All-organic field-effect transistors made of π-conjugated oligomers and polymeric insulators

All-organic field-effect transistors (FET) based on various oligothiophenes and insulators are reviewed. These devices generally work in the accumulation mode, but can also be operated in the depletion regime. The performances of organic FETs, as mirrored by their field-effect mobility μ_FET, are strongly dependent on the nature of both the semiconducting oligomer and the insulator. A general trend is the increase of μ_FET as the chain length increases. α-Substitution with alkyl chains also increases the mobility, whereas ß-substitution leads to an almost complete disappearance of the field effect. The use of an organic insulator with a high dielectric constant, cyanoethylpullulane (CYEP), resulted in a considerable increase of μ_FET. All these behaviors are attributed to structural effects, and more specifically to changes in crystallinity. The study of the depletion regime gives indications on the concentration of dopants and free carriers. Free carriers are two to three orders of magnitude less numerous than dopants. Finally, measurements on freshly made and aged devices give information on the process of oxygen doping in thiophene oligomers.