Should they stand or should they lie: Molecular control of organic interfaces?

In the past years, the field of organic electronics, [comprising light emitting diodes (OLEDs), field effect transistors (OTFTs), photovoltaic cells (OPVCs)], has made tremendous progress in structure, functionality, and processability. In addition to these devices, further functions have been demonstrated, for example, sensors, memory cells, or light-emitting transistors. The most intriguing benefits from using organic materials include mechanical flexibility and light weight, which makes this type of device most attractive for mobile and flexible applications, while new design concepts for consumer electronics, architecture and fashion have emerged, as organic electronic devices can be adopted to follow complex surface shapes with almost no restriction in size as well.
Improved insight into organic material properties can often instantly be implemented in novel devices. An essential ingredient for this success is the understanding of interfaces between dissimilar materials—organic/organic and organic/inorganic—since they are not only already inherent in current organic electronic devices, but the rapidly ongoing miniaturization will lead to interface-only devices in near future. In fact, it has been recognized that these interfaces are a key for device function and efficiency, and detailed investigations of interface physics and chemistry are at the focus of research.

Experimentally, the supreme tool to investigate interface electronic properties is photoelectron spectroscopy, since it allows for direct measurement of the organic material’s ionization energy, the hole injection barrier between the organic and the substrate material, and the sample work function. Interaction of organics and substrate covering a wide range from the weak physisorption regime to strong chemisorption, involving charge transfer and/or covalent bond formation can be identified. In many cases, molecular conformation changes can be observed, which directly impact the interface electronic structure and charge injection across the organic/metal contact. Moreover, coverage dependent measurements offer the opportunity to asses information beyond the substrate/adsorbate interface as inter- and intramolecular interactions. Angle resolved measurements allow to measure band structures of crystalline organic thin films, which is an important step in getting insight in the complex transport mechanisms of organic semiconductors.

Here we wish to draw your attention to several highlights in this area of research, performed by our HZB-users using the end-station SurICat at the Optics beamline:

[1] S. Duhm, G. Heimel, I. Salzmann, H. Glowatzki, R. L. Johnson, A. Vollmer, J. P. Rabe, N. Koch, "Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies", Nature Mater. 7 (2008) 326

[2] H. Glowatzki, B. Bröker, R.-P. Blum, O. T. Hofmann, A. Vollmer, R. Rieger, K. Müllen, E. Zojer, J. P. Rabe, N. Koch, "Soft" metallic contact to isolated C60 molecules", Nano Letters, 8 (2008) 3825

[3] B. Bröker, R.-P. Blum, J. Frisch, A. Vollmer, O. T. Hofmann, R. Rieger, K. Müllen, J. P. Rabe, E. Zojer, N. Koch, "Gold work function reduction by 2.2 eV with an air-stable molecular donor layer", Appl. Phys. Lett. 93 (2008) 243303

[4] B. Bröker, R.-P. Blum, L. Beverina, O. T. Hofmann, M. Sassi, R. Ruffo, G. A. Pagani, G. Heimel, A. Vollmer, J. Frisch, J. P. Rabe, E. Zojer, N. Koch, "A high molecular weight donor for electron injection interlayers on metal electrodes",
ChemPhysChem 10 (2009) 2947