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Flexible OLED encapsulated with gas barrier film and adhesive gasket
Synthetic Metals 193 (2014) 77–80
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Flexible OLED encapsulated with gas barrier film and adhesive gasket A Ra Cho a , Eun Hye Kim b , Soo Young Park a , Lee Soon Park c,∗ a
School of Applied Chemical Engineering, Major in Polymer Science and Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea Department of Sensor and Display Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea c School of Material Science and Engineering, Ulsan National Institute of Science and Technology, 689-798, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 25 September 2013 Received in revised form 21 March 2014 Accepted 26 March 2014 Available online 6 May 2014 Keywords: Flexible OLED Encapsulation of OLED Gas barrier layer Life time of OLED
a b s t r a c t Flexible OLEDs have been studied actively for their potential application to both large size OLEDs display and general lighting. The efficiency and life time of flexible OLEDs based on film substrate are still major problem to overcome compared to the OLEDs devices based on glass substrate. The short life time of flexible OLEDs are attributed to the increased penetration of oxygen and water vapor to the thin organic layer of OLEDs device through the flexible polymer film substrate. In this work, we have used polyethylene telephthalate (PEN) film as flexible substrate for OLEDs. The AlNx /UVR (UV curable resin)/AlNx type gas barrier layers were formed on top of PEN film, followed by OLEDs panel fabrication. After deposition of Al cathode, the upper part of flexible OLEDs panel was encapsulated by laminating the same PEN film with the aid of optically clear adhesive (OCA) in the form of gasket. The completed flexible OLEDs were examined through the mechanical bending and life time tests for comparison to the rigid OLEDs based on glass substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Organic light emitting diodes (OLEDs) displays based on glass substrate have been used in mobile devices such as smart phones, smart pads and recently OLEDs TVs which have been introduced in the market. Due to their simple, organic thin-film design, OLED can also be easily built on not only glass substrates but also flexible substrates [1–5]. However, on film substrates, the short life time and low efficiency of the flexible OLEDs are still the major problem to be solved [6,7]. The short life time of flexible OLEDs is attributed to the increased penetration of oxygen and water vapor to the thin organic layers of OLEDs devices through the substrate film and the seal layer between the flexible substrate and upper encapsulated film [8]. Therefore the formation of inorganic/organic multiple gas barrier layers on the flexible substrate film and the choice of side seal material are important issue for the flexible OLEDs [9]. We used poly(ethylene naphthalate) (PEN) film as flexible substrate since it had a balanced property such as high transmittance, high thermal and mechanical property [10]. However, it is not suitable for the flexible substrates of OLEDs flexible substrates because it has higher penetration of oxygen and water vapor (about
∗ Corresponding author at: School of Material Science and Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea. Tel.: +82 52 217 2349. E-mail address: [email protected] (L.S. Park). http://dx.doi.org/10.1016/j.synthmet.2014.03.027 0379-6779/© 2014 Elsevier B.V. All rights reserved.
1.64 g/m2 day) than glass (about 10−6 g/m2 day). So the inorganic and organic multiple gas barrier layers are required on the PEN substrate films [11]. In our work, these films were used both as the flexible substrate for the OLEDs devices and as the encapsulation film for covering upper aluminum cathode for the long term flexible property [12–14]. The encapsulation of OLEDs device was conducted by lamination process utilizing optically clear adhesive (OCA) in the form of gasket [15]. The use of optically clear adhesive (OCA) as gasket could simplify the encapsulation process while securing the gas barrier property in the sealing section. The materials and process of the flexible encapsulation were examined from the viewpoint of increasing the life time of flexible OLEDs.
