S.-Y. LEE

A New Circuit for high definition AMOLED employing simultaneous emission driving

Soo-Yeon Lee, Moon-Kyu Song, Seung-Min Song, Dong-Won Kang and Min-Koo Han School of Electrical Engineering and Computer Science, Seoul National University, Gwanakno 599, Gwanak-gu, Seoul, 151-742, Korea Tel.:82-2-880-7992, E-mail: [email protected]

Abstract progressive emission, because the progressive emission has almost 100 % emission ratio on one frame. However, in the We have proposed a new pixel circuit for aspect of the pixel circuit, the simultaneous emission is organic light emitting diode (AMOLED) employing more suitable for high-resolution AMOLED display simultaneous emission driving. The simultaneous because the circuit can be simplified with the simultaneous emission driving can reduce the pixel area compared to emission driving. the widely used progressive emission driving. With the We proposed a new p-type poly-Si pixel circuit for proposed pixel circuit, OLED emission time can be simultaneous emission method, which is suitable for high increased as much as 85 % than the previous work. The resolution AMOLED. In the previous work, we fabricated proposed pixel circuit, which consists of 3 PMOS TFTs the 1280 x 720 2D AMOLED panel employing the and 2 , compensates the threshold voltage simultaneous emission driving method for the first time8. variation of low temperature poly-silicon TFTs. However, because the emission time is closely related to the OLED life time, emission time should be increased. The 1. Introduction proposed pixel circuit can produce about 85 % OLED emission time during one frame. Active organic light emitting diode display (AMOLED) employing Low temperature polycrystalline silicon (LTPS) 2. Simultaneous emission driving thin film transistor (TFT) have attracted a considerable attention due to high brightness, wide viewing angle and In the previous work, we proposed a new Vth low power consumption 1, 2. LTPS TFTs employing excimer compensation pixel circuit for the simultaneous emission laser annealing (ELA) are widely used in AMOLED driving as shown in Figure 1. The pixel consists of 4-TFT displays2-4. However, the excimer laser annealed poly-Si and 1-. Because all OLED of the panel are turned TFTs suffer from a threshold voltage (Vth) variation in the off in the scanning period, VDATA, VINIT and ELVDD line pixel which causes non-uniform OLED current. Various can be merged, so that 2 TFTs and 1 power line of the pixel compensation circuits have been reported2-5. Usually, 5-6 is reduced compared to the conventional pixel circuit. TFTs and 1-2 capacitors are required for Vth compensation. However, the external circuit is required as shown in figure However, as displays increase in resolution, the pixel size 1. The gate voltage of the driving TFT of all pixels is decreases due to the increase of pixel number on the display. initialized simultaneously when the source node of T1 is For the high resolution AMOLED display, the connected to VINIT by external circuit. During the scanning compensation pixel circuit should be simplified. period, the source node of is connected to VDATA by signal In this paper, we proposed a new compensation pixel D and it is connected to ELVDD during the emission period. circuit for simultaneous emission driving. With the pixel structure of Figure 1, 4.0-inch high The simultaneous emission diving has advantages for the resolution AMOLED panel was fabricated. stereoscopic 3D display6. Most of AMOLED display employs the progressive emission driving scheme, which 3. Proposed Pixel Circuit for Simultaneous increases the pixel area due to the compensation of Vth Emission Driving variation and decreases the emission time in the stereoscopic 3D display. In the progressive emission In this paper, we proposed a new pixel design for driving, the reset, Vth storage, data scan and emission steps improving the previous work. As shown in Figure 2, during progress line-by-line. On the other hand, the simultaneous initializing step, the gate of the driving TFT is initialized emission can simplify the pixel structure and increase the and prepared for Vth detection. Due to the simultaneous emission time in the 3D display. The panel is turned off at emission driving, Vss power line can be changed during the the reset, Vth storage and data scan steps and it is turned on scanning time. When Vss is high, OLED is turned off and simultaneously at the emission step. Vth is detected by diode connection of the driving TFT. Vth However, for 2D display, the simultaneous emission is memorized in Cvth. Then, data signal is memorized in Cst. driving method has shorter emission time than the The storage time of data writing is faster than that of Vth

