 nm/40 nm)multilayer film grown on PET substrate.

thin films were also prepared using pure ITO target (99.99%purity). The thickness of the multilayer films was determined withhigh resolution transmission electron microscopy (HR-TEM, JEM-ARM, 200 F, JEOL). For instance, Fig. 1 shows a cross-sectionHR-TEM image of a ZnO/Ag/ZnO (40 nm/18.8 nm/40 nm) multi-layer grown on the PET substrate. It can be seen that the individuallayers are well defined. Hall measurements by the van der Pauwtechnique were carried out with a magnetic field of 0.55 T (HMS3000, Ecopia). The four-point-probe technique was used for sheetresistance measurements. Transmittance of the multilayers wasmeasured with a UV/visible spectrometer (UV-1800, Shimadzu).The crystal structure of the multilayers was determined with X-raydiffraction (XRD, ATX-G, Rigaku). The mechanical flexibility ofthe samples was analyzed using a bending test system (ZBT-200,Z-tec). The samples were clamped between two parallelsemicircular-plates. One plate was mounted to the shaft of amotor, while the other was fixed to a supporter. The distance of thestretched mode was 80 mm and that of the bent position was35 mm. The bending radius was approximately 9 mm and thebending frequency was 1 Hz. Finally, the resistance of the samplesduring the bending was measured using a multimeter.

3. Results and discussion

Fig. 2 shows the XRD patterns from the reference 100 nm-thick ITO film and optimized ZnO/Ag/ZnO (40 nm/18.8 nm/40 nm) multilayer film deposited on the PET substrates. TheZnO/Ag/ZnO sample has peaks at 2θ¼34.21 and 64.61 thatcorrespond to the (002) and (103) planes of ZnO, respectively.In addition there is a peak at 2θ¼38.21, corresponding to the(111) plane of Ag (JCPDS no. 65-3411 and 87-0720). On theone hand, the ITO sample have peaks at 2θ¼30.51, 35.41, and511, corresponding to the (222), (400), and (440) planes(JCPDS card no. 76-0152).Fig. 3 shows the transmittance spectra obtained from the ITO

single film and the ZnO/Ag/ZnO multilayer film. The transmittanceof the multilayer film reaches an overall maximum and thengradually decreases with increasing wavelength, while that of ITOreaches an overall maximum and then slightly decreases. The

Fig. 3. Transmittance spectra obtained from ITO single and ZnO/Ag/ZnOmultilayer films.

Table 1The carrier concentration, Hall mobility, resistivity and sheet resistanceobtained from ITO single and ZnO/Ag/ZnO multilayer films.

Sample Carrier concentration[cm–3]

Mobility[cm2/V s]

Resistivity[Ω cm]

Sheet resistance[Ω/sq.]

ITO 1.19� 1020 32.06 1.62� 10–3 175.99ZnO/Ag/ZnO

6.68� 1021 21.06 4.43� 10–5 4.98

J. Ho Kim et al. / Ceramics International 41 (2015) 7146–71507148

maximum transmittance is measured to be 92.9% and �95% atabout 530 nm for the ITO and ZnO/Ag/ZnO samples, respectively.

The carrier concentration and Hall mobility of the ITO singleand ZnO/Ag/ZnO multilayer films were characterized, as shownin Table 1. The carrier concentration was determined to be1.19� 1020 and 6.68 � 1021 cm–3 for the ITO single and ZnO/Ag/ZnO multilayer films, respectively. On the one hand, themobility was 32.06 and 21.06 cm2/V s for the ITO single andZnO/Ag/ZnO multilayer films, respectively. Carrier mobilitybehavior is usually described in terms of scattering mechanismssuch as phonon scattering, grain-boundary scattering, surfacescattering, interface scattering, and ionized-impurity scattering[24]. Since the crystalline ZnO films are undoped, interfacescattering may be dominant at the ZnO/Ag interfaces.

The resistivity and sheet resistance of the ITO single andZnO/Ag/ZnO multilayer films were characterized, as shown inTable 1. The sheet resistance was measured to be 175.99 and4.98 Ω/sq for the ITO single and ZnO/Ag/ZnO multilayerfilms, respectively. Meanwhile, the resistivity was 1.62� 10–3

and 4.43� 10–5 Ω cm for the ITO single and ZnO/Ag/ZnOmultilayer films, respectively. It is noted that the ITO singlefilm shows higher sheet resistance and resistivity than the ZnO/Ag/ZnO multilayer film. The resistivity is inversely propor-tional to the mobility and the carrier concentration [25]. Thisimplies that in our samples the resistivity is dominated by thecarrier concentration, as shown in Table 1.

