Flexible transparent electrodes for organic light-emitting diodes

Abstract

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

Keywords:

• organic light-emitting diodes
• flexible electrode
• flexible display

1. Introduction

Organic light-emitting diodes (OLEDs) have attracted a great deal of attention for their potential use for the next-generation flexible displays and solid-state lightings. OLEDs are flexible, lightweight, and thin and can be produced in large sheets; as such, they have potential applications in flexible and wearable displays [1–5]. For such applications, however, they must also have flexible electrodes. The conventional bottom-emission OLEDs are fabricated on a substrate coated with a transparent conducting electrode (TCE) and emit light through it. Transparent conducting oxides (TCOs) such as indium–tin-oxide (ITO) are commonly used as anodes in the bottom-emission OLEDs. A high work function (WF) and high electrical conductivity are also requisites for efficient charge injection into OLEDs. An anode with a low WF forms a large hole injection energy barrier between the anode and the overlying organic layers, and as such it cannot provide efficient hole injection into OLEDs. The low electrical conductivity of TCE also prevents charge conduction from the TCE and thereby increases the operating voltage of OLEDs. ITO has been widely used in OLEDs because it fulfills the requirements of TCE (WF: 4.7–4.9 eV; optical transmittance (OT): >90% at a 550 nm wavelength; sheet resistance (R_sh): ∼10 Ω/sq) [6,7]. ITO, however, is not suitable for use as a TCE in flexible OLEDs because it has limited flexibility [8–10], is increasingly expensive, and causes device degradation due to the metal atom diffusion from ITO into the adjacent organic layers during device operation [11,12]. Therefore, finding a flexible TCE to replace ITO is a main objective in the development of flexible OLEDs.

So that it can be used in reliable flexible electrodes, the TCE must have high OT (>80%) in the visible spectrum range, low R_sh for the reduction of ohmic power loss, high mechanical strength, thermal stability, adhesion with the substrate, and chemical resistance against organic solvents for electrode stability. In this paper, the candidate flexible electrodes are classified into four categories: conducting polymer, silver nanowire (Ag NW), carbon nanotube (CNT) and graphene. These have a variety of properties and will be reviewed sequentially in the following sections.

2. Conducting polymer

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

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

The insufficient electrical conductivity of PEDOT:PSS is an impediment to its use as a TCE in an OLED. The electrical conductivity of PEDOT:PSS comes from the charged doping in the PEDOT backbone, and the PEDOT chains are positively doped. The PSS stabilizes the positive charges and makes the PEDOT chains dispersible in water. The main method of increasing the electrical conductivity of PEDOT:PSS is to effectively separate the conductive PEDOT domains from the insulating PSS domains and to enhance the π–π coupling of the PEDOT chains. To separate the PEDOT and PSS domains, several types of polar solvent additives have been used [13,25–35]. Methods of removing the PSS chains in PEDOT:PSS thin films have been reported and can effectively improve their electrical conductivity [18–24]. Several polar solvents that can be used to improve the electrical conductivity of PEDOT:PSS electrodes have been reported (Table 1) [13,28–35]. Small amounts of polar solvents can effectively separate the PEDOT and PSS chains by reducing the Coulomb interaction between the positively charged PEDOT chains and the negatively charged PSS chains [36]; as a result, the π–π coupling of the conducting PEDOT chains can be enhanced and the electrical conductivity of the PEDOT:PSS films can be increased by several orders of magnitude [32].

Table 1. Summary of polar solvent additives for improving the electrical conductivity of PEDOT: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]

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

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) are phase images while (c) and (d) are topographic images. (e) Conventional polar solvent additive method and PSVA. (f) Schematic diagram of the structural reorganization of the PEDOT:PSS films through H₂SO₄ treatment. [Reprinted from Mengistie et al. [13], © 2013, with permission from The Royal Society of Chemistry; Yeo et al. [23], © 2012, with permission from American Chemical Society; and Kim et al. [18], © 2014, with permission from Wiley-VCH]

