Effect of deposition rate on the morphology, chemistry and electroluminescence of tris-(8-hydroxyqiunoline) aluminum films

The effect of Alq₃ deposition rate on the performance of the devices has been investigated by using the organic light-emitting diodes of indium-tin-oxide/N,N′-bis-(1-naphthyl)-N,′-diphenyl-1,1′- biphenyl-4,4′-diamine/tris-(8-hydroxyquinoline) aluminum (Alq₃)/Mg:Ag. When the Alq₃ deposition rate decreased from about 1.33 to 0.05 Å/s, the luminance efficiency of the devices decreased from 4.75 to 2.0 cd/A. Atomic force microscopy observations showed that Alq₃ films prepared at the deposition rates of 1.33, 0.05, and 0.01 Å/s had a root-mean-square roughness of 12.0, 32.0, and 36.6 Å, respectively. X-ray photoelectron spectroscopy measurements showed that as Alq₃ deposition rate decreased from 1.33 to 0.01 Å/s, the film contained more N-containing species. These changes in film morphology and chemistry are suspected to be responsible for the change in the electroluminescent performance of the devices.