Direct Evidence for Delocalization of Charge Carriers at the Fermi Level in a Doped Conducting Polymer

Abstract

The infrared absorption spectrum of the polaron charges at the Fermi level $E_F$ in a heavily $p$-doped conducting poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) film has been measured using interferogram-modulated Fourier-transform charge-modulation spectroscopy. The spectrum indicates softer phonons and weaker electron-phonon coupling riding on a strongly redshifted Drude-like electronic transition, different from the population-averaged “bulk” spectrum. This provides direct evidence that the $E_F$ holes are sufficiently delocalized even in such disordered materials to reside in an energy continuum (band states) while the rest of the hole population resides in self-localized gap states.