2. Experimental 2.1. Fabrication of flexible OLEDs In order to make flexible OLEDs which had PEN film with gas barrier layers both as flexible substrate and encapsulation film, the gas barrier layers were deposited on PEN film utilizing roll-to-roll sputter. The structure of the PEN film with gas barrier layers was PEN (110 m)/AlNx (25 nm)/UVR (1.2 m)/AlNx (25 nm) which was optimized in our previous work [16]. The aluminum nitride (AlNx ) thin film was deposited by reactive sputtering with Al target and controlled by flow of N2 gas as shown in Table 1. The UVR layer was obtained by spin coating and then exposed to UV light. The spin
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Table 1 Deposition condition of AlNx gas barrier thin film by using roll-to-roll sputter. Base pressure
l × l0−5 [torr]
Working pressure Power Winding speed Gas flow rate = N2 /Ar + N2 Target
3 [mtorr] DC plus 600 [W] 0.05 [m/mim] 0–75 [%] Al (99.99%) (250 mm × 100 mm × 6 mm)
Table 2 Deposition condition of ITO/Ag/ITO transparent anode thin film by using roll-to-roll sputter. Target
ITO
Ag
Composition Power Winding speed Gas flow rate
90:10 DC pulse 600[W] 0.3 [m/min] Ar:O2 = 50:1.5 [sccm]
Ag 99.99% DC 600 [W] 0.9 [m/min] Ar = 50 [sccm]
2.2. Film encapsulation of flexible OLEDs and measurement The encapsulation of flexible OLEDs device was carried out by using the same film, PEN (110 m)/AlNx (25 nm)/UVR (1.2 m)/AlNx (25 nm), which was used as substrate of flexible OLEDs, in order to reduce the stress difference under bending motions. The encapsulation film was fixed on the rectangular edge of flexible OLEDs substrate utilizing OCA used in the touch panel fabrication. The light transmittance of the PEN film with gas barrier layers was measured by UV–vis spectrophotometer (CM-3600d, Konica Minolta Co., Japan). The water vapor transmission rate (WVTR) of the gas barrier films was measured by Mocon (Permatran-WR 3/33MA) equipment under the condition of 37.8 ◦ C, RH 100%, and N2 gas rate of 10 ± 0.2 sccm for 24 h [17,18]. The bending test of PEN film with gas barrier layers was conducted with the bending machine (Zeetech, ZBT-200, Korea). The gas barrier film size was 100 mm × 100 mm and the test conditions were 10,000 cycles under 100 mm/s speed in the range of 40–80 mm. 3. Results and discussion
coated UVR is NLR06 which is obtained from Nanopolychem Co., Korea. After forming gas barrier layers on PEN film, the ITO (20 nm)/Ag (17 nm)/ITO (20 nm) multilayer anode was also deposited by rollto-roll sputter under the condition shown in Table 2. The patterning of ITO/Ag/ITO anode was conducted by photolithographic process utilizing a positive-PR (DS-i1000) and an etchant solution (DA-300) obtained from Dongjin Chemical Co., Korea. After patterning of ITO/Ag/ITO anode, the insulator layer was patterned by using polyimide-PR supplied by Dongjin Chemical Co. The structure of the 4-pixel back-plane of flexible OLEDs is shown in Fig. 1. The fabrication of flexible OLEDs on patterned 4-pixel flexible back-plane was carried out with SUNIC EL Plus 200, a cluster type OLEDs panel fabrication system. The flexible OLEDs device on patterned flexible back-plane without film encapsulation had ˚ ˚ (Alq3 ;C545T, 400 A)/ETL configurations of HTL (NPB, 1000 A)/EML ˚ ˚ ˚ (Alq3 , 250 A)/EIL (LiF, 10 A)/Cathode (Al, 1000 A).
3.1. Property of flexible substrates The performance of flexible OLEDs is quite dependent on the materials, especially on the quality of the flexible substrate and encapsulation film [19,20]. The important requirements of the flexible substrate for OLEDs include high visible light transmittance, low water vapor transmittance rate (WVTR) and low sheet resistance of transparent conducting anode thin film (ITO/Ag/ITO) [21]. We used the PEN (100 m)/AlNx (25 nm)/UVR (1.2 m)/AlNx (25 nm) film as encapsulation film after OLEDs panel fabrication. The properties of PEN based films after gas barrier layer and ITO/Ag/ITO anode formation are shown in Table 3. The PEN/AlNx /UVR/AlNx film after gas barrier formation exhibited 86.8% visible light transmittance and low WVTR value (0.008 g/m2 day before bending and 0.02 g/m2 day after bending test). The WVTR of bare PEN, PEN/AlNx (before bending), PEN/AlNx (after bending), PEN/UVR, PEN/AlNx /UVR(before bending) and
Fig. 1. The structure of 4 pixel back plane for flexible OLED (a) patterned ITO layer, (b) patterned ITO and insulator layer, (c) working flexible OLED device and (d) the cross-sectional structure patterned ITO and insulator layer.
