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We present the results of a study of flat and uniform organic electroluminescent (EL) layers produced using a simple premetered horizontal-dipping process. It is shown that this process can produce high quality organic semiconductor thin films by utilizing the downstream meniscus of the solution, which may be controlled by adjusting the gap height and the carrying speed. It is also shown that the organic light emitting devices (OLEDs) produced using this method exhibit a peak brightness in excess of 52,000 cd/m2 and a maximum efficiency of 24 cd/A, with a large active area. From these results, we show that this premetered process for solution coating offers considerable promise for the production of highly efficient, reliable, and large-area solution-processed OLEDs.
©2009 Optical Society of America
1. Introduction
Recent research has focused on the development of organic materials and device structures for use in organic light-emitting devices (OLEDs), with the aim of realizing cost-efficient, lightweight, and large–area flat panel displays [1–6]. In order to achieve this aim, the scientific developments of greatest interest to researchers are the improved efficiency, stability, and simplicity of the device fabrication process. In respect of the efficiency of these devices, for example, their internal quantum efficiency has been improved significantly of late, and is currently typically near 100%, as a result of incorporating phosphorescent dopant into the electroluminescent (EL) layer. This innovation has resulted in strong spin-orbit coupling, which leads to a rapid intersystem crossing and a radiative transition from triplet states to a ground state, thus promoting enhanced EL emissions [3–6]. By making use of the electrophosphorescent Ir complex, it has been possible to create phosphorescent OLEDs (PHOLEDs) with an increased peak luminescence of up to ~ 50,000 cd/m2 [3–6]. In contrast, relatively little progress has been made to date in designing a reliable and simple fabrication process that ensures the formation of a flat and uniform EL layer over a large area, which is particularly important for achieving the highly efficient and reliable device performance that is required for OLEDs. During the fabrication of OLEDs, the organic layers used are typically prepared using physical vapor deposition or wet solution-coating processes [1–11]. To date, OLEDs manufactured using vapor-deposited organic multi-layers of small molecular materials have the best performance record. However, the vapor deposition process is quite complex and expensive. Solution-processed devices made of polymeric or small molecular materials are also of interest, because these techniques make possible a simple production technique that uses a non-vacuum process [7–11]. For solution-processed devices, spin-coating has until now been the most popular method of forming organic layers. This method is convenient, but has several disadvantages, such as the high stress caused by the spinning motion, the poor uniformity at the edges of large areas, and the large amounts of wasted solution [8–12]. These factors make spin-coating unsuitable for application to large active areas. An alternative method for depositing the solution is to use such techniques as screen printing [9, 10], ink-jet printing [11], or blade coating [12]. By using these techniques, organic or polymeric layers may be formed on substrates in a controlled fashion. However, despite the recent developments in such solution-processed devices, an alternative solution-coating process is nevertheless required, because of the continued difficulty of controlling the uniformity of the organic semiconducting layers resulting from the conventional coating methods that have so far been proposed. Hence, further research on solution deposition techniques is required in order to achieve simpler and more reliable fabrication of OLEDs.
We herein present a novel premetered solution process for the fabrication of OLEDs. The advantage of using premetered coating is that the coating thickness is predetermined, in contrast to more typical metered methods such as fixed-gap blade, knife, or wire-bar coatings. We recently described a solution-processed, highly efficient polymer solar cell that was fabricated using a premetered solution-process [13]. In the study described herein, we used this process to demonstrate the fabrication of efficient, high-performance solution-processed OLEDs.
2. Horizontal-dipping process
It is well-known that coating flows can be divided into two categories, metered or premetered, according to whether the thickness of the coated film is determined by the process or imposed externally. The thickness of the film coated by the metered process is independent of the capillary number, while that produced by premetered process usually increases with increasing capillary number (Ca=(μU/σ)), where μ and σ represent the viscosity and surface tension of the coating solution, respectively, and U is the carrying (coating) speed. Examples of premetered coating flow include meniscus and dip coatings. A photograph and schematic illustration of the premetered solution coating process under investigation are shown in Fig. 1(a) . This figure shows a cylindrical coating barrier hanging at a specific height (h0) above a rigid substrate laid on a carrying stage that transports the substrate in a horizontal direction. The coating process occurs in the following sequence. (1) The substrate is attached to the carrying stage, and the coating barrier is placed at the front edge of the substrate. A blended organic semiconducting molecular solution is then introduced into the empty space between the barrier and the substrate by capillary action, so that a uniform meniscus of the solution may be formed on the substrate by attraction to the barrier (i.e. by surface tension). (2) The substrate is then transported horizontally at constant velocity whilst maintaining the shape of the downstream meniscus. A thin solution layer of the downstream meniscus is then spread evenly on the substrate. While the substrate is being transported, the blended organic solution may be supplied into the gap space at an appropriate injection rate. (3) Having been spread on the substrate, the wet film is dried, and a heater may be used to assist the evaporation of the residual solvent in the wet film on the substrate. Following this process, it is possible to obtain a substrate coated with a solid organic film of uniform thickness. The transport of the substrate through the meniscus of the solution is similar to that which occurs in the typical dip-coating method [14, 15]. In that method, the substrate is immersed in the coating solution and a wet layer is then formed by withdrawing the substrate vertically through the meniscus of the coating solution. Our proposed coating method differs from the conventional dip-coating method, however, because the wet film is formed by withdrawing the substrate horizontally. As a result, we call the proposed process horizontal dipping (H-dipping) [13].
