Highly Conductive and Uniform Graphene hybrid Electrode with Chemical Reduction for Flexible Organic Light-Emitting Diodes

Xinkai Wu, Jun liu, Jing Wang, Xindong Shi, Saijun Huang, Yikai Su, Gufeng He* National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering,

Shanghai Jiao Tong University, Shanghai 200240, China

Abstract

Highly conductive, uniform, transparent and flexible films were produced by mixing graphene oxide (GO) with poly (3,4- ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and then chemically reduced by HI-AcOH. Using the optimized graphene hybrid film as anode, a fascinating flexible organic light-emitting diode (FOLED) has been demonstrated on polyethylene naphthalate (PEN) substrate.

1. Introduction Recently the flexible organic light-emitting diodes (FOLEDs) have attracted strong attentions owing to their low weight, thinness and high mechanical flexibility [1]. Among all components in FOLEDs, the transparent conductive electrode is an essential part that has strong influences on device performance. Indium tin oxide (ITO) has been used for many years as transparent electrode due to its high conductivity and high transparence. However a few drawbacks of ITO including high price due to indium scarcity, complicated processing requirements, sensitivity to acid and basic environments, prone to cracking upon bending, render it unsuitable for application that requires stretchability or flexibility [2].

Graphene, a single layer of two-dimensional carbon lattice, has unique electrical, optical and mechanical properties [3], and is considered as one of the most potential candidates for flexible transparent electrode. However, the achievement of highly conductive and flexible large-area graphene film is still a big challenge. One of the most potential methods to prepare large-area graphene electrode film is to reduce solution processed graphene oxide (GO), which delivers high throughput, low cost, and the simple fabrication [4]. In contrast, graphene obtained by other methods, including mechanical exfoliation [5], epitaxial growth [6] and chemical vapor deposition (CVD) [7], is high quality but limited size, quantity and complex processing. GO in aqueous solution can easily assemble into films by simple processes such as spin coating [8], spray coating [9] and dip-coating [10], but oxygen-containing functional groups attached to GO make the assembled film almost insulating [4]. GO assembled films can transform to reduced graphene oxide (rGO) by traditional high temperature thermal annealing and low temperature chemical reduction. However, high temperature thermal reduction is not compatible with flexible plastic substrates, and some chemical reduction agents including hydrazine and sodium borohydride (NaBH4) may stiffen and disintegrate the GO flexible film. On the other hand, these deoxygenating processes are time-consuming and complicated. Recently, Kyu [11] has reported the synthesis of high-quality rGO films through dipping the film into the hydroiodic acid and acetic acid mixture solution (HI-AcOH) at temperature below 100 °C. The rGO films are integrated and flexible, and the conductivity of thin film can reach 78 S cm-1. This process is simple and efficient to produce large-area and flexible rGO films, but the conductivity is not high enough for application in OLEDs.

In this paper, we mixed GO with different ratios of primitive poly (3,4-ethylenedioxythiophene) : poly (styrene sulfonate) (PEDOT:PSS), then treated with HI-AcOH. The conductivity and uniformity of resulted film increase significantly. The lowest sheet resistance of the rGO hybrid film is 300 ohm per square with the transmittance of about 60% at 550 nm wavelength. Using the hybrid film as anode, the FOLEDs with fascinating performance can be obtained.

2. Experimental Graphite powder was oxidized by the Hummers method to form GO [12]. The PEDOT:PSS (PH1000) was filtered through a syringe filter (0.45 μm pore size), then various ratios of PEDOT:PSS were added into the GO solution and stirred for two hours at room temperature. A homogenous film was obtained by spin coating the composite solution on pre-cleaned polyethylene naphthalate (PEN) or glass substrates, and then was annealed on a hot plate at 150 °C for 15 min in ambient atmosphere. The reduction was carried out by immersing GO hybrid films into a HI-AcOH solution at 85 °C for 30 s after drying at room temperature. Later, thin layer of PEDOT:PSS (AI4083) was spin coated on the glass with ITO or PEN substrates with rGO hybrid films as hole injection layers, then the composite electrodes were transferred into a high-vacuum system at a base pressure of 10−6 Torr for the following organic and metal layers deposition to finish the OLED fabrication. The OLED devices use 60 nm N, N'-Bis (naphthalen-1-yl)- N,N'-bis (phenyl) -benzidine (NPB) as the hole transporting layer, 40 nm tris-8-hydroxyquinoline aluminum (Alq3) as both electron transporting and emitting layer, 1 nm 8-hydroxy-quinolinato lithium (Liq) and 100 nm aluminum (Al) as cathode. The active area defined by the overlap of the anode and the Al cathode was around 3 mm × 3 mm. The deposition rate was monitored in-situ by a quartz-crystal monitor. Figure 1 shows the schematic section of the fabricated device stack.

