- Email: [email protected]
Al2O3 hybrid multi-layers
Organic Electronics 10 (2009) 1352–1355
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Thin-film encapsulation of top-emission organic light-emitting devices with polyurea/Al2O3 hybrid multi-layers Young Gu Lee a, Yun-Hyuk Choi a, In Seo Kee a, Hong Shik Shim a, YongWan Jin a, Sangyoon Lee a, Ken Ha Koh b, Soonil Lee b,* a b
Reformable Display Group, Samsung Advanced Institute of Technology, Samsung Electronics, P.O. Box 111, Suwon 440-600, Republic of Korea Division of Energy Systems Research, Ajou University, Suwon 442-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 June 2009 Received in revised form 13 July 2009 Accepted 15 July 2009 Available online 19 July 2009 PACS: 85.60.Pg 85.60.Jb 78.66.QN Keywords: Encapsulation Hybrid multi-layers Polyurea Top-emission OLED
a b s t r a c t We developed a room-temperature encapsulation process based on multi-stack of ultra thin Al2O3 and polyurea layers for top-emission organic light-emitting devices (TEOLEDs). Device structure, including a capping layer for refractive-index matching and a thick polyurea buffer layer, was optimized to enhance light extraction without distorting electroluminescence spectrum. The efficiency of a TEOLED encapsulated with 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers was better than that of a glass-encapsulated TEOLED, whereas their color coordinates were almost identical. Moreover, the half-decay lifetime of a TEOLED encapsulated with 5 pairs of Al2O3/polyurea layers was 86% of that of a glassencapsulated TEOLED. Water vapor transition rate of 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers on PET film was measured as low as 5 104 g/m2 day. Ó 2009 Elsevier B.V. All rights reserved.
Organic light-emitting device (OLED) has emerged as a very promising flat-panel display technology because of its high efficiency, fast response time, and wide viewing angle. One of challenging technical problems that hinders further progress of OLED is a thin-film encapsulation. Encapsulation is important for OLED to prevent oxidation of light-emitting materials and electrodes by blocking permeation of water vapor and ambient oxygen, and to protect devices from external shocks. Typically, metal or glass is used with UV-curable sealant for OLED encapsulation, and thin-film encapsulation is not popular yet. In particular, no commercial active-matrix OLED (AMOLED) has been developed based on thin-film encapsulation. However, thin-film encapsulation is indispensible in realizing
* Corresponding author. Tel.: +82 31 219 2582; fax: +82 31 219 1748. E-mail address: [email protected] (S. Lee). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.07.015
flexible OLED or top-emission OLED (TEOLED) without desiccant [1]. Previously, a number of different schemes for thin-film encapsulation were reported in the literature [2–8], but still the following two issues remain unresolved. First, barrier layers against water vapor and ambient oxygen have to be formed via low-temperature processes that are compatible with OLED. Second, device lifetime and transmittance in visible wavelength range have to be improved to the level that makes commercialization viable [9,10]. In this work, we relied on sputtered Al2O3 layers for effective blocking of water vapor and ambient oxygen, and to minimize damage to TEOLED and to release stress due to inorganic layer, we deposited organic layers of polyurea via vapor condensation polymerization. The alternating deposition of polyurea and Al2O3 layers was repeated to form different numbers of polyurea/Al2O3 pairs, and
Y.G. Lee et al. / Organic Electronics 10 (2009) 1352–1355
the performance of TEOLEDs encapsulated with each of these polyurea/Al2O3 pairs were tested to find optimal thin-film encapsulation design in regard to efficiency and lifetime of TEOLEDs. In this study, we fabricated TEOLEDs by using Ag(100 nm)/ITO(10 nm) as a reflective anode, LiF(1 nm)/ Mg:Ag(10:1 mass ratio, 18 nm) as a semitransparent cathode [11], and tris(8-hydroxyquinolinato)aluminum (Alq3) doped with 2-wt% 2,3,6,7-tetrahydro-1H, 5H, 11H-10-(2benzothiazolyl)quinolizino-[9,9a,1gh] Coumarin (C-545T) as an emitting layer (EML). N,N0 -Bis(1-naphthyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (NPB) and Alq3 were used as a hole transport layer (HTL) and an electron transport layer (ETL), respectively. Thickness of HTL, EML, and ETL were selected to optimize microcavity effect, so that the structure of TEOLEDs was Ag(100 nm)/ITO(10 nm)/ NPB(190 nm)/Alq3(33 nm) + C-545T(2 wt%)/Alq3(30 nm)/ LiF(1 nm)/Mg:Ag(18 nm)/NPB(60 nm). The 60-nm NPB capping layer was added for refractive-index matching to maximize light extraction [12]. Prior to deposit organic layers, reflective anode Ag/ITO was treated with 60-watt oxygen plasma for 90 s. All the organic layers and the Mg:Ag semitransparent cathode were deposited via thermal evaporation in vacuum better than 1.33 104 Pa. We tested 6 different encapsulation schemes for identical TEOLEDs. The first TEOLED, Device A, was encapsulated
Fig. 1. Cross-sectional SEM image of 5 pairs of Al2O3/polyurea layers on top of a 0.8-lm-thick polyurea layer. Polyurea and Al2O3 layers were deposited by alternating sputtering and VPD processes, respectively.
