Elsevier

Organic Electronics

Volume 14, Issue 12, December 2013, Pages 3348-3354
Organic Electronics

The application of single-layer graphene modified with solution-processed TiOx and PEDOT:PSS as a transparent conductive anode in organic light-emitting diodes

https://doi.org/10.1016/j.orgel.2013.10.003Get rights and content

Highlights

  • A single layer graphene was modified with TiOx and PEDOT:PSS.

  • The sheet resistance of the modified graphene was reduced by 86% from 628 Ω/sq to 86 Ω/sq.

  • The work function of the modified graphene was increased by 0.82 eV from 4.30 eV to 5.12 eV.

  • Enhanced charge injection and transport were achieved from the modified graphene.

Abstract

There are many challenges for a direct application of graphene as the electrodes in organic electronics due to its hydrophobic surfaces, low work function (WF) and poor conductance. The authors demonstrate a modified single-layer graphene (SLG) as the anode in organic light-emitting diodes (OLEDs). The SLG, doped with the solution-processed titanium suboxide (TiOx) and poly(3,4-ethylenedio-xythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS), exhibits excellent optoelectronic characteristics with reduced sheet resistance (Rsq), increased work function, as well as over 92% transmittance in the visible region. It is notable that the Rsq of graphene decreased by ∼86% from 628 Ω/sq to 86 Ω/sq and the WF of graphene increased about 0.82 eV from 4.30 eV to 5.12 eV after a modification by using the TiOx–PEDOT:PSS double interlayers. In addition, the existence of additional TiOx and PEDOT:PSS layers offers a good coverage to the PMMA residuals on SLG, which are often introduced during graphene transfer processes. As a result, the electrical shorting due to the PMMA residues in the device can be effectively suppressed. By using the modified SLG as a bottom anode in OLEDs, the device exhibited comparable current efficiency and power efficiency to those of the ITO based reference OLEDs. The approach demonstrated in this work could potentially provide a viable way to fabricate highly efficient and flexible OLEDs based on graphene anode.

Introduction

Transparent indium-tin oxide (ITO) is conventionally used as the anode in the organic electronics such as OLEDs and organic photovoltaic devices (OPVs) owing to its high conductivity and excellent transmission of light. However, the brittleness of ITO and the dwindling supplies of indium limit its industrial applications. Therefore, there is an urgent need to develop alternative transparent electrode materials. Graphene has been supposed to a promising candidate in replacing ITO for some advantages. For example, graphene is mechanically compliant enough to inherently compatible with the flexibility nature of organic materials. In addition, being consisted with only one type element of carbon, it exhibits excellent bio-compatibility for possible applications in bio-electronics in the near future, as well as excellent optical transparency and low fabrication cost for the applications of organic optoelectronics [1], [2].

In addition to the mechanical flexibility and optically transparent, conductivity and work function are also important for one material as a transparent conducting electrode. It is known that the Rsq of a single-layer graphene (SLG) is usually larger than 500 Ω/sq [3], [4], and the WF of graphene is commonly about 4.3–4.7 eV [5], [6], [7]. Unfortunately, the two properties of graphene are not enough to act as an anode in OLEDs. Therefore, additional modifications of graphene are necessary with the goal of getting a lower Rsq (less than 100 Ω/sq) and a proper WF to match the requirement of electrodes in organic electronics. Many doping methods, such as chemical doping of HNO3 [8], [9], [10], SOCl2 [11], TFSA, [12] MoO3 [13], [14] AuCl3 [15] into graphene films, have been devoted to decreasing the sheet resistance of graphene films and adjusting the WF of graphene films. Nevertheless, the doping effects are limited (as shown in Table S1) and further lowering of sheet resistance is necessary for graphene.

