ABSTRACT
In the currently prevalent active matrix organic light-emitting diodes (AMOLEDs), pixel driving circuits usually involve four or more backplane thin-film transistors (TFTs) combined with a capacitor. Such a large number of TFTs placed on the substrate competes with organic light-emitting diodes (OLEDs) in terms of space available on the chip, and inevitably reduces the aperture ratio of each pixel of a bottom-emission organic light-emitting diode (BEOLED), as shown in Fig. 7.1a,c. This reduced aperture ratio in turn results in the need to drive the OLED at a higher current density (which shortens device lifetime) in order to achieve the same level of luminance as a display having larger-apertureratio pixels. Fortunately, top-emission active matrix organic lightemitting diodes (TEOLEDs) can be employed to circumvent this issue since the TEOLED can be fabricated on top of any opaque substrate like silicon, thereby making it possible to place the TFTs below the reflective anode, as shown in Fig. 7.1b,d. Hence, using TEOLEDs, the aperture ratio can be significantly increased and the operating voltage required to achieve a desired luminance can be reduced, which further lead to longer device operational lifetime. Indeed this approach has been adopted with high success across current portable OLED display industries.
