Determination of charge carrier mobility in tris(8-hydroxy-quinolinato) aluminum by means of impedance spectroscopy measurements

Abstract The hole mobility of tris(8-hydroxy-quinolinato) aluminum (Alq 3 ) thin films and double layer films of a 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPB) and Alq 3 , have been determined using impedance spectroscopy (IS) measurements. The theoretical basis of mobility measurement with IS rests on a theory for single-injection space-charge limited current. The NPB thin film operates as an injection layer to reduce the hole injection barrier between an injection electrode and the Alq 3 thin film. The hole mobilities of Alq 3 thin films measured with IS are identical to those measured by a time-of-flight (TOF) transient photocurrent technique. The hole drift mobility determined using IS measurements in NPB/Alq 3 double layer hole-only devices where the thickness of NPB is comparable to that of Alq 3 are also identical to the hole drift mobility of Alq 3 layer. The origin of the observation is discussed using a device simulation and the results are useful for the determination of charge carrier mobility in multilayer devices.