POLARON TO BIPOLARON TRANSITION IN A CONJUGATED POLYMER - RUBIDIUM-DOPED POLY(P-PHENYLENEVINYLENE)

Photelectron spectroscopy has been used to study the evolution of the electronic structure induced by n-type doping of two pi-conjugated sys tems: poly(p-phenylenevinylene), or PPV, and trans,trans-1,4-distyrylb enzene, a model molecule for PPV. Doping was carried out in UHV by phy sical vapor deposition of rubidium atoms. In both systems, two differe nt doping regimes are observed. At low doping levels, for both materia ls, one new electronic state appears in the originally forbidden energ y gap, and a finite density of states is observed at the Fermi level. Upon increased doping, the observed state moves to lower binding energ y and a second state appears within the original energy gap. The evolu tion of the gap states, together with the appearance of a finite densi ty of states at Fermi level at low doping levels, indicates a transiti on from polaron to bipolaron charge storage states in these conjugated systems. It should be stressed that these results constitute the firs t direct measurements of a polaron to bipolaron transition in a conjug ated polymer using ultraviolet photoelectron spectroscopy.