Photoelectron spectroscopic studies of poly(9,9-dioctylfluorene)–potassium interface, and its influence by oxygen

The electronic structure of poly(9,9-dioctylfluorene) (PFO) coated with potassium has been studied by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The low work function of K led to low or no injection barrier at the K/PFO interface and induced bipolaron gap states. With increasing K coverage, the bipolaron states and the highest-occupied molecular orbitals gradually broadened, and the two shake-up peaks of C 1s XPS peaks associated with the lowest-unoccupied molecular orbitals of PFO also broadened, diminished and eventually vanished. Upon slight oxygen exposure, the two bipolaron states disappeared and the deformed features in the UPS and XPS spectra were partially recovered.