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with glass under nitrogen atmosphere while placing getter on one side not to block the light path. The other 5 TEOLEDs were encapsulated with alternating layers of Al2O3 and polyurea, and labeled according to the number of Al2O3/polyurea layer pairs: Device B-1, B-2, B-3, B-4, and B-5. Inorganic Al2O3 layers were deposited by RF magnetron sputtering under the condition of 42-sccm argon flow, 0.29-Pa working pressure, and 300-W RF power. Organic polyurea layers were deposited at room-temperature from isocyanate monomer and diamine precursor via vapor deposition polymerization (VDP) at the working pressure of 0.49 Pa, and the deposition rate was 1 nm/s. For thinfilm encapsulation, we first transferred a TEOLED to the VDP chamber to deposit 0.8-lm thick polyurea that was necessary to minimize damage to TEOLED during Al2O3 sputtering. Next, the TEOLED was transferred back and forth between the sputtering and VDP chambers to deposit 50-nm Al2O3 and 20-nm polyurea layers in sequence. The structure of encapsulation layer for Device B-1 was polyurea(0.8 lm)/Al2O3(50 nm)/polyurea(20 nm), and those for other devices were similar except the number of Al2O3(50 nm)/polyurea(20 nm) pairs on top of the 0.8-lm thick polyurea layer. The active area of all the TEOLEDs were 2 2 mm2. Keithley238 and PR650 were used as a source-measurement unit and a luminance meter, respectively, in measuring I–V–L(current-voltage-luminance) characteristics. Fig. 1 shows a cross-sectional scanning electron microscopy (SEM) image of 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers on top of the 0.8-lm thick polyurea layer, which is equivalent to the thin-film encapsulation for Device B-5. Polyurea and Al2O3 layers appear darker and less dark in the SEM image, respectively. This SEM picture confirms the formation of a good quality stack of Al2O3 and polyurea layers by alternating sputtering and VPD processes. In Fig. 2a, we compare operation characteristics of Device B-5 (thin-film-encapsulated TEOLED) and Device A (glass-encapsulated TEOLED). It is interesting to note that current density variation with respect to driving voltage is almost identical between Device A and Device B-5. This observation indicates that our thin-film encapsulation pro-
Fig. 2. Comparison of the operating characteristics of Device B-5 (thin-film-encapsulated TEOLED) with those of Device A (glass-encapsulated TEOLED): (a) current density and luminance versus applied voltage, and (b) luminance yield (EL efficiency) versus current density.
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Y.G. Lee et al. / Organic Electronics 10 (2009) 1352–1355
cess did not induce any damage to the TEOLEDs. However, there is a noticeable increase in luminance of Device B-5 compared to that of Device A at identical bias voltages. We attribute the luminance increase of Device B-5 to the optical effects of the thin-film stack. The luminance increase results in improvement of electroluminance (EL) efficiency as shown in Fig. 2b. At the current density of 10 mA/cm2, the EL efficiency of Device B-5 is 17.4 cd/A that corresponds to 23.4% improvement from 14.1 cd/A of Device A. Table 1 shows the position and width of EL peaks, from which CIE color coordinates are deduced, and the EL efficiency of the aforementioned six TEOLEDs at the current density of 10 mA/cm2. It is obvious that there is a periodic dependence of TEOLED performance on the number of Al2O3/polyurea pairs for the thin-film encapsulation devices. In particular, when even number of Al2O3/polyurea pairs was used, the EL efficiency was improved only 11% (Device B-2) and 15% (Device B-4) compared to that of Device A with glass encapsulation. However, there were respectively 30%, 25%, and 23% improvement in the EL efficiency when one (Device B-1), three (Device B-3), and five (Device B-5) pairs of Al2O3/polyurea layers were used for encapsulation. It is apparent that the optics of multi-layer stacks resulted in the observed periodic variation of the EL efficiency, and the position and width of EL peaks. We simulated the experimentally observed EL peaks through the combination of the photoluminance (PL) spectrum of the Coumarin-doped Alq3, which we used to represent the original emission from the EML, with the rigorous modeling of multi-layer optical effects that stem from the structure of each device. It was surprising to find that slight thickness adjustment of two NPB layers and a thick polyurea layer was sufficient to simulate the EL spectra of all five thin-film-encapsulated TEOLEDs. The thickness values of these layers, which we used to produce the simulated EL spectra in Fig. 3a, are listed in Table 1. To manifest the origin of the periodic variation of the EL efficiency, we compared the simulated transmittance of the five encapsulating thin films at the wavelengths corresponding to EL peaks. As shown in Fig. 3b, the transmittance varies periodically with respect to the increase of number of Al2O3-polyurea pairs, and higher transmittance is available from odd number of Al2O3-polyurea pairs. It is worth emphasizing that the EL efficiency of each of the five thin-film-encapsulated TEOLEDs is better than that of Device A. In the case of glass-encapsulated TEOLEDs, waveguide-mode loss is inevitable because the light emitted from the EML has to enter an encapsulating glass, having
Fig. 3. (a) Comparison of EL spectra (dash lines) simulated based on the structural parameters in Table 1 with measured EL spectra (solid lines): (b) systematic variation of simulated transmittance of the five encapsulating thin films at the wavelengths corresponding to EL peaks.
