Electronic structure of poly(9,9-dioctylfluorene) in the pristine and reduced state

The electronic structure of the conjugated polymer poly (9,9-dioctylfluorene) and the charge storage mechanism upon doping with lithium atoms have been studied using a combined experimental-theoretical approach. The density of states in the valence band region was measured using ultraviolet photoelectron spectroscopy, and the spectra interpreted with the help of the results of quantum-chemical calculations carried out at the Austin Model 1/Valence Effective Hamiltonian level. The valence band spectra as well as the calculated band structure and derived density of valence states of poly(9,9-dioctylfluorene) are compared to those of poly(p-phenylene) and a ladder-type derivative of poly(p-phenylene). The deposition of lithium atoms. in situ, onto the surface of the polymer films resulted in uniform (bulk) diffusion of the lithium atoms, charge transfer to the polymer (resulting in lithium ions), and the appearance of new electronic states in the gap. At low doping levels (up to about 0.1 Li-atom per repeat unit of the polymer), these states are polarons, and, as the doping level increases, they evolve into bipolarons. (C) 2002 American Institute of Physics.