In Situ Measurement of Effective Gate Bias Voltage in Ionic Liquid-Gated Organic Field-Effect Transistors: Exploring Intrinsic Performance and Trap Density of States.

Electric double layer (EDL)-mediated transistors with ionic liquid (IL) gating have garnered substantial interest due to their exceptional properties, such as high transconductance and low-voltage operation, positioning them as promising candidates for organic electronics. In this study, we present an in situ measurement of effective gate bias voltage ( V GS,eff ) in IL-gated organic field-effect transistors (IL-OFETs) using a modified current-voltage measurement configuration. The results reveal a significant deviation between V GS,eff and the applied gate bias ( V GS,app ), indicating that the EDL at the gate/IL interface screens the applied voltage. It is observed that the screening effect varies depending on the specific cation and anion present in the IL. The evaluation of V GS,eff plays a pivotal role in understanding the intrinsic behavior of IL-OFETs and addresses the challenges associated with accurate performance assessment. Inherently, IL-OFETs demonstrate high transconductance, achieving values of approximately 9 mS while operating at a low threshold voltage of around 0.55 V. Through the acquisition of V GS,eff , we have successfully addressed the limitations impeding the numerical estimation of the trap density of states (trap DOS) in IL-OFETs. Remarkably, our calculations reveal an exceptionally low density of deep traps, which serves as a crucial factor contributing to the near-ideal subthreshold swing (61-68 mV dec -1 ) observed in IL-OFETs. Further investigations unveil the neutral electrical nature of the IL bulk during OFET operation, confirming the hypothesis that the applied gate bias voltage in electrolyte-gated OFETs drops across the EDLs formed at the interfaces. The impedance spectroscopic (IS) analysis confirms the low contact resistance (≈1 Ω·m) of IL-OFETs calculated using the transition voltage method. The IS analysis also reveals the low-transmissive nature of the IL/organic semiconductor interface. The knowledge gained from this study holds significant implications for realizing high-performance electrolyte-gated OFETs in various applications including digital electronics, energy storage, and sensing. By unraveling the factors influencing the device performance, such as V GS,eff and trap DOS, this research contributes to the advancement of organic electronics and paves the way for future developments in the field.