Abstract
For active-matrix organic light-emitting diode (AMOLED) displays, the pixel circuit utilizes thin film transistors (TFTs) as the driving and switching components. However, variation in the threshold voltage (VTH) of the driving TFT due to inherent grain boundaries or long-term operation, and luminance decay caused by OLED degradation as well as power line current-resistance (IR) voltage drop directly influence the image quality of the AMOLED displays.This dissertation proposes five novel pixel circuits and verifies their effectiveness by simulations and experiments. The first pixel circuit is a 4T1C hydrogenated amorphous silicon (a-Si:H) structure that compensates for the threshold voltage shift of TFT using an internal compensated structure and reduces luminance decay by external detection method, based on the interdependence between the luminance degradation of OLED and the decrease in current under constant voltage bias stress. Experimental results demonstrate that the luminance of the OLED device with the proposed external detection method is more stable than that with the conventional 2T1C pixel circuit. Due to high mobility and stability of low-temperature polycrystalline-silicon thin film transistors (LTPS TFTs), the second 3T1C structure uses all p-type TFTs for panel requirements of high resolution and high aperture ratio. According to the results of a simulation, over the entire range of tested data voltages (4.5 to 2.5 V), the relative errors of the OLED current of the proposed circuit are below 1.2% when the driving TFT threshold voltage varies from 0.5 to -0.5 V. The third pixel circuit is a 4T1C LTPS structure with a novel driving scheme that is based on the simultaneous emission and reverse-biased methods. During high-speed three-dimensional (3D) operation at 240 Hz, the proposed circuit can successfully compensate for the TFT threshold voltage variation and improve the effects of IR voltage drop in the power line. Simulation and experimental results confirm the stability of the OLED current and the amelioration of the OLED lifetime. Since most prior voltage-programmed methods can not solve the impacts of mobility variation in LTPS TFTs, the fourth 3T2C pixel circuit uses the voltage-programmed method to compensate for the electrical characteristic variations in LTPS TFTs and the IR voltage drop in the power line. Based on the measurement results of the transient waveforms of the source and gate nodes in the driving TFT, the functionality of the proposed pixel circuit can be operated correctly during each operation phase. Moreover, the simulation results demonstrate that the pixel current exhibits high uniformity against the threshold voltage and mobility variations in TFTs. As for the last pixel circuit, an amorphous indium-gallium-zinc-oxide (a-IGZO) pixel circuit with 3T1C structure is proposed for large-sized and high-resolution 3D AMOLED displays. The proposed pixel circuit can compensate for the threshold voltage shifts in both enhancement-mode and depletion-mode a-IGZO TFTs. Based on the simulation results with ±1 V threshold voltage shifts of the driving TFT, the relative errors of the OLED current are less than 2% for the entire range of tested data voltages.
In conclusion, a complex external system must be designed to achieve practical application of the first pixel circuit, ultimately increasing the system costs. The second, third, and fourth pixel circuits use LTPS TFTs and a simple structure for the application of high-resolution displays. The last pixel circuit is promising for use in large-sized and high-resolution 3D AMOLED displays, owing to its improved large area uniformity, superior scalability with a low production cost, and lower leakage current than LTPS TFTs.
| Date of Award | 2014 |
|---|---|
| Original language | Chinese (Traditional) |
| Supervisor | Chih-Lung Lin (Supervisor) |