Investigating the exciton formation zone and its roles in phosphorescent organic light emitting diodes

The exciton formation zone has been studied in phosphorescent organic light emitting diodes (OLEDs). It is found that the width (W) of the exciton formation zone can be increased by reducing the transit time for holes (electrons) from the anode (cathode) into the emissive layer (EML) by decreasing the thickness of the hole (electron) transport layer. The increase in the thickness of the EML enables the exciton formation zone to be widened. The efficiency roll-off of the device is relieved when W increases. The device stability is found to be directly proportional to the product of W and the thickness of the electron transport layer. A thicker electron transport layer better suppresses diffusion of the metal cathode into the EML, increasing device stability but simultaneously decreasing the transit time for electrons from the cathode into the EML, and thereby W, so decreasing device stability. We also investigated the effect of charge carrier mobilities in the EML, modified by dye concentration, on W. The current research provides novel insights into guiding the delicate designs of OLEDs, hopefully pushing OLED technology towards high-luminance applications.