Device characteristics of organic Light-emitting diodes based on electronic structure of the Ba-Doped Alq ₃ layer

Organic light-emitting diodes (OLEDs) with a Ba-doped tris(8-quinolinolato) aluminum(lll) (Alq ₃) layer were fabricated to reduce the barrier height for electron injection and to improve the electron conductivity. In the OLED consisting of glass/ITO/4,4′,4″-tris[2-naphthylphenyl-1- phenylamino]triphenyl-amine (2-TNATA, 30 nm)/4,4′-bis[N-(1-napthyl)-N- phenyl-amino]-biphenyl (NPB, 18 nm)/Alq ₃ (42 nm)/Ba-doped Alq ₃ (20 nm, x% :x = 0, 10, 25, and 50)/AI (100 nm), the device with the Alq ₃ layer doped with 10% Ba showed the highest light out-coupling characteristic. However, as the Ba dopant concentration was increased from 25% to 50%, this device characteristic was largely reduced. The characteristics of these devices were interpreted on the basis of the chemical reaction between Ba and Alq ₃ and the electron injection property by analyzing the electronic structure of the Ba-doped Alq ₃ layer. At a low Ba doping of 10%, mainly the Alq ₃ radical anion species was formed. In addition, the barrier height for electron injection in this layer was decreased to 0.6 eV, when compared to the pristine Alq ₃ layer. At a high Ba doping of 50%, the Alq ₃ molecules were severely decomposed. When the Ba dopant concentration was changed, the light-emitting characteristics of the devices were well coincided with the formation mechanism of Alq ₃ radical anion and Alq ₃ decomposition species.