Flexible Dual‐Gate Organic Field‐Effect Transistors for Pressure Sensing

The development of sensors based on organic field-effect transistors (OFETs) for use in flexible sensors has attracted research interest. The research targets include OFET-based low-voltage sensors capable of highly sensitive detection of chemical or physical signals. Here, we developed a flexible pressure sensor based on flexible OFETs. The low-voltage (−5 V) dual-gate OFET operates by strongly modulating the drain current ( I D ) using a novel device architecture composed of a piezoelectric sensing layer and a low-voltage OFET readout. Dual-gate OFET-based pressure sensors consist of a pressure sensing element made of piezoelectric poly(vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)] and a low-voltage OFET readout. The pressure-induced response of I D , which is due to the depletion of charge carriers accumulated in the channel of the OFET, depends on the magnitude of the pressure load. This response indicates that a voltage is generated in the piezoelectric P(VDF-TrFE) layer by the application of pressure, which causes a shift in the threshold voltage ( V TH ). The magnitude of the generated voltage is derived as the magnitude of the V TH shift, and then the piezoelectric constant d 33 of the piezoelectric P(VDF-TrFE) layer is derived. As the d 33 value is in good agreement with that obtained by direct measurement, the operation mechanism of the dual-gate OFET-based pressure sensor is identified as the piezoelectric behavior of the P(VDF-TrFE) layer. A flexible dual-gate OFET-based pressure sensor that employs a thin polyethylene naphthalate (PEN, 25 μm) film as a substrate is also introduced. Because the substrate is flexible, the pressure response of the device on a curved surface was observed. The performance was equivalent to that of a device fabricated on a glass substrate. I D varied in response to a pressure load even without the application of the gate voltage. The magnitude of the change in I D in response to the applied pressure was approximately 2.5 times that of the device on the glass substrate.