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Investigation on slot-die coating of hybrid material structure for OLED lightings
Journal of Physics and Chemistry of Solids 95 (2016) 119–128
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Investigation on slot-die coating of hybrid material structure for OLED lightings Kwang-Jun Choi, Jin-Young Lee, Dong-Kyun Shin, Jongwoon Park n School of Electrical, Electronics & Communication Engineering, Korea University of Technology and Education, Cheonan 330-708, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 8 March 2016 Accepted 25 April 2016 Available online 26 April 2016
With an attempt to fabricate large-area OLED lighting panels, we investigate slot-die coating of a small molecule (SM) hole transport layer (HTL). It is observed that SM HTL films formed by spin coating exhibit pinhole-like surface, whereas the films by slot-die coating show micro-sized hillocks due to agglomeration. As the plate temperature of the slot coater is increased, smaller hillocks appear more densely. To tackle it, a small amount of a polymer HTL is added into the SM HTL (Hybrid HTL). By the aid of entangled polymer chains, small molecules are prohibited from migrating and thus agglomerations disappear. The peak-to-peak roughness of the slot-coated hybrid HTL films is measured to be about 11.5 nm, which is slightly higher than that ( 7 nm) of the polymer HTL film, but much lower than that ( 1071 nm) of the SM HTL film. Similar results are also observed in spin-coated films. It is also addressed that OLED with the hybrid HTL shows higher luminous efficacy, compared to OLED with the SM HTL or the polymer HTL. We have further demonstrated that the dissolution problem occurring between two stacked layers with different solvents during slot-die coating can be suppressed to a great extent using such a combination of materials in hybrid structure. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Slot-die coating Organic light-emitting diodes (OLEDs) Small molecules Polymer Agglomeration
1. Introduction Organic light-emitting diode (OLED) technology can be utilized mainly for flat panel display and lighting applications by virtue of its superior features such as surface emission, flexibility, transparency, roll-to-roll compatibility, etc [1–4]. However, commercial active-matrix OLED (AMOLED) displays still use cost-ineffective thermal evaporation and fine metal mask, making it difficult to scale up the glass size. To reduce fabrication costs, solution-processable techniques are highly demanded. AMOLED display panels were fabricated using solution-coating process; namely, the blanket layers were formed by slot-die coating and the patterned emission layers by nozzle printing [4]. Similarly, large-area OLED lighting panels fabricated by vacuum evaporation are still highly priced, hindering their successful market entry and viability. Of many solution-coating methods (e.g., inkjet printing, nozzle printing, gravure offset printing, spin coating, spray coating, blade coating, and slot-die coating) [5–11], pre-metered slot-die coating is preferred since OLED lighting panels require a multilayer formation of large-area uniform organic films. It provides large scale roll-to-roll production and the simultaneous coating of multiple layers of different solutions [7–9]. It is also capable of coating a n
Corresponding author. E-mail address: [email protected] (J. Park).
http://dx.doi.org/10.1016/j.jpcs.2016.04.006 0022-3697/& 2016 Elsevier Ltd. All rights reserved.
wide range of process materials and depositing organic thin films having the thickness as low as 20 nm. It was demonstrated that homogeneous small molecule (SM) OLED layers could be produced at gap-to-film-thickness ratios of up to 50 [12]. As depicted in Fig. 1, however, it is known that solution coating of SM films entails pinhole-like surface due to the absence of physical entanglement in soluble SMs or/and the crystallization of SMs. Such problems make it difficult to fabricate large-area uniform SM films. It can be suppressed by attaching flexible chains, by adopting an asymmetric molecular structure, or by substituting bulky side groups [13]. Even so, they bring in a negative effect on the device performance (high driving voltage induced by low carrier mobility). Unlike SMs, polymer materials have entangled chain structure and can form stable amorphous film during thermal annealing (Fig. 1). The entanglement of the polymer chains suppresses the crystallization of the polymer chains and the chains are closely packed by thermal annealing without crystallization [13], enabling the formation of large-area uniform polymer films. Compared with SMs, however, polymer materials result in relatively low quantum efficiency of OLEDs. As such, we come up with a hybrid material structure (Fig. 1) where a small amount of chain-entangled polymer is doped to a SM solution in such a way that the crystallization of SMs can be suppressed without compromise on the device performance. In fact, a mixture of a polymer and SM has been used for hybrid host system of an emission layer [14,15]. However, such hybrid host
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Fig. 1. Schematic view of solution coating of hybrid material structure featuring pinhole-free surface morphology.
films were fabricated mainly to enhance the quantum efficiency of spin-coated small-area OLEDs. In this paper, we investigate the feasibility of large-area uniform multilayer coating of SM-based organic materials by the addition of a chain-entangled polymer and the possibility of fabricating large-area OLEDs without sacrificing the device performance. To this end, we have slot-coated a SM hole transport layer (HTL) where a small amount of a polymer HTL is doped. Such a hybrid material system is shown to enhance the film quality. Namely, it suppresses the occurrence of pinholes and ribbing defects to a great extent. We have also addressed that the dissolution problem occurring between two stacked layers is evitable. Furthermore, it is demonstrated that OLEDs with such a hybrid HTL layer show higher device performance (current efficiency, luminous efficacy, device resistance, etc.), compared to OLEDs with a SM HTL or polymer HTL.
