Analysis and characterization of self-compensating current programmed a-Si:H active matrix organic light-emitting diode pixel circuits

The organic light-emitting diode (OLED) drive current provided by a simple 2 thin film transistor (TFT) hydrogenated amorphous silicon (a-Si:H) active matrix OLED (AMOLED) voltage programmed pixel circuit drops over time due to threshold voltage (VT) increase in the drive TFT. VT shift compensating current programmed 4-TFT circuits have been developed to overcome this problem and to keep the drive current constant, thereby prolonging the life of the display. In this article, we present a thorough analysis of the static and dynamic operation of such circuits and derive theoretical limits for the extent of compensation. Measurement results of circuit characteristics and stability are presented which demonstrate that it is feasible to use a-Si:H for AMOLED back planes. The advantages and drawbacks of current programmed circuits are also outlined from the perspective of active matrix back-plane design.