OLED技术论文

P-11 / S.-M. ChoiP-11: An Improved Voltage Programmed Pixel Structure for Large Size andHigh Resolution AM-OLED DisplaysSang-Moo Choi and Oh-Kyong KwonDivision of Electrical and Computer Engineering, Hanyang University, Hangdang-Dong, Seongdong-Gu,Seoul, KoreaHo-Kyun ChungR&D Center, Samsung SDI, Co., Ltd, Yongin-City, Kyungki-Do, KoreaAbstractWe propose an improved pixel structure for large size and high resolution AM-OLED(Active matrix-Organic Light Emitting Diode) displays. The proposed structure is composed of 5 TFT and 1 capacitor. It can compensate not only the threshold voltage variation of LTPS(Low Temperature Poly Silicon) TFTs but also the voltage drop of supply voltage on panel. Moreover, it operates with simple structure and control signals. In this paper, we describe the operating principle and the characteristics of the proposed pixel structure and verify the performance by HSPICE simulation comparing with those of previously reported structures..1. IntroductionAM-OLED(Active Matrix-Organic Light Emitting Diode) display has been studied intensively because of its superior characteristics for display such as light, thin and self emissive characteristics, wide viewing angle and fast response time. In spite of those outstanding characteristics it is still difficult to implement AM-OLED panel with good image quality because of the threshold voltage and mobility variation of LTPS(Low Temperature Poly-Silicon) TFTs. In addition, as the panel size and brightness grow up, the degradation of supply voltage on panel(VDD IR drop) becomes an another critical issue because it may occur an image degradation and crosstalk. Since it is expected to obtain many advantages to apply AM-OLED display for large size TV application, the driving method and pixel structure should be applicable for large size panel [1]. Recently, AM-OLED displays are demonstrated with several solutions to obtain uniform images, such as voltage programming method that can compensate the variation of threshold voltage [2-4], current programming method [5-6] and digital driving method [7-9]. Even though the current programming method can be applied to achieve excellent image quality, its panel driving speed is too slow to implement high resolution displays. Digital driving method can reduce the threshold voltage sensitivity of display images, but it needs very fast addressing speed so that it may not be the good solution for high gray scale displays. Several voltage programming methods have been reported to earn uniform images [2-4]. However, any of those voltage programming methods can not achieve both simple driving method and low sensitivity to the degradation of supply voltage.So we propose an improved voltage programmed pixel structure for large size and high resolution AM-OLED displays, which can compensate the threshold voltage variation of TFTs and is less sensitive to the degradation of supply voltage with simple driving method.2. Conventional pixel structure and programming methodFigure 1 shows the conventional pixel structure and timing diagram of that structure which can compensate threshold voltage variation and the degradation of supply voltage [2]. But it needs 3 control lines and complex driving signals for data line and control lines. Moreover the data line must be alternated to supply voltage level with every row line time to store a threshold voltage. Figure 2 shows a modified structure and the timing diagram of that structure [3]. It reduces one control line by more complex controlling. But as the previous one, it wastes the row line time to store the threshold voltage of TFT. Figure 3 shows an another structure that simplify the driving method and pixel structure [4] compared with previously reported one[2]. And the panel that is applied that structure is already demonstrated successfully withscan[n]AZAZBdata[m](b)Timing diagramFigure 1. Conventional voltage programming pixel structure and timing diagram for operation by R. Dawson [2]ISSN/0004-0966X/04/3501-0260-$1.00+.00 © 2004 SID 260 • SID 04 DIGESTP-11 / S.-M. Choiremarkable image quality. However, it is sensitive to the degradation of panel supply voltage because the actual programmed data of each pixel is set by VDD and V DATA .3. Proposed pixel structure and programming methodThe proposed pixel structure having 5 TFTs, 1 capacitor and 1 control line is shown in figure 4 with a timing diagram for operation. T2, T3, T4 and T5 are switching TFTs, and T1 is driving TFT, which supplies constant current to OLED for a frame time. V SUS produces a constant voltage which is lower than the programmed data to sustain the gate node voltage of T1 for a frame time. The gate node voltage of T1, the right side of C ST , increases to ‘V DD -V TH,T1’ by diode connected T1 and the left side of C ST is set to programmed data voltage during the current scan line time. After the row line time T4 and T5 are turned on and the potential of both side of C ST decreases as a level of ‘V DATA -V SUS ’. From that time OLED current flows by driving TFT, T1, as following equation (1).21,1,|)||(|2T TH T GS OLED V V I −=β(){}[]21,1,||||2T TH SUS DATA T TH V V V VVDD VDD −−−−−=β2)(2SUS DATA V V −=β(1)It has only one control line, scan[n], for pixel operation and can use a whole row line time to program the data. And the OLED current is controlled only by the V DADA and V SUS so that it has good immunity against the degradation of panel supply voltage. Moreover, it can utilize the driving circuit of poly-Si TFT-LCDs and then fast charging speed can be obtained. It has enough programming speed so that the demultiplexing method for panel programming is available, which makes it possible to reduce the manufacturing cost of display module by reducing the interconnection part and the number of driver ICs.4. Simulation resultsWe verify the performance of the proposed pixel structure by HSPICE simulation. In order to confirm the possibility whether the proposed structure can be used to implement high resolution and large size AM-OLED displays, we assume the panel specification and simulation condition as shown in Table 1. The resolution, panel size and brightness are WXGAscan[n]AZB data[m](b) Timing diagramFigure 2. Conventional voltage programming pixel structure and timing diagram for operation by W. J. Nam [3](b) Timing diagramscan[n-1]data[m]scan[n]data[n-1]data[n]Figure 3. (a) Conventional pixel structure by N. Komiya [4] (b) simplified control signals for operation.SID 04 DIGEST • 261P-11 / S.