JOURNAL OF DISPLAY TECHNOLOGY, VOL. 5, NO. 12, DECEMBER

JOURNAL OF DISPLAY TECHNOLOGY, VOL. 5, NO. 12, DECEMBER 2009 541 Dual-Plate OLED Display (DOD) Embedded With White OLED Chang-Wook Han, Hwa Kyung Kim, Hee Suk Pang, Sung-Hoon Pieh, Chang Je Sung, Hong Seok Choi, Woo-Chan Kim, Myung-Seop Kim, and Yoon-Heung Tak Abstract White organic light-emitting diode (WOLED) with color filter adopting dual-plate OLED display (DOD) structure is proposed. In order to prevent outgassing from color filter and overcoat, the SiN passivation film was deposited on the overcoat film. This structure does not show any defects after it has been kept over 500 hours of storage tests at 90 C. By fabricating 1 stacked WOLED consisting of fluorescent blue layer/ phosphorescent red:green layer, luminance efficiency of 20 cd/a with CIEx =029, CIEy =037 was achieved. Index Terms Barrier layer, color filter, dual-plate organic lightemitting diode (OLED) display (DOD), white OLED (WOLED). I. INTRODUCTION I T IS WELL KNOWN that fine-metal mask (FMM) method is not suitable for full color active-matrix organic lightemitting diode (AMOLED) display prepared on a large-sized substrate, due to its rather inaccurate precision in alignment of the mask with the substrate. Also, dark spots, which are caused as the glass substrate is in contact with FMM, are considered as serious problem. Considering these problems, white organic light-emitting diode (WOLED) is expected to become a strong candidate for AMOLED display adopting larger than 5th Generation substrate, owing to their advantages of high resolution, low cost and FMM-less method [1] [4]. In general, when WOLED and color filter (CF) are embedded into AMOLED panel, two types of panel structure, such as, bottom and top emitting structure may be considered, depending on emitting direction. In the bottom emitting structure, WOLED, CF and thin-film transistor (TFT) are fabricated on the same substrate. This structure has some drawbacks in respect to fabrication process, aperture ratio and CF-outgassing. On the other hand, top emitting structure has a serious problem in electrical resistance of semitransparent cathode to get uniform brightness. Moreover, due to the unwanted microcavity effect between a reflective lower electrode and a semi-reflective upper electrode, there is limitation to emitting the visible light in the wide wavelength range [5], [6]. Manuscript received February 16, 2009; revised May 05, 2009. Current version published November 18, 2009. The authors are with the OLED Technology Development Division, LG Display Company, Ltd., Paju-si Gyeonggi-do, 413-811, Korea (e-mail: hcw@lgdisplay.com; hinter@lgdisplay.com; eldlab@lgdisplay.com; shpieh@lgdisplay.com; cjsung@lgdisplay.com; redchoi@lgdisplay.com; wckim@lgdisplay.com; gokms@lgdisplay.com; yhtak@lgdisplay.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JDT.2009.2024009 Fig. 1. Cross section of DOD after encapsulation. The DOD structure consists of two plates, i.e., the bottom plate involving TFT and the top plate involving OLED. We already published dual-plate OLED display (DOD) as an alternative top emitting structure [7]. In DOD structure, OLED and TFT backplane are fabricated independently as shown in Fig. 1, so that we can utilize the whole area of upper plate for OLED and lower plate for TFT arrays. Thus, high aspect ratio in upper OLED plate is obtained as much as the top emission structure in favor of long lifetime and we can employ a-si TFT backplane which is more competitive than LTPS at a point of cost. And, it can show a uniform brightness over the entire panel with large size, because low electrical resistance metal can be patterned beneath anode.[8] Moreover, the structure of WOLED embedded in DOD is completely the same as that of the conventional bottom emitting WOLED, except the emitting direction. Therefore, such a microcavity effect in conventional top emitting device employing thin metal upper electrode can be avoided in DOD structure. In this paper, DOD structure embedded with WOLED and CF is proposed for the large-size AMOLED display. We also investigate silicon nitride SiN passivation film inserted between CF and anode to prevent formation and growth of dark spot. II. FABRICATION OF DOD EMBEDDED WITH CF The DOD structure consists of two plates, i.e., TFT on bottom plate and WOLED and CF on top plate which are connected by contact spacer as illustrated in Fig. 1. Both substrates are separately made and encapsulated afterwards. This way provides the higher yield than the conventional method since it is possible to screen bad substrates separately. The conventional manufacturing flow of AMOLED display using WOLED is that TFT components are made first on the glass substrate and then 1551-319X/$26.00 2009 IEEE