refractive-index of 1.53, through the air gap. However, similar refractive-indexes of Al2O3 and polyurea, 1.66 for Al2O3 and 1.52 for polyurea, makes thin-film-encapsulated devices free from wave-guide loss. Fig. 4 shows the decrease of luminance, starting from the initial value of 1000 cd/m2, with respect to operation time for Device A, and Device B-1, B-3, and B-5. We find that the half-decay lifetime of Device B-1 was only 592 hours, whereas that of Device A was 2985 h. It seems that 1-lm thick polyurea and one pair of Al2O3/polyurea layers were not sufficient enough to block permeation of water vapor and oxygen. However, as we increased the number of Al2O3/polyurea pairs to three (Device B-3) and five (Device B-3), the half-decay lifetime improved to 1618 and 2570 h, respectively. It is worth emphasizing that the half-decay lifetime of Device B-5 (2570 h) is about 86% of that of Device A (2985 h). The variation of lifetime indicates that protection against the permeation of water va-
Table 1 Details of EL performance for a series of top-emission OLEDs at the current densities of 10 mA/cm2. Device
Structure (nm) Ag/ITO
EL performance
NPB EML Alq3 LiF
A (Glass Encap) 100/10 189 B-1 188 B-2 190 B-3 188 B-4 189 B-5 189
33
30
1
Mg:Ag NPB Polyurea Al2O3/polyurea EL effi. (cd/A) EL peak (nm) FWHM (nm)
CIE (CIEx, CIEy)
18
(0.148, (0.143, (0.156, (0.144, (0.152, (0.146,
62 62 62 62 61 61
– 800 806 795 790 810
– 1 2 3 4 5
Basic unit for thin-film encapsulation is Al2O3(50 nm) and polyurea(20 nm).
pair pairs pairs pairs pairs
14.1 18.3 15.6 17.6 16.2 17.4
520 520 524 520 524 520
23 28 22 25 22 23
0.763) 0.768) 0.758) 0.766) 0.759) 0.763)
Y.G. Lee et al. / Organic Electronics 10 (2009) 1352–1355
Fig. 4. Comparison of half-decay lifetime of the TEOLEDs encapsulated with 1, 3, and 5 pairs of Al2O3 and polyurea layers (Device B-1, B-3, and B5, respectively) to that of the glass-encapsulated TEOLED (Device A). Initial luminance L0 was 1000 cd/m2. Inset shows a photograph of the device with 5 pairs of Al2O3 and polyurea layers (Device B-5) after the lifetime test.
Fig. 5. The result of WVTR measurement at 37.8 °C and 100% RH for a set of Al2O3(50 nm)/polyurea(20 nm) layers on a PET film. Each data point represents the average of three-sample measurements. The WVTR of one, two, three, four, and 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers on a PET film were measured as 5, 0.23, 0.04, 0.008, and 0.0005 g/m2 day, respectively, whereas the WVTR of a bare PET film was 8.2 g/m2 day.
