9耐熱性を有する塗布型有機半導体材料の開発
福渡羽田辺村真

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人＊ 1
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A Soluble Thermally-Stable Organic Semiconductor Material forField-Effect Transistors
Takashi FUKUDA
Makoto WATANABE
Satoshi HAMURA

A novel solution-processable low molecular weight organic semiconductor material TS-5, which has high thermal
stability and solubility with various aromatic solvents, was developed. Field effect transistors (FET) fabricated using
TS-5 as an active layer, exhibited p-type transistor responses with mobilities greater than 1.0 cm2/Vs and are stable
at 150℃ in air condition.

1．Introduction

TIPS-pentacene undergoes a structural phasetransition at 124℃ and the mobilities are reduced

Organic field-effect transistors (OFETs) areafter heat treatment at 120℃ [3]. Benzene-thiophene

particularly attractive as their fabrication processes are

fused heteroacene compounds such as 2,7-dialkyl[1]simple compared with conventional silicon technology,

benzothieno[3,2-b]benzothiophenes (Cn-BTBTs)

which involve high-temperature and high-vacuum

show hole mobilities greater than 1.0 cm 2/Vs [4].

deposition processes. The low process temperature of

Furthermore Cn-BTBTs are highly stable to oxidation

OFETs enables the compatibility with plastic substrates

because of their low lying highest occupied molecular

for lightweight and flexible products.

orbitals (HOMOs). However, low HOMO levels lead

For high performance OFETs, organic

to high injection barriers for carriers from electrodes

semiconductors with a high mobility, stability,

and such materials are thermally unstable because ofand processability are needed. The solution-

their low melting points (ca. 130℃ ). Therefore, there

processed organic materials in transistors have tois a demand for thermally stable organic materials withbe stable for practical applications. Recently, lowhigh mobilities. Here we report the properties of a novel

molecular weight organic semiconductors, suchlow molecular-weight fused ring compound TS-5, andas bis(triisopropylsilylethynyl)pentacene (TIPS-

electrical characteristics of FETs.

pentacene), have been shown to yield high performancedevices [1]. Although the possibility of photooxidationof pentacene analogs cannot be completely excluded[2], polyacene compounds give high performances
because of increased π-stacking. It is known that＊1四日市研究所

2．Experimental
FET Fabrication
Field effect transistors were fabricated in top-contact
configuration on a doped n+-Si wafer with a thermallyTOSOH Research & Technology Review Vol.57（2013）

10grown oxide layer (200 nm). A TS-5 thin film (50 nm)
was deposited using the drop cast method from a 0.2
wt% toluene solution onto the Si/SiO 2 substrate inair. The solvent was evaporated under atmosphericpressure. On top of the organic thin film, gold drain
and source electrodes were deposited in vacuo to yielddevices with a channel length (L ) of 15–20 μ m and achannel width (W ) of 500 μ m (Fig. １).

［eV］−1−1.49−1.50−2−2.39

−3

2.21

3.91
−4

4.05

−4.60
−5.15

−5

−5.54

−6Source

Drain

Semiconductor−7Vds

DielectricTS−5

Fig.２

GatePentacene

BTBT

HOMO and LUMO energy levels of TS−5,pentacene and BTBT

Vg

Fig.１

at room temperature (Table 1), solution processed

FET device structuretechniques can be used to incorporate it as the active
layer in OFETs. Liquid chromatography analysis

The electrical characteristics of TS-5 were measured
in ambient conditions. The field-effect mobilities wererevealed that no oxidized compounds of TS-5 wereformed after 2 weeks in air.

achieved in the saturation region from the square rootof the drain current versus gate voltage curve, using
Id＝［2］Electrical Characteristics of FETThe FETs exhibited typical p-type transistor behavior,

CμsatW（Vg−Vth）22L

with hole mobilities as high as 1.27 cm2/Vs and on/off

where I d is drain current, C is capacitance per unit

ratios of 3.6 × 107 (Fig. 4). When the FETs were heatedarea, V g is gate voltage, and V th is threshoud voltage.

at 150℃ for 15 min in air, the electrical performances

The current on/off ratios were determined from the I ddid not change. These results indicate that TS-5 FETs

between V g = 0 V and -50 V.

are stable.