-M. Choi(1280×RGB ×768), 15-inch diagonal and 300 cd/m 2, respectively, to achieve an acceptable simulation result for degradation of supply voltage. Estimated load condition of data line and resistance of supply voltage line(VDD) of that specifications are applied to verification. The actual brightness of displayed image is assumed as 30% of peak brightness level in case of natural image.According to simulation results, the deviation of OLED current is less than 1.3 % under the described fluctuation of threshold voltage(±0.3 V) and it can verify that the proposed pixel structure has good compensating performance against the threshold voltage variation. Figure 5 shows a comparison of the deviation of OLED current by the degradation of panel supply voltage between the conventional structures[4,10] and the proposed one. The x-axis shows the distance from external power supply point to each pixel by the number of row line. The ‘0’ position means the nearest pixel from the external power supply point. The maximum variation level of OLED current resulted from degradation of supply voltage is 2.7 % when the proposed structure is applied. Therefore, we can observe from that result that the proposed structure is much less sensitive to the degradation of supply voltage than conventional pixel structures.5. ConclusionsWe propose an improved pixel structure for voltage programmed AM-OLED display and show the simulation results of the structure. It is composed of 5 TFT and 1 capacitor to compensate the threshold voltage variation of LTPS TFTs and the degradation of supply voltage. Though the proposed pixel has a simple structure, simple driving signals and fast charging time, it can reduce the deviation of OLED current under 1.3 % and has a much less sensitivity about the degradation of supply voltage than previously reported pixel structures. Therefore, it can be applied for the large size and high resolution AM-OLED displays.6. References[1] O. K. Kwon, “Driving and System Considerations of PM-and AM- OLEDs ,” in IMID ’02 Tech. Dig., 2002, pp. 963-968. [2] R. Dawson, Z. Shen, D.A. Furst, S. Connor, J. Hsu, M.G.Kane, R.G. Stewart, A. Ipri, C.N. King, P.J. Green, R.T.(b) Timing diagram(a) Pixel structurescan[n]data[m]Figure 4. (a) Proposed pixel structure with 5 TFT and 1 capacitor (b) control signals for operationO L E D c u r r e n t [n A ]Row line numberConventional 2-TFT structure [10]Figure 5. OLED current fluctuation by degradation of supply voltage when VDD is supplied from row line 1 to row line 768 and the brightness is 30% of peak level.Table 1. Assumed panel specification and fluctuationfactors for HSPICE simulationResolution WXGA Panel size 15-inch Brightness 300 cd/m 2OLED current (30% of full white level) 540 nADeviationof threshold voltage± 0.3 VThreshold voltage of p-type TFT-3.41 V262 • SID 04 DIGESTP-11 / S.-M. ChoiFlegal, S. Pearson, W.A. Barrow, E. Dickey, K. Ping, S.Robinson, C.W. Tang, S. Vanslyke, F. Chen, J. Shi, J.C.Sturm and M.H.Lu, “The Impact of the Transient Responseof Organic Light Emitting Diodes on the Design of ActiveMatrix OELD Displays ,” in IEDM ’98 Tech. Dig., 1008,pp.875-878.[3]W. J. Nam, S. H. Jung, K. C. Park, and M. K. Han,“Reduction of Control Signal Lines in VTH VariationCompensation Scheme of AM-OLED Driving Circuit,” inIDW ’01 Tech. Dig., 2001, pp. 1727-1728.[4]N. Komiya, C. Y. Oh, K. M. Eom, J. T. Jeong, H. K. Chung,S. M. Choi, and O. K. Kwon, “Comparison of VthCompensation Ability among Voltage Programming circuits for AMOLED Panels,” in IDW ’03 Tech. Dig.,2003.[5]T. Sasaoka, M. Sediya, A. Yumoto, J. Yamada, T. Hirano, Y.Iwase, T. Yamada, T. Ishibashi, T. Mori, M. Asano, S.Tamura and T. Urabe, “A 13.0-inch AM-OLED Displaywith Top Emitting Structure and Adaptive Current ModeProgrammed Pixel Circuit(TAC),” in SID ’01 Tech. Dig,2001, pp. 384-386. [6]M. Shimoda, K. Abe, H. Haga, H. Asada, H. Hayama, K.Iguchi, D. Iga, Y. Iketsu, H. Imura and S. Miyano, “NewPixel-Driving Scheme with Data-Line Pre-Charge Functionfor Active Matrix Organic Light Emitting Diode Displays,”in IDW ’02 Tech. Dig., 2002, pp.239-242.[7]M. Mizukami, K. Inukai, H. Yamagata, T. Konuma, T. Nishi,J. Koyama, S. Yamazaki and T. Tsutsui, “6-Bit Digital VGA OLED,” in SID ’00 Tech. Dig., 2000, pp. 912-915. [8]K. Inukai, H. Kimura, M Mizukami, J. Maruyama, S.Murakami, J. Koyama, T. Konuma and S. Yamazaki, “4.0-in. TFT-OLED Displays and a Novel Digital Driving Method,” in SID ’00 Tech. Dig., 2000, pp.924-927.[9]T.Ouchi, Y. Mikami, T. Satou and N. Kasai, “A 2.2-in. ColorTFT-OLED display Using a Continuous Sub-Field With aMultiplex Scanning System,” in IDW ’02 Tech. Dig., 2002,pp. 247-250.[10]M. Stewart, R. S. Howell, L. Pires, M. K. Hatalis, W.Howard, and O. Prache, “Polysilicon VGA active matrix OLED displays-Technology and performance,” in IEDMTech. Dig., 1998, pp. 871-874.SID 04 DIGEST • 263。