Imprinted nonoxidized graphene sheets as an efficient hole transport layer in polymerlight-emitting diodesChun-Yuan Huang, I-Wen Peter Chen, Chih-Jung Chen, Ray-Kuang Chiang, and Hoang-Tuan Vu Citation: Applied Physics Letters 104, 073111 (2014); doi: 10.1063/1.4866341 View online: http://dx.doi.org/10.1063/1.4866341 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Hole injection layer containing PolyanilinePoly (4styrenesulfonate) for efficient organic lightemitting diodes AIP Conf. Proc. 1255, 342 (2010); 10.1063/1.3455629 Hole-injecting conducting-polymer compositions for highly efficient and stable organic light-emitting diodes Appl. Phys. Lett. 87, 231106 (2005); 10.1063/1.2132072 Inorganic solution-processed hole-injecting and electron-blocking layers in polymer light-emitting diodes J. Appl. Phys. 92, 7556 (2002); 10.1063/1.1522812 Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transportingmolecules Appl. Phys. Lett. 76, 1810 (2000); 10.1063/1.126173 Novel main-chain poly-carbazoles as hole and electron transport materials in polymer light-emitting diodes Appl. Phys. Lett. 71, 1921 (1997); 10.1063/1.119981

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Imprinted nonoxidized graphene sheets as an efficient hole transport layerin polymer light-emitting diodes

Chun-Yuan Huang,1,a) I-Wen Peter Chen,1 Chih-Jung Chen,2 Ray-Kuang Chiang,2

and Hoang-Tuan Vu3

1Department of Applied Science, National Taitung University, Taitung 950, Taiwan2Department of Material Science and Engineering, Far East University, Tainan 744, Taiwan3Institute of Microelectronics and Advanced Optoelectronic Technology Center,National Cheng Kung University, Tainan 701, Taiwan

(Received 25 December 2013; accepted 7 February 2014; published online 20 February 2014)

Nonoxidized graphene sheets (NGSs) with single- and multilayered structures were generated by

direct exfoliation of highly oriented pyrolytic graphite in a water-ethanol mixture with the

assistances of pyridinium salt (PyþBr3–) and sonication. Raman spectrum exhibited a low intensity

ratio (0.055) of D and G bands, indicating that the NGSs were nearly defect-free. Their application

for the fabrication of polymer light-emitting diodes (PLEDs) was also demonstrated. The PLEDs

that used an imprinted NGS film as a hole transport layer show a luminance exceeding

13000 cd/m2, which was comparable to that of devices using the typical hole transport material:

poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4866341]

Graphene, an emerging carbon material comprising

monolayered sp2-hybridized carbon atoms, has attracted sub-

stantial attention due to its impressive physical, chemical,

and mechanical properties. Because of its two dimensional

conjugated chemical structure and atomic thickness, gra-

phene is particularly promising as a transparent conductive

electrode to be used in electronic and optoelectronic

applications.1–3 Nowadays, single- and multilayered gra-

phene films with sizes of tens of inches have been achieved

using chemical vapor deposition,4 indicating that this mate-

rial can be applied over large areas and is viable for indus-

trial production. However, lacking efficient patterning and

transfer methods, devices with graphene electrodes remain

impractical. To improve the processability of graphene, gra-

phene oxide (GO) or highly reduced graphene oxide (HRG)

sheets have been rapidly developed to be compatible with

standard device-processing procedures. Regarding GO,

although the chemical oxidation of graphite results in

delamination and solubility, it also introduces intrinsic

defects and deteriorates these excellent conducting proper-

ties of the material.5,6 Consequently, GO is electrically insu-

lating because of its heavy oxygenation and not suitable as

transparent electrodes. Despite the reduction effect, which

partially removes the oxygenated groups and ameliorates the

induced defects, the same condition inevitably applies to

HRG sheets.6 Here we demonstrated a facile and elegant

method of preparing high-quality nonoxidized graphene

sheets (NGSs) that exhibit single- and multilayered struc-

tures; the process involves directly exfoliating highly ori-

ented pyrolytic graphite (HOPG), by exploiting cation–pinteractions7,8 through the adsorption of pyridinium ions on

the graphene sheets. Colloidal stability is obtained in the gra-

phene sheets in a water-ethanol mixture through the

Coulomb repulsion between the pyridinium-adsorbed sheets.

