**Note to Chair**The presenting author, Bhooshan C. Popere, is a faculty candidate.
Polymerized ionic liquids (PILs) have recently emerged as a new class of materials with potential applications in energy conversion and storage, electromechanical actuators, catalysis and separations, and high k dielectrics, amongst many others. The large electrochemical stability window, non-flammability and lack of volatile organics impart these materials with properties heretofore lacking in conventional ionic materials. An attractive feature of such polymeric materials is their ability to form electrical double layers (EDLs) upon polarization at the interface with an electrode. The large electric fields due to the small EDL thickness results in unusually large areal capacitance values in comparison with most organic materials. This makes PILs attractive candidates as high k dielectric materials for low-voltage operable field effect transistors (FETs). However, the fundamental understanding of how the molecular structure of these PILs affects their dielectric properties is only beginning to emerge.
To gain a deeper insight into the dielectric behavior of these materials, we have investigated a class of random copolymers based on dialkyl imidazolium pendant groups with varying compositions of the PIL component. Our experiments reveal that in presence of a relatively non-polar comonomer, such as butylacrylamide, the effective capacitance of the resulting copolymers varies as a function of the ionic liquid (IL) monomer content. Analyses of the complex dielectric spectra (obtained from Dielectric Relaxation Spectroscopy) indicate three primary responses: a high-frequency (104 – 106 Hz) relaxation of ion dipoles, an intermediate-frequency (102 –104 Hz) relaxation due to DC ion conduction and a low-frequency (0.1 – 102 Hz) relaxation due to polarization of ions at the electrode/PIL interface. We observe that an increase in the IL content depresses the glass transition temperature (Tg) of the resulting copolymer, which concomitantly shifts the responses of the above-mentioned processes to higher frequencies at a given temperature. The onset of electrode polarization also marks the onset of the frequency-independent capacitive charging of the electrical double layers. Thus, lower Tg results in less impeded ion motion leading to more facile electrode polarization. We also find that the primarily resistive response of the DC ion conduction can be minimized by decreasing the thickness of the dielectric. We hypothesize that in the limiting sample thickness comparable to that of the EDL, the only material response to applied potential should be the charging and the discharging of the EDL. Indeed, the onset of electrode polarization for a given polymer composition and temperature shifts to higher frequencies with decreasing film thickness.
Our preliminary experiments with using these PILs as actual gate dielectrics in organic FETs indeed suggest a dependence of operational threshold voltage on the specific capacitance as determined at low frequencies. Specifically, for conventional organic semiconducting polymers like P3HT and PBTTT, the threshold voltages with our PILs as gate dielectrics are less than 1V indicating a true electrolyte gating mechanism.