Oligothiophenes of DDnT and DPnT type: Morphology, field effect transistors, performance in vacuum and in air

Field effect transistors were prepared by vacuum sublimation of the oligothiophenes DDnT and DPnT. Their electrical performance was investigated in vacuum ("in situ") and after exposure to air ("ex situ"). Their morphology was compared with that of end-capped oligothiophenes, ECnT, and the influence of the number of monomer units and of the chain end molecules was studied. The in situ as well as ex situ drain current-voltage characteristics exhibit linear and saturation regions, however, with a large drain current increase after exposure to air due to oxygen doping. A change in layer thickness from 10 to 120 nm resulted in only a small drain current change in vacuum indicating that most of the OFET conduction takes place near the interface with the gate insulator (SiO2). In situ values for threshold voltage, field effect mobility, transconductance, and carrier density are reported. The carrier density increases by a factor of 10 after exposure to air. The highest field effect mobility (4.8x10(-3) cm(2)/Vs) was observed for DP5T, which had the lowest carrier concentration and the smoothest surface morphology. The influence of the gate voltage on the transconductance is discussed. (C) 2005 Elsevier B.V. All rights reserved.