Since no chemical oxidation or reduction is involved in

exfoliation, defect-associated deterioration in the electronic

properties of graphene can be avoided. Although graphene

layers have been demonstrated as electrodes in organic

light-emitting diodes, graphene or GO have rarely been used

as charge transport layers in optoelectronic devices; never-

theless, pristine graphene has a mismatched work function

with that of common organic emitters.9,10 The processable

NGSs prepared in this study shows their multifunctionality

in polymer light-emitting diodes (PLEDs), in which the

imprinted NGS layer works as a successful hole transport

layer (HTL), replacing typical hole transport materials such

as poly (3,4-ethylenedioxythiophene)-polystyrenesulfonic

acid (PEDOT-PSS). By using this imprinting technique, the

proposed NGS layer can be integrated into hybrid

light-emitting diodes (LEDs) incorporating metal oxides and

inorganic quantum dots in the future.11–13

First, proper amounts of HOPG (approximately 1 mg)

and PyþBr3– (1 M) were mixed with a water-ethanol mixture

(1:1, 7 ml), in which PyþBr3– dissociated into Pyþ and Br–.

After 45 min of sonication in a low-power sonic bath, a

transparent NGS suspension formed. The resulting suspen-

sion was centrifuged (5000 rpm, 5 min) to remove any unex-

foliated species. The decanted dispersion was further diluted

for subsequent deposition of the HTL. No precipitation or

agglomeration of the NGSs was observed after the solution

was stored for several months. For fabricating the device, the

patterned indium tin oxide (ITO)/glass substrates were pre-

cleaned in sequence by using detergent, acetone, isopropa-

nol, and deionized water. An NGS layer was then

spin-coated onto a polydimethylsiloxane (PDMS) stamp at

2000 rpm for 30 s and dried in an oven at 70 �C. The NGS

layer on the PDMS stamp was subsequently transferred to a

substrate pretreated with O2-plasma (120 W, 1 min), by ther-

mal imprinting at 80 �C. The emissive poly (9,9-dioctylfluor-

ene-co-benzothiadiazole) (F8BT) layer was then spin-coated

onto the graphene HTL at 3000 rpm for 30 s. Finally, a Ca/Al

cathode (30/120 nm thick) was thermally deposited toa)Electronic mail: laputa@nttu.edu.tw

0003-6951/2014/104(7)/073111/3/$30.00 VC 2014 AIP Publishing LLC104, 073111-1

APPLIED PHYSICS LETTERS 104, 073111 (2014)

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complete the device. For comparison, devices respectively

using spin-coated NGS and PEDOT-PSS (Baytron PH500

from HC Starck) HTLs were fabricated by the aforemen-

tioned processes. The overlap of the ITO and Al electrodes

defined the active device area as 1.5 mm2. For the conven-

ience of discussion, the devices that involved imprinted gra-

phene, spin-coated graphene, and PEDOT-PSS HTLs were

named as Devices A, B, and C, respectively.

As shown schematically in Fig. 1, exfoliation is

achieved using amphiphilic Pyþ cations in an aqueous solu-

tion; the Pyþ tends to adsorb on the exposed edges of the

graphene surfaces through cation–p interactions, and the

repulsion force between Pyþ cations on adjacent graphene

layers competes with the original van der Waals forces, caus-

ing increased interlayer spacing. The arrows in Fig. 1 indi-

cate that large spaces allow increasing numbers of cations to

penetrate and adsorb. The cations act as molecular wedges,

separating the graphene layer lying outside and eventually

causing the exfoliation of NGSs. Electrostatic stabilization

further affords the NGSs as a stable aqueous suspension.

