Fabrication of OLEDs without photolithography patterning

Organic Electronics 12 (2011) 745–750

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Fabrication of OLEDs without photolithography patterning Hyokyun Ham a, Jongwoon Park a,⇑, Dongchan Shin b, Junghwan Park c a b c

OLED Lighting Team, National Center for Nanoprocess and Equipment, Korea Institute of Industrial Technology, Gwangju 500-480, Republic of Korea Department of Advanced Material Engineering, Chosun University, Gwangju 501-759, Republic of Korea Corporated R&D Center, Duksan Hi-Metal Co. Ltd., Seongnam 463-420, Republic of Korea

a r t i c l e

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Article history: Received 16 November 2010 Received in revised form 22 January 2011 Accepted 13 February 2011 Available online 26 February 2011 Keywords: Organic light-emitting diodes (OLEDs) Photolithography Leakage current Sputter

a b s t r a c t We report on a new cost-cutting fabrication process for organic light-emitting diodes (OLEDs), requiring no photolithography patterning for indium-tin-oxide (ITO). We formed the patterned ITO electrodes using a sputtering deposition system. However, sputter-patterned ITO-based OLEDs caused leakage current on the slope of the ITO edges due to spikelike surface. To suppress it, an organic molecule was thermally evaporated as an insulating layer that covers the sputter-patterned ITO edges. Such an insulating organic molecule demands low carrier mobility. We found that the reverse current density of OLEDs based on this scheme was comparable to that of photolithography-patterned ITO-based OLEDs. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic light-emitting diodes (OLEDs) come into the spotlight due to their potential applications in flat-panel displays and lightings [1–5]. They have great design freedom in producing new concept lightings by virtue of salient features such as surface emission, flexibility, and transparency. Continuous advance in the OLED industry (i.e., organic materials, OLED equipments, OLED panels, etc.) has now put them on the market. The lifetime of OLEDs has been extended up to 200 Kh [6] and their efficiency has reached 124 lm/W with a 3D light extraction system [7]. As such, they are believed to lead a paradigm shift in the lighting industry. However, they still suffer from cost issue and consequently have uncompetitive prices compared to other lighting sources (bulbs, fluorescent lamps, and LED lightings). One can improve their price competitiveness by reducing the production cost. To this end, a new costcutting fabrication process is indispensible. Usually, patterning indium-tin-oxide (ITO) used as a transparent anode for OLEDs involves a standard photolithography process. This process requires many steps; a ⇑ Corresponding author. Tel.: +82 62 600 6520; fax: +82 62 600 6509. E-mail address: [email protected] (J. Park). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.02.009

photo-resist (PR) coating, exposure and developing, etching, and PR removal. It is repeated three times for the patterning of an auxiliary metal, ITO, and insulator. Though this process is matured, yet it entails a lot of fabrication cost. Therefore, it would be desirable to eliminate those steps for the cost down. One can fabricate the patterned ITO electrodes using a sputtering deposition system without any photolithography involved. The advantage of this scheme is that it can be integrated with an in-line type organic evaporation system, possibly reducing a tact time and thus the production cost. Without an insulating material covering the ITO edges, however, OLEDs fabricated based on such a scheme exhibit a great deal of leakage current under the reverse bias condition due to the rough surface state of ITO and the interface state between ITO and the organic layers [8–12]. The occurrence of reverse leakage current means that the forward current can flow along that pathway without any contribution on light emission [11]. Therefore, we need to reduce the leakage current in order to avoid unnecessary power loss and to enhance device stability. To simplify the patterning process and also reduce leakage current, we fabricate the patterned ITO electrode using a sputtering deposition system and employ an organic molecule as an insulating material that covers

