Determination of Electron Mobility in Small Molecular 1,4-di(bis(8-hydroxyquinoline)aluminum-oxy)benzene by Transient Electroluminescence

Abstract

Transient electroluminescence is an important tool to determine the charge carrier dynamics in light emitting organic semiconductors. We have used this method to determine the electron mobility in one of the important organic semiconductors 1,4-di(bis(8 hydroxyquinoline)aluminum-oxy)benzene (Alq(1)), used as emissive layer in organic light emitting diodes (OLEDs). For transient electroluminescence studies, we prepared OLEDs using Alq(1) as the emitter. The OLEDs were prepared on indium tin oxide (ITO) coated glass substrates using N, N’-diphenyl -N,N’- bis (3-methylphenyl)-(1,1’-biphenyl)-4,4- diamine (TPD) as hole transport layer (HTL) and lithium fluoride (LiF) as electron injecting buffer layer. The temporal evaluation of the electroluminescence (EL) was studies with respect to a voltage pulse of different amplitudes applied to the device at different temperatures. A delay was observed in the onset of EL from the device with respect to the applied voltage pulse. The EL exhibited a fast initial rise followed by tending to saturate. The EL decayed rapidly as the applied voltage became zero and the decay did not depend upon the amplitude of the applied voltage pulse. The delay time in the onset of EL with respect to the applied voltage pulse is correlated to the electron mobility in Alq(1). The electron mobility in Alq(1) calculated by transient EL method, showed strong dependency on the applied electric field and temperature at low electric fields however at quite high electric fields, the electron mobility in Alq(1) showed poor dependency on the applied electric field and temperature. This behavior of electron mobility in Alq(1) has been explained in terms of shallow charge carrier traps in Alq(1) film. The electron mobility in Alq(1) at 295 K and 2.7´10⁶ V/cm, has been determined to be 5.4´10^-6 cm²V^-1s^-1, which is much higher than that in the well-studied Alq₃.