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Flexible transparent electrodes for organic light-emitting diodesTae-Hee Hana, Su-Hun Jeonga, Yeongjun Leea, Hong-Kyu Seoa, Sung-Joo Kwona, Min-Ho Parka

& Tae-Woo Leeaa Department of Materials Science and Engineering, Pohang University of Science andTechnology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang 790-784, Gyeongbuk, Republicof KoreaPublished online: 19 Mar 2015.

To cite this article: Tae-Hee Han, Su-Hun Jeong, Yeongjun Lee, Hong-Kyu Seo, Sung-Joo Kwon, Min-Ho Park & Tae-Woo Lee (2015): Flexible transparent electrodes for organic light-emitting diodes, Journal of Information Display, DOI:10.1080/15980316.2015.1016127

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Journal of Information Display, 2015http://dx.doi.org/10.1080/15980316.2015.1016127

INVITED PAPER

Flexible transparent electrodes for organic light-emitting diodes

Tae-Hee Han, Su-Hun Jeong, Yeongjun Lee, Hong-Kyu Seo, Sung-Joo Kwon, Min-Ho Park and Tae-Woo Lee∗

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong,Nam-gu, Pohang 790-784, Gyeongbuk, Republic of Korea

(Received 2 October 2014; accepted 22 January 2015 )

The use of flexible organic light-emitting diodes (OLEDs) for the next-generation displays and solid-state lightings has beenconsidered, but the widely used transparent conducting electrode (TCE), indium–tin-oxide (ITO), should be replaced byflexible electrodes due to its brittleness and increasing cost. Therefore, many kinds of alternative TCEs have been increas-ingly studied. In this paper, the properties and applications of the candidate transparent flexible electrodes classified into fourcategories (conducting polymer, silver nanowire, carbon nanotube and graphene) are described. This paper finally suggestshow to develop alternative TCEs for replacing the conventional ITO electrode.

Keywords: organic light-emitting diodes; flexible electrode; flexible display

1. IntroductionOrganic light-emitting diodes (OLEDs) have attracted agreat deal of attention for their potential use for thenext-generation flexible displays and solid-state lightings.OLEDs are flexible, lightweight, and thin and can beproduced in large sheets; as such, they have potentialapplications in flexible and wearable displays [1–5]. Forsuch applications, however, they must also have flexibleelectrodes. The conventional bottom-emission OLEDs arefabricated on a substrate coated with a transparent conduct-ing electrode (TCE) and emit light through it. Transparentconducting oxides (TCOs) such as indium–tin-oxide (ITO)are commonly used as anodes in the bottom-emissionOLEDs. A high work function (WF) and high electricalconductivity are also requisites for efficient charge injec-tion into OLEDs. An anode with a low WF forms a largehole injection energy barrier between the anode and theoverlying organic layers, and as such it cannot provideefficient hole injection into OLEDs. The low electrical con-ductivity of TCE also prevents charge conduction fromthe TCE and thereby increases the operating voltage ofOLEDs. ITO has been widely used in OLEDs because itfulfills the requirements of TCE (WF: 4.7–4.9 eV; opti-cal transmittance (OT): > 90% at a 550 nm wavelength;sheet resistance (Rsh): ∼ 10 �/sq) [6,7]. ITO, however, isnot suitable for use as a TCE in flexible OLEDs becauseit has limited flexibility [8–10], is increasingly expensive,and causes device degradation due to the metal atom dif-fusion from ITO into the adjacent organic layers duringdevice operation [11,12]. Therefore, finding a flexible TCE

to replace ITO is a main objective in the development offlexible OLEDs.

So that it can be used in reliable flexible electrodes,the TCE must have high OT ( > 80%) in the visible spec-trum range, low Rsh for the reduction of ohmic powerloss, high mechanical strength, thermal stability, adhe-sion with the substrate, and chemical resistance againstorganic solvents for electrode stability. In this paper, thecandidate flexible electrodes are classified into four cat-egories: conducting polymer, silver nanowire (Ag NW),carbon nanotube (CNT) and graphene. These have a vari-ety of properties and will be reviewed sequentially in thefollowing sections.

2. Conducting polymerConducting polymers have properties, including flexi-bility, that make them candidates for TCEs in flexibleOLEDs. The most widely studied conducting polymer is acomplex of poly(3,4-ethylenedioxythiophene) and poly(4-syrenesulfonate) (PEDOT:PSS) (Figure 1), which is com-mercially available. Thin films of PEDOT:PSS dispersed inwater as gel particles can be easily fabricated using simpleand cheap solution-based processes, such as spin coating,bar coating [14], inkjet printing [15], and stamping print-ing [16,17], which are suitable for roll-to-roll production.Recently, the electrical conductivity of PEDOT:PSS filmswas increased to 4380 S/cm through H2SO4 post-treatment[18]. Many research groups have reported flexible OLEDsin which PEDOT:PSS thin films are used as TCEs [19–24].

∗Corresponding author. Email: twlee@postech.ac.kr

ISSN (print): 1598-0316; ISSN (online): 2158-1606

© 2015 The Korean Information Display Society

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Figure 1. Chemical structure of PEDOT:PSS. [Reprinted fromMengistie et al. [13], © 2013, with permission from The RoyalSociety of Chemistry]

The insufficient electrical conductivity of PEDOT:PSSis an impediment to its use as a TCE in an OLED. Theelectrical conductivity of PEDOT:PSS comes from thecharged doping in the PEDOT backbone, and the PEDOTchains are positively doped. The PSS stabilizes the positivecharges and makes the PEDOT chains dispersible in water.The main method of increasing the electrical conductiv-ity of PEDOT:PSS is to effectively separate the conductivePEDOT domains from the insulating PSS domains and toenhance the π–π coupling of the PEDOT chains. To sep-arate the PEDOT and PSS domains, several types of polarsolvent additives have been used [13,25–35]. Methods ofremoving the PSS chains in PEDOT:PSS thin films havebeen reported and can effectively improve their electricalconductivity [18–24]. Several polar solvents that can beused to improve the electrical conductivity of PEDOT:PSSelectrodes have been reported (Table 1) [13,28–35]. Small

amounts of polar solvents can effectively separate thePEDOT and PSS chains by reducing the Coulomb interac-tion between the positively charged PEDOT chains and thenegatively charged PSS chains [36]; as a result, the π–πcoupling of the conducting PEDOT chains can be enhancedand the electrical conductivity of the PEDOT:PSS films canbe increased by several orders of magnitude [32].

The phase separation between the PEDOT and PSSchains can be observed through atomic force microscope(AFM) images. The phase AFM image of the PEDOT:PSSfilms with 6% ethylene glycol showed more fiber-likeinterconnected conductive PEDOT chains compared withthe pristine PEDOT:PSS films (Figure 2(a)). Also, thetopographic AFM images show that the addition of ethy-lene glycol increases the sizes of the particles formed bythe PEDOT:PSS (Figure 2) [17].

The polar solvent vapor annealing (PSVA) of thePEDOT:PSS films (Figure 2(e)) induces phase separationbetween the PEDOT and PSS chains more effectively thandoes annealing in an ambient atmosphere,and improves theelectrical conductivity of the PEDOT:PSS films [23]. Com-pared with annealing in an ambient atmosphere, PSVAtakes a much longer time to achieve phase separationbetween the PEDOT and PSS chains due to the slow evap-oration of the polar solvents in the PEDOT:PSS films. As aresult, PSVA with dimethyl sulfoxide (DMSO) increasedthe electrical conductivity of the PEDOT:PSS films to1050 S/cm, whereas the electrical conductivity of the filmsspin-coated from the water solution containing a DMSOadditive and annealed in an ambient atmosphere increasedto only 725 S/cm.

