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Two-in-One Method for Graphene Transfer: Simplified Fabrication Process for Organic Light-Emitting Diodes Lihui Liu, Wenjuan Shang, Chao Han, Qing Zhang, Yao Yao, Xiaoqian Ma, Minghao Wang, Hongtao Yu, Yu Duan, Jie Sun, Shufen Chen, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19039 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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ACS Applied Materials & Interfaces

Two-in-One Method for Graphene Transfer: Simplified Fabrication Process for Organic Light-Emitting Diodes Lihui Liu,†, § Wenjuan Shang,†, § Chao Han,† Qing Zhang,† Yao Yao,† Xiaoqian Ma,† Minghao Wang,† Hongtao Yu,† Yu Duan,‖ Jie Sun,‡,﹟ Shufen Chen†,* and Wei Huang†,⊥,*

†

Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China

‡

Key Laboratory of Optoelectronics Technology, College of Microelectronics, Beijing University of Technology, Beijing100124, China.

﹟

Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Göteborg 41296, Sweden

‖

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China.

⊥

Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, Shaanxi, China.

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ABSTRACT: Graphene as one of the most promising transparent electrode materials has been successfully applied in organic light-emitting diodes (OLEDs). However, traditional poly (methyl methacrylate) (PMMA) transfer method usually results in hardly removed polymeric residue on the graphene surface, which induces unwanted leakage current, poor diode behavior and even device failure. In this work, we proposed a facile and efficient two-in-one method to obtain clean graphene and fabricate OLEDs, in which the poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene) (TFB) layer was inserted between graphene and PMMA film as both a protector during graphene transfer and a hole-injection layer in OLEDs. Finally, green OLED devices were successfully fabricated on the PMMA-free graphene/TFB film, and the device luminous efficiency was increased from 64.8 to 74.5 cd/A by using the two-in-one method. Therefore, the proposed two-in-one graphene transfer method realizes a high efficient graphene transfer and device fabrication process, which is also compatible with the roll-to-roll manufacturing. It is expected that this work can enlighten the graphene based optoelectronic device design and fabrication.

KEYWORDS: graphene, PMMA, transfer method, hole-injection layer, F4TCNQ, OLEDs


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INTRODUCTION Recently, graphene, a one-atom-thick sp2-hybridized network of carbon atoms, has drawn tremendous interest because of its extraordinary electrical, optical and mechanical properties including high intrinsic mobility of 200 000 cm2 V−1 s−1, excellent optical transparency of >97% and high Young’s modulus of 1100 Tpa.1-5 Thus, the rich diversity of the physical and chemical properties of graphene merits a wide range of applications such as organic light-emitting diodes (OLEDs),6-7 solar cells,8-9 field-effect transistors (FETs),10-11 memory devices,12 supercapacitors,13 sensors and so on.14 Graphene has demonstrated to be a promising candidate to replace the commercial transparent electrodes, such as indium tin oxide (ITO), which hinders the future development of flexible organic electronics due to its increasing cost and the lack of mechanical flexibility.15-17 Various methods have been proposed for the preparation of high quality graphene films, and one of the more promising technologies is the chemical vapor deposition (CVD) method, which synthesizes graphene films of the large-area, low sheet resistance and high transparency.18-19

In the decades after realizing large area production, graphene has been successfully applied in OLEDs and tremendous developments have been achieved. In 2012, Han et al. achieved extremely high current efficiency (CE) of 98.1 cd/A and power efficiency (PE) of 102.7 lm/W in flexible phosphorescent OLEDs with four-layered graphene by modifying the surface with conducting polymer, which constructed a gradient work function and enabled holes to be injected easily.20 In 2013, Li et al. fabricated white OLEDs by using triethyloxonium hexachloroantimonate-doped graphene and light out-coupling structures, 3 ACS Paragon Plus Environment


