The effect of ketonic defects on electrical properties, i.e., the performance of organic field-effect transistors (OFETs) was examined in fluorene end capped fused bithiophene oligomers (BFTT). The long wavelength emission at 2.1-2.3 eV resulting from the ketonic defects was observed in photoluminescence spectra of BFTT films after UV irradiation in air. In addition, the peak corresponding to the carbonyl stretching mode of the fluorenone moiety at 1721 cm(-1) was also apparent after UV irradiation for periods longer than 6 h in air. These observations confirm that ketonic defects are present in the fluorene units of BFTT after photo-oxidation. The threshold voltage (V-th), i.e., switch-on voltage, of OFETs was increased and field-effect mobility (mu(FET)) was decreased after the formation of the ketonic defects, since these defects induce the formation of numerous trap sites in the bandgap of the semiconducting conjugated oligomer. (C) 2004 American Institute of Physics.