Polythiophene - Mechanism of Conductivity and Doping

Mechanism of Conductivity and Doping

Electrons are delocalized along the conjugated backbones of conducting polymers, usually through overlap of π-orbitals, resulting in an extended π-system with a filled valence band. By removing electrons from the π-system (“p-doping”), or adding electrons into the π-system (“n-doping”), a charged unit called a bipolaron is formed (see Figure 1).

Doping is performed at much higher levels (20–40%) in conducting polymers than in semiconductors (<1%). The bipolaron moves as a unit up and down the polymer chain, and is responsible for the macroscopically observed conductivity of the polymer. For some samples of poly(3-dodecylthiophene) doped with iodine, the conductivity can approach 1000 S/cm. (In comparison, the conductivity of copper is approximately 5×105 S/cm.) Generally, the conductivity of PTs is lower than 1000 S/cm, but high conductivity is not necessary for many applications of conducting polymers (see below for examples).

Simultaneous oxidation of the conducting polymer and introduction of counterions, p-doping, can be accomplished electrochemically or chemically. During the electrochemical synthesis of a PT, counterions dissolved in the solvent can associate with the polymer as it is deposited onto the electrode in its oxidized form. By doping the polymer as it is synthesized, a thick film can build up on an electrode—the polymer conducts electrons from the substrate to the surface of the film. Alternatively, a neutral conducting polymer film or solution can be doped post-synthesis.

Reduction of the conducting polymer, n-doping, is much less common than p-doping. An early study of electrochemical n-doping of poly(bithiophene) found that the n-doping levels are less than those of p-doping, the n-doping cycles were less efficient, the number of cycles required to reach maximum doping was higher, and the n-doping process appeared to be kinetically limited, possibly due to counterion diffusion in the polymer.

A variety of reagents have been used to dope PTs. Iodine and bromine produce high conductivities but are unstable and slowly evaporate from the material. Organic acids, including trifluoroacetic acid, propionic acid, and sulfonic acids produce PTs with lower conductivities than iodine, but with higher environmental stabilities. Oxidative polymerization with ferric chloride can result in doping by residual catalyst, although matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) studies have shown that poly(3-hexylthiophene)s are also partially halogenated by the residual oxidizing agent. Poly(3-octylthiophene) dissolved in toluene can be doped by solutions of ferric chloride hexahydrate dissolved in acetonitrile, and can be cast into films with conductivities reaching 1 S/cm. Other, less common p-dopants include gold trichloride and trifluoromethanesulfonic acid.