Operational Stability of Electrolyte-Gated Organic Field-Effect Transistors

This work studied the operational stability of electrolyte-gated organic feldefect transistors (EGOFETs) made from diferent materials, using pure water as the electrolyte. The inclusion of an electrolyte in an OFET greatly increases the capacitance but introduces many more unknowns into a device that itself is not yet fully understood. The EDLs themselves constitute an entire feld of study, as does the interactions between water and the metallic gate electrode. Furthermore, water is known to cause issues in conventional OFETs under bias, and thus it is
not obvious that a water-gated device could be operated for any length of time. Nonetheless, the water-gated EGOFETs studied in this work exhibited usable lifetimes on the order of several weeks or months, with the short-, medium- and long-term stability varying signifcanty between devices prepared from diferent materials.

We investigated the stability of two diferent P3HT EGOFETs. The frst device gradually degraded, but nonehteless remained usable for biosensing experiments for two months. We showed that the degradation was caused by electrical stress and not only exposure to water. Interestingly, the second P3HT device was much more stable, refusing to degrade even during a considerably more intense electrical measurement regime, although it did show small signs of doping during use. We highlighted some possible explanations for this. Firstly, the first device used chromium/gold contacts, which have been shown to be susceptible to galvanic corrosion, while the titanium/gold contacts of the second device are more resistant to this. Secondly, the sensing measurements performed with the frst device may have introduced some contaminant that caused gradual degradation.
During the first study on a P3HT EGOFET, we identifed the gold gate electrode as having a signifcant efect on the short-term stability of the transistor. When the gate was introduced to water after sitting in ambient air, the threshold voltage of the EGOFET shifted signifcantly to more positive values, before gradually returning to more negative ones. We were able to link this to reported changes in the work function of gold due to water adsorption. Expanding the study to several diferent gate electrode materials, we found that both evaporated and bulk gold behaves in the same way, but that the efect was greatly reduced with a platinum gate or by encapsulating the gold with an organic semiconductor.

IDT-BT was investigated as an alternative to P3HT, due to its high performance and stability in conventional OFETs. While IDT-BT EGOFETs showed better performance that their P3HT counterparts, especially with regards to sweep rate, they exhibited signifcant bias-induced doping and a smaller degree of charge trap generation. These effects, ocurring under operation, where gradually reversed when measurements stopped. We proposed that the doping is the same oxygen-related hole doping/electron trapping commonly observed in IDTBT, while the charge trapping is consistent with water-induced traps in the semiconductor. We also show that when the EGOFET is operated consistently, the effects reach their respective dynamic equilibriums, and the device becomes highly stable.

Finally, we demonstrated a method for extracting the density and energetic distribution of localized states in electrolyte-gated transistors from transfer measurements. The function provides more accurate knowledge about the depth and distribution of created traps than rough estimates from the fgures of merit, but the methods commonly used to extract this in conventional OFETs are not applicable to electrolyte-gated devices. We repeated the derivation of one of these methods, commonly referred to as the Grünewald method, using the Gouy-Chapman model of the EDL to create appropriate boundary conditions for the electrolyte. We validated the method on data from a P3HT EGOFET, illustrating the utility of the method as a tool for future works on the stability of electrolyte-gated transistors.