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Optical and barrier properties of thin-film encapsulations for transparent OLEDs
Organic Electronics 13 (2012) 1956–1961
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Optical and barrier properties of thin-film encapsulations for transparent OLEDs Jongwoon Park a,⇑, Yong-Young Noh b, Jin Woo Huh c, JeongIk Lee c, Hyeyong Chu c a
School of Electrical, Electronics & Communication Engineering, Korea University of Technology and Education, Cheonan 330-708, Republic of Korea Department of Chemical Engineering, Hanbat National University, Daejeon 305-71, Republic of Korea c OLED Lighting Team, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 April 2012 Received in revised form 7 June 2012 Accepted 7 June 2012 Available online 27 June 2012 Keywords: Organic light-emitting diodes (OLEDs) Thin-film encapsulation (TFE) Transmittance dip Barrier property
a b s t r a c t We investigate the optical and barrier properties of thin-film encapsulations (TFEs) for transparent organic light-emitting diodes (TOLEDs). To improve the barrier property of OLEDs, the number of dyads (Al2O3/polymer) and the thickness of polymer layer in the TFE structure are required to be increased. It is, however, demonstrated that a sharp dip appears in the transmittance of TFE films due to the interference of light caused by organic/inorganic multi-layered configuration, resulting in a dip in the top emission spectrum of TOLEDs. We have found that such a transmittance dip deepens when the number of dyads is large. What is worse, the number of transmittance dips and their sharpness are raised with increased thickness of the polymer layer. When the number of dyads is small, however, the effect of the polymer layer thickness on such a transmittance dip is weak. Therefore, we have addressed that the number of dyads needs to be reduced, but the thickness of the polymer layer should be increased to meet both optical and barrier properties of TOLEDs at the same time. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted much attention for their potential applications in flat panel lightings and backlight units of flat panel displays [1–3]. They exhibit salient features such as surface emission, high efficiency, low cost, flexibility and transparency, etc. Continuous technological progress in OLED industry enables us to overcome major technology setbacks such as a relatively low power efficiency and short lifetime. Some OLEDs show the lifetime more than 200 khrs [4]. Moreover, the power efficiency of OLEDs developed by Novaled has reached 124 lm/W with a 3D light extraction system [5], which is high enough to replace a bulb and even fluorescent lamp. Recently, OSRAM Opto Semiconductors has developed transparent OLED (TOLED) lighting panels, exhibiting the transparency of 55% and power efficiency ⇑ Corresponding author. Tel.: +82 41 560 1425; fax: +82 41 564 3261. E-mail address: [email protected] (J. Park). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.010
of 20 lm/W at a brightness of 1000 cd/m2 [6]. However, there still remain some technical issues in the development of OLED devices. One of the most critical issues is to protect them from moisture and oxygen. In the literature, various encapsulation schemes have been introduced. The most commonly employed method utilizes an ultraviolet (UV)-cured epoxy resin together with a desiccant inside the glass-capped OLEDs [7]. However, this scheme is not applicable to ultra-thin and flexible OLEDs. In recent years, many studies have been done on a thin-film encapsulation (TFE) [8–12]. It is required to have a water vapor transmission rate (WVTR) of below 106g m2 day1 to achieve the lifetime of OLEDs above 10,000 h. Of many TFE methods, TFE based on vacuum based organic/inorganic multi-layers is very promising [8,13–15]. The TFE structure also showed the best thermal performance due to its short heat transfer pathway [16]. In this configuration, the inorganic (Al2O3) layers form the basis of the moisture and oxygen barrier, the typical thickness of which is of the order of 50 nm. Alternating organic
J. Park et al. / Organic Electronics 13 (2012) 1956–1961
(polymer) layers provide a smooth surface to facilitate defect free alumina deposition. In this case, thicker polymer layers (>500 nm) are favored to lengthen the penetration pathway of oxygen and moisture. To obtain the satisfactory barrier property, it is known that the TFE structure with more than 3.5 dyads (Al2O3/polymer) is required. Due to the organic/inorganic multi-layered configuration with different refractive index, however, the TFE structure inevitably brings in the interference of light. Consequently, the OLED light emitted through the TFE layer may be different from that through the conventional glass encapsulation. A TOLED panel emits light through the glass substrate (i.e., bottom emission) and also through the glass cap (i.e., top emission). In the presence of TFE films as an encapsulation for TOLEDs, a dip or peak appears in their top emission spectrum because there arises a dip or peak in the transmittance of TFE films. In this paper, we investigate the effect of the TFE structure (e.g., the number of dyads and the thickness of polymer layer) on the optical property of TOLEDs. We have found that a sharp dip appears in the transmittance of TFE films, causing a dip in the top emission spectrum of TOLEDs. It is also observed that such a transmittance dip deepens as the number of dyads is increased. What is worse, the number of transmittance dips appearing within the visible wavelength range and their sharpness are raised with increased thickness of the polymer layer. To reduce the effect of such transmittance dips on the optical property of TOLEDs, therefore, one has to reduce the number of dyads and the thickness of the polymer layer. In this case, however, the gas barrier property of the TFE films would be degraded. Namely, there exists a tradeoff between the optical and barrier properties of TOLEDs on the optimization of the TFE structure. It is highly desirable to optimize the TFE structure in such a way that both optical and barrier properties are satisfied at the same time. In this paper, we demonstrate that the optical and barrier properties of TFE films can be improved by reducing the number of dyads, yet increasing the thickness of the polymer layer.
