Effect of Molecular Alignment in Smectic Mesophases on Carrier Transport of Self-Organizing Molecular Semiconductors

Abstract

The general features of electrical properties of a new class of organic photoconductors, i.e., photoconductive liquid crystals, to which we prefer “Self-organizing Molecular Semiconductors” as a conceptual name, are reviewed briefly. And in two series of smectic mesophases, i.e., from SmA to SmE and from SmC to SmG in which the liquid crystalline molecules sit perpendicular to the smectic layer and tilted towards the layer, respectively, the effect of molecular alignments on the carrier transport was investigated by transient photocurrent measurements with 2-phenylnaphtalene and terthiophene derivatives corresponding to the two series. All the transient photocurrents measured were non-dispersive and ambipolar irrespective of the mesophases. The mobility increased stepwise in accordance with the upgrade of the molecular alignment and ranged from 10−4 cm2/Vs to 10−2 cm2/Vs, which was independent of electric field and temperature. The highest mobility was observed in SmE phase of 2-phenylnaphtalenes and SmG phase of terthiophenes. Judging from the analysis with two-dimensional hopping model in the smectic layers and the results from X-ray diffraction study, it was concluded that the upgrade of mobility at the phase transition from mesophase to mesophase is attributed primarily to the reduction of the molecular distance in the smectic layer irrespective of the type of the series. In addition, it was suggested that the hopping probability is enhanced possibly through a local overlap of molecular orbitals in highly ordered mesophases in which the molecular motion is limited.