The polar solvent vapor annealing (PSVA) of the PEDOT:PSS films (Figure 2(e)) induces phase separation between the PEDOT and PSS chains more effectively than does annealing in an ambient atmosphere,and improves the electrical conductivity of the PEDOT:PSS films [23]. Compared with annealing in an ambient atmosphere, PSVA takes a much longer time to achieve phase separation between the PEDOT and PSS chains due to the slow evaporation of the polar solvents in the PEDOT:PSS films. As a result, PSVA with dimethyl sulfoxide (DMSO) increased the electrical conductivity of the PEDOT:PSS films to 1050 S/cm, whereas the electrical conductivity of the films spin-coated from the water solution containing a DMSO additive and annealed in an ambient atmosphere increased to only 725 S/cm.

Removing the PSS chains from the PEDOT:PSS films greatly increases its electrical conductivity [18,29–36]. Recently, the electrical conductivity of the PEDOT:PSS films was increased to 4380 S/cm through the removal of the PSS chains by immersing the films in highly concentrated H₂SO₄ solutions [18]. Highly concentrated H₂SO₄ molecules yield two ions, , and stabilize the segregated states of the positively charged PEDOT and the negatively charged PSS chains (Figure 2(f), middle). After the rinsing of the H₂SO₄-treated PEDOT:PSS films in a deionized water bath, the excess PSS chains are removed, and a minimal amount of PSS chains remains with the PEDOT; these chains act as counter-ions. After the H₂SO₄ treatment, the amorphous PEDOT:PSS grains were (Figure 2(f), left) reorganized into crystalline PEDOT:PSS nanofibrils (Figure 2(f), right).

Several research groups have tried to use conducting polymers as the TCEs of flexible OLEDs [19–23]. Due to the poor conductivity and low WF of the conducting polymers, however, the first devices that used them as TCEs showed inferior performance (i.e. luminance, current efficiency (CE), power efficiency (PE), and turn-on voltage) compared with the conventional ITO-based ones. A recent research, however, produced conducting polymer TCE-based OLEDs that show better performance than the ITO-based devices [20–23].

PEDOT:PSS transparent anode-based red, green, and blue OLEDs with better luminance–voltage characteristics and PE than the ITO transparent anode-based ones have been reported [20]. A PEDOT:PSS film with 5 wt% DMSO was used as a transparent anode instead of the conventional ITO. The PEDOT:PSS-based red, green, and blue OLEDs required 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) to reach 100 cd/m² than did the ITO-based ones (2.64 ± 0.03 V for red, 3.16 ± 0.05 V for green, and 3.25 ± 0.06 V for blue). As a result, the calculated PEs of the PEDOT:PSS anode-based OLEDs (15.9 ± 3.1 lm/W for red, 63.5 ± 3.3 lm/W for green, and 4.0 ± 0.1 lm/W for blue) were higher than those of the ITO-based devices (17.3 ± 4.1 lm/W for red, 53.8 ± 1.9 lm/W for green, and 3.3 ± 0.1 lm/W for blue), except the red-emitting OLEDs. The increase in the light-emitting properties of the PEDOT:PSS anode-based devices was attributed to the advantageous optical properties 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 in ITO-free OLEDs was simulated using a classical dipole model (Figure 3(a)) [21]. The simulation results suggest that the improved CE in the PEDOT:PSS anode-based devices occur due to the suppression of the waveguide modes. In the waveguide modes, the large refractive index differences between the substrates and the anode/organic layers trap the emitted light inside the OLED devices [37]. The refractive index of ITO (∼2.0) is higher than that of PEDOT:PSS (∼1.6) and than those of the overlying organic layers (∼1.7); as a result, the situation becomes similar to a waveguide with a large core, which tends to allow more modes to get excited. On the other hand, ITO-free, PEDOT:PSS-based OLEDs are similar to a waveguide with a small core, reducing the optical power coupled 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) than the devices on ITO electrodes (82 lm/W).

