Design of an Organic Pixel Addressing Circuit for Active-matrix Oled Displays

In this paper, we present a new model for an organic pixel addressing circuit based on four-transistor configuration. The proposed circuit doesn’t have the problems of regular configurations which are based on four-thin film transistors (TFT). Circuits based on four-TFT have many advantages, but the only problem is the existence of current peaks at the beginning of each pixel luminescence. Both 4-TFT based configuration and proposed one were simulated using HSPICE. Simulation results indicate the superiority of the proposed method compared with existing technique. INTRODUCTION Organic Light Emitting Diodes (OLED) are strong candidates for the next generation flat panels. OLED displays are light weight, durable, power efficient and have a wider viewing angle [1]. Because of these properties, OLED displays could be a good alternative for liquid crystal displays (LCD). Since now, properties of matrix displays comprising OLEDs basically meet automotive and consumer product requirements. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. Several attempts have been carried out to improve the efficiency and quality of OLEDs and their interface circuits like stabilizing the uniformity of scattered light. Of that worth mentioning works was the 4-TFT configuration introduced in reference [5]. In this paper, we propose a modified version of this configuration in order to achieve a more uniform light by eliminating overshoots at the beginning of each luminescence. THE STRUCTUER OF OLED Currently, conventional OLED displays are made on glass substrates. Also efforts are underway to develop displays on flexible plastic substrates. OLED has a multilayer structure that consists of doped emitting layers, novel transparent Fig.1. The structure of OLED. Fig.2. The circuit model for OLED [3]. and luminescence materials. In a typical device, an indium-tinoxide (ITO) anode, several organic layers and a metal cathode are deposited on the glass substrate. Electro luminescence (EL) is generated from the organic layers and will be emitted from the ITO side (see Fig.1). OLED’s life time is strongly depended on its current density and current peaks can decrease it concisely [2]. The easiest way for addressing a matrix display is passive matrix addressing. The main disadvantage of this method is that it requires a high current density for a good 1 Copyright © 2008 by ASME luminescence and as a result, OLED’s lifetime would be decreased significantly [3]. To avoid this problem it is necessary to decrease the imposed current by Active Matrix Addressing method. Fig.2 illustrates the circuit model of an OLED which could be implemented for simulation purposes. Indicated values are obtained experimentally. Several attempts have been carried out to implement the active matrix addressing method instead of the passive one. We will discuss some of them in the next section. ACTIVE MATRIX ADDRESSING In the active matrix addressing method, one or more active devices are added to each pixel [5]. Active matrix displays have lower drive currents compared to passive matrix displays and therefore they have longer life time, however, active matrix addressing method has its own problems. OLED displays are current drive devices and in order to have a uniform brightness, it is necessary to obtain a constant current when the pixel is on. Two factors that cause non-uniform brightness are: (1) Variation of the threshold voltage (2) Current peaks at the beginning of pixel luminescence Followings are brief reviews of existing techniques used in active matrix addressing method. A. SINGLE-TFT STRUCTURE In Single-TFT configuration (see Fig.3) the data signal is imposed to the OLED through a transistor. Since a very high current is required to derive the OLED, the data line should be able to source such a high density current, similar to Passive Matrix Addressing method. Moreover, the variation of the threshold voltage will result in the variation in emitted luminescence. B. 2-TFT STRUCTURE In two-TFT configuration (see Fig.4), the data signal is applied to the gate of transistor T2 through T1 and then imposed to OLED indirectly through T2. The high current requirement problem would be overcome but the circuit is very sensitive to the variation of the threshold voltage of TFTs [4]. The OLED current is expressed by following equation: