AbstractFor 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 adopts thin film transistors (TFTs) as the driving and switching components, the VTH and mobility of TFTs vary due to process variation and long-term operation, leading to the decrease of current uniformity of the panel. Moreover, the OLED material degrades under long-term operation which causes drops in luminance and uniformity of the panel, hence influencing the image quality of the display. Several compensating pixel circuits have been proposed to improve the shortcomings of two-dimensional (2D) AMOLED displays. However, for high-resolution or high-speed operation three-dimensional (3D) AMOLED displays, the capability to solve all the aforementioned problems within the limited programming time has become an important issue.
This thesis proposes three new compensating pixel circuits which address the aforementioned problems. The first circuit adopts a simultaneous emission (SE) driving scheme to be suitable for use in 3D displays. The circuit utilizes a 5T2C structure to compensate for the VTH variations of low temperature poly-silicon (LTPS) TFTs, and additional current is provided to maintain luminance stability as the OLED degrades. Moreover, the driving current of the pixel circuit will not be influenced by current-resistance supply voltage drops and TFT parasitic capacitances. Based on simulation results, the circuit successfully compensates for the VTH variation of TFT, while the current error rates all fall below 1% as different parasitic capacitances of TFTs and different gray levels are provided. Furthermore, the circuit can effectively maintain luminance stability as the OLED degrades. However, this circuit adopts LTPS TFT which has the drawbacks of high cost and high processing temperature. Thus, the second 6T2C circuit is proposed to apply amorphous indium gallium zinc oxide (a-IGZO) TFTs to the pixel circuit. In conditions necessitating use of one additional component, the circuit can be suitable for a-IGZO TFTs and retain the compensating functions with regards to VTH variations of TFTs and OLED degradations in the first pixel circuit. The electrical characteristics of a-IGZO TFTs are measured to establish a simulation model. According to simulation results, the normalized luminance of OLED maintains stability, while the OLED shifts by 0.5 V. Consequently, this circuit can compensate for the VTH shifts of TFTs and increase the lifetime of displays. Due to the fact that the second circuit uses an excessive number of elements, it results in complex control lines and a low aperture ratio. Therefore, the third proposed circuit uses the 4T2C structure to compensate for the mobility and VTH shifts of TFTs, while the OLED luminance drops and adopts the parallel addressing method in order to render the circuit suitable for high-resolution displays. The simulation model is established through measuring the electrical characteristics of another a-IGZO TFT. Based on simulation results, current error rates all fall below 4%, as the shift of TFT VTH is 1 V or the shift of TFT mobility is 20%. Moreover, when the OLED material degrades and the VTH of OLED shifts by 0.9 V, current error rates also fall below 4%. Hence, the proposed circuit can compensate for the VTH and mobility shifts of TFTs, increasing the resolution and lifetime of displays. The proposed three circuits successfully reduce the non-uniformity and luminance decay caused by the degradation of TFTs and OLEDs. Consequently, these circuits make a significant contribution to the applications of AMOLED displays.
|Date of Award
|Chih-Lung Lin (Supervisor)