Abstract
For an active matrix organic light emitting diode (AMOLED) display, the pixel circuit utilizes a thin film transistor (TFT) as the driving and switching component. However, variation in the VTH of the driving TFT due to process variation or long-term operation, in addition to luminance decay caused by OLED aging, directly influence the image quality of the AMOLED display.This dissertation proposes four novel pixel circuits and verifies their effectiveness by simulations and experiments. The first system integrates a simplified 2T1C pixel structure and external compensation circuit to ameliorate VTH variations and OLED degradation. The pixel current in the proposed system remains constant, whereas current degradation in the conventional pixel circuit is over 20%. Additionally, the luminance degrades by 15% in the proposed pixel circuit versus 30% in the conventional 2T1C circuit. The second circuit can stabilize the OLED current against TFT degradation and provide an additional driving current to compensate for the brightness of the AMOLED. Measurement results indicate that the current degradation of the proposed circuit is less than 5%, over more than 50000s at 60 ℃, whereas that of a conventional 2T1C pixel circuit is over 34%. Additionally, the luminance of the fabricated OLED device with the proposed 3T1C driving scheme can remain at the initial value. The third proposed pixel circuit with three TFTs utilizes the external compensation circuit to detect VTH variations, and employs the increase in VTH_OLED as voltage feedback to compensate for luminance degradation. Simulation results indicate that the normalized luminance of this work is 96.4%, while ΔVTH_OLED is 0.9 V, while those of the conventional 2T1C circuit with P-type driving TFT and N-type driving TFT are 79.4% and 47.4%, respectively. Thus, the proposed driving scheme can ameliorate the luminance degradation via the detection of VTH_OLED through the external compensated circuit. Because of the complexity in the operation of the external detection method, the fourth proposed 3T1C circuit adopts a novel ac driving method to ameliorate the brightness uniformity. Experimental results indicate that adjusting the duty ratio of the ac driving scheme can ameliorate the brightness uniformity; meanwhile, the normalized luminance of a 20% duty ratio after a 3600s stress is 94.3% and that of dc stress is 76.4%. The normalized luminance of measured OLED is 94.3% under a higher voltage (-6V), while it is only 77.15% under a lower voltage (0V). Therefore, the applications of a reversed-bias voltage under an ac driving scheme ameliorate luminance uniformity variation. The proposed pixel circuit has high immunity to variations of electrical characteristics of TFTs and can compensate for OLED degradation.
| Date of Award | 2011 |
|---|---|
| Original language | Chinese (Traditional) |
| Supervisor | Chih-Lung Lin (Supervisor) |