Transport mechanisms in tris(8-hydroxyquinoline)aluminium (Alq3) electronic layers: a study by photodipolar absorption

Abstract

This paper describes the role of traps in the electronic conductivity of tris(8-hydroxyquinoline)aluminium (Alq3) in a conventional sandwich structure with indium tin oxide and aluminium electrodes. New results obtained by photodipolar absorption techniques and impedance spectroscopy are presented. The former method acts as a probe to highlight the role of traps. It is shown that optical pumping of electrons to trap levels gives a clear increase in dielectric absorption due to the reorientation of dipoles associated with trapped charges. The trap depth is estimated to be around E _t = 0.19 eV, a value in good agreement with theoretical calculations and thermoluminescence measurements. The latter method permits a representation of the sample in terms of a circuit composed of a parallel capacitor (C _p) and resistor (R _p) both in series with a resistor R _s ≈ 50 Ω located on the anode side. A logarithmic plot of R _p as a function of the dc bias voltage gives a linear law that is recognized, for the first time, to be a consequence of a trapped charge limited (TCL) current. The linearity can be improved by the introduction of a field-dependent mobility.