Fang-Chi Hsu

Electric-field induced ion-leveraged metal–insulator transition in conducting polymer-based field effect devices

Abstract The field effect devices prepared completely from conducting polymers, especially poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS), were studied. Normally in a conductive “on” state, the transistor-like device has a transition to a substantially less conductive “off” state at an applied positive gate voltage, typically ∼15–25 V. The current ratio I off / I on can exceed 10 −4 at room temperature. We have found that the field effect is strongly temperature dependent and is substantially reduced upon decreasing the temperature by only a 10 °C. This loss of current reduction upon application of a gate voltage is not due to the temperature dependence of the electrical conductivity of polymers of which the devices are made. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage, though with an increased activation energy, T 0 . We suggest that the conducting polymer is near the metal–insulator transition and that the field effect in the device is related to the electric field modulating this transition in the region underneath the gate electrode. The transition is controlled and leveraged by ion motion. The time dynamics of the current with the gate modulation strongly supports our conjecture. We demonstrate the generality of the phenomena by presenting similar results for devices fabricated from the conducting polypyrrole doped with Cl.

Doped conducting polymer-based field effect devices

Field effect devices (FEDs) prepared completely from conducting polymers PEDOT/PSS (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) and polypyrrole doped with Cl - (PPy/Cl - ) are reported. In the on (conductive) state when the gate voltage V G is applied, these FEDs have threshold turn-off positive V g s of - 15 V (varying with polymer, geometry, and preparation conditions). The current ratio I on /I off can exceed 10 +4 at room temperature. We have found this field effect is strongly temperature dependent and nearly entirely disappears with decrease of temperature by as little as ten degrees. The conductance of the active channel has stronger temperature dependence when V G exceeds the threshold voltage. Time dynamics of drain current with gate voltage modulation and its temperature dependence supports that this transition is coupled with ion motion. We suggest that the conducting polymer is near the insulator-metal transition (IMT) and this field effect caused by positive V g in the FED may be related with this IMT in the region underneath the gate.