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

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 O-2 exposure, the interaction between Ca and C was reduced and the LUMO of PFO was partially recovered. However, too much O-2 exposure (>10(5) L in our experiments) would again deteriorate the original LUMO and result ina very wide band gap, which might be due to changes in the chemical structures of PFO in the Ca-doped region after extra 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 O-2 exposure.