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 $R_{s} \approx $ 50 $\rm \Omega $ 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.