Abstract
For three dimensional (3D) active matrix organic light emitting diode (AMOLED) displays, the display frame rate should be increased to send images to the left and right eyes within 16.6 ms for producing depth perception. Since the pixel circuit for 3D AMOLED displays utilizes a thin film transistor (TFT) as the driving and switching component, the variation in the VTH of the driving TFT due to process variation or long-term operation result in non-uniform driving current. Furthermore, luminance decay caused by OLED aging also directly influences the image quality of the AMOLED display. Therefore, to resolve the above problems with high speed operation is necessary for pixel circuits applied in 3D AMOLED displays.This thesis proposes three novel 3D AMOLED pixel circuits and verifies their effectiveness by simulations and experiments. The first proposed 4T2C low temperature poly-silicon (LTPS) pixel circuit adopts the simultaneous emission driving scheme to compensate for both the threshold voltage variation of TFTs and power line IR-drops for implementation in high-speed 3D displays. Based on the simulation results, the relative current error rate of the proposed pixel circuit is below 5% over the entire data voltage range. In addition, while the voltage of the power line of the proposed pixel circuit drops from 10V to 9.5V, the maximum error rate is nearly 3.8%. Thus, the proposed pixel circuit has high immunity to the threshold voltage variation of the driving TFT and the IR drop of the power line. However, this circuit can not compensate for the luminance degradation of OLED material decay. Thus, the proposed second circuit uses the 5T2C structure to compensate for the luminance drop. To verify the operation of the proposed circuit with a frame rate of 240Hz in 3D AMOLED displays, the simulation includes the load effects of parasitic capacitance and resistance of the buslines. Simulation results show the current error rates at the four corners of the display are suppressed to below 5%. Moreover, based on the experimental results of OLED degradation, the second proposed circuit provides enough current to compensate for the degradation of luminance at various gray levels as the OLED degrades. Recently, amorphous indium gallium zinc oxide (a-IGZO) TFTs have gained special attention in pixel circuit design due to their favorable characteristics. Thus, the third 4T2C pixel circuit uses a-IGZO TFTs to compensate for the threshold voltage shift in 3D AMOLED displays. The parameters of the model card are established by the measurement results of a-IGZO TFTs. To verify the feasibility of the proposed circuit, a 70×70 a-IGZO pixel circuit array is fabricated. Experimental results demonstrate that the current of the proposed circuit only decays about 3.3% under six hours stress as the initial current is 0.62μA, and thus the OLED current stability of the proposed circuit can be investigated. The presented three circuits effectively improve the nonuniformity and degradation of luminance caused by degradation of TFTs and OLED. Thus, these circuits can significantly contribute to AMOLED applications.
Date of Award | 2012 |
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Original language | Chinese (Traditional) |
Supervisor | Chih-Lung Lin (Supervisor) |