Lifetime Amelioration for an AMOLED Pixel Circuit by Using a Novel AC Driving Scheme

This paper describes how a reversed-bias voltage affects organic light-emitting diodes (OLEDs) with green emitting Alq₃:C545T (40 nm). The luminance efficiency and the active area of the test OLED devices are 8-11 cd/A and 2.5 mm × 2.5 mm, respectively. The OLED lifetime under an ac driving scheme can be apparently improved, in contrast with that under a dc driving scheme. Experimental results indicate that adjusting the duty ratio of the ac driving scheme can ameliorate the brightness uniformity; meanwhile, the normalized luminance of a 20% duty ratio after a 3600-s stress is 94.3%, and that of dc stress is 76.4%. Therefore, a novel pixel circuit with three p-type thin-film transistors (TFTs) for an active-matrix OLED application is proposed to ameliorate the brightness uniformity. Variations of the threshold voltage of the driving TFT are compensated for, and the performance and lifetime of the OLED are improved by using an ac driving method. Simulation results indicate that the output current is almost constant over the data range. Furthermore, the current error rate of the proposed pixel circuit is less than 6.5%.