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Invited Paper

Large Area Organic Light Emitting Diodes with Multilayered Graphene Anodes Jaehyun Moon 1, Joohyun Hwang 1, Hong Kyw Choi 2, Taek Yong Kim3, Sung-Yool Choi 3, Chul Woong Joo 1, Jun-Han Han 1, Jin-Wook Shin 1, Bong Joon Lee 1, Doo-Hee Cho 1, Jin Woo Huh 1, Seung Koo Park1, Nam Sung Cho1, Hye Yong Chu, and Jeong-Ik Lee1,* 1

OLED Research Team, Electronics and Telecommunications Research Institute (ETRI), Daejeon 305700, Korea,2University of Science and Technology, Daejeon 305-500, Korea,3Dept. of Electrical Engineering, Korean Advanced Institute of Science and Technology, Daejeon 305-701, Korea

*Jeong-Ik Lee: [email protected]

ABSTRACT In this work, we demonstrate fully uniform blue fluorescence graphene anode OLEDs, which have an emission area of 10u7 mm2. Catalytically grown multilayered graphene films have been used as the anode material. In order to compensate the current drop, which is due to the graphene’s electrical resistance, we have furnished metal bus lines on the support. Processing and optical issues involved in graphene anode OLED fabrications are presented. The fabricated OLEDs with graphene anode showed comparable performances to that of ITO anode OLEDs. Our works shows that metal bus furnished graphene anode can be extended into large area OLED lighting applications in which flexibility and transparency is required. Keywords: Organic Light Emitting Diode (OLED), Graphene, Flexible electrode, Lighting

1. INTRODUCTION Organic light emitting diodes (OLEDs) offer high quality display and lightings. [1-6] The generated light escapes the transparent electrode to become externally perceivable. Thus the transparent electrode is a component of prime importance in OLED applications. Conventionally, indium tin oxide (ITO) electrodes have been used as the transparent anode in OLEDs. As an alternative choice for transparent anode, we investigate multi layered graphene. [7],[8] We choose graphene, because it is optically transparent and electrically conductive. [9-13] From the perspective of realizing flexible OLEDs, graphene also offers mechanical compliance. The sheet resistances of multi-layered graphene films are higher than 200 /Ƒ. Thus, on large area emission, the resistance will cause emission non-uniformity across the emission surface. As an effort to reduce the expected non-uniformity we have furnished the glass substrate with auxiliary metal bus lines. We demonstrate fully uniform blue fluorescence graphene anode OLED on an actual emission area of 10 u 7 mm2.

2. EXPERIMENTS Multilayered graphene films were obtained by using a thermal chemical vapor deposition method. Multilayered graphene films were grown on a 300 nm thick catalytic Ni film, which has been deposited by a sputtering method on SiO2/Si substrates. The Ni/SiO2/Si substrate was exposed to a reaction gas of mixture of CH4:H2:Ar (50:50:200 sccm) for 5 s at 1000 °C, and rapidly cooled to room temperature at a rate of ~10 °C s-1 using flowing Ar. The sample was immersed in a FeCl3 (1M) solution to etch away the catalytic Ni films and to collect multi-layered graphene films. The detached graphene films were transferred to the substrates for OLED fabrications.

Organic Light Emitting Materials and Devices XVI, edited by Franky So, Chihaya Adachi, Proc. of SPIE Vol. 8476, 84760U · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.928065

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Figure 1(a) shows an optical photo of the substrate we have used in this work. The width and thickness of auxiliary metal bus lines are 26 Pm and 30 nm, respectively. The metal bus lines cover approximately 5 % of the emission area. Figure 1(b) shows the cross-sectional structure of our OLED. All organic layers were deposited in a high vacuum chamber below 6.67 u 10-5 Pa and thin film of LiF/Al were deposited as a cathode electrode. Blue fluorescent organic (20 nm) was used as the light emitting material. We have also fabricated ITO anode OLEDs as reference samples. From the view point of the operation principles of OLED, it is desirable to have high work function anodes to enhance the hole injection. In our previous works, we have shown that improvement in hole injection may be achieved by applying mild oxygen plasma on the graphene surface. [9] Thus, prior to the deposition of organics, in order to improve the hole injection property of the graphene anode, we have performed mild oxygen plasma treatment on the graphene film surface. The electro-luminescence (EL) spectra were measured using a spectroradiometer (Minolta CS-2000). The current density–voltage (J–V) and luminescence–voltage (L–V) characteristics were measured with a current/voltage source/measure unit (Keithley 238) and the afomermentioned spectroradiometer.

Figure 1. (a) An optical photo of metal bus furnished substrate. (b) Schematics of graphene anode OLED.

