Novel Amorphous-Si AMOLED Pixels with OLED-independent Turn-on Voltage and Driving Current

In this abstract, we present a novel integration approach that allows conventional bottom-anode OLEDs to be integrated with n-channel a-Si TFT´s in such a way that programming becomes independent of the OLED I-V curve. The circuit schematic of a conventional AMOLED pixel with p-channel poly-Si TFT´s is shown in Fig. 1(a). The TFT is processed first with temperatures up to 300 degC, followed by the OLED anode (patterned ITO, connected to the TFT drain), the organic (blanket) (Fig. 2(a)) and cathode (blanket) (Fig. 2(b)). Because the best OLED´s are made with the anode (ITO) deposited first, and because processing (photolithography) of the organic is not feasible, p-channel devices are required for direct programming of the pixel. With the OLED at the drain (not source) of the driving FET, the data voltage appears directly across the gate-source of the driving TFT and therefore the pixel current depends only on the characteristics of the driving FET, independent of the OLED characteristics when the FET is in saturation. If n-channel FET´s (such as a-Si TFT´s) are used instead with the same integration process (Fig. 1 (b)), since the only accessible contact is the anode of the OLED, the OLED will be connected to the source of the driving FET, and therefore the data voltage will be divided across the gate-source of the driving FET and the OLED, based on the TFT and OLED characteristics. This is undesirable because the gate-source voltage of the driving TFT (and thus the driving current) will depend on the OLED characteristics which vary with time and in manufacturing. Also, higher programming voltages will be required for the same gate-source voltages.