Electron drift mobility and electroluminescent efficiency of tris(8-hydroxyquinolinolato) aluminum

The electron drift mobility in films of tris(8-hydroxyquinolinolato) aluminum (Alq) deposited at different rates (0.2, 0.4, and 0.7 nm/s) on silicon has been determined by the time-of-flight technique. It has been found that the drift mobility of electrons in Alq increased by about two orders of magnitude as the deposition rate decreased from 0.7 to 0.2 nm/s. Further, the electron drift mobility in all Alq samples increased linearly with the square root of the applied electric field. Electroluminescent devices with a structure of indium tin oxide/alpha-naphthylphenylbiphenyl amine (NPB, 90 nm)/Alq (90 nm)/Mg:Ag were fabricated at different Alq deposition rates. The device efficiency was found to increase with increasing electron mobility in Alq. As the electron is the minority carrier in the present device, an increase in electron mobility in Alq would thus lead to an increase in device efficiency. (C) 1999 American Institute of Physics. [S0003- 6951(99)02251-2].