Removing the PSS chains from the PEDOT:PSS filmsgreatly increases its electrical conductivity [18,29–36].Recently, the electrical conductivity of the PEDOT:PSS

Table 1. Summary of polar solvent additives for improving the electrical conductivity ofPEDOT:PSS.

Polar solvent additive Chemical structure Max. electrical conductivity (S/cm) Ref.

Dimethyl sulfoxide 526 (Clevios PH510) [27]

Ethylene glycol 1418 (Clevios PH1000) [31]

Polyethylene glycol 805 (Clevios PH1000) [13]

Formic acid 2050 (Clevios PH1000) [29]

Glycerol monostearate 1019 (Clevios PH1000) [35]

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(a) (b)

(c) (d)

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Figure 2. AFM images (1 × 1 μm) of the PEDOT:PSS films: (a, c) pristine and (b, d) treated with 6% ethylene glycol. (a) and (b) arephase images while (c) and (d) are topographic images. (e) Conventional polar solvent additive method and PSVA. (f) Schematic diagramof the structural reorganization of the PEDOT:PSS films through H2SO4 treatment. [Reprinted from Mengistie et al. [13], © 2013, withpermission from The Royal Society of Chemistry; Yeo et al. [23], © 2012, with permission from American Chemical Society; and Kimet al. [18], © 2014, with permission from Wiley-VCH]

films was increased to 4380 S/cm through the removal ofthe PSS chains by immersing the films in highly concen-trated H2SO4 solutions [18]. Highly concentrated H2SO4molecules yield two ions, H3SO+4 and HSO

−4 , and stabi-

lize the segregated states of the positively charged PEDOTand the negatively charged PSS chains (Figure 2(f), mid-dle). After the rinsing of the H2SO4-treated PEDOT:PSSfilms in a deionized water bath, the excess PSS chains areremoved, and a minimal amount of PSS chains remainswith the PEDOT; these chains act as counter-ions. After theH2SO4 treatment, the amorphous PEDOT:PSS grains were(Figure 2(f), left) reorganized into crystalline PEDOT:PSSnanofibrils (Figure 2(f), right).

Several research groups have tried to use conductingpolymers as the TCEs of flexible OLEDs [19–23]. Dueto the poor conductivity and low WF of the conductingpolymers, however, the first devices that used them as

TCEs showed inferior performance (i.e. luminance, cur-rent efficiency (CE), power efficiency (PE), and turn-onvoltage) compared with the conventional ITO-based ones.A recent research, however, produced conducting polymerTCE-based OLEDs that show better performance than theITO-based devices [20–23].

PEDOT:PSS transparent anode-based red, green, andblue OLEDs with better luminance–voltage characteristicsand PE than the ITO transparent anode-based ones havebeen reported [20]. A PEDOT:PSS film with 5 wt% DMSOwas used as a transparent anode instead of the conventionalITO. The PEDOT:PSS-based red, green, and blue OLEDsrequired lower driving voltages (2.61 ± 0.02 V for red,3.07 ± 0.06 V for green, and 3.25 ± 0.06 V for blue) toreach 100 cd/m2 than did the ITO-based ones (2.64 ± 0.03V for red, 3.16 ± 0.05 V for green, and 3.25 ± 0.06 V forblue). As a result, the calculated PEs of the PEDOT:PSS

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anode-based OLEDs (15.9 ± 3.1 lm/W for red, 63.5 ± 3.3lm/W for green, and 4.0 ± 0.1 lm/W for blue) were higherthan those of the ITO-based devices (17.3 ± 4.1 lm/W forred, 53.8 ± 1.9 lm/W for green, and 3.3 ± 0.1 lm/W forblue), except the red-emitting OLEDs. The increase in thelight-emitting properties of the PEDOT:PSS anode-baseddevices was attributed to the advantageous optical prop-erties of PEDOT:PSS, which has a low refractive index( ∼ 1.6) of PEDOT:PSS compared with ITO (1.8–2.2).

The optical enhancement from PEDOT:PSS anodes inITO-free OLEDs was simulated using a classical dipolemodel (Figure 3(a)) [21]. The simulation results suggestthat the improved CE in the PEDOT:PSS anode-baseddevices occur due to the suppression of the waveguidemodes. In the waveguide modes, the large refractive indexdifferences between the substrates and the anode/organiclayers trap the emitted light inside the OLED devices [37].The refractive index of ITO ( ∼ 2.0) is higher than thatof PEDOT:PSS ( ∼ 1.6) and than those of the overlying

organic layers ( ∼ 1.7); as a result, the situation becomessimilar to a waveguide with a large core, which tendsto allow more modes to get excited. On the other hand,ITO-free, PEDOT:PSS-based OLEDs are similar to awaveguide with a small core, reducing the optical powercoupled to the waveguide modes. Consequently, the ITO-free green phosphorescent OLEDs fabricated on a two-layer stacked PEDOT:PSS (with 6 vol% ethylene glycol)electrode showed a higher maximum PE (118 lm/W) thanthe devices on ITO electrodes (82 lm/W).

To additionally reduce the waveguide modes in OLEDsthat have PEDOT:PSS anodes, efficient light extractionstructures have been introduced [22]. The light scatteringof metal oxide nanostructures increased the external quan-tum efficiency (EQE) of the white OLEDs on top of thePEDOT:PSS anodes by a factor of 1.7, even at a high lumi-nance of 10,000 cd/m2. The metal oxide nanostructureswere fabricated (Figure 3(b)) by depositing 300-nm-thickSn and then melting and agglomerating the Sn films by

(a) (b)

(c)

Figure 3. Application of the PEDOT:PSS electrodes to OLEDs. (a) Angular emission profiles of OLEDs with ITO and PEDOT:PSS.(b) Schematic process for forming metal-oxide-based light extraction systems. (c) Schematic surface morphology modification of thePEDOT:PSS films through solvent vapor annealing, and the luminance–voltage characteristics of PLEDs using them as anodes. [Reprintedfrom Cai et al. [21], © 2012, with permission from Wiley-VCH; Kim et al. [22], © 2013, with permission from Wiley-VCH; and Yeoet al. [23], © 2012, with permission from American Chemical Society]

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annealing them in vacuum. Sn nanostructures were formedand were then annealed at 500°C in air to complete theSn-based metal oxide nanostructures. The planarization ofthe PEDOT:PSS anodes on top of the rough surface ofthe nanostructures prevents electrical shorts in the result-ing OLEDs. With the EQE increase, the nanostructuresimproved the color stability over the viewing angle, whichis one of the key challenges in white OLED lighting.This improvement was attributed to the high diffuse trans-mittance due to the light scattering by the metal oxidenanostructures.

