- Nano Express
- Open Access
Highly Stretchable and Flexible Graphene/ITO Hybrid Transparent Electrode
© Liu et al. 2016
- Received: 9 December 2015
- Accepted: 26 January 2016
- Published: 27 February 2016
The flexible hybrid transparent electrode was prepared by a two-step process: graphene film was firstly grown on Cu foil by modified thermal chemical vapor deposition (CVD) and then transferred onto indium tin oxide (ITO) electrode on the polyethylene terephthalate (PET) substrate. The quality of the graphene is characterized by various analytic techniques, including the AFM, SEM, TEM, and Raman spectroscopy. The gradient flux was found to be beneficial to decrease defect. The thickness, morphology, light transmittance, and electromechanical properties of three conductive electrodes were investigated and compared. The outcomes show that the hybrid electrode could resist mechanical force and the results are better than original ITO electrode. It may be a potential trend to apply the graphene to other conducts in the flexible transparent conductive field.
- Flexible transparent electrode
- Composite materials
- Electromechanical properties
A new generation of flexible devices has been extensively studied for electronics, optoelectronics, and energy harvesting applications [1–4]. The key component for such devices is a flexible and stretchable electrode, which is able to maintain original electrical properties after bending or stretching process. Indium tin oxide (ITO) has been the industrial standard for transparent electrodes in traditional optoelectronic devices . However, ITO is difficult to apply in flexible devices because of its brittleness [6, 7]. To fulfill the growing requirements of flexible and transparent electrodes, several alternative materials have been developed recently. For example, conducting polymers (specifically poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT: PSS)) [8, 9], metallic nanowires [10, 11], and carbon nanotubes  have attracted extensive attention and employed to utilize as flexible electrode materials. But their environmental instability, surface roughness, and surface uniformity hinder the widespread applications [13–15].
Recently, graphene becomes a novel kind of two-dimensional (2D) carbon allotrope with a unique band structure, which shows outstanding thermal, mechanical, and electrical properties . Besides, it is well known that graphene has high transmittance and high electron mobility [15, 16]. There have been efforts to utilize the outstanding properties of graphene for transparent electrodes . Synthesis or deriving process of graphene is one of the most important issues in electrode fabrication. Several methods have been reported . Currently, graphene films prepared by chemical vapor deposition (CVD) method have been recognized as the most promising materials for flexible transparent electrode. However, owing to the high resistance value of graphene films, the stacked multilayer graphene films are required for electrodes, which are fabricated by complicated multiple growth and transfer processes . Combining graphene with other conductor to achieve transparent flexible electrodes is an effective way, owning to its outstanding electrical and optical properties . Previous works of combining CVD-grown graphene with metal microstructure has been done. For instance, the combination of Ag nanowires and graphene for use as flexible transparent electrodes has been reported . While the difficulty of precisely controlling Ag nanowire distribution on film limits the further development. The above studies show that hybrid electrodes not only reduce resistance but also improve flexibility effectively. Moreover, transparent conductive oxide films are widely used in electrodes, such as ITO and aluminum-doped zinc oxide (AZO). The rigidity of these materials could be enhanced by the introduction of graphene with excellent mechanical properties. However, to our knowledge, the properties of these hybrid electrodes have not been systematically researched.
Herein, the graphene/ITO flexible hybrid transparent electrode was prepared by a two-step process: graphene film was grown on Cu foil by modified CVD method and then transferred onto ITO electrode on polyethylene terephthalate (PET) substrate. The transmission electron microscope (TEM) image, atomic force microscopy (AFM) image, and the Raman spectra of graphene films were analyzed firstly. The morphology, light transmittance, and electromechanical properties of the hybrid electrode were investigated. It is found that the gradient flux of carbon source modifies the synthesis of graphene and reduces the defect of surface structure. Moreover, the graphene layer of hybrid electrode enhances the ITO layer with excellent mechanical characteristic, and hence the electromechanical properties are improved compared to the ITO electrode.
The graphene film was synthesized using the CVD method similar to procedures reported previously [21–23]. According to the growth kinetics and reaction mechanisms, the flow rate of precursor gas is vital for synthesis of high-quality graphene films [24, 25]. Herein, a gradient flux of the methane was developed to achieve high-quality graphene films in the growth. Briefly, a 25-μm-thick Cu foil was inserted into a 25-mm quartz tube in a tube furnace and heated to 1024 °C with 16 sccm H2 (pressure ~ 150 mTorr) flows. After reaching 1024 °C, the sample was left in H2 for 15 min and then a gradient flux of methane (CH4) was added to the tube for another 15 min (10 sccm for 5 min, 20 sccm for 5 min, 33 sccm for 5 min). After that, the furnace was turned off to cool down the chamber with both CH4 and H2 presence. The CH4 pump was turned off when the furnace temperature reached 700 °C. Then, the sample cooled down to room temperature in the presence of H2 continually. Meanwhile, another graphene films were prepared by the same process with constant methane flux at annealing process of 1024 °C.
