Field-effect transistors based on thiophene hexamer analogues with diminished electron donor strength

Several new p-type semiconducting materials with lower electron-donating ab ility than the parent sexithiophene were synthesized and their thermal, mor phological, and FET properties were investigated. The incorporation of thia zole rings into oligothiophenes was designed to lower the highest occupied molecular orbital (HOMO) level of the molecules and hence make them less su sceptible 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 10(4) for gate voltages of -100 and 0 V) were routine ly obtained from devices operating in air, eliminating the need for strict exclusion of oxygen. The threshold voltage of devices made from this compou nd showed no signs of shifting toward more positive gate voltages after mor e than one month's exposure to the air. The thiazole-containing oligomers g enerally had lower field-effect mobilities than the corresponding oligothio phenes, possibly due to larger charge injection barrier and. less favorable morphologies of the evaporated films. The electrical characteristics of th e previously uninvestigated alpha,omega-dihexyl quinquethiophene (DH alpha 5T) are also reported in this paper. Single-crystal morphologies were obser ved in evaporated films of DH alpha 5T. Films evaporated at an optimal depo sition temperature (T-D = 155 degrees C) gave mobilities as high as 0.1 cm( 2)/Vs. As in the cases of pentacene and DH alpha 4T, we believe that the gr eatly enhanced mobility is probably correlated with the single-crystal morp hology. DH alpha 5T molecules also have an orientation different from the e ssentially perpendicular one of other oligothiophenes; they were found to b e tilted at similar to 30 degrees to the substrate.