Printed organic transistors: Toward ambient electronics
Takao Someya1,2*, Tsuyoshi Sekitani1, Makoto Takamiya3, Takayasu Sakurai3,
Ute Zschieschang4 and Hagen Klauk4
1 Department of Electric and Electronic Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
2 Institute for Nano Quantum Information Electronics, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, Japan
3 Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, Japan
4 Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany*
Email: someya@ee.t.u-tokyo.ac.jp, Phone: +81(Japan)-3-5841-6820, Fax: +81(Japan)-3-5841-6828
Abstract
In the forthcoming ambient electronics era, multiple
electronic objects are scattered on walls, ceilings or in
imaginative locations and interact each other to enhancesafety, security and convenience. For implementation ofmany electronic objects in our daily life, large-area, flexible
devices, which would be printed on plastic sheet, cloth,
and/or paper, are expected to play an important role. In thispaper, we describe recent progress and future prospects offlexible, large-area electronics based on printed organic
transistors.
Introduction
In view of the tremendous technical challenges for realizing
next-generation
information
technology,
organic
semiconductors have attracted significant attention since theemerging electronics based on them have features that arecomplimentary to main stream electronics based on silicon.
Thanks to the recent advent of organic transistors (FETs) [15], the emergence of a new class of electronics makes full useof the unique features of organic semiconductors, such as theultralow cost, low weight, and flexibility, is becoming more
realistic. With this background, our group discerned thatlarge-area circuits could be easily fabricated using organic
transistors, which are essential for certain applications, andhas developed large-area sensors and actuators using organic
transistors. More accurately, we have integrated various typesof sheet-type sensors and sheet-type actuators with organic
transistors on plastic films and have demonstrated the world’s
first electronic artificial skin (E-skin) [6-8], a sheet-type
scanner [9], a sheet-type Braille display [10], a wirelesspower transmission sheet [11], a communication sheet [12],an ultrasonic proximity sensor sheet [13], stretchable displays[14], and many other sheet-type devices (Fig. 1).
In the forthcoming ambient electronics era, multiple
electronic objects are scattered on walls, ceilings or in
imaginative locations and interact each other to enhancesafety, security and convenience. For implementation ofmany electronic objects in our daily life, large-area sheet-typedevices are needed and organic transistors are expected toplay an important role.
In this paper, we will report recent progress and future
prospects of organic transistor-based flexible, large-area
sensors and actuators. Moreover, the issues and the future
prospect of organic transistors will be addressed from theview point of ambient electronics.

E-skinPocket scanner

Braille display

Power sheet

Communication sheet

Ultrasonic sheet

Stretchable OLED

2003
Pressure sensor+ OFETs2004
Photodetectors+ OFETs2005
Actuators+ OFETs2006
Coil + MEMS
+ OFETs2007
OFETs + Si
co-design

2008
High freq. OFETs+ ultrasonic2009
Elastic conductors+ OLED + AM OFETs

Fig. 1: Large-area, flexible sensors, actuators, and displays that are realized by organic field-effect transistors. (From the left) an electronics artificial
skin (E-skin), a sheet-type image scanner (pocket scanner), a sheet-type Braille display for blind persons, a wireless power transmission sheet, a sheet-typecommunication system, a sheet-type ultrasonic system, and a stretchable active matrix OLED display.