FOM (φTC) of the ITO single and ZnO/Ag/ZnO multilayerfilms was calculated using the equation defined by Haacke

[26], as given below

φTC ¼ T10av

Rsð1Þ

where Rs is the sheet resistance and Tav is the average opticaltransmittance (in the range 450–780 nm). Tav can be estimatedusing the relation shown below

Tav¼RVðλÞTðλÞdλRVðλÞdλ ð2Þ

where T(λ) is the transmittance and V(λ) is the photopicluminous efficiency function defining the standard observerfor photometry [12,27]. The ZnO/Ag/ZnO multilayer filmgives FOM of 104.5� 10�3Ω�1. However, the FOM issignificantly lower for the ITO single film, with a value of2.36� 10�3Ω�1. As both the samples show similar transmit-tance, the higher FOM for the ZnO/Ag/ZnO sample can beattributed to the dominant contribution of low sheet resistance.Fig. 4 shows the change in resistance of the ITO single and

ZnO/Ag/ZnO multilayer films on a PET substrate as a functionof cycle. The insets of Fig. 4 exhibit the bending testers used.The change in resistance was defined as R�R0/R0, where R0 isthe initial resistance and R is the measured resistance afterbending. The value of R�R0/R0 for the ITO single filmdramatically increases even after two cycles, Fig. 4(a). Thisis directly attributable to the generation and propagation ofcracks. On the other hand, the value of R�R0/R0 for the ZnO/Ag/ZnO multilayer film remains almost constant, as can beseen in Fig. 4(b). This implies that the resistance of the ZnO/Ag/ZnO multilayer film remains almost unchanged during thebending test. This electrical stability is due to the presence ofthe ductile Ag middle layer [28]. Lewis et al. [28] investigatedthe optical, electrical, and bending properties of ITO/Ag/ITOmultilayer electrodes for organic light-emitting diodes andreported that the ITO/Ag/ITO multilayer electrodes exhibitedsignificantly lower sheet resistance and higher average trans-mittance (in the range 450–650 nm) than ITO only films. Thesignificantly improved mechanical property of the ITO/Ag/ITOmultilayer electrodes was attributed to the use of ductile Aglayer with higher failure strain (4–50%) [29].It was shown that the ZnO/Ag/ZnO samples yielded rather high

optical transmittance at 550 nm. It is known that a large differencebetween the refractive indices of Ag and the dielectric layer cancause efficient plasmon coupling, resulting in a visible transmittancehigher than 80% [30]. Thus, the high transmittance of our ZnO/Ag/ZnO multilayers may also be attributed to surface plasmonresonance in the Ag layers when using an optimal ZnO thicknessof 24–56 nm. Furthermore, the optical transmittance at wavelengthslonger than 700 nm is higher in the ZnO/Ag/ZnO multilayers thanin ITO. This may be caused by plasmon-absorption-dependentreflections due to the increasing carrier concentration [12].

4. Summary and conclusion

The opto-electrical and bending characteristics of ITO andZnO(40 nm)/Ag(18.8 nm)/ZnO(40 nm) multilayer film deposited

Fig. 4. Change in resistance of (a) ITO single and (b) ZnO/Ag/ZnO multilayer films on PET substrates as a function of cycle. The insets show a bending test systemused in this work.

J. Ho Kim et al. / Ceramics International 41 (2015) 7146–7150 7149

on polyethylene terephthalate (PET) substrate by RF sputteringwere characterized. The ZnO/Ag/ZnO multilayer films hadimproved transmittance at 550 nm and significantly better elec-trical properties compared to the 100-nm-thick ITO single film.The ZnO/Ag/ZnO multilayer film exhibited much higherHaacke's FOM than the ITO. The bending test results showedthat the ZnO/Ag/ZnO multilayer electrode demonstrated drama-tically improved mechanical stability than the ITO single film.The result shows that ZnO/Ag/ZnO multilayer film can be usedas an important transparent multilayer electrode in flexibleorganic photovoltaic and photonic devices.

Acknowledgments

This work was supported by the Brain Korea 21 PlusProgram funded by the Ministry of Science, ICT and FuturePlanning, Korea, and Ministry of Trade, Industry and Energyunder Grant no. 10049601.

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