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 ligh t-emitting diodes (OLEDs) was investigated by driving the devices with shor t, rectangular voltage pulses. The single-layer devices consist of indium-t in oxide (ITO)/tris(8-hydroxy-quinoline)aluminum (Alq(3))/magnesium (Mg):si lver (Ag), whereas the structure of the multilayer OLEDs are ITO/copper pht halocyanine (CuPc)/N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB)/ Alq(3)/Mg:Ag. Apparent model-dependent values of the electron mobility (mu (e)) in Alq(3) have been calculated from the onset of EL for both device st ructures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias volta ges indicated that the EL delay time is determined by the accumulation of c harge carriers inside the device rather than by transport of the latter. Th is interpretation is supported by the observation of delayed EL after the v oltage pulse is turned off. In the multilayer OLED the EL onset-dependent o n the electric field-is governed by accumulated charges (holes) at the inte rnal organic-organic interface (NPB/Alq(3)) or is transport limited. Time-o f-flight measurements on 150-nm-thin Alq(3) layers yield weak field-depende nt mu (e) values of the order of 1x10(-5) cm(2)/Vs at electrical fields bet ween 3.9x10(5) and 1.3x10(6) V/cm. (C) 2001 American Institute of Physics.