97-4244-5640-6/09/$26.00 ©2009 IEEE

1.2.1

IEDM09-9

Printed organic transistor active matrix
Organic transistors were manufactured using inkjet printing(Ricoh Printing Systems Co., Ltd.; IJP-1), screen printing(Micro-tec Co., Ltd.; MT-550), and vacuum evaporation
methods. The base film (substrate) was a polyimide film witha thickness of 75 μm. Ag nanoparticles (Harima ChemicalCo., Ltd.; NPS-J-HP) were patterned by inkjet printing withpicoliter accuracy and cured at 180 °C to form 300-nm-thickgate electrodes. Epoxy partitions were formed around thegate electrodes by using screen printing. Poly-chloro-paraxylylene (i.e., parylene, Daisan-kasei Co., Ltd.; diX-SR) were
coated at room temperature to form 500-nm-thick gate
dielectric layers. Ag paste was patterned by screen printing
and cured at 150 °C to form source and drain electrodes. A
50-nm-thick pentacene layer was deposited using vacuum
evaporation with rotation mechanics through printed shadow
masks [15] to form a channel layer. The channel length andwidth were typically 30 μm and 500 μm, respectively.
Finally, a 5-μm-thick layer of poly-chloro-para-xylylene,
parylene was uniformly coated to form a passivation layer.
The fabrication process is similar to the method reported inref. [16]. Figure 2 shows a magnified view of organic
transistors. Channel length with 30 μm and printed electrodewidth with 500 μm were clearly formed using state-of-the-art
screen printing.

We characterized pentacene transistors using printed
electrodes. The drain current of the transistors was measured
as a function of the source-drain voltage (VDS) as shown in
Fig. 2c: The gate-source voltage (VGS) was varied from 0 V
to −15 V in steps of −3 V. Figure 1f shows the correspondingtransfer curves of the same device: VGS is varied from 2 V to
−15 V with the application of VDS = −15 V. A hysteresis wasvery small. Note in Fig. 2d that on-off ratio was about 106,
showing that leakage current through gate insulator was
sufficiently low. The mobility in the saturation regime was
0.1 cm2/Vs when gate voltage was −15 V. In multilayered
structures formed by solution processes, it is important tochoose a solvent carefully so that a solvent may not melt afront layer. The present result shows that the choice of thesolvents is reasonable.
Printed organic transistor with 1-μm channel length
In order to fabricate very fine top contact organic transistors
using printing technology, we employed sub-femtoliter inkjet
technology [17,18]. A sub-femtoliter inkjet can pattern veryfine Ag nanoparticles (NPS-J-HP) directly on pentacene. Thelines having a width of 2 μm can be written uniformly andreproducibly.
(b)

( a)
(b)

Source/drain electrode (Ag)

Source & drain electrode (Ag)
Organic
semiconductor(Pentacene)

One transistor

Organic semiconductor(Pentacene)

AlOx
Gate electrode (Al)Gate electrode (Ag) Substrate
Gate dielectric (Parylene)(Polyimide)

VGS = -15 V

-12 V

-0.2

0

10

-9 V

0

-5
-10
-15
Drain-source voltage (V)Drain current (A)

-0.4

-6

10-6

-7

-7

10

10-8

10-8

10-9

10-9

10-10

10-10

10-11

10-11

10-12

10-12

10-13

0
-4
-8
-12
Gate-source voltage (V)-6

VGS = -2.5 V-5-4-2.0 V-3-2-1.5 V-10

Pentacene

0

Drain

( d)

-7

-1.0 V
0-1-2
Drain-source voltage (V)

-3

10

-5

10

-7

10

-9

10

-11

10

-13

0

W = 300 μm
L = 1 μm

10

-5

10

-7

10

-9

10

-11

VDS = -2.5 V
-13
10
-1
-2
-3
Gate-source voltage (V)10-13

Fig. 3: Organic transistor manufactured using sub-femtoliter inkjet on
self-assembled monolayer gate dielectric: (a) An illustration of one
organic transistor using a self-assembled monolayer as a gate dielectric. (b)AFM image and magnified picture of printed source/drain electrodes. (c)Drain currents as a function of drain-source voltage. Gate-source voltage(VGS) is changed. (d) Drain and gate currents as a function of gate-source
voltage.

Fig. 2: Organic transistor active matrix using printing
technologies: (a) Cross-sectional illustration of one organic
transistor. (b) Top view of transistor’s channel. Linewidth and spaceare 30 μm. (c) Drain currents as a function of drain-source voltage.
(d) Transfer curve and gate currents obtained from ten transistors.