The WF of PEDOT:PSS is a key parameter for deter-mining the turn-on voltage and luminance of the OLEDsfabricated on PEDOT:PSS anodes [23]. PSVA, in whichPEDOT:PSS films are annealed under a polar solvent(DMSO) atmosphere, induces phase separation betweenthe PEDOT and PSS chains (Figure 3(c)). On top of thePEDOT:PSS films, the PSS chains were enriched; as aresult, the surface WF of the PEDOT:PSS films increasedfrom 4.99 ± 0.02 to 5.30 ± 0.05 eV, in addition to theconductivity improvement. The high-WF PEDOT:PSSanodes can make ohmic contact with the overlying organiclayers. As a result, the polymer light-emitting diodes(PLEDs) fabricated on high-WF (5.3 eV) PEDOT:PSSanodes showed a lower turn-on voltage (4.5 V) and highermaximum luminance (247.9 cd/m2) than did the devices onlow-WF ( ≤ 5.0 eV) PEDOT:PSS anodes (turn-on voltage:7.5 eV; maximum luminance: 101.3 cd/m2).

3. Silver nanowireThe use of an Ag nanowire (NW)/polymer composite filmas a flexible electrode is feasible due to the high electricalconductivity of Ag NWs and the flexibility of the poly-mer composite film [38–47]. Ag NW/polymer compositefilms have low Rsh due to the high free-electron density ofAg NWs and the high figure of merit (σ /α = 1/�), whereσ is the material’s DC conductivity and α is its absorp-tion coefficient [40]. Thus, its electrical conductivity givesit great potential for practical use as a flexible TCE. Therandomly dispersed Ag NW/ polymer composite film isfabricated through a solution-based process and is suitablefor roll-to-roll manufacturing.

The diameter and length of the Ag NW and the perco-lation of the Ag NW network have strong effects on the Rshand OT of the Ag NW/polymer composite film. The smalldiameter of the Ag NW can decrease its light scattering andits long length enables good connection of the Ag NW net-work, and low Rsh. Moreover, embedding the Ag NWs ina polymer film can greatly reduce the surface roughness ofthe electrode, which has been a critical drawback of the AgNW electrodes.

The PVA (polyvinyl alcohol) film has been usedas a transparent polymer matrix with Ag NWs fabri-cated by a polyol process [38]. The Ag NWs had anaverage diameter of 49 nm and an average length of

5.4 μm. The composite transparent electrode had 1–5nm surface roughness, 87.5% OT (at 550 nm), and 63�/sq Rsh. The Rsh of the Ag NWs/PVA film was notchanged much by friction, tape adhesion, and bendingwith a 100–200 μm curvature. Moreover, the compos-ite film showed good thermal stability at 330°C andchemical stability in the Na2S solution. Using this AgNW/PVA film, vacuum-deposited simple OLED devices[PVA/Ag NWs/PEDOT:PSS/NPB/Alq3/LiF/Al] were fab-ricated using 55- or 67-nm-thick PEDOT:PSS layers. APEDOT:PSS layer (WF: ∼ 5.0 eV) was used to flatten thesurface and to compensate for the low WF of Ag (4.26 eV).Both Ag-NW-based OLED devices showed lower lumi-nance, a higher turn-on voltage, and a higher leakagecurrent than ITO-based devices. The PE of an Ag-NW-based OLED device with a 67-nm PEDOT:PSS layer,however, was higher (2.43 lm/W at a 45.8 mA/cm2 cur-rent density) than that of the ITO-based device (1.62 lm/Wat a 277.2 mA/cm2 current density). The Ag-NW-based-OLED device with a 55-nm PEDOT:PSS layer had a verylow PE because of its significant leakage current. The Ag-based device had a high PE because it had a high localcurrent density due to its highly conductive Ag NWs,and increased light extraction efficiency due to the lightscattering by the Ag NWs.

Instead of PVA, cross-linked polyacrylate, which has ashape-memory property, was used as the polymer matrixof the Ag NW/polymer transparent electrode in high-performance PLEDs [39]. The polyacrylate film swellsless in organic solvents such as acetone, dichloroben-zene, chloroform, toluene, and tetrahydrofuran than doesthe PVA film; therefore, the Ag NW/polyacrylate filmis suitable for solution-based polymer electronic devices.Moreover, the shape-memory capability allows the AgNW/polyacrylate film to assume various deformed shapeswith a small resistance change when heated to > 120°Ctemperatures (Figure 4(a)–(c)). This property is suitablefor use in variously shaped flexible light-emitting devices(Figure 4(d)–(f)). The Ag NWs were 60 nm in diame-ter and 6 μm long. The surface roughness of the AgNW/polyacrylate film was < 5 nm. The composite filmshowed 86% OT and 30 �/sq Rsh; these values wereunchanged after an adhesion test. Solution processedPLED devices [polyacrylate/Ag NWs/PEDOT:PSS/superyellow(SY-PPV)/CsF/Al] were fabricated and showedlower current density and luminance characteristics at thesame applied voltages than did the devices with ITO elec-trodes, because the Ag NW/polyacrylate film has a higherRsh (30 �/sq) than does ITO (10 �/sq) (Figure 5(a)). TheAg-NW-based device, however, showed a higher max-imum CE (14.0 cd/A) than the ITO-based device (12.5cd/A) because the light scattering by the Ag NWs increasedthe out-coupling efficiency (Figure 5(b)). These elec-trical characteristics of Ag-NW-based PLEDs were notaffected by a bending-recovery cycle test (Figure 5(c)and 5(d)).

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(a) (b) (c)

(d) (e) (f)

Figure 4. Photographs of the shape memory property of the Ag NW/polyacrylate film and the PLEDs. (a) Curved Ag NW/polyacrylatefilm. (b) Intermediate relaxation state at 120°C. (c) Recovered film. (d) Bent PLEDs. (e) Recovered shape after annealing at 120°C. (f)Bent in the direction opposite (d). [Reprinted from Yu et al. [39], © 2011, with permission from Wiley-VCH]

(a)

(b)

(c)

(d)

Figure 5. Performance of PLEDs using ITO or Ag NW/polyacrylate electrodes: (a) I–V–L curve and (b) luminous efficiency–currentdensity curve of the PLEDs using ITO or Ag NW/polyacrylate electrodes; (c) I–V–L curve and (d) luminous efficiency–current densitycurve of the PLEDs using Ag NW/polyacrylate electrodes before and after the bending-recovery cycles. [Reprinted from Yu et al. [39],© 2011, with permission from Wiley-VCH]

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White tandem OLEDs were fabricated using anAg NW/polymethylmethacrylate (PMMA) composite filmelectrode, which had 6–8 nm RMS roughness and 92% OTat 12.5 �/sq Rsh [40]. The low roughness of the Ag-NW-based OLED device resulted in a small leakage current, andthe device had current–voltage–luminance characteristicssimilar to those of an ITO-based OLED device. The operat-ing voltage of the Ag-NW-based OLED (6 V) was slightlyhigher than that of ITO (5.9 V) at 1000 cd/m2 luminancebecause carriers were injected through the PEDOT:PSSlayer, which was used for the compensation of the highinjection barrier between the low WF of the Ag NW elec-trode (4.3 eV) and the HOMO of the HTL (5.1 eV). TheAg-NW-based OLED, however, had a higher CE than didthe ITO-based OLED at the same current density, becausethe NW device showed a higher spectral-emission contri-bution in the green and yellow range than did the ITOdevice. Moreover, due to the haze and light scattering bythe Ag NW electrode, the Ag NW device showed a moreconsistent white color regardless of the viewing angle, andan emission closer to the ideal Lambertian distribution thandid the ITO-based OLED.

