N and P-type zwitterion gated organic field effect transistors†

Low voltage operation in organic field effect transistors (OFETs) requires dielectric materials with extremely large capacitance. We explored a novel zwitterion-based dielectric material prepared using 4-(3-Butyl-1-imidazolio)-1-butanesulfonate (ZI) in a poly(vinyl alcohol) (PVA) polymer matrix. P-type OFET devices were fabricated with poly(3-hexylthiophene-2,5-diyl) (P3HT), their performance was found to be strongly humidity dependent with humidified devices producing roughly the same current at a voltage nearly 30 times lower than devices tested under an inert atmosphere. N-type OFETs based on poly{[N,N′-bis(2-octyldodecyl)napthlene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-2T)) also showed improved current levels in humidified devices, but possessed a low on/off ratio. Impedance measurements of the dielectric film showed a marked increase in the magnitude and frequency response of the capacitance with increasing humidity. The process can be modelled in terms of a single rate-limiting process using the Havriliak–Negami equation. Infrared spectroscopy was used to further examine the intermolecular interactions responsible for the humidity-dependent capacitance. Changes were observed in the spectrum of PVA with ZI inclusion and with respect to humidity. We hypothesize that the ZI molecules rotate in response to an applied field and that rotation is inhibited by strong intermolecular interactions between ZI molecules and the polymer matrix under dry conditions. This hypothesis also can be used to rationalize the low on/off ratio of the P(NDI2OD-2T) transistors. In sum, we demonstrate a material with capacitance values approaching those of an electrostatic double layer and demonstrated that local intermolecular interactions are central to understanding material behavior.