An investigation of the electronic properties of poly(4,4′-dipentoxy-2,2′-bithiophene)

A study of the electronic properties of a polythiophene (poly(4,4'-dipentoxy-2,2/-bithiophene) as functions of the positive doping level has been carried out. The oxidative cyclic voltammogram and the electron spin resonance (ESR) spectrum show the consecutive formation of polaron and bipolaron states on increasing the doping level of the polymer. From the optical spectra it was found that the pi-pi* band undergoes a blue shift with the doping. The Fermi level of the polymer was derived from the diffusion potential of the heterojunction between n-doped silicon and poly(4,4'-dipentoxy-2, 2'-bithiophene). As expected for p-type inorganic semiconductors, the Fermi level was found to be pushed towards the valence band by increasing the doping level. The trend of the experimental Fermi level is in good agreement with the one calculated for a p-type semiconductor with the same doping level and bandgap as those of poly(4,4'-dipentoxy-2,2'-bithiophene).