Experimental and theoretical studies of the electronic structure of Na-doped poly (para-phenylenevinylene)

The electronic structure of sodium-doped poly (p-phenylenevinylene), or PPV, has been studied using photoelectron spectroscopy, UPS and XPS. Upon doping, two new states are created in the previously forbidden electronic bandgap. No finite density- of-states is observed at the Fermi energy. The UPS spectra are analysed with the help of VEH-level quantum chemical calculations. It is determined that the Na-doping of PPV results in the formation of bipolaron bands in the otherwise forbidden energy gap at saturation doping. These results are in contrast with the case of poly-hexyl-thiophene doped from NOPF₆, where the existence of a finite density-of-states at E_F and a stable polaron lattice was observed at saturation doping at room temperature. This work represents the first direct measure of multiple, resolved gap states in a doped conjugated polymer.