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Table 3 The properties of PEN films with gas barrier layer and ITO/Ag/ITO deposition. Film structure Transmittance WVTR
PEN/AlNx /UVR/AlNx 86% Before bending After bending
0.008 g/m2 day 0.02 g/m2 day
Film structure Transmittance Sheet resistance
PEN/AlNx/UVR/AINx/ITO/Ag/ITO 82.1% Before bending After bending
8.3/sq 7.8/sq
Fig. 3. The importance of various film encapsulations on flexible OLEDs.
3.2. Performance of flexible OLEDs
Fig. 2. The structure of flexible OLEDs after film encapsulation.
PEN/AlNx /UVR (after bending) were 1.6356, 0.1937, 0.1953, 0.0395, 0.0395 and 0.0485 g/m2 day, respectively. But the combination of organic and inorganic thin layers such as PEN/AlNx /UVR/AlNx exhibited an excellent result of WVTR, 0.008 g/m2 day. On the other hand, multilayer has lower transmittance than just glass. It is also important to find optimum conditions between transmittance and WVTR. After ITO/Ag/ITO thin film deposition on outer PEN/AlNx /UVR/AlNx layer, the sheet resistance values were 8.3 /sq. at room temperature and 7.8 /sq. after heat treatment at 100 ◦ C for 30 min, which was as good as the ITO glass used for OLEDs panel fabrication. The transmittance to visible light was slightly decreased to 82.1% while the WVTR values were almost same as the one before ITO/Ag/ITO thin film deposition. After confirmation of basic properties of PEN films, the flexible OLEDs devices were fabricated through the processes of ITO/Ag/ITO anode patterning, polyimide-PR insulator patterning, organic layers and cathode deposition followed by film encapsulation. The structure of the final flexible OLEDs device is shown in Fig. 2.
The importance of various film encapsulations on the life time tests of flexible OLEDs is shown in Fig. 3. The life time tests were conducted under the condition of 85% relative humidity and 85 ◦ C temperature. The OLEDs device encapsulated with the same flexible substrate, PEN/AlNx /UVR/AlNx film, showed 91% luminance after 750 h compared to OLEDs made on ITO glass substrate and standard glass encapsulation, while the OLEDs encapsulated with PEN/AlNx film exhibited 86% luminance after 750 h. The flexible OLEDs device encapsulated with PEN base film and the OLEDs device without film encapsulation, i.e. Al cathode open to the air, exhibited 50% luminance (corresponding to life time) after 360 h and 320 h, respectively. Therefore from these static tests it was noted that the deposition of multiple gas barrier layer is very important for the long life time of flexible OLEDs. The dynamic test of flexible OLEDs was conducted with the bending machine shown in Fig. 4 at room temperature. Before bending, the flexible OLEDs device exhibited both high current efficiency (6.3 cd/A) and external efficiency (2.8%) as shown in Fig. 5. The flexible OLEDs encapsulated with PEN/AlNx /UVR/AlNx film could withstand bending motion of 10,000 times, however, the OLEDs without any film encapsulation (open cathode) did not work after bending 10,000 times. The differences of OLEDs with PEN/AlNx /UVR/AlNx film encapsulation and without any encapsulation after bending 5000 times are shown in Fig. 5 with green and white upside triangle, respectively. It was of interest to note that the flexible OLEDs without any encapsulation could maintain emission up to 5000 times bending. In case of life time test of OLED under 85% RH and 85 ◦ C, the OLEDs device without any encapsulation exhibited life time of only 320 h as shown in Fig. 3. These data suggest that flexible OLED had
Fig. 4. Images of bending machine (a) the maximum width and (b) the minimum width.
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Fig. 5. The performance of flexible OLEDs (a) current efficiency and (b) external quantum efficiency.
high potential in flexible display application, if they were properly encapsulated. 4. Conclusions The performance of flexible OLED is quite dependent on the materials compared to the glass-based OLED, especially on the quality of the flexible substrate and encapsulation film. We used PEN films with same gas barrier layers for both as substrate of flexible OLED and as encapsulation film after OLED panel fabrication in order to reduce the stress difference under bending motion. The flexible OLED encapsulated with PEN/AlNx/UVR/AlNx film could withstand bending motion of 10,000 times, however, the OLED without any film encapsulation (open cathode) did not work after bending 10,000 times. It was also noted that the flexible OLED device without any encapsulation could maintain emission up to 5000 times bending, while the same OLED exhibited life time of only 320 h under the condition of 85% RH and 85 ◦ C temperature. These data suggest that flexible OLEDs had high potential in flexible display application if they were properly encapsulated. Acknowledgement This work was supported by the industrial strategic technology development program [10035225, Development of core technology for high performance AMOLED on plastic] funded by MKE/KEIT, Korea. References [1] T.K. Hatwar, J.P. Spindler, M. Kondakova, D. Giesen, J. Deaton, J.R. Vargas, SID Symp. Digest. 41 (2010) 778.