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 the PEDOT:PSS films through solvent vapor annealing, and the luminance–voltage characteristics of PLEDs using them as anodes. [Reprinted from Cai et al. [21], © 2012, with permission from Wiley-VCH; Kim et al. [22], © 2013, with permission from Wiley-VCH; and Yeo et al. [23], © 2012, with permission from American Chemical Society]

To additionally reduce the waveguide modes in OLEDs that have PEDOT:PSS anodes, efficient light extraction structures have been introduced [22]. The light scattering of metal oxide nanostructures increased the external quantum efficiency (EQE) of the white OLEDs on top of the PEDOT:PSS anodes by a factor of 1.7, even at a high luminance of 10,000 cd/m². The metal oxide nanostructures were fabricated (Figure 3(b)) by depositing 300-nm-thick Sn and then melting and agglomerating the Sn films by annealing them in vacuum. Sn nanostructures were formed and were then annealed at 500°C in air to complete the Sn-based metal oxide nanostructures. The planarization of the PEDOT:PSS anodes on top of the rough surface of the nanostructures prevents electrical shorts in the resulting OLEDs. With the EQE increase, the nanostructures improved the color stability over the viewing angle, which is one of the key challenges in white OLED lighting. This improvement was attributed to the high diffuse transmittance due to the light scattering by the metal oxide nanostructures.

The WF of PEDOT:PSS is a key parameter for determining the turn-on voltage and luminance of the OLEDs fabricated on PEDOT:PSS anodes [23]. PSVA, in which PEDOT:PSS films are annealed under a polar solvent (DMSO) atmosphere, induces phase separation between the PEDOT and PSS chains (Figure 3(c)). On top of the PEDOT:PSS films, the PSS chains were enriched; as a result, the surface WF of the PEDOT:PSS films increased from 4.99 ± 0.02 to 5.30 ± 0.05 eV, in addition to the conductivity improvement. The high-WF PEDOT:PSS anodes can make ohmic contact with the overlying organic layers. As a result, the polymer light-emitting diodes (PLEDs) fabricated on high-WF (5.3 eV) PEDOT:PSS anodes showed a lower turn-on voltage (4.5 V) and higher maximum luminance (247.9 cd/m²) than did the devices on low-WF (≤5.0 eV) PEDOT:PSS anodes (turn-on voltage: 7.5 eV; maximum luminance: 101.3 cd/m²).

3. Silver nanowire

The use of an Ag nanowire (NW)/polymer composite film as a flexible electrode is feasible due to the high electrical conductivity of Ag NWs and the flexibility of the polymer composite film [38–47]. Ag NW/polymer composite films have low R_sh due to the high free-electron density of Ag NWs and the high figure of merit (σ/α = 1/Ω), where σ is the material's DC conductivity and α is its absorption coefficient [40]. Thus, its electrical conductivity gives it great potential for practical use as a flexible TCE. The randomly dispersed Ag NW/ polymer composite film is fabricated through a solution-based process and is suitable for roll-to-roll manufacturing.

The diameter and length of the Ag NW and the percolation of the Ag NW network have strong effects on the R_sh and OT of the Ag NW/polymer composite film. The small diameter of the Ag NW can decrease its light scattering and its long length enables good connection of the Ag NW network, and low R_sh. Moreover, embedding the Ag NWs in a polymer film can greatly reduce the surface roughness of the electrode, which has been a critical drawback of the Ag NW electrodes.

The PVA (polyvinyl alcohol) film has been used as a transparent polymer matrix with Ag NWs fabricated by a polyol process [38]. The Ag NWs had an average diameter of 49 nm and an average length of 5.4 µm. The composite transparent electrode had 1–5 nm surface roughness, 87.5% OT (at 550 nm), and 63 Ω/sq R_sh. The R_sh of the Ag NWs/PVA film was not changed much by friction, tape adhesion, and bending with a 100–200 µm curvature. Moreover, the composite film showed good thermal stability at 330°C and chemical stability in the Na₂S solution. Using this Ag NW/PVA film, vacuum-deposited simple OLED devices [PVA/Ag NWs/PEDOT:PSS/NPB/Alq₃/LiF/Al] were fabricated using 55- or 67-nm-thick PEDOT:PSS layers. A PEDOT:PSS layer (WF: ∼5.0 eV) was used to flatten the surface and to compensate for the low WF of Ag (4.26 eV). Both Ag-NW-based OLED devices showed lower luminance, a higher turn-on voltage, and a higher leakage current 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 current density) than that of the ITO-based device (1.62 lm/W at a 277.2 mA/cm² current density). The Ag-NW-based-OLED device with a 55-nm PEDOT:PSS layer had a very low PE because of its significant leakage current. The Ag-based device had a high PE because it had a high local current density due to its highly conductive Ag NWs, and increased light extraction efficiency due to the light scattering by the Ag NWs.

