Voltage Distribution of Power Source in Large AMOLED Displays

Journal of the Korean Physical Society, Vol. 48, January 2006, pp. S5 S9 Voltage Distribution of Power Source in Large AMOLED Displays Myoung-Hoon Jung and Ohyun Kim Electronic and Electrical Engineering Division, Pohang University of Science and Technology, Gyeongbuk 790-784 Hoon-Ju Chung School of Electronic Engineering, Kumoh National Institute of Technology, Gyeongbuk 730-701 (Received 12 October 2005) The voltage distribution of power lines in large Active Matrix Organic Light-Emitting Diode (AMOLED) displays was investigated as a function of panel size, OLED material, power line material and VDD structure in order to improve global brightness uniformity. In addition, the power source voltage distribution was investigated to determine the effect on crosstalk. These results indicate that new OLED materials with high efficiency and VDD lines made of a low-resistance material are essential for AMOLED panels larger than 20 diagonally. Crosstalk was found to be due mainly to the abrupt voltage change of the power source on panels. A new mesh VDD structure is suggested as a solution to this problem. PACS numbers: 42.79.Kr, 85.60.Pg Keywords: AMOLED, Voltage distribution, Crosstalk, Power lines, Mesh I. INTRODUCTION II. SIMULATION AMOLED displays have been studied for decades, due For the simulation, the luminance efficiency of each to their wide viewing angle, fast response time, low cost OLED device was investigated. Based on this, current requirements of sub-pixels were calculated. Table and thinness [1, 2]. They are already being adopted as displays for digital still cameras [3] and PDAs. Recently, 1 shows the luminance efficiencies and the CIE color coordinates for red, green and blue materials for the simula- many research groups have reported on AMOLED displays larger than 13 inches [4 6]. This suggests that tion [9]. PHOLEDs are composed of phosphorescent red AMOLED displays could become a leading TV display and green materials with high quantum efficiency and a device. However, many problems must be solved for that fluorescent blue material for white color balance. Table application. For TFT back planes, the technical issues 2 shows the calculated maximum sub-pixel currents for are non-uniform characteristics of LTPS TFTs and V T H 500-nit full white brightness. A circular polarizer with 50 instability and low mobility of a-si TFTs [7]. In terms of % efficiency is assumed, to calculate the maximum subpixel currents. The simplified equivalent circuit models OLED materials, there exist some problems in efficiency and lifetime of the OLED [3, 8]. In addition, there are shown in Figure 1 were used to simulate the VDD distribution for the conventional horizontal VDD structure. technical challenges in large-area color patterning technology, pixel-driving methods and VDD drop in power They are composed of discrete resistors and ideal current lines. In particular, as the panel size increases, the IR sources. Figure 1(a) shows an equivalent circuit model of drop of power sources on the panel causes image degradations such as non-uniform brightness and crosstalk. VDD is supplied only from the left side of the panel. Fig- a single bank horizontal (SBH) VDD structure in which Therefore, the voltage distribution of the power source ure 1(b) shows an equivalent circuit model of a double in display panels is a very important factor. bank horizontal (DBH) VDD structure in which VDD is In this paper, the voltage distribution of power lines supplied from both sides of the panel. In general, the in AMOLED panels is investigated for conventional horizontal VDD structures. Panel sizes, OLED materials, the DBH VDD structure is suitable for large-size panels. SBH VDD structure is suitable for small-size panels and power line materials, and VDD structures are varied in The symbols shown in Figure 1 are defined as follows: order to identify any trends. A new mesh VDD structure R P = the resistance of the power line between the is suggested for improving global brightness uniformity outside of the panel and the panel; and reducing crosstalk. R P V = the resistance of the power line per pixel unit (vertical direction) outside the display area; E-mail: majent@postech.ac.kr; Fax.: +82-54-279-5894 R H = the resistance of the power line per pixel unit -S5-