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which showed CE larger than 120 cd/A at 10000 cd/m2, yielding OLEDs with brightness and efficiency sufficient for general lighting.21 In 2016, Lee et al. proposed an ideal electrode structure based on a synergetic interplay of high-index TiO2 layers and low-index hole-injection layers sandwiching graphene electrodes, which leads to ultrahigh external quantum efficiency (EQE) of 40.8% for single-junction OLEDs.22 Han et al. realized flexible tandem OLEDs based on graphene anode with a very high CE of ~205.9 cd/A, and EQE of 45.2%.23 However, the pristine graphene is suffering from the mismatch work-function and high sheet resistance drawbacks compared with ITO.24-25 Extensive researches have been carried out to adjust the work function and improve the electrical conductivity via chemical doping treatments or hybridization.26-32 Additionally, the electrical properties of transferred graphene are heavily dependent on the transfer process, the batch-to-batch variation of the graphene film quality induces to the low yield of the graphene based OLEDs.33-35 The traditional CVD-graphene transfer method is to employ poly (methyl methacrylate) (PMMA) as a supporting layer, which will be removed with acetone after transferring graphene/PMMA to the target substrates.

PMMA, the most commonly used graphene transfer material, can cause local rehybridization of carbon atoms from sp2 to sp3 on graphene defects because of their long-chain structure.36 Therefore, a large amount of PMMA residue is usually left on the graphene surface along with many defects, which will result in bad film formation, leakage current, poor diode behavior and even device failure.37 In order to obtain a clean graphene film with less PMMA residue, some approaches have been developed, such as hot acetone 4 ACS Paragon Plus Environment

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vapor treatment and thermal annealing in hydrogen environment.36-40 Nevertheless, the annealing is not suitable for flexible substrates because of the high annealing temperature, and acetone vapor treatment usually takes several hours to remove PMMA residue, which is a time-consuming process and reduces the production efficiency of graphene transfer and device fabrication. Therefore, it is urgent needed to develop effective and efficient transfer technologies for high quality and clean graphene. In order to resolve the PMMA residue problem, Han et al. developed a sandwich structure of graphene/2-(diphenylphosphory) spirouorene (SPPO1)/PMMA.41 Zhang et al. reported a transfer method by using rosin as a support layer and claimed a decreased amount of residue from the polymer.42 These developed support materials have weaker interaction with graphene and better solubility, cleaner graphene can be achieved. However, the support materials still need to be removed after transferring onto arbitrary substrates, which may induce the risk of unremoved residue of support materials on graphene surface.

In this work, we demonstrated a two-in-one method to transfer graphene for OLED application. Different from the traditional transfer approach, the conductive polymer poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene) (TFB) doped with tetrafluoro-tetracyano-quinodimethane (F4TCNQ) was inserted between PMMA and graphene. After removing PMMA with acetone that is an orthogonal solvent for TFB, TFB layer is left on graphene, which will act as hole injection layer in OLEDs, and then the OLED devices can be fabricated onto the TFB layer directly. Using this kind of transfer method, the influence of support material residues on graphene can be reduced to a minimum. 5 ACS Paragon Plus Environment


OLEDs based on the two-in-one transferred graphene were fabricated successfully with CE of 74.5 cd/A, which exhibits a much enhanced performance compared with the counterpart one based on the traditional transferred graphene.

MATERIALS AND METHODS

Materials

Graphene films were synthesized with a CVD process using copper (Cu) foils as substrates. The detailed synthesis process refers to previous reports.18-19 PMMA with molecular weight of 30 000 000 was dissolved in anisole with 6 wt. % concentration. F4TCNQ, TFB, 1,10-bis(di-4-tolylaminophenyl) cyclohexane (TAPC), 1,3-bis(carbazol-9-yl)benzene (mCP), tris(2-phenylpyridine)

iridium

(Ir(ppy)3)

and

1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene

(TmPyPB) were purchased from Lumtec Tech. Corp. All materials were used as received without further purification. Material chemical structure and energy level were shown in Figure S1.