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the sputter-patterned ITO edges. Such an insulating organic molecule is preferred to have low carrier (both electron and hole) mobility. It is demonstrated that the reverse current density of OLEDs based on this scheme is comparable to that of photolithography-patterned ITO-based OLEDs. 2. Results and discussion For a comparative study, we have used two different ITO-coated glass substrates. One was purchased from Shinan SNP Co. Ltd., showing the sheet resistance of 12 X/h. The other one was fabricated using a sputtering deposition system (SUNICOAT-IS3000, SUNIC SYSTEM) under the DC power of 500 W and at the substrate heater temperature of 250 °C. The sputtered ITO at the rate of 2.3 Å/s exhibits the sheet resistance of 17 X/h. The commercial ITO was patterned using the standard photolithography process with or without a patterned polyimide insulation layer (TORAY, Photoneece™ DL-1602). In this paper, a device fabricated based on this method is referred to as a photolithography-patterned and insulator-coated (or insulator-uncoated) ITO-based OLED (PP/IC-OLED or PP/IU-OLED). Meanwhile, the sputtered ITO was patterned using a metal open mask during deposition. To make a comparison with PP/IC- and PP/IU-OLEDs, the sputterpatterned ITO edges were covered with the patterned polyimide insulation layer (Fig. 1(a)). A device fabricated based on this method is named as a sputter-patterned and insulator-coated ITO-based OLED (SP/IC-OLED), whereas a device fabricated based on this method with the insulator excluded is named as a sputter-patterned and insulatoruncoated ITO-based OLED (SP/IU-OLED) (Fig. 1(b)). In replacement of the polyimide insulation layer, thermally evaporated insulating organic molecules can be employed to cover the sputter-patterned ITO edges. A device fabricated based on this method is referred to as a sputter-

patterned ITO-based and insulator-evaporated OLED (SP/ IE-OLED) (Fig. 1(c)). In the SP/IE-OLED configuration, the insulating organic layer can be evaporated either right on the ITO edges or right before the cathode layer. It is noted that the insulating organic molecule evaporated right on the ITO edges should have low hole mobility (i.e., blocking hole injection), whereas it should have low electron mobility when evaporated right before the cathode (i.e., blocking electron injection). We have fabricated phosphorescent green OLED devices that consist of a 10-nm-thick LG-101 for a hole injection layer (HIL), 25-nm-thick 4,40 -bis[N-(1-nathyl)-N-phenylamino]biphenyl (NPB) for a hole transport layer (HTL), 25nm-thick 4,40 -N,N0 -dicarbazolylbiphenyl (CBP) doped with tris(2-phenylpyridinate) iridium(III) (Ir(ppy)3, 8 wt.%) for an emitting layer (EML), 10-nm-thick m-bis-(triphenylsilyl)benzene (UGH3) for a hole/exciton blocking layer (HBL), 30-nm-thick (2,9-dimethyl-4, 7-diphenyl-1, 10phenanthroline) (Bphen) for an electron transport layer (ETL), 1-nm-thick lithium fluoride (LiF) for an electron injection layer (EIL), and 100-nm-thick aluminum (Al). In SP/IE-OLEDs, NPB is used as an insulating material, which has very low electron mobility (1  108 cm2/Vs) [13]. The sheet resistance, surface roughness, and cross-sectional image of ITO were measured by aging package (Mcscience Polaronix, M6101), atomic force microscopy (AFM, XE-200 system, PSIA), and field emission scanning electron microscope (FE-SEM, Quanta 200FEG, FEI Co.), respectively. As depicted in Fig. 1, the width of the photolithographyand sputter-patterned ITOs is 4 mm. The Al cathode has the same width as the patterned ITO anode. Insulatorcoated ITO-based OLEDs (PP/IC- and SP/IC-OLEDs) have the emission area of 2  2 mm. Meanwhile, insulatoruncoated ITO-based OLEDs (PP/IU- and SP/IU-OLEDs) have the emission area of 4  4 mm. As such, the leakage

Fig. 1. Cross-sectional view of (a) SP/IC-OLED, (b) SP/IU-OLED, and (c) SP/IE-OLED structures.

