A photoemission study of the Ooct-OPV5/SiO2/Si(1 1 1) interface: Effect of the SiO2 interlayer thickness

The influence of ultrathin (11–50 Å) SiO2 interlayers at the interface between n-doped Si(1 1 1) and a conjugated oligomer (Ooct-OPV5) film was studied by X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS). The Ooct-OPV5 [(2,5-bis(4-styryl) styryl) 1,4-dioctyloxybenzene] oligomer resembles PPV [poly(p-phenylenevinylene)], one of the more widely used polymers in organic light emitting diodes. High purity oligomer films of up to ∼10 nm thick were stepwise grown on clean SiO2 layers with various thickness under ultrahigh vacuum (UHV) conditions. A detailed study of the SiO2/Si(1 1 1) oxide film was made in order to elucidate possible effects on the electronic properties of the organic/SiO2 interface. Furthermore, the possibility of electrostatic charging of the insulating films was thoroughly examined, given that a photoemission study of the organic interface with charged SiO2 substrates is unreliable. Negligible surface electrostatic charging was found for oxide thickness up to 36 Å, and the Ooct-OPV5 interfacial properties with these SiO2 films were thoroughly studied. The band line-up at the Ooct-OPV5 interfaces with the oxide substrates was deduced from the combination of XPS and UPS results. Charge transfer between the oligomer and oxide surfaces with thickness up to 18 Å occurred by the formation of interfacial dipole layers, whereas practically vacuum level alignment was observed for thicker oxide films. The interfacial band diagrams revealed that the oxide interlayers change the eventual oligomer HOMOcut-off offset with respect to the silicon substrate VB edge at the interface region.