n- and p-doped poly(3,4-ethylenedioxythiophene): Two electronically conducting states of the polymer

Neutral poly(3,4-ethylenedioxythiophene) (PEDOT) thin films can be switched to an electronically conducting form either by oxidation (p-doping) or red uction (n-doping) in anhydrous organic solvents. The maximum attainable n-c onductivity is ca. 1% of the maximum p-conductivity. However, based on spec troelectrochemical and in-situ conductance measurements, the p-conductivity regime can be divided into two domains, in which either positive polarons or bipolarons and free carriers are the major charge carriers. In the n-con ductivity regime, voltammetric, spectral, and conductance data suggest only the generation of negative polaron-type carriers. These results imply that the conductivity due to positive or negative polarons is of the same order of magnitude and that the higher maximum p-conductivity may be attributed to the generation of other charge carriers in the highly stable oxidized PE DOT films. The reduced form is not stable even in an extremely dry oxygen-f ree environment, which severely hampers the use of n-doped PEDOT in practic al applications. The conductivity of pristine PEDOT films can be markedly e nhanced by successive p-doping and undoping. In addition, the conductance s lowly increased after an anodic potential step. The origin of these effects is not known.