Controlling interface electronic properties with organic molecules

Interfaces play a key role for the function and efficiency of organic (opto-) electronic devices. Electrical contacts to organic semiconductors must feature lowest possible contact resistance, which can be achieved by designing the electrode such that its Fermi level pins at the semiconductor valence or conduction levels. The various methods and materials that allow establishing such a situation will be discussed in the light of the underlying fundamental physical mechanisms. The focus is on employing strong molecular acceptors and donors and their charge transfer interactions with metals and conductive oxides on work function tuning.

For inorganic/organic and organic/organic semiconductor heterojunctions, the level of understanding of the mechanisms that govern the energy level alignment is less established. However, for selected well defined model systems long-range charge transfer from a metal electrode through one organic layer to a subsequently deposited one has been evidenced. It remains yet to be clarified if there exists a thickness-limit of this mechanism for establishing electronic equilibrium across heterojunctions. Furthermore, doping of the semiconductors appears as interesting approach to tune energy levels at interfaces, which will be exemplified for an inorganic/organic semiconductor interface.

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