2. Experiment For experiments, we fabricated a hybrid white OLED device (Fig. 1(a)) that consists of a 150-nm-thick indium tin oxide (ITO) pre-coated on a glass substrate, 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), 10-nm-thick BH036 for a fluorescent blue-emitting layer (Dow Advanced Display Materials Ltd.), 20-nm-thick PGH-02 for a phosphorescent green- and redemitting layer, 10-nm-thick TMM-004 for a hole/exciton blocking layer (HBL) (Merck), 30-nm-thick LG-201 for an electron transport layer (ETL) doped with 50 wt.% lithium quinolate (LiQ), and 150-nm-thick Al. In the blue-emitting layer, 3 wt.% GDI4669 is doped (Dow Advanced Display Materials Ltd.). In the green- and red-emitting layer, 8 wt.% Ir(mpp)3 and 0.3 wt.% Ir(mphmq)2acac [17] are codoped. We have then formed alternating Al2O3/monomer layers using Barix™ thin-film encapsulation equipment.
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Alumina layers are deposited by reactive sputtering of an alumina target. Polymer layers are deposited by organic acrylate evaporation and condensation, followed by a UV cure. For TOLEDs, we have fabricated the semitransparent cathode that is composed of 1.5-nm-thick Al, 12-nm-thick Ag, and 50-nm-thick NPB. NPB is evaporated to suppress an oxidization of Ag. The sheet resistance of Al/Ag cathode was measured to be as low as 4.5 X/h. Presented in Fig. 1(b) is the measured luminance of the small-area (2 2 mm2) hybrid white TOLED device. The luminous intensity of bottom emission was measured to be 1000 cd/m2 at the bias voltage of 5.7 V, whereas the luminous intensity of top emission was 380 cd/m2 at the same bias voltage. The luminance of top emission was observed to be lower than that of bottom emission because the transparency of the Al/Ag cathode (60% at 550 nm) is much lower than that (90%) of ITO. We also measured the luminous efficiency of the device and presented the result in Fig. 1(c). The current efficiency of bottom emission was measured to be 8.8 cd/A at 10 mA/cm2, whereas the current efficiency of top emission was 2.7 cd/A.
3. Results and discussion For a comparison, we have also fabricated TOLEDs based on the conventional glass encapsulation. Fig. 2 shows the electroluminescence (EL) spectra measured from top emission of the glass-capped and thin-filmencapsulated TOLEDs at the bias voltage of 5.7 V. Compared with the glass-capped TOLED, the 3.5 dyad TFETOLED exhibits a dip in the emission spectrum around 535 nm. Upon emergence of such a dip, the color rendering index (CRI) of white OLEDs would be degraded because the bandwidth of green emission is narrowed. Namely, bottom and top emissions of TOLEDs would show different color property. In reality, however, it is demanded that the OLED light from top emission is similar to that from bottom emission for many applications including lightings. Therefore, a thorough investigation of the effect of TFE films on the optical property of TOLEDs deserves to be made. To analyze the cause for such a dip in the emission spectrum, we have investigated the optical property of TFE films by varying their structure (e.g., the number of dyads and the thickness of the polymer layer). Presented in Fig. 3 are the scanning electron microscope (SEM) images of a cross-section of 3.5 dyad TFE films with the polymer layer thickness of (a) 464 nm, (b) 826 nm, and (c) 1.55 lm when the alumina thickness is 50 nm. We intended to fabricate TFE films with the polymer layer thickness of 500 nm, 1000 nm, and 1500 nm, respectively. However, the measured average thickness of the polymer layers was a little smaller and the thickness of each polymer layer is slightly different. In practice, the condition of monomer dispensing and UV-curing may be varied slightly during the deposition of each monomer layer. As such, much care is taken to ensure the thickness uniformity of the polymer layer. We have measured the transmittance of TFE films using a UV-spectrometer (T70+ UV/VIS Spectrometer, PG Instruments Ltd.) and presented the results in Fig. 4. One can see that there arise sharp dips in the transmittance
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J. Park et al. / Organic Electronics 13 (2012) 1956–1961
Fig. 2. Comparison of EL spectra measured from top emission of glasscapped TOLED and TFE-TOLED at the bias voltage of 5.7 V.