4. CNT and grapheneCNTs are cylindrical nanostructures composed of carbon;they have good mechanical and electrical properties [48]and exceptionally high length-to-diameter ratios of up to132,000,000 [49]. The chemical bonds that constitute theCNTs are sp2 hybridized bonds, which give unique elec-trical and mechanical properties. Thin CNT films have> 80% OT and ∼ 300 �/sq Rsh (Figure 6(a)); both quan-tities can vary in terms of the SWNT (single-walled carbonnanotube) thickness (Figure 6(b)) [50]. Their intrinsic WFis 4.50–5.1 eV [51], which can be modified by the chargetransfer doping of either the n- or p-type [52].

The arc discharge, laser ablation, and chemical vapordeposition (CVD) methods are representative methods ofsynthesizing CNTs. The arc discharge method synthesizes

CNTs at temperatures > 1700°C. In this technique, a DCarc is discharged between two graphite electrodes, and thencarbon clusters separate from the graphite anode and con-dense on the graphite cathode. CNTs synthesized by arcdischarge usually have fewer structural defects than dothose synthesized by other methods. The laser ablationmethod can grow nanotubes with high yield and purity[53]. The principle of this technique is similar to that ofthe arc discharge method; the difference is that the energyis provided by a laser-striking graphite that contains cat-alyst elements (Ni, Co, or Fe) [54]. Inert gas molecules,including He and Ar, are used to transport vaporized car-bon particles, which are consequently adsorbed by thecollector. CVD is the most standard method of synthe-sizing CNTs. High-purity nanotubes can be obtained bythis technique, and their diameter, length, density, struc-ture, and crystallinity are easily controlled. A substratedeposited with a catalytic metal (Fe, Ni, and Co) is usedfor CNT growth. The thermal CVD method requires ahigh temperature ( > 1000°C) to synthesize CNTs, but theplasma-enhanced CVD (PECVD) can synthesize CNTsat a low temperature [55]. Reactive hydrocarbon gases,including C2H2, CH4, C2H4, and C2H6, are used as car-bon sources to grow CNTs. CVD can achieve large areaand highly reproducible synthesis of CNTs.

Several groups have used CNT sheets as an anode toreplace ITO in OLEDs. A PLED that uses a CNT anodewas first demonstrated in 2005 [56]. Transparent nanotubesheets were synthesized by CVD methods using acetyleneas the carbon source. The prepared nanotube sheets showeda WF of ∼ 5.2 eV and bending stability. Flexible PLEDswere also fabricated on flexible plastic substrates. PLEDswith CNT anodes [CNT/PEDOT:PSS/poly(2-methoxy-5-(2′ethyl-hexyloxy)-p-phenylene vinylene)/Ca/Al] obtaineda 2.4 V turn-on voltage and 500 cd/m2 maximum lumi-nance. A small-molecule OLED with an SWNT anodewas demonstrated [57]. The SWNT electrodes were pro-duced using pulsed laser vaporization. The SWNT elec-trode was 130 nm thick and had 60 �/sq Rsh. A small

(a) (b)

Figure 6. (a) Transmittance vs. wavelength of the SWNT films in the visible and near-infrared region. Inset: SWNT films on a PETsubstrate. (b) Sheet resistance vs. transmittance of the SWNT films. [Reprinted from Li et al. [50], © 2006, with permission from AmericanChemical Society]

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molecule-based OLED device showed 2800 cd/m2 max-imum luminance, which was roughly half of that of anOLED device with an ITO anode, and a CE (1.4 cd/A)comparable with that of the ITO anode ( ∼ 1.9 cd/A).CNT electrodes passivated by the PEDOT and doped withSOCl2 for a low Rsh were reported [58]. SOCl2 dop-ing increases the conductivity of CNT films. This dopingtechnique had a negligible effect on the transmittance.CNT films with ∼ 160 �/sq Rsh and 87% transmittancewere fabricated and applied to OLEDs. The OLED deviceshowed 17 cd/m2 maximum luminance. The low maxi-mum luminance can be attributed to the rough surfaceof the CNT film, which can produce a leakage cur-rent in OLEDs. A rough SWNT anode was planarizedby spin-coating PEDOT:PSS:methanol (MeOH) [50]. TheAFM images of the SWNT film coated with PEDOT:PSS(Figure 7(a)) and PEDOT:PSS:MeOH (Figure 7(b)) showthat planarization with PEDOT:PSS:MeOH reduced thesurface roughness of the SWNT film. The OLED deviceswith a CNT anode planarized by PEDOT:PSS:MeOHshowed 3500 cd/m2 maximum luminance and 1.6 cd/ACE. A morphology-controlled CNT anode obtained bycoating PEDOT:PSS films several times was reported [60].As the number of coated PEDOT:PSS layers increased,

the film smoothness also increased. The OLED devicewith the PEDOT:PSS-coated CNT anode was fabricatedand showed 4500 cd/m2 maximum luminance and 2.3 cd/ACE. A high-performance OLED with a CNT anode wasreported [59]. A 5 nm-thick polyvinylpyrrolidone (PVP)layer was used to improve the adhesion between the sub-strate and the CNT. The PEDOT:PSS coating on theCNT films coated with a 5 nm polymeric layer had amore uniform surface than did the PEDOT:PSS coatingon the pristine CNT films. After PEDOT:PSS coating,the CNT films showed 102.9 �/sq Rsh. An OLED fab-ricated on a polyethylene naphthalate (PEN) substrateshowed 9000 cd/m2 maximum luminescence (Figure 7(c))and 10 cd/A CE (Figure 7(d)), similar to a device withthe same structure but with an ITO anode. Comparedwith other electrode materials, including graphene, metalnanowires, and conducting polymers, the CNT anode hasa rough surface and low electrical conductivity and trans-mittance. So that it could be used as an efficient electrodematerial of OLEDs, its conductivity and transmittanceshould be increased, and its surface morphology should besmoothed.

Graphene is a two-dimensional sheet of sp2-bondedcarbon atoms and is another possible alternative TCE.

(a) (b)

Figure 7. (a) AFM image of the SWNT film coated with PEDOT:PSS. (b) AFM image of the SWNT film coated withPEDOT:PSS:MeOH. [Reprinted from Li et al. [50], © 2006, with permission from American Chemical Society] (c) Luminescence vs.voltage characteristics of OLED on a PET and PEN substrate with and without polymer coating. (d) CE vs. voltage characteristics of anOLED device with PEN/CNT/PEDOT:PSS. [Reprinted from Ou et al. [59], © 2009, with permission from American Chemical Society]

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(a) (b)

(c) (d) (e)

Figure 8. (a) Schematic of the patterned graphene synthesis process on thin nickel layers. (b) Image of a 50-μm aperture partiallycovered by single-layer and bilayer graphene. (c) Transmittance of white light vs. number of graphene layers. (d) Transmittance ofa graphene (red, 10 nm thick) ITO (black) and FTO (blue) film. (e) Sheet resistances of the graphene films transferred using severalgraphene transfer methods. [Reprinted from Kim et al. [75], © 2009, with permission from Nature Publishing Group; Nair et al. [79],© 2008, with permission from American Association for the Advancement of Science; Wang et al. [80], © 2008, with permission fromAmerican Chemical Society; and Bae et al. [81], © 2010, with permission from Nature Publishing Group]

Graphene has outstanding electrical [61,62], physical [63],and chemical [64] properties. It has > 15,000 cm2/(V s)electron mobility [65] and 97.7% transparency to whitelight [66]. These characteristics make graphene feasible foruse as a transparent electrode.

