A unified model for metal/organic interfaces: IDIS, ‘pillow’ effect and molecular permanent dipoles
Introduction
Organic semiconductors have been attracting much attention for over a decade, partly due to their electronic structure, in between localized states of molecules and delocalized states of solids, and partly because these materials can be used in new electronic or optoelectronic devices, such as light-emitting diodes, organic solar cells or field-effect transistors [1], [2].
The issue of charge injection at metal/organic interfaces has too been the subject of considerable theoretical and experimental research: since many electronic devices use metallic electrodes, the formation of metal/organic contacts and the key parameters which control charge injection have been intensively studied. The alignment of the relevant electronic levels at metal/organic interfaces has become therefore the subject of increased interest [3], [4]: this alignment is interesting both from a fundamental point of view and also due to its relevance in organic-based devices. The most important parameter determining charge injection at such interfaces is the Schottky barrier. Thus, theoretical models are needed to understand the formation of the Schottky barrier and the energy level alignment at metal/organic interfaces.
When the metal and the organic material are isolated, the position of the relevant electronic levels (the metal work function and the molecular highest occupied or lowest unoccupied molecular orbitals—HOMO and LUMO, respectively) relative to the vacuum level can be measured or calculated. As the interface is formed, however, the vacuum levels of both materials do not, in general, align. This phenomenon, known as the breakdown of the vacuum alignment rule, can be caused by many factors taking place at the interface, such as defects, chemical reaction, charge transfer, the presence of molecular permanent dipoles, or the reduction of the metal work function due to the presence of adsorbed molecules. The observation that, in general, the vacuum level rule is not followed [5], [6] implies the existence of interface dipoles at these junctions, which shift the electronic levels of one material relative to those of the other. Since injection barriers depend on the magnitude of these interface dipoles, the ability to model and predict the formation of these dipoles is of considerable theoretical interest. Interfaces of inorganic semiconductors offer some inspiration as to the behaviour at these junctions, though the models developed for those systems are not directly applicable.
In this paper, we present a unified model which incorporates several mechanism operating at metal/organic interfaces: the induced density of interface states (IDIS) [7], [8], [9], [10], the ‘pillow’ effect [11], [12], [13], [14], [15] and we describe how to include the case of molecules exhibiting molecular permanent dipoles within our formalism. We focus on ‘ideal’ interfaces and take gold as a prototypical unreactive metal, which forms clean and abrupt interfaces with the organic semiconductor. We have considered the following organic materials: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI), 4,4′,N,N′-dicarbazolyl biphenyl (CBP), copper phthalocyanine (CuPc) and tris-(8-hydroxyquinoline) aluminium (Alq3).
Section snippets
Theoretical model
In a recent paper [15], we described how the reduction of the metal work function due to the compression of the metal electronic tail by the adsorbed molecules (‘pillow’ effect), could be incorporated into our IDIS formalism. The main idea was to calculate the electrostatic ‘pillow’ dipole induced at the interface, which translates into a reduction of the metal work function, and then to include this modification in the IDIS equations. Here, we show how the same approach can be extended to
Unified IDIS-pillow-permanent-dipoles model
The unified model which incorporates the IDIS, ‘pillow’ and molecular dipole effects is a straightforward extension of that presented in ref. [15].
The ‘bare’ IDIS mechanism (as discussed in Section 2.3) shows how the organic CNL tends to align with the work function of the metal, where the degree of (mis)alignment depends on the initial potential offset and on the screening at the interface S. The effect of the ‘pillow’ mechanism and of molecular dipoles can be straightforwardly included as a
Results and discussion
The unified model presented above allows the study of interface properties by looking at the sign and magnitude of the potential drop induced at the metal/organic interface. Since most organic solids are composed of molecules which do not exhibit a permanent dipole, molecular permanent dipoles are not responsible for the energy level alignment taking place at most metal/organic interfaces. We have analyzed interfaces of two types, corresponding to organic molecules lacking or exhibiting
Conclusions
We have presented a unified model which incorporates three important mechanisms taking place at metal/organic interfaces. The compression of the metal electronic wavefunction due to the orthogonalization with that of the adsorbed organic molecule leads to a reduction of the metal work function. Using the same formalism, molecular permanent dipoles (if present) are taken into account, which give rise to a variation of the metal work function. Finally, using this value of corrected by the
Acknowledgements
We gratefully acknowledge financial support by the Universidad Autónoma de Madrid, the Juan de la Cierva project and the Spanish CICYT under projects MAT 2001-0665, MAT 2004-01271 and NAN-2004-09183-C10-07.
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