Electrostatics do the trick
When inserting an ultrathin dielectric between metal electrode and organic semiconductor, charge carriers (shown here for a positively charged holes in red) are, counter intuitively, more efficiently extracted from their transport level (blue) in the organic to the Fermi level (black) in the metal than without the interlayer. © M. Oehzelt/HZB
A simple model describes what happens between organic semiconductors and metals
Organic semiconductors allow for flexible displays (OLEDs), solar cells (OPVCs), and other interesting applications. One common problem in these devices, however, is the interface between the metallic contacts and the organic semiconductor material, where undesirable losses occur. Now Dr. Martin Oehzelt has shown what these losses between the metal and the organic semiconductors depend upon and how to minimize them. In particular, his model also explains why a thin, electrically insulating layer between the two materials can even facilitate the transition of charge carriers. His results have recently been published in Nature Communications.
Currently, there are many different approaches describing the interface between organic semiconductor materials and metallic contacts. These somewhat contradictory theories, none of which is universally valid for all cases, have now been unified by Oehzelt and developed into a single coherent model based on the electrostatic potential caused by the charge carriers in the metal and the organic semiconductor. “I calculated the impact of the charge carrier distribution on the electronic states at the interface and how these changes feed back onto the charge carrier distribution”, he explains. Oehzelt is presently conducting research with Dr. Georg Heimel as a postdoc for Prof. Norbert Koch, who works at the Humboldt-Universität zu Berlin and the Helmholtz-Zentrum Berlin.
Such calculations have never been so comprehensively carried out before. Performing them, Oehzelt states: “it was surprising to me that the quantum physical level was not that important. The electrostatic effects predominated! The agreement between our model and the experimental data were astonishing.” On the example of pentacene, a common organic semiconductor, Oehzelt has quantitatively checked the model’s predictions for interface losses.
The energy distribution of the electronic states in organic semiconductors determines the minimum energy barrier the charge carriers have to overcome in transitioning from or into the metal. The calculation demonstrates that the shape of this energy barrier can vary, from a step-function to slow, continuously rising curves that lead to considerably lower losses. The latter can be achieved by introducing an extremely thin insulating layer between the organic semiconductor and the metal. Contrary to the general expectation, the introduction of an insulator thus improves the electrical contact.
The results of this work could notably simplify optimization of interfaces and contacts and, thereby, the development of more efficient organic electronic devices.
The work has recently been published in Nature Communications:
doi 10.1038/ncomms5174
- BESSY II: Phosphorous chains – a 1D material with 1D electronic propertiesFor the first time, a team at BESSY II has succeeded in demonstrating the one-dimensional electronic properties of a material through a highly refined experimental process. The samples consisted of short chains of phosphorus atoms that self-organise at specific angles on a silver substrate. Through sophisticated analysis, the team was able to disentangle the contributions of these differently aligned chains. This revealed that the electronic properties of each chain are indeed one-dimensional. Calculations predict an exciting phase transition to be expected as soon as these chains are more closely packed. While material consisting of individual chains with longer distances is semiconducting, a very dense chain structure would be metallic.
- Did marine life in the palaeocene use a compass?Some ancient marine organisms produced mysterious magnetic particles of unusually large size, which can now be found as fossils in marine sediments. An international team has succeeded in mapping the magnetic domains on one of such ‘giant magnetofossils’ using a sophisticated method at the Diamond X-ray source. Their analysis shows that these particles could have allowed these organisms to sense tiny variations in both the direction and intensity of the Earth’s magnetic field, enabling them to geolocate themselves and navigate across the ocean. The method offers a powerful tool for magnetically testing whether putative biological iron oxide particles in Mars samples have a biogenic origin.
- What vibrating molecules might reveal about cell biologyInfrared vibrational spectroscopy at BESSY II can be used to create high-resolution maps of molecules inside live cells and cell organelles in native aqueous environment, according to a new study by a team from HZB and Humboldt University in Berlin. Nano-IR spectroscopy with s-SNOM at the IRIS beamline is now suitable for examining tiny biological samples in liquid medium in the nanometre range and generating infrared images of molecular vibrations with nanometre resolution. It is even possible to obtain 3D information. To test the method, the team grew fibroblasts on a highly transparent SiC membrane and examined them in vivo. This method will provide new insights into cell biology.