OLED TFT의 문턱전압 및 모빌리티 편차 보상가능한 순차주사 방식 화소 개발

Abstract

An active matrix organic light emitting diode (AMOLED) display is a self-emissive display that has many advantages, such as low power consumption, wide color range, and fast response time. As a result, the market for AMOLED displays has been growing rapidly. However, this type of display still has some problems, such as the variations in OLED emission currents caused by the electrical characteristic variations of low temperature poly-silicon (LTPS) thin film transistors (TFTs), which are mainly used for the active matrix driving method. This non-uniform property of TFTs causes a degradation in image quality. Solving the non-uniformity of the TFT backplane requires an additional compensation operation. Compensation operations are classified into two types: external and internal. In the external compensation operation, the electrical characteristics of the TFTs are stored in the external memory block, and the data is modified based on the stored information of each TFT. In the internal type of operation, the electrical characteristics of the TFTs are sensed in the pixels, and compensated before or during data programming. Internal compensation is preferable for its lower cost and system complexity. This operation itself can be classified into two types: current programming and voltage programming. Voltage programming is preferable because the current programming method has the disadvantage of requiring a long programming time when the data representing the low gray level are programmed. Programming time is of great importance, especially when the progressive programming method, in which all the lines of each frame are drawn in sequence, is employed. As higher resolution displays become increasingly in demand by the market, the reduction of programming time becomes more crucial, an issue that has driven much research into compensation for the non-uniformities of TFTs. In this thesis, three voltage programming type pixel structures using internal compensation method are proposed. All of proposed pixel structures compensate threshold voltage and mobility variations, main causes of non-uniform luminance problem, of each TFTs and are verified through the simulation. In each proposed pixel structure, the maximum current errors due to threshold voltage variation are about 10% or less when the threshold voltage variation is ±0.4 V and the maximum current errors due to mobility variation are 43%, 7.3%, and 5.4% when the mobility variation is ±10%. As the results, firstly proposed pixel structure increases current error, but the secondly and thirdly proposed pixel structures decrease current error caused by mobility variation. The reason of this results is because the firstly proposed one compensates mobility variation only in highest gray level, then the mobility variation is overcompensated in the other gray level, but the secondly proposed one and thirdly proposed one compensates mobility variation in every gray level, so there is no overcompensating operation. And also, the thirdly proposed one has smaller capacitor than secondly proposed one. Therefore the thirdly proposed pixel structure compensates threshold voltage and mobility variations with high aperture ratio about 40%.