Instead of PVA, cross-linked polyacrylate, which has a shape-memory property, was used as the polymer matrix of the Ag NW/polymer transparent electrode in high-performance PLEDs [39]. The polyacrylate film swells less in organic solvents such as acetone, dichlorobenzene, chloroform, toluene, and tetrahydrofuran than does the PVA film; therefore, the Ag NW/polyacrylate film is suitable for solution-based polymer electronic devices. Moreover, the shape-memory capability allows the Ag NW/polyacrylate film to assume various deformed shapes with a small resistance change when heated to >120°C temperatures (Figure 4(a)–(c)). This property is suitable for use in variously shaped flexible light-emitting devices (Figure 4(d)–(f)). The Ag NWs were 60 nm in diameter and 6 µm long. The surface roughness of the Ag NW/polyacrylate film was <5 nm. The composite film showed 86% OT and 30 Ω/sq R_sh; these values were unchanged after an adhesion test. Solution processed PLED devices [polyacrylate/Ag NWs/PEDOT:PSS/super yellow(SY-PPV)/CsF/Al] were fabricated and showed lower current density and luminance characteristics at the same applied voltages than did the devices with ITO electrodes, because the Ag NW/polyacrylate film has a higher R_sh (30 Ω/sq) than does ITO (10 Ω/sq) (Figure 5(a)). The Ag-NW-based device, however, showed a higher maximum CE (14.0 cd/A) than the ITO-based device (12.5 cd/A) because the light scattering by the Ag NWs increased the out-coupling efficiency (Figure 5(b)). These electrical characteristics of Ag-NW-based PLEDs were not affected by a bending-recovery cycle test (Figure 5(c) and 5(d)).

Figure 4. Photographs of the shape memory property of the Ag NW/polyacrylate film and the PLEDs. (a) Curved Ag NW/polyacrylate film. (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]

Figure 5. Performance of PLEDs using ITO or Ag NW/polyacrylate electrodes: (a) I–V–L curve and (b) luminous efficiency–current density curve of the PLEDs using ITO or Ag NW/polyacrylate electrodes; (c) I–V–L curve and (d) luminous efficiency–current density curve 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]

White tandem OLEDs were fabricated using an Ag NW/polymethylmethacrylate (PMMA) composite film electrode, which had 6–8 nm RMS roughness and 92% OT at 12.5 Ω/sq R_sh [40]. The low roughness of the Ag-NW-based OLED device resulted in a small leakage current, and the device had current–voltage–luminance characteristics similar to those of an ITO-based OLED device. The operating voltage of the Ag-NW-based OLED (6 V) was slightly higher than that of ITO (5.9 V) at 1000 cd/m² luminance because carriers were injected through the PEDOT:PSS layer, which was used for the compensation of the high injection barrier between the low WF of the Ag NW electrode (4.3 eV) and the HOMO of the HTL (5.1 eV). The Ag-NW-based OLED, however, had a higher CE than did the ITO-based OLED at the same current density, because the NW device showed a higher spectral-emission contribution in the green and yellow range than did the ITO device. Moreover, due to the haze and light scattering by the Ag NW electrode, the Ag NW device showed a more consistent white color regardless of the viewing angle, and an emission closer to the ideal Lambertian distribution than did the ITO-based OLED.

4. CNT and graphene

CNTs are cylindrical nanostructures composed of carbon; they have good mechanical and electrical properties [48] and exceptionally high length-to-diameter ratios of up to 132,000,000 [49]. The chemical bonds that constitute the CNTs are sp² hybridized bonds, which give unique electrical and mechanical properties. Thin CNT films have >80% OT and ∼300 Ω/sq R_sh (Figure 6(a)); both quantities can vary in terms of the SWNT (single-walled carbon nanotube) thickness (Figure 6(b)) [50]. Their intrinsic WF is 4.50–5.1 eV [51], which can be modified by the charge transfer doping of either the n- or p-type [52].