The graphene synthesis methods can be assigned to fourcategories: mechanical [65,66] and chemical exfoliation[71–73], epitaxial growth [74], and CVD using Ni [75,76]or Cu [77,78]. The CVD method is mostly used to syn-thesize graphene for practical applications and can providelarge-area and high-quality graphene. Patterned large-scalegraphene can be grown by passing CH4/H2/Ar gases overa patterned Ni thin film as a catalyst at a high temperature( ∼ 1000°C), then using a polydimethylsiloxane (PDMS)stamp to transfer the graphene from the Ni film to the targetsubstrate (Figure 8(a)) [75].

The high optical transparency and electrical conduc-tivity of graphene make it a good candidate for TCE.The optical opacity of single-layer graphene was experi-mentally determined to be 2.3% [79,82]. In the exfoliatedsamples, the light transmittance of bilayer graphene wasfound to be 4.6% (Figure 8(b)) and decreased linearly withan increase in the number of graphene layers (Figure 8(b)

and 8(c)) [79]. In the visible and infrared ranges, thegraphene electrodes had flatter spectra than did the ITO andfluorine-doped tin oxide (FTO) electrodes (Figure 8(d)). Assuch, graphene may be a good alternative TCE for flexibleOLEDs [80].

Doping is the most common way to increase the con-ductivity of graphene, by increasing the number of chargecarriers without changing its OT. The HNO3 doping ofgraphene attained graphene with 30 �/sq Rsq and 90%transmittance, and 30-inch graphene films were producedthrough a roll-to-roll process that uses CVD on a flexibleCu foil (Figure 8(e)) [81].

Graphene is more flexible and less fragile than ITO[6,81]. Therefore, the use of graphene as a transparent andflexible electrode allows the fabrication of flexible OLEDs.For this application to be feasible, however, two majorproblems must be solved. First, the relatively high Rsh ofgraphene increases the operating voltage of OLEDs andcan therefore decrease their PE. Second, the relatively lowWF of graphene ( ∼ 4.4 eV) as an anode causes a largehole injection energy barrier at the interface between theorganic layer ( > 5.4 eV) and the anode. A large hole injec-tion barrier prevents efficient hole injection from the anode

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to the organic layers, and may therefore decrease the CEconsiderably.

Although early studies demonstrated the potential ofgraphene for use as an electrode in OLEDs, it has provedto be less applicable than expected. A spin-coated grapheneoxide dispersion on a quartz substrate with a PEDOT:PSSlayer on top of the graphene was used as an anode inOLEDs, but it had very low EQE ( ∼ 0.2%) and PE ( ∼ 0.3lm/W) [83]. The top-emitting small-molecule OLEDs withCVD-grown multi-layer graphene electrodes (WF: ∼ 4.6eV) and a transition metal oxide hole injection layer (HIL)(i.e. V2O3) also had very low CE ( ∼ 0.75 cd/A) and PE( ∼ 0.38 lm/W) [70].

Remarkable advances have been made of late in thefabrication of OLEDs with graphene electrodes. WF-tunable n-doped reduced graphene electrodes for PLEDshave been demonstrated [69]. The n-type graphene wasobtained by spin-coating a graphene oxide dispersion,followed by sequential hydrazine (N2H4) treatment andthermal reduction in an NH3 atmosphere. Due to theoptimal doping of quaternary nitrogen and the effectiveremoval of the oxygen functionalities, the n-doped reducedgraphene showed 300 �/sq Rsq, 90% transmittance, and alow WF ( ∼ 4.25 eV). PLEDs with n-doped graphene as theTCE exhibited a higher maximum CE (7.0 cd/A at 17,000cd/m2) than did FTO-based devices (4.0 cd/A at 17,000cd/m2) (Figure 9).

Figure 9. OT, sheet resistance, and WF of FTO, N-dopedgraphene, and reduced graphene electrodes, and luminous effi-ciency vs. luminance (ηEL–L) curves of iPLEDs. [Reprinted fromHwang et al. [69], © 2010, with permission from AmericanChemical Society]

Highly efficient flexible OLEDs with modified grapheneanodes were fabricated on a polyethylene terephtha-late (PET) substrate [6]. The CVD-grown single-layergraphene from a Cu catalyst was transferred onto the PETsubstrates four times; the resulting four-layer graphenewas chemically p-doped using HNO3 or AuCl3. Thefour-layered graphene anode doped with chemical p-typedopants showed a low Rsh (HNO3: ∼ 54 �/sq; AuCl3:∼ 34 �/sq). By coating the graphene anode with apolymer-blended gradient hole injection layer (GraHIL),the WF from the graphene to the organic layer graduallyincreased to ∼ 6.0 eV (Figure 10(a)). The hole injection

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Figure 10. (a) Schematic of a hole injection process from thegraphene anode to the NPB layer through self-organized HILwith a WF gradient (GraHIL). (b) CE vs. current density of phos-phorescent OLED devices using 4 L-G-HNO3 and ITO anodes.(c) Luminous efficiencies of phosphorescent OLED devices.[Reprinted from Han et al. [6], © 2012, with permission fromNature Publishing Group]

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Figure 11. PE and CE of phosphorescent white organiclight-emitting diodes (WOLEDs) on a single-layer graphene elec-trode and ITO with enhanced light out-coupling, and photos ofWOLEDs on a single-layer graphene electrode. [Reprinted fromLi et al. [85], © 2013, with permission from Nature PublishingGroup]

from the graphene anode to the organic layer was effi-ciently improved due to the modified graphene anode withlow Rsh and high WF [6,84], and it showed higher CE thandid ITO (Figure 10(b)). The flexible OLEDs with grapheneanodes exhibited high PEs (37.2 lm/W in the fluores-cent OLEDs; 102.7 lm/W in the phosphorescent OLEDs),which are significantly higher than those of the ITO-baseddevices (24.1 lm/W in the fluorescent OLEDs; 85.6 lm/Win the phosphorescent OLEDs) (Figure 10(c)). This paperdemonstrated that flexible OLEDs with modified grapheneanodes can outperform those with the conventional oxide-based anode by overcoming the high Rsh and low WF,and circumventing the metal atom diffusion from the ITOanode. The metal atoms released from the ITO anode can

act as interfacial hole-trapping sites that degrade the holeinjection efficiency from the anode [6].

White OLEDs for general lighting with high bright-ness and efficiency were fabricated using a single-layergraphene anode [85]; they had 80 lm/W PE at 3000cd/m2 (Figure 11). To meet the general lighting require-ments, the charge trapping, which induces charge imbal-ance and exciton quenching from the anode to the hostmaterials in the light-emitting layers, was decreased.The single-layer graphene was p-doped by soakingthe graphene anode in a triethyloxonium hexachloroan-timonate (OA)/dichloroethene solution to achieve anenhanced WF ( ∼ 5.1 eV); to further increase the WF ofthe graphene anode, a PEDOT:PSS layer was spin-coatedon the anode, then a transition metal oxide (i.e. MoO3) wasdeposited on it.

As graphene is very thin, it has an additional advantage:it causes almost no reflection or trapping of light, whereasITO shows significant light reflection and trapping. Thus,the use of ITO requires the use of light extraction structuresto overcome these problems.