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current is expected to occur on the slope of the ITO edges. In SP/IE-OLEDs, the insulating NPB layer is deposited before the Al cathode, with the width of 1 mm centered at the ITO edges and with the length of 5 mm. Therefore, the emission area of SP/IE-OLEDs is as large as 3  4 mm. It is noteworthy that as the emission area of those devices is all different, a direct comparison should be made at the same current density or the current density instead of the current should be considered. Shown in Fig. 2(a)–(c) are the images of green light emission from those devices (SP/IC-, SP/IU-, and SP/IEOLEDs) when the current density is 10 mA/cm2. It is clearly seen to the naked eye that the SP/IE-OLED has smaller emission area (3  4 mm) than the SP/IU-OLED (4  4 mm), indicating that the insulating organic molecule prevents the current flow effectively near the ITO edges. Fig. 2(d) shows the current efficiency of those devices as a function

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of the current density. Summarized in Table 1 are the luminance and current efficiency of those devices at a current density of 10 mA/cm2. As expected, the PP/IC-OLED is shown to have the highest luminance (6114 cd/m2) and current efficiency (54 cd/A). A small reduction (2 cd/A) in the current efficiency appears on the PP/IU-OLED due possibly to increased leak channels. Though the emission area is increased four times, yet the leakage current is little increased for photolithography-patterned ITO-based OLEDs. Meanwhile, the SP/IC-OLED shows slightly lower current efficiency (51.2 cd/A at 10 mA/cm2) than the PP/IC-OLED. It may arise from the fact that the sheet resistance (17 X/ h) of the sputtered ITO is higher than that (12 X/h) of the commercial ITO. However, the current efficiency is substantially reduced (39.6 cd/A at 10 mA/cm2) in the SP/IU-OLED. Considering that the emission area of the SP/IU-OLED is the same as that of PP/IU-OLED, such a large

Fig. 2. Images of green light emission from (a) SP/IC-OLED, (b) SP/IU-OLED, and (c) SP/IE-OLED at 10 mA/cm2, (d) measured current efficiency versus current density, and (e) reverse current density versus bias voltage. The insulating organic (NPB) layer of SP/IE-OLED is as thick as 50 nm.

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Table 1 Measured luminance and current efficiency of those devices at a current density of 10 mA/cm2. @10 (mA/cm2)

PP/ IU OLED

PP/IC OLED

SP/IU OLED

SP/IC OLED

SP/IE OLED

Luminance (cd/m2) Current efficiency (cd/ A)

5837 52

6114 54

4062 39.6

5685 51.2

4983 45.4

reduction (52 cd/A ? 39.6 cd/A) in the current efficiency is due most likely to the rough surface state of the ITO edges. From the results in Fig. 2(d), we can see that the leakage current can be suppressed to a great extent by evaporation of insulating organic molecules (SP/IE-OLED), especially at low current density (<10 mA/cm2) or below the luminance of 5000 cd/m2. The leakage current is further demonstrated by measuring the current density of those devices under the reverse

Fig. 3. FE-SEM images of (a) photolithography-patterned ITO and (b) sputter-patterned ITO and AFM images of (c) photolithography-patterned ITO and (d) sputter-patterned ITO (scanning area of 5  5 lm).

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Fig. 4. Measured reverse current density of SP/IE-OLEDs for different thicknesses of insulating NPB molecule.

