Structural effects on the characteristics of organic field effect transistors based on new oligothiophene derivatives

We report on a comparative investigation of a new series of oligothiophene derivatives as active semiconductor in organic field effect transistors (OFET). Quater- and sexithiophenes end-capped with linear hexyl chains or fused phenyl rings have been synthesized. The oligomers have been deposited by vacuum evaporation onto Si-p(++) substrates. The gate dielectric was a bilayer PMMA/Ta2O5 which ensures both good field effect mobility and rather low operating voltage. The thickness of the oligothiophene films was 80 nm. OFET performances were analyzed as a function of the nature of molecules. The analysis of the characteristics of the various devices shows that the nature and the number of the end group exert a considerable influence on the field effect mobility of the resulting devices. Adding hexyl end group improves the mobility up to 0.1 cm(2) V-1 s(-1). Reference di-hexyl-sexithiophene OFETs exhibit a mobility of 0.13 cm(2) V-1 s(-1). Fused phenyl ring end terminations have also been studied. Poorer performances are obtained with this termination. Our results are discussed in terms of steric interactions and also on the basis of the influence of the end group in the oligothiophene on the molecular arrangement on the surface of the substrate. (C) 2004 Elsevier B.V. All rights reserved.