Graphene transfer

For the traditional transferred graphene, PMMA was spin-coated onto the Cu/graphene surface with the thickness of ~550 nm, followed by annealing at 120 oC for 15 min. While for our TFB/PMMA transferred graphene, before preparing the PMMA layer, TFB was firstly spin-coated onto the Cu/graphene surface from chlorobenzene solution with the thickness of ~40 nm, followed by annealing at 130 oC for 30 min. With a subsequent coating of PMMA, the graphene/PMMA or graphene/TFB/PMMA layers were delaminated from Cu foil by 6 ACS Paragon Plus Environment

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using a high efficient and nondestructive electrochemical bubbling delamination method in NaOH electrolyte.43-44 Afterwards, the graphene/PMMA or graphene/TFB/PMMA films were rinsed with deionized water to remove residual electrolyte, and then transferred onto the target glass substrate followed by annealing at 160 °C for 30 min. Finally, glass/graphene/PMMA or glass/graphene/TFB/PMMA samples were rinsed with acetone to remove the PMMA layer.

Film characterization

Raman investigations were performed with Renishaw inVia Raman Microscope equipped with a 532 nm He-Ne laser. The surface topographies of the graphene, graphene/TFB (spin-coated on the transferred graphene), graphene/TFB/PMMA and graphene/TFB (removed PMMA) samples were characterized by optical microscopy. The thicknesses of the films were determined using a Bruker DektakXT Stylus Profiler. The transmittance spectra and sheet resistance of transferred graphene were measured with an ultraviolet-visible spectrophotometer (Shimadzu, UV-3600) and a four-point probe (RTS-9, China).

Device fabrication and measurement

The OLEDs were fabricated with a structure of graphene/TFB: F4TCNQ (x %) (40 nm)/TAPC (40 nm)/mCP: Ir(ppy)3 (10%) (30 nm)/TmPyPB (50 nm)/LiF (0.5 nm)/Al (100 nm). For the TFB/PMMA-transferred graphene, TAPC, mCP, Ir(ppy)3, TmPyPB, LiF, and Al were thermally evaporated on the graphene/TFB surface directly at a pressure of less than 4×10-4 Pa in the vacuum chamber. For the PMMA-transferred graphene anode, TFB: 7 ACS Paragon Plus Environment


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F4TCNQ (x %) was spin-coated on graphene surface from chlorobenzene solution, followed by annealing at 130 oC for 30 min in a glove box. The hole-only devices were fabricated with the structure of ITO/TFB: F4TCNQ (x %) (105 nm)/ MoO3 (5 nm)/Al (100 nm). The current density-voltage-luminance (J-V-L) characteristics and electroluminescence (EL) spectra of the graphene based OLEDs were measured with a Keithley 2400 source meter and a coupled PR655 spectroscan photometer. All measurements were carried out at room temperature under ambient condition.

RESULTS AND DISCUSSION The processes of the two-in-one method proposed in this work are illustrated in Figure 1, with a traditional approach also attached. In the traditional graphene transfer method (Figure 1a), the PMMA layer was spin-coated on the Cu/graphene surface, which contacted with the graphene film directly. After the delamination of the PMMA-protected graphene from the Cu foil, PMMA was removed from graphene with acetone solvent. Then ~40 nm of TFB was spin-coated on graphene surface to play a role as the hole-injection layer and other OLED functional layers were fabricated subsequently. In the two-in-one graphene transfer method as shown in Figure 1b, the TFB layer with a thickness of ~40 nm was deposited on the graphene surface firstly, subsequently the PMMA layer was spin-coated onto the TFB film to provide sufficient mechanical protection during transfer and prevent TFB from contacting with electrolyte solution. After the PMMA/TFB-protected graphene being transferred onto the target substrates, PMMA was removed with acetone. At last, the OLED devices were fabricated onto the TFB layer. This TFB layer not only can protect the graphene during the 8 ACS Paragon Plus Environment

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transfer process, but also can act as the hole-injection layer in the OLEDs, thus it was named as the two-in-one graphene transfer method.