2. Experiment Slot-die coating has been done under ambient air with a table slot coater (TSDC-KTEU, DCN) equipped with a heating plate, dry unit, syringe pump system, and slot head module (head size: 240 mm 30 mm 56.5 mm) having the effective coating width of 150 mm. Summarized in Table 1 are typical values of coating process variables. Unless otherwise specified, those process variables are kept unchanged. Of many OLED layers, the HTL layer is formed using the table slot coater. A SM HTL (KHT-001, Duksan Neolux Co., Ltd.) is mixed with a polymer HTL (Poly(N-vinylcarbazole) (PVK), purchased from Sigma Aldrich) at the ratio of 7:3 (i.e., SM is dominant in the hybrid HTL material). In fact, PVK has been widely used as a HTL and host for phosphorescent dyes
Table 1 Fixed values variables.
of
coating
process
Variables
Value
Die shim thickness Coating gap Coating speed Flow rate Plate temperature Ambient temperature
0.05 mm 150 mm 1.8 mm/sec 0.2 ml/min 90 °C 24 °C
[13,16,17]. We have used a co-solvent for the hybrid HTL material to improve the coated film quality. Namely, chlorobenzene (CB) with lower vapor pressure (11.8 mm Hg at 25 °C) and toluene (TE) with higher vapor pressure (26 mm Hg at 25 °C) are mixed at the ratio of 3:7. Before coating, we used PTFE syringe filter (0.5 mm, 25JP050AN) to filter out aggregated particles existing in the solution. For comparison, we have also fabricated the SM HTL films and the polymer HTL films using the same co-solvent. The mass fraction of SM HTL and polymer HTL is 0.015 (1.5% by mass). For a comparative study, those films were also fabricated using a spin coater (ACE-200, DONG AH Trade Corp.). Spin coating has been done at the speed of 3000 rpm for 10 s. Glass substrates used for coating of organic materials were cleaned with isopropyl alcohol (IPA) and treated with UV/O3 for 90 s at a power of 150 W. The coated films by the slot and spin coaters were dried at 80 °C for 30 min in a vacuum oven (ThermoStable OV-30, DAIHAN Scientific Co., Ltd.). The film thickness was measured by scanning electron microscope (SEM, Helios nanolab 600i) and the surface roughness by atomic force microscope (AFM, XE-100).
K.-J. Choi et al. / Journal of Physics and Chemistry of Solids 95 (2016) 119–128
To evaluate the film quality at the device level, we have fabricated a green phosphorescent OLED device that consists of a 150nm-thick ITO pre-coated on a glass substrate (purchased from Geomatech Co., Ltd), HTL (SM HTL, polymer HTL, or hybrid HTL), 15-nm-thick 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) for an emission layer (EML), 7-nm-thick 4,7-diphenyl-1,10-phenanthroline (Bphen) for a hole/exciton blocking layer (HBL), 30-nm-thick LG201(LG Chem., Ltd.) for an electron transport layer (ETL), 1-nmthick lithium fluoride (LiF) for an electron injection layer (EIL), and 100-nm-thick aluminum (Al). In the green-emitting layer, 6 wt% fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) is doped. ITO was also treated with UV/O3 for 90 s at a power of 150 W. Except for the HTL, all layers are deposited sequentially at a rate of 0.5 nm/s under a base pressure of 2 10 6 Torr by thermal evaporation. We have fabricated two OLED devices that are different in size. The larger one (emission area of 43 mm 29 mm, substrate size of 50 mm 50 mm) was fabricated to investigate the size-related issues such as the emission uniformity, pinholes, dark spots, shortcircuits, etc. The smaller one (emission area of 2 mm 2 mm) was fabricated in order to measure the IVL characteristics. They were encapsulated by a glass cap in an inert-gas environment (a glove box). The EL property was measured using a source meter (Keithley 2400) and a spectroradiometer (CS-1000, Konica Minolta).
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3. Results and discussion 3.1. Film Quality of spin-coated HTLs It is well known that a spin coating method shows the most excellent surface uniformity due to the horizontal spreading of solutions by centrifugal force. By way of spin coating, we have first fabricated the HTL films on a glass substrate and observed the film quality. The film thickness is measured to be about 8574 nm by a laser confocal microscope (VK-9710, KEYENCE). Presented in Fig. 2 are the measured AFM images of spin-coated SM HTL, hybrid HTL, and polymer HTL films. Of those, the SM HTL film exhibits the roughest surface. As summarized in Table 2, the average and peakto-peak roughness (Ra and Rpv) values are as high as 5.8 nm and 201 nm, respectively, which are too high for large-area OLEDs. It is due to the fact that the absence of physical entanglement in soluble SMs results in pinhole-like surface in the dried film [18]. However, no pinhole appears by the addition of chain-entangled polymers into the SM (Fig. 2(b)). As a result, the Ra (Rpv) value of the hybrid HTL film is measured to be as low as 0.6 nm (11.1 nm), which is slightly higher than that (0.4 nm (8.3 nm)) of the polymer HTL film. Similar behaviors are also observed in the SEM images presented in Fig. 3. There appear a number of holes in the SM HTL film, yet they totally disappear in the hybrid HTL film, a feature highly desirable for large-area OLEDs. As expected, the most excellent film quality is achieved with the polymer HTL.
Fig. 2. Measured AFM images of spin-coated (a) SM HTL film, (b) hybrid HTL film, and (c) polymer HTL film.
