a-Si:H pixel electrode circuits for AM-OLEDs

A new pixel electrode circuit based on hydrogenated amorphous silicon technology has been designed, fabricated and characterized for the active-matrix organic light- emitting displays. This circuit has four thin-film- transistors (TFTs) and only two external terminals. In addition, a current driver is used to provide the signals to the data line and automatically adjust the current level to compensate for the threshold voltage shifts of both the organic light-emitting devices and the drive TFT. Consequently, this pixel electrode circuit has an excellent electrical reliability even when a large threshold voltage shift is present. Experimental results indicate that a continuous pixel electrode excitation can be achieved with these circuits. Even after a long time circuit aging stress, this circuit only shows an output current level variation less than 1 percent at the high current level and less than 5 percent at the low current levels. Two additional pixel electrode circuits have also been proposed. These new circuits can achieve a higher output current level and a better output-input current linearity without sacrificing the circuit's electrical reliability. All these circuits can potentially be used for the active-matrix organic light- emitting displays.