30

3．Results and Discussion

169.4［1］Characteristics of TS-5
Molecular orbital (MO) calculations were carriedout using the DFT method at the B3LYP/6-31G＊level.

The calculated HOMO and LUMO levels for TS-5,DSC［mW］

20
100pentacene, and BTBT are shown in Fig. ２. The HOMO−10level of TS-5 is -5.15 eV, while that of pentacene is -4.60−20−20eV and BTBT is -5.54 eV. Since the work function of189.73080130180

Temp.［℃］

gold is 5.1 eV, a smooth carrier injection from the gold

Fig.３

electrode to TS-5 is expected. As TS-5 has a large band

DSC trace of TS−5gap, it is stable in air.
Figure 3 shows a tracing differential scanningcalorimetry (DSC) measurement. No signal could beassigned to a structural phase transition, and only amelting point (ca. 190℃ ) was observed. This indicatesthat TS-5 is thermally stable.
Since TS-5 is soluble in various aromatic solvents

Table１TS−5 solubility in aromatic solvents

Solvent
toluenetetralinem−xylenemesitylene
pentylbenzene

Solubility（wt％），25℃
1.13
1.10
0.971
0.749
0.479

東ソー研究・技術報告

1.0E−030.018
μh＝1.27cm2／Vs

1.0E−040.016

Vth＝22.1V

1.0E−05

0.014

7

on／off＝3.6×10

1.0E−06

0.012

1.0E−070.01

1.0E−08

0.008

1.0E−090.006

1.0E−101.0E−110.004

1.0E−120.002

1.0E−1300（B）Id 1／2−Idg［A］

11−−（A）

第 57 巻（2013）−10−20−30
Vg［V］−40−50−1.8E−4
Vg＝−50V−1.6E−4−1.4E−4−Id［A］−1.2E−4−1.0E−4
Vg＝−40V−8.0E−5−6.0E−5−4.0E−5

Vg＝−30V−2.0E−5

Vg＝0, −20V

0.0E＋02.0E−5100−10−20−30−40−50Vg［V］

Fig.４ （A）Transfer characteristics（Vd＝−50V）and（B）output characteristics of a TS−5 FET

substrate and the ab plane is parallel to the substrate.

［3］Characterization of TS-5 thin films
The X-ray diffraction (XRD) spectrum of the TS-5 thin

Figure 6 shows the AFM image of the thin film. Afilm on a Si/SiO2 substrate is shown in Fig. 5. A series

series of flat terraces were observed over a range ofof 12 diffraction peaks, assigned to the (00l) reflections,micrometers on the surface. The height of the step isindicate the film’s high crystallinity. The inter layer

1.8 nm, which is consistent with the d-spacing fromdistance (d-spacing) calculated from these reflections

the XRD pattern. The flat surface and the well-definedis 17.8 Å, which correlates with the dimensions ofmolecular steps indicate that TS-5 forms a molecularly

the c axis. The c axis is oriented perpendicular to theflat thin film extending across a micrometer area on the106

1040［nm］
10103
210024210
10102030

4050

60

66881070

2θ［degree］

Fig.５

4
［μm］

［μm］

Intensity［cps］

105

XRD reflections for a drop−casted TS−5 thin film

Fig.６

AFM image of a TS−5 thin film

TOSOH Research & Technology Review Vol.57（2013）

12SiO2 surface. This structure enhances the charge carrier
mobilities.

4．Conclusion
A novel low molecular-weight semiconductor material
TS-5 was developed. The compound is soluble in various
aromatic solvents with an oxidation resistance in air,and useful using as an active layer of FET devices bysolution process. FETs fabricated with TS-5 exhibited
p-type transistor responses with mobilities greater than
1.0 cm2/Vs and on/off ratios 3.6 × 107.

In addition,

TS-5 thin films have high thermal stabilities in anatmosphere. As a highly oriented TS-5 crystalline thinfilm was obtained using the simple drop casting method
under ambient conditions, TS-5 is highly promisingfor practical usage. We will continue to examine othercoating methods and ink formulation of TS-5 to further
improve electrical FET performances.

5．Acknowledgment
The authors would like to appreciate Dr. Satoshi
Hoshino of National Institute of Advanced Industrial
Science and Technology (AIST) for helpful suggestionand discussion.

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