3. RESULTS and DISCUSSION Figure 2(a) shows a cross-sectional SEM image of our graphene anode OLED. Because the thickness of the auxiliary metal bus line is comparable to that of the OLED, the suspension of the OLED can be observed. However, such suspension has been observed to cause no operational problem. As will be shown in following part, the OLED is fully operational with not observable defects, such as dark spots or electrical failure. This feature implies the good ductility of multi layered graphene films and good adhesion to the adjacent organic layer. The graphene films were physically put on the metal bus line arrays. Thus, bad electrical contact can be expected. However, the J-V measurement results show that the contact between graphene and metal is acceptably good. The J-V measurements are presented in Fig. 3(b). Figure 2(b) shows the measured optical transmittance of multi layered graphene films. In the visible range (400~700 nm), the transmittance is higher than 87 %, which is comparable to that of ITO films. Figure 3(a) shows the change in the surface morphology of the graphene film before and after oxygen plasma surface treatment. As results of oxygen plasma surface treatment the surface roughens. The root-mean-square roughness increases from 1.6nm to 2.8 nm. The sheet resistance also increase from 289 :/Sq. to 550 :/Sq.. The increase in sheet resistance indicates that graphene film has been peeled-off to become thinner. While the surface roughness is acceptable for OLED fabrication the sheet resistance is not. If graphene alone is used in large emission area, non-uniformity in light emission is inevitable. Thus, in order to avoid such non-uniformity, we have furnished the substrate with auxiliary metal


bus lines are to compensate the sheet resistance of graphene. Figure 3(b) shows the J-V characteristics of graphene OLEDs, which have been plasma treated and not. The ITO anode OLED J-V characteristic is also shown. For all anode cases, in the voltage range of 0~ 2.5V, fairly low hole injection is taking place. The Js do not exceed 10-7A/cm2. Around applied voltage of ~ 3V clear difference is noticeable in the J-V characteristics between the plasma treated and un-treated samples. The J of the plasma treated sample increases at much faster rate than that of the un-treated sample. Compared to J of the treated sample J of the un-treated sample is lower in two orders in magnitude. This difference does not change up to 6 V.

Figure 2. (a) A cross-sectional SEM image of graphene anode OLED. (b) Transmittance of graphene film.

The treated sample shows even higher J in the voltage range of 3.5~5.5 V. At a voltage higher than 5.5 V the J of the treated sample saturates, while the J of untreated sample keeps increasing upon voltage application. Such feature may be attributed to the higher sheet resistance of plasma treated graphene films. Based on the experimental results, mild oxygen plasma treatments on graphene film can be summarized to as giving two effects in the J-V characteristics. First, in the low voltage regime, treatment improves hole injection. The improvement can be attributed to the improved graphene/organic interface. Detailed surface science is required to elucidate the improved interface. In this course, ultraviolet photoelectron spectroscopy studies can be very effective. Second, presumably due to the high sheet resistance of the treated graphene films, saturation in J occurs at faster rate than the J of the un-treated graphene films.

Figure 3. (a) Oxygen plasma effect on the surface morphology of graphene films. (b)The J-V characteristics of graphene OLEDs with and without oxygen plasma treatment. The ITO anode OLED J-V characteristic is also shown.

Figure 4(a) shows an optical photograph of our sky-blue graphene anode OLED, which has been fabricated on the metal bus furnished substrate of Fig. 1(a). As can be seen, the graphene anode OLED shows uniform emission. This demonstrates that by compensating the resistance of graphene films using metal bus lines, it is possible to achieve uniform emission in large area graphene anode OLEDs. Figure 4(b) shows the electro-luminescence (EL) spectra of treated and un-treated graphene anode OLEDs, and conventional ITO anode OLED. As can be seen, difference in EL


spectra between treated and un-treated samples is negligible. This implies that slight difference in the graphene film thickness is not causing any optical effect. However, differences in the intensity ratio (R) of the two main EL peaks, R=IO1=470 nm/I O2=500 nm, are observed, indicating the presence of different internal optical effects in OLEDs with graphene and ITO anodes. Compared to the R of the of ITO anode OLED, the Rs of the graphene anode OLEDs have bigger values. Technologically, this is important in the OLED stack designs. In other words, the graphene anode OLED stack structure has to be tuned to replicate that of the ITO anode OLED. Figure 4(c) shows the variations in luminance as a function of applied voltage. OLEDs with graphene and ITO anodes exhibit very similar variation trend. Figure 4(d) shows the external quantum efficiencies (EQEs). In the given luminance range (3~104 cd/m2), the ITO anode device displays higher EQEs. The EQEs at 103 cd/m2 were modest with 5.1 % and 2.9 % for ITO anode and graphene anode OLEDs, respectively.

Figure 4. (a) An optical photo of metal bus furnished graphene anode OLED. The emitting area is 10u7 mm2. (b) The electroluminescence spectra. (c) The L-V characteristics. (d) The external quantum efficiencies.

4. SUMMARY In this paper, we demonstrated larger area uniform emission of OLEDs with the graphene anode. In order to overcome the high resistance of the graphene films, we have furnished the substrate with auxiliary metal bus lines. Also the hole injection was improved by treating the graphene surface with mild oxygen plasma. Our graphene anode OLEDs showed device performances comparable to those of conventional ITO anode OLED. We believe that further process optimization and better understanding of graphene treatments will lead to improved OLED efficiency and stability. These results show that metal furnished graphene films can offer a practical solution for flexible large area OLED panels for lighting, in which emission uniformity is very important.

ACKNOWLEDGEMENT This work was financially supported from IT R&D program of Development of Key Technology for Interactive Smart OLED Lighting, which is a part of ETRI Internal Research Fund from Ministry of Knowledge and Economy.


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