5. ConclusionsFor use in flexible displays, the conventional oxide-basedTCEs have inherent problems, including brittleness andincreasing cost. Therefore, the development of flexibleTCEs and the improvement of the related technology arevery important for the development of flexible displaysand lightings. There have actually been more studies aboutflexible TCEs for replacing ITO than those that are men-tioned in this paper (conducting polymer, Ag NW, CNT,and graphene), such as studies on the metal grid elec-trode [86] and the oxide/metal/oxide multilayer electrode[87]. All physical properties of various TCEs studied inthis paper are summarized in Table 2. Although alter-native flexible TCEs based on conducting polymers, AgNWs, CNTs, and graphene sheets have been studied asreplacements for the conventional oxide-based TCE (i.e.ITO), the devices have inferior device performance with-out further modification of electrodes compared with thosethat use oxide-based TCEs. There has been a remarkableprogress in TCEs based on PEDOT:PSS in terms of theTCE’s electrical conductivity, but it is still low comparedto that of the conventional ITO, Ag NW, and grapheneTCEs, and the fabrication of these devices entails a com-plex process. Even though PEDOT:PSS has a relatively

Table 2. Physical properties of TCEs.

TCE Transmittance (%) Rsh (�/sq) Work function (eV) Failure strain (%)

ITO ∼ 90 [59] 10–30 [59] 4.7–4.9 [59] 1 ∼ 3 [67]CNT 80–90 [50,58] 150 ∼ 300 [50,58] 4.5–5.1[51] 25 [68]Conducting polymer 85–90 [22] 30–130 [18] 4.9–5.3 [23] 3–5 [67]Ag NW 80–90 [40] 10–50 [40] 4.2 ∼ 4.3 [40] ∼ 16 [39]Graphene 80–90 [6,69] 30–300 [6,69] 4.4–4.6 [6,70] ∼ 15 [21]

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high WF compared with the CNTs and the graphene TCEs,it must be further increased before the use of PEDOT:PSSin high-efficiency flexible OLEDs can be made feasible.The possible methods of increasing the WF include thedevelopment of a high-WF conducting polymer or surfacemodification for efficient hole injection. Another drawbackof devices that use conducting-polymer TCEs is the aggre-gation of polymer chains, which are dispersed in water; thisphenomenon can degrade the device stability. Ag NW andCNT TCEs consist of junctions of one-dimensional materi-als; the junctions can have rough surfaces that can producea leakage current in OLEDs. As this kind of leakage cur-rent at the protruding regions of electrodes degrades theluminous efficiency and operational stability of OLEDs,planarization methods should be further developed for thefabrication of efficient flexible OLEDs. The relatively lowelectrical conductivity and transmittance of CNT TCEs arealso critical problems that should be solved before CNTscan be used in flexible devices. Although graphene and AgNW TCEs have relatively high electrical conductivity ata high transmittance, a method of further increasing theelectrical conductivity to a level higher than that of the con-ventional oxide-based TCEs should be developed to reducethe operating voltage with high luminous PE. Methods ofgrowing defect-free, high-quality graphene composed oflarge single domains must be developed to enhance theelectrical properties of graphene. Graphene’s low WF seri-ously impedes the hole injection from the anode to theorganic materials, and thus novel graphene-doping pro-cesses and surface modification must be developed beforegraphene can be used as a TCE in high-efficiency flexibleOLEDs.

Notes on contributorsTae-Hee Han received his B.S. inthe Department of Materials Scienceand Engineering in 2010 from PohangUniversity of Science and Technology(POSTECH). He is a graduate stu-dent in POSTECH since 2010. Hiscurrent research work is focused on flex-ible organic light-emitting diodes usinggraphene electrodes.

Su-Hun Jeong received his B.S. inMaterial Science and Engineering degreefrom POSTECH in 2012 and has beena graduate student in POSTECH since2012. His current research work isfocused on polymeric electrodes for flex-ible organic optoelectronic devices.

Yeongjun Lee received his B.S. andM.S. in Material Science and Engineer-ing degrees from Hanyang Universityin 2012 and from POSTECH in 2014,respectively. He is currently a Ph.D.candidate in the same institute. He hasbeen researching on the printed metalnanofiber transparent electrode and itselectronic applications.

Hong-Kyu Seo received his B.S. inInformation Display degree from KyungHee University in 2011, and has beena graduate student in POSTECH since2011. His current research work isfocused on graphene electronics andorganic light-emitting diodes.

Sung-Joo Kwon received his B.S.in Material Science and Engineeringdegree from POSTECH in 2013 and iscurrently a Ph.D. candidate in the sameinstitute. He has been researching ongraphene and OLEDs.

Min-Ho Park received his B.S. andM.S. in Material Science and Engineer-ing degrees from Inha University in 2011and from POSTECH in 2013, respec-tively, and is currently a Ph.D. candi-date in the same institute. He has beenresearching on organic light-emittingdiodes related to flexible encapsulationand the tandem structure and solutionprocess.

Tae-Woo Lee is an associate profes-sor in the Department of Material Sci-ence and Engineering at POSTECH,South Korea. He received his Ph.D.in Chemical Engineering degree fromKorea Advanced Institute of Scienceand Technology (KAIST), South Koreain February 2002. He then joined BellLaboratories, USA as a postdoctoral

researcher in 2002. From September 2003 to August 2008, heworked in Samsung Advanced Institute of Technology as a mem-ber of the research staff. He received the prestigious Korea YoungScientist Award from the President of South Korea in 2008, andthe Scientist of the Month Award from the Ministry of Science,ICT, and Future Planning in 2013. His research is focused onprinted and organic electronics based on organic and carbon mate-rials for flexible electronics, displays, solid-state lightings, andsolar energy conversion devices.

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References[1] a) S. Kim, H.-J. Kwon, S. Lee, H. Shim, Y. Chun, W. Choi,

J. Kwack, D. Han, M. Song, S. Kim, S. Mohammadi, I.Kee, and S.Y. Lee, Adv. Mater. 23, 3511–3516 (2011); b)S.J. Lee, J.R. Koo, S.E. Lee, H.J. Yang, S.S. Yoon, J. Park,and Y.K. Kim, Electron. Mater. Lett. 10, 1127–1131 (2014);c) J.-L. Lee and K. Hong, Electron. Mater. Lett. 7, 77–91(2011).

[2] H. Cho, C. Yun, J.-W. Park, and S. Yoo, Org. Electron. 10,1163–1169 (2009).

[3] A. Kumar and C. Zhou, ACS Nano. 4, 11–14 (2010).[4] Y. Ji, S. Lee, B. Cho, S. Song, and T. Lee, ACS Nano. 5,

5995–6000 (2011).[5] M.-C. Choi, Y. Kim, and C.-S. Ha, Polym. Sci. 33, 581–630

(2008).[6] T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B.H.

Hong, J.-H. Ahn, and T.-W. Lee, Nat. Photonics. 6, 105–110(2012).

[7] S. Pang, Y. Hernandez, X. Feng, and K. Müllen, Adv. Mater.23, 2779–2795 (2011).

[8] H.H. Yu, S.-J. Hwang, and K.-C. Hwang, Opt. Commun.248, 51–57 (2005).

[9] R. Paetzold, K. Heuser, D. Henseler, S. Roeger, and G.Wittmann, Appl. Phys. Lett. 82, 3342–3344 (2003).

[10] Z. Chen, B. Cotterell, W. Wang, E. Guenther, and S.-J. Chua,Thin Solid Films. 394, 202–206 (2001).