IEDM09-10

Pentacene
100
L = 1 μm
SourcePlastic substrate(Polyimide)( c)Gate current (A)Drain current (μA)

-0.6

250
30
(μm) 20Gate current (A)

(d)
10

( c)

Pentacene
Printed Ag electrode

Drain current (μA)

100 μmHeight (nm)500

Agelectrodes

SAM

Drain current (A)

(a)

1.2.2

Taking full advantages of self-assembled monolayer gate
dielectric and sub-femtoliter inkjet, we fabricated very fineprinted organic transistors that can operate with very small
voltages. A cross-sectional illustration of the organic
transistors is shown in Fig.3a. Ag nanoparticles were
patterned using sub-femtoliter inkjet to form the source anddrain electrodes. The manufactured organic transistor has achannel length of 1 μm and width of 300 μm (Fig. 3b), whichis the smallest channel length among printed organic
transistors without pre-patterning, pre-treatments, or surface
modifications.
The DC characteristics of the manufactured transistors were
measured in air using a semiconductor parameter analyzer(Agilent 4156C). Figure 3c shows the drain current (IDS) as afunction of the drain-source voltage (VDS). The gate-source
voltage (VGS) shifts from 0 to –3 V in steps of –0.5 V.
Saturation was clearly observed even at 3 V. Figure 3d shows
the corresponding transfer curves under an applied VDS of –1.5 V. The transistors exhibit a high mobility of as high as
0.3 cm2/Vs, Vth of –1.1 V, and their on/off current ratio
exceeds 105. The present sub-femtoliter inkjet technologies
can be applied not only to p-type organic semiconductors
such as pentacene but also to n-type organic semiconductors
such as F16-CuPc. The transistors exhibit a mobility of 0.02
cm2/Vs in air, Vth of –0.32 V, and an on/off current ratio inexcess of 104.
Despite the short channel length of 1 µm, the off state current
at VGS = 0 V is less than 20 pA, and the on/off current ratio is
about 106. Despite the small thickness of the room
temperature gate dielectric, the maximum gate current at VGS= 3 V is only about 100 pA.
These mobilities are comparable to those obtained with
evaporated Au source/drain contacts, suggesting that themobility is not affected by the 130 °C calcination. This isconsistent with our previous work [19] and with our

(a)

(b)

observation that the morphology of the pentacene films(examined by AFM) does not change during sintering at
130 °C.
E-skins
The first application of large-area sensors is a flexible
pressure sensor [6-8], which is suitable for electronic
artificial skin (E-skin) for the next-generation robots.
Although the mobility of organic semiconductors isapproximately two or three orders of magnitude less than thatof poly- and single-crystalline silicon, the slower speed istolerable for most applications of large-area sensors. In
particular, for the fabrication of E-skins, the integration ofpressure sensors and organic peripheral electronics avoids the
drawbacks of organic transistors, while taking advantage of
their mechanical flexibility, large area, low cost, and relative
ease of fabrication.
Figure 4 (a) shows an image of an artificial skin system and(b) its corresponding circuit diagram. A 16 × 16 active matrixof organic transistors, row decoder, and column selector areassembled by a physical cut-and-paste procedure to develop
integrated circuits for data readout. Three functional films —an interconnection layer, a pressure-sensitive rubber sheet,
and a top electrode for power supply— are then laminated
together with the organic ICs. Pressure images were obtainedby a flexible active matrix of organic transistors whose
mobility is as high as 1.4 cm2/Vs. These sensors can be bent
to a radius of 2 mm, which is sufficiently small for the
fabrication of human-sized robot fingers.
However, human skin is more complex than transistor-based
imitations demonstrated thus far. It performs certain functions
includingthermalsensing.
Furthermore,
without
conformability, the application of E-skins to threedimensional surfaces is impossible. Based on an organic
semiconductor, we have developed conformable, flexible,
wide-area networks of thermal and pressure sensors. A plastic
film with organic transistor-based electronic circuits was

16x16 FET matrix

Bit-out (DX)(bit line)

line)(c) Decoder-out (word
23ms
20V

40V

With pressure

No pressure

Voltage [V]40

Row
decoders
Column selectors

20V
21ms
3ms

10V

L=100µm
L=25µm0

Fig. 4: An artificial skin for robots. (a) A picture of an artificial skin system and (b) a corresponding circuit diagram. A 16×16 organic transistor active
matrix, a row decoder and a column selector are manufactured on plastic films separately and then assembled to make integrated circuits for data readout.
(c) a measured waveform (upper panel) and a simulation (lower panel). The delay from the row decoder activation to bit-out is measured to be 23ms. The
cycle time can be within 30 ms, which means that the time needed to scan the whole 16 × 16 sensor matrix is about 2 seconds.

