Organic Electrochemical Transistors Based on Room Temperature Ionic Liquids: Performance and Stability

Abstract

Organic electrochemical transistors (OECTs) are becoming a key device in the field of organic bioelectronics. For many applications of OECTs, in particular for enzymatic sensing, a complex mixture of room temperature ionic liquids (RTILs) combined with other electrolytes is used as a gate electrolyte, making the interpretation of experimental trends challenging. Here, the switching mechanism of OECTs using such RTILs is studied. It shows that ions smaller in size than the ions contained in the RTIL (e.g., Na+) have to be added to the ionic liquid to ensure switching of the OECTs. Furthermore, it is shown that OECTs based on RTILs exhibit noticeable gate‐bias stress effects and a hysteresis in the electrical transfer characteristics. A model based on incomplete charging/discharging of the effective gate capacitance during operation of the OECT and a dispersion in the ion mobilities is proposed to explain these instabilities, and thus it shows that the hysteresis can be minimized by optimizing the geometry of the device. Overall, a better understanding of the underlying mechanisms of switching and stability of OECTs based on RTILs is the first step toward various applications such as lactate acid sensors and neurotransmitter recording.

Tuning the Transconductance of Organic Electrochemical Transistors

“…Some devices, however, failed to show any modulation in drain current or showed unusually small on‐off ratios compared to similar devices and are discarded in the analysis. As reported previously, [ 33 ] the transistors show a hysteresis between the forward and backward sweep (cf. Figure S9, Supporting Information).…”

“…The area of hysteresis depends on the purity of channel materials, gate area, distance of gate from the channel, type of electrolyte, and the measuring speed. [ 33 ] To be consistent, only the forward sweep is shown in the following and taken into account for further analysis.…”

Finding the equilibrium of organic electrochemical transistors

“…Most importantly, a better description of extraction of holes at the drain contact in the presence of large cation concentrations has to be found. Furthermore, the experimental characterization of OECTs usually shows hysteresis effects 25 , which leads to sample to sample variations. Still, the argument used here, that is, that the concentration of ions inside the channel is determined by the equilibrium condition of drift and diffusion currents and not by a gate capacitance, is independent of the particular modeling result.…”

Sub‐3 V ZnO Electrolyte‐Gated Transistors and Circuits with Screen‐Printed and Photo‐Crosslinked Ion Gel Gate Dielectrics: New Routes to Improved Performance

“…Electrolyte‐gated transistors (EGTs), including electric double layer transistors (EDLTs) and organic electrochemical transistors (OECTs), are being developed by a number of research groups for applications in sensing and printed electronics . Attractive features of EGTs are their very high transconductances and low voltage characteristics, both of which derive from the huge specific capacitance of the electrolyte gate dielectrics that they employ, which varies from ≈10 μF cm −2 in the double layer operating regime to over 100 μF cm −2 in the electrochemical regime …”