Strong adhesion and high optoelectronic performance hybrid graphene/carbon nanotubes transparent conductive films for green-light OLED devices

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Abstract

Carbon nanotubes (CNTs) and graphene (Gr) are promising materials for flexible transparent conductive films (TCFs) in optoelectronic application because of their exceptional electrical, optical, and mechanical properties. Here, high-quality graphene nanosheets were synthesized by the one-step green method which using natural polyphenols-gallic acid (GA) as the exfoliation medium through high-pressure homogenization; besides, GA molecules can also be used as a non-covalent modifier to obtain GA modified CNTs (GCNTs). Then, the Au decorated Gr/GCNT-PET (Au-Gr/GCNT-PET) TCFs were obtained by a simple spray-coating process. The TCFs exhibited an excellent optoelectronic performance (Rs = 45.6 Ω/sq., T = ca. 80% at 550 nm), relatively flat surface (roughness = 12.43 nm), good bending resistance (the ΔR Au-Gr/GCNT-PET TCF/R0 was 0.07 after 1000 times bending tests) and strong adhesion (the fT of all Au-Gr/GCNT PET films was higher than 0.8). Finally, the Au-Gr/GCNT-PET films were used as an anode to prepare flexible green-light OLED devices; the maximum luminance of the Au-Gr/GCNT-PET device was 6554.1 cd/m2 at 15V, and the maximum current efficiency was 4.03 cd/A. It confirmed that this type of TCFs has considerable potential application in photovoltaic devices.

Introduction

Traditional Indium tin oxide (ITO) film [1] is widely used as transparent electrodes (TEs) in various applications such as solar cells, touch panels, and organic light-emitting diode (OLED) devices [2], [3], [4] due to its good light transmittance and high conductivity. However, the indium scarcity issue, its inherently brittle nature, high cost, and complicated fabrication process have boosted the search for substitutes for ITO. Many researchers claim that carbon nanotubes (CNTs), graphene (Gr), conductive polymers, and metal nanowires [5], [6], [7], [8] have many excellent properties and can be used as ITO alternative materials in the next-generation flexible display device.

CNTs are among the strongest candidates for the replacement of commonly used transparent conductive films (TCFs) based on ITO. CNTs possess unique multifunctional nature, which is based on their outstanding combination of mechanical strength, chemical stability, exceptional electrical conductivity, and optical properties. But for pure CNT films, the high contact resistance caused by the loose bond between tubes is the main factor restricting the film performance. According to the reports of many scholars, some natural polyphenols such as dopamine [9] and tannic acid [10] can be adsorbed to the surface of CNTs as adhesives, and our previous work [11] has also proved that such non-covalent modification can make tubes stick together and make the film dense. Similar to dopamine and tannic acid, gallic acid (GA) is a cheap, nontoxic natural polyphenolic compound, which can also be adsorbed onto CNTs via the π−π stacking interactions to realize non-covalent modification of CNTs [12]. Besides, the phenolic groups on GA-functionalized-CNTs (GCNTs) provide the site for the next reaction (the structure of the GA molecule was shown in Fig. S1) [13].

Graphene (Gr), a novel two-dimensional (2D) material, exhibits a single layer of carbon atoms in a hexagonal lattice, whose unique structure makes it the most promising material for TEs and OLEDs. Therefore, the environmentally friendly preparation technology of high-quality graphene is frequently significant for graphene itself and its application. Among all the graphene preparation methods, liquid-phase exfoliation (LPE) of graphite to produce graphene is the most promising way because of its simplicity, scalability, versatility, and low cost. Up to date, most LPE methods produce graphene via sonication or high-speed shear. However, the graphene prepared by these two methods has low quality and thick layers [14]. Rapid progress in the utilization of high-pressure homogenization to produce graphene has been noted recently. The homogenization technique is based on the use of high pressure to force a fluid through a narrow channel, where three simultaneous effects (cavitation, collision, and high shear stress) occur to peel graphene material off from the bulk graphite. The high-pressure homogenization method was chosen here due to its low energy consumption and availability. Also, exfoliation media directly affects exfoliation efficiency. Particularly, in aqueous system, it is essential to add stabilizers into graphene suspensions against re-aggregation. A good stabilizer needs to be water-soluble and has sufficient interactions with graphene. Here, we creatively chose GA as the exfoliation medium to prepare high-quality graphene through high-pressure homogenization. GA molecules can also generate a strong interfacial force (π-π interaction) with graphene; thereby reducing the van der Waals force between graphene layers to achieve green preparation of graphene. This efficient method of preparing Gr can successfully avoid environmental pollution problems caused by traditional preparation methods.

Based on our previous work [11], the functional CNTs and Gr nanosheets were combined to form an efficient 3D conducting network, which showed excellent properties such as high light transmittance, high electrical conductivity, strong bending resistance, and adhesion. This film has fully met the requirements for flexible OLED devices. To further improve the performance of the devices, increasing the conductivity of the film is one of the most effective methods. Recently, HAuCl4 is one of the most commonly used and effective dopants for graphene-based TCFs [15,16]. Then the HAuCl4 can be used as p-type dopants for CNT/Gr-TCFs to further optimize the properties of the device by increasing its carrier concentration.

In this article, we innovatively propose to use GA as a medium for liquid-phase exfoliation of Gr and a non-covalent modifier for CNTs. The obtained Gr nanosheets and GCNTs were characterized by Fourier transform-infrared (FT-IR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Then the Gr/GCNT-PET films were fabricated using a simple spray-coating method. To improve the conductivity of the film, HAuCl4 was used as the dopant to modify Gr/GCNT-PET films. Finally, the flexibility of green-lighting OLED devices using these hybrid films as an anode was successfully prepared.

Section snippets

Materials

Pristine graphite (medium particle size of 5-10 μm, carbon purity of 95%), sodium dodecylbenzene sulfonate (SDBS), GA (gallic acid), HAuCl4•3H2O (purity > 99%), and nitric acid (HNO3) were purchased from Shanghai Aladdin Chemical Reagent Co., Ltd., China. Single-walled CNTs (SWCNTs, purity of 95 wt.%, diameter ≤ 2 nm, length >5 μm) were provided from Carbon Star Technology (Tianjin) Co., Ltd. The polyethylene terephthalate (PET) was purchased from Tianjin Wanhua Co., Ltd, China with an average

Results and Discussion

GA is a natural polyphenol aromatic compound. When graphite powders were dispersed in the GA solution, GA molecules could enter the interlayer of graphite to reduce the van der Waals force between the layers [19]. Subsequently, the mixed solution was successfully produced graphene through multiple high-pressure homogenizer cycles. The obtained Gr sheets were characterized by TEM, AFM, Raman, and XRD. As shown in Fig. 2a-c, with the increase in the number of cycles, the thickness and size of Gr

Conclusion

In summary, GA was used as a medium for liquid-phase exfoliation of Gr and a non-covalent modifier for CNTs. The Gr/GCNT-PET films were fabricated by the spray-coating method, the HAuCl4 was used as the dopant to modify Gr/GCNT-PET films and to further enhance the conductivity. The obtained Au-Gr/GCNT-PET TFCs exhibited good properties such as low sheet resistance, relatively flat surface, bending resistance, and strong adhesion. Besides, the flexible green-light OLED devices that using

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge financial support from the Natural Science Foundation of Tianjin China (Grant No. 20YDTPJC01690 and 19JCZDJC37800).

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