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

Rubidium carbonate (Rb₂CO₃)-doped tris(8-quinolinolato)aluminum (III) (Alq₃) 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 Alq₃ (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 Rb ₂CO₃ into the Alq₃ 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 Alq₃ interfaces. Also, the x-ray photoemission spectra showed that as the doping concentration was increased at the interfaces, Alq₃ molecules decomposed in a chemical reaction with Rb₂CO₃. The OLED device, having the glass/ITO/molybdenum oxide (MoO_x, 25)-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4, 4′-diamine (NPB, 5 nm)/NPB (63 nm)/Alq₃ (32 nm)/Rb ₂CO₃-doped Alq₃ (10, 10 nm)/Al (100 nm) structure, showed the best performance at the optimal doping concentration of Rb₂CO₃-doped Alq₃, both the maximum luminance of 114 400 cd/m² at the bias voltage of 9.8 V and the power efficiency of 2.7 lm/W at the luminance of 1000 cd/m² were obtained.