542 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 5, NO. 12, DECEMBER 2009 Fig. 2. Fabrication process flow of the proposed DOD structure employing CF. WOLED and CF components are made on top of the TFT substrate. Therefore, if just one part of two components fails, it will directly affect the total yield. By fabricating each substrate separately, we can choose only good substrates from each set of substrates, which results in increasing yields. To evaluate white OLED and CF in the DOD structure, we fabricated DOD panel whose diagonal size is 2 inch and resolution is 90 RGBW 90. Pixel size of the DOD panel is equivalent to that of 32-in full HD AMOLED. The a-si TFT was fabricated on a glass substrate and it was made by using a conventional five-photomask process. For RGBW pixel formation, schematics of the fabrication process are illustrated in Fig. 2. Black matrix (BM) layer was fabricated on a glass substrate by photolithography. CF resin was prepared in an aqueous solution and then coated on the glass substrate followed by prebaking at 90 C for 90 s. It was patterned through exposure at 60 mj/cm2 followed by post-baking at 230 C for 30 min for each color. In order to achieve good flatness in white sub-pixel, the transparent negative photoresist (PR) was patterned, instead of colored resin. (Fig. 2(a)) Overcoat (OC) was coated to planarize color filter layer. After baking at 230 C for 60 min to remove the remaining moisture of the OC film, the SiN passivation layer was deposited on the OC film by plasma-enhanced chemical vapor deposition (PE-CVD) at 230 C temperature (Fig. 2(b)). The auxiliary electrode of molybdenum (Mo) with 200-nm-thick was formed in shape of mesh network. This network decreases the electrical resistance of the anode and enhances the uniformity of the brightness of panel. 150-nm-thick anode of ITO was prepared by sputtering on the Mo auxiliary electrode. And then, SiN thin film was deposited at 230 C and patterned by photolithography to separate adjacent sub-pixels (Fig. 2(c)). 3.2 m-thick contact spacer was made from positive PR which was spin-coated and photo-patterned. Lastly, 2- m-thick separator layer which isolates cathode layer into each sub-pixels was made from negative PR by spin-coating and photolithography [Fig. 2(d)]. Fig. 3 shows a scanning electron microscopy (SEM) image of the pixels. Cathode layer was deposited on contact spacer connected to the contact metal of driving TFT and the cathode Fig. 3. SEM images of: (a) separator and (b) contact spacer after detaching the top plate form the panel. The thickness of this separator should be lower than that of contact spacer. Lower taper angle of the separator is preferred to define the cathode accurately. layer of the individual sub-pixel was successfully disconnected by the separator. The contact resistance between OLED cathode and the TFT contact metal was measured around 10 k. This value is somewhat higher, compared to the contact resistance between OLED anode and TFT contact metal in conventional bottom emission structure. But in region of real driving condition, voltage drop due to the contact resistance is just below 10 mv, which is sufficiently low regarding the voltage applied across TFT and OLED. III. CHARACTERISTICS OF WOLED The organic layers of WOLED were prepared in the following structures: anode, hole-injection layer (HIL), hole-transport layers (HTL), fluorescent blue emitting layer (B EML), interlayer, phosphorescent green and red emitting mixed layer (G R EML), electron-transport layer (ETL) and electron-injection layer (EIL) next to the cathode metal electrode. All of organic and metal layers on the top of the anode surface were deposited by a thermal evaporation method under a vacuum at approximately 1 10 torr. In this structure, the red and green mixed layer was co-deposited with 1 host and 2 phosphorescent dopants where doping ratios of red and green dopants are 0.4 % and 10 %, respectively. The interlayer between B EML and G R EML consists of -tris(carbazol-9-yl)-triphenylamine (TCTA) and the host material of G R EML which we will call PH. Mixing ratio of TCTA and PH is 3:1. TCTA plays the role of blocking not only