por and ambient oxygen improves in accordance with the increase of the number of Al2O3/polyurea pairs. Nevertheless, the EL peak positions of Device B-1, B-3, and B-5 are identical, and the widths of EL peaks of these TEOLEDs are similar, so that their CIEx,y color coordinates are only slightly different, because thin-film encapsulation is designed in such a way to minimize the change in color coordinates from those of the original glass-encapsulated TEOLED, Device A. Inset in Fig. 4 is a photograph of the device with 5 pairs of Al2O3 and polyurea layers (Device B-5) after the lifetime test, which shows that the degradation of the Al2O3/polyurea-encapsulated OLEDs occurred typically from the edge due to the permeation of water vapor and ambient oxygen through the polyurea layers. Fig. 5 shows the result of independent measurement of water vapor transition rate (WVTR) for a set of Al2O3(50 nm)/polyurea(20 nm) layers on PET films. This WVTR measurement was carried out at 37.8 °C and 100% RH by using the AQUTRAN Model-1 (MOCON Co. Ltd.) with the detection range from 5 104 to 60 g/m2 day. The average of three-sample measurements for each number of Al2O3(50 nm)/polyurea(20 nm) pairs showed that the WVTR of one, two, three, four, and 5 pairs
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of Al2O3(50 nm)/polyurea(20 nm) layers on a PET film are 5, 0.23, 0.04, 0.008, and 0.0005 g/m2 day, respectively. It should be noted that the WVTR of the four pairs of Al2O3/ polyurea layers on a PET film is about three orders of magnitude smaller than that of a bare PET film, which is 8.2 g/m2 day, and that the WVTR of the 5 pairs of Al2O3/ polyurea layers on a PET film is close to the detection limit of the apparatus we used. In summary we developed a room-temperature process for thin-film encapsulation of TEOLEDs via alternating stacking of organic polyurea and inorganic Al2O3 layers. Initial deposition of a 0.8-lm-thick polyurea layer on top of TEOLEDs turned out to be effective to prevent TEOLEDs from experiencing noticeable damage during sputtering processes. We were able to prove that 5 pairs of Al2O3/polyurea layers were sufficient for significant improvement of half-decay lifetime of the TEOLED, to the 86% of the lifetime of the glass-encapsulated TEOLED. Moreover, we achieved 23% improvement in EL efficiency without noticeable change in color coordinates because the thickness of the Al2O3/polyurea pairs, the thick polyurea buffer layer, the transparent Mg:Ag layer, and the two NPB layers were selected to maximize light extraction without color distortion. Independent measurement of WVTR for a set of Al2O3/polyurea layers on PET films showed that the barrier property against water vapor can be so good that the WVTR can be as low as 5 104 g/m2 day with 5 pairs of Al2O3/ polyurea layers, which indicates that the hybrid thin-film encapsulation based on Al2O3/polyurea layers has the application potential in developing flexible top-emitting OLEDs. Acknowledgements S.L. acknowledges the financial support by the Ministry of Science and Technology through the Nanoscopia Center of Excellence at Ajou University and by the Korea Research Foundation (Grant No. KRF-2007-412-J04003). References [1] Sung Mook Chung, Chi-Sun Hwang, Jeong-Ik Lee, Sang Hee Ko Park, Yong Suk Yang, Lee-Mi Do, Hye Yong Chu, Synth. Met. 158 (2008) 561. [2] Anna B. Chwang, Mark A. Rothman, Sokhanno Y. Mao, Richard H. Hewitt, Michael S. Weaver, Jeff A. Silvernail, Kamala Rajan, Michael Hack, Julie J. Brown, Xi Chu, Lorenza Moro, Todd Krajewski, Nicole Rutherford, Appl. Phys. Lett. 83 (2003) 413. [3] S.-N. Lee, S.-W. Hwang, C.H. Chen, Jpn. J. Appl. Phys. 46 (2007) 7432. [4] 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) 014509. [5] J. Granstrom, J.S. Swensen, J.S. Moon, G. Rowell, J. Yuen, A.J. Heeger, Appl. Phys. Lett. 93 (2008) 193304. [6] H.-K. Kim, M.S. Kim, J.-W. Kang, J.-J. Kim, M.-S. Yi, Appl. Phys. Lett. 90 (2007) 13502. [7] A.P. Ghosh, L.J. Gerenser, C.M. Jarman, J.E. Fornalik, Appl. Phys. Lett. 86 (2005) 223503. [8] H. Lifka, H.A. van Esch, J.J.W.M. Rosink, SID 04 Digest, pp. 1384–1387. [9] L. Moro, T.A. Krajewski, N.M. Rutherford, O. Philips, R.J. Visser, M. Gross, W.D. Bennett, G. Graff, SPIE 5214 (2004) 83. [10] M. Hemerik, R. van Erven, R. Vangheluwe, J. Yang, T. van Rijswijk, R. Winter, B. van Rens, SID 06 DIGEST, pp. 1571–1574. [11] B.J. Chen, X.W. Sun, K.S. Wong, X. Hu, Opt. Exp. 13 (2005) 2631. [12] L.S. Hung, C.W. Tang, M.G. Mason, P. Raychaudhuri, J. Madathil, Appl. Phys. Lett. 78 (2001) 544.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Thin-film encapsulation of top-emission organic light-emitting devices with polyurea/Al2O3 hybrid multi-layers Young Gu Lee a, Yun-Hyuk Choi a, In Seo Kee a, Hong Shik Shim a, YongWan Jin a, Sangyoon Lee a, Ken Ha Koh b, Soonil Lee b,* a b
