Controlling the electronic structure at interfaces in opto-electronic devices with functional molecules
Prof. Norbert Koch, Humboldt-Universität zu Berlin
The electronic structure at interfaces in electronic and optoelectronic devices is decisive for their function and efficiency. For instance, charge carrier injection and selective carrier extraction rely on a proper line-up of the electrode Fermi level and the semiconductor valence or conduction band edges. As well, the relative energy position of conduction and valence levels at semiconductor heterojunctions determines whether the junction facilitates charge separation or energy transfer. If, for technological reasons, one would favor employing certain material combinations in a device, methods to modify the interface level alignment are required.
The introduction of an interfacial layer comprising functional molecules enables controlling the electronic structure at interfaces in opto-electronic devices. Hereby, the “functionality” of the molecules can be rather simple: strong electron donor and acceptor molecules substantially modify the work function of electrodes, covering a range from 3.5 eV to over 6 eV. This, in turn, allows the realization of ohmic contacts to most organic and inorganic semiconductors. Inorganic/organic semiconductor heterojunction energy levels can also be adjusted with molecular donor/acceptor interlayers, based on the work function tuning of the inorganic semiconductor surface. Dynamic adjustment of interfacial energy levels can be achieved with molecular photochromic switches. Reversible switching between two conformations by optical excitation changes the frontier energy levels available for charge transport and transfer. This can be used to realize multifunctional devices, e.g., addressability by electrical and optical means.