[11] M. Kim, Y.S. Lee, Y.C. Kim, M.S. Choi, and J.Y. Lee,Synth. Met. 161, 2318–2322 (2011).

[12] J. Wu, M. Agrawal, H.A. Becerril, Z. Bao, Z. Liu, Y. Chen,and P. Peumans, ACS Nano. 4, 43–48 (2010).

[13] D.A. Mengistie, P.-C. Wang, and C.-W. Chu, J. Mater.Chem. A, 1, 9907–9915 (2013).

[14] Y. Jang, J. Jo, and D.-S. Kim, J. Polym. Sci. Part B Polym.Phys. 49, 1590–1596 (2011).

[15] J.Z. Wang, Z.H. Zheng, H.W. Li, W.T.S. Huck, and H.Sirringhaus, Nat. Mater. 3, 171–176 (2004).

[16] C. Piliego, M. Mazzeo, B. Cortese, R. Cingolani, and G.Gigli, Org. Electron. 9, 401–406 (2008).

[17] T. Granlund, T. Nyberg, L.S. Roman, M. Svensson, and O.Inganäs, Adv. Mater. 12, 269–273 (2000).

[18] N. Kim, S. Kee, S.H. Lee, B.H. Lee, Y.H. Khang, Y.-R. Jo,B.-J. Kim, and K. Lee, Adv. Mater. 26, 2268–2272 (2014).

[19] M. Gross, D.C. Müller, H.-G. Nothofer, U. Scherf, D.neher, C. Bräuchle, and K. Meerholz, Nature. 405, 663–665(2000).

[20] K. Fehse, K. Walzer, K. Leo, W. Lövenich, and A. Elscher,Adv. Mater. 19, 441–444 (2007).

[21] M. Cai, Z. Ye, T. Xiao, R. Liu, Y. Chen, R.W. Mayer, R.Biswas, K.-M. Ho, R. Shinar, and J. Shinar, Adv. Mater. 24,4337–4342 (2012).

[22] Y.H. Kim, J. Lee, W.M. Kim, C. Fuchs, S. Hofmann, H.-W.Chang, M.C. Gather, L. Müller-Meskamp, and K. Leo, Adv.Funct. Mater. 24, 2553–2559 (2014).

[23] J.-S. Yeo, J.-M. Yun, D.-Y. Kim, S. Park, S.-S. Kim, M.-H.Yoon, T.-W. Kim, and S.-I. Na, ACS Appl. Mater. Interfaces4, 2551–2560 (2012).

[24] M.S. White, M. Kaltenbrunner, E.D. Głowacki, K. Gut-nichenko, G. Kettlgruber, I. Graz, S. Aazou, C. Ulbricht,D.A.M. Egbe, M.C. Miron, Z. Major, M.C. Scharber, T.Sekitani, T. Someya, S. Bauer, and N.S. Sariciftci, Nat.Photon. 7, 812–816 (2013).

[25] J.Y. Kim, J.H. Jung, D.E. Lee, and J. Joo, Syn. Met. 126,311–316 (2002).

[26] J.-H. Huang, D. Kekuda, C.-W. Chu, and K.-C. Ho, J. Mater.Chem. 19, 3704–3712 (2009).

[27] S.-I. Na, G. Wang, S.-S. Kim, T.-W. Kim, S.-H. Oh, B.-K.Yu, T. Lee, and D.-Y. Kim, J. Mater. Chem. 19, 94045–9053(2009).

[28] M. Reyes-Reyes, I. Cruz-Cruz, and R. Lópex-Sandoval, J.Phys. Chem. C. 114, 20220–20224 (2010).

[29] J.E. McCarthy, C.A. Hanley, L.J. Brennan, V.G. Lamber-tini, and Y.K. Gun’ko, J. Mater Chem. C. 2, 764–770(2014).

[30] D.A. Mengistie, P.-C. Wang, and C.-W. Chu, ACS Appl.Mater. Interfaces. 6, 2292–2299 (2014).

[31] Y.H. Kim, C. Sachse, M.L. Machala, C. May, L. Müller-Meskamp, and K. Leo, Adv. Funct. Mater. 21, 1076–1081(2011).

[32] N. Kim, B.H. Lee, D. Choi, G. Kim, H. Kim, J.-R. Kim, J.Lee, Y.H. Kahng, and K. Lee, Phys. Rev. Lett. 109, 106405(2012).

[33] Y. Xia and J. Ouyang, ACS Appl. Mater. Interfaces 4, 4131–4140 (2012).

[34] Y. Xia, K. Sun, and J. Ouyang, Energy Environ. Sci. 5,5325–5332 (2012).

[35] W. Zhang, B. Zhao, Z. He, X. Zhao, H. Wang, S. Yang, H.Wu, and Y. Cao, Energy Environ. Sci. 6, 1956–1964 (2013).

[36] D. Alemu, H.-Y. Wei, K.-C. Ho, and C.-W. Chu, EnergyEnviron. Sci. 5, 9662–9671 (2012).

[37] A. Chuitinan, K. Ishiharam, T. Asano, M. Fujita, and S.Noda, Org. Electron. 6, 3–9 (2005).

[38] X.-Y. Zeng, Q.-K. Zhang, R.-M. Yu, and C.-Z. Lu, Adv.Mater. 22, 4484–4488 (2010).

[39] Z. Yu, Q. Zhang, L. Li, Q. Chen, X. Niu, J. Liu, and Q. Pei,Adv. Mater. 23, 664–668 (2011).

[40] W. Gaynor, S. Hofmann, M.G. Christoforo, C. Sachse, S.Meha, A. Salleo, M.D. McGhee, M.C. Gather, B. Lüssem,L. Müller-Meskamp, P. Peumans, and K. Leo, Adv. Mater.25, 4006–4013 (2013).

[41] J. Liang, L. Li, X. Niu, Z. Yu, and Q. Pei, Nature Photonics7, 817–824 (2013).

[42] D.-S. Leem, A. Edwards, M. Faist, J. Nelson, D.D.C.Bradley, and J.C. de Mello, Adv. Mater. 23, 4371–4375(2011).

[43] W. Gaynor, J.-Y. Lee, and P. Peumans, ACS Nano 4, 30–34(2010).

[44] M. Song, D.S. You, K. Lim, S. Park, S. Jung, C.S. Kim, D.-H. Kim, D.-G. Kim, J.-K. Kim, J. Park, Y.-C. Kang, J. Heo,S.-H. Jin, J.H. Park, and J.-W. Kang, Adv. Funct. Mater. 23,4177 (2013).

[45] W. Gaynor, G.F. Burkhard, M.D. McGehee, and P. Peumans,Adv. Mater. 23, 2905–2910 (2011).

Dow

nloa

ded

by [

The

Kor

ean

Info

Dis

play

Soc

iety

] a

t 00:

14 0

8 Ju

ne 2

015

14 T.-H. Han et al.

[46] L. Yang, T. Zhang, H. Zhou, S.C. Price, B.J. Wiley, and W.You, ACS Appl. Mater. Interfaces 3, 4075–4084 (2011).

[47] J.-Y. Lee, S.T. Connor, Y. Cui, and P. Peumans, Nano Lett.8, 689–692 (2008).

