A crucial step in the fabrication of organic photovoltaic or electronic devices is represented by the growth of the first few molecular layers. In fact, these determine the structure of the molecular film and the energy level alignment at the interface between the organic layer and the conductive substrate.In this talk, we will explore the formation of model metal-organic junctions by analysing the interplay between molecule-substrate charge transfer and supramolecular self-assembly. By using ad-hoc designed molecules with specific ionisation energies and electron affinities and by varying the work function of the substrate, we will show that it is possible to finely tune the resulting molecular assembly. In particular, we will demonstrate a novel growth mode characterised by anomalous coarsening and show that it derives from a competition between the omnipresent short-range attractive van der Waals interactions and long-range repulsive forces due to charge-transfer induced molecular dipoles. Moreover, we will analyse the behaviour of codeposited acceptor and donor molecular units, showing that the resulting complex assemblies are a consequence of the interactions between oppositely oriented molecular interfacial dipoles.While allowing to rationalise also previously reported experimental findings by identifying the precise conditions that molecule and substrate must meet in order for these novel growth modes to occur, our work further shows that specific molecular linkage patterns can reveal the occurrence of charge transfer. This provides a novel means for obtaining crucial information on the electronic properties of metal-organic interfaces.  Due to the fundamental nature of the processes at play, we anticipate that our results could be generalised to a large class of metal-organic systems.

Interfacial molecular dipoles and their effect on 2D molecular self-assembly

247th National Meeting (2014)

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