Principal Research Results

High-mobility, Low-power and Fast-switching Organic Field-effectTransistors with Ionic Liquids
BackgroundAn important challenge in the study of organic field-effect transistors (OFETs) is to realize low-cost, high-mobility, lowpower, and fast-switching devices. In this regard, there has been significant interest in developing a new kind of organic transistor withelectric double layers (EDLs) of electrolytes (Fig. 1). Recently, it was shown that the EDLs of polymer electrolytes can gate organic
transistors and that high-density carriers are injected indeed with the application of small gate voltages. However such devices show
relatively large hysteresis, because the high viscosity of the polymer electrolyte prevents the devices from forming the EDLs immediately.

Objectives
To realize the high performance OFETs, this study aims to use ionic liquids (Fig. 2) as the electrolyte layer in the EDL-gated
OFETs. An advantage of the ionic-liquid electrolyte is its rapid ionic diffusion and large capacitance. Rapid ionic diffusion of ionicliquids in the OFETs translates into quick-switching devices and the large capacitance into high density of induced charge withminimum gate voltages, meaning that sufficiently large ON current can be achieved even with the switching voltage less than 0.5 V.

Principal Results
We put an ionic liquid electrolyte on top of rubrene single crystals to make organic single crystal field-effect transistors, sothat formation of the EDLs in the electrolyte could induce high-density carriers at the surface of the crystal and obtained the following
results:1) Low-power operation
We found that rather large current in the order of μA was generated with the application of less than 0.5 V for both gate
voltage VG and drain voltage VD, demonstrating the very low-power operation of the organic transistors due to the high-density carrier
accumulation at the ionic liquid/rubrene liquid-to-solid interfaces (Fig. 3). Same drain current could be generated with 2 orders ofmagnitude larger gate voltage in SiO2 gated devices that are the most common gate dielectrics used for the OFETs.
2) High-mobility
The obtained mobility became as large as 1.0 - 1.2 cm2 /Vs as a statics of several similarly prepared samples. These values
were higher than those ever reported for EDL-gated OFETs and those required for organic flexible displays (0.1〜 0.5 cm2/Vs).
3) Fast-switching operation
EDL capacitance of the IL was measured over the range from 0.1 Hz to 1 MHz (Fig. 4). The EDL capacitance of the IL
became as large as 11 μF/cm2 at 0.1 Hz and remained large even at 1 MHz, demonstrating the fast ionic diffusion in response to thevoltage application. Therefore, the EDL-gated OFETs incorporating the IL allowed switching operation at such a high frequency,
which none of the previously reported devices do.

Future Developments
It is planned to demonstrate that the use of ionic-liquid electrolytes in organic single- crystal FETs enables high-density
carrier doping with minimum gate voltages without sacrificing the carrier mobility. Therefore, the ionic-liquid gating can be a promising technology to realize high-mobility, fast-switching and low-power organic transistors. Owing to varieties of ILs that have beensynthesized so far, higher performances are likely to emerge by elaborate search for compounds incorporated in OFETs. Due to thecapability of the high-density carrier doping, the present technique is also useful in more basic material sciences such as carrier-density driven phase transition in strongly correlated electron systems.
Main Researcher: Shimpei Ono, Ph. D.,
Research Scientist, Advanced Functional Materials, Materials Science Research Laboratory
Reference
S. Ono, S. Seki, R. Hirahara, Y. Tominari and J. Takeya “High-mobility, low-power, and fast-switching organic field-effect transistors
with ionic liquids” Applied Physics Letter 92 (2008) 103313.

13010. Advanced Basic Technologies

Fig.1 A schematic illustration of the single
crystal field-effect transistor with ionicliquid electrolyte (left).
When the gate voltage VG is applied to the electrolyte, theEDLs are formed after the ionic redistribution (right). Gate
electric field is confined only to the EDLs with 1 nm.

Fig.2

Chemical structure of the ionic liquid
used in this device.
The ionic liquid used in this experiment is 1-ethyl3methylimidazolium bis (trifluoromethanesulfonyl) imide(emimTFSI).

Fig.4 Capacitance of the ionic liquid as afunction of frequency measured by
the ac impedance technique.
The EDLs are formed even at the high frequency.
Fig.3 Transfer characteristics (drain current
ID as a function of gate voltage VG) of theEDL-gated OFETs with ionic liquid(Bottom).
Same drain current in IL gated OFETs can be generated
with 2 orders of magnitude larger gate voltage in SiO2
gated devices (upper).

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