Polaron to bipolaron transition in a conjugated polymer: Rubidum-doped poly(para-phenylenevinylene)

Photelectron spectroscopy has been used to study the evolution of the electronic structure induced by n-type doping of two pi-conjugated systems: poly(p-phenylenevinylene), or PPV, and trans,trans-1,4-distyrylbenzene, a model molecule for PPV. Doping was carried out in UHV by physical vapor deposition of rubidium atoms. In both systems, two different doping regimes are observed. At low doping levels, for both materials, one new electronic state appears in the originally forbidden energy gap, and a finite density of states is observed at the Fermi level. Upon increased doping, the observed state moves to lower binding energy and a second state appears within the original energy gap. The evolution of the gap states, together with the appearance of a finite density of states at Fermi level at low doping levels, indicates a transition from polaron to bipolaron charge storage states in these conjugated systems. It should be stressed that these results constitute the first direct measurements of a polaron to bipolaron transition in a conjugated polymer using ultraviolet photoelectron spectroscopy.