2, Institute of Materials Science, University of Stuttgart, Stuttgart, , Germany
3, Department of Mechanical Engineering, University of Hong Kong, Hong Kong, , Hong Kong
The optimization of organic thin-film transistors (TFTs) to address specific analog and digital circuit design requirements depends heavily on the choice of the materials employed, particularly the organic semiconductor and the gate dielectric. For a particular organic semiconductor, the performance must be reviewed with different combinations of substrate material, fabrication conditions and the choice of the gate dielectric material in order to achieve the optimum TFT characteristics for particular device and circuit needs. We have fabricated and characterized organic TFTs using the small-molecule organic semiconductor 2,7-diphenylbenzothieno[3,2-b]-benzothiophene (DPh-BTBT)  in combination with a hybrid gate dielectric composed of aluminum oxide and a fluoroalkyl-phosphonic acid self-assembled monolayer. We find that the optimum substrate temperature during the vacuum deposition of the DPh-BTBT layer depends on the type of substrate on which the organic TFTs are fabricated. For vacuum-deposited DPh-BTBT to form a closed layer with the highest possible charge-carrier mobility, the semiconductor deposition should be carried out with the substrate held at a temperature of 100°C on silicon substrates and without substrate heating on flexible plastic substrates. XRD measurements indicate different orientations of the DPh-BTBT molecules under different fabrication conditions, which can be correlated to the TFT performance. Furthermore, fluoroalkyl-phosphonic acids with fluoroalkyl chain lengths ranging from 6 to 14 carbon atoms have been used to vary the thickness of the self-assembled monolayer in the gate dielectric, and its influence on the TFT characteristics has been studied. The fluoroalkyl terminal groups facilitate a shift of the TFTs’ turn-on voltage  in a deterministic manner, e.g., to exactly 0 V, if so desired from a circuit-design perspective, while taking advantage of the possibility to optimize the other TFT parameters by selecting the optimum fluoroalkyl chain length. We find that a medium fluoroalkyl chain length (10 carbon atoms in the fluoroalkyl chain) leads to the highest charge-carrier mobility (0.4 cm2/Vs on flexible plastic substrates, 1 cm2/Vs on silicon substrates) and the largest on/off current ratio (above 106).  K. Takimiya et al., J. Am. Chem. Soc. 128, 12604, 2006.  U. Kraft et al., J. Mater. Chem. 20, 6416, 2010.