Francesca Soavi

Conducting Polymer Transistors Making Use of Activated Carbon Gate Electrodes

The characteristics of the gate electrode have significant effects on the behavior of organic electrochemical transistors (OECTs), which are intensively investigated for applications in the booming field of organic bioelectronics. In this work, high specific surface area activated carbon (AC) was used as gate electrode material in OECTs based on the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS). We found that the high specific capacitance of the AC gate electrodes leads to high drain-source current modulation in OECTs, while their intrinsic quasi-reference characteristics make unnecessary the presence of an additional reference electrode to monitor the OECT channel potential.

Low voltage electrolyte-gated organic transistors making use of high surface area activated carbon gate electrodes

In electrolyte-gated transistors, the exceptionally high capacitance of the electrical double layer forming at the electrolyte/transistor channel interface permits current modulations of several orders of magnitude, at relatively low gate voltages. The effect of the nature of the gate electrode on the performance of electrolyte-gated transistors is still largely unclear, despite recent intensive efforts. Here we demonstrate that the use of high surface area, low cost, activated carbon gate electrode enables low voltage (sub-1 V) operation in ionic liquid-gated organic transistors and renders unnecessary the presence of an external reference electrode to monitor the channel potential, thus dramatically simplifying the device structure. We used the organic electronic polymer MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene), as the channel material, and the high ionic conductivity, low viscosity ionic liquid [EMIM][TFSI] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), as the electrolyte gating material. We believe that this will prove to be the first of a new generation of low voltage electrolyte-gated transistors for applications in organic printable electronics.

Effect of channel thickness, electrolyte ions, and dissolved oxygen on the performance of organic electrochemical transistors

We investigated the device characteristics of organic electrochemical transistors based on thin films of poly(3,4-ethylenedioxythiophene) doped with poly(styrene-sulfonate). We employed various channel thicknesses and two different electrolytes: the micelle forming surfactant cetyltrimethyl ammonium bromide (CTAB) and NaCl. The highest ON/OFF ratios were achieved at low film thicknesses using CTAB as the electrolyte. Cyclic voltammetry suggests that a redox reaction between oxygen dissolved in the electrolytes and PEDOT:PSS leads to low ON/OFF ratios when NaCl is used as the electrolyte. Electrochemical impedance spectroscopy reveals that doping/dedoping of the channel becomes slower at high film thickness and in the presence of bulky ions.

Ionic liquid–water mixtures and ion gels as electrolytes for organic electrochemical transistors

Organic Electrochemical Transistors (OECTs) are widely investigated for applications in bioelectronics. Ionic liquids (ILs) are, in principle, interesting candidates as gating media in OECTs. Nevertheless, ILs can exhibit excessively high viscosity that prevents their straightforward application in OECTs. Here we report two processing approaches to apply the highly viscous ionic liquid triisobutyl(methyl)phosphonium tosylate (Cyphos® IL 106) in OECTs based on poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS), namely IL–H2O binary mixtures and ion gels. The use of Cyphos® IL 106–H2O binary mixtures and ion gels as gating media determines an increase of the OECT modulation, with respect to the pure ionic liquid. This increase cannot be explained simply by the change of the viscosity and ionic conductivity of the ionic liquid–H2O mixtures with the increase of the H2O content. Using high surface area activated carbon gates, ON/OFF ratios as high as 5000 are achieved with Cyphos® IL 106–H2O mixtures at 5 and 10% H2O v/v.

Electrolyte-gated polymer thin film transistors making use of ionic liquids and ionic liquid-solvent mixtures

Electrolyte-Gated (EG) transistors, making use of electrolytes as the gating medium, are interesting for their low operation voltage. Furthermore, EG polymer transistors offer the advantage of solution processing, low cost, and mechanical flexibility. Despite the intense research activity in EG transistors, clear guidelines to correlate the properties of the materials used for the transistor channel and electrolytes with the doping effectiveness of the transistor channel are yet to be clearly established. Here, we investigate the use of room temperature ionic liquids (RTILs) based on the [TFSI] anion (namely, [EMIM][TFSI], [BMIM][TFSI], and [PYR14][TFSI]), to gate transistors making use of MEH-PPV as the channel material. Morphological studies of MEH-PPV and RTIL films showed a certain degree of segregation between the two components. All the EG transistors featured clear drain-source current modulations at voltages below 1 V. Polar solvent additives as propylene carbonate were used to improve the transis...