Fig. 1. (a) Layer structure, (b) measured luminance versus bias voltage, and (c) current efficiency of the 2 mm 2 mm transparent white OLED device based on the thin-film encapsulation.
of TFE films. Those sharp dips are positioned at the wavelengths of 407 nm and 535 nm for 3.5 dyad TFE films, whereas they appear at 411 nm and at 547 nm for 5.5 dyad TFE films. Therefore, it is obvious that a dip appeared at 535 nm in the top emission spectrum of 3.5 dyad
TFE-TOLED in Fig. 2 originated from a transmittance dip (at 535 nm) of 3.5 dyad TFE film in Fig. 4. It is also observed that those dips deepen and sharpen as the number of dyads is increased, which would distort the emission spectrum further. Meanwhile, those transmittance dips of 3.5 and 5.5 dyad TFE films must appear at the same wavelengths. However, they are positioned at slightly different wavelengths (i.e., roughly 10 nm shift occurs). It is attributed that the thickness of each polymer layer deposited is slightly different, as shown in Fig. 3. In other words, they would appear at the same wavelengths, provided that the thickness uniformity of the polymer layers is high. For more systematic analysis of the optical property of TFE films, we have performed simulations using an OLED optical simulator (SimOLED [18–20]). Briefly, the program has inputs of refractive index and thickness of every layer and employs thin film optics. In order to obtain realistic simulation results, we have used all measured optical constants (n, k) of organic materials, which were obtained using an ellipsometer (UVISEL ER Benchtop AGAS, Horiba Korea Ltd.). Presented in Fig. 5 is the measured refractive index of alumina and polymer layers. At the wavelength of 520 nm, the refractive index of alumina and polymer layers is measured to be 1.56 and 1.51, respectively. Shown in Fig. 6 are the simulation results of the transmittance of TFE films for different TFE structure (i.e., different number of dyads). As observed in experiment results (Fig. 4), the transmittance dips also appear in simulation results at similar wavelengths (419 and 551 nm). Furthermore, the simulation results clearly show that such transmittance dips deepen as the number of dyads increases. From the experiment results in Fig. 4 and the simulation results in Fig. 6, therefore, we can draw a conclusion that one needs to reduce the number of dyads to suppress the effect of such dips on the optical property of TOLEDs. We have further investigated the effect of the polymer layer thickness on the transmittance of TFE films. We have increased the polymer layer thickness of 3.5 dyad TFE films from 100 to 500 nm and performed simulations first. From
J. Park et al. / Organic Electronics 13 (2012) 1956–1961
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Fig. 4. Measured optical transmittance of TFE films for different numbers of dyads (50-nm-thick Al2O3, 500-nm-thick polymer) with the air reference.
Fig. 5. Measured refractive index of Al2O3 and polymer thin films.
Fig. 3. Scanning electron microscope (SEM) images of a cross-section of 3.5 dyad TFE films with the polymer layer thickness of (a) 464 nm, (b) 826 nm, and (c) 1.55 lm.
Fig. 7, it is observed that the transmittance dip sharpens and the number of dips increases as the polymer layer thickness is increased. Similar result is also obtained experimentally in Fig. 8. For experiments, we have increased the polymer layer thickness of 3.5 dyad TFE films
from 500 to 1500 nm. It is found that the transmittance dips are getting spikier and appear more within the same wavelength range. From these results, we can conclude that the polymer layer thickness needs to be reduced to suppress the effect of those dips on the top emission spectrum of TOLEDs. Other than the effect of increasing the number of dyads, however, it is noted that increasing the polymer layer thickness does not deepen noticeably those transmittance dips (Fig. 7). From Figs. 4 and 8, it is found that the number of dyads and the thickness of the polymer layer are desired to be reduced. However, this would degrade the gas barrier property of OLEDs. To satisfy both optical and barrier properties, we can consider two different TFE structures. One structure can have the larger number of dyads but smaller thickness of the polymer layer. The other structure can have the smaller number of dyads but thicker polymer layer. Within the framework of mass production, the latter
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J. Park et al. / Organic Electronics 13 (2012) 1956–1961
Fig. 6. Simulation results of transmittance of TFE films for different TFE structures.
Fig. 8. Measured transmittance from 3.5 dyad TFE films for different polymer layer thicknesses.
Fig. 7. Simulation results of transmittance of 3.5 dyad TFE films for different polymer layer thicknesses.