Figure 1. Schematic section of OLED device

3. Results and Discussion The GO films with primitive PEDOT:PSS have high sheet resistance ( > 3×106 ohm per square ), which limits their application as an electrode. After reduction by HI-AcOH, the sheet resistance of GO hybrid film can lower down dramatically. Figure 2 gives the variation of sheet resistance and transmittance of rGO hybrid films which are obtained by GO mixing different ratios of

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PEDOT:PSS after reduction by HI-AcOH. As shown in Figure 2 (a), the sheet resistance of rGO hybrid film decreases with the increasing ratios of PEDOT:PSS, and the value can go well below 300 ohm per square with a mixing weight ratio at 1:2. This improvement may be attributed to the bonding of graphene nanosheets by PEDOT chains, which increases the conductive pathways and reduce the contact resistance of graphene nannosheets. However, the sheet resistance does not exhibit remarkable change when the mixing ratio was over 1:1. Figure 2 (b) shows that the transmittance of the rGO hybrid films has a little reduction with the increase of PEDOT:PSS. It indicates that PEDOT:PSS as mixing agent may bond with graphene nanosheets and fill the spaces between the graphene sheets. The addition of PEDOT:PSS in the gap will improve the dispersity of graphene sheets and affect the transmission of light. As a result, the rGO with PEDOT:PSS hybrid film at a 1:2 weight ratio can reach 300 ohm per square just with the transmittance of about 60% at 550 nm.

Figure 2. (a) The conductivity of the rGO hybrid films with different ratios of the PEDOT:PSS; (b) Transmittance of rGO hybrid films with different ratios of the PEDOT:PSS

The surface morphologies of fabricated rGO hybrid films were examined by scanning electron microscopy (SEM). The variable dispersion of rGO nanosheets and PEDOT:PSS can be observed in Figure 3 (a) – (d), which indicate the GO mixing different ratios of

PEDOT:PSS after reduction by HI-AcOH, 1:0 (a), 2:1 (b), 1:1 (c), 1:2 (d). As shown in Figure 2 (a), rGO film has integrated and uniform surface with spin coating process, and many bright lines can be clearly observed. These bright lines are the overlap of GO sheets, where some of the graphene edges are scrolled or folded during film fabrication. After mixing more PEDOT:PSS, the rGO hybrid films become flatter with less observable graphene edges. It means that graphene nanosheets may bond with the PEDOT:PSS, therefore the polymer can fill the spaces between the graphene sheets and the edges become smoother. Such smooth surface morphology is particularly critical for high performances of OLEDs, otherwise any conductive nanostructures protruding toward the organic layer will form preferential current pathways through the device, leading to shunting and a reduced performance [13]. Therefore, the thicker PEDOT:PSS are embedded into graphene edges, improving the conductivity and uniformity of GO hybrid film and preventing this occurring.

Figure 3. SEM images of graphene hybrid electrodes with different ratios of PEDOT:PSS: (a) 1:0, (b) 2:1, (c) 1:1, (d) 1:2.

Figure 4 (a) shows the current density–voltage (J–V) characteristics of OLEDs using ITO and rGO hybrid films with different ratios of PEDOT:PSS ( 1:0, 5:1, 2:1, 1.25:1, 1:1, 1:1.5, 1:2) as anodes. As one can see, the current density of OLEDs with rGO hybrid anodes enhances with the increase of mixing concentration, and the variation trend is similar to the characteristics of conductivity. It means increasing the conductivity of rGO hybrid anodes has a significant effect on charge injection and transport. A considerably high sheet resistance limits the current flow through the OLED device, and results in reduced current density [14]. Similar trend can also be observed for luminance characteristics of OLEDs. Figure 3 (b) shows the luminance–voltage curves, and the largest luminance of devices with rGO hybrid electrodes were enhanced with the increase ratios of PEDOT:PSS. The device using rGO hybrid anode with PEDOT:PSS at a 1:2 weight ratio has the lowest optical turn-on voltage (3V) and the related luminance value is higher than other devices. The reachable maximum luminance for the device with rGO hybrid electrode is 900 cd/m2 at the voltage of 8V, and the value is much larger than the device with rGO electrode. The inset in Figure 4 (b) shows optical images of green FOLEDs with rGO hybrid films made on a PEN substrate.

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Figure 4. (a) Current density to voltage curves and (b) Luminance to voltage curves of OLED devices with anode made by ITO and rGO hybrid films with different weight ratios of PEDOT:PSS.

Figure 5. The current efficiency to current density curve of OLED devices with anode made by ITO and Graphene hybrid anodes with different weight ratios of PEDOT:PSS.

Figure 5 shows the current efficiency to current density curves. It indicates that the current efficiency is augmented with the increase of PEDOT:PSS mixing concentration, and the device using the rGO hybrid anode with PEDOT:PSS at a weight ratio of 1.25:1 has a highest efficiency at 3.5 cd/A. When the ratios of PEDOT:PSS are further increased, the current efficiency shows the trend to reduction. The efficiency is mainly decided by the charge balance of the whole device. As discussed above, the conductivity of device is enhanced with the increase mixing concentration. More charges can flow through the OLED device, and the balance of hole-electron combination is altered. The location of carrier combination in the emitting layer is changed toward to cathode. Therefore, the efficiency is reduced for the poor recombination balance here. The device performance could be further enhanced by optimization of device architecture in the future studies.

4. Conclusion Using HI-AcOH low temperature chemical reduction, highly conductive, uniform, transparent and flexible graphene mixing PEDOT:PSS hybrid films were obtained . Using the rGO hybrid films as anode, FOLEDs were fabricated. The devices have been demonstrated on PEN substrates, showing a fascinating performance. This work provides a potential solution for high performance, large area, low cost and flexible display and lighting technologies.

5. Acknowledgements This research work was supported by 973 Program (2013CB328803, 2013CB328804), the National Natural Science Foundation of China (61377030) and the Science and Technology Commission of Shanghai Municipal (12JC1404900).

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