Eur. Phys. J. Appl. Phys.
Volume 32, Number 2, November 2005
|Page(s)||95 - 104|
|Section||Physics of Organic Materials and Devices|
|Published online||26 October 2005|
Study of trap levels in Alq3 layers by photodipolar absorption
UMOP, CNRS – FRE 2701, Faculté des Sciences et Techniques,
87060 Limoges Cedex, France
Corresponding author: email@example.com
Revised: 6 June 2005
Accepted: 24 June 2005
Published online: 26 October 2005
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.
PACS: 73.61.Ph – Polymers; organic compounds / 73.61.-r – Electrical properties of specific thin films / 73.50.-h – Electronic transport phenomena in thin films / 78.90.+t – Other topics in optical properties, condensed matter spectroscopy and other interactions of particles and radiation with condensed matter (restricted to new topics in section 78)
© EDP Sciences, 2005
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