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

The arc discharge, laser ablation, and chemical vapor deposition (CVD) methods are representative methods of synthesizing CNTs. The arc discharge method synthesizes CNTs at temperatures >1700°C. In this technique, a DC arc is discharged between two graphite electrodes, and then carbon clusters separate from the graphite anode and condense on the graphite cathode. CNTs synthesized by arc discharge usually have fewer structural defects than do those synthesized by other methods. The laser ablation method can grow nanotubes with high yield and purity [53]. The principle of this technique is similar to that of the arc discharge method; the difference is that the energy is provided by a laser-striking graphite that contains catalyst elements (Ni, Co, or Fe) [54]. Inert gas molecules, including He and Ar, are used to transport vaporized carbon particles, which are consequently adsorbed by the collector. CVD is the most standard method of synthesizing CNTs. High-purity nanotubes can be obtained by this technique, and their diameter, length, density, structure, and crystallinity are easily controlled. A substrate deposited with a catalytic metal (Fe, Ni, and Co) is used for CNT growth. The thermal CVD method requires a high temperature (>1000°C) to synthesize CNTs, but the plasma-enhanced CVD (PECVD) can synthesize CNTs at a low temperature [55]. Reactive hydrocarbon gases, including C₂H₂, CH₄, C₂H₄, and C₂H₆, are used as carbon sources to grow CNTs. CVD can achieve large area and highly reproducible synthesis of CNTs.

Several groups have used CNT sheets as an anode to replace ITO in OLEDs. A PLED that uses a CNT anode was first demonstrated in 2005 [56]. Transparent nanotube sheets were synthesized by CVD methods using acetylene as the carbon source. The prepared nanotube sheets showed a WF of ∼5.2 eV and bending stability. Flexible PLEDs were also fabricated on flexible plastic substrates. PLEDs with CNT anodes [CNT/PEDOT:PSS/poly(2-methoxy-5-(2′ethyl-hexyloxy)-p-phenylene vinylene)/Ca/Al] obtained a 2.4 V turn-on voltage and 500 cd/m² maximum luminance. A small-molecule OLED with an SWNT anode was demonstrated [57]. The SWNT electrodes were produced using pulsed laser vaporization. The SWNT electrode was 130 nm thick and had 60 Ω/sq R_sh. A small molecule-based OLED device showed 2800 cd/m² maximum luminance, which was roughly half of that of an OLED 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 with SOCl₂ for a low R_sh were reported [58]. SOCl₂ doping increases the conductivity of CNT films. This doping technique had a negligible effect on the transmittance. CNT films with ∼160 Ω/sq R_sh and 87% transmittance were fabricated and applied to OLEDs. The OLED device showed 17 cd/m² maximum luminance. The low maximum luminance can be attributed to the rough surface of the CNT film, which can produce a leakage current in OLEDs. A rough SWNT anode was planarized by spin-coating PEDOT:PSS:methanol (MeOH) [50]. The AFM images of the SWNT film coated with PEDOT:PSS (Figure 7(a)) and PEDOT:PSS:MeOH (Figure 7(b)) show that planarization with PEDOT:PSS:MeOH reduced the surface roughness of the SWNT film. The OLED devices with a CNT anode planarized by PEDOT:PSS:MeOH showed 3500 cd/m² maximum luminance and 1.6 cd/A CE. A morphology-controlled CNT anode obtained by coating PEDOT:PSS films several times was reported [60]. As the number of coated PEDOT:PSS layers increased, the film smoothness also increased. The OLED device with the PEDOT:PSS-coated CNT anode was fabricated and showed 4500 cd/m² maximum luminance and 2.3 cd/A CE. A high-performance OLED with a CNT anode was reported [59]. A 5 nm-thick polyvinylpyrrolidone (PVP) layer was used to improve the adhesion between the substrate and the CNT. The PEDOT:PSS coating on the CNT films coated with a 5 nm polymeric layer had a more uniform surface than did the PEDOT:PSS coating on the pristine CNT films. After PEDOT:PSS coating, the CNT films showed 102.9 Ω/sq R_sh. An OLED fabricated on a polyethylene naphthalate (PEN) substrate showed 9000 cd/m² maximum luminescence (Figure 7(c)) and 10 cd/A CE (Figure 7(d)), similar to a device with the same structure but with an ITO anode. Compared with other electrode materials, including graphene, metal nanowires, and conducting polymers, the CNT anode has a rough surface and low electrical conductivity and transmittance. So that it could be used as an efficient electrode material of OLEDs, its conductivity and transmittance should be increased, and its surface morphology should be smoothed.

