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 lower off currents than those of typical sexithiophenes. Relatively high on/off current ratios (greater than 10 sup 3 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 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 alpha,omega-dihexyl quinquethiophene (DHalpha5T) are also reported in this paper.