Since 1997, when Heeger, Shirakawa and MacDiarmid et al have discovered high electrical conductivity of doped polyacetylene, organic conjugated systems have been subject of extensive interest. Nowadays, conjugated polymers are applied in organic electronic devices, like light emitting diodes (OLED) or field effect transistors (OFET), to give few examples. Principal advantages of organic electronics consist of facile processibility of organic semiconductors and of the possibility of precise tuning of their electronic and/or optoelectronic properties by the modification of the molecular structure. However, the next important factor which has to be taken into account is their supramolecular organization. Low dimensional character of conjugated macromolecules combined with their specific supramolecular organization may result in anisotropy of the charge carriers mobility.

Recent results concerning STM studies of the supramolecular organisation of poly(4,4”-dioctyl-2,2’:-5’,2”-terthiophene) - a very promising polymer for the construction of OFETs will be presented. The post-reaction mixture was fractionated into fractions of low polydispersity. Then, the monolayers prepared for each fraction were imaged in molecular resolution. By this approach, the transformation of the film structure, caused by the increase of molecular weight (corresponding to the elongation of conjugated chain) could be observed. Shorter macromolecules form well defined two-dimensional islands. Interactions between longer macromolecules become anisotropic. As a consequence the supramolecular organization is columnar and consists of more or less separated columns of ordered macromolecules. The conductivity of organic films is a compromise between two counteracting factors: extension of the conjugation length and significantly more difficult supramolecular ordering expected for longer macromolecules. The presented discussion based on STM results is therefore of interest for potential applications.

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