Multilayer Graphene with Chemical Modification as Transparent Conducting Electrodes in Organic Light-Emitting Diode
© The Author(s). 2017
Received: 13 January 2017
Accepted: 19 March 2017
Published: 5 April 2017
Graphene is a promising candidate for the replacement of the typical transparent electrode indium tin oxide in optoelectronic devices. Currently, the application of polycrystalline graphene films grown by chemical vapor deposition is limited for their low electrical conductivity due to the poor transfer technique. In this work, we developed a new method of preparing tri-layer graphene films with chemical modification and explored the influence of doping and patterning process on the performance of the graphene films as transparent electrodes. In order to demonstrate the application of the tri-layer graphene films in optoelectronics, we fabricated the organic light-emitting diodes (OLEDs) based on them and found that plasma etching is feasible with certain influence on the quality of the graphene films and the performance of the OLEDs.
Transparent conductive materials play a significant role in optoelectronic devices such as solar cells [1–6], sensors [7–10], and organic light-emitting diodes (OLEDs) [11–13] and have attracted wide attention from the research community. The most conventionally used transparent conductive material is indium tin oxide (ITO), which has high optical transparency and electrical conductivity . Nevertheless, the increasing cost of indium, brittleness, and photoelectric attenuation due to indium diffusion limited the development of the OLEDs. Graphene, as a member of the two-dimensional material community, has excellent photoelectric performance, such as ultra-high carrier mobility and transparency, which shows tremendous potential to replace ITO as transparent conductive electrodes in photoelectric devices [15, 16]. The common method of preparing large-area graphene films is through chemical vapor deposition (CVD) on metallic substrates, which typically contains the transfer of the as-grown graphene onto target substrates for further device fabrication. However, the quality of graphene is greatly influenced by the growth conditions, and the high sheet resistance is still caused by the polycrystallinity, wrinkles, and impurities introduced during the transfer process [17–19]. It has attracted wide attention to further improve the conductivity of graphene films on the premise of ensuring high transmittance and achieving the application of graphene films as the anodes of the OLEDs. Yu Wang et al.  reported that they explored the application of laminated graphene as the transparent electrodes in organic solar cells. Chen Nie et al.  reported that they used vinylidene chloride/acrylic ester copolymer (OA)/dichloromethane solution to modify graphene and reduce the sheet resistance to 300 Ω/□. Moreover, they utilized the graphene/PEDOT:PSS composite transparent conductive films as the anodes to fabricate flexible blue OLED devices with high current efficiency. However, the conductivity of graphene films decreases over time as the OA doping is not stable; it is difficult to satisfy the needs of industrial application. Jaehyun Moon et al.  recently reported that the graphene patterning using laser or plasma methods turned out to be problematic and could not preserve graphene quality. In this article, we developed a new method of preparing multilayer graphene with the chemical modification, which can improve its sheet resistance to 150 Ω/□ with the optical transmittance above 91% in the visible spectrum. We explored the influence of the patterning process using plasma on the graphene transparent electrodes. In addition, we prepared OLED devices based on the graphene anodes, and achieved reasonable luminous efficiency.
Preparation of Single-Layer Graphene Films
The solution of polymethylmethacrylate (PMMA) in anisole was spin-coated on the copper foils with graphene for 30–60 s at 3000–6000 rpm (the thickness of PMMA films is about 200 nm).
The samples of Cu/graphene/PMMA were floated on FeCl3 aqueous solution to remove the copper foils.
The PMMA/graphene film was washed with deionized water and moved onto the glass substrate.
Finally, the PMMA/graphene films were immersed in acetone to remove the PMMA.
Preparation of the Tri-Layer Graphene Film with Chemical Modification
Fabrication and Measurement of OLEDs
The tri-layer-doped graphene film was patterned by photolithography technology and etching. OLED devices were fabricated with a structure of graphene/N,N′-diphenyl-N,N′-bis (1-naphthyl)-(1,1′-biphenyl)-4,4′-diamin (NPB) (60 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/lithium fluoride (LiF) (1 nm)/Al (80 nm), where NPB was used as the hole transport material, Alq3 was used as both the host material and the electron transport material, and LiF and Al were used as the electron inject material and the cathode material, respectively.
To characterize the graphene films, field-emission scanning electron microscope (SEM, FEI Quanta 600), transmission electron microscope, and atomic force microscope (NT-MDT) were used to examine the surface morphology. Raman measurements were performed using Thermo Scientific DXR Raman microscope spectrometer with a laser wavelength of 532 nm. For sheet resistance measurements, we used a semiconductor analyzer (Agilent, B1500A) combined with a four-probe station (CASCADE, alessi REL-4800). The optical transmittance in the wavelength range of 300–1100 nm was obtained by a PV Measurements QEX10. The current–voltage (I–V) characteristics of the fabricated OLEDs were measured with an experimental setup including a Keithley 2400 source meter. A spectroradiometer (PR750) was also employed to measure the electroluminescence spectrum of the 3 × 3 mm2 emitting area of the devices. The reference OLEDs with the same layer structures, except that the graphene film was replaced by a conventional ITO layer (20 Ω/□), were also fabricated for comparison.
Results and Discussion
Characterization of the Graphene Films
Characterization of the Tri-Layer Graphene Film with Chemical Modification
Sheet resistance and transmittance at 550 nm of the pristine and doped graphene films, as shown in Fig. 3c
No. of layers
Characterization of the OLEDs
In conclusion, we developed a new method to prepare tri-layer graphene films with chemical modification and explored the influence of doping and patterning process on the performance for the graphene films as transparent electrodes. We have demonstrated that the tri-layer graphene films can be used as transparent and conductive anodes in the OLEDs. The performance of the devices based on the tri-layer graphene films doped with AuCl3 is higher than that of the devices based on the tri-layer pristine graphene film due to the contribution of high conductivity. These results suggest that the tri-layer-doped graphene films are a promising candidate as the transparent conductive electrode in optoelectronic devices.
This research was supported by the Shanghai Sailing Program (15YF1413200) and the CAS/SAFEA International Partnership Program for Creative Research Teams.
YX and XF designed the experiments and analyzed the data. YX and CW prepared the manuscript. HY and JC prepared the fabrication of the OLEDs. YC, ZM, and XC supervised the project and led the overall effort. YX, YY, and JW prepared the samples and carried out the characterization of the graphene film. All authors discussed the experimental results and commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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