Fig. 9. Measured transmittance from 1.5 dyad TFE films for different polymer layer thicknesses.
configuration is more preferable. From the viewpoint of the optical and gas barrier properties, however, it is not clear which structure is more favored. To investigate it, we have decreased the number of dyads (1.5 dyads) and measured their transmittance for different polymer layer thicknesses. As seen in Fig. 9, more transmittance dips appear as the polymer layer thickness is increased. However, the depth of those dips is kept unchanged. It is rather more sensitive to the number of dyads. As discussed above (Fig. 7), the polymer layer thickness does not deepen noticeably those transmittance dips. When the number of dyads is small, therefore, the effect of increasing the polymer layer thickness on the depth of the transmittance dips is negligible. In this respect, the latter structure with the smaller number of dyads and thicker polymer layer is preferred. To inquire into which structure exhibits superior gas barrier property, we have fabricated two different
thin-film encapsulated OLEDs (TFE-OLEDs) and compared their barrier property. The 3.5 dyad TFE-OLED has the polymer layer thickness of 500 nm. Therefore, the total thickness of TFE film is 1.7 lm. Meanwhile, the 1.5 dyad TFEOLED has the polymer layer thickness of 2 lm. Therefore, the total thickness of TFE film is 2.1 lm. Fig. 10 shows the initial drop of their luminous intensity as a function of time measured at 85 °C and 85% relative humidity. The initial luminance of the devices was 1000 cd/m2. It is observed that both devices exhibit a very short lifetime of about 100 h at T80 (the duration from the start of testing to the time at which luminance is decreased to 80% of its initial value). One may expect much longer lifetime of TFE-OLEDs. It can be achieved by increasing the number of dyads (typically, 5.5 dyad TFE film is required to ensure no significant variation in the L–I–V characteristics of OLEDs for 1000 h at high temperature with high humidity condition). As mentioned above, however, it entails a
J. Park et al. / Organic Electronics 13 (2012) 1956–1961
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of those transmittance dips without sacrificing the barrier property of TFE films by decreasing the number of dyads but increasing the polymer layer thickness. If the difference of refractive index between the inorganic and organic (polymer) layers in TFE films is small, the optical interference effect of TFE films can be suppressed. Therefore, the development of organic (polymer) materials with the same refractive index as the inorganic layer might be the way to achieve TFE films without the negative optical effect on TOLED, which is under investigation. Acknowledgements This work was supported by the IT R&D program of MKE/IITA [2009-F-016-01, Development of Eco-Emotional OLED Flat-Panel Lighting]. Fig. 10. Luminance of TFE-OLEDs with different TFE structures as a function of time measured under conditions of 85 °C and 85% relative humidity.
negative effect on the optical property of TFE-TOLEDs. Even if they showed a relatively short lifetime, it was observed that there was no big difference in the lifetime of those devices. Namely, the barrier property of the 1.5 dyad TFEOLED with thicker polymer layer is comparable with that of the 3.5 dyad TFE-OLED. From the results in Figs. 9 and 10, therefore, it can be concluded that we can suppress the effect of those transmittance dips without sacrificing the device lifetime by decreasing the number of dyads, yet increasing the polymer layer thickness. We believe that surface smoothening was not satisfactory for the TFE film with the thin polymer layer, which resulted in poor lifetime characteristics. 4. Conclusion By way of experiments and simulations, we have investigated the effects of thin-film encapsulations on the optical and barrier properties of hybrid white TOLEDs and provided a guideline that may be employed in the design of TFE structures. We have found that the number of dyads and the thickness of the polymer layer need be reduced to suppress a sharp dip appearing in the transmittance of TFE films, which causes a dip in the top emission spectrum of TOLEDs. It is demonstrated that we can suppress the effect