For the PMMA transferred graphene, the transmittance at 550 nm was 97.6% as shown in Figure S2a, indicating the single layer intrinsic property of the pristine graphene. It was noted that there were obvious PMMA residue and cracks after cleaning with acetone. Figure S2b illustrated the statistical sheet resistance values of graphene, which converged from 1050 to 1400 Ω/sq. The surface morphology and thickness of graphene/TFB (spin-coated on the transferred graphene), graphene/TFB/PMMA and graphene/TFB (removed PMMA) films were collected with optical microscopy and stylus profiler as shown in Figure 2. The wettability and spreadability of TFB solution on the graphene surface is excellent indicated by the contact angle measurement in Figure S3, the TFB film spin-coated on graphene transferred via the traditional method is uniform and continuous (Figure 2a). Figure 2b shows that the graphene/TFB/PMMA film is continuous and crack-free, indicating the successful transfer of graphene via our two-in-one method. Besides, it should be noted that the polygonal pattern in the graphene/TFB/PMMA film was attributed to the polycrystalline Cu foil for graphene synthesis. After removal of PMMA, the TFB film showed a smooth and homogenous surface (Figure 2c). Experiments to investigate the influence of acetone rinsing process on TFB film were also carried out. In order to illustrate this point clearly, surface morphology and film thickness of TFB film spin-rinsed with chlorobenzene (a good solvent for TFB) were also investigated, and the results in Figure S4 shown that most of the TFB film was removed away, a discontinuous film with thickness of ~ 17 nm was remained. 9 ACS Paragon Plus Environment


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Differently, the surface morphology and film thickness of TFB film before and after rinsing in acetone were unchanged, indicating that TFB was completely remained on the graphene surface since acetone is an orthogonal solvent for TFB. As the three films performed in Figure 2d, the thickness of the graphene/TFB/PMMA film was ~550 nm, and the thickness decreased to ~40 nm after removing PMMA, which was almost the same as that of TFB directly spin-coated on the graphene with the traditional transfer method (Figure 2a). The proposed two-in-one method can be carried out into practice successfully with the smooth and uniform graphene/TFB film, which offers the possibility to fabricate OLED devices subsequently.

Furthermore, the surface chemical composition of the TFB film was needed to be clarified considering that it was challengeable to remove PMMA completely. Based on the Raman spectra shown in Figure 3a, the pristine PMMA film showed the characteristic band at 2952 cm−1.43 This band indicates the C-H stretching vibration, which is the most prominent in the PMMA structure. The other Raman bands were conformity with literature, such as those appearing at 1451 cm−1 were from δa(C-H) of α-CH3 and O-CH3, 1728 cm−1 were from υ(C=O) of (C-COO).45 The neat TFB showed the characteristic band at 1609 cm-1, which indicated the fluorene ring stretching. These unique Raman peaks of TFB and PMMA can be used as chemical signatures for the materials to qualitatively determine the compositions and distributions of the TFB film surface.46 For the graphene/TFB (removed PMMA) film, there was a strong peak at 1609 cm-1 from TFB and a band at 2690 cm-1. In order to clarify the originate of Raman peak at 2690 cm-1, Raman characterizations for neat TFB, graphene/TFB 10 ACS Paragon Plus Environment

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(removed PMMA) and graphene samples were compared systematically, and it was found that there was Raman band at 2687 and 2690 cm-1 for graphene and graphene/TFB samples respectively, which was absent for the neat TFB film. Thus, the Raman band at 2690 cm-1 of the graphene/TFB (removed PMMA) film was attributed to the under graphene layer. And the slight shift in wavenumber was due to the p-doping effect from TFB. Finally, Raman characterization on selected 16 different zones of the graphene/TFB (removed PMMA) film was carried out (Figure 3b). It can be found that there was no unique Raman peaks of PMMA over all the film surface, indicating that the PMMA has been removed completely via the two-in-one transfer method.

