Oxygen effect on the interface formation between calcium and a polyfluorene film

Abstract

The effect of oxygen on the interface formation between Ca electrode and a poly (9,9-dioctylfluorene) (PFO) film was investigated using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The XPS results indicated that strong interaction between Ca and C occurred and the lowest unoccupied molecular orbitals (LUMO's) of PFO were affected upon a submonolayer Ca deposition. After ${\mathrm{O}}_{2}$ exposure, the interaction between Ca and C was reduced and the LUMO of PFO was partially recovered. However, too much ${\mathrm{O}}_{2}$ exposure $(>{10}^{5}\mathrm{L}$ in our experiments) would again deteriorate the original LUMO and result in a very wide band gap, which might be due to changes in the chemical structures of PFO in the Ca-doped region after extra ${\mathrm{O}}_{2}$ exposure. The UPS results confirmed that the deformed UPS spectrum after Ca deposition can be partially recovered and the Ca-induced bipolaron states in the former forbidden energy gap could be removed upon ${\mathrm{O}}_{2}$ exposure.