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Table 2 Surface roughness values of spin-coated HTL films in Fig. 2. Sample
Ra(nm)
Rpv(nm)
SM HTL Hybrid HTL Polymer HTL
5.8 0.6 0.4
201 11.1 8.3
3.2. Film quality of slot-coated HTLs To fabricate large-area OLED lighting panels using slot-die coating, the film quality should be comparable with that by spin coating. To confirm it, we have slot-coated the SM HTL, polymer HTL, and hybrid HTL films on a glass substrate, measured the film quality, and presented the results in Fig. 4, Fig. 5, and Table 3. As seen in Table 3, the SM HTL film also shows the roughest surface (i.e., Ra of 122.5 nm and Rpv of 1071 nm). By the addition of the chain-entangled polymer HTL into the SM HTL, however, the Ra
(Rpv) value is reduced to 0.6 nm (11.5 nm), which is slightly higher than that (0.5 nm (7.0 nm)) of the polymer HTL film. It is obvious from Table 2 and Table 3 that the surface uniformity (Ra ¼ 0.6 nm, Rpv ¼ 11.5 nm) of slot-coated hybrid HTL film is as good as that (Ra ¼0.6 nm, Rpv ¼11.1 nm) of spin-coated one, indicating that the process variables of slot coating in Table 1 are appropriately set. However, the big difference from the spin-coated films is that there appear a number of small hillocks rather than pinholes in the SM HTL films, as evident in Figs. 4(a) and 5(a) and (b). The diameter and height of those hillocks are measured to be about 7.23 mm and 1.1 mm, respectively. It is attributed that the agglomeration of SMs occurs quickly before drying. Namely, the occurrence of agglomeration is somewhat different from spin coating because heat is applied to the plate during slot coating. Nevertheless, the doping of entangled polymer chains hinders small molecules from migrating and thus agglomerating, thereby providing excellent film quality (Fig. 5(c)). Therefore, such a hybrid material system offers an effective way of improving the operating
Fig. 3. Measured SEM images of spin-coated (a) SM HTL film, (b) hybrid HTL film, and (c) polymer HTL film.
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Fig. 4. Measured AFM images of slot-coated (a) SM HTL film, (b) hybrid HTL film, and (c) polymer HTL film.
window in which stable and uniform slot coating is feasible. The thickness of the polymer and hybrid HTL films is measured to be about 5875 nm. To investigate the emergence of micro-sized hillocks in the SM HTL films further, we have varied the plate temperature from 60 °C to 105 °C. As evident in Fig. 6, smaller hillocks appear more densely as the plate temperature is increased. It is noted that the quality of the hybrid film is getting degraded with decreasing amount of the polymer HTL. When the amount of the polymer HTL is much less than 30%, some stripe patterns (ribbing defects) appear in the dried films. 3.3. Effect of coated HTLs on OLED performance Though the addition of the polymer HTL enables us to fabricate large-area uniform SM HTL films, yet it is uncertain whether or not such a hybrid material system brings in a positive effect on OLED device performance. To investigate it, we have fabricated OLEDs without any hole injection layer (HIL) involved using the slotcoated HTL films on ITO. It is noted that the surface tension between the HTL solution and the glass substrate is different from the surface tension between the solution and ITO. Since UV/O3treated ITO is more hydrophilic, the film quality of HTLs on ITO is good enough to fabricate large- and small-area OLEDs. In reality, however, the HTL layer is formed not directly on ITO but on various HIL or other layers. Even if the surface tension is varied between the HTL solution and a certain HIL or any layer, the addition of the polymer HTL into the SM HTL solution renders slot-die
coating stable and uniform. Fig. 7 shows the images of light emission from large-area OLED devices with the slot-coated HTL films. Dark stripes induced by ribbing defects occurring during slot-die coating are invisible. From large-area OLEDs, we have evaluated the emission uniformity by measuring the luminance at five different spots (four corners and one center) when the luminance is about 1000 cd/m2 (calculated in terms of the maximum luminance). All three devices exhibit the emission uniformity higher than 87%, indicating that the thickness uniformity of those HTL films fabricated by slot-die coating is also excellent (higher than 84%). From small-area OLEDs with slot-coated HTL films, we have measured the current–voltage (J–V) characteristics, current efficiency versus current density, and luminous efficacy as a function of luminance. For comparison, we have also fabricated OLEDs with vacuum-evaporated HTL film. As evident in Fig. 8, OLED with the polymer HTL shows the highest current efficiency (42.8 cd/A), yet the lowest luminous efficacy (14.6 lm/W) due to very high driving voltage (9.1 V at 1000 cd/m2). The highest occupied molecular orbital (HOMO) level (5.8 eV) of PVK is slightly lower than that (5.9 eV) of the SM HTL. Therefore, such a high driving voltage arises mainly from the low hole mobility (2.5 10 6 cm2V 1s 1) of PVK [17]. From this, it is obvious that the hole mobility of the polymer HTL is much lower than that of the SM HTL. Therefore, it is doubtful whether the addition of the polymer HTL into the SM HTL solution would degrade the J–V characteristics. As apparent in Fig. 8(a), however, there was no increase in the driving voltage of
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Fig. 5. Measured SEM images of slot-coated (a) SM HTL film, (b) magnified hillocks on SM HTL film, (c) hybrid HTL film, and (d) polymer HTL film.