[2] P.A. Levermore, V. Adamovich, K. Rajan, W. Yeager, C. Lin, S. Xia, G.S. Kottas, M.S. Weaver, R. Kwong, R. Ma, M. Hack, J.J. Brown, SID Symp. Digest. 41 (2010) p786. [3] K. Yamashita, T. Mori, T. Mizutani, J. Phys. D. Appl. Phys. 34 (2001) 740. [4] Y. Hu, V. Topolkaraev, A. Hiltner, E. Baer, J. Appl. Polym. Sci. 81 (2001) 1624. [5] Y.J. Kim, Y.H. Son, J.H. Kwon, J. Inform. Display 14 (2013) 109. [6] B. Comiskey, J.D. Albert, H. Yoshizawa, J. Jacobson, Nature 394 (1998) 253. [7] E.H. Lueder, Proc. SPIE 3297 (1998) 64. [8] J.I. Lee, H.Y. Chu, H. Lee, J. Oh, L.M. Do, T. Zyung, J. Lee, H.K. Shim, ETRI J. 27 (2005) 181. [9] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [10] M. Gross, D.C. Muller, H.G. Nothofer, U. Scherf, D. Neher, C. Brauchle, K. Meerholz, Nature 405 (2000) 661. [11] A. Feng, B.J. McCoy, Z.A. Munir, D.E. Cagliostro, J. Colloid Interface Sci. 180 (1996) 276. [12] M.S. Weaver, L.A. Michalski, K. Rajan, M.A. Rothman, J.A. Silvernail, J.J. Brown, P.E. Burrows, G.L. Graff, M.E. Gross, P.M. Martin, M. Hall, E. Mast, C. Bonham, W. Bennett, M. Zumhoff, Appl. Phys. Lett. 81 (2002) 2929. [13] A.G. Erlat, B.M. Henry, J.J. Ingram, C.R.M. Grovenor, G.A.D. Briggs, R.J. Chater, Y. Tsukahara, J. Phys. Chem. B 883 (2004) 108. [14] C.S. Deng, H.E. Assender, F. Dinelli, O.V. Kolosov, G.A.D. Briggs, T. Miyamoto, Y. Tsukahara, J. Polym. Sci. B: Polym. Phys. 38 (2000) 3151. [15] P.E. Burrows, G.L. Graff, M.E. Gross, P.M. Martin, M.K. Shi, M. Hall, E. Mast, C. Bonham, W. Bennet, M.B. Sullivan, Displays 22 (2001) 65. [16] C.S. Oh, H.J. Lee, E.J. Lee, L.S. Park, Mol. Cryst. Liquid Cryst. 551 (2011) 78. [17] W.H. Koo, S.M. Jeong, S.H. Choi, H.K. Baik, S.M. Lee, S.J. Lee, J. Phys. Chem. B 108 (2004) 18884. [18] V. Dimitrov, T. Komatsu, J. Sol. State Chem. 163 (2003) 100. [19] S. Takeda, M. Fukawa, Y. Hayashi, K. Matsumoto, Thin Solid Films 339 (1999) 220. [20] S. Jeon, H. Hwang, J. Appl. Phys. 93 (2003) 6393. [21] A.P. Roberts, B.M. Henry, A.P. Sutton, C.R.M. Grovenor, G.A.D. Briggs, T. Miyamoto, M. Kano, Y. Tsukahara, M. Yanaka, J. Membr. Sci. 208 (2002) 75.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Flexible OLED encapsulated with gas barrier film and adhesive gasket A Ra Cho a , Eun Hye Kim b , Soo Young Park a , Lee Soon Park c,∗ a
School of Applied Chemical Engineering, Major in Polymer Science and Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea Department of Sensor and Display Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea c School of Material Science and Engineering, Ulsan National Institute of Science and Technology, 689-798, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 25 September 2013 Received in revised form 21 March 2014 Accepted 26 March 2014 Available online 6 May 2014 Keywords: Flexible OLED Encapsulation of OLED Gas barrier layer Life time of OLED
