Large-Area and Flexible Sensors
with Organic Transistors
Hiroshi Fuketa, Koichi Ishida, Tsuyoshi Sekitani, Makoto Takamiya, Takao Someya, and Takayasu Sakurai
University of Tokyo and JST/ERATO
Tokyo, Japan
fuketa@iis.u-tokyo.ac.jp
Abstract—Organic transistor circuits are attracting a lot of
attention for large-area and flexible electronics applications, suchas sensors for human interface applications, because organic
transistors can be fabricated with printable/printed technology
on arbitrary substrates, which enables low-cost production andhigh mechanical flexibility. In this paper, recent progresses oflarge-area and flexible sensors with organic transistors arereviewed.

I.

INTRODUCTION

Organic transistor circuits have been drawing attentions forlarge-area and flexible electronics applications. Table I
summarize a comparison between our organic FETs (OFETs)
and state-of-the-art 45-nm Si MOSFETs [1]. OFETs are muchslower than Si MOSFETs, whereas OFETs have the following
two advantages; 1) OFETs are mechanically flexible, since
OFETs are fabricated with printable/printed technology on afilm, such as a polyimide film and a polyethylene naphthalate(PEN) film, and 2) the cost per area of OFET is lower than thatof Si MOSFETs. Therefore, large area and flexible electronics
applications are suitable for OFETs. Sensors for humaninterface applications are one of the promising candidates,
since large area and flexibility are extremely important forhuman interfaces. In this paper, some examples of large areaand flexible sensors with organic transistors are shown.
II.

LARGE AREA AND FLEXIBLE SENSORS WITH ORGANICELECTRONICS

In this section, recent progresses of large-area and flexible
sensors with organic transistors are briefly reviewed.
A. Electronic Artificial Skin (E-skin)
The importance of pressure sensing is increasing inapplications such as area sensor networks and robots for the
next generation. We developed a large area and flexible
pressure-sensor array, called E-skin [2-4], as shown in Fig. 1.
Fig.2 shows the developed E-skin system consisting of 16 x
16 sensor cell matrix, row decoders and column selectors glued
with connecting tapes. Each sensor cell consists of a pressure
sensor and an OFET for selecting the sensor cell as shown in
the inset of Fig. 2. The OFETs are stacked with the pressure
sensors. The pressure sensors are made of pressure-sensitive

978-1-4799-0041-1/13/$31.00 ©2013 IEEE

Table I. Comparison between organic FETs (OFETs)
developed in our group and state-of-the-art 45-nm Si
MOSFETs [1].

OFETs

Si MOSFETs

Minimum gate length

20 μm

45 nm

Mechanical flexibility

Flexible

Very limited

Normalized ON current

3 nA / μm @ 3 V

1 mA / μm @ 1 V

Gate delay

0.1 s @ 3 V

10 ps @ 1 V

Cost / area

Low

High

Cost / transistor

High

Low

.

Fig. 1. Implementation example of electronic artificial skin(E-skin), which is sheet containing array of pressure
sensors and OFETs.

conductive rubbery sheets sandwiched between a coppercoated polyimide film (electrode sheet in Fig. 2) and anotherpolyimide film with via holes (through-hole sheet in Fig. 2).
The measured pressure dependence of current flowing throughthe sensor cell is also shown in the inset of Fig. 2. When thesensor is pushed by a rectangular object, only thecorresponding part of the pressure-sensitive rubber turns on
and the corresponding cells pull the bit lines (BLs). The size ofthe sensor cell is 2.54 x 2.54 mm2.

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Sensor cellPressure-sensitive
conducting rubber

Through-hole sheet
Column selectors

Top electrode sheet
IDS

16x16 sensor cell matrix

Row decoders

Connecting tapes

Fig. 2. Photograph of electronic artificial-skin (E-skin) system, and detailed structure of sensor cell.