The transmission electron microscopy (TEM) micrograph

(Fig. 2) shows the low contrast plates of the NGSs, demon-

strating single- or multilayered structures that exhibit lateral

dimensions of several tens to hundreds of nanometers. The

wrinkles (indicated by the arrow) should be the scrolled

edges or crumples of a continuous graphene sheet14 that pre-

vent the NGS layer from collapsing back to a stacked gra-

phitic structure. To evaluate the structural disorder and

defects in NGSs, the Raman spectra of the HOPG and NGSs

were analyzed using a dispersive Raman spectrometer at an

excitation of 532 nm. As shown in Fig. 3, the Raman spec-

trum of the HOPG sample showed an intense G band at

1580 cm�1 and a broad second-order D band (2D) at approx-

imately 2700 cm�2. By contrast, the NGS sample exhibited a

weak D band at 1339 cm�1 and an upshifted G band at

1591 cm�1, demonstrating a D/G intensity ratio of 0.055;

this indicates that the exfoliated NGSs were nearly

defect-free, yielding a low degree of functionalization and

density of disorder in the sp2 carbon lattice15 although the

NGSs were small. In addition, the symmetric 2D band was

centered at 2676 cm�1, with a full width at half maximum of

15 cm�1. This symmetrical and narrow width in the 2D band

of the NGSs was caused by the single-layered graphene.15

To highlight the importance of the imprinting process in

NGS film deposition, it is worth mentioning that the gra-

phene layer imprinted on the ITO substrate demonstrated a

superior transmittance of light at 550 nm compared with the

spin-coated graphene layer (93% versus 84%), as shown in

Fig. 4. The matted surface was caused by the chemical reac-

tion of the acidic PyþBr3– solution and ITO. Figure 5 shows

the current-voltage (I-V) and luminance-voltage (L-V) char-

acteristics of Devices A, B, and C. Device A, which involved

an imprinted NGS layer, showed a similar injection current

to Device C. In Device B (the spin-coated NGS HTL), the

abrupt increase of current at bias voltages exceeding 12 V

behaves analogous to the breakdown of junction of

NGS/F8BT, leading to device failure. This may arise from

the distinct surface morphologies of the two films because

FIG. 1. Exfoliating HOPG into single- and multilayered NGSs through

Pyþ-cation adsorption. The Pyþ cations were represented in exaggerated

sizes to demonstrate their distribution and effects.

FIG. 2. TEM image of the exfoliated NGSs.

FIG. 3. Raman spectra comparison for the bulk HOPG and NGSs.

FIG. 4. Transmission spectra of the imprinted and spin-coated NGS layers.

073111-2 Huang et al. Appl. Phys. Lett. 104, 073111 (2014)

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the imprinted layer has a glare surface whereas the

spin-coated layer has a matted one. It is easily appreciated

that the morphology of emissive F8BT layer on spin-coated

NGS HTL with a matted surface was rough and defective.

A rough morphology is not good for definitive junction for-

mation due to the barriers for hole injection.16 The substan-

tial imbalance between injected holes and electrons and

dense nonradiative recombination centers within the defec-

tive F8BT layer inhibits photon generation. Furthermore, the

similar light turn-on voltages of Devices A and C suggests

that the holes in the NGS/F8BT and PEDOT-PSS/F8BT

interfaces encountered similar energy barriers. Considering

the known work function of graphene (approximately

4.56 eV), it is difficult to understand the high hole transport

efficiency of the imprinted NGS layer in Device A.17 To

evaluate the work function of the imprinted NGS film, the

photoelectron emission of NGS and PEDOT-PSS HTLs

were measured by a Riken Keiki AC-2 spectrometer.18 As

shown in the inset of Fig. 5(a), the derived work functions

were 5.6 and 5.1 eV for the imprinted NGS and

PEDOT-PSS, respectively. Studies have demonstrated that

the work function of graphene can be modified using metal

contacting,19 doping,20 or noncovalently functionalizing the

graphene with organic molecules.21 Based on these studies,

we proposed that the adhesion of Pyþ ions or even Py on the

NGS surfaces may have influenced their electronic band

structures. Figure 5(b) shows that the luminance of Device A

biased at 13.2 V was as high as 13615 cd/m2, corresponding

to an efficiency of 1.2 cd/A. The inset of Fig. 5(b) shows that

the F8BT emitted yellow-green light in Device A, which was

operated at 3 mA.

In conclusion, single- and multilayered NGSs dispersed

in a water-methanol mixture were fabricated by exfoliating

Pyþ-ion-adsorbing graphite. The Raman spectra of the NGSs

exhibited a low intensity ratio (0.055) of D and G bands,

implying that the exfoliated NGSs have few defects. When

the ITO substrates were coated with imprinted NGSs for hole

transport, the transmittance level of the glare surface

increased (>90%). The PLED fabricated using this imprinted

NGS layer exhibited a luminance exceeding 13 000 cd/m2,

which was comparable to that of a PLED with a typical

PEDOT-PSS HTL.

