Elsevier

Carbon

Volume 98, March 2016, Pages 457-462
Carbon

Crackless transfer of large-area graphene films for superior-performance transparent electrodes

https://doi.org/10.1016/j.carbon.2015.11.041Get rights and content

Abstract

Graphene is a promising candidate as transparent electrode owing to its high optical transmittance and conductivity. Graphene transfer is still one of critical issues for achieving the high-performance transparent graphene electrodes. Here, we report a crackless transfer method without the removal of polymer, which is usually utilized as the carrier for graphene transfer. This method enables the successful transfer of single-layer graphene onto polymethyl-methacrylate (PMMA)/AB-glue/polyethylene-terephthalate (PET) up to 14-inch with a sheet resistance of 219 Ω sq−1 at 96.5% transmittance. The conductivity is comparable with that of the doped graphene films, and is 5 times higher than that of undoped graphene films obtained by the transfer with polymer removal. Our experiment results show that the sheet resistance is proportional to the area of graphene crack, which is unavoidably caused during removing the polymer carrier. So the high conductivity is attributed to crackless graphene films produced by our new transfer method.

Introduction

Graphene, a two-dimensional single-layer of sp2 bonded carbon atoms, possesses high optical transmittance, conductivity and chemical resistance [1]. These features make graphene an ideal candidate to replace indium tin oxide (ITO) [2], [3], [4], [5], [6], [7], a conventional transparent electrode material, which suffers from some considerable drawbacks, including increased costs, high vacuum conditions and brittleness [8], [9], [10]. In particular, high-quality and large-area single-layer or few-layer graphene films have been successfully synthesized on certain metals by chemical vapor deposition (CVD) method, which remarkably advances the development of large-area graphene electrode [11], [12], [13], [14].

However, the electrode application requires an insulator substrate supporting graphene films, which means that graphene grown on metal substrate should be transferred onto other appropriate target substrate. Despite the high-quality graphene growth, the sheet resistance of transferred CVD-grown single-layer graphene films is reported to be as high as 2100 Ω sq−1 at 97% optical transmittance [7]. Such a value is much higher than the theoretical value of graphene resistance (30 Ω sq−1 at 97.4% transmittance) and remains too high for graphene to be used as electrode [15]. It indicates that graphene transfer is still one of critical issues for achieving high-performance transparent electrodes. Several approaches can be pursued to reduce the sheet resistance of transferred graphene films, including layer-by-layer stacking of graphene, chemical doping and combining graphene with metal nanowires [15], [16], [17], [18], [19], [20]. For example, Bae et al. reported four-layer graphene film with a sheet resistance of 30 Ω sq−1 at 90% transmittance by layer-by-layer stacking of graphene, in comparison with the sheet resistance of 125 Ω sq−1 at 97.4% transmittance for single-layer graphene film [15]. Kim et al. demonstrated that AuCl3 can reduce the sheet resistance of single-layer graphene film to 150 Ω sq−1 [17]. Deng et al. reported roll-to-roll encapsulation of metal nanowires between graphene and plastic substrate for the production of transparent electrodes with 8 Ω sq−1 at 94% transmittance [20]. However, the current approaches have their own different drawbacks. The stacking of graphene adds the channels for charge transport, at the cost of degrading the optical transmittance. The dopant materials absorbed on the graphene surface affects the stability. The metal nanowires are easily oxidized and thus affect the electrode stability.

In fact, there is a huge room for reducing the sheet resistance of intrinsic graphene by comparing the reported experimental values of several thousand Ω sq−1 with the theoretic value of 30 Ω sq−1 [15]. We believe that the crack produced in the transferred graphene films is one of the key points that cause the degrading conductivity (See the solid evidence below), because cracks in graphene films reduce the channels for charge transport [19]. After metal substrate etching, the CVD-grown graphene is first transferred by the polymer carrier (normally polymethyl methacrylate (PMMA)) onto the target substrate with graphene between polymer and substrate, and subsequently the polymer should be removed to expose graphene, as shown in Fig. S1(a). During the PMMA removal process, cracks are unavoidably produced [7]. In order to reduce the sheet resistance of transferred graphene films as much as possible, the cracks should be completely avoided. Very recently, Chandrashekar et al. reported the transfer of graphene films from Cu foil onto plastic substrates by delamination process to minimize the damage introduced by the carrier removal [21]. Here, we propose a different way to transfer graphene without the carrier removal, as shown in Fig. S1(b). After metal etching, the graphene/PMMA is transferred onto the target substrate with graphene above PMMA/substrate. It should be noted that the PMMA underlying graphene will not affect the graphene's application as transparent electrode since the substrate is normally transparent polymer, such as polyethylene terephthalate (PET). In this way, the polymer is not needed to be removed, and thus it can be expected that no cracks are produced in the transferred graphene films and thereby the sheet resistance of intrinsic graphene is reduced without any other treatment. To achieve large-area transfer, before Cu etching, the PMMA carrier is adhered with PET by AB glue to overcome the weak mechanical strength of PMMA which limits the transfer scale [15]. As well, the spin coater is replaced by a rolling coater, which is similar to roll-to-roll process [15], [20]. The detailed process is shown in Fig. 1. In Fig. 1(a,b), PMMA and AB glue are coated on the graphene/Cu and PET by a rolling coater, respectively. After AB glue is solidified, they are combined by a rolling coater to form Cu/graphene/PMMA/AB-glue/PET structure, as shown in Fig. 1(c). Afterward, the obtained layer structure was placed on the etchant surface to etch Cu (Fig. 1(d)). Finally, graphene grown on Cu foil was transferred onto PMMA/AB-glue/PET substrate, as shown in Fig. 1(e). Obviously, the transfer size using our method is in principle unlimited. Therefore, the employed approach provides a simple way for the crackless transfer of large-area graphene films toward superior-performance large-area transparent electrodes.