References [1] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 4759 (2009) 234. [2] K. Leo, Science 310 (2005) 1762. [3] Y. Sun, N. Giebink, H. Kanno, B. Wa, M.E. Thompson, S.R. Forrest, Nature (London) 440 (2006) 908. [4] B. D’Andrade, J. Esler, C. Lin, M. weaver, J. Brown, Digest 940 (2008). [5] [6] Osram transparent OLED offers high efficacy, LEDs magazine (Dec. 2007) . [7] P.E. Burrows, V. Bulovic, S.R. Forrest, L.S. Sapochack, D.M. McCarty, M.E. Thompson, Appl. Phys. Lett. 65 (1994) 2922. [8] J.D. Affinito, M.E. Gross, C.A. Coronado, G.L. Graff, E.N. Greenwell, P.M. Martin, Thin Solid Films 290–291 (1996) 63. [9] A.P. Ghosh, L.J. Gerenser, C.M. Jarman, J.E. Fornalik, Appl. Phys. Lett. 86 (2005) 223501–223503. [10] J. Meyer, D. Schneidenbach, T. Winkler, S. Hamwi, T. Weimann, P. Hinze, S. Ammermann, H.-H. Johannes, T. Riedl, W. Kowalsky, Appl. Phys. Lett. 94 (2009) 233301–233305. [11] A.B. Chwang, M.A. Rothman, S.Y. Mao, R.H. Hewitt, M.S. Weaver, J.A. Silvernail, K. Rajan, M. Hack, J.J. Brown, X. Chu, L. Moro, T. Krajewski, N. Rutherford, Appl. Phys. Lett. 83 (2003) 413. [12] F.L. Wong, M.K. Fung, S.L. Tao, S.L. Lai, W.M. Tsang, K.H. Kong, W.M. choy, C.S. Lee, S.T. Lee, J. Appl. Phys. 104 (2008) 014501–14509. [13] G.L. Graff, R.E. Williford, P.E. Burrows, J. Appl. Phys. 96 (2004) 1840. [14] C. Charton, N. Schiller, M. Fahland, A. Hollander, A. Wedel, K. Noller, Thin Solid Films 502 (2006) 99. [15] L.L. Moro, T.A. Krajewski, N.M. Rutherford, O. Philips, R.J. Visser, M.E. Gross, W.D. Bennett, G.L. Graff, Proc. SPIE 5214 (2004) 83. [16] J.W. Park, H.K. Ham, C.Y. Park, Org. Electron. 12 (2011) 227–233. [17] D.H. Kim, N.S. Cho, H.-Y. Oh, J.H. Yang, W.S. Jeon, J.S. Park, M.C. Suh, J.H. Kwon, Adv. Mater. 23 (2011) 2721. [18] Sim4tec GmbH, . [19] J. Staudigel, M. Stoessel, F. Steuber, J. Simmere, J. Appl. Phys. 86 (1999) 3895–3910. [20] R. Meerheim, R. Nitsche, K. Leo, Appl. Phys. Lett. 93 (2008) 043310.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Optical and barrier properties of thin-film encapsulations for transparent OLEDs Jongwoon Park a,⇑, Yong-Young Noh b, Jin Woo Huh c, JeongIk Lee c, Hyeyong Chu c a
School of Electrical, Electronics & Communication Engineering, Korea University of Technology and Education, Cheonan 330-708, Republic of Korea Department of Chemical Engineering, Hanbat National University, Daejeon 305-71, Republic of Korea c OLED Lighting Team, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 April 2012 Received in revised form 7 June 2012 Accepted 7 June 2012 Available online 27 June 2012 Keywords: Organic light-emitting diodes (OLEDs) Thin-film encapsulation (TFE) Transmittance dip Barrier property
a b s t r a c t We investigate the optical and barrier properties of thin-film encapsulations (TFEs) for transparent organic light-emitting diodes (TOLEDs). To improve the barrier property of OLEDs, the number of dyads (Al2O3/polymer) and the thickness of polymer layer in the TFE structure are required to be increased. It is, however, demonstrated that a sharp dip appears in the transmittance of TFE films due to the interference of light caused by organic/inorganic multi-layered configuration, resulting in a dip in the top emission spectrum of TOLEDs. We have found that such a transmittance dip deepens when the number of dyads is large. What is worse, the number of transmittance dips and their sharpness are raised with increased thickness of the polymer layer. When the number of dyads is small, however, the effect of the polymer layer thickness on such a transmittance dip is weak. Therefore, we have addressed that the number of dyads needs to be reduced, but the thickness of the polymer layer should be increased to meet both optical and barrier properties of TOLEDs at the same time. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted much attention for their potential applications in flat panel lightings and backlight units of flat panel displays [1–3]. They exhibit salient features such as surface emission, high efficiency, low cost, flexibility and transparency, etc. Continuous technological progress in OLED industry enables us to overcome major technology setbacks such as a relatively low power efficiency and short lifetime. Some OLEDs show the lifetime more than 200 khrs [4]. Moreover, the power efficiency of OLEDs developed by Novaled has reached 124 lm/W with a 3D light extraction system [5], which is high enough to replace a bulb and even fluorescent lamp. Recently, OSRAM Opto Semiconductors has developed transparent OLED (TOLED) lighting panels, exhibiting the transparency of 55% and power efficiency ⇑ Corresponding author. Tel.: +82 41 560 1425; fax: +82 41 564 3261. E-mail address: [email protected] (J. Park). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.010
of 20 lm/W at a brightness of 1000 cd/m2 [6]. However, there still remain some technical issues in the development of OLED devices. One of the most critical issues is to protect them from moisture and oxygen. In the literature, various encapsulation schemes have been introduced. The most commonly employed method utilizes an ultraviolet (UV)-cured epoxy resin together with a desiccant inside the glass-capped OLEDs [7]. However, this scheme is not applicable to ultra-thin and flexible OLEDs. In recent years, many studies have been done on a thin-film encapsulation (TFE) [8–12]. It is required to have a water vapor transmission rate (WVTR) of below 106g m2 day1 to achieve the lifetime of OLEDs above 10,000 h. Of many TFE methods, TFE based on vacuum based organic/inorganic multi-layers is very promising [8,13–15]. The TFE structure also showed the best thermal performance due to its short heat transfer pathway [16]. In this configuration, the inorganic (Al2O3) layers form the basis of the moisture and oxygen barrier, the typical thickness of which is of the order of 50 nm. Alternating organic