Reformable Display Group, Samsung Advanced Institute of Technology, Samsung Electronics, P.O. Box 111, Suwon 440-600, Republic of Korea Division of Energy Systems Research, Ajou University, Suwon 442-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 June 2009 Received in revised form 13 July 2009 Accepted 15 July 2009 Available online 19 July 2009 PACS: 85.60.Pg 85.60.Jb 78.66.QN Keywords: Encapsulation Hybrid multi-layers Polyurea Top-emission OLED
a b s t r a c t We developed a room-temperature encapsulation process based on multi-stack of ultra thin Al2O3 and polyurea layers for top-emission organic light-emitting devices (TEOLEDs). Device structure, including a capping layer for refractive-index matching and a thick polyurea buffer layer, was optimized to enhance light extraction without distorting electroluminescence spectrum. The efficiency of a TEOLED encapsulated with 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers was better than that of a glass-encapsulated TEOLED, whereas their color coordinates were almost identical. Moreover, the half-decay lifetime of a TEOLED encapsulated with 5 pairs of Al2O3/polyurea layers was 86% of that of a glassencapsulated TEOLED. Water vapor transition rate of 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers on PET film was measured as low as 5 104 g/m2 day. Ó 2009 Elsevier B.V. All rights reserved.
Organic light-emitting device (OLED) has emerged as a very promising flat-panel display technology because of its high efficiency, fast response time, and wide viewing angle. One of challenging technical problems that hinders further progress of OLED is a thin-film encapsulation. Encapsulation is important for OLED to prevent oxidation of light-emitting materials and electrodes by blocking permeation of water vapor and ambient oxygen, and to protect devices from external shocks. Typically, metal or glass is used with UV-curable sealant for OLED encapsulation, and thin-film encapsulation is not popular yet. In particular, no commercial active-matrix OLED (AMOLED) has been developed based on thin-film encapsulation. However, thin-film encapsulation is indispensible in realizing
* Corresponding author. Tel.: +82 31 219 2582; fax: +82 31 219 1748. E-mail address: [email protected] (S. Lee). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.07.015
flexible OLED or top-emission OLED (TEOLED) without desiccant [1]. Previously, a number of different schemes for thin-film encapsulation were reported in the literature [2–8], but still the following two issues remain unresolved. First, barrier layers against water vapor and ambient oxygen have to be formed via low-temperature processes that are compatible with OLED. Second, device lifetime and transmittance in visible wavelength range have to be improved to the level that makes commercialization viable [9,10]. In this work, we relied on sputtered Al2O3 layers for effective blocking of water vapor and ambient oxygen, and to minimize damage to TEOLED and to release stress due to inorganic layer, we deposited organic layers of polyurea via vapor condensation polymerization. The alternating deposition of polyurea and Al2O3 layers was repeated to form different numbers of polyurea/Al2O3 pairs, and
Y.G. Lee et al. / Organic Electronics 10 (2009) 1352–1355
the performance of TEOLEDs encapsulated with each of these polyurea/Al2O3 pairs were tested to find optimal thin-film encapsulation design in regard to efficiency and lifetime of TEOLEDs. In this study, we fabricated TEOLEDs by using Ag(100 nm)/ITO(10 nm) as a reflective anode, LiF(1 nm)/ Mg:Ag(10:1 mass ratio, 18 nm) as a semitransparent cathode [11], and tris(8-hydroxyquinolinato)aluminum (Alq3) doped with 2-wt% 2,3,6,7-tetrahydro-1H, 5H, 11H-10-(2benzothiazolyl)quinolizino-[9,9a,1gh] Coumarin (C-545T) as an emitting layer (EML). N,N0 -Bis(1-naphthyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (NPB) and Alq3 were used as a hole transport layer (HTL) and an electron transport layer (ETL), respectively. Thickness of HTL, EML, and ETL were selected to optimize microcavity effect, so that the structure of TEOLEDs was Ag(100 nm)/ITO(10 nm)/ NPB(190 nm)/Alq3(33 nm) + C-545T(2 wt%)/Alq3(30 nm)/ LiF(1 nm)/Mg:Ag(18 nm)/NPB(60 nm). The 60-nm NPB capping layer was added for refractive-index matching to maximize light extraction [12]. Prior to deposit organic layers, reflective anode Ag/ITO was treated with 60-watt oxygen plasma for 90 s. All the organic layers and the Mg:Ag semitransparent cathode were deposited via thermal evaporation in vacuum better than 1.33 104 Pa. We tested 6 different encapsulation schemes for identical TEOLEDs. The first TEOLED, Device A, was encapsulated
Fig. 1. Cross-sectional SEM image of 5 pairs of Al2O3/polyurea layers on top of a 0.8-lm-thick polyurea layer. Polyurea and Al2O3 layers were deposited by alternating sputtering and VPD processes, respectively.
