Electrical transport in crystalline perylene derivatives films for electronic devices
Graphical abstract
Introduction
The organic semiconductors are very attractive for electronic applications [1], [2], [3], [4] because of their large structural variety, special properties, compatibility and complementarity with the silicon VLSI (Very-Large-Scale-Integration) technology. The particularities of the charge carrier injection and transport across the inorganic/organic and organic/organic heterojunctions are determinant for the device's properties.
Perylene and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films are characterized by stacked arrangements with distances of 3.46 Å [5] and 3.21 Å [6], respectively between the parallel planes of molecules. This assures a significant overlapping between the π electron clouds of the neighbouring molecules from successive planes and sustains a stronger intermolecular coupling perpendicularly to the substrate that favours highly anisotropic electrical transport properties. A way to control the conduction properties through perylene and PTCDA based heterostructures is to introduce a buffer organic layer of zinc phthalocyanine (ZnPc) or N,N′-di-[(1-naphtalenyl)-N,N′-diaphenyl]-(1,1′-biphenyl)-4,4′-diamine (a-NPD), with special structural and energetic characteristics, between the electrode and the mentioned PTCDA or perylene organic layer. These organic/organic junctions favourable to the injection of the charge carriers open new perspectives in the field of organic based electronic devices. The quality of the PTCDA and perylene films can also be controlled by the morphology of this intermediate layer and the influence of the substrate could be minimized.
This paper presents some investigations on the influence of the electrode on the charge injection properties of perylene and PTCDA thin films and on the effect of an intermediate organic/organic heterostructure (ZnPc/perylene; ZnPc/PTCDA; α-NPD/perylene) on the injection and transport properties of inorganic (Cu; Al; Si)/organic (perylene; PTCDA)/inorganic (Si) heterostructures.
Section snippets
Experimental
We have prepared SIS (Semiconductor Insulator Semiconductor) sandwich type Si/(single or double organic layer)/Si and MIS (Metal Insulator Semiconductor) sandwich type (Cu or Al)/(single or double organic layer)/Si structures in two-step process. Firstly we have manufactured the inorganic/organic (SI) and metal/organic (MI) junctions and subsequently the SIS and MIS structures putting into direct contact the organic films of two (SI) and, one (SI) and one (MI) individual junctions [7], [8].
We
Results and discussions
The carrier transport in the direction parallel to the molecular plane of perylene and PTCDA is limited by the distance between the adjacent molecules and by the polar binding of the hydrogen and oxygen atoms. The perpendicular contact geometry is used because of the stacked arrangement of these compounds during the crystallization that increases the conduction in the direction perpendicular to the substrate. In the inorganic/organic/inorganic heterojunctions the charge carrier flow can be
Conclusions
For all the samples prepared with perylene and PTCDA layers and Si, Cu, Al electrodes we have evidenced a dominant ohmic behaviour at low voltage (<0.1 V). The Poole–Frenkel mechanism has not been remarked on the experimentally drawn I–V characteristics. The limitations of the current introduced by the space charge and trap charge for Si/perylene derivative/Si structures at voltages >1 V are sustained by the values deduced for the hole mobility. In our experiments the highest current was obtained
Acknowledgements
This investigation has been supported financially by Ministry of Education and Research through CEEX Programme, contract number CEX-05-D11-63.
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