Figure 7. (a) AFM image of the SWNT film coated with PEDOT:PSS. (b) AFM image of the SWNT film coated with PEDOT: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 an OLED device with PEN/CNT/PEDOT:PSS. [Reprinted from Ou et al. [59], © 2009, with permission from American Chemical Society]

Graphene is a two-dimensional sheet of sp²-bonded carbon atoms and is another possible alternative TCE. 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 white light [66]. These characteristics make graphene feasible for use as a transparent electrode.

Table 2. Physical properties of TCEs.

TCE	Transmittance (%)	Rsh (Ω/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]

The graphene synthesis methods can be assigned to four categories: 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 synthesize graphene for practical applications and can provide large-area and high-quality graphene. Patterned large-scale graphene can be grown by passing CH₄/H₂/Ar gases over a 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 target substrate (Figure 8(a)) [75].

Figure 8. (a) Schematic of the patterned graphene synthesis process on thin nickel layers. (b) Image of a 50-µm aperture partially covered by single-layer and bilayer graphene. (c) Transmittance of white light vs. number of graphene layers. (d) Transmittance of a graphene (red, 10 nm thick) ITO (black) and FTO (blue) film. (e) Sheet resistances of the graphene films transferred using several graphene 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 from American Chemical Society; and Bae et al. [81], © 2010, with permission from Nature Publishing Group]

The high optical transparency and electrical conductivity of graphene make it a good candidate for TCE. The optical opacity of single-layer graphene was experimentally determined to be 2.3% [79,82]. In the exfoliated samples, the light transmittance of bilayer graphene was found to be 4.6% (Figure 8(b)) and decreased linearly with an increase in the number of graphene layers (Figure 8(b) and 8(c)) [79]. In the visible and infrared ranges, the graphene electrodes had flatter spectra than did the ITO and fluorine-doped tin oxide (FTO) electrodes (Figure 8(d)). As such, graphene may be a good alternative TCE for flexible OLEDs [80].

Doping is the most common way to increase the conductivity of graphene, by increasing the number of charge carriers without changing its OT. The HNO₃ doping of graphene attained graphene with 30 Ω/sq R_sq and 90% transmittance, and 30-inch graphene films were produced through a roll-to-roll process that uses CVD on a flexible Cu foil (Figure 8(e)) [81].

Graphene is more flexible and less fragile than ITO [6,81]. Therefore, the use of graphene as a transparent and flexible electrode allows the fabrication of flexible OLEDs. For this application to be feasible, however, two major problems must be solved. First, the relatively high R_sh of graphene increases the operating voltage of OLEDs and can therefore decrease their PE. Second, the relatively low WF of graphene (∼4.4 eV) as an anode causes a large hole injection energy barrier at the interface between the organic layer (>5.4 eV) and the anode. A large hole injection barrier prevents efficient hole injection from the anode to the organic layers, and may therefore decrease the CE considerably.