Table 3 Surface roughness values of slot-coated HTL films in Fig. 4. Sample
Ra(nm)
Rpv(nm)
SM HTL Hybrid HTL Polymer HTL
122.5 0.6 0.5
1070.9 11.5 7.0
OLED with the hybrid HTL film. As a result, the current and luminous efficacies (37 cd/A and 28.7 lm/W) of OLED with the coated hybrid HTL is measured to be even slightly higher than those (34.1 cd/A and 25.3 lm/W) of OLED with the coated SM HTL at the luminance of 1000 cd/m2. This result indicates that the addition of chain-entangled polymers can improve the coating window of SM films without any sacrifice in the device performance. It is also observed that OLED with the evaporated HTL shows the lowest driving voltage, but almost the same luminous efficacy as OLED with the coated hybrid HTL at high luminances because of relatively low current efficiency induced by the
imbalance of charges in the emission layer. We have also evaluated the device stability by measuring the device resistance. The films with poor surface morphology reduce the device resistance and thus cause a short-circuit phenomenon. Those OLED devices with the coated hybrid HTL film exhibit the device resistance of higher than 10 MΩ, representing that high device stability can be achieved by slot-die coating. 3.4. Effect of hybrid material structure on interface dissolution Using orthogonal solvents, the dissolution problems occurring between two stacked layers may be avoidable. In reality, however, it is known that a coated and dried SM film would be dissolved even by the orthogonal solvent of the second layer because SMs typically attach to each other only by weak intermolecular forces. Such a dissolution phenomenon may be suppressed by enhancing the solvent resistance of SMs. It is known that the solvent resistance can be increased with molecular weight [19]. Therefore, the hybrid HTL is such a case in point because the addition of PVK
K.-J. Choi et al. / Journal of Physics and Chemistry of Solids 95 (2016) 119–128
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Fig. 6. Measured SEM images of slot-coated SM HTL films at the plate temperature of (a) 60 °C, (b) 75 °C, (c) 90 °C, and (d) 105 °C.
into the SM HTL increases the molecular weight and thus the solvent resistance. To demonstrate it, we have employed aqueous poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS, Clevios AI 4083) because its solvent (water) is chemically orthogonal to the solvent (CB) for the hybrid HTL. We have spin- and slot-coated PEDOT:PSS (2nd layer) atop the HTLs (1st layer). Normally, PEDOT:PSS lies in the first layer as a HIL. For this experiment, however, we have deliberately coated it on the HTL in order to investigate whether the addition of PVK into the SM HTL can suppress the dissolution phenomenon occurring even by the orthogonal solvent of the second layer. Before coating, we used a cellulose acetate disposable syringe filter (0.2 mm, DISMIC-25CS) to filter out aggregated particles existing in the PEDOT:PSS solution. Spin coating of HTL and PEDOT:PSS has been done at the speed of 3500 rpm and 1500 rpm, respectively, for 20 s. The process variables of slot coating for both HTL and PEDOT:PSS are as follows; the die shim thickness of 0.03 mm, coating gap 200 mm, flow rate 0.1 ml/min, coating speed 6 mm/ sec, and plate temperature 60 °C.
Presented in Fig. 9 are the SEM images of HTL/PEDOT bilayers fabricated using spin and slot coating. The spin-coated SM HTL/ PEDOT bilayer film exhibits the wavy interface due to dissolution. Therefore, it is evident that dissolution occurs even by the orthogonal solvent due to weak intermolecular forces between SMs. However, a very clear interface (boundary line) can be observed in the spin-coated hybrid HTL/PEDOT bilayer film, implying that the addition of PVK into the SM HTL indeed suppresses the dissolution phenomenon. Such a dissolution phenomenon yields a noticeable difference in the layer thickness. As evident in Fig. 9(a) and (b), the total thickness (130.8 nm) of the SM HTL/PEDOT bilayer is almost the same as that (132.3 nm) of the hybrid one. However, the PEDOT:PSS layer becomes thicker and, in turn, the SM HTL becomes thinner. It appears that the dissolution phenomenon is moderated to some extent in the slot-coated SM HTL/PEDOT bilayer film because such a wavy interface is not observed. Even so, its interface is less distinct compared to the slot-coated hybrid HTL/PEDOT one (Fig. 9(c) and (d)). Therefore, such a combination of materials in hybrid structure provides the effective way of
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Fig. 7. Image of green light emission from large-area (43 mm 29 mm) OLED devices with (a) SM HTL layer, (b) hybrid HTL layer, and (c) polymer HTL layer at 6 V.
multi-layer slot coating of large-area uniform SM films for OLED lightings.
4. Conclusion Using slot-die coating, we have fabricated the large-area uniform SM HTL films by the addition of a small amount of the polymer HTL. By suppressing the occurrence of pinhole-like surface and micro-sized hillocks appeared in SM HTL films, we have fabricated the HTL films with the peak-to-peak roughness value as
Fig. 8. Measured (a) J–V curve, (b) current efficiency versus current density, and (c) luminous efficacy versus luminance of OLEDs with different HTLs.
low as 11.5 nm. Using the hybrid HTL film, we have fabricated large-area OLEDs showing the emission uniformity as high as 87%. It was demonstrated that the addition of entangled polymer chains does not bring in any degradation in the device
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Fig. 9. SEM image of HTL/PEDOT:PSS bilayer; (a) spin-coated SM HTL, (b) spin-coated hybrid HTL, (c) slot-coated SM HTL, and (d) slot-coated hybrid HTL.
performance. OLED with the hybrid HTL showed the luminous efficacy of 28.7 lm/W at 1000 cd/m2, which is higher than those (25.3 lm/W, 14.6 lm/W) of OLEDs with the SM HTL or the polymer HTL. In addition to the improved operating window of slot-die coating, such a hybrid material system is demonstrated to suppress the dissolution problem occurring between two stacked layers even with orthogonal solvents.
Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) (NRF2015R1D1A1A01057266) funded by the Ministry of Education.
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Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Investigation on slot-die coating of hybrid material structure for OLED lightings Kwang-Jun Choi, Jin-Young Lee, Dong-Kyun Shin, Jongwoon Park n School of Electrical, Electronics & Communication Engineering, Korea University of Technology and Education, Cheonan 330-708, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 8 March 2016 Accepted 25 April 2016 Available online 26 April 2016
With an attempt to fabricate large-area OLED lighting panels, we investigate slot-die coating of a small molecule (SM) hole transport layer (HTL). It is observed that SM HTL films formed by spin coating exhibit pinhole-like surface, whereas the films by slot-die coating show micro-sized hillocks due to agglomeration. As the plate temperature of the slot coater is increased, smaller hillocks appear more densely. To tackle it, a small amount of a polymer HTL is added into the SM HTL (Hybrid HTL). By the aid of entangled polymer chains, small molecules are prohibited from migrating and thus agglomerations disappear. The peak-to-peak roughness of the slot-coated hybrid HTL films is measured to be about 11.5 nm, which is slightly higher than that ( 7 nm) of the polymer HTL film, but much lower than that ( 1071 nm) of the SM HTL film. Similar results are also observed in spin-coated films. It is also addressed that OLED with the hybrid HTL shows higher luminous efficacy, compared to OLED with the SM HTL or the polymer HTL. We have further demonstrated that the dissolution problem occurring between two stacked layers with different solvents during slot-die coating can be suppressed to a great extent using such a combination of materials in hybrid structure. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Slot-die coating Organic light-emitting diodes (OLEDs) Small molecules Polymer Agglomeration
1. Introduction Organic light-emitting diode (OLED) technology can be utilized mainly for flat panel display and lighting applications by virtue of its superior features such as surface emission, flexibility, transparency, roll-to-roll compatibility, etc [1–4]. However, commercial active-matrix OLED (AMOLED) displays still use cost-ineffective thermal evaporation and fine metal mask, making it difficult to scale up the glass size. To reduce fabrication costs, solution-processable techniques are highly demanded. AMOLED display panels were fabricated using solution-coating process; namely, the blanket layers were formed by slot-die coating and the patterned emission layers by nozzle printing [4]. Similarly, large-area OLED lighting panels fabricated by vacuum evaporation are still highly priced, hindering their successful market entry and viability. Of many solution-coating methods (e.g., inkjet printing, nozzle printing, gravure offset printing, spin coating, spray coating, blade coating, and slot-die coating) [5–11], pre-metered slot-die coating is preferred since OLED lighting panels require a multilayer formation of large-area uniform organic films. It provides large scale roll-to-roll production and the simultaneous coating of multiple layers of different solutions [7–9]. It is also capable of coating a n
Corresponding author. E-mail address: [email protected] (J. Park).
http://dx.doi.org/10.1016/j.jpcs.2016.04.006 0022-3697/& 2016 Elsevier Ltd. All rights reserved.
wide range of process materials and depositing organic thin films having the thickness as low as 20 nm. It was demonstrated that homogeneous small molecule (SM) OLED layers could be produced at gap-to-film-thickness ratios of up to 50 [12]. As depicted in Fig. 1, however, it is known that solution coating of SM films entails pinhole-like surface due to the absence of physical entanglement in soluble SMs or/and the crystallization of SMs. Such problems make it difficult to fabricate large-area uniform SM films. It can be suppressed by attaching flexible chains, by adopting an asymmetric molecular structure, or by substituting bulky side groups [13]. Even so, they bring in a negative effect on the device performance (high driving voltage induced by low carrier mobility). Unlike SMs, polymer materials have entangled chain structure and can form stable amorphous film during thermal annealing (Fig. 1). The entanglement of the polymer chains suppresses the crystallization of the polymer chains and the chains are closely packed by thermal annealing without crystallization [13], enabling the formation of large-area uniform polymer films. Compared with SMs, however, polymer materials result in relatively low quantum efficiency of OLEDs. As such, we come up with a hybrid material structure (Fig. 1) where a small amount of chain-entangled polymer is doped to a SM solution in such a way that the crystallization of SMs can be suppressed without compromise on the device performance. In fact, a mixture of a polymer and SM has been used for hybrid host system of an emission layer [14,15]. However, such hybrid host
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Fig. 1. Schematic view of solution coating of hybrid material structure featuring pinhole-free surface morphology.
films were fabricated mainly to enhance the quantum efficiency of spin-coated small-area OLEDs. In this paper, we investigate the feasibility of large-area uniform multilayer coating of SM-based organic materials by the addition of a chain-entangled polymer and the possibility of fabricating large-area OLEDs without sacrificing the device performance. To this end, we have slot-coated a SM hole transport layer (HTL) where a small amount of a polymer HTL is doped. Such a hybrid material system is shown to enhance the film quality. Namely, it suppresses the occurrence of pinholes and ribbing defects to a great extent. We have also addressed that the dissolution problem occurring between two stacked layers is evitable. Furthermore, it is demonstrated that OLEDs with such a hybrid HTL layer show higher device performance (current efficiency, luminous efficacy, device resistance, etc.), compared to OLEDs with a SM HTL or polymer HTL.
