Light-Emitting Characteristics of Organic Light-Emitting Diodes with Ba/Al Cathode and Effect of Ba Thickness by Measuring their Built-in Potential

Abstract

The electronic nature of metal-organic semiconductor contacts is a fundamental issue in the field of organic semiconductor device physics, because these contacts control the charge injection. The built-in potential in organic light-emitting diodes (OLEDs) with a Ba/Al cathode was investigated by using the modulated photocurrent technique. To measure the built-in potential, a device with a glass/tin-doped indium oxide (ITO)/tris(8-quinolinolato)aluminum (III) (Alq(3), 150 nm)/Ba (x nm, x = 3, 2, 1, and 0)/Al (150 nm) structure was fabricated and encapsulated in a nitrogen atmosphere. The device with Ba/Al cathode showed a higher built-in potential, compared with the Al-only device, which reduced the barrier height for electron injection from the Ba/Al cathode to Alq(3). For the device with a Ba thickness of 3 nm, the barrier height for electron injection showed a low value of 0.1 eV. On the basis of the built-in potential data, the device with the ITO/4,4',4 ''-tris(2-naphthylphenyl-1-phenylamino)triphenylamine (2-TNATA, 30nm)/4,4'-bis(N-(1-napthyl)-N-phenyl-amino)-biphenyl (NPB, 18 nm)/Alq(3) (62 nm)/Ba (3 nm)/Al (100 nm) structure showed the best characteristics with the highest luminance of 54,000 cd/m(2) and the highest efficiency of 2.7 Im/W, as compared to the other devices with Ba thicknesses of less than 3 nm. (C) 2009 The Japan Society of Applied Physics