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) (Alq3, 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 Alq3. 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, 30 nm)/4,4'-bis(N-(1-napthyl)-N-phenyl-amino)-biphenyl (NPB, 18 nm)/Alq3 (62 nm)/Ba (3 nm)/Al (100 nm) structure showed the best characteristics with the highest luminance of 54,000 cd/m2 and the highest efficiency of 2.7 lm/W, as compared to the other devices with Ba thicknesses of less than 3 nm.
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