Bulk and interface doping of organic semiconductors
Prof. Norbert Koch, Humboldt-Universität, Berlin
Doping organic semiconductors is a key technique for expanding the functionality of organic (opto-) electronic devices. It allows adjusting the position of the Fermi level relative to the semiconductor transport levels and it reduces ohmic losses in transport layers through increased conductivity. Based on concepts adopted from inorganic semiconductor physics, often integer charge transfer from the highest occupied molecular orbital (HOMO) of the semiconductor to the lowest unoccupied molecular orbital (LUMO) of a p-dopant, and vice versa for n-type doping, is assumed. Recently, an alternative model was proposed , which is based on frontier molecular orbital hybridization between the semiconductor HOMO and the dopant LUMO, leading to the formation of a supramolecular complex with a reduced energy gap between a doubly occupied bonding and an unoccupied antibonding hybrid orbital. Evidence for the existence for such complexes will be provided, which allows deriving guidelines for designing new dopants .
Another important issue is realizing ohmic electrical contacts to organic semiconductors. Fermi level pinning at electrode/organic interfaces provides for induced charge carriers on the semiconductor side, which can be viewed as interface doping with concomitant band bending . However, depending on the electronic coupling strength between the electrode and the organic semiconductor, the band bending region may collapse into one monolayer. A global electrostatic treatment helps rationalizing the transition between models that employ either extended band bending or interface dipoles.
 I. Salzmann, G. Heimel, S. Duhm, M. Oehzelt, P. Pingel, B. M. George, A. Schnegg, K. Lips, R. Blum, A. Vollmer, N. Koch, Phys. Rev. Lett. 108 (2012) 035502.
 H. Méndez, G. Heimel, A. Opitz, K. Sauer, P. Barkowski, M. Oehzelt, J. Soeda, T. Okamoto, J. Takeya, J.-B. Arlin, J.-Y. Balandier, Y. Geerts, N. Koch, I. Salzmann, Angew. Chem. Int. Ed. 52 (2013) 7751.
 P. Amsalem, J. Niederhausen, A. Wilke, G. Heimel, R. Schlesinger, S. Winkler, A. Vollmer, J. P. Rabe, N. Koch, Phys. Rev. B. 87 (2013) 035440.