High performance photolithographically-patterned polymer thin-film transistors gated with an ionic liquid/poly(ionic liquid) blend ion gel

Abstract

We demonstrate the fabrication of polymer thin-film transistors gated with an ion gel electrolyte made of the blend of an ionic liquid and a polymerised ionic liquid. The ion gel exhibits a high stability and ionic conductivity, combined with facile processing by simple drop-casting from solution. In order to avoid parasitic effects such as high hysteresis, high off-currents and slow switching, a fluorinated photoresist is employed in order to enable high-resolution orthogonal patterning of the polymer semiconductor over an area that precisely defines the transistor channel. The resulting devices exhibit excellent characteristics, with an on/off ratio of 10 6 , low hysteresis and a very large transconductance of 3 mS. We show that this high transconductance value is mostly the result of ions penetrating the polymer film and doping the entire volume of the semiconductor, yielding an effective capacitance per unit area of about 200 µF cm −2 , one order of magnitude higher than the double layer capacitance of the ion gel. This results in channel currents larger than 1 mA at an applied gate bias of only -1 V. We also investigate the dynamic performance of the devices and obtain a switching time of 20 ms, which is mostly limited by the overlap capacitance between the ion gel and the source and drain contacts.Electrolyte-gated organic thin-film transistors (OTFTs) have received a lot of attention over the past years, thanks to their ability to operate at very low voltages (< |1| V), 1-3 which makes them particularly attractive for use in portable devices powered by low supply voltage sources such as thin-film batteries, and interfacing with conventional Si CMOS electronics. If an electrolyte is placed in contact with two electrodes, to which a small voltage is applied, ionic species within the electrolyte migrate towards the electrode of opposite polarity, forming ultra-thin electrical double layers at both electrolyte/electrode interfaces. When one of these electrodes is the channel of a transistor, large charge densities arise within the semiconductor, as a result of the ionic accumulation within a small volume close to the interface or within the bulk of the semiconductor, depending on the mode of operation of the device. In the case where ionic charges are contained at the interface with the semiconductor, for example by using a polyelectrolyte in which the ionic charges are covalently linked to the polymer chains 4 or a crystalline semiconductor impermeable to ion penetration, 5 the device operates much like a field-effect transistor. On the other hand, when ions penetrate inside the bulk of a polymer semiconductor, the capacitance must be considered as a volumetric parameter and the doping process is electrochemical in nature, 6-8 resulting in very large currents flowing through the whole channel volume. As a) Electronic mail: alasdair.campbell@imperial.ac.uk a consequence, large transconductances surpassing that of silicon-based TFTs can be obtained, 9 even in OTFTs employing polymer semiconductors with modest ...

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

“…The V G dependences of the channel‐width‐normalized transconductance ( g m / W ) at V D = 1 and 0.1 V, respectively, for the same device are shown in Figure d and Figure S5b in the Supporting Information. For an ideal transistor, g m / W should be independent of V G beyond threshold, but as is typical for EGTs, g m / W is peaked, as discussed further below. Maximum channel‐width‐normalized transconductance ( g m / W) max is the peak value of the Figure d plot.…”

“…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 …”

Organic Field‐Effect Transistor for Energy‐Related Applications: Low‐Power‐Consumption Devices, Near‐Infrared Phototransistors, and Organic Thermoelectric Devices

“…Organic transistors that can operate below 1 V have been demonstrated, [70,82,85,86,[108][109][110][111][112] but many research efforts have focused on devices involving electrolyte gate insulators, including ionic liquid, poly (ionic liquid), and electrochemical transistors. [108,109,111,112] The electrolyte gate insulator will form electrical double layers (EDLs) at both the electrolyte/semiconductor and electrolyte/gate electrode interfaces due to the ion movement under gate bias. Within each EDL, the redistributed ions are balanced by the electronic charges from the semiconductor or gate electrode, respectively, and the distance between the ionic and electrical charges is less than 1 nm, equivalent to a large-value capacitor.…”

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

“…Room Temperature Ionic Liquids (RTILs) such as C2MIM EtSo4 are an alternative to water‐based electrolytes . So far, ionic liquids have been used extensively in electrolyte gated organic field‐effect transistors . In general, the slow polarization response of electrolytes limits the switching speed of these transistors to a few hertz at room temperature.…”

“…[23,10] So far, ionic liquids have been used extensively in electrolyte gated organic field-effect transistors. [24][25][26][27] In general, the slow polarization response of electrolytes limits the switching speed of these transistors to a few hertz at room temperature. To decrease the polarization time with a large specific capacitance, different ionic liquids were used, [28] which show different frequency-dependent double layer capacitances.…”