OLED技术论文

P-11 / S.-M. Choi

P-11: An Improved Voltage Programmed Pixel Structure for Large Size and

High Resolution AM-OLED Displays

Sang-Moo Choi and Oh-Kyong Kwon

Division of Electrical and Computer Engineering, Hanyang University, Hangdang-Dong, Seongdong-Gu,

Seoul, Korea

Ho-Kyun Chung

R&D Center, Samsung SDI, Co., Ltd, Yongin-City, Kyungki-Do, Korea

Abstract

We 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. Introduction

AM-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 method

Figure 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 with

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(b)Timing diagram

Figure 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 DIGEST