Electron mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via transient electroluminescence from single- and multilayer organic light-emitting diodes

Transient electroluminescence (EL) from single- and multilayer organic light-emitting diodes (OLEDs) was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide (ITO)/tris(8-hydroxy-quinoline)aluminum (Alq3)/magnesium (Mg):silver (Ag), whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine (CuPc)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/Alq3/Mg:Ag. Apparent model-dependent values of the electron mobility (μe) in Alq3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the EL onset—dependent on the electric field—is governed by accumulated charges (holes) at the internal organic-organic interface (NPB/Alq3) or is transport limited. Time-of-flight measurements on 150-nm-thin Alq3 layers yield weak field-dependent μe values of the order of 1×10−5 cm2/Vs at electrical fields between 3.9×105 and 1.3×106 V/cm.