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with glass under nitrogen atmosphere while placing getter on one side not to block the light path. The other 5 TEOLEDs were encapsulated with alternating layers of Al2O3 and polyurea, and labeled according to the number of Al2O3/polyurea layer pairs: Device B-1, B-2, B-3, B-4, and B-5. Inorganic Al2O3 layers were deposited by RF magnetron sputtering under the condition of 42-sccm argon flow, 0.29-Pa working pressure, and 300-W RF power. Organic polyurea layers were deposited at room-temperature from isocyanate monomer and diamine precursor via vapor deposition polymerization (VDP) at the working pressure of 0.49 Pa, and the deposition rate was 1 nm/s. For thinfilm encapsulation, we first transferred a TEOLED to the VDP chamber to deposit 0.8-lm thick polyurea that was necessary to minimize damage to TEOLED during Al2O3 sputtering. Next, the TEOLED was transferred back and forth between the sputtering and VDP chambers to deposit 50-nm Al2O3 and 20-nm polyurea layers in sequence. The structure of encapsulation layer for Device B-1 was polyurea(0.8 lm)/Al2O3(50 nm)/polyurea(20 nm), and those for other devices were similar except the number of Al2O3(50 nm)/polyurea(20 nm) pairs on top of the 0.8-lm thick polyurea layer. The active area of all the TEOLEDs were 2 2 mm2. Keithley238 and PR650 were used as a source-measurement unit and a luminance meter, respectively, in measuring I–V–L(current-voltage-luminance) characteristics. Fig. 1 shows a cross-sectional scanning electron microscopy (SEM) image of 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers on top of the 0.8-lm thick polyurea layer, which is equivalent to the thin-film encapsulation for Device B-5. Polyurea and Al2O3 layers appear darker and less dark in the SEM image, respectively. This SEM picture confirms the formation of a good quality stack of Al2O3 and polyurea layers by alternating sputtering and VPD processes. In Fig. 2a, we compare operation characteristics of Device B-5 (thin-film-encapsulated TEOLED) and Device A (glass-encapsulated TEOLED). It is interesting to note that current density variation with respect to driving voltage is almost identical between Device A and Device B-5. This observation indicates that our thin-film encapsulation pro-
Fig. 2. Comparison of the operating characteristics of Device B-5 (thin-film-encapsulated TEOLED) with those of Device A (glass-encapsulated TEOLED): (a) current density and luminance versus applied voltage, and (b) luminance yield (EL efficiency) versus current density.
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Y.G. Lee et al. / Organic Electronics 10 (2009) 1352–1355
cess did not induce any damage to the TEOLEDs. However, there is a noticeable increase in luminance of Device B-5 compared to that of Device A at identical bias voltages. We attribute the luminance increase of Device B-5 to the optical effects of the thin-film stack. The luminance increase results in improvement of electroluminance (EL) efficiency as shown in Fig. 2b. At the current density of 10 mA/cm2, the EL efficiency of Device B-5 is 17.4 cd/A that corresponds to 23.4% improvement from 14.1 cd/A of Device A. Table 1 shows the position and width of EL peaks, from which CIE color coordinates are deduced, and the EL efficiency of the aforementioned six TEOLEDs at the current density of 10 mA/cm2. It is obvious that there is a periodic dependence of TEOLED performance on the number of Al2O3/polyurea pairs for the thin-film encapsulation devices. In particular, when even number of Al2O3/polyurea pairs was used, the EL efficiency was improved only 11% (Device B-2) and 15% (Device B-4) compared to that of Device A with glass encapsulation. However, there were respectively 30%, 25%, and 23% improvement in the EL efficiency when one (Device B-1), three (Device B-3), and five (Device B-5) pairs of Al2O3/polyurea layers were used for encapsulation. It is apparent that the optics of multi-layer stacks resulted in the observed periodic variation of the EL efficiency, and the position and width of EL peaks. We simulated the experimentally observed EL peaks through the combination of the photoluminance (PL) spectrum of the Coumarin-doped Alq3, which we used to represent the original emission from the EML, with the rigorous modeling of multi-layer optical effects that stem from the structure of each device. It was surprising to find that slight thickness adjustment of two NPB layers and a thick polyurea layer was sufficient to simulate the EL spectra of all five thin-film-encapsulated TEOLEDs. The thickness values of these layers, which we used to produce the simulated EL spectra in Fig. 3a, are listed in Table 1. To manifest the origin of the periodic variation of the EL efficiency, we compared the simulated transmittance of the five encapsulating thin films at the wavelengths corresponding to EL peaks. As shown in Fig. 3b, the transmittance varies periodically with respect to the increase of number of Al2O3-polyurea pairs, and higher transmittance is available from odd number of Al2O3-polyurea pairs. It is worth emphasizing that the EL efficiency of each of the five thin-film-encapsulated TEOLEDs is better than that of Device A. In the case of glass-encapsulated TEOLEDs, waveguide-mode loss is inevitable because the light emitted from the EML has to enter an encapsulating glass, having
Fig. 3. (a) Comparison of EL spectra (dash lines) simulated based on the structural parameters in Table 1 with measured EL spectra (solid lines): (b) systematic variation of simulated transmittance of the five encapsulating thin films at the wavelengths corresponding to EL peaks.