2. Experiment Slot-die coating has been done under ambient air with a table slot coater (TSDC-KTEU, DCN) equipped with a heating plate, dry unit, syringe pump system, and slot head module (head size: 240 mm 30 mm 56.5 mm) having the effective coating width of 150 mm. Summarized in Table 1 are typical values of coating process variables. Unless otherwise specified, those process variables are kept unchanged. Of many OLED layers, the HTL layer is formed using the table slot coater. A SM HTL (KHT-001, Duksan Neolux Co., Ltd.) is mixed with a polymer HTL (Poly(N-vinylcarbazole) (PVK), purchased from Sigma Aldrich) at the ratio of 7:3 (i.e., SM is dominant in the hybrid HTL material). In fact, PVK has been widely used as a HTL and host for phosphorescent dyes
Table 1 Fixed values variables.
of
coating
process
Variables
Value
Die shim thickness Coating gap Coating speed Flow rate Plate temperature Ambient temperature
0.05 mm 150 mm 1.8 mm/sec 0.2 ml/min 90 °C 24 °C
[13,16,17]. We have used a co-solvent for the hybrid HTL material to improve the coated film quality. Namely, chlorobenzene (CB) with lower vapor pressure (11.8 mm Hg at 25 °C) and toluene (TE) with higher vapor pressure (26 mm Hg at 25 °C) are mixed at the ratio of 3:7. Before coating, we used PTFE syringe filter (0.5 mm, 25JP050AN) to filter out aggregated particles existing in the solution. For comparison, we have also fabricated the SM HTL films and the polymer HTL films using the same co-solvent. The mass fraction of SM HTL and polymer HTL is 0.015 (1.5% by mass). For a comparative study, those films were also fabricated using a spin coater (ACE-200, DONG AH Trade Corp.). Spin coating has been done at the speed of 3000 rpm for 10 s. Glass substrates used for coating of organic materials were cleaned with isopropyl alcohol (IPA) and treated with UV/O3 for 90 s at a power of 150 W. The coated films by the slot and spin coaters were dried at 80 °C for 30 min in a vacuum oven (ThermoStable OV-30, DAIHAN Scientific Co., Ltd.). The film thickness was measured by scanning electron microscope (SEM, Helios nanolab 600i) and the surface roughness by atomic force microscope (AFM, XE-100).
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To evaluate the film quality at the device level, we have fabricated a green phosphorescent OLED device that consists of a 150nm-thick ITO pre-coated on a glass substrate (purchased from Geomatech Co., Ltd), HTL (SM HTL, polymer HTL, or hybrid HTL), 15-nm-thick 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) for an emission layer (EML), 7-nm-thick 4,7-diphenyl-1,10-phenanthroline (Bphen) for a hole/exciton blocking layer (HBL), 30-nm-thick LG201(LG Chem., Ltd.) for an electron transport layer (ETL), 1-nmthick lithium fluoride (LiF) for an electron injection layer (EIL), and 100-nm-thick aluminum (Al). In the green-emitting layer, 6 wt% fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) is doped. ITO was also treated with UV/O3 for 90 s at a power of 150 W. Except for the HTL, all layers are deposited sequentially at a rate of 0.5 nm/s under a base pressure of 2 10 6 Torr by thermal evaporation. We have fabricated two OLED devices that are different in size. The larger one (emission area of 43 mm 29 mm, substrate size of 50 mm 50 mm) was fabricated to investigate the size-related issues such as the emission uniformity, pinholes, dark spots, shortcircuits, etc. The smaller one (emission area of 2 mm 2 mm) was fabricated in order to measure the IVL characteristics. They were encapsulated by a glass cap in an inert-gas environment (a glove box). The EL property was measured using a source meter (Keithley 2400) and a spectroradiometer (CS-1000, Konica Minolta).
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3. Results and discussion 3.1. Film Quality of spin-coated HTLs It is well known that a spin coating method shows the most excellent surface uniformity due to the horizontal spreading of solutions by centrifugal force. By way of spin coating, we have first fabricated the HTL films on a glass substrate and observed the film quality. The film thickness is measured to be about 8574 nm by a laser confocal microscope (VK-9710, KEYENCE). Presented in Fig. 2 are the measured AFM images of spin-coated SM HTL, hybrid HTL, and polymer HTL films. Of those, the SM HTL film exhibits the roughest surface. As summarized in Table 2, the average and peakto-peak roughness (Ra and Rpv) values are as high as 5.8 nm and 201 nm, respectively, which are too high for large-area OLEDs. It is due to the fact that the absence of physical entanglement in soluble SMs results in pinhole-like surface in the dried film [18]. However, no pinhole appears by the addition of chain-entangled polymers into the SM (Fig. 2(b)). As a result, the Ra (Rpv) value of the hybrid HTL film is measured to be as low as 0.6 nm (11.1 nm), which is slightly higher than that (0.4 nm (8.3 nm)) of the polymer HTL film. Similar behaviors are also observed in the SEM images presented in Fig. 3. There appear a number of holes in the SM HTL film, yet they totally disappear in the hybrid HTL film, a feature highly desirable for large-area OLEDs. As expected, the most excellent film quality is achieved with the polymer HTL.
Fig. 2. Measured AFM images of spin-coated (a) SM HTL film, (b) hybrid HTL film, and (c) polymer HTL film.
