Sub-3 V, MHz-Class Electrolyte-Gated Transistors and Inverters

Tuning Gate Potential Profiles and Current-Voltage Characteristics of Polymer Electrolyte-Gated Transistors by Capacitance Engineering

We demonstrate that the transfer characteristics of electrolyte-gated transistors (EGTs) with polythiophene semiconductor channels are a strong function of gate/electrolyte interfacial contact area, i.e., gate size. Polythiophene EGTs with gate/electrolyte areas much larger than the channel/electrolyte areas show a clear peak in the drain current vs gate voltage (ID-VG) behavior, as well as peak voltage hysteresis between the forward and reverse VG sweeps. Polythiophene EGTs with small gate/electrolyte areas, on the other hand, exhibit current plateaus in the ID-VG behavior and a gate-size-dependent hysteresis loop between turn on and off. The qualitatively different transport behaviors are attributed to the relative sizes of the gate/electrolyte and channel/electrolyte interface capacitances, which are proportional to interfacial area. These interfacial capacitances are in series with each other such that the total capacitance of the full gate/electrolyte/channel stack is dominated by the interface with the smallest capacitance or area. For EGTs with large gates, most of the applied VG is dropped at the channel/electrolyte interface, leading to very high charge accumulations, up to ∼0.3 holes per ring (hpr) in the case of polythiophene semiconductors. The large charge density results in sub-band-filling and a marked decrease in hole mobility, giving rise to the peak in ID-VG. For EGTs with small gates, hole accumulation saturates near 0.15 hpr, band-filling does not occur, and hole mobility is maintained at a fixed value, which leads to the ID plateau. Potential drops at the interfaces are confirmed by in situ potential measurements inside a gate/electrolyte/polymer semiconductor stack. Hole accumulations are measured with gate current-gate voltage (IG-VG) measurements acquired simultaneously with the ID-VG characteristics. Overall, our measurements demonstrate that remarkably different ID behavior can be obtained for polythiophene EGTs by controlling the magnitude of the gate-electrolyte interfacial capacitance.

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 (VGS,eff) in IL-gated organic field-effect transistors (IL-OFETs) using a modified current-voltage measurement configuration. The results reveal a significant deviation between VGS,eff and the applied gate bias (VGS,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 VGS,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 VGS,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 VGS,eff and trap DOS, this research contributes to the advancement of organic electronics and paves the way for future developments in the field.

Integrated internal ion-gated organic electrochemical transistors for stand-alone conformable bioelectronics

Organic electronics can be biocompatible and conformable, enhancing the ability to interface with tissue. However, the limitations of speed and integration have, thus far, necessitated reliance on silicon-based technologies for advanced processing, data transmission and device powering. Here we create a stand-alone, conformable, fully organic bioelectronic device capable of realizing these functions. This device, vertical internal ion-gated organic electrochemical transistor (vIGT), is based on a transistor architecture that incorporates a vertical channel and a miniaturized hydration access conduit to enable megahertz-signal-range operation within densely packed integrated arrays in the absence of crosstalk. These transistors demonstrated long-term stability in physiologic media, and were used to generate high-performance integrated circuits. We leveraged the high-speed and low-voltage operation of vertical internal ion-gated organic electrochemical transistors to develop alternating-current-powered conformable circuitry to acquire and wirelessly communicate signals. The resultant stand-alone device was implanted in freely moving rodents to acquire, process and transmit neurophysiologic brain signals. Such fully organic devices have the potential to expand the utility and accessibility of bioelectronics to a wide range of clinical and societal applications.