refractive-index of 1.53, through the air gap. However, similar refractive-indexes of Al2O3 and polyurea, 1.66 for Al2O3 and 1.52 for polyurea, makes thin-film-encapsulated devices free from wave-guide loss. Fig. 4 shows the decrease of luminance, starting from the initial value of 1000 cd/m2, with respect to operation time for Device A, and Device B-1, B-3, and B-5. We find that the half-decay lifetime of Device B-1 was only 592 hours, whereas that of Device A was 2985 h. It seems that 1-lm thick polyurea and one pair of Al2O3/polyurea layers were not sufficient enough to block permeation of water vapor and oxygen. However, as we increased the number of Al2O3/polyurea pairs to three (Device B-3) and five (Device B-3), the half-decay lifetime improved to 1618 and 2570 h, respectively. It is worth emphasizing that the half-decay lifetime of Device B-5 (2570 h) is about 86% of that of Device A (2985 h). The variation of lifetime indicates that protection against the permeation of water va-
Table 1 Details of EL performance for a series of top-emission OLEDs at the current densities of 10 mA/cm2. Device
Structure (nm) Ag/ITO
EL performance
NPB EML Alq3 LiF
A (Glass Encap) 100/10 189 B-1 188 B-2 190 B-3 188 B-4 189 B-5 189
33
30
1
Mg:Ag NPB Polyurea Al2O3/polyurea EL effi. (cd/A) EL peak (nm) FWHM (nm)
CIE (CIEx, CIEy)
18
(0.148, (0.143, (0.156, (0.144, (0.152, (0.146,
62 62 62 62 61 61
– 800 806 795 790 810
– 1 2 3 4 5
Basic unit for thin-film encapsulation is Al2O3(50 nm) and polyurea(20 nm).
pair pairs pairs pairs pairs
14.1 18.3 15.6 17.6 16.2 17.4
520 520 524 520 524 520
23 28 22 25 22 23
0.763) 0.768) 0.758) 0.766) 0.759) 0.763)
Y.G. Lee et al. / Organic Electronics 10 (2009) 1352–1355
Fig. 4. Comparison of half-decay lifetime of the TEOLEDs encapsulated with 1, 3, and 5 pairs of Al2O3 and polyurea layers (Device B-1, B-3, and B5, respectively) to that of the glass-encapsulated TEOLED (Device A). Initial luminance L0 was 1000 cd/m2. Inset shows a photograph of the device with 5 pairs of Al2O3 and polyurea layers (Device B-5) after the lifetime test.
Fig. 5. The result of WVTR measurement at 37.8 °C and 100% RH for a set of Al2O3(50 nm)/polyurea(20 nm) layers on a PET film. Each data point represents the average of three-sample measurements. The WVTR of one, two, three, four, and 5 pairs of Al2O3(50 nm)/polyurea(20 nm) layers on a PET film were measured as 5, 0.23, 0.04, 0.008, and 0.0005 g/m2 day, respectively, whereas the WVTR of a bare PET film was 8.2 g/m2 day.
