Alkyl side chain length modulates the electronic structure and electrical characteristics of poly(3-alkylthiophene) thin films

The alkyl side chain length in poly(3-alkylthiophene) (P3AT) was found to affect the electrical properties and molecular electronic structures in thin films. The self-assembly and morphology of a P3AT film was easily controlled through the solvent vapor pressure (varied over the range 0-55 kPa) during solidification. Under high solvent vapor pressure conditions, long dense nanowires formed in the P3AT thin films, and the electrical properties of field-effect transistors (FETs) based on these films improved. The enhancement in the electrical properties of FETs based on a P3AT nanowire structure was strongly affected by the alkyl side chain length. Alkyl side chains in the disordered P3AT thin film act as a barrier to charge movement; however, they assist in the self-assembly of P3AT under high solvent vapor pressures via alkyl chain interactions. Sufficiently long alkyl chains in P3AT molecules, however, form an insulating barrier between the conjugated backbone and the Au electrode, thereby preventing carrier injection and reducing the electrical characteristics of an FET device.