The authors are grateful to the National Science Council

of the Republic of China, Taiwan, for funding this research

(No. NSC 102-2221-E-143-005).

1P. Kumar, A. K. Singh, S. Hussain, K. N. Hui, K. S. Hui, J. Eom, J. Jung,

and J. Singh, Rev. Adv. Sci. Eng. 2, 238 (2013).2J. Wu, M. Agrawal, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen, and P.

Peuman, ACS Nano 4, 43 (2010).3M. P. Ramuz, M. Vosgueritchian, P. Wei, C. Wang, Y. Gao, Y. Wu, Y.

Chen, and Z. Bao, ACS Nano 6, 10384 (2012).4S. Bae, H. Kim, Y. Lee, X. Xu, J. Park, Y. Zheng, J. Balakrishnan, T. Lei,

H. Kim, Y. Song, Y. Kim, K. Kim, B. €Ozyilmaz, J. Ahn, B. Hong, and S.

Iijima, Nature Nanotechnol. 5, 574 (2010).5O. C. Compton and S. T. Nguyen, Small 6, 711 (2010).6H. R. Thomas, S. P. Day, W. E. Woodruff, C. Valles, R. J. Young, I. A.

Kinloch, G. W. Morley, J. V. Hanna, N. R. Wilson, and J. P. Rourke,

Chem. Mater. 25, 3580 (2013).7S. P. Lonkar, A. Bobenrieth, J. De Winter, P. Gerbaux, J. Raquez, and P.

Dubois, J. Mater. Chem. 22, 18124 (2012).8I. W. P. Chen, S. H. S. Jhou, and Y. W. Chen, J. Mater. Chem. C 1, 5970

(2013).9D. Yang, L. Zhou, L. Chen, B. Zhao, J. Zhang, and C. Li, Chem.

Commun. 48, 8078 (2012).10S. Shi, V. Sadhu, R. Moubah, G. Schmerber, Q. Bao, and S. R. P. Silva,

J. Mater. Chem. C 1, 1708 (2013).11T. P. Chen, F. Y. Hung, S. P. Chang, S. J. Chang, Z. S. Hu, and K. J.

Chen, Appl. Phys. Express 6, 045201 (2013).12C. Y. Huang and Y. C. Chen, Int. J. Electrochem. Sci. 8, 3572 (2013).13C. Y. Huang, T. H. Wu, C. Y. Cheng, and Y. K. Su, J. Nanopart. Res. 14,

866 (2012).14J. Yan, J. Liu, Z. Fan, T. Wei, and L. Zhang, Carbon 50, 2179 (2012).15A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F.

Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim,

Phys. Rev. Lett. 97, 187401 (2006).16J. Liu, Y. Shi, L. Ma, and Y. Yang, J. Appl. Phys. 88, 605 (2000).17R. Yan, Q. Zhang, W. Li, I. Calizo, T. Shen, C. A. Richter, A. R. Hight-

Walker, X. Liang, A. Seabaugh, D. Jena, H. G. Xing, D. J. Gundlach, and

N. V. Nguyen, Appl. Phys. Lett. 101, 022105 (2012).18T. Ikuno, R. Suzuki, K. Kitazumi, N. Takahashi, N. Kato, and K. Higuchi,

Appl. Phys. Lett. 102, 193901 (2013).19K. M. McCreary, K. Pi, and R. K. Kawakami, Appl. Phys. Lett. 98,

192101 (2011).20T. Han, Y. Lee, M. Choi, S. Woo, S. Bae, B. H. Hong, J. Ahn, and T. Lee,

Nature Photon. 6, 105 (2012).21Y. Wang, X. Chen, Y. Zhong, F. Zhu, and K. P. Loh, Appl. Phys. Lett. 95,

063302 (2009).

FIG. 5. (a) Current-voltage and (b) luminance-voltage curves of Devices A,

B, and C. Inset of (a) shows the curves of square root of the photoelectron

emission intensity for the NGS and PEDOT-PSS films as a function of irra-

diated photon energy. Inset of (b) is an image of the light emission from

Device A.

073111-3 Huang et al. Appl. Phys. Lett. 104, 073111 (2014)

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