IMID 2012 DIGEST S.-Y. LEE

7 detection . Vth detection takes relatively long time. Thus, ·ΥΙ͑ΕΖΥΖΔΥΚΠΟ ͶΞΚΤΤΚΠΟ ͺΟΚΥΚΒΝΚΫΚΟΘ ͵ΒΥΒ͑ΨΣΚΥΚΟΘ Vth is detected simultaneously so that the scanning time can be dereased. In this manner, the emission time can be ͺͿ΅ increased as much as 85 %. Not only emission time, but also external circuit is improved. Compared to previous ·ΤΤ work, the VINIT part can be eliminated. Ͷ;

·͵Ͳ΅Ͳ Ͷͽ·͵͵ ·ͺͿͺ΅ ΄ΔΒΟ

͵ Ͷ; ͺͿ΅ ͶΩΥΖΣΟΒΝ ΔΚΣΔΦΚΥ

·͵͵ ΁ΚΩΖΝ ͺͿ΅ (b) ʹΤΥ ΔΚΣΔΦΚΥ͑

΅ͥ Fig. 2. (a) Proposed pixel schematic and (b) the ΅͢ timing diagram for the simultaneous emission ΅ͣ driving. ΅ͤ Ͷ;͙Ο͚

΄ΔΒΟ͙Ο͚ ΀ͽͶ͵ 5. Summary

·΄΄ We proposed a new p-type poly-Si pixel circuit for ͺͿ΅ improve the previous work employing simultaneous ͵ emission driving method. The proposed pixel is suitable for high resolution AMOLED due to the simple structure and ΄ΔΒΟ sufficient OLED emission time. It consists of 3 TFTs and 2 capacitors. Because diode connected Vth detection takes Ͷ; rather long time, Vth of all driving TFT of the panel is detected simultaneously. Then, data is memorized by scan Fig. 1. Pixel schematic and the timing diagram of signal. Compared to the previous work, the emission time is one frame for the simultaneous emission driving. increased as much as 85 % of one frame. References ͶΩΥΖΣΟΒΝ͑ ·͵Ͳ΅Ͳ ·͵͵ ΔΚΣΔΦΚΥ 1. C. Tang and S. VanSlyke, Applied physics letters 51 (12), 913-915 (1987). ͵͙Ο͚ Ͷ;͙Ο͚ 2. R. Dawson, Z. Shen, D. Furst, S. Connor, J. Hsu, M. Kane, R. Stewart, A. Ipri, C. King and P. Green, presented at the International Electron Devices Meeting ΄ΔΒΟ͙Ο͚ 1998, 1998. 3. S. H. Jung, W. J. Nam and M. K. Han, Electron Device Letters, IEEE 25 (10), 690-692 (2004). 4. S. M. Choi, O. K. Kwon and H. K. Chung, presented at the SID Symposium Digest of Technical Papers, 2004. ʹΤΥ ʹΧΥΙ 5. S. Choi, O. Kwon, N. Komiya and H. Chung, presented ΅΃Ε at the IDW Tech Dig., 2003. 6. B.-W. Lee, I.-H. Ji, S.-M. Han, S.-D. Sung, K.-S. Shin, J. D. Lee, B. H. Kim, B. H. Berkeley and S. S. Kim, SID Symposium Digest of Technical Papers 41 (1), 758-761 ͺͿ΅ ΀ͽͶ͵ (2010). 7. D.-W. Park, C.-K. Kang, Y. Park, B. Chung, K.-H. Chung, B.-H. Kim and S. S. Kim, SID Symposium ·΄΄ Digest of Technical Papers 41 (1), 806-809 (2010). (a) 8. S. Y. Lee, J. S. Lee, M. K. Song, S. M. Song, S. H. Kuk and M. K. Han, presented at the SID Symposium Digest of Technical Papers, 2012.

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