Furthermore, in order to investigate the mechanism for the cleaner surface of graphene/TFB than graphene after PMMA removing, the interfacial free energies γ1/2 between polymers and graphene were calculated.47, 48 The relative surface free energies γ of graphene, PMMA, and TFB are listed in Table 1. The interfacial free energies γ1/2 between polymers and graphene can be calculated by the following formalism:

ߛଵ/ଶ = ߛଵ + ߛଶ −

ସఊభ೏ ఊమ೏

೏ −

ఊభ೏ାఊమ

೛ ೛

ସఊభ ఊమ

೛ ೛ ఊభ ାఊమ

(1)

where γd and γp are the dispersion and polar component of surface free energy, respectively. The values of the interfacial free energies calculated are as follows: γgraphene/PMMA= 32.3 mN/m; γgraphene/TFB= 15.2 mN/m; γTFB/PMMA=54.6 mN/m, respectively. Thus, γgraphene/TFB< γgraphene/PMMA< γTFB/PMMA, indicating TFB/PMMA has the most unstable interface since it shows much higher interfacial free energy than others, the relative higher 11 ACS Paragon Plus Environment


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interfacial free energy is not energetically favored.48 Therefore, the PMMA should be much easier removed from TFB film than graphene surface, which is accordance with the aforementioned cleaner surface of graphene/TFB than graphene after PMMA removing.

The two-in-one method transferred graphene without PMMA residue offers the possibility to fabricate high-performance OLEDs compared with the traditional one. The OLEDs have been fabricated with the structure of graphene/TFB: F4TCNQ (x %) (40 nm)/TAPC (40 nm)/mCP: Ir(ppy)3 (10%) (30 nm)/TmPyPB (50 nm)/LiF (0.5 nm)/Al (100 nm) as illustrated in Figure 4a. Because of the low conductivity of TFB polymer, F4TCNQ was doped into TFB to improve the conductivity.50 As shown in Figure S5, the hole carrier current density was increased by one order with adding 7.5 wt. % F4TCNQ in TFB, and two orders with adding 10 wt. % F4TCNQ. First of all, OLEDs with various concentrations of F4TCNQ (0, 5, 7.5 and 10 wt. %) were fabricated on ITO substrates. The device with F4TCNQ of 7.5 wt. % performed the best efficiency of 79.3 cd/A as shown in Figure S6 and Table S1. Secondly, OLEDs based on traditional and two-in-one method transferred graphene were fabricated with the aforementioned optimized F4TCNQ concentration of 7.5 wt. %. Figure 4b depicted the J-L-V characteristics, in which the J and L of the OLEDs based on two-in-one method transferred graphene were higher than those of the traditional one at each specific driven voltage. The turn-on voltage (Von) and maximum luminance (Lmax) for the device based on two-in-one transfer method were 3.9 V and 39100 cd/m2, while 5.3 V and 31190 cd/m2 for the traditional one. The resulting maximum CE and PE were improved from 64.8 cd/A and 18.5 lm/W to 74.5 cd/A and 26.6 lm/W as shown in Figure 4c. And EQE was calculated from 12 ACS Paragon Plus Environment

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CE and EL spectra, which increased from 18.1% to 20.7% as shown in Table 2. It has been reported that the insulating PMMA residue on graphene is an inhibitor for charge extraction, resulting in larger sheet resistance and non-uniform lighting.36-42 Thus, the reduced driving voltage and improved device efficiency can be attributed to the much less PMMA residue on graphene/TFB than graphene surface. Device performances for OLEDs based on both traditional and two-in-one methods were summarized in Table 2. Figure 4d shows the normalized EL spectra of the green OLED measured at voltages from 7 to 15 V. The spectra for both of the devices were similar with a pure green emission from Ir(ppy)3, indicating that different transfer methods of graphene films didn’t cause any significant alteration in the EL spectra. And the identical EL spectra obtained at different voltages indicate the good stability of the graphene based OLEDs.

CONCLUSIONS

In summary, we have proposed a facile and efficient two-in-one method, in which the TFB layer plays a role as both the protector during graphene transfer process and the hole-injection layer in the subsequently fabricated OLEDs. After removing PMMA with acetone-the orthogonal solvent for TFB, the TFB film totally remained, showing a smooth, homogenous and PMMA-free surface. Furthermore, green OLED devices were directly fabricated on the PMMA residue-free TFB films. OLEDs based on the two-in-one method exhibited a much better device performance than those control ones with a traditional approach due to the clean TFB surface without PMMA residue, such as the Von was decreased from 5.3 to 3.9 V, while the CE and PE were improved from 64.8 cd/A and 18.5 lm/W to 74.5 cd/A and 26.6 lm/W. 13 ACS Paragon Plus Environment


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Therefore, the as-proposed two-in-one transfer method of graphene realizes a highly efficient device fabrication process and decreases the production cost. It is expected that this work can enlighten the design and fabrication process of optoelectronic devices based on graphene electrodes.