a b s t r a c t Flexible OLEDs have been studied actively for their potential application to both large size OLEDs display and general lighting. The efficiency and life time of flexible OLEDs based on film substrate are still major problem to overcome compared to the OLEDs devices based on glass substrate. The short life time of flexible OLEDs are attributed to the increased penetration of oxygen and water vapor to the thin organic layer of OLEDs device through the flexible polymer film substrate. In this work, we have used polyethylene telephthalate (PEN) film as flexible substrate for OLEDs. The AlNx /UVR (UV curable resin)/AlNx type gas barrier layers were formed on top of PEN film, followed by OLEDs panel fabrication. After deposition of Al cathode, the upper part of flexible OLEDs panel was encapsulated by laminating the same PEN film with the aid of optically clear adhesive (OCA) in the form of gasket. The completed flexible OLEDs were examined through the mechanical bending and life time tests for comparison to the rigid OLEDs based on glass substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Organic light emitting diodes (OLEDs) displays based on glass substrate have been used in mobile devices such as smart phones, smart pads and recently OLEDs TVs which have been introduced in the market. Due to their simple, organic thin-film design, OLED can also be easily built on not only glass substrates but also flexible substrates [1–5]. However, on film substrates, the short life time and low efficiency of the flexible OLEDs are still the major problem to be solved [6,7]. The short life time of flexible OLEDs is attributed to the increased penetration of oxygen and water vapor to the thin organic layers of OLEDs devices through the substrate film and the seal layer between the flexible substrate and upper encapsulated film [8]. Therefore the formation of inorganic/organic multiple gas barrier layers on the flexible substrate film and the choice of side seal material are important issue for the flexible OLEDs [9]. We used poly(ethylene naphthalate) (PEN) film as flexible substrate since it had a balanced property such as high transmittance, high thermal and mechanical property [10]. However, it is not suitable for the flexible substrates of OLEDs flexible substrates because it has higher penetration of oxygen and water vapor (about
∗ Corresponding author at: School of Material Science and Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea. Tel.: +82 52 217 2349. E-mail address: [email protected] (L.S. Park). http://dx.doi.org/10.1016/j.synthmet.2014.03.027 0379-6779/© 2014 Elsevier B.V. All rights reserved.
1.64 g/m2 day) than glass (about 10−6 g/m2 day). So the inorganic and organic multiple gas barrier layers are required on the PEN substrate films [11]. In our work, these films were used both as the flexible substrate for the OLEDs devices and as the encapsulation film for covering upper aluminum cathode for the long term flexible property [12–14]. The encapsulation of OLEDs device was conducted by lamination process utilizing optically clear adhesive (OCA) in the form of gasket [15]. The use of optically clear adhesive (OCA) as gasket could simplify the encapsulation process while securing the gas barrier property in the sealing section. The materials and process of the flexible encapsulation were examined from the viewpoint of increasing the life time of flexible OLEDs.