-S6- Journal of the Korean Physical Society, Vol. 48, January 2006 Table 2. Full white sub-pixel current at 500-nit with a 50 % efficiency circular polarizer. Display size PHOLED [µa/sub-pixel] OLED (F) [µa/sub-pixel] 8.0, WVGA IR = 1.21 IR = 4.91 IG = 0.86 IG = 2.72 (800 480) IB = 4.48 IB = 4.51 13.0, SVGA IR = 2.79 IR = 11.30 IG = 1.98 IG = 6.26 (800 600) IB = 10.31 IB = 10.38 20.1, WXGA IR = 2.91 IR = 11.82 IG = 2.07 IG = 6.55 (1280 800) IB = 10.79 IB = 10.86 Table 3. Unit resistance of the horizontal VDD structure. Fig. 1. Equivalent circuit models of AMOLED panels. Horizontal VDD structure Display size R H AlNd Cu 8.0 1.06 Ω 0.549 Ω 13.0 0.39 Ω 0.201 Ω 20.1 0.38 Ω 0.197 Ω Table 1. Luminance Efficiencies and color coordinates of phosphorescent OLEDs and fluorescent OLEDs. Material PHOLED OLED (F) Red Green Blue [cd/a, CIE] [cd/a, CIE] [cd/a, CIE] 11 24 3 (0.65, 0.35) (0.30, 0.63) (0.15, 0.17) 3 7 3 (0.63, 0.37) (0.31, 0.63) (0.15, 0.17) (horizontal direction); I P IXEL = OLED current of one pixel. I P IXEL = I R + I G + I B ; and N = The total number of input power lines at one side of the panel. The simulation is based on two assumptions. First, the pixel structure is composed of two thin film transistors and one capacitor (2T1C). Second, the aperture ratio of a sub-pixel is 35 %. The unit resistances for various display sizes and VDD line materials are summarized in Table 3. The input VDD is 15 V, and R P is 0.1 Ω, and R P V is 0.01 Ω. Finally, N is 2, 3 and 4 for 8.0, 13.0 and 20.1, respectively. Figure 2 shows the simulation data for 13.0 AM- PHOLED displays. The VDD drop of the SBH VDD structure is larger than that of the DBH VDD structure. The SBH VDD distribution gradually decreased along the horizontal direction, with a minimum VDD voltage of 12.25 V at the right edge of the display area. On the other hand, the DBH VDD distribution had vertical and Fig. 2. VDD distributions for 13.0 AM-PHOLEDs with AlNd VDD material.

Voltage Distribution of Power Source in Large AMOLED Displays Myoung-Hoon Jung et al. -S7- Fig. 4. Equivalent circuit models of AMOLED panels with the proposed mesh VDD structures. Fig. 3. VDD drops of SBH or DBH VDD structure, phosphorescent OLEDs or fluorescent OLEDs, and AlNd or Cu VDD materials. horizontal symmetries, and the minimum VDD voltage was 14.10 V. The simulation results indicate that the global brightness uniformity of the DBH VDD structure is better than that of the SBH VDD structure. Figure 3 shows the histograms of VDD drop for various display sizes, VDD structures, OLED materials and VDD metal materials. VDD drop increased significantly as the panel size increased. For the DBH VDD structure, PHOLEDs and Cu VDD line had minimum VDD drops, while for the SBH VDD structure, fluorescent OLEDs and AlNd VDD line had maximum VDD drops. By assuming AMOLED displays with a 10 % VDD drop tolerance, these simulation results indicate that the DBH VDD structure is adequate for 8.0 and 13.0 AMOLED displays. However, a 20.1 AMOLED display also requires PHOLED pixels and Cu VDD line. Horizontal and vertical crosstalk are caused by the voltage distribution of the VDD line in a large AMOLED display. In order to solve this problem, a pixel structure with low dependency on the power line IR drop or a new VDD structure with high crosstalk immunity should be developed. Figure 4(a) shows the equivalent circuit model of the proposed double bank mesh (DBM) VDD structure in which VDD is supplied from both sides of the panel. In this structure, a pixel interconnects with neighboring pixels so that pixel current is supplied from both the horizontal direction and the vertical direction, unlike the ordinary horizontal VDD structure. Figure 4(b) shows the equivalent circuit model of the quadruple bank mesh (QBM) VDD structure. In this case, VDD is supplied from all four sides of the panel, which suppresses the VDD drop efficiently. Figure 5 shows the simulation results for a full white 13.0 AM-PHOLED with the mesh VDD structure. This

