High-efficiency organic electroluminescent device with multiple emitting units

In this paper, we report on the fabrication of multilayer organic light emitting diodes (OLEDs) with high electroluminescent (EL) yield achieved by integrating two units of green-emissive devices in series. The architecture of the device used in the experiment is Indium-Tin-Oxid (ITO)/CuPc/NPB/C545T: Alq₃/Alq₃/Mg:Alq₃/WO₃/NPB/C545T: Alq₃/Alq₃/LiF/Al. We found the efficiency of the two-unit devices can be controllable by the thickness of WO₃. The two-unit devices with 1-nm-thick WO₃ produces the luminance efficiency of 49.2 cd/A at 20mA/cm², which is around four times that of the controlled single-unit device (ITO/CuPc/NPB/C545T:Alq₃/Alq₃/LiF/Al). Compared with reported research data, the "amplification effect" discovered in our device is a rather unexpected result. The external quantum efficiency of 12.6%, with near-saturated Commission Internationale d'Eclairage coordinates (CIEx = 0.27, CIEy = 0.68), is one of the best ever reported for a fluorescent dye-doped OLED. We also demonstrate that the electron injection layer of Mg:Alq₃ is a necessary component for the enhancement of EL efficiency. These results may prove to be an effective method to enhance the efficiency as well as the lifetime of current OLEDs.