1.2.3

IEDM09-11

(a)

(b)

Bit line (BL)

Word line (WL)(c)

Intersection area
(Sensor cell)

IDS
(μA)

P
(kPa)

Strut
(Electrical wiring)pressedPunched area

30

6

20

4

10

2

0

0

Fig. 5: A conformable network of pressure sensors. (a) A plastic film with organic transistors and pressure sensitive rubber is processed mechanically toform a unique net-shaped structure, which makes a film device extendable by 25%. (b) A picture of the 3×3 sensor cells. Scale is 4 mm. (c) An image(upper panel) of pressure sensor matrix put on an egg is shown together with a spatial distribution of pressure (lower panel). The current of each sensor cellis measured by applying a voltage bias of VDS = –20 V and VGS = –20 V under the application of local pressure.

In order to realize truly rubber-like stretchable e-skins, elastic
conductors have been developed [20] using single-walledcarbon nanotubes (SWNTs) [21,22] as a conducting dopant.
SWNTs were uniformly dispersed as chemically stable dopantsin a vinylidene fluoride-hexafluoropropylene copolymer matrix
by using an ionic liquid [24] of 1-butyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide, and manufactured SWNT
composite films. The measured value of conductivity is as highas 57 S/cm. Such high conductivity is achieved because theSheet-type ultrasonic sensors
The second example is an ultrasonic sensor, which realizes thethree-dimensional (3D) sensing of the position of people andobjects in free space. Although position sensing in free space
has commonly been carried out using light, radio, andmillimeter waves, ultrasonic imaging can offer complimentary
attractive features such as a simple architecture and real-timenondestructive 3D imaging in free space at an ultralow cost
[23].
(a)

Ionic liquids(b) 15

VGS = −40 to 30 V

-10 W/L = 330

10

-5

5

IDS (nA)

R = n-C4H9,
X=(CF3SO2)2N

-15

0
5

-1.2

Vout (mV)Moreover, a similar network of thermal sensors wasdeveloped using organic semiconductors. A possible
implementation of both pressure and thermal sensors on
various surfaces is presented. By using laminated sensor
networks, the distributions of pressure and temperature are
simultaneously obtained.

content of SWNTs in the conductor can be increased up to 20
wt% without sacrificing mechanical flexibility and softness.
The SWNT elastic conductor can be stretched to approximately
134% of its original size without significant mechanicaldamage. The elastic conductors are used as wirings in largearea stretchable organic FET ICs (Fig. 6). These ICs, whichhave a high electronic performance, can be stretched by up to
80% without any degradation in their mechanical or electronic
properties. This is an important step in the development of ICs
that can be used on freely curved surfaces and in smart surfaces.

V GS = −40 to 0 V-0.8

IDS (mA)

processed to form a net-shaped structure that allows the Eskin films to be stretched by 25%. The net-shaped pressure
sensor matrix was attached to the surface of an egg andpressure images were successfully obtained in thisconfiguration (Fig. 5).

-0.4

10
SWNTs

15
1

Fluorinated copolymer

VGS V

0
-5
VGS = +20V

-10
-20
VDS (V)0.5
0 -0.5
VDS (mV)

fin = 1 MHz

-40

-1

GS = −40V

-15

0

2

4

6

8

10

Time (μsec)

Fig. 7: High-speed printed organic FETs for ultrasonic sheet. (a)Operation of the organic FETs with a small source-drain voltage (VDS).
A plot of the source-drain current (IDS) vs. VDS. (b) 1 MHz operation of
the organic FETs with a grounded-gate.

Fig. 6: A truly rubber-like stretchable organic FET active matrix.
Elastic conductors, which are fabricated with mixing single-walledcarbon nanotubes (SWCNs), ionic liquids, and fluorinated copolymers,
are used as wirings to connect contacts of printed organic FETs.