Fig. 1 (a) Photograph (left) with schematic illustration (right) of the premetered horizontal-dip (H-dip) coating process described herein: a cylindrical coating barrier (SUS steel) with a diameter R, a gap height h0, and a carrying speed U. (b) Photograph of the photoluminescent spin-coated film (left, at 1000 rpm) and the H-dip-coated film (right) on patterned 2” substrates.
Photoluminescence images obtained from spin- and H-dip-coated organic films on 2-inch glass substrates (ITO patterned) using a UV light source of 365 nm are shown in Fig. 1(b). It may be seen from the figure that the luminescent intensity and thickness of the spin-coated films vary near the edges of the substrates due to the Bernoulli effect [16]. In order to produce a fully smooth spin-coated film, the speed of rotation must be adjusted and the amount of spreading solution increased. On the other hand, it may be seen that the H-dip-coated film is very smooth and uniform. Variation in the thickness of the film was observed only at the very rear edge of the substrate. The surface morphology investigation using AFM showed clearly that the topography was fairly uniform, the root mean square roughness for the H-dip-coated film being only ca. 0.9 nm, which was comparable to that (ca. 1.0 nm) of the spin-coated films. Moreover, the surface roughness of the H-dip-coated films was identical at different positions. This uniformity was achieved because no external centrifugal force was applied during the formation of the film. Thus, compared to spin-coating, it is possible to achieve a uniformity in the film thickness of the EL layer in a reliable way when using H-dipping, even on a large-area substrate. This is a result of the control of the undesirable free-surface flow that occurs at the top organic solution-air interface, via the surface tension effect between the solution and the coating barrier.
The film thickness that results from the H-dipping process may be explained by the description of the associated drag-out problem suggested by Landau and Levich [15]. Based on their description, for a small capillary number (Ca << 1), a useful relationship may be obtained that relates the thickness of the film emerging from a coating bead to the radius of the associated meniscus and carrying speed, U [13, 15]:
where Rd represents the radius of curvature of the downstream meniscus. Here, R and h0 represent the radius of the cylindrical coating barrier and the minimum gap height, respectively, and n is 1 for a contact angle of 90° or 2 for a contact angle of 0° measured on the contact line at the interface between the solution and the coating barrier. In our study, n was assumed to be 2, as shown in the photograph [Fig. 1(a)].It is worthy of note that the thickness of the H-dip-coated film is much less than the gap height. This is characteristic of the main way in which the premetered H-dipping process differs from the conventional metered doctor-blade (or wire-bar) coating [17]. In the conventional approach, the doctor-blade (or wire-bar) coating process produces a film thickness of the order of the gap size whose thickness is independent of the carrying speed of the substrate. In our proposed method, the premetered H-dipping process allows the critical control of the thickness and can produce superior quality and extremely thin films at line speeds of the order of a few meters per minute.
