Ion gels as dielectrics in organic thin-film transistors

Ion gels composed of ionic liquids and block copolymers can be used as gate dielectrics in organic thin-film transistors. Conventional dielectrics used in OTFTs do not have capacitances large enough to enable transistor operation at low voltages, which is needed in many organic electronics applications. Mobile ions in the gel form electric double layers, giving rise to a very high capacitance and therefore high charge carrier concentrations at low operating voltages. The goal of this study was to fabricate a functioning OTFT using an ion gel as gate dielectric, and to examine its properties. The OTFT was prepared with a top-gate geometry, where the ion gel was sandwiched between the gate electrode and the substrate with the semiconductor channel. The semiconductor was amorphous poly(triarylamine), which was deposited by spin-coating. The ion gel gated OTFT properties were compared with a reference OTFT structure, which had a conventional polymeric gate dielectric. The ion gel gated transistor operated with voltages below 2 V and the output current was three orders of magnitude larger than that of the reference OTFT, which required more than ten times the voltage. The capacitance of the ion gel was over 5 uF/cm^2 up to 1 MHz. The charge carrier mobility in the OTFT was on the order of 10^-3 cm^2/(Vs), which was similar to that in previous studies with the semiconductor used. The transistor could be switched on and off at the frequency of 1 kHz, but there was a large overlap capacitance across the gel. Hysteresis and leakage were challenges in the ion gel gated OTFT preparation. The ionic liquid is hygroscopic, making the gel susceptible to water impurities. The impurities can cause leakage and hysteresis due to electrochemical reactions at the electrodes even at low voltages. Future work with ion gel gated OTFTs will require a more controlled environment in terms of ambient humidity. /Kir1