STRUCTURAL BASIS FOR HIGH CARRIER MOBILITY IN CONJUGATED OLIGOMERS

In order to understand the structural parameters which control the charge transport properties in conjugated molecular semiconductors, we have analysed the ohmic conductivity and field-effect mobility in a series of short conjugated thiophene oligomers, alpha-nT, ranging from the trimer, alpha-3T, to the octamer, alpha-8T. It is first shown that a critical conjugation length, corresponding to alpha-5T, is required for observing a field-enhanced current in thin film transistors based on these materials. Field-effect mobility, together with conductivity, then increases with conjugation length, up to a very high value of the order of 10(-1) cm2 V-1 s-1 for alpha-6T. This high mobility value is confirmed by results from space-charge limited current analysis of Au/alpha-6T/Au sandwich structures, which furthermore indicate the presence of a shallow trapping of the charges, located at 0.2 eV above valence band, and associated to a hopping or to a multiple trapping mechanism for charge transport. A levelling of electrical properties appears for alpha-8T, which is interpreted by the increase of conjugation defects in thiophene oligomers as their chain length increases. Furthermore, the absence of any field-effect mobility in the distorted chains of para-sexiphenylene confirms that the extent of conjugation and absence of structural defects are the most significant parameters for achieving high carrier mobility in these molecular materials.