por and ambient oxygen improves in accordance with the increase of the number of Al2O3/polyurea pairs. Nevertheless, the EL peak positions of Device B-1, B-3, and B-5 are identical, and the widths of EL peaks of these TEOLEDs are similar, so that their CIEx,y color coordinates are only slightly different, because thin-film encapsulation is designed in such a way to minimize the change in color coordinates from those of the original glass-encapsulated TEOLED, Device A. Inset in Fig. 4 is a photograph of the device with 5 pairs of Al2O3 and polyurea layers (Device B-5) after the lifetime test, which shows that the degradation of the Al2O3/polyurea-encapsulated OLEDs occurred typically from the edge due to the permeation of water vapor and ambient oxygen through the polyurea layers. Fig. 5 shows the result of independent measurement of water vapor transition rate (WVTR) for a set of Al2O3(50 nm)/polyurea(20 nm) layers on PET films. This WVTR measurement was carried out at 37.8 °C and 100% RH by using the AQUTRAN Model-1 (MOCON Co. Ltd.) with the detection range from 5 104 to 60 g/m2 day. The average of three-sample measurements for each number of Al2O3(50 nm)/polyurea(20 nm) pairs showed that the WVTR of one, two, three, four, and 5 pairs
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of Al2O3(50 nm)/polyurea(20 nm) layers on a PET film are 5, 0.23, 0.04, 0.008, and 0.0005 g/m2 day, respectively. It should be noted that the WVTR of the four pairs of Al2O3/ polyurea layers on a PET film is about three orders of magnitude smaller than that of a bare PET film, which is 8.2 g/m2 day, and that the WVTR of the 5 pairs of Al2O3/ polyurea layers on a PET film is close to the detection limit of the apparatus we used. In summary we developed a room-temperature process for thin-film encapsulation of TEOLEDs via alternating stacking of organic polyurea and inorganic Al2O3 layers. Initial deposition of a 0.8-lm-thick polyurea layer on top of TEOLEDs turned out to be effective to prevent TEOLEDs from experiencing noticeable damage during sputtering processes. We were able to prove that 5 pairs of Al2O3/polyurea layers were sufficient for significant improvement of half-decay lifetime of the TEOLED, to the 86% of the lifetime of the glass-encapsulated TEOLED. Moreover, we achieved 23% improvement in EL efficiency without noticeable change in color coordinates because the thickness of the Al2O3/polyurea pairs, the thick polyurea buffer layer, the transparent Mg:Ag layer, and the two NPB layers were selected to maximize light extraction without color distortion. Independent measurement of WVTR for a set of Al2O3/polyurea layers on PET films showed that the barrier property against water vapor can be so good that the WVTR can be as low as 5 104 g/m2 day with 5 pairs of Al2O3/ polyurea layers, which indicates that the hybrid thin-film encapsulation based on Al2O3/polyurea layers has the application potential in developing flexible top-emitting OLEDs. Acknowledgements S.L. acknowledges the financial support by the Ministry of Science and Technology through the Nanoscopia Center of Excellence at Ajou University and by the Korea Research Foundation (Grant No. KRF-2007-412-J04003). References [1] Sung Mook Chung, Chi-Sun Hwang, Jeong-Ik Lee, Sang Hee Ko Park, Yong Suk Yang, Lee-Mi Do, Hye Yong Chu, Synth. Met. 158 (2008) 561. [2] Anna B. Chwang, Mark A. Rothman, Sokhanno Y. Mao, Richard H. Hewitt, Michael S. Weaver, Jeff A. Silvernail, Kamala Rajan, Michael Hack, Julie J. Brown, Xi Chu, Lorenza Moro, Todd Krajewski, Nicole Rutherford, Appl. Phys. Lett. 83 (2003) 413. [3] S.-N. Lee, S.-W. Hwang, C.H. Chen, Jpn. J. Appl. Phys. 46 (2007) 7432. [4] 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) 014509. [5] J. Granstrom, J.S. Swensen, J.S. Moon, G. Rowell, J. Yuen, A.J. Heeger, Appl. Phys. Lett. 93 (2008) 193304. [6] H.-K. Kim, M.S. Kim, J.-W. Kang, J.-J. Kim, M.-S. Yi, Appl. Phys. Lett. 90 (2007) 13502. [7] A.P. Ghosh, L.J. Gerenser, C.M. Jarman, J.E. Fornalik, Appl. Phys. Lett. 86 (2005) 223503. [8] H. Lifka, H.A. van Esch, J.J.W.M. Rosink, SID 04 Digest, pp. 1384–1387. [9] L. Moro, T.A. Krajewski, N.M. Rutherford, O. Philips, R.J. Visser, M. Gross, W.D. Bennett, G. Graff, SPIE 5214 (2004) 83. [10] M. Hemerik, R. van Erven, R. Vangheluwe, J. Yang, T. van Rijswijk, R. Winter, B. van Rens, SID 06 DIGEST, pp. 1571–1574. [11] B.J. Chen, X.W. Sun, K.S. Wong, X. Hu, Opt. Exp. 13 (2005) 2631. [12] L.S. Hung, C.W. Tang, M.G. Mason, P. Raychaudhuri, J. Madathil, Appl. Phys. Lett. 78 (2001) 544.