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FIGURES

Figure 1. The scheme of (a) traditional and (b) two-in-one graphene transfer approaches.



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Figure 2. Optical microscope images of (a) graphene/TFB (spin-coated on traditional transferred graphene), (b) graphene/TFB/PMMA, (c) graphene/TFB (removed PMMA); and (d) their corresponding film thickness values.


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Figure 3. (a) Raman spectra of PMMA, TFB, graphene/TFB (removed PMMA), and pristine graphene; (b) Raman spectra of graphene/TFB (removed PMMA) film conducted on selected 16 different zones. The Raman bands at 1451, 1728 and 2952 cm−1 of PMMA were marked.



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Figure 4. (a) The device configuration of the graphene based OLEDs. (b) J-L-V, (c) CE-PE-J curves, and (d) voltage-dependent EL spectra of OLEDs based on the traditional and two-in-one method transferred graphene. Insert image shows the photograph of the OLED based on the two-in-one method.


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TABLES.

Table 1 The surface free energies of graphene, PMMA and TFB. Where γd and γp were the dispersion and polar component of the surface free energy, respectively. γ (mN/m)

γp (mN/m)

γd (mN/m)

Graphene49

53.0

13.9

39.1

PMMA47

35.1

30.1

5.0

TFB48

36.2

0.1

36.2

Table 2 Summarized device performances of OLEDs fabricated by traditional and two-in-one method transferred graphene. Graphene transfer method

Von (V)

Lmax (cd/m2)

CEmax (cd/A)

PEmax (lm/W)

EQEmax (%)

C.I.E. (x, y) @ 1000 cd/m2

Traditional

5.3 ± 0.1

31190 ± 1240

64.8 ± 1.7

18.5 ± 1.8

18.1 ± 0.5

(0.317 ± 0.001, 0.619 ± 0.001)

Two-in-one

3.9 ± 0.1

39100 ± 1010

74.5 ± 2.3

26.6 ± 1.1

20.7 ± 0.6

(0.319 ± 0.001, 0.619 ± 0.001)



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ASSOCIATED CONTENT

Supporting Information.

The supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx. Additional experimental results and discussion (Table S1 and Figures S1-S6) (PDF)

AUTHOR INFORMATION Corresponding Author *SF Chen, E-mail: [email protected]

*W Huang, E-mail: [email protected]

Author Contributions L.L. and S.C. initiated the research. The transfer of graphene and the fabrication process of OLEDs were conducted by L.L., Y.Y., Q.Z. and W.S. The characteristics of OLEDs were measured by X.M., M.W., H.Y. and C.H. L.L., Y.D., S.J., S.C. and W.H. organized and analyzed all of the data and prepared the manuscript. Notes §

The authors contributed equally to this work.

ACKNOWLEDGMENT


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The authors acknowledge financial supports from National Foundation for Science and Technology Development (973 project, Grant No. 2015CB932203), the National Key Research and Development Program of China (Grant No. 2017YFB0404501), the National Natural Science Foundation of China (Grant Nos. 61274065, 61505086, 61705111, and 61704091), the Science Fund for Distinguished Young Scholars of Jiangsu Province of China (Grant No. BK20160039), the Natural Science Foundation of Jiangsu Province (Grant Nos. BM2012010), the Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. YX030002), the Jiangsu National Synergetic Innovation Center for Advanced Materials, the Synergetic Innovation Center for Organic Electronics and Information Displays, and the Open Foundation from Jilin University (Grant No. IOSKL2017KF04).

J.

Sun

acknowledges

NSFC

(11674016),

Nat.

Key

R&D

(2017YFB0403102), Beijing M. Sci. Tech. (Z161100002116032), and STINT (CH2015– 6202).

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