The polaron and bipolaron contributions to the electronic structure of polypyrrole films

UPS and NEXAFS spectroscopy have been used to study the changes in the electronic structure of polypyrrole films by varying the dopant anion. In particular, we find the density of states within the electronic gap to be influenced by the chemical structure of the dopant anion. From the position of the Fermi level in the bandgap, the charge carriers in the polymer can be identified either as polarons or as bipolarons. For the first time, we used copper phthalocyanine trisulfonate as dopant, resulting in a high concentration of bipolarons which have a high contribution from the wavefunctions of the pyrrole–nitrogen atoms. Interaction of the π-systems of the polymer and the dopant causes a change in the chemical nature of the nitrogen from amino-like to imino-like.