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Table 2 Surface roughness values of spin-coated HTL films in Fig. 2. Sample
Ra(nm)
Rpv(nm)
SM HTL Hybrid HTL Polymer HTL
5.8 0.6 0.4
201 11.1 8.3
3.2. Film quality of slot-coated HTLs To fabricate large-area OLED lighting panels using slot-die coating, the film quality should be comparable with that by spin coating. To confirm it, we have slot-coated the SM HTL, polymer HTL, and hybrid HTL films on a glass substrate, measured the film quality, and presented the results in Fig. 4, Fig. 5, and Table 3. As seen in Table 3, the SM HTL film also shows the roughest surface (i.e., Ra of 122.5 nm and Rpv of 1071 nm). By the addition of the chain-entangled polymer HTL into the SM HTL, however, the Ra
(Rpv) value is reduced to 0.6 nm (11.5 nm), which is slightly higher than that (0.5 nm (7.0 nm)) of the polymer HTL film. It is obvious from Table 2 and Table 3 that the surface uniformity (Ra ¼ 0.6 nm, Rpv ¼ 11.5 nm) of slot-coated hybrid HTL film is as good as that (Ra ¼0.6 nm, Rpv ¼11.1 nm) of spin-coated one, indicating that the process variables of slot coating in Table 1 are appropriately set. However, the big difference from the spin-coated films is that there appear a number of small hillocks rather than pinholes in the SM HTL films, as evident in Figs. 4(a) and 5(a) and (b). The diameter and height of those hillocks are measured to be about 7.23 mm and 1.1 mm, respectively. It is attributed that the agglomeration of SMs occurs quickly before drying. Namely, the occurrence of agglomeration is somewhat different from spin coating because heat is applied to the plate during slot coating. Nevertheless, the doping of entangled polymer chains hinders small molecules from migrating and thus agglomerating, thereby providing excellent film quality (Fig. 5(c)). Therefore, such a hybrid material system offers an effective way of improving the operating
Fig. 3. Measured SEM images of spin-coated (a) SM HTL film, (b) hybrid HTL film, and (c) polymer HTL film.
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Fig. 4. Measured AFM images of slot-coated (a) SM HTL film, (b) hybrid HTL film, and (c) polymer HTL film.
window in which stable and uniform slot coating is feasible. The thickness of the polymer and hybrid HTL films is measured to be about 5875 nm. To investigate the emergence of micro-sized hillocks in the SM HTL films further, we have varied the plate temperature from 60 °C to 105 °C. As evident in Fig. 6, smaller hillocks appear more densely as the plate temperature is increased. It is noted that the quality of the hybrid film is getting degraded with decreasing amount of the polymer HTL. When the amount of the polymer HTL is much less than 30%, some stripe patterns (ribbing defects) appear in the dried films. 3.3. Effect of coated HTLs on OLED performance Though the addition of the polymer HTL enables us to fabricate large-area uniform SM HTL films, yet it is uncertain whether or not such a hybrid material system brings in a positive effect on OLED device performance. To investigate it, we have fabricated OLEDs without any hole injection layer (HIL) involved using the slotcoated HTL films on ITO. It is noted that the surface tension between the HTL solution and the glass substrate is different from the surface tension between the solution and ITO. Since UV/O3treated ITO is more hydrophilic, the film quality of HTLs on ITO is good enough to fabricate large- and small-area OLEDs. In reality, however, the HTL layer is formed not directly on ITO but on various HIL or other layers. Even if the surface tension is varied between the HTL solution and a certain HIL or any layer, the addition of the polymer HTL into the SM HTL solution renders slot-die
coating stable and uniform. Fig. 7 shows the images of light emission from large-area OLED devices with the slot-coated HTL films. Dark stripes induced by ribbing defects occurring during slot-die coating are invisible. From large-area OLEDs, we have evaluated the emission uniformity by measuring the luminance at five different spots (four corners and one center) when the luminance is about 1000 cd/m2 (calculated in terms of the maximum luminance). All three devices exhibit the emission uniformity higher than 87%, indicating that the thickness uniformity of those HTL films fabricated by slot-die coating is also excellent (higher than 84%). From small-area OLEDs with slot-coated HTL films, we have measured the current–voltage (J–V) characteristics, current efficiency versus current density, and luminous efficacy as a function of luminance. For comparison, we have also fabricated OLEDs with vacuum-evaporated HTL film. As evident in Fig. 8, OLED with the polymer HTL shows the highest current efficiency (42.8 cd/A), yet the lowest luminous efficacy (14.6 lm/W) due to very high driving voltage (9.1 V at 1000 cd/m2). The highest occupied molecular orbital (HOMO) level (5.8 eV) of PVK is slightly lower than that (5.9 eV) of the SM HTL. Therefore, such a high driving voltage arises mainly from the low hole mobility (2.5 10 6 cm2V 1s 1) of PVK [17]. From this, it is obvious that the hole mobility of the polymer HTL is much lower than that of the SM HTL. Therefore, it is doubtful whether the addition of the polymer HTL into the SM HTL solution would degrade the J–V characteristics. As apparent in Fig. 8(a), however, there was no increase in the driving voltage of
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Fig. 5. Measured SEM images of slot-coated (a) SM HTL film, (b) magnified hillocks on SM HTL film, (c) hybrid HTL film, and (d) polymer HTL film.
