Indium-tin oxide (ITO) thin layers are obtained by an IBS (Ion Beam Sputtering) deposition process. We elaborated ITO films on flexible substrates of polyethylene terephthalate (PET), under soft conditions of low temperatures and fulfilling the requirements of fabrication processes of the organic optoelectronic components. With a non thermally activated (20 °C) ITO deposition assisted by an oxygen flow (1 cm3/min), we got an optical transmittance of 90% in the visible range, a resistivity around 10−3 Ω.cm and a surface roughness lower than 1.5 nm. Thus we realized flexible organic light-emitting diodes (FOLEDs) with good performances: a maximum luminance of 12000 cd/m2 at a voltage of 19 V and a maximum luminous power efficiency around 1 lm/W at a voltage of 10 V (or a maximum current efficiency of 4 cd/A at 14 V) for the (PET(50 μm)/ITO(200 nm)/TPD(40 nm)/Alq3(60 nm)/Ca/Al) structure.

Tris(8-hydroxyquinoline) aluminium (Alq3) inserted in the structure ITO/Alq3/Al is used to show how dielectric spectroscopy and a $\log \varepsilon '' ={\rm f}(\log \omega)$ representation can separate dielectric conductivity by bound charges from $\gamma _{0 }$ conductivity due to residual pseudo free charges. Coupled with the I(V) characteristic, the $\log \varepsilon '' ={\rm f}(\log \omega)$ representation allow to study the mobility of these carriers as a function of the applied bias voltage V; then the mobility was estimated to be $\mu \approx 2 \times 10^{-6}$  cm2 V−1 s−1 (with V = 0), and we demonstrate that $\mu $ approximately follows a Poole-Frenkel like law. Consequently, these results show that mobility determined by field effects is generally overestimated by around one order of magnitude for an electric field at around 1  MV cm−1. More precisely, we demonstrate that pseudo free charges exhibit a field and weakly temperature dependent mobility, which is in contrast to carriers involved in the trap–charge limited current regime which exhibit a temperature dependent and field independent mobility.

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 C p and parallel resistor R p network with a series resistance $R_{s} \approx $ 50 $\rm \Omega $ located on the anode side; the log – log plot R p 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 E t = 0.19 eV, which is a value in good agreement with theoretical calculations or thermoluminescence measurements.