IEDM09-12

0
0

Vout

1.2.4

(b)

(c)
Semisphere(a)Actuator(Nafion)On

Surface(PDMS)

Off

Fig. 8: A Braille sheet display. (a) An image of a pocket Braille sheet display. It was manufactured on a plastic film by integrating the active matrix ofhigh-quality organic transistors with a plastic sheet actuator array based on a perfluorinated polymer electrolyte membrane. The device is mechanically
flexible, very thin, and lightweight. (b) One character is displayed by a 3 × 2 array of rectangular actuators (4 mm in length and 1 mm in width). A
semisphere of radius 0.9 mm is attached to each actuator, which bends and lifts the semisphere. The scale is 4 mm. (c) Principle of Braille motion.

A large-area, flexible, ultrasonic 3D imaging system, byintegrating an ultrasonic transducer array sheet with a printedorganic FET active matrix [13]. The ultrasonic sensing cellcomprises one organic transistor and one ultrasonic transducer.
A transistor active matrix sheet and an ultrasonic transducer
array sheet are manufactured separately and electricallyconnected to each other. The system comprises 8 × 8 ultrasonicsensing cells or others, with a printed area of 25 × 25 cm2. It ismechanically flexible and can be wrapped around a cylindricalbar; this configuration is suitable for obtaining a viewing angleof 360º for a medium-length (~ several meters) proximity
robotic skin-like sensor.
The organic FETs are fabricated on a 75-µm-thick polyimide
film. The source, drain, and gate electrodes are deposited by an
inkjet printing or a vacuum evaporation. A 500-nm-thickpolyimide gate dielectric layer is formed by inkjet or spin
coating, and a 50-nm-thick pentacene channel layer isdeposited by vacuum evaporation by using a printed shadowmask. The mobility in the saturation regime is 0.5 cm2/Vs,
while an on/off ratio is 107 in DC measurements. The
transistors operate efficiently in the small signal regime of theultrasonic transducers (~1 mVp-p). The transistors with thegrounded gate can switch 40 kHz signals with an on/off ratio ofgreater than 104.
A 28-µm-thick piezoelectric polyvinylidene fluoride (PVDF)
film [24], whose surfaces are covered by metal, is used as asheet-type ultrasonic transducer. The transducers are used in
the bending mode. To adjust the mechanical resonancefrequency to 40 kHz, the transducer is bended to improve thesensitivity using a spacer sheet. The ultrasonic transducer has
an output voltage of ~0.8 Vp-p and a narrow pulse width of~100 µs after an amplification of 40 dB by the incidence of an
ultrasonic wave. The ultrasonic transducers can detect a stiff
object that is hidden inside a cloth. It also shows broad
directivity (> –5 dB for ± 60º).

examined. The signal from the ultrasonic transducer isswitched by the organic FET at 40 kHz. The integrated sensing
cells have an on/off ratio of 104 at 40 kHz. 3D ultrasonicimages can be obtained for multiple target objects over thissheet. This system can detect target objects behind a cloth anda paper.
A pocket scanner
As a new development in the field of large-area electronics,
we have developed a large-area, flexible, and lightweight
sheet image scanner based on organic semiconductors [9].
We report its principle of imaging, manufacturing process,and electronic performance. The device is manufactured onplastic films by integrating organic FETs and organic
photodiodes. Organic photodetectors distinguish between theblack and white parts of paper based on the difference in theirreflectivity. The effective sensing area of the integrated
device is 5 × 5 cm2; resolution, 36 dpi; and the total numberof sensor cells, 5,184. Furthermore, by using twodimensional arrays of a photodiode matrix without theorganic transistors, we have successfully obtained images ofseveral characters whose size is approximately 1 × 1 mm2.
The present sheet-image scanners are mechanically flexible,
lightweight, and very thin; therefore, suitable for humanfriendly mobile electronics.
A sheet-type Braille display
Organic transistors are suitable for applications to plastic
large-area actuators. We have developed a flexible,
lightweight Braille sheet display that is fabricated on a plastic
film, by integrating high-quality organic FETs with plastic
actuators [10]. A small hemisphere which projects upwards
from the rubber-like surface of the display is attached to thetip of each rectangular actuator (Fig. 8).
A wireless power transmission sheet

