Issue Eur. Phys. J. Appl. Phys. Volume 32, Number 2, November 2005 Page(s) 95 - 104 Section Microelectronics and Optoelectronics DOI 10.1051/epjap:2005072 Published online 26 October 2005

Eur. Phys. J. Appl. Phys. 32, 95-104 (2005)
DOI: 10.1051/epjap:2005072

Study of trap levels in Alq3 layers by photodipolar absorption

A. Moliton, W. Rammal and B. Lucas

UMOP, CNRS - FRE 2701, Faculté des Sciences et Techniques, 87060 Limoges Cedex, France

amoliton@unilim.fr

(Received: 19 May 2005 / Received in final form: 6 June 2005 / Accepted: 24 June 2005 / Published online: 26 October 2005 )

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 C_p and parallel resistor R_p network with a series resistance 50 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.

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).

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