CHARACTERIZATION OF THE INTRACHAIN CHARGE-GENERATION MECHANISM OF ELECTRONICALLY CONDUCTIVE POLY (3-METHYLTHIOPHENE)

The charge carriers and band structure that evolve upon doping of electrochemically grown poly (3-methylthiophene) (PMeT) films are characterized as a function of film preparation conditions by optical absorption, infrared, and electron spin resonance measurements. Regardless of the solvent (acetonitrile or propylene carbonate) or dopant anion (ClO4-, PF6-, BF4-, or CF3SO3-) used in preparing these films, bipolarons are found to play a prominent role in the charge-generation mechanism of PMeT at doping levels above 2 mol %. At low doping concentrations (< 2 mol %), structural disorder effects in the polymer are evident. The amount of structural disorder depends on the nature of the solvent and dopant anion. The binding energy of bipolarons to dopant anions was evaluated and found to be independent of the identity of the dopant anion. Solvent-dependent shifts in the bipolaron gap states and the binding energy are observed. These results are interpreted in terms of current theories of charge-carrier production in conductive polymers.