J. Park et al. / Organic Electronics 13 (2012) 1956–1961
(polymer) layers provide a smooth surface to facilitate defect free alumina deposition. In this case, thicker polymer layers (>500 nm) are favored to lengthen the penetration pathway of oxygen and moisture. To obtain the satisfactory barrier property, it is known that the TFE structure with more than 3.5 dyads (Al2O3/polymer) is required. Due to the organic/inorganic multi-layered configuration with different refractive index, however, the TFE structure inevitably brings in the interference of light. Consequently, the OLED light emitted through the TFE layer may be different from that through the conventional glass encapsulation. A TOLED panel emits light through the glass substrate (i.e., bottom emission) and also through the glass cap (i.e., top emission). In the presence of TFE films as an encapsulation for TOLEDs, a dip or peak appears in their top emission spectrum because there arises a dip or peak in the transmittance of TFE films. In this paper, we investigate the effect of the TFE structure (e.g., the number of dyads and the thickness of polymer layer) on the optical property of TOLEDs. We have found that a sharp dip appears in the transmittance of TFE films, causing a dip in the top emission spectrum of TOLEDs. It is also observed that such a transmittance dip deepens as the number of dyads is increased. What is worse, the number of transmittance dips appearing within the visible wavelength range and their sharpness are raised with increased thickness of the polymer layer. To reduce the effect of such transmittance dips on the optical property of TOLEDs, therefore, one has to reduce the number of dyads and the thickness of the polymer layer. In this case, however, the gas barrier property of the TFE films would be degraded. Namely, there exists a tradeoff between the optical and barrier properties of TOLEDs on the optimization of the TFE structure. It is highly desirable to optimize the TFE structure in such a way that both optical and barrier properties are satisfied at the same time. In this paper, we demonstrate that the optical and barrier properties of TFE films can be improved by reducing the number of dyads, yet increasing the thickness of the polymer layer.
2. Experiment For experiments, we fabricated a hybrid white OLED device (Fig. 1(a)) that consists of a 150-nm-thick indium tin oxide (ITO) pre-coated on a glass substrate, 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), 10-nm-thick BH036 for a fluorescent blue-emitting layer (Dow Advanced Display Materials Ltd.), 20-nm-thick PGH-02 for a phosphorescent green- and redemitting layer, 10-nm-thick TMM-004 for a hole/exciton blocking layer (HBL) (Merck), 30-nm-thick LG-201 for an electron transport layer (ETL) doped with 50 wt.% lithium quinolate (LiQ), and 150-nm-thick Al. In the blue-emitting layer, 3 wt.% GDI4669 is doped (Dow Advanced Display Materials Ltd.). In the green- and red-emitting layer, 8 wt.% Ir(mpp)3 and 0.3 wt.% Ir(mphmq)2acac [17] are codoped. We have then formed alternating Al2O3/monomer layers using Barix™ thin-film encapsulation equipment.
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Alumina layers are deposited by reactive sputtering of an alumina target. Polymer layers are deposited by organic acrylate evaporation and condensation, followed by a UV cure. For TOLEDs, we have fabricated the semitransparent cathode that is composed of 1.5-nm-thick Al, 12-nm-thick Ag, and 50-nm-thick NPB. NPB is evaporated to suppress an oxidization of Ag. The sheet resistance of Al/Ag cathode was measured to be as low as 4.5 X/h. Presented in Fig. 1(b) is the measured luminance of the small-area (2 2 mm2) hybrid white TOLED device. The luminous intensity of bottom emission was measured to be 1000 cd/m2 at the bias voltage of 5.7 V, whereas the luminous intensity of top emission was 380 cd/m2 at the same bias voltage. The luminance of top emission was observed to be lower than that of bottom emission because the transparency of the Al/Ag cathode (60% at 550 nm) is much lower than that (90%) of ITO. We also measured the luminous efficiency of the device and presented the result in Fig. 1(c). The current efficiency of bottom emission was measured to be 8.8 cd/A at 10 mA/cm2, whereas the current efficiency of top emission was 2.7 cd/A.
