Organic Transistors Making Use of Room Temperature Ionic Liquids as Gating Medium

The ability to couple ionic and electronic transport in organic transistors, based on π conjugated organic materials for the transistor channel, can be particularly interesting to achieve low voltage transistor operation, i.e. below 1 V.The operation voltage in typical organic transistors based on conventional dielectrics (200 nm thick SiO2) is commonly higher than 10 V. Electrolyte-gated (EG) transistors, i.e. employing an electrolyte as the gating medium, permit current modulations of several orders of magnitude at relatively low gate voltages thanks to the exceptionally high capacitance at the electrolyte/transistor channel interface, in turn due to the low thickness (ca. 3 nm) of the electrical double layers forming at the electrolyte/semiconductor interface.Electrolytes based on room temperature ionic liquids (RTILs) are promising in EG transistor applications for their high electrochemical stability and good ionic conductivity.The main motivation behind this work is to achieve low voltage operation in organic transistors by making use of RTILs as gating medium.First we demonstrate the importance of the gate electrode material in the EG transistor performance.The use of high surface area carbon gate electrodes limits undesirable electrochemical processes and renders unnecessary the presence of a reference electrode to monitor the channel potential.This was demonstrated using activated carbon as gate electrode, the electronic conducting polymer MEH-PPV, poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4phenylene vinylene] channel material, and the ionic liquid [EMIM][TFSI] (1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide), as gating medium.Using high surface area gate electrodes resulted in sub-1 V operation and charge carrier mobilities of (1.0 ± 0.5) ×10 -A challenge in the field of EG transistors is to decrease their response time, a consequence of the slow ion redistribution in the transistor channel upon application of electric biases.We systematically investigated EG transistors employing RTILs belonging to the same family, i.e. based on a common anion and different cations.The transistor characteristics showed a limited cation influence in establishing the p-type doping of the conducting polymer.Interestingly, we observed that the transistor response time depends on at least two processes: the redistribution of ions from the electrolyte into the transistor channel, affecting the gate-source current (Igs); and the redistribution of charges in the transistor channel, affecting the drain-source current (Ids), as a vii function of time.The two processes have different rates, with the latter being the slowest.Incorporating propylene carbonate in the electrolyte proved to be an effective solution to increase the ionic conductivity, to lower the viscosity and, consequently, to reduce the transistor response time.Finally, we were able to demonstrate a multifunctional device integrating the transistor logic function with that of energy storage in a supercapacitor: the TransCap.The polymer/electrolyte/carbon vertical stacking of the EG transistor features the cell configuration of a hybrid supercapacitor.Supercapacitors are high specific power systems that, for their ability to store/deliver charge within short times may outperform batteries in applications having high power demand.When the TransCap is ON (open transistor channel), the polymer and the carbon gate electrodes store charge (Q) at a given Vgs, hence the stored energy equals Q•Vgs.When the TransCap is switched OFF, the channel and the gate are discharged and the energy can be delivered back to power other electronic components.EG transistors, making use of activated carbon as gate electrode and different RTILs as well as RTIL solvent mixtures as electrolyte gating medium, are interesting towards low voltage printable electronics.The high capacitance at the interface between the electrolyte and the transistor channel enables energy storage within the EG transistor architecture.

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