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

Abstract

Modification by ultra-thin fullerene (C60) 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 (SiO2) 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 C60 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 C60 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 C60 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 C60/C8-BTBT than that between the C8-BTBT molecules. Compared to without C60 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 EF than that at an almost perfectly vertically standing up configuration. Our results show that the introduction of C60 insertion layer lifts up the HOMO of C8-BTBT and improves the energy level alignment at the interface.