Field-Effect Transistors Based on Thiophene Hexamer Analogues with Diminished Electron Donor Strength

Several new p-type semiconducting materials with lower electron-donating ability than the parent sexithiophene were synthesized and their thermal, morphological, and FET properties were investigated. The incorporation of thiazole rings into oligothiophenes was designed to lower the highest occupied molecular orbital (HOMO) level of the molecules and hence make them less susceptible to p-doping. FET devices based on a dihexylated six-ring compound with thiazoles as the central rings indeed showed enhanced stability to p-doping over those of typical sexithiophenes. Relatively high on/off current ratios (greater than 104 for gate voltages of −100 and 0 V) were routinely obtained from devices operating in air, eliminating the need for strict exclusion of oxygen. The threshold voltage of devices made from this compound showed no signs of shifting toward more positive gate voltages after more than one month's exposure to the air. The thiazole-containing oligomers generally had lower field-effect mobilities than the corresponding oligothiophenes, possibly due to larger charge injection barrier and less favorable morphologies of the evaporated films. The electrical characteristics of the previously uninvestigated α,ω-dihexyl quinquethiophene (DHα5T) are also reported in this paper. Single-crystal morphologies were observed in evaporated films of DHα5T. Films evaporated at an optimal deposition temperature (TD = 155 °C) gave mobilities as high as 0.1 cm2/Vs. As in the cases of pentacene and DHα4T, we believe that the greatly enhanced mobility is probably correlated with the single-crystal morphology. DHα5T molecules also have an orientation different from the essentially perpendicular one of other oligothiophenes; they were found to be tilted at ∼30° to the substrate.