Fullerene (C60) interlayer modification on the electronic structure and the film growth of 2,7-diocty[1]benzothieno-[3,2-b]benzothiophene on SiO2

Modification by ultra-thin fullerene (C 60 ) insertion layer of about 1 monolayer on the electronic structure and the film growth of 2,7-diocty[1]benzothieno-[3,2-b]benzothiophene (C8-BTBT) on silicon oxide (SiO 2 ) is investigated using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is found that the ultra-thin C 60 insertion layer doesn’t induce an ambipolar charge transport characteristic but it indeed lifts up the energy levels of C8-BTBT, resulting in p-doping and hole accumulation at the semiconductor-dielectric interface. The charge transfer from trap states residing in the band gap of C8-BTBT to the lowest unoccupied molecular orbital (LUMO) of C 60 most likely leads to a decrease of threshold voltage for corresponding organic field effect transistors (OFETs) based on such a configuration. Band bending and decrease of the ionization potential (IP) are observed in the C8-BTBT film attributed to the increasing ordering of molecular orientation with the deposition of C8-BTBT. The HOMO downward shift is clearly bigger with the C 60 insertion layer than that without. A typical Volmer-Weber (VW) growth mode is detected due to the weaker van der Waals interaction between C8-BTBT and the underlying layer of C 60 /C8-BTBT than that between the C8-BTBT molecules. Compared to without C 60 interlayer, the molecular packing is a little more disordered at low coverages, resulting in more C8-BTBT molecules to lean away from the interface normal. The C8-BTBT film in such a structure phase has the HOMO closer to the E F than that at an almost perfectly vertically standing up configuration. Our results show that the introduction of C 60 insertion layer lifts up the HOMO of C8-BTBT and improves the energy level alignment at the interface.