Study on the Pixel Designs of Low-Temperature Polycrystalline Silicon Thin Film Transistors for Active Matrix Organic Light Emitting Diodes
In this thesis, based on the low-temperature polycrystalline silicon thin film transistors (LTPS TFTs), the influences of LTPS TFTs on the active matrix organic light emitting diodes (AMOLEDs) have been investigated. From the topics of device dimension, layout method and circuit design, systematic study and two pixel compensation circuits have been proposed to further enhance the image quality of AMOLEDs. First, SPICE simulation and testing pixel circuits of conventional pixel schematics have been accomplished and fabricated base on LTPS TFTs. Besides the effects of layout method, the dimensional effects of transistors and storage capacitors are investigated in detail. According to the results of simulation and experiments, it is observed that for the conventional layout method, the dimension of switching TFT has no impact on the anode voltage of OLED. When the size of driving TFT is increased, the anode voltage of OLED will be increased due to the larger driving capability. Therefore, the brighter image can be obtained. As to the storage capacitor, the appropriate determination of the magnitude for storage capacitor can control the charging time and storage capability effectively. Compared with conventional layout method, the transistor slicing layout of driving TFT in this work can promote the output current for display effectively. Besides, the non-uniformity across the panel can be reduced. By experimental results, it is verified that the transistor slicing layout can enhance the driving capability and the uniformity effectively at the same time. However, the conventional pixel structure suffers from pixel to pixel brightness non-uniformity problem resulted from the electrical characteristic variations of the transistors. In order to further enhance the image quality of AMOLEDs, the different driving methods have been compared and evaluated. Although digital driving methods can compensate the variation of threshold voltage and mobility, it is not suitable for them to be applied to high resolution products due to the limited process ability and the high driving speed. Analog driving circuits can be divided into current programmed circuits and voltage programmed circuits. Current programmed circuits not only can compensate the variation of threshold voltage and mobility but can control the gray scale directly. However, they have the limitation of the writing time and the data driver IC needs more complicated design. Compared with digital driving method and current programmed method, voltage programmed circuits show great potential for high resolution and low cost applications in the future because of the simple structure. By means of experimental results of conventional pixel design, because of the variation of threshold voltages in LTPS TFTs, the different anode voltages of OLED are obtained. As a result, the different current flow through OLED would result in the non-uniform brightness across the panel. In order to improve the image quality of AMOLED by voltage programmed circuits, two new pixel circuits for compensating the variation of threshold voltage in LTPS TFTs are proposed. In the first circuit design, the driving scheme is divided into three stages. In the first period, all TFTs in the pixel are on state in order to reset the pixel circuit. The second period is for compensation. At this time, data signal is transferred into the circuit. Because driving TFT acts as a diode, data voltage and threshold voltage will be stored in the capacitor. The last period is for emitting. The OLED output current will be determined by the stored voltage in the capacitor. By means of experimental results, it is verified that the proposed pixel circuit can reduce the problem of device variation. In addition, higher output voltage can be obtained. However, the power consumption is increased because there is a current flow through OLED device. Therefore, modified circuit design is further proposed to overcome the problem. By using the modified pixel design which replaced a n channel TFT with a p channel TFT in order to block the current flow path through OLED during pre-charge period, lower power consumption can be obtained. In conclusion, in the proposed circuit designs in this work, besides the higher driving capability, the better uniform image quality can be obtained. Therefore, the circuit designs possess the potential for the AMOLED panel application in the future.
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