High-Efficiency Organic Electroluminescent Device with Multiple Emitting Units

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Abstract
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 20 mA/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.