HAN et al.: DUAL-PLATE OLED DISPLAY EMBEDDED WITH WOLED 543 Fig. 5. Photographs of the emitting areas for two kinds of DOD panels with CF/OC/SiNx passivation film: (a) quad-type and (b) stripe-type. Fig. 4. Electroluminescent spectra of WOLED with red doping ratios of 0.2% (dashed line) and 0.4% (solid line), in which thickness of HIL is 500 A. (inset: device structure of WOLED). electron transfer but also triplet-exciton from the phosphorescent emitting layer (G R EML) to the fluorescent emitting layer (B EML), due to its high LUMO level and triplet energy level. In case that the interlayer consists of TCTA only, the electron blocking property is so strong that electron hole recombination occurs mostly at G R EML and WOLED emits weak blue light. As mixing PH into the interlayer, electrons are transferred from G R EML to B EML so that electron hole recombination occurs at B EML as well. It is found that our WOLED shows optimal white color at the mixing ratio of 3:1. Before fabricating entire WOLED, we studied to find the optimum concentration of the dopants in G R EML for improving efficiency and color tunability. When green and red dopants are 10% and 0.4%, respectively, we found that the WOLED shows the best result. Although amount of red dopant is quite small, Fig. 4 shows that intensity of red region in white spectrum is considerable. These phenomena can be explained by the exciton energy transfer from green dopant to red dopant. In other words, green dopant acts as the host of red dopant. It was confirmed by the fact that intensity of red region is enhanced as the green dopant increases [9]. Fig. 4 displays the electroluminescent spectra of the WOLED of red dopant with 0.2% and 0.4% respectively, with green dopant kept at 10% and 500 -thick HTL in both devices. As amount of red dopant is raised, intensity of red region is increased, but that of green is decreased. It means that more exciton energy transfer occurs from green dopant to red dopant in G R EML. The WOLED with 0.4% red doping ratio shows efficiency of 18.7 cd/a and CIEx, CIEy at 3.9 ma cm. For 0.2% red doping ratio, the efficiency is 20 cd/a with CIEx, CIEy at the same current density. IV. 2-IN DOD PANEL EMBEDDED WITH WOLED AND CF A. Optimization of WOLED in DOD Panel Previously mentioned, we fabricated the 2-in panel of which pixel size is equivalent to that of 32-in FHD panel, to evaluate WOLED and CF in the DOD structure. Two kinds of pixel configuration were investigated, i.e., (a) quad-type and (b) stripetype, as shown in Fig. 5. We obtained the high aperture ratio up Fig. 6. Electroluminescent spectra of WOLED in the DOD panel with HTL thicknesses of 500 A (solid line) and 800 A (dashed line), in which red doping ration is 0.4%. (inset: device structure of WOLED in the DOD panel). to 67% for quad-type and 65% for stripe-type which is one of the powerful advantages of DOD structure. Fig. 6 shows the electroluminescent (EL) spectra of the white sub-pixel where emitted light goes through OC/SiN barrier layer illustrated in the inset. The thickness of SiN layer is 3000. WOLED, which the EL spectrum denoted with solid line in Fig. 6 belongs to, has the completely same structure with WOLED whose EL spectrum is represented with solid line in Fig. 4, except of OC/SiN below ITO anode. However, the white spectrum in Fig. 6 is utterly different from that in Fig. 4, i.e. main peak of green is close to the blue. Interestingly, as thickness of HTL is changed from 500 to 800, it is found that main peak of green is shifted toward longer wavelength and intensity is increased as shown in Fig. 6. So, three peaks of red, green and blue of white EL spectrum become more distinctive and CIE coordinates of EL spectrum of WOLED changed (0.29, 0.36) into (0.30, 0.38), which are more desirable for an application of display. Additionally, color gamut is increased by 8.4% from 61.4% to 70% in DOD panel with CF. Influences of insertion OC/SiN layer and increase of HTL thickness could be explained by optical interference effect. [10] To make sure the assumption, simulated emittance spectrum was calculated by an optical simulation program. The simulated emittance curves are determined by optical constant (n, k) and thickness of each layers consisting in WOLED and OC/SiN, but is irrespective of photoluminescence (PL) of emitting layers. Expected EL spectra can be obtained by multiplying PL of emitting layer in WOLED with the simulated emittance [11].

