Characterization and simulation of electrolyte-gated organic field-effect transistors

In this work we fabricate and characterize field-effect transistors based on the solution-processable semiconducting polymer poly(3-hexylthiophene) (P3HT). Applying two independent gate potentials to the electrolyte-gated organic field-effect transistor (EGOFET), by using a conventional SiO(2) layer as the back-gate dielectric and the electrolyte-gate as the top-gate, allows the measurement of the electrical double layer (EDL) capacitance at the semiconductor-electrolyte interface. We record the transfer curves of the transistor in salt solutions of different concentration by sweeping the bottom gate potential for various constant electrolyte-gate potentials. A change of the electrolyte-gate potential towards more negative voltages shifts the threshold voltage of the bottom-gate channel towards more positive back-gate potentials, which is directly proportional to the capacitive coupling factor. By operating the EGOFET in the dual-gate mode, we can prove the dependency of the EDL capacitance on the molarity of the electrolyte according to the Debye-Hückel theory, and additionally show the difference between a polarizable and non-polarizable electrolyte-gate electrode. With the experimentally obtained values for the EDL capacitance at the semiconductor-electrolyte interface we can model the electrolyte-gate transfer characteristics of the P3HT OTFT.