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Understanding metal/organic interfacial properties

Quantum-mechanical calculations can provide fundamental insights into structure for improving organic devices and molecular electronics.
30 January 2008, SPIE Newsroom. DOI: 10.1117/2.1200801.1006

The field of organic electronics has recently attracted significant attention with its promise of efficient, versatile, and cheap devices. Displays, solar cells, transistors, and sensors can be fabricated on flexible and large-area substrates. For such applications, the crucial role of the interface between the organic semiconductor and the charge-injecting electrodes, other organic layers, or gate dielectric is now widely acknowledged. In molecular electronics, an entire device usually consists of a single molecule or monomolecular layer, and the role of the interface is even more important since the interface is essentially the device itself.

Over the years, several approaches for tuning interfacial properties in organic devices have been developed. These include matching the electronic levels of electrodes and organic semiconductors, using charge-transport or blocking layers, or doping the interfacial region with strong donors or acceptors. Of particular relevance to our work is the use of polar self-assembled monolayers1 and highly electron-poor (or rich) molecules that experience strong charge transfer from or to the metals.2 In this context, our research seeks to provide a fundamental understanding of the microscopic electronic properties of metal/organic interfaces. Our goal is to develop general guidelines for molecular interfacial design and to help avoid pitfalls that would result in optimizing the wrong parameters.

Figure 1. Simplified schematic representation of how tuning the metal electrode work function (EF, blue arrow)—e.g., by using a polar self-assembled monolayer (SAM)—modifies the EIBs and HIBs of an organic semiconductor (left). The right panel illustrates the effects on the current-voltage characteristics of the metal/semiconductor contact. The voltages for driving a hole (electron) current are reduced when the HIB is decreased. LUMO: Lowest unoccupied molecular orbital. HOMO: Highest occupied molecular orbital.

We are particularly interested in studying how the work function of an electrode is modified by an interfacial layer. The immediate consequences for the electron and hole injection barriers (EIBs and HIBs) are shown in Figure 1 (left), and the resulting shift in the current-voltage characteristics of an organic diode structure in Figure 1 (right).

We are also investigating how the electronic levels within a self-assembled monolayer (SAM) align relative to the metal states. This effect is of particular importance in molecular electronics applications, where the SAM is the active element of the device.

In our work, we used first-principles band structure calculations based on density functional theory. We focused on developing systematic relations between the chemical structure of molecules attached to metal surfaces and the electronic structure of the resulting interfaces. The applied methods were carefully benchmarked by calculating various experimentally known quantities.3

Figure 2. Density of states projected onto the molecular region (PDOS) for the conjugated part (green) and the alkyl spacer (blue, at much higher binding energy) of a SAM on a gold—Au(111)—surface. The left part of the plot shows the geometric structure of the SAM and the relative energies for the first peaks in the DOS associated with the different regions. I: Insulator. S: Semiconductor.

Figure 3. Isodensity representation of charge rearrangement on adsorption of F4TCNQ on copper: Cu(111). Electrons flow from the blue to the red regions.

We first showed that modifying the ionization potentials and electron affinities of individual molecules by end-group substitution made it possible to tune the work function of the SAM-covered metal and of the EIBs and HIBs. To our surprise, however, the alignment between molecular and metallic states was not modified.4 In other words, energetic level alignment cannot be tuned by tinkering the chemistry of the backbone but rather by altering the docking chemistry of the groups that link the conjugated molecules to the metal.5 Combining both schemes should therefore in principle allow independent tuning of the work function and the level alignment. Note that our results are valid for densely packed monolayers, and can be substantially different when reducing the coverage6 or applying mechanical stress to the molecules.7

Another interesting case is encountered when inserting an alkyl spacer between the metal and the conjugated part of the SAM. In this scheme, the metal electrons are decoupled from the π-electrons (electrons that are weakly bound by nuclear reactions) in the conjugated system. The result is the ‘metal/insulator/semiconductor’ structure shown in Figure 2.8

The situation can become considerably more complex when molecules lie flat on metal surfaces. For example, in the case of the strong acceptor F4TCNQ, we observed a delicate balance between forward and backward charge transfer between the metal, the π-states of the molecular core, and the –CN substituents that, together with molecular distortions, determines the work function modification.9 Figure 3 presents a 3D representation of the simulated charge rearrangements that occur on bonding.

In conclusion, we have found that a fundamental understanding of the electronic structure of metal/organic interfaces is essential for knowledge-based tuning of the relevant electronic quantities. Since some relevant parameters cannot be determined by experiment, quantum-mechanical modeling can significantly contribute to achieving that understanding. Our future work will focus on the role of monolayer and substrate imperfections, as well as extending calculations to the interfaces between organic and inorganic semiconductors.

The authors gratefully acknowledge Jean-Luc Br´edas, Gerold Rangger, Oliver Hofmann, LinJun Wang, Peter Pacher, Norbert Koch, and Alexander Gerlach. Funding was provided by the Marie Curie program (contract 021511), the Austrian Science Foundation (FWF: N-702- SENSPHYS and Erwin Schr¨odinger grant J2419-N02), the European Community IControl project (EC-STREP-033197), and the US National Science Foundation (grants CHE-0342321, CHE-0443564, and CHE-0443564).

Egbert Zojer
Institute for Solid State Physics
Graz University of Technology
Graz, Austria

Egbert Zojer received his MS (1996) and PhD (1999) from Graz University of Technology and obtained an associate professor appointment in 2002. He also worked as an assistant staff scientist at the University of Arizona (2002/2003) and as a senior research scientist at the Georgia Institute of Technology (2003–2005).

Georg Heimel
Department of Materials Science and Engineering
Massachusetts Institute of Technology
Cambridge, MA

Georg Heimel received his MS (2000) and PhD (2003) from the Graz University of Technology. He was then awarded Erwin Schödinger (2004) and Marie Curie (2006) postdoctoral fellowships for pursuing research at the Georgia Institute of Technology and the Massachusetts Institute of Technology, where he presently works.

Lorenz Romaner
Institute of Solid State Physics
Graz University of Technology
Graz, Austria
Department of Material Physics
University of Leoben
Leoben Austria

Lorenz Romaner obtained his MS (2003) and PhD (2007) from the Graz University of Technology. He spent two years at the Georgia Institute of Technology during his PhD studies. He is presently a postdoctoral fellow at the University of Leoben, Austria.