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, and luminance decay caused by OLED aging directly influence the image quality of the AMOLED display.This thesis proposes three novel pixel circuits and verifies their effectiveness by experiments and simulations. The first proposed 3T1C circuit adopts a novel scheme that combines the conventional voltage-programmed method with the clamped inverter driving scheme to compensate for the VTH shift of the driving TFT. Based on the experimental results, the normalized pixel current degradation of the proposed pixel circuit is 5% of the initial current over 18 hours at 60 ℃, while that of the conventional 2T1C pixel circuit is 35%. Thus, the proposed pixel circuit has high immunity to the threshold voltage degradation of a-Si:H TFT, and thus the OLED current stability can be improved.
The second proposed 4T1C circuit compensates for threshold voltage shift using internal compensation method and ameliorates luminance decay by external detection method based on the interdependence between the luminance degradation of the OLED and its current drop under bias stress. According to the experiment results, the normalized luminance of the conventional 2T1C pixel circuit with a stable driving current decreases by 15% of its initial value for over 500 hours. However, the simulation results show that deviation of normalized luminance of the proposed pixel circuit with external detection method is within ±2% of the initial value. Thus, the proposed pixel circuit with external detection method improves the luminance stability of AMOLED.
Because of the complexity in the operation of external detection method, the third proposed 5T1C circuit adopts a novel clamped inverter pixel circuit with a voltage feedback structure that detects the OLED degradation and increases the OLED emission time in the emission period to reduce the effect of the material decay. Simulation results demonstrate that the voltages of the related nodes can be approximately set to their required values when the threshold voltages of the TFT and the OLED shift. Therefore, the proposed pixel circuit has high immunity to variations of electrical characteristics of TFTs and can compensate for OLED degradation.
| Date of Award | 2010 |
|---|---|
| Original language | Chinese (Traditional) |
| Supervisor | Chih-Lung Lin (Supervisor) |