2011 Volume 52 Issue 6 Pages 1316-1319
Red electrophosphorescent organic light-emitting devices using red phosphorescent dopant, and tris-(8-hydroxyquinoline)-aluminum (Alq3) and 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) electron injecting have been fabricated. A device with Alq3 as electron transport layer shows high turn-on voltage (@ 1 cd/m2) of 6.25 V, but a device with BCP does lower turn-on voltage of 5.0 V. Alq3 and BCP have silimar lowest unoccupied molucular obital, 3.0 eV and 2.9 eV, which are the higher values than work function of Ba (2.7 eV). So, Ba cathode does not form energy barrier to inject electron to organic layer from simple energy band of view. However, these devices represent different injection properties, therefore we analyze the interface between organic layer and cathode. X-ray photoelectron spectroscopy depth profiling measurements reveal the strong chemical reaction and bonding of Ba with the C and O present in the Alq3 molecules. Results show that the performance of the electrophosphorescent devices is mainly governed by the structure and the chemical reaction at the interface between Ba metal and electron-injecting material.