544 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 5, NO. 12, DECEMBER 2009 In case of reliability test on DOD panel without the SiNx barrier layer, we failed to obtain consistent results. Some DOD panel without SiNx layer showed as good reliability as DOD panel with SiN layer, but some DOD panel showed several defects such as non-emission region at edge of pixel. On the contrary, when SiN layer was fabricated over the overcoat layer in DOD panel, consistent result could be obtained. It is concluded that the barrier function to prevent outgassing coming from the color filter was enhanced by the SiN layer combined with the overcoat layer. V. CONCLUSION Fig. 7. Simulated emittance spectra with respect to HTL thickness; 500 A (solid line) and 800 A (dashed line). The key to the success of AMOLED in the application of large sized display is to fabricate AMOLED at lower cost of manufacturing. In order to achieve this goal, many efforts are going on from the industrial and academic sectors. We expect that the DOD structure embedded with WOLED and CF will be suitable for the large-sized AMOLED display with low cost. REFERENCES Fig. 8. Photographs of the emitting areas in a DOD panel which was stored at 90 C during the indicated time. The simulated emittance spectrum acts on modifying/modulating the emission intensity of white EL spectrum. Fig. 7 reveals emittance spectra calculated by our optical simulation program. As the HTL thickness is increased from 500 to 800, the peak position of green region moves closer to the peak wavelength of the original PL of green dopant (about 540 nm). This result well explains the shift of green peak of white EL spectrum as shown in Fig. 6. B. Reliability Test of 2-in DOD Panel High temperature reliability test of the 2-in DOD panels was carried out to examine if SiNx barrier layer is effective to prevent outgassing from CF and OC. The panels were stored in convection oven maintaining temperature at 90 C. By every 50 hours, they were observed with microscope. As shown in Fig. 8(a)-(d), there is no change in emitting area after over 500 hours of keeping at high temperature. [1] C. C. Chu, J. K. Ha, and K. H. Cjung, Advanced and issues in white OLED and color filter architecture, in Proc. Soc. Inf. Display, 2007, p. 1118. [2] S.-J. Su, E. Gonmori, J. Sasabe, and J. Kido, Highly efficient organic blue-and white-light emitting devices having a carier-and exciton-confining structure for reduced efficiency roll-off 2008, p. 4189. [3] T.-W. Lee, T.Y. Noh, and D.W. Shin, High-efficiency stacked white organic light-emitting diodes, Appl. Phys. Lett., vol. 92, p. 5071, 2003. [4] Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R. Forrest, Management of singlet and triplet excitons for efficient white organic light-emitting devices, Nature Letters, p. 908, 2006. [5] Reason behind the new structure used in Sony s organic EL panel, Flat Panel Display, p. 128, 2005. [6] S. Y. Kim, M. G. Kim, S. H. Lee, and J. M. Kim, 3.0-in 308-ppi WVGA AMOLED by Top-emitting white OLED white color filter, in Proc. Soc. Inf. Display, 2008, p. 937. [7] C.-W. Han, Y.-H. Tak, I. B. Kang, B.-C. Ahn, and I. J. Cjung, Dualplate OLED display (DOD) based on amorphous-silicon(a-si) TFT backplane, J. SID, p. 101, 2009. [8] C. C. Wu, S. D. Theiuss, G. Gu, M. H. Lu, J. C. Sturm, S. Wagner, and S. R. Forrest,, IEEE Electron Device Lett., vol. 18, no., pp. 609 612, 1997. [9] S.-H. Pieh, H. S. Choi, and Y. H. Tak, Two-stacked white organic light-emitting diodes consisting of fluorescent and phosphorescent hybrid structure with high efficiency and good color characteristics, in Proc. Soc. Inf. Display, 2009. [10] J. W. Lee, N. Chopra, and F. So, Cavity effects on light extraction in organic light emitting devices, Appl. Phys. Lett., vol. 92, p. 033303, 2008. [11] H. K. Kim, H. S. Choi, D. H. You, and J. W. Yang, Method to enhance color gamut up to 89% in bottom emission active-matrix organic light emitting device, in Proc. Int. Meeting on Inf. Display, 2007, p. 43. Chang-Wook Han received the B.S. and M.S. degrees in material science from Seoul National University, Seoul, Korea, in 1987 and 1989, respectively, and the Ph.D. degree in electrical engineering from Seoul National University in 2007, researching a-si TFTs and the pixel structure of AMOLEDs on a flexible metal substrate. OLED Business, Paju, Korea. Since joining LG Display in 1990, he has worked on the device and process development of TFT backplanes for AMLCDs and also on the TFT backplane, circuit, and panel structure of AMOLEDs. Now, he is responsible for research on AMOLEDs as a Team Leader at the OLED R&D Department, OLED Business, LG Display.