Section snippets

Graphene growth

The growth of graphene films on Cu foil was carried out in a low-pressure CVD system. A 25 μm thick Cu foil placed in the CVD chamber was gradually heated to 1050 °C within 1 h in 20 sccm H2, and then annealed for 30 min under this condition. Graphene growth was conducted for 40 min after introducing 20 sccm CH4.

Transfer

1) Conventional transfer: A 5% solution of 996K PMMA (Sigma Aldrich, #182265) in anisole was spin-coated onto the graphene/Cu-foil, and then the sample coated with PMMA was baked on a

Results and discussions

The photograph image of the produced 14-inch graphene film on PMMA/AB-glue/PET is shown in Fig. 2(a). The underlying badge can be clearly seen, which demonstrates that the graphene film on PMMA/AB-glue/PET exhibits excellent transparence. It should be noted that graphene transferred by AB-glue/PET without PMMA assistance has high resistance in our work. That is why we combined PMMA and AB glue, in which PMMA facilities the graphene transfer and AB glue makes graphene/PMMA strongly attached to

Conclusions

In summary, we report a crackless transfer method without the removal of polymer. This method enables the successful transfer of single-layer graphene onto PMMA/AB-glue/PET up to 14-inch with a sheet resistance of 219 Ω sq−1 at 96.5% transmittance. The conductivity is comparable with that of the doped graphene films, and is 5 times higher than that of undoped graphene films obtained by the transfer with polymer removal. Our experiment results show that the sheet resistance is proportional to

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (Nos. 21473047, 51402080 and 51471162), and the Fundamental Research Funds for the Central Universities (2015HGCH0007).

References (29)

  • H.A. Becerril et al.

    Evaluation of solution-processed reduced graphene oxide films as transparent conductors

    ACS Nano

    (2008)
  • X. Li et al.

    Transfer of large-area graphene films for high-performance transparent conductive electrodes

    Nano Lett.

    (2009)
  • Q. Yu et al.

    Graphene segregated on nickel surfaces and transferred to insulators

    Appl. Phys. Lett.

    (2008)
  • K.S. Kim et al.

    Large-scale pattern growth of graphene films for stretchable transparent electrodes

    Nature

    (2009)
  • Cited by (54)

    • Wearable and flexible electrodes in nanogenerators for energy harvesting, tactile sensors, and electronic textiles: novel materials, recent advances, and future perspectives

      2022, Materials Today Sustainability
      Citation Excerpt :

      Transparent carbon-based materials are cheaper and more abundant compared to ITO, which is a transparent and widely used material in the field of photovoltaic and display applications. ITO, as one of the most widely used electrodes in recent years, has faced problems such as lack of indium, poor mechanical properties, high production costs, and uncontrollable reflections, which have led to the rapid replacement of carbon-based materials [286–291]. Chen et al. [292] reported a transparent and stretchable layer of graphene which could be used as an electrode in energy storage devices and supercapacitors.

    • A new twist in graphene research: Twisted graphene

      2020, Carbon
      Citation Excerpt :

      More conventionally followed method is based on stacking of two SLGs prepared by chemical vapor deposition (CVD) [64] (see Fig. 1a). It typically involves transfer of a SLG onto a substrate using the conventional procedure of transfer [80–84] where SLG grown on Cu is coated with a polymer (PMMA) followed by etching of Cu. SLG/PMMA is then placed on the substrate, followed by removal of PMMA either by acetone [85–88] or by thermal annealing [89–91].

    View all citing articles on Scopus
    View full text