Active matrix organic light emitting diode pixel design using polysilicon thin film transistors

Organic light emitting diodes (OLEDs) are presently of great interest due to their potential application in high efficiency displays. However, as the number of pixels in a passively addressed display increases, the time available to drive each pixel decreases, the peak pixel brightness increases and the OLED drive current increases. Thin film transistors made with polysilicon can be used to generate a constant current source at each pixel. Each pixel is programmed to provide a constant current throughout the entire frame time, eliminating the high currents encountered in the passive matrix approach. However, polysilicon thin film transistors vary widely in their initial output characteristics due to the nature of the polysilicon crystal growth. A description of some of the important considerations in the design of such an AMOLED pixel and results on improved pixel designs are presented.