3. Results and discussion For a comparison, we have also fabricated TOLEDs based on the conventional glass encapsulation. Fig. 2 shows the electroluminescence (EL) spectra measured from top emission of the glass-capped and thin-filmencapsulated TOLEDs at the bias voltage of 5.7 V. Compared with the glass-capped TOLED, the 3.5 dyad TFETOLED exhibits a dip in the emission spectrum around 535 nm. Upon emergence of such a dip, the color rendering index (CRI) of white OLEDs would be degraded because the bandwidth of green emission is narrowed. Namely, bottom and top emissions of TOLEDs would show different color property. In reality, however, it is demanded that the OLED light from top emission is similar to that from bottom emission for many applications including lightings. Therefore, a thorough investigation of the effect of TFE films on the optical property of TOLEDs deserves to be made. To analyze the cause for such a dip in the emission spectrum, we have investigated the optical property of TFE films by varying their structure (e.g., the number of dyads and the thickness of the polymer layer). Presented in Fig. 3 are the scanning electron microscope (SEM) images of a cross-section of 3.5 dyad TFE films with the polymer layer thickness of (a) 464 nm, (b) 826 nm, and (c) 1.55 lm when the alumina thickness is 50 nm. We intended to fabricate TFE films with the polymer layer thickness of 500 nm, 1000 nm, and 1500 nm, respectively. However, the measured average thickness of the polymer layers was a little smaller and the thickness of each polymer layer is slightly different. In practice, the condition of monomer dispensing and UV-curing may be varied slightly during the deposition of each monomer layer. As such, much care is taken to ensure the thickness uniformity of the polymer layer. We have measured the transmittance of TFE films using a UV-spectrometer (T70+ UV/VIS Spectrometer, PG Instruments Ltd.) and presented the results in Fig. 4. One can see that there arise sharp dips in the transmittance
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Fig. 2. Comparison of EL spectra measured from top emission of glasscapped TOLED and TFE-TOLED at the bias voltage of 5.7 V.
Fig. 1. (a) Layer structure, (b) measured luminance versus bias voltage, and (c) current efficiency of the 2 mm 2 mm transparent white OLED device based on the thin-film encapsulation.
of TFE films. Those sharp dips are positioned at the wavelengths of 407 nm and 535 nm for 3.5 dyad TFE films, whereas they appear at 411 nm and at 547 nm for 5.5 dyad TFE films. Therefore, it is obvious that a dip appeared at 535 nm in the top emission spectrum of 3.5 dyad
TFE-TOLED in Fig. 2 originated from a transmittance dip (at 535 nm) of 3.5 dyad TFE film in Fig. 4. It is also observed that those dips deepen and sharpen as the number of dyads is increased, which would distort the emission spectrum further. Meanwhile, those transmittance dips of 3.5 and 5.5 dyad TFE films must appear at the same wavelengths. However, they are positioned at slightly different wavelengths (i.e., roughly 10 nm shift occurs). It is attributed that the thickness of each polymer layer deposited is slightly different, as shown in Fig. 3. In other words, they would appear at the same wavelengths, provided that the thickness uniformity of the polymer layers is high. For more systematic analysis of the optical property of TFE films, we have performed simulations using an OLED optical simulator (SimOLED [18–20]). Briefly, the program has inputs of refractive index and thickness of every layer and employs thin film optics. In order to obtain realistic simulation results, we have used all measured optical constants (n, k) of organic materials, which were obtained using an ellipsometer (UVISEL ER Benchtop AGAS, Horiba Korea Ltd.). Presented in Fig. 5 is the measured refractive index of alumina and polymer layers. At the wavelength of 520 nm, the refractive index of alumina and polymer layers is measured to be 1.56 and 1.51, respectively. Shown in Fig. 6 are the simulation results of the transmittance of TFE films for different TFE structure (i.e., different number of dyads). As observed in experiment results (Fig. 4), the transmittance dips also appear in simulation results at similar wavelengths (419 and 551 nm). Furthermore, the simulation results clearly show that such transmittance dips deepen as the number of dyads increases. From the experiment results in Fig. 4 and the simulation results in Fig. 6, therefore, we can draw a conclusion that one needs to reduce the number of dyads to suppress the effect of such dips on the optical property of TOLEDs. We have further investigated the effect of the polymer layer thickness on the transmittance of TFE films. We have increased the polymer layer thickness of 3.5 dyad TFE films from 100 to 500 nm and performed simulations first. From
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Fig. 4. Measured optical transmittance of TFE films for different numbers of dyads (50-nm-thick Al2O3, 500-nm-thick polymer) with the air reference.
Fig. 5. Measured refractive index of Al2O3 and polymer thin films.
Fig. 3. Scanning electron microscope (SEM) images of a cross-section of 3.5 dyad TFE films with the polymer layer thickness of (a) 464 nm, (b) 826 nm, and (c) 1.55 lm.