bias condition (from 0 V to 10 V). As shown in Fig. 2(e), significant leakage current occurs in the SP/IU-OLED device. Such leakage current is substantially suppressed in the SP/IE-OLED device. The reverse current density of SP/ICOLED, SP/IE-OLED, and SP/IU-OLED is measured to be 118 lA/cm2, 537 lA/cm2, and 3.6 mA/cm2, respectively, at the reverse bias voltage of 6.6 V. Within the range between 0 V and 7 V, the reverse current density of SP/IE-OLED is comparable to that of SP/IC-OLED, indicating that the insulating organic molecule prevents the flow of leakage current as effectively as the polyimide insulation material. It appears that the leakage current is highly related with the surface morphology of ITO, especially on the slope of the ITO edges. To investigate it, we have measured the cross-sectional image and the surface morphology of the patterned ITOs and presented the results in Fig. 3. It was observed that the photolithography-patterned ITO has a very steep slope (the slope length is about 0.64 lm), whereas the sputter-patterned ITO has a very gentle slope (the slope length is about 400 lm). The gentle slope of the sputter-patterned ITO edges is induced by the spatial distance (5.5 cm) between the metal open mask and the substrate in the in-line sputter. The slope length could be reduced further by placing them more closely. Due to their gentle slope, we are capable of measuring the surface morphology of the sputter-patterned ITO on the slope of the ITO edges, though the whole ITO slope cannot be measured by FE-SEM. The SEM image in Fig. 3(b) was measured on the sputter-patterned ITO slope. It is likely that the ITO thickness (109.3 nm) measured on the slope is a little smaller than that (150 nm) on the flat surface. Meanwhile, we have measured the surface morphology of the photolithography-patterned ITO near the ITO edges. From Fig. 3(c) and (d), grain structure of ITO surfaces are clearly confirmed. It is evident that many rugged regions appear on the sputter-patterned ITO slope during the sputtering process. While the surface morphology of the photolithogra-

phy-patterned ITO is good, with a root mean square roughness (Rrms) of 1.13 nm and peak to valley roughness (Rpv) of 10.01 nm, the surface roughness of the sputterpatterned ITO is poor, with Rrms of 1.837 nm and Rpv of 39.028 nm. A high Rpv indicates the existence of spikes. In Fig. 3(d), several spikes that have very large height were observed. Spike-like surface induces more injection of holes into the device, which may increase the chance of causing a short-circuit problem. It is known that Rpv rather than Rrms directly affects a leakage current under reverse bias condition [14,15]. Therefore, more leakage current appears in SP/IU-OLEDs due to high Rpv. In other words, nonemissive current component exists at the rugged edges of the electrode. Another cause for the occurrence of more leakage current in SP/IU-OLEDs lies in the gentle slope of the patterned ITO edges (i.e., broader spiky surface). Since the surface morphology of the photolithography-patterned ITO is relatively good, it was found that the current efficiency of the PP/IU-OLED was comparable to that of the PP/IC-OLED (Table 1). With attempt to investigate further the effect of the insulating organic layer on the leakage current of SP/IE-OLEDs, we have varied its thickness and measured the reverse leakage current. As shown in Fig. 4, less leakage current occurs with increasing insulating organic layer thickness. It indicates that thicker insulating organic layer prevents more effectively the flow (or injection) of electrons through (into) the slope of the ITO edges. It is noteworthy that any organic molecules such as hole injecting, hole transporting, and electron transporting materials can be used as an insulating material given that their electron or hole mobility is low enough to suppress leakage current. Ideally, organic molecules that have both low electron and hole mobility are preferred. This scheme allows us to implement an in-line type organic evaporation system that does not require any photolithography process, thereby bringing in a reduction of a tact time and fabrication cost.

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3. Conclusion We have shown that, without a standard photolithography process, ITO can be patterned by a sputtering deposition and thermal evaporation of an insulating organic molecule. OLEDs based on this scheme exhibit much less leakage current, which is even comparable to that of photolithography-patterned ITO-based OLEDs. The most salient feature of this technique is that it is applicable to in-line type organic evaporation system, a reduction of fabrication cost by which is feasible. References [1] K. Leo, Science 310 (2005) 1762. [2] Y. Sun, N. Giebink, H. Kanno, B. Wa, M.E. Thompson, S.R. Forrest, Nature (London) 440 (2006) 908.

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