Table 3 Surface roughness values of slot-coated HTL films in Fig. 4. Sample
Ra(nm)
Rpv(nm)
SM HTL Hybrid HTL Polymer HTL
122.5 0.6 0.5
1070.9 11.5 7.0
OLED with the hybrid HTL film. As a result, the current and luminous efficacies (37 cd/A and 28.7 lm/W) of OLED with the coated hybrid HTL is measured to be even slightly higher than those (34.1 cd/A and 25.3 lm/W) of OLED with the coated SM HTL at the luminance of 1000 cd/m2. This result indicates that the addition of chain-entangled polymers can improve the coating window of SM films without any sacrifice in the device performance. It is also observed that OLED with the evaporated HTL shows the lowest driving voltage, but almost the same luminous efficacy as OLED with the coated hybrid HTL at high luminances because of relatively low current efficiency induced by the
imbalance of charges in the emission layer. We have also evaluated the device stability by measuring the device resistance. The films with poor surface morphology reduce the device resistance and thus cause a short-circuit phenomenon. Those OLED devices with the coated hybrid HTL film exhibit the device resistance of higher than 10 MΩ, representing that high device stability can be achieved by slot-die coating. 3.4. Effect of hybrid material structure on interface dissolution Using orthogonal solvents, the dissolution problems occurring between two stacked layers may be avoidable. In reality, however, it is known that a coated and dried SM film would be dissolved even by the orthogonal solvent of the second layer because SMs typically attach to each other only by weak intermolecular forces. Such a dissolution phenomenon may be suppressed by enhancing the solvent resistance of SMs. It is known that the solvent resistance can be increased with molecular weight [19]. Therefore, the hybrid HTL is such a case in point because the addition of PVK
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Fig. 6. Measured SEM images of slot-coated SM HTL films at the plate temperature of (a) 60 °C, (b) 75 °C, (c) 90 °C, and (d) 105 °C.
into the SM HTL increases the molecular weight and thus the solvent resistance. To demonstrate it, we have employed aqueous poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS, Clevios AI 4083) because its solvent (water) is chemically orthogonal to the solvent (CB) for the hybrid HTL. We have spin- and slot-coated PEDOT:PSS (2nd layer) atop the HTLs (1st layer). Normally, PEDOT:PSS lies in the first layer as a HIL. For this experiment, however, we have deliberately coated it on the HTL in order to investigate whether the addition of PVK into the SM HTL can suppress the dissolution phenomenon occurring even by the orthogonal solvent of the second layer. Before coating, we used a cellulose acetate disposable syringe filter (0.2 mm, DISMIC-25CS) to filter out aggregated particles existing in the PEDOT:PSS solution. Spin coating of HTL and PEDOT:PSS has been done at the speed of 3500 rpm and 1500 rpm, respectively, for 20 s. The process variables of slot coating for both HTL and PEDOT:PSS are as follows; the die shim thickness of 0.03 mm, coating gap 200 mm, flow rate 0.1 ml/min, coating speed 6 mm/ sec, and plate temperature 60 °C.
Presented in Fig. 9 are the SEM images of HTL/PEDOT bilayers fabricated using spin and slot coating. The spin-coated SM HTL/ PEDOT bilayer film exhibits the wavy interface due to dissolution. Therefore, it is evident that dissolution occurs even by the orthogonal solvent due to weak intermolecular forces between SMs. However, a very clear interface (boundary line) can be observed in the spin-coated hybrid HTL/PEDOT bilayer film, implying that the addition of PVK into the SM HTL indeed suppresses the dissolution phenomenon. Such a dissolution phenomenon yields a noticeable difference in the layer thickness. As evident in Fig. 9(a) and (b), the total thickness (130.8 nm) of the SM HTL/PEDOT bilayer is almost the same as that (132.3 nm) of the hybrid one. However, the PEDOT:PSS layer becomes thicker and, in turn, the SM HTL becomes thinner. It appears that the dissolution phenomenon is moderated to some extent in the slot-coated SM HTL/PEDOT bilayer film because such a wavy interface is not observed. Even so, its interface is less distinct compared to the slot-coated hybrid HTL/PEDOT one (Fig. 9(c) and (d)). Therefore, such a combination of materials in hybrid structure provides the effective way of
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Fig. 7. Image of green light emission from large-area (43 mm 29 mm) OLED devices with (a) SM HTL layer, (b) hybrid HTL layer, and (c) polymer HTL layer at 6 V.
multi-layer slot coating of large-area uniform SM films for OLED lightings.
4. Conclusion Using slot-die coating, we have fabricated the large-area uniform SM HTL films by the addition of a small amount of the polymer HTL. By suppressing the occurrence of pinhole-like surface and micro-sized hillocks appeared in SM HTL films, we have fabricated the HTL films with the peak-to-peak roughness value as
Fig. 8. Measured (a) J–V curve, (b) current efficiency versus current density, and (c) luminous efficacy versus luminance of OLEDs with different HTLs.
low as 11.5 nm. Using the hybrid HTL film, we have fabricated large-area OLEDs showing the emission uniformity as high as 87%. It was demonstrated that the addition of entangled polymer chains does not bring in any degradation in the device
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Fig. 9. SEM image of HTL/PEDOT:PSS bilayer; (a) spin-coated SM HTL, (b) spin-coated hybrid HTL, (c) slot-coated SM HTL, and (d) slot-coated hybrid HTL.
performance. OLED with the hybrid HTL showed the luminous efficacy of 28.7 lm/W at 1000 cd/m2, which is higher than those (25.3 lm/W, 14.6 lm/W) of OLEDs with the SM HTL or the polymer HTL. In addition to the improved operating window of slot-die coating, such a hybrid material system is demonstrated to suppress the dissolution problem occurring between two stacked layers even with orthogonal solvents.
Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) (NRF2015R1D1A1A01057266) funded by the Ministry of Education.
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