HAN et al.: DUAL-PLATE OLED DISPLAY EMBEDDED WITH WOLED 545 Hwa-Kyung Kim received the B.S. and M.S. degrees in material science engineering from Hanyang University and Pohang University for Science and Technology (POSTECH), respectively. She is a Research Engineer at LG Display, OLED Business, Paju, Korea. Since joining LG Electronics Company in 2004, she has worked on the device development for AMOLED. Now, she is responsible for research on WOLED and CF as research engineer at OLED Technology Development Team 1, OLED Business, LG Display. Hong Seok Choi received the B.S., M.S., and Ph.D. degrees in physics from Seoul National University, Korea, in 1992, 1994, and 1999, respectively. His Ph.D. work was on optical properties of transition metal oxides. OLED Business, Paju, Korea.In 2001, he joined LG Electronics Company and worked on technical development of OLED. In 2008, he transferred to LG Display and has researched OLED. Heesuk Pang received the M.S. degree in physics from Sungkyunkwan University. Seoul, Korea, in which year?? He is an Senior Research Engineer at the LG Display, OLED Business, Paju, Korea. He is responsible for the research and development of AMOLED displays. He joined LG Display in 2000, he has worked on process development of TFT back plane for AMOLED as a process engineer. Now He is developing the new structure and process for large size AMOLED. Woo Chan Kim e received the B.S. degrees in metallurgical engineering from Korea University, Korea, in 1994, and the M.S. and Ph.D. degrees in material science from KAIST, Korea, in 1996 and 2002, respectively. OLED Business, Paju, Korea. Since joining LG Display in 2002, he has worked on the process development and also on panel structure of AMOLED displays. Sung-Hoon Pieh received the B.S. and M.S. degrees in electrical engineering from Soong-Sil and Korea University, respectively. He is a junior Research Engineer at LG Display, OLED Business, Paju, Korea. His M.S. work was on fabrication of Nano Device using selective patterning with e-beam Lithography, LCD projection Lithography. Since joining LG Electronics Co. in 2005, he has worked on the device development for AMOLED. Now, he is responsible for research on WOLED as research engineer at OLED Technology Development Team 1, LG Display. Chang Je Sung is Research Engineer at LG Display, OLED Business, Paju, Korea. received the B.S. and M.S. degrees in Physics from Yonsei University. His work was on carrier injection and transport in OLEDs. In 2004, he joined the LG Electronics Company and since then he worked on the organic device technology and process engineering of AMOLED display. Myung-Seop Kim received the M.S. degrees in Physics and the Ph.D. in Applied Physics from Yonsei University, Korea, in 1997 and 2007, respectively. OLED Business, Paju, Korea. Since joining LG Electronics Co. in 1997, he has worked on the development of AMOLED device. Now, he is researching on the degradation mechanism of OLED device and is responsible for process technology of AMOLED displays at OLED Business, LG Display. Yoon-Heung Tak received the B.S. degree in chemistry and M.S. degree and his Ph.D. in physical chemistry from Philips Marburg University, Germany, in 1991, 1994, and 1997, respectively. His Ph.D. work was on the injection, transport, and recombination of charge carriers in OLEDs. He is a Vice President at LG Display, OLED Business, Gumi, Korea. He is responsible for the process and production department of AMOLED displays. In 1997, he joined the LG Electronics Co. and since then he has worked on the technology, process, and product development of AMOLED displays. He is currently the Head of the OLED Process & Production Department, OLED Business, LG Display.