Supramolecular organization in ultra-thin films of alpha-sexithiophene on silicon dioxide

The supramolecular organization of organic semiconductors on the dielectric layer of thin-film field-effect transistors is a crucial factor in achieving good device performance. Charge transport in these devices occurs near the interface with the gate dielectric. By confocal spectroscopy and microscopy we study the supramolecular organization in ultra-thin films of a prototype organic semiconductor, alpha-sexithiophene, on silicon dioxide, a widely used transistor gate dielectric. We demonstrate that in submonolayer films of sexithiophene (T6), regions where the molecules stand on their long molecular axis coexist with regions where the molecules lie flat on the substrate. When the first monolayer is completed, all T6 molecules stand on the substrate, and the flat molecules detected in the submonolayer films are no longer present. In films thicker than two monolayers, the photoluminescence spectra of standing molecules show a molecular H-like aggregation as in the single crystal.