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 m aterials on their structural organization, three sexithiophene derivat ives were studied, unsubstituted 6T and its di-hexyl substituted compo unds, both on the terminal alpha positions (alpha,omega)DH6T) and as p endant groups in beta position (beta, beta'DH6T). X-ray diffraction ch aracterization of vacuum evaporated thin films of these oligomers show s that 6T and alpha,omegaDH6T consist of layered structures in a monoc linic arrangement, with all-trans planar molecules standing on the sub strate. A large increase of molecular organization at the mesoscopic l evel is observed when passing from 6T to alpha,omegaDH6T, as shown by a much longer range ordering. Electrical characterizations show that t he conductivity of alpha,omegaDH6T is anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, ie alon g the stacking axis. The charge carrier mobility, determined on field- effect transistors realized from these conjugated oligomers, also show s 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 whic h presents very low conductivity and mobility. These results are attri buted to the self-assembly properties brought by alkyl groups in the a lpha,omega position.