Fig. 7, it is observed that the transmittance dip sharpens and the number of dips increases as the polymer layer thickness is increased. Similar result is also obtained experimentally in Fig. 8. For experiments, we have increased the polymer layer thickness of 3.5 dyad TFE films
from 500 to 1500 nm. It is found that the transmittance dips are getting spikier and appear more within the same wavelength range. From these results, we can conclude that the polymer layer thickness needs to be reduced to suppress the effect of those dips on the top emission spectrum of TOLEDs. Other than the effect of increasing the number of dyads, however, it is noted that increasing the polymer layer thickness does not deepen noticeably those transmittance dips (Fig. 7). From Figs. 4 and 8, it is found that the number of dyads and the thickness of the polymer layer are desired to be reduced. However, this would degrade the gas barrier property of OLEDs. To satisfy both optical and barrier properties, we can consider two different TFE structures. One structure can have the larger number of dyads but smaller thickness of the polymer layer. The other structure can have the smaller number of dyads but thicker polymer layer. Within the framework of mass production, the latter
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Fig. 6. Simulation results of transmittance of TFE films for different TFE structures.
Fig. 8. Measured transmittance from 3.5 dyad TFE films for different polymer layer thicknesses.
Fig. 7. Simulation results of transmittance of 3.5 dyad TFE films for different polymer layer thicknesses.
Fig. 9. Measured transmittance from 1.5 dyad TFE films for different polymer layer thicknesses.
configuration is more preferable. From the viewpoint of the optical and gas barrier properties, however, it is not clear which structure is more favored. To investigate it, we have decreased the number of dyads (1.5 dyads) and measured their transmittance for different polymer layer thicknesses. As seen in Fig. 9, more transmittance dips appear as the polymer layer thickness is increased. However, the depth of those dips is kept unchanged. It is rather more sensitive to the number of dyads. As discussed above (Fig. 7), the polymer layer thickness does not deepen noticeably those transmittance dips. When the number of dyads is small, therefore, the effect of increasing the polymer layer thickness on the depth of the transmittance dips is negligible. In this respect, the latter structure with the smaller number of dyads and thicker polymer layer is preferred. To inquire into which structure exhibits superior gas barrier property, we have fabricated two different
thin-film encapsulated OLEDs (TFE-OLEDs) and compared their barrier property. The 3.5 dyad TFE-OLED has the polymer layer thickness of 500 nm. Therefore, the total thickness of TFE film is 1.7 lm. Meanwhile, the 1.5 dyad TFEOLED has the polymer layer thickness of 2 lm. Therefore, the total thickness of TFE film is 2.1 lm. Fig. 10 shows the initial drop of their luminous intensity as a function of time measured at 85 °C and 85% relative humidity. The initial luminance of the devices was 1000 cd/m2. It is observed that both devices exhibit a very short lifetime of about 100 h at T80 (the duration from the start of testing to the time at which luminance is decreased to 80% of its initial value). One may expect much longer lifetime of TFE-OLEDs. It can be achieved by increasing the number of dyads (typically, 5.5 dyad TFE film is required to ensure no significant variation in the L–I–V characteristics of OLEDs for 1000 h at high temperature with high humidity condition). As mentioned above, however, it entails a
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of those transmittance dips without sacrificing the barrier property of TFE films by decreasing the number of dyads but increasing the polymer layer thickness. If the difference of refractive index between the inorganic and organic (polymer) layers in TFE films is small, the optical interference effect of TFE films can be suppressed. Therefore, the development of organic (polymer) materials with the same refractive index as the inorganic layer might be the way to achieve TFE films without the negative optical effect on TOLED, which is under investigation. Acknowledgements This work was supported by the IT R&D program of MKE/IITA [2009-F-016-01, Development of Eco-Emotional OLED Flat-Panel Lighting]. Fig. 10. Luminance of TFE-OLEDs with different TFE structures as a function of time measured under conditions of 85 °C and 85% relative humidity.
negative effect on the optical property of TFE-TOLEDs. Even if they showed a relatively short lifetime, it was observed that there was no big difference in the lifetime of those devices. Namely, the barrier property of the 1.5 dyad TFEOLED with thicker polymer layer is comparable with that of the 3.5 dyad TFE-OLED. From the results in Figs. 9 and 10, therefore, it can be concluded that we can suppress the effect of those transmittance dips without sacrificing the device lifetime by decreasing the number of dyads, yet increasing the polymer layer thickness. We believe that surface smoothening was not satisfactory for the TFE film with the thin polymer layer, which resulted in poor lifetime characteristics. 4. Conclusion By way of experiments and simulations, we have investigated the effects of thin-film encapsulations on the optical and barrier properties of hybrid white TOLEDs and provided a guideline that may be employed in the design of TFE structures. We have found that the number of dyads and the thickness of the polymer layer need be reduced to suppress a sharp dip appearing in the transmittance of TFE films, which causes a dip in the top emission spectrum of TOLEDs. It is demonstrated that we can suppress the effect
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