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Photoemission spectroscopy study of the interface formation in organic thin film transistors

Pentacene, perylene, and sexithiophene are all materials being used in organic thin film transistors due to their relatively large mobilities. It has been suggested that the functional behavior in these devices occurs within the first few molecular layers of the organic at the interfaces between the organic and the dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation using photoemission spectroscopy to examine layer by layer growth of pentacene, perylene, and sexithiophene on conductors, dielectrics, and charge transfer agents and in some cases vice versa. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition. There appears to be a linear relation between the interface dipole and metal workfunction with the observed dipoles ranging from a 1 eV dipole at the interface between sexithiophene and gold to a -0.46 eV dipole at the interface between pentacene and calcium. We also observed that more complex material intermixing takes place during metal on organic deposition than during organic deposition onto metal and as a result, the electronic structure of the interface differs from that of organic on metal deposition. Possible charge transfer, dipole formation and energy level bending at these interfaces will be discussed.