Electron-injecting properties of Rb2CO3-doped Alq3 thin films in organic light-emitting diodes

Rubidium carbonate (Rb2CO3)-doped tris(8-quinolinolato)aluminum (III) (Alq(3)) thin films have been investigated as electron-injecting materials for organic light-emitting diodes (OLEDs). Electron-only devices consisting of glass/tin-doped indium oxide (ITO)/Rb2CO3-doped Alq(3) (10 nm)/aluminum (Al) showed an electron-ohmic contact property between the electrode and the organic layer at the doping concentration of 10% and higher. The electron-injecting ability of these contacts was largely enhanced by the n-doping effect of Rb2CO3 into the Alq(3) layer. The ultraviolet photoemission spectra revealed that when the doping concentration was increased, the n-doping effect reduced the carrier-injecting barrier height by lowering the work function at the Rb2CO3-doped Alq(3) interfaces. Also, the x-ray photoemission spectra showed that as the doping concentration was increased at the interfaces, Alq(3) molecules decomposed in a chemical reaction with Rb2CO3. The OLED device, having the glass/ITO/molybdenum oxide (MoOx, 25%)-doped N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB, 5 nm)/NPB (63 nm)/Alq(3) (32 nm)/Rb2CO3-doped Alq(3) (10%, 10 nm)/Al (100 nm) structure, showed the best performance at the optimal doping concentration of Rb2CO3-doped Alq(3), both the maximum luminance of 114 400 cd/m(2) at the bias voltage of 9.8 V and the power efficiency of 2.7 lm/W at the luminance of 1000 cd/m(2) were obtained. (C) 2013 American Vacuum Society.