Localized and extended states in doped polyaniline: The effect of bond-length alternation

We present numerical studies of localization properties of oxidized and protonated disordered polyaniline. The calculations are performed on the single electron level, using H\"uckel theory for {π} electrons. The spatial extension of the eigenstates at the Fermi energy is determined by a fast and accurate transfer-matrix scheme. The spatial extension of eigenstates diverges for a large degree of protonation, provided all bonds within the benzoid and quinoid can be described with the same set of tight-binding parameters. Localization effects become important for small degrees of protonation and in systems that show bond-length alternation in quinoid rings and aromatic bonds in benzoid rings.