Active-matrix Organic Light-emitting Diode Pixel Research Articles

AMOLED Pixel Circuit Using LTPO Technology Supporting Variable Frame Rate from 1 to 120 Hz for Portable Displays.

This paper proposes a new 6T1C pixel circuit based on low-temperature polycrystalline oxide (LTPO) technology for portable active-matrix organic light-emitting diode (AMOLED) displays with variable refresh rates ranging from 1 to 120 Hz. The proposed circuit has a simple structure and is based on the design of sharing lines of switch-controlling signals. It also provides low-voltage driving and immunity to OLED degeneration issues. The calculation and analysis of programming time are discussed, and the optimal storage capacitor for the proposed circuit’s high-speed driving is selected. The results of the simulation reveal that threshold voltage variations in driving thin-film transistors of ±0.33 V can be well sensed and compensated with a 1.8% average shift of OLED currents in high-frame-rate operation (120 Hz), while the maximum variation in OLED currents within all gray levels is only 3.56 nA in low-frame-rate operation (1 Hz). As a result, the proposed 6T1C pixel circuit is a good candidate for use in portable AMOLED displays.

Separate Frame Compensated Current-Biased Voltage-Programmed Active Matrix Organic Light-Emitting Diode Pixel

We propose a new active matrix organic light-emitting diode (AMOLED) pixel circuit in which the compensation for the thin film transistor and OLED performance variation and/or degeneration is performed periodically in a separate frame time. In the other frame time, the AMOLED pixel works without any compensation operation, so that the whole row time is used for driving. Only three transistors and two capacitors are required to construct the pixel circuit. It is demonstrated that both accurate compensation and high driving speed can be realized in the separate frame compensated AMOLED pixel circuit.

Characterization of AMOLED pixel circuit without power line

We fabricated and evaluated the simple active matrix organic light emitting diode (AMOLED) pixel circuits without power line and proved that it is useful for the AMOLED display. Without power line in the pixel circuit we got higher-aperture ratio of emission area than the pixel with power line and the pixel with high aperture ratio can give a long life time due to lower current density of organic light emitting diode. However, the lack of power line requires the verification of the driving scheme of the pixel circuit. After fabrication of two types of AMOLED with and without power line in the pixel, we evaluated the pixel currents under various conditions. The operation of the pixel circuit without power line gave similar characteristics to that of the pixel circuit with power line. By the comparison, we verified that the pixel without power line is acceptable for the application to the AMOLED display combined with feedback compensation scheme for the uniform brightness.

External Compensation of Nonuniform Electrical Characteristics of Thin-Film Transistors and Degradation of OLED Devices in AMOLED Displays

The variation of electrical characteristics of polycrystalline-silicon thin-film transistor (TFT) and degradation of organic light-emitting-diode (OLED) device cause nonuniform intensity of luminance and image sticking in active-matrix OLED (AMOLED) displays. An external compensation method that senses and compensates variations of threshold voltage and mobility of TFTs and degradation of OLED device is proposed. The effect of the external compensation method on AMOLED pixel is experimentally verified by measuring the luminance of OLEDs and the electrical characteristics of TFTs in AMOLED pixels.

Driving scheme for stable operation of 2-TFT a-Si AMOLED pixel

The amorphous silicon (a-Si) thin film transistor (TFT) backplane is becoming the technology of choice for active matrix organic light emitting diode (AMOLED) displays in view of its industrial accessibility and low fabrication cost over large area. However, the a-Si TFT suffers from threshold voltage (V/sub T/) shift. Presented is the simplest voltage-programmed pixel with an ability to compensate for the V/sub T/-shift in the drive TFT of the AMOLED pixel circuit. Simulation results also show that the pixel circuit can tolerate the spatial mismatch, temperature variation, and mechanical stress.