3. Experimental methods
In all the experiments, an ITO layer (80 nm, 30 ohm/square, RMS roughness ~ 1.98 nm, UID Co. Ltd.) on a glass substrate was used as the transparent anode. After routine cleaning of the substrate using ultraviolet-ozone treatment, a blended EL solution was used to coat the ITO layer, which was precoated with a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) buffer layer. The PEDOT:PSS and the organic EL layers were successively deposited by H-dipping on an ITO-coated glass substrate. The PEDOT:PSS solution used was a mixture of 1 % PEDOT:PSS solution (CLEVIOS P VP AI 4083, H.C. Starck) and isopropyl alcohol with a weight ratio of 2:1. The viscosity of the mixed PEDOT:PSS solution, measured by viscometer (RVDVII + , Brookfield Inc.), was about 11.6 cp. For the blended EL solution, we used N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'biphenyl-4,4'-diamine (TPD), 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (Bu-PBD), tris(2- Phenylpyridinato) iridium (Ir(ppy)3), and poly(vinylcarbazole) (PVK) without further purification (2.5 wt%), in mixed solvents of 1,2-dichloroethane and chloroform (3:1) [18]. The viscosity of the EL solution was about ~1.0 cp at a temperature of 25°C. The apparatus used for H-dipping had a maximum work space of 8 × 10 cm2. A small volume of the solution (~ 6 μl) per unit coating area (1 × 1 cm2) was fed into the gap between the cylindrical barrier (SUS steel, R = 6.35 mm) and the glass substrate using a syringe pump (Pump Systems Inc. NE-1000). The height of the gap, h0 was adjusted vertically using two micrometer positioners, and the carrying speed U was controlled using a computer-controlled translation stage (SGSP26-200, Sigma Koki Co., Ltd). After a meniscus had formed on the solution, the substrate was transported horizontally, so that the barrier spread the solution on the transporting substrate. The transporting speed U was 1.5 cm/s. It took 2 seconds to prepare a complete film on a substrate with an area of 1.8 × 2.0 cm2. The H-dip-coated PEDOT:PSS layer and EL layer were then dried using a heating plate at 110°C for 60 minutes and at 60°C for 5 minutes, respectively, in order to remove the remaining solvents. For comparison, conventional devices were fabricated by spin coating the blend solutions. 1 nm CsF and 60 nm of Al were evaporated sequentially on the EL layer via thermal deposition (0.5 nm/s) at a base pressure below 2.7 × 10−4 Pa. The OLED fabricated in our study thus had a device configuration of Glass// ITO// PEDOT:PSS// EL layer// CsF// Al. The fabrication and characterization of the device were carried out at room temperature under ambient conditions, without encapsulation. In order to investigate the surface morphologies of the fabricated films, the variation in the surface roughness of the film was monitored using an Atomic Force Microscope (AFM, Nanosurf easyscan2 FlexAFM, Nanosurf AG Switzerland Inc.). During the measurements, a contact mode was used with a cantilever (CONTR-10 point probe-silicon, Nanoworld, Inc.). A Chroma Meter CS-200 (Konica Minolta Sensing, INC.), a spectrometer (Ocean's Optics), and a source meter (Keithley 2400) were used to measure the EL characteristics.
4. Results and discussion
First, using the AFM, we investigated the dependence of the film thickness, h of the H-dip-coated EL layer on the transporting speed U and the gap height h0. The results obtained are shown in Fig. 2(a) . As shown in the figure, for a gap height, h0 of 0.8 mm, the thickness of the H-dip-coated EL layer increases continuously as the speed U increases in the observed region (red circles). Furthermore, when h0 was increased from 0.8 mm to 0.9 mm, the thickness of the H-dip-coated EL layer also increased with increasing speed U. These results may be explained by the description of the associated drag-out problem, using Eq. (1). The theoretical curves resulting from Eq. (1) are shown in the figure as solid lines. The observed data fitted the theoretical values predicted by Eq. (1) rather well, indicating that the thickness of the H-dip-coated EL film may be controlled by adjusting the gap height h0 and the carrying speed U. These results indicate that the H-dipping process may be used to produce an EL layer at least as well as spin-coating can. It is further evident that the thickness of the H-dip-coated EL layer follows nearly the same trends as those shown in previous results using the H-dip-coated PV and PEDOT:PSS layers [13]. For comparison, the experimental results and theoretical curves for the PEDOT:PSS layer are also shown in Fig. 2(b).
Fig. 2 (a) Coated film thickness data of the EL layer as a function of carrying speed for two gap heights (0.9 and 0.8 mm). (b) Film thickness of the H-dip coated PEDOT:PSS layer as a function of carrying speed for two gap heights (0.9 and 0.8 mm). The solid curves show the theoretical predictions of the Landau & Levich equation [13,15].