2. Experimental 2.1. Fabrication of flexible OLEDs In order to make flexible OLEDs which had PEN film with gas barrier layers both as flexible substrate and encapsulation film, the gas barrier layers were deposited on PEN film utilizing roll-to-roll sputter. The structure of the PEN film with gas barrier layers was PEN (110 m)/AlNx (25 nm)/UVR (1.2 m)/AlNx (25 nm) which was optimized in our previous work [16]. The aluminum nitride (AlNx ) thin film was deposited by reactive sputtering with Al target and controlled by flow of N2 gas as shown in Table 1. The UVR layer was obtained by spin coating and then exposed to UV light. The spin
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Table 1 Deposition condition of AlNx gas barrier thin film by using roll-to-roll sputter. Base pressure
l × l0−5 [torr]
Working pressure Power Winding speed Gas flow rate = N2 /Ar + N2 Target
3 [mtorr] DC plus 600 [W] 0.05 [m/mim] 0–75 [%] Al (99.99%) (250 mm × 100 mm × 6 mm)
Table 2 Deposition condition of ITO/Ag/ITO transparent anode thin film by using roll-to-roll sputter. Target
ITO
Ag
Composition Power Winding speed Gas flow rate
90:10 DC pulse 600[W] 0.3 [m/min] Ar:O2 = 50:1.5 [sccm]
Ag 99.99% DC 600 [W] 0.9 [m/min] Ar = 50 [sccm]
2.2. Film encapsulation of flexible OLEDs and measurement The encapsulation of flexible OLEDs device was carried out by using the same film, PEN (110 m)/AlNx (25 nm)/UVR (1.2 m)/AlNx (25 nm), which was used as substrate of flexible OLEDs, in order to reduce the stress difference under bending motions. The encapsulation film was fixed on the rectangular edge of flexible OLEDs substrate utilizing OCA used in the touch panel fabrication. The light transmittance of the PEN film with gas barrier layers was measured by UV–vis spectrophotometer (CM-3600d, Konica Minolta Co., Japan). The water vapor transmission rate (WVTR) of the gas barrier films was measured by Mocon (Permatran-WR 3/33MA) equipment under the condition of 37.8 ◦ C, RH 100%, and N2 gas rate of 10 ± 0.2 sccm for 24 h [17,18]. The bending test of PEN film with gas barrier layers was conducted with the bending machine (Zeetech, ZBT-200, Korea). The gas barrier film size was 100 mm × 100 mm and the test conditions were 10,000 cycles under 100 mm/s speed in the range of 40–80 mm. 3. Results and discussion
coated UVR is NLR06 which is obtained from Nanopolychem Co., Korea. After forming gas barrier layers on PEN film, the ITO (20 nm)/Ag (17 nm)/ITO (20 nm) multilayer anode was also deposited by rollto-roll sputter under the condition shown in Table 2. The patterning of ITO/Ag/ITO anode was conducted by photolithographic process utilizing a positive-PR (DS-i1000) and an etchant solution (DA-300) obtained from Dongjin Chemical Co., Korea. After patterning of ITO/Ag/ITO anode, the insulator layer was patterned by using polyimide-PR supplied by Dongjin Chemical Co. The structure of the 4-pixel back-plane of flexible OLEDs is shown in Fig. 1. The fabrication of flexible OLEDs on patterned 4-pixel flexible back-plane was carried out with SUNIC EL Plus 200, a cluster type OLEDs panel fabrication system. The flexible OLEDs device on patterned flexible back-plane without film encapsulation had ˚ ˚ (Alq3 ;C545T, 400 A)/ETL configurations of HTL (NPB, 1000 A)/EML ˚ ˚ ˚ (Alq3 , 250 A)/EIL (LiF, 10 A)/Cathode (Al, 1000 A).
3.1. Property of flexible substrates The performance of flexible OLEDs is quite dependent on the materials, especially on the quality of the flexible substrate and encapsulation film [19,20]. The important requirements of the flexible substrate for OLEDs include high visible light transmittance, low water vapor transmittance rate (WVTR) and low sheet resistance of transparent conducting anode thin film (ITO/Ag/ITO) [21]. We used the PEN (100 m)/AlNx (25 nm)/UVR (1.2 m)/AlNx (25 nm) film as encapsulation film after OLEDs panel fabrication. The properties of PEN based films after gas barrier layer and ITO/Ag/ITO anode formation are shown in Table 3. The PEN/AlNx /UVR/AlNx film after gas barrier formation exhibited 86.8% visible light transmittance and low WVTR value (0.008 g/m2 day before bending and 0.02 g/m2 day after bending test). The WVTR of bare PEN, PEN/AlNx (before bending), PEN/AlNx (after bending), PEN/UVR, PEN/AlNx /UVR(before bending) and
Fig. 1. The structure of 4 pixel back plane for flexible OLED (a) patterned ITO layer, (b) patterned ITO and insulator layer, (c) working flexible OLED device and (d) the cross-sectional structure patterned ITO and insulator layer.
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Table 3 The properties of PEN films with gas barrier layer and ITO/Ag/ITO deposition. Film structure Transmittance WVTR
PEN/AlNx /UVR/AlNx 86% Before bending After bending
0.008 g/m2 day 0.02 g/m2 day
Film structure Transmittance Sheet resistance
PEN/AlNx/UVR/AINx/ITO/Ag/ITO 82.1% Before bending After bending
8.3/sq 7.8/sq
Fig. 3. The importance of various film encapsulations on flexible OLEDs.
3.2. Performance of flexible OLEDs
Fig. 2. The structure of flexible OLEDs after film encapsulation.
