CONTROL OF THE MESOSCOPIC ORGANIZATION OF CONJUGATED THIOPHENE OLIGOMERS, INDUCED BY SELF-ASSEMBLY PROPERTIES

In order to analyse the dependence of charge transport in conjugated materials on their structural organization, three sexithiophene derivatives were studied, unsubstituted 6T and its di-hexyl substituted compounds, both on the terminal alpha positions (alpha,omega)DH6T) and as pendant groups in beta position (beta, beta'DH6T). X-ray diffraction characterization of vacuum evaporated thin films of these oligomers shows that 6T and alpha,omegaDH6T consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. A large increase of molecular organization at the mesoscopic level is observed when passing from 6T to alpha,omegaDH6T, as shown by a much longer range ordering. Electrical characterizations show that the conductivity of alpha,omegaDH6T is anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, ie along the stacking axis. The charge carrier mobility, determined on field-effect transistors realized from these conjugated oligomers, also shows a large increase when passing from 6T to alpha,omegaDH6T, reaching a value of 5 x 10(-2) cm2 V-1 s-1, in contrast with beta,beta'DH6T which presents very low conductivity and mobility. These results are attributed to the self-assembly properties brought by alkyl groups in the alpha,omega position.