Recently, Liu et al. reported that the Rsq of SLG could be decreased from 550 Ω/sq to 96 Ω/sq and the WF could be increased from 4.6 eV to 5.0 eV by doping the SLG with Au and PEDOT:PSS [4]. Han et al. demonstrated a successful application of graphene in OLEDs by doping the graphene films with HNO3 or Au. However, there are some issues on Au or HNO3 used as dopants although good device performance were obtained in their works. In the case of Au doping, some large Au particles (up to 50 nm in diameter) will be introduced into graphene films, which may create shorting pathways through the device [16]. In addition, the HNO3 doping process is accompanied by a gradually weakened doping effect due to the volatility of HNO3. Zhang et al. recently reported that the composite interface layer of aluminum (Al) nanoclusters and titanium oxide (TiO2) were introduced to modify the SLG as a transparent cathode for inverted OPVs [17]. The thin Al nanoclusters formed by vacuum evaporation in the Al–TiO2 composite can reduce the WF for better energy alignment. In addition, Wang et al. demonstrated a graphene-based OPV with a modification by MoO3/PEDOT:PSS double layer [14]. Nevertheless, the key materials such as Al and MoO3 in these two literatures are all realized by a thermally evaporating process. From a manufacturing point of view, the solution method such as inkjet printing and/or spin coating is superior to the thermal-evaporation method due to its simple process, high throughput fabrication and low cost of production. Therefore, it is desired to develop some new solution-processed interlayers for graphene modifications.

In this work, we demonstrate a solution-processing-modified SLG transparent conductive film with an Rsq of 86 Ω/sq, high work function of 5.12 eV, over 92% transmittance in the visible region and a smooth flat surface. The SLG was firstly modified by a 35 nm TiOx with Rsq reduced from 628 Ω/sq to 228 Ω/sq. Then the surface wettability of SLG–TiOx was improved after the treatment of ozone. Furthermore, the TiOx-modified SLG was capped with a 40 nm PEDOT:PSS, resulting in a further decrease of the Rsq from 228 Ω/sq to 86 Ω/sq. Compared with the pristine SLG, there was an decrease about 86% in Rsq. What’s more, an OLED with the combined films of SLG–TiOx–PEDOT:PSS as the bottom anode was successfully fabricated with comparable electroluminescence (EL) performance to that using conventional ITO electrode.

Section snippets

Preparation of SLG–TiOx–PEDOT:PSS tri-layer films

SLG samples were grown on a 25-μm-thick copper foil in a quartz tube furnace using LPCVD. The foils were heated at 1040 °C in H2 atmosphere (10 sccm, 80 mTorr) for 30 min. And then, the precursor gas CH4 was infused with a flow rate of 15 sccm by keeping the same temperature and at a pressure of ∼1.6 Torr in the furnace for 30 min. Finally, the copper foils were rapidly cooled to room temperature.

The graphene films were transferred by following procedure: (1) The copper foils with graphene were

Results and discussion

Fig. 1a shows the Raman spectrum of the graphene film to verify the number of graphene layers. The sharp peaks at 1595 cm−1 and 2690 cm−1 are corresponding to the G band and 2D band of SLG, respectively [19]. The intensity ratio of I2D/IG  3.7 and the weak peak at ∼1344 cm−1 (D band) suggest a high crystalline quality of the monolayer graphene film [20].

To increase the conductivity of the graphene films, the sol–gel TiOx solutions were spin-coated on the graphene films. As a result, the Rsq was

Conclusion

Single-layer graphene was doped with titanium suboxide (TiOx) following covering by PEDOT:PSS layer. The Rsq of graphene decreased by ∼86% from 628 Ω/sq to 86 Ω/sq. In addition, the single-layer graphene based transparent conductive film has a good transparency (over 92% transmittance in the visible region), high work function (∼5.12 eV) and a smooth flat surface. As a result, the OLEDs fabricated based on both the SLG–TiOx–PEDOT:PSS films and the ITO exhibit the comparable electroluminescence

Acknowledgment

We acknowledge financial support from the Natural Science Foundation of China (Nos. 61036009, 21161160446, and 61177016), the National High-Tech Research Development Program (No. 2011AA03A110), the Natural Science Foundation of Jiangsu Province (No. BK2010003). This is also a project supported by the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2011KFJ006), by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and by the

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    These authors contributed to the work equally and should be regarded as co-first authors.

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