Interface formation between poly(9,9-dioctylfluorene) and Ca electrode investigated using photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS) have been used to study the interface formation between poly(9,9-dioctylfluorene) (PFO) and Ca electrode. As the Ca coverage increases, the vacuum energy level of PFO decreases gradually in order to match the Fermi level (EF) of Ca. The original highest occupied molecular orbital of PFO moves away from the EF, and vanishes eventually. Bipolaron states with 1.8 eV in peak interval are formed in the former energy gap. The original lowest occupied molecular orbitals of PFO are broadened resulting in a featureless shake-up in the XPS C1s core level.