The PET substrate with ITO films (ITO/PET films, thickness = 0.125 mm) were purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd. The graphene film was transferred to ITO/PET film by means of reported reference [26-28]. Briefly, the synthesized graphene film on Cu foil was coated with the poly (methyl methacrylate) (PMMA) solution. After totally bake, PMMA was solidified to the membrane upon the graphene layer. The underlying Cu foil of hybrid film is chemically etched in the ammonium persulfate solution. Completing the etching of Cu foil, the PMMA/graphene stack was transferred to the top of the ITO/PET film and the sacrificial PMMA is removed to form the final hybrid transparent electrode.
The graphene films were characterized by Raman spectroscopy (Confocal Raman Microspectroscopy-Renishaw RM-1000) with an Ar+ laser excitation wavelength of 514.5 nm. The laser beam was focused onto the sample through an ×50 objective lens. The Raman spectra were collected in the frequency range of 1000 to 3000 cm−1 with an acquisition time of 10 s. The morphology of hybrid electrodes of graphene/ITO on PET substrate was analyzed by scanning electron microscope (CARL ZEISS-∑IGMA/VP) and eclipse inverted microscope (Nikon-TE2000U). TEM imaging was carried out by JEM-2100 (HR) with 200-kV acceleration voltage and LaB6 emitter. AFM imaging was conducted with tapping mode on Multimode 8 SPM system (Bruker Inc.) to test the morphology and thickness. The optical transmittance of the samples was investigated by using X-Rite spectrometer (Coloreye 7000A). The sheet resistance and other electrical data were measured by four-probe meter (KDY-1), Hall effect measurement system (Lakeshore 7704A), and LCR meter (Agilent U1731B). The mechanical force in the stretching test was conducted by an electromechanical universal testing machine (MTS, CMT-8502).
Figure 1b–d shows the TEM images of graphene prepared with holey carbon grids with different magnification and position. Figure 1b shows the typical TEM image of graphene grown with gradient flux by CVD method. Besides, Fig. 1c shows high resolution and magnification of graphene and the inset selected area electron diffraction pattern shows hexagonal patterns, which indicates the crystallization. Figure 1d depicts the edge of graphene film. The surface topography seems smooth.
The measured electrical data of the conductive electrode test in Hall effect measurement system
Sheet resistance (Ω/sq)
Surface carrier concentration (cm−2)
Carrier mobility (cm2/V s)
2.295 × 1015
5.37 × 1013
1.403 × 104
2.551 × 1015
2.366 × 1015
A comparative experiment for resistance change trend measurements was also performed under stable bending process at different curvature. The bending radius was measured by the scale, which is shown in Fig. 4b. When the bending radius is 0.1 cm by adjusting the screw smoothly, the value of ITO electrode rises to ~112.51 while that of graphene/ITO hybrid electrodes to ~1.11 and hybrid film to ~11.65. When the electrodes recover to be flat, the value of ITO electrode is back to be ~6.92, while the graphene/ITO hybrid electrode is back to ~0.27. Such result shows the benefit of graphene in terms of mechanical flexibility over ITO electrode, which is favorable to ITO. Figure 4c presents the sheet resistance of the samples with the same dimension, as they are bent multiple times at the bending radius of 0.1 cm. The smooth force makes sure of the secular change of PET substrate and decreases the test error. After 5 cycles of bending test, the sheet resistance was measured by four-probe meter. Original sheet resistance value of graphene electrode is bigger than the hybrid electrode and ITO. Such situation changed after 15 cycles of bending. We obtained a sheet resistance of ITO with 683.98 Ω/sq, graphene with 570.24 Ω/sq, and hybrid electrode with 506.37 Ω/sq. Currently, the three kinds of samples share similar resistance value. As more cycles of bending was made, the sheet resistance of ITO changed greatly and rise to ~3757.2 Ω/sq finally.
In summary, this work demonstrates a feasible two-step process method to employ the CVD graphene in fabricating the graphene/ITO flexible hybrid transparent electrode. With this method, as-fabricated graphene shows lower defect and improves the mechanical stability of single ITO electrode. Such hybrid electrode makes a significant advancement toward traditional transparent flexible film both in electrical and optical properties. The hybrid method meets the criteria of low cost, conductivity, and mechanical stability. The graphene/ITO hybrid film as a flexible transparent electrode has important implications for future devices.
This work was supported in part by the National Key Technology R&D Program of the Ministry of Science and Technology of China (2013BAH03B01), the Special Project on the Integration of Industry, Education and Research of Guangdong Province of China (2012A090300017), China Postdoctoral Science Foundation (2015M582277), and the Fundamental Research Funds for the Central Universities of China (2042015kf0059).
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