The characteristics of the integrated ultrasonic sensing cellcomprising one ultrasonic transducer and one organic FET are

We have also demonstrated the implementation of a large-

1.2.5

IEDM09-13

(c) Power transmission coil sheet(a) Position-sensing coil sheet

BL

Cu

Cu

Via

PolyimideCu (BL)
Output10 mmParylene
Epoxy Au (D)S

Pentacene
Polyimide
Ag (G)

Ag-paste(d) MEMS switching sheet

Via

BL

Au (S)

Via

5 mm

Input

Ag
PolyimideA

Ag (to BL)

Ag
B

WL

Polyimide
Ag (WL)Ag
Pentacene

Cu

10 mm

Ag

G

D

PolyimideCu

Ag-paste(b) Organic FET sheet
1 mm

Via
Cu

C
Parylene

PolyimideAg
Ag (to WL)
Polyimide
Input

Ag

Fig. 9: A wireless power transmission sheet. The contactless position-sensing sheet comprising (a) position-sensing coil sheet and (b) organic FET sheet.
S, D, and G represent the electrodes for source, drain, and gate, respectively. The organic FET sheet is laminated with the position-sensing coil sheet using
silver paste islands. A resonance frequency of 2.95 MHz is used in the position-sensing system. WL and BL represent word-line and bit-line, respectively.
The power transmission sheet comprising (c) power transmission coil sheet and (d) MEMS switching sheet. Electrodes for electrostatic attraction
connected to the word-line (WL) and bit-line (BL) of MEMS switch. B and C: Electrodes for power transmission connected to coils and a power generator.

area wireless power transmission system [11]. The sheet-typelarge-area wireless power transmission system has been
manufactured using organic transistors and MEMS switches(Fig. 9). The position of electronic objects on this sheet canbe contactlessly sensed by electromagnetic coupling using anorganic transistor active matrix. Then, power is selectively
fed to the objects by an electromagnetic field using a twodimensional array of copper coils that are driven by a printed
plastic switching matrix. The effective power transmission
area is 21 × 21 cm2. Due to selective power transmission, we
achieved a coupling efficiency of power transmission of
81.4%, and a power of 40 W was wirelessly received. The
thickness and weight of the entire sheet are 1 mm and 50 g,
respectively.

actuators. Our prototypes demonstrate that organic transistors
can be used to extend the field of electronics into large areas.
One day high-speed integrated circuits will be printed
everywhere you want such as surfaces on plastic films, paper,and cloth. Subsequently, electronics can jump out of aconfined small box of a computer into a real world andsubstantial progresses in electronics will continue.
Acknowledgements
The authors greatly appreciate invaluable contributions thathave been made by their group members. This study waspartially supported by KAKENHI; WAKATE S, NEDO,
CREST/JST and the Special Coordination Funds for
Promoting and Technology.

Remaining issues and future prospects
References
We describe the issues of organic transistors from theviewpoint of ambient electronics. Stability and reliability are
the main concerns for the organic transistors. Degradation may
be induced by oxygen and/or moisture like electroluminescent(EL) devices; therefore, it can be suppressed drastically byappropriate encapsulation. Although plastic films usuallyexhibit high gas penetration, applications that require
mechanical flexibility, such as electronic skins and sheet image
scanners, also require flexible substrates with low gas
permeability. Thus, it is very important to develop flexible basefilms with low gas permeability.
Finally, we would like to briefly describe a futuristic life thatwould be possible by ambient electronics. Please image thatthe interiors of a car will be covered with large-area sensors.
Based on flexible biometrics installed in the steering wheel,
the driver is identified. When I fasten the seat belt, which isan invisible flexible sensor, it will start monitoring theheartbeat. If the grip pressure and the heart rate decrease, thecar perceives that the driver’s concentration is declining.
Subsequently, stimulus was returned to the drivers by

IEDM09-14

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1.2.6