[48] S. Iijima, Nature, 354, 7 (1991).[49] X. Wang, Q. Li, J. Xie, Z. Jin, J. Wang, Y. Li, K. Jiang, and

S. Fan, Nano Lett., 9, 3137–3141 (2009).[50] J. Li, L. Hu, L. Wang, Y. Zhou, G. Gruner, and T.J. Marks,

Nano Lett. 6, 2472–2477 (2006).[51] M. Shirashi and M. Ata, Carbon, 39, 1913–1917 (2001).[52] T. Takenobu, T. Takano, M. ShirashimY. Murakami, M. Ata,

H. Kataura, Y. Achiba, and Y. Iwasa, Nat. Mater. 2, 683(2003).

[53] T. Guo, P. Nikolaev, A. Thess, D.T. Colvert, and R.E.Smalley, Chem. Phys. Lett., 243, 49–54 (1995).

[54] T. Ikegami, F. Nakanishi, M. Uchiyama, and K. Ebihara,Thin Solid Films 457, 7–11 (2004).

[55] E.J. Bae, Y.S. Min, D.H. Kang, J.H. Ko, and W.J. Park,Chem. Mater. 17, 5141–5145 (2005).

[56] M. Zhang, S. Fang, A.A. Zakhidov, S.B. Lee, A.E. Aliev,C.D. Williams, K.R. Arkinson, and R.H. Baughman, Sci-ence. 309, 1215 (2005).

[57] C.M. Aguirre, S. Auvray, S. Pigeon, R. Izquiedo, P.Deshardins, and R. Matel, Appl. Phys. Lett. 88, 183104(2006).

[58] D. Zhang, K. Ryu, X. Liu, E. Polikarpov, J. Ly,M.E. Tompson, and C. Zhou, Nano. Lett. 6, 1880–1886(2006).

[59] E.C. Ou, L. Hu, G.C. Raymond, O.K. Soo, J. Pan, Z. Zheng,Y. Park, D. Hecht, G. Irvin, P. Drzaic, and G. Gruner, ACSNano. 3, 2258–2264 (2009).

[60] C.D. Williams, R.O. Robles, M. Zhang, S. Li, R.H. Baugh-man, and A.A. Zakhidov, Appl. Phys. Lett. 93, 183506(2008).

[61] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y.Zhang, S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov,Science. 306, 666–669 (2004).

[62] E.H. Hwang, S. Adam, and S.D. Sarma, Phys. Rev. Lett. 98,186806 (2007).

[63] K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg,J. Hone, P. Kim, and H.L. Stormer, Solid State Commun.146, 351–355 (2008).

[64] C. Lee, X. Wei, J.W. Kysar, and J. Hone, Science. 321, 385–388 (2008).

[65] A.K. Geim, Science. 324, 1530–1534 (2009).[66] M.J. Allen, V.C. Tung, and R.B. Kaner, Chem. Rev. 110,

132–145 (2010).[67] C. Peng, Z. Jia, D. Bianculli1, T. Li, and J. Lou, J. Appl.

Phys. 109, 103530 (2011).[68] F. Zhang, M. Johansson, M.R. Andersson, J.C. Hummelen,

and O. Inganas, Adv. Mater. 14, 662 (2002).[69] J.O. Hwang, J.S. Park, D.S. Choi, J.Y. Kim, S.H. Lee, K.E.

Lee, Y.-H. Kim, M.H. Song, S. Yoo, and S.O. Kim, ACSNano. 6, 159–167 (2012).

[70] T. Sun, Z.L. Wang, Z.J. Shi, G.Z. Ran, W.J. Xu, Z.Y. Wang,Z.Y. Li, L. Dai, and G.G. Qin, Appl. Phys. Lett. 96, 133301(2010).

[71] S. Park and R.S. Ruoff, Nat. Nanotechnol. 4, 217–224(2009).

[72] V.C. Tung, M.J. Allen, Y. Yang, and R.B. Kaner, Nat.Nanotechnol. 4, 25–29 (2009).

[73] G. Eda, G. Fanchini, and M. Chhowalla, Nat. Nanotechnol.3, 270–274 (2008).

[74] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud,D. Mayou, T. Li, J. Hass, A.N. Marchenkov, A.H. Con-rad, P.N. First, and W.A. de Heer, Science. 312, 1191–1196(2006).

[75] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim,J.-H. Ahn, P. Kim, J.-Y. Choi, and B.H. Hong, Nature. 457,706–710 (2009).

[76] A. Reina, X.T. Jia, J. Ho, D. Nezich, H.B. Son, V. Bulovic,M.S. Dresselhaus, and J. Kong, Nano Lett. 9, 30–35 (2009).

[77] X.S. Li, W.W. Cai, J.H. An, S. Kim, J. Nah, D.X. Yang,R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S.K. Baner-jee, L. Colombo, and R.S. Ruoff, Science. 324, 1312–1314(2009).

[78] X. Li, W. Cai, L. Colombo, and R.S. Ruoff, Nano Lett. 9,4268–4272 (2009).

[79] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J.Booth, T. Stauber, N.M.R. Peres, and A.K. Geim, Science.320, 1308 (2008).

[80] X. Wang, L.J. Zhi, and K. Mullen, Nano Lett. 8, 323–327(2008).

[81] S. Bae, H. Kim, Y.H. Lee, X.F. Xu, J.S. Park, Y. Zheng, J.Balakrishnan, T. Lei, H.R. Kim, Y.I. Song, Y.J. Kim, K.S.Kim, B. Ozyilmaz, J.H. Ahn, B.H. Hong, and S. Iijima, Nat.Nanotechnol. 5, 574–578 (2010).

[82] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, andY. Chen, ACS Nano. 2, 463, 1487–1491 (2008).

[83] J. Wu, M. Agrawal, H.c.A. Becerril, Z. Bao, Z. Liu, Y. Chen,and P. Peumans, ACS Nano 4, 43–48 (2009).

[84] a) M.-R. Choi, T.-H. Han, K.-G. Lim, S.-H. Woo, D.H.Huh, and T.-W. Lee, Angew. Chem. Int. Ed. 50, 6274–6277 (2011); b) T.-H. Han, M.-R. Choi, S.-H. Woo, S.-Y.Min, C.-L. Lee, and T.-W. Lee, Adv. Mater. 24, 1487–1493(2012); c) T.-W. Lee, Y. Chung, O. Kwon, and J.-J. Park,Adv. Funct. Mater. 17, 390–396 (2007); d) M.-R. Choi,S.-H. Woo, T.-H. Han, K.-G. Lim, S.-Y. Min, W.M. Yun,O.K. Kwon, C.E. Park, K.-D. Kim, H.-K. Shin, M.-S. Kim,T. Noh, J.H. Park, K.-H. Shin, J. Jang, and T.-W. Lee,ChemSusChem. 4, 363–368 (2011).

[85] N. Li, S. Oida, G.S. Tulevski, S.-J. Han, J.B. Hannon,D.K. Sadana, and T.-C. Chen, Nat. Commun. 4, 2294–2301(2013).

[86] J.H. Park, D.Y. Lee, Y.-H. Kim, J.K. Kim, J.H. Lee, J.H.Park, T.-W. Lee, and J.H. Cho, ACS Appl. Mater. Interfaces,6, 12380–12387 (2014).

[87] H.K. Kim and J.-W. Lim, Nanoscale Res. Lett. 7, 67 (2012).

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1. Introduction2. Conducting polymer3. Silver nanowire4. CNT and graphene5. ConclusionsReferences