Ionic-Liquid-Originated Carrier Trapping Dynamics at the Interface in Electric Double-Layer Organic FET Revealed by Operando Interfacial Analyses

Abstract

Ionic liquids (ILs) with their high dielectric constants (k) are promising dielectrics of organic field-effect transistors (OFETs). However, it is known that interfacial polarizations of dielectrics sometimes cause carrier trappings. In this paper, we investigated the carrier trapping behavior at the interface by using a benchmark system of electric double-layer OFETs (EDL-OFETs) with high device performance and stability: rubrene single crystal as an organic semiconductor showing high hole career mobility and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIM-FSI) as an IL gate dielectric. Apparent bias stress was observed for the system: the operational voltage of the EDL-OFET increased to more than double in ∼2 h. Operando electrochemical frequency modulation atomic force microscopy revealed the gradual structuring of the first IL layer facing the carrier-injected rubrene surface. Molecular dynamics (MD) simulations suggested the reorientations of ions in the first IL layer with electrostatic stabilization by forming a patchwork structure. In this stable configuration, negatively charged O and F atoms of the FSI anions pointing to the positively charged rubrene surface come closer, leading to the gradual hole carrier trapping. It was also found that the structured IL layer is deformed and IL-originated bias stress recovers immediately by applying a large OFF-state gate voltage for detrapping the hole carrier.