The Study For AMOLED Pixel Circuit Based On Metal Oxide TFTs

Active matrix organic light emitting diode(AMOLED) display have attracted much attention and become the research focus in the new generation displays because of its favorable properties, such as the high brightness, high contrast, wide viewing angle, light weight, simple manufacturing process, fast response speed, low power consumption, and flexibility. In recent years, metal oxide thin film transistor(MOTFT) has demonstrated the enormous superiority in the application of new displays technology due to reasonably high mobility, good uniformity, low temperature process and good process compatibility with a-Si TFTs. It is well known that the conventional two-TFTs and one-capacitor(2T1C) pixel circuit is not suitable for AMOLED displays because the OLED current is non-uniform due to the variations in the threshold voltage of the driving TFT. Therefore, the study of compensation pixel circuit has a great significance for the application of AMOLED display.Firstly, we analyzed the performance of the main components(TFT and OLED) in AMOLED, mainly including test the device’s performance curve and extract its main electrical parameters. And then, we fit the electrical properties of MOTFT and OLED by using mature low-temperature polysilicon TFT(LTPS TFT) model using the H-Spice circuit simulation software, and get good fitting results of devices simulation model. This work provides accurate simulation results for evaluating the performance of pixel circuit.Secondly, we propose a threshold voltage one-time detection method and corresponding voltage-programmed compensation pixel circuit. This programming method can improve the programming speed, and make the pixel circuit be better applicable to high resolution, large size and three-dimensional AMOLED displays. However, in this programming method, the programming time of compensation frame and non-compensation frame is not conformity, which leads to the light-emitting time inconsistency and the programming scheme complicated. Therefore, we develop a new programming scheme named the mixed parallel addressing scheme, which can perfectly solve the problem of the one-time detection method. We also propose compensation pixel circuit corresponding to the driving scheme. One of the pixel circuits can effectively compensate for the threshold-voltage shift of the driving TFT and keep the current through OLED constant. And the other one can not only compensate for the threshold-voltage shift of the driving TFT, but also can increase current through OLED with the aging of OLED, which can solve the problem of OLED brightness recession even in the case of constant current drive. What’s more, the several kinds of the compensation pixel circuits can work well and get a good compensation effect drived by corresponding high speed programming scheme through the simulation and experimental verification.Finally, aiming at the characteristics of MOTFT may have a positive or a negative threshold voltage, we propose a compensation pixel circuit. This pixel circuit can not only adapt to n-type LTPS TFT only have the positive threshold voltage, but also can compensate the threshold voltage of driving thin-film transistor whether it is positive or negative, which is very suitable MOTFT have positive or negative threshold voltage characteristics at the same time. This compensation pixel circuit can compensate the threshold voltage of driving TFT whether it is positive or negative through simulation and experimental verification.