Although early studies demonstrated the potential of graphene for use as an electrode in OLEDs, it has proved to be less applicable than expected. A spin-coated graphene oxide dispersion on a quartz substrate with a PEDOT:PSS layer on top of the graphene was used as an anode in OLEDs, but it had very low EQE (∼0.2%) and PE (∼0.3 lm/W) [83]. The top-emitting small-molecule OLEDs with CVD-grown multi-layer graphene electrodes (WF: ∼4.6 eV) 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 the fabrication of OLEDs with graphene electrodes. WF-tunable n-doped reduced graphene electrodes for PLEDs have been demonstrated [69]. The n-type graphene was obtained by spin-coating a graphene oxide dispersion, followed by sequential hydrazine (N₂H₄) treatment and thermal reduction in an NH₃ atmosphere. Due to the optimal doping of quaternary nitrogen and the effective removal of the oxygen functionalities, the n-doped reduced graphene showed 300 Ω/sq R_sq, 90% transmittance, and a low WF (∼4.25 eV). PLEDs with n-doped graphene as the TCE exhibited a higher maximum CE (7.0 cd/A at 17,000 cd/m²) than did FTO-based devices (4.0 cd/A at 17,000 cd/m²) (Figure 9).

Figure 9. OT, sheet resistance, and WF of FTO, N-doped graphene, and reduced graphene electrodes, and luminous efficiency vs. luminance (η_EL–L) curves of iPLEDs. [Reprinted from Hwang et al. [69], © 2010, with permission from American Chemical Society]

Highly efficient flexible OLEDs with modified graphene anodes were fabricated on a polyethylene terephthalate (PET) substrate [6]. The CVD-grown single-layer graphene from a Cu catalyst was transferred onto the PET substrates four times; the resulting four-layer graphene was chemically p-doped using HNO₃ or AuCl₃. The four-layered graphene anode doped with chemical p-type dopants showed a low R_sh (HNO₃: ∼54 Ω/sq; AuCl₃: ∼34 Ω/sq). By coating the graphene anode with a polymer-blended gradient hole injection layer (GraHIL), the WF from the graphene to the organic layer gradually increased to ∼6.0 eV (Figure 10(a)). The hole injection from the graphene anode to the organic layer was efficiently improved due to the modified graphene anode with low R_sh and high WF [6,84], and it showed higher CE than did ITO (Figure 10(b)). The flexible OLEDs with graphene anodes exhibited high PEs (37.2 lm/W in the fluorescent OLEDs; 102.7 lm/W in the phosphorescent OLEDs), which are significantly higher than those of the ITO-based devices (24.1 lm/W in the fluorescent OLEDs; 85.6 lm/W in the phosphorescent OLEDs) (Figure 10(c)). This paper demonstrated that flexible OLEDs with modified graphene anodes can outperform those with the conventional oxide-based anode by overcoming the high R_sh and low WF, and circumventing the metal atom diffusion from the ITO anode. The metal atoms released from the ITO anode can act as interfacial hole-trapping sites that degrade the hole injection efficiency from the anode [6].

Figure 10. (a) Schematic of a hole injection process from the graphene anode to the NPB layer through self-organized HIL with a WF gradient (GraHIL). (b) CE vs. current density of phosphorescent 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 from Nature Publishing Group]

White OLEDs for general lighting with high brightness and efficiency were fabricated using a single-layer graphene anode [85]; they had 80 lm/W PE at 3000 cd/m² (Figure 11). To meet the general lighting requirements, the charge trapping, which induces charge imbalance and exciton quenching from the anode to the host materials in the light-emitting layers, was decreased. The single-layer graphene was p-doped by soaking the graphene anode in a triethyloxonium hexachloroantimonate (OA)/dichloroethene solution to achieve an enhanced WF (∼5.1 eV); to further increase the WF of the graphene anode, a PEDOT:PSS layer was spin-coated on the anode, then a transition metal oxide (i.e. MoO₃) was deposited on it.

