New approach to eliminate losses between metallic contacts and organic semiconductors

  A visualisation of the transition. Image: M Oehzelt/HZB
A visualisation of the transition. Image: M Oehzelt/HZB

In applications like OLEDs and OPV there are known problems at the interface between the metallic contacts and the organic semiconductor, where undesirable losses occur. Now Dr Martin Oehzelt of the German Helmholtz Zentrum Berlin has shown what these losses between the different materials depend upon and how to minimise them.

According to Oehzelt 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.

Up to now there were many different approaches describing the interface between organic semiconductor materials and metallic contacts, but none of them was universally valid for all cases. Oehzelt managed to unify them and developed them 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. “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.”

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