Study of trap levels in Alq3 layers by photodipolar absorption

Abstract

In this paper we develop arguments about the part of traps involved in the electronic conductivity of the 8-tris-hydroxyquinoline aluminium (Alq3) studied in the conventional electronic structure ITO/Alq3/Al. After the presentation of general models and some topical discussions about the expression of the mobility in organic materials, we present results obtained by photodipolar absorption, which is a thermo-photo-dielectric effect, and by impedance spectroscopy measurements. This last method permits to define the equivalent circuit that can be designed as a single capacitor Cp and parallel resistor Rp network with a series resistance 50 located on the anode side; the log – log plot Rp as a function of the dc bias voltage gives a linear law that can be seen in a first time as a consequence of a Trapped Charge Limited current (TCL); this TCL law could be improved with the introduction of a field dependent mobility. Indeed, the photodipolar absorption leads to more convincing arguments because this method acts as a probe to highlight the traps: in particular, we show that the optical pumping of electrons on trap levels gives a clear increase in the dielectric absorption generated by the reorientation of dipoles associated with trapped charges; the trap depth is located around Et = 0.19 eV, which is a value in good agreement with theoretical calculations or thermoluminescence measurements.