One of the problems when integrating organic elements into conventional electronic devices is achieving good contact between the active organic layer and metal conductors. For this task, frequently organic molecules are used. However, hitherto it was not possible to predict which molecules effectively can be used. For this reason, engineers basically had to follow a trial-and-error approach. The results of the Berlin research teams around Georg Heimel and professor Norbert Koch could show a way to improve the contact layers between metal electrodes and active organic material.
After several year’s work with combinations of diverse measurement techniques and theoretical calculations, the team has worked out what Georg Heimel calls a consistent image. In order to find these results, the researchers systematically got granular on molecules with a "backbone" formed of a number of aromatic carbon rings. The materials only varied in one detail: Different numbers of oxygen atoms protruded out of this backbone. These modified molecules then were applied to the most common contact metals gold, silver and copper.
By means of photoelectron spectroscopy (UPS and XPS) of HZB’s BESSY II synchrotron radiation source, they were able to determine the chemical bonds between metal surface and organic molecules and to measure the energy levels of the conduction electrons. The exact distance between the molecules has been determined by a colleague team of the Tübingen university using X-Ray standing wave measurements conducted at the ERSF synchrotron radiation source ESRF in Grenoble.
As a result, it was found that the molecules investigated change their inner structure after contact between the electrons protruding and some metallic surfaces in the sense that they lost their semiconducting properties and instead adopted the metallic properties of the surface. Despite comparable conditions the "bare" backbone molecule did not show this effect. From the observation which molecules underwent these kinds of drastic changes on what metal, the researchers could derive general guidelines. "At this point, we have a pretty good sense of how molecules ought to look like and what their properties should be if they are to be good mediators between active organic materials and metal contacts, or, as we like to call it, good at forming soft metallic contacts," says Heimel.
Besides the Berlin research teams mentioned above, further researchers from German universities as well as from research centres in Suzhou (China), Iwate and Chiba (Japan) and ESRF (France) participated in the research activities.