PEN/AlNx /UVR (after bending) were 1.6356, 0.1937, 0.1953, 0.0395, 0.0395 and 0.0485 g/m2 day, respectively. But the combination of organic and inorganic thin layers such as PEN/AlNx /UVR/AlNx exhibited an excellent result of WVTR, 0.008 g/m2 day. On the other hand, multilayer has lower transmittance than just glass. It is also important to find optimum conditions between transmittance and WVTR. After ITO/Ag/ITO thin film deposition on outer PEN/AlNx /UVR/AlNx layer, the sheet resistance values were 8.3 /sq. at room temperature and 7.8 /sq. after heat treatment at 100 ◦ C for 30 min, which was as good as the ITO glass used for OLEDs panel fabrication. The transmittance to visible light was slightly decreased to 82.1% while the WVTR values were almost same as the one before ITO/Ag/ITO thin film deposition. After confirmation of basic properties of PEN films, the flexible OLEDs devices were fabricated through the processes of ITO/Ag/ITO anode patterning, polyimide-PR insulator patterning, organic layers and cathode deposition followed by film encapsulation. The structure of the final flexible OLEDs device is shown in Fig. 2.
The importance of various film encapsulations on the life time tests of flexible OLEDs is shown in Fig. 3. The life time tests were conducted under the condition of 85% relative humidity and 85 ◦ C temperature. The OLEDs device encapsulated with the same flexible substrate, PEN/AlNx /UVR/AlNx film, showed 91% luminance after 750 h compared to OLEDs made on ITO glass substrate and standard glass encapsulation, while the OLEDs encapsulated with PEN/AlNx film exhibited 86% luminance after 750 h. The flexible OLEDs device encapsulated with PEN base film and the OLEDs device without film encapsulation, i.e. Al cathode open to the air, exhibited 50% luminance (corresponding to life time) after 360 h and 320 h, respectively. Therefore from these static tests it was noted that the deposition of multiple gas barrier layer is very important for the long life time of flexible OLEDs. The dynamic test of flexible OLEDs was conducted with the bending machine shown in Fig. 4 at room temperature. Before bending, the flexible OLEDs device exhibited both high current efficiency (6.3 cd/A) and external efficiency (2.8%) as shown in Fig. 5. The flexible OLEDs encapsulated with PEN/AlNx /UVR/AlNx film could withstand bending motion of 10,000 times, however, the OLEDs without any film encapsulation (open cathode) did not work after bending 10,000 times. The differences of OLEDs with PEN/AlNx /UVR/AlNx film encapsulation and without any encapsulation after bending 5000 times are shown in Fig. 5 with green and white upside triangle, respectively. It was of interest to note that the flexible OLEDs without any encapsulation could maintain emission up to 5000 times bending. In case of life time test of OLED under 85% RH and 85 ◦ C, the OLEDs device without any encapsulation exhibited life time of only 320 h as shown in Fig. 3. These data suggest that flexible OLED had
Fig. 4. Images of bending machine (a) the maximum width and (b) the minimum width.
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Fig. 5. The performance of flexible OLEDs (a) current efficiency and (b) external quantum efficiency.
high potential in flexible display application, if they were properly encapsulated. 4. Conclusions The performance of flexible OLED is quite dependent on the materials compared to the glass-based OLED, especially on the quality of the flexible substrate and encapsulation film. We used PEN films with same gas barrier layers for both as substrate of flexible OLED and as encapsulation film after OLED panel fabrication in order to reduce the stress difference under bending motion. The flexible OLED encapsulated with PEN/AlNx/UVR/AlNx film could withstand bending motion of 10,000 times, however, the OLED without any film encapsulation (open cathode) did not work after bending 10,000 times. It was also noted that the flexible OLED device without any encapsulation could maintain emission up to 5000 times bending, while the same OLED exhibited life time of only 320 h under the condition of 85% RH and 85 ◦ C temperature. These data suggest that flexible OLEDs had high potential in flexible display application if they were properly encapsulated. Acknowledgement This work was supported by the industrial strategic technology development program [10035225, Development of core technology for high performance AMOLED on plastic] funded by MKE/KEIT, Korea. References [1] T.K. Hatwar, J.P. Spindler, M. Kondakova, D. Giesen, J. Deaton, J.R. Vargas, SID Symp. Digest. 41 (2010) 778.
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