We then investigated the EL characteristics of the OLED produced by the H-dipping process. Using the results described above, we prepared H-dipped OLEDs with a device configuration of Glass// ITO// PEDOT:PSS// EL layer// CsF// Al. Both the PEDOT:PSS and the organic EL layers were coated using the H-dipping process on an ITO-coated glass substrate. The thicknesses of the PEDOT:PSS and the EL layers were adjusted to about 40 nm and 80 nm, respectively. Figure 3(a) shows the observed current density-luminance-voltage (J-L-V) characteristics of the fabricated OLED. The slope of the J–V curve between 0 and 18 V shows the excellent diode behavior of the fabricated OLED and thus indicates good coverage of the H-dip-coated PEDOT:PSS buffer layer and the EL layer. It is clear from the J-L-V curves that both the charge injection and turn-on voltages are below 3.7 V, with sharp increases in the J-L-V curves occurring at higher applied voltages. An operating voltage of about 4.5 V yields a brightness of 100 cd/m2, 6.2 V yields 1,000 cd/m2, and 9.4 V yields 10,000 cd/m2. The luminescence reached ca. 52,300 cd/m2 (at 16.5 V), which is higher than those (ca. 10,000 cd/m2 at 11.0 V) of the control device made by spin-coating. It is worthy of note that the maximum brightness of over 52,000 cd/m2 is among the highest values reported to date for OLEDs manufactured using a solution process. In order to confirm the high performance of the sample devices, we also calculated the efficiency of the devices studied, as shown in Fig. 3(b). For the H-dip-coated OLED, a current efficiency ηc of 9.4 cd/A was obtained at 1,000 cd/m2, reaching ηc = 23.7 cd/A at 6,000 cd/m2. We also calculated the power efficiency ηp of the H-dip-coated device, which reached a maximum of 9 lm/W. For the spin-coated device, ηp reached a maximum of only 6 lm/W. These results clearly indicated that the EL layer manufactured by H-dip-coating possesses bright and efficient EL characteristics due to the formation of a uniform layer. The observed results from the devices are summarized in Table 1 .
Fig. 3 (a) Current density-voltage and luminance-voltage characteristics of the OLED made using the H-dipping process. (b) Current efficiency-voltage and power efficiency-voltage characteristics of the studied OLED.
Table 1. Summary of performance of OLEDs made by H-dipping process.
Next, in order to check the processing ability of large-area OLEDs, we also fabricated a 2” × 2” OLED device using the H-dipping process on an ITO-coated glass substrate. A photographic image of the fabricated device is shown in Fig. 4 . A PEDOT:PSS layer and an EL layer were deposited on a strip-patterned 2” × 2” ITO-coated glass substrate by H-dipping, in order to fabricate a passive-matrix display device. The pixel array was 7 × 7 and the pixel size was 5 × 5 mm2. It may be seen from the figure that the fabricated OLEDs were fairly luminous. The EL spectra were collected from each of the 49 individual pixels on the substrate, and were almost identical for each pixel (not shown), the emission peak wavelength being about 510 nm with a full width at half maximum of about 60 nm. The variation of the emitting intensity at different pixels was less than 10%. This result implies that the variation in the thickness of the organic thin film was small, because the EL intensity from an OLED is quite sensitive to the layer thickness. The low variation of EL intensity is quite acceptable for large-scale fabrication. These results confirm that the H-dipping method shows considerable promise for use in simple fabrication techniques that may easily be scaled up to a larger size at a lower cost than other processes. It should be noted that we were not able to form a homogeneous and uniformly thin EL layer by spin-coating for EL solutions on a 2” × 2” substrate. This result also implies that the H-dipping process is also suitable for fabricating large and homogeneous organic semiconducting films. It is worth noting that the performance of OLEDs may be further enhanced by, for example, the selection of more suitable materials, solvents, solution concentrations and viscosities, and by optimizing the gap height between the barrier and the substrate.
Fig. 4 Photographs of the operating 7 × 7 OLED pixels made by the H-dipping method at 10 V on a glass substrate (2” × 2”).
From the results reported above, it is clear that the H-dipping process for solution coating shows considerable promise for the fabrication of bright and large-area OLEDs. Furthermore, the H-dipping process used in this study can be also applied to the design of new electric organic devices, such as organic transistors, special organic lighting devices, memory devices, and sensors.
5. Conclusions
In summary, we investigated a simple premetered H-dipping process as a promising organic thin-film coating process for the manufacture of cost-efficient and large-area OLEDs. Organic semiconducting thin films were fabricated successfully on a 2” × 2” substrate with a high uniformity using H-dipping in a solution whose meniscus was controlled by adjusting the gap height and coating speed. It was also shown that bright and efficient OLEDs were produced that perform comparably to OLEDs made using the spin-coating process. Our experimental results indicate that the H-dipping method shows great potential for applications involving large-area OLEDs. This novel process for depositing the solution on the substrate can be expanded to slot-die and slit-die coatings, and will provide a solid foundation for extending the fabrication of large-area organic devices to include various advanced OLEDs. Combining the H-dipping method reported here with luminous organic materials reported elsewhere will surely lead to the development of highly luminous large-area OLEDs, which will render the use of such devices possible for many applications, such as lighting, displays, and/or optoelectronic devices.
Acknowledgments
This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. 2009-0077378). This work was supported by the Brain Korea 21 Project 2009.
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