High-capacitance polyurethane ionogels for low-voltage operated organic transistors and pressure sensors

In this paper, we report a facile method to fabricate high-capacitance polyurethane ionogel-based bilayer dielectrics for low-voltage and high-performance pressure-sensitive top-gate organic thin-film transistors (OTFTs). These elastomeric bilayer dielectrics are made of a neat polyurethane top layer and a bottom ionogel layer composed of a polyurethane-ionic liquid gel prepared by a simple, cost-effective dissolution process. Utilizing the various formulated ionogels with different ionic contents, controlled high capacitance values between 10 and 30 μF cm-2 are achieved, which is attributed to the formation of a combined electric double layer and dipole polarization in the ionogel/polyurethane layers, respectively. Remarkably increased hole mobilities up to ∼2 cm2 V-1 s-1 and a low operation voltage less than 6 V are achieved with a poly(9,9-dioctylfuorene-co-bithiophene) (F8T2) liquid-crystalline polymer semiconductor and by optimizing the ionic content at the bottom ionogel layer of the OTFTs. Additionally, the optimized F8T2 OTFTs show a low threshold voltage of -2 V, a high on/off ratio of ∼105, and excellent operational stability. Finally, we investigate the pressure sensing properties of the OTFTs by applying pressure on top of the polyurethane ionogel-based bilayer gate dielectric. The OTFTs showed a pressure sensitivity of 0.12 kPa-1 over a wide pressure range. This study demonstrates that employing a thin polyurethane overlayer on an ionogel dielectric is a simple and effective approach to enhance the interface contact for both printing and thermal top-gate electrode deposition for high-performance ionogel-based OTFTs and pressure sensors.