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

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

5. Conclusions

For use in flexible displays, the conventional oxide-based TCEs have inherent problems, including brittleness and increasing cost. Therefore, the development of flexible TCEs and the improvement of the related technology are very important for the development of flexible displays and lightings. There have actually been more studies about flexible TCEs for replacing ITO than those that are mentioned in this paper (conducting polymer, Ag NW, CNT, and graphene), such as studies on the metal grid electrode [86] and the oxide/metal/oxide multilayer electrode [87]. All physical properties of various TCEs studied in this paper are summarized in Table 2. Although alternative flexible TCEs based on conducting polymers, Ag NWs, CNTs, and graphene sheets have been studied as replacements for the conventional oxide-based TCE (i.e. ITO), the devices have inferior device performance without further modification of electrodes compared with those that use oxide-based TCEs. There has been a remarkable progress in TCEs based on PEDOT:PSS in terms of the TCE's electrical conductivity, but it is still low compared to that of the conventional ITO, Ag NW, and graphene TCEs, and the fabrication of these devices entails a complex process. Even though PEDOT:PSS has a relatively high WF compared with the CNTs and the graphene TCEs, it must be further increased before the use of PEDOT:PSS in high-efficiency flexible OLEDs can be made feasible. The possible methods of increasing the WF include the development of a high-WF conducting polymer or surface modification for efficient hole injection. Another drawback of devices that use conducting-polymer TCEs is the aggregation of polymer chains, which are dispersed in water; this phenomenon can degrade the device stability. Ag NW and CNT TCEs consist of junctions of one-dimensional materials; the junctions can have rough surfaces that can produce a leakage current in OLEDs. As this kind of leakage current at the protruding regions of electrodes degrades the luminous efficiency and operational stability of OLEDs, planarization methods should be further developed for the fabrication of efficient flexible OLEDs. The relatively low electrical conductivity and transmittance of CNT TCEs are also critical problems that should be solved before CNTs can be used in flexible devices. Although graphene and Ag NW TCEs have relatively high electrical conductivity at a high transmittance, a method of further increasing the electrical conductivity to a level higher than that of the conventional oxide-based TCEs should be developed to reduce the operating voltage with high luminous PE. Methods of growing defect-free, high-quality graphene composed of large single domains must be developed to enhance the electrical properties of graphene. Graphene's low WF seriously impedes the hole injection from the anode to the organic materials, and thus novel graphene-doping processes and surface modification must be developed before graphene can be used as a TCE in high-efficiency flexible OLEDs.

Additional information

Notes on contributors

Tae-Hee Han received his B.S. in the Department of Materials Science and Engineering in 2010 from Pohang University of Science and Technology (POSTECH). He is a graduate student in POSTECH since 2010. His current research work is focused on flexible organic light-emitting diodes using graphene electrodes.

Su-Hun Jeong received his B.S. in Material Science and Engineering degree from POSTECH in 2012 and has been a graduate student in POSTECH since 2012. His current research work is focused on polymeric electrodes for flexible organic optoelectronic devices.

Yeongjun Lee received his B.S. and M.S. in Material Science and Engineering degrees from Hanyang University in 2012 and from POSTECH in 2014, respectively. He is currently a Ph.D. candidate in the same institute. He has been researching on the printed metal nanofiber transparent electrode and its electronic applications.

Hong-Kyu Seo received his B.S. in Information Display degree from Kyung Hee University in 2011, and has been a graduate student in POSTECH since 2011. His current research work is focused on graphene electronics and organic light-emitting diodes.

Sung-Joo Kwon received his B.S. in Material Science and Engineering degree from POSTECH in 2013 and is currently a Ph.D. candidate in the same institute. He has been researching on graphene and OLEDs.

Min-Ho Park received his B.S. and M.S. in Material Science and Engineering degrees from Inha University in 2011 and from POSTECH in 2013, respectively, and is currently a Ph.D. candidate in the same institute. He has been researching on organic light-emitting diodes related to flexible encapsulation and the tandem structure and solution process.

Tae-Woo Lee is an associate professor in the Department of Material Science and Engineering at POSTECH, South Korea. He received his Ph.D. in Chemical Engineering degree from Korea Advanced Institute of Science and Technology (KAIST), South Korea in February 2002. He then joined Bell Laboratories, USA as a postdoctoral researcher in 2002. From September 2003 to August 2008, he worked in Samsung Advanced Institute of Technology as a member of the research staff. He received the prestigious Korea Young Scientist Award from the President of South Korea in 2008, and the Scientist of the Month Award from the Ministry of Science, ICT, and Future Planning in 2013. His research is focused on printed and organic electronics based on organic and carbon materials for flexible electronics, displays, solid-state lightings, and solar energy conversion devices.