-S8- Journal of the Korean Physical Society, Vol. 48, January 2006 Fig. 5. Voltage distribution for 13.0 AM-PHOLED displays with the proposed mesh VDD structures. Fig. 7. VDD distribution under the window pattern for 13.0 SVGA AM-PHOLED displays. Fig. 6. Window pattern for the crosstalk test. simulation used AlNd VDD lines (R V = 1.73 Ω, R H = 0.59 Ω, R P H = 0.01 Ω, and M = 3). With the mesh VDD structure, the power-line voltage distribution spreads in all directions, giving a circular shape. As expected, the VDD drop of the QBM VDD structure is smaller than that of the DBM VDD structure. However, the minimum VDD voltage of the DBM structure was 14.04 V, which is similar to that of the DBH VDD structure. In contrast, the minimum VDD voltage of the QBM structure, 14.31 V, is definitely superior to the other structures. The effect of VDD distribution on crosstalk for the four different VDD structures was simulated for 13.0 SVGA AM-PHOLED displays with AlNd VDD lines. Figure 6 shows the window pattern for the crosstalk test. The inner box has 400 300 pixels with full white brightness (500 nit) and the brightness of the background is mid-level gray. Line A-A and line B-B represent one dimensional voltage drop measurement lines in the panel. Figure 7 shows the voltage distributions of VDD lines when the window pattern shown in Figure 6 is displayed on the display panel. There are abrupt voltage drops from A to A and B to B, where horizontal crosstalk appears. Also, the voltage drop at B-B was larger than that at A-A, which means that the horizontal crosstalk is stronger at B-B. Figure 7(b) shows the voltage distribution and the voltage profile of the VDD lines for

Voltage Distribution of Power Source in Large AMOLED Displays Myoung-Hoon Jung et al. -S9- the DBH VDD structure. In this case, VDD changed but the voltage drop at the edges of the window pattern was smaller than that of the SBH VDD structure. Due to the smaller voltage drop, the DBH VDD structure had a better crosstalk characteristic. The same result is expected in the case of vertical cross talk with the conventional vertical VDD structure. Figure 7(c) shows the voltage distribution of the DBM VDD structure for a crosstalk test. The voltage profile of the mesh VDD structure changed more smoothly and continuously than that of the horizontal structures. Figure 7(d) shows the voltage distribution of the QBM VDD structure for a crosstalk test, which is similar to that of the DBM VDD structure. These results suggest that crosstalk can be avoided by adopting the proposed mesh VDD structure for large AMOLED displays. III. CONCLUSION The VDD distribution of AMOLED display panels was investigated by varying panel size, OLED materials, VDD line materials and VDD structure to improve global brightness uniformity. The simulation results indicate that the VDD line voltage drop increases as panel size increases. The combination of the QBM VDD structure, PHOLEDs, and Cu VDD line results in the best VDD distribution. Crosstalk was found to be due mainly to the abrupt VDD drop in display panels. Simulation results verify that the proposed mesh VDD structure gives a smoother voltage distribution at the edges of the crosstalk test pattern. The results indicate that the combination of the QBM VDD structure, high efficiency OLED materials and low resistance VDD wiring is the best solution to realize large-size OLED displays with high image quality. REFERENCES [1] Woo Young Kim, J. Korean Phys. Soc. 35, S1115 (1999). [2] Han-Su Pae and Oh-Kyong Kwon, J. Korean Phys. Soc. 40, 26 (2002). [3] Soo-Woong Hwang, Hwan-Sool Oh and Seong-Jong Kang, J. Korean Phys. Soc. 47, 34 (2005). [4] Tatsuya Sasaoka, Mitsunobu Sekiya, Akira Yumoto, Jiro Yamada, Takashi Hirano, Yuichi Iwase, Takao Yamada, Tadashi Ishibashi, Takao Mori, Mitsuru Asano, Shinichiro Tamura and Tetsuo Urabe, SID 2001 DIGEST, 384 (2001). [5] Takatoshi Tsujimura, Yoshinao Kobayashi, Kohji Murayama, Atsushi Tanaka, Mitsuo Morooka, Eri Fukumoto, Hiroki Fujimoto, Junichi Sekine, Keigo Kanoh, Keizo Takeda, Koichi Miwa, Motohiko Asano, Nami Ikeda, Sayuri Kohara, Shinya Ono, Chia-Tin Chung and Ruey- Min, SID 2003 DIGEST, 6 (2003). [6] Shoji Terada, Gaku Izumi, Yukio Sato, Masayuki Takahashi, Mitsuru Tada, Kimitaka Kawase, Koji Shimotoku, Hitoshi Tamashiro, Nobuo Ozawa, Takanori Shibasaki, Chiyoko Sato, Tadakatsu Nakadaira, Yuichi Iwase, Tatsuya Sasaoka and Tetsuo Urabe, SID 2003 DIGEST, 1463 (2003). [7] Y. K. Lee, K. M. Kim, J. I. Ryu, Y. D. Kim, K. H. Yoo, J. Jang, H. Y. Jeong and D. J. Choo, J. Korean Phys. Soc. 39, S291 (2001). [8] Eugene Kim and Sook Jung, J. Korean Phys. Soc. 45, 1361 (2004). [9] M. Hack, M. Lu, R. Kwong, M. S. Weaver, J. J. Brown, J. A. Nichols and T. N. Jackson, Eurodisplay 2002, 21 (2002).