Modification of an ultrathin C₆₀ interlayer on the electronic structure and molecular packing of C8-BTBT on HOPG

X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM) and X-ray diffraction (XRD) were applied to investigate the electronic structure and molecular packing of C8-BTBT on HOPG with an ultrathin C₆₀ interlayer. It was found that C8-BTBT displays a Vollmer-Weber (V-W) growth mode on HOPG, with an ultrathin C₆₀ interlayer (0.7 nm). Compared to the uniform lying-down growth mode as directly grown on HOPG, the C8-BTBT molecules here adopt a lying-down orientation at low coverage with some small tilt angles because the π-π interaction between C8-BTBT and HOPG is partly disturbed by the C₆₀ interlayer, delivering a higher highest occupied molecular orbital (HOMO) in C8-BTBT. An interface dipole of 0.14 eV is observed due to electron transport from C8-BTBT to C₆₀. The upward and downward band bending in C8-BTBT and C₆₀, respectively, near the C8-BTBT/C₆₀ interface reduces the hole transport barrier at the interface, facilitating the hole injection from C₆₀ to C8-BTBT, while a large electron transfer barrier from C₆₀ to C8-BTBT is detected at this interface, which effectively limits electron injection from C₆₀ to C8-BTBT. The HOMO of C8-BTBT near the interface is largely lifted up by the C₆₀ insertion layer, which causes a p-doping effect and increases the hole mobility in C8-BTBT. Furthermore, owing to the lowest occupied molecular orbital (LUMO) of C₆₀ residing in the gap of C8-BTBT, charge transfer occurs between C₆₀ and the trap states in C8-BTBT to effectively passivate the trapping states. Our efforts aid a better understanding of the electron structure and film growth of anisotropic molecules and provide a useful strategy to improve the performance of C8-BTBT-based devices.