STRUCTURE EFFECT ON TRANSPORT OF CHARGE-CARRIERS IN CONJUGATED OLIGOMERS

The carrier mobility mu(FET) of a series of unsubstituted, alpha alkyl -susbtituted and beta alkyl-susbtituted thiophene oligomers nT has bee n measured on field-effect transistors based on these organic semicond uctors. A large increase of mu(FET) with conjugation length n has been observed up to the hexamer 6T. For a given length n, alkyl susbtituti on in alpha position leads to a steep rise of mobility, which reaches values close to 1 cm2V-1 s-1. These important values, which require hi gh efficiency in intermolecular charge transfer, are attributed to the long range structural ordering appearing in thin films of these molec ules, as shown by X-ray characterizations. On the other hand, the decr ease of mu(FET) observed in the case of beta-substituted oligothiophen es is related to the large structural modifications originating from t he pendant bulky alkyl groups. The occurrence of radical cation or dic ation charged states is also discussed for understanding the effect of conjuagtion length, n, on carrier mobility.