Controlling the Doping Mechanism in Thin Film Transistors Through Design of Polymeric Ionic Liquid Gate Dielectrics

Two disparate modes of operation can occur when gating an organic thin film transistor (OTFT) with an electrolyte. Field-effect (FE) doping occurs when the semiconductor is impermeable to ions, whereas electrochemical (EC) doping of the bulk occurs when the active layer is permeable to ions. Here, we present a method to control the mode of charge accumulation in an OTFT with a constant semiconducting layer by gating with polymeric ionic liquids (PILs) of opposite polarity. Ion infiltration into the active layer is driven by attraction between ions and electronic carriers of opposite charge. As a result, tethering either the anion or the cation to a polymer backbone in the dielectric enables a direct comparison between FE and EC doping. Two PILs of opposite polarity have been synthesized and employed as the dielectric in p-type OTFTs. Tethering either the anion or the cation is shown to dictate whether ions infiltrate the active layer. Interfacial FE doping decreases the accumulation layer thickness and increases the carrier concentration as a function of injected charge. The local carrier concentration in the accumulation layer is found to be the main factor effecting the threshold voltage and the conductivity of the devices, despite the difference in doping mechanisms.