80mm

80mm

64 x 64
pixels

OFET sheet#2
OFET sheet#1

Fig. 3. Sheet-type image scanner, which is sheet integratingarray of photodetectors and OFETs.
In the developed E-skin, a scalable-circuit concept based on
cut-and-paste programmability, which enables all of thecircuits to be scalable in size, was proposed. This concept ispreferable in terms of mask cost because there is no need tomake new masks depending on the required sizes and shapes.
Using the E-skin system, a pressure image with resolutionof 10 dpi was successfully taken. The system can be bent down
to 5 mm in radius, which is sufficient to wrap around thesurface of a round object like a robot as shown in Fig. 1.
B. Sheet-Type Image ScannerFig. 3 shows a sheet-type image scanner integrated with an
OFET and organic photodiode (OPD) [5-8]. The sheet-type
scanner can capture a black-and-white image on a paper
without heavy mechanical components or optical lens.
Fig. 4 shows the photograph of the sheet-type scanner. The
scanner contains 64 x 64 pixel arrays, which occupy 80 x 80
mm2, and the structure of each pixel is also illustrated in Fig. 4.
One OPD sheet and two OFET sheets are separately fabricatedand glued together with silver paste. All base films are

OPD sheetSide view of each pixel
Silver paste
OFET sheet#2

Light

OFET sheet#1
OPD sheet
Paper: black.
Peripheral
OFET
Pixel
selector

Photodiodewhite

Fig. 4. Photograph of fabricated sheet-type image scannersystem and device structure of each pixel.

transparent PEN, through which light can pass. On the OPD
sheet, the base film is covered with ITO as a common anode.
CuPc is p-type semiconductor and PTCDI is n-type one,
forming the OPD [9, 10]. The aperture (open-area ratio) is 45%
of a total pixel area. The pixel size is 1.27 x 1.27 mm2, whichcorresponds to 20 dpi. Three sheets are stacked as shown in Fig.
4 and the total thickness and weight are 0.4 mm and 1 g,
respectively. The minimum bending radius is 30 mm.

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Scanned

Analog circuits (20V organic
CMOS with floating gate)

100V organic PMOS
Power
supply(full-wave
rectifier)

100V
50Hz

VDD (20V)
GND (0V)
VSS (-20V)

Clock
generator Clock(half-wave (50Hz)
rectifier)Q9 0.05Hz

10bit
frequency
dividerFig. 5. Original image of “F” and image scanned bydeveloped sheet-type image scanner.

Q6 0.4Hz

Current sense R

Original

100V AC load current iL V

v=iLR

+

MOS
diode

SENSE

VTRI
EN

-

Opamp (FG)Count up (0.4Hz)
VSENSE

Triangularwave
generator

0.05Hz
VTRI

-

EN

+

Comparator (FG)
Q9

EN
10bit
counter

Q5

Logic circuits (20V organic CMOS)
Analog circuits( 20V organic CMOS with floating gate)

Unfolded form

Bar indicator (OLED)

Fig. 7. Block diagram of energy meter.

Flexible PCB (Polyimide)

Prosthetic hand

AC connector

AC-to-DC Power convertor(100V organic PMOS)
Bar indicator (OLED)

Surface electromyogram measurement sheet (SEMS)

Using form

Logic circuits( 20V organic CMOS)

Fig. 6. Photograph of 100-V AC energy meter on flexible
film.
Fig. 5 shows an example of scanned image using thedeveloped sheet-type image scanner. The scanner is so thin andflexible that it can take an image of a round object such as alabel on a wine bottle, as shown in Fig. 3, which is impossible
for the conventional commercial scanners.
C. AC Energy Meter
A smart meter is essential for realizing the smart grid. Inorder to further reduce the energy loss in the power grid, an
extremely fine-grain power monitoring system is desirable andit will require an enormous number of low-cost power meters.
Since printable organic devices on flexible films have great
potential to realize low-cost and flexible energy meters, we
proposed a 100-V AC energy meter based on the system-on-afilm concept (SOF) [11, 12].
Fig. 6 shows a photograph of the proposed organic 100-V
AC energy meter on a flexible film. Fig. 7 illustrates a block
diagram of the energy meter. The key components of theenergy meter include; 1) a rectifier composed of a 100-V
organic pMOS, 2) analog circuits, such as operational amplifier,fabricated with 20-V organic CMOS transistors, 3) logic
circuits composed of 20-V organic CMOS transistors for thefrequency divider and the counter accumulating the measured
power, 4) an organic LED (OLED) [13] bar indicator for

8 x 2 amplifier array8 x 8 EMG electrode array

45 mm

Fold

Stacked2 sheets

EMG electrodes(0.7mm x 0.7mm)

Organic transistors

1µm-thick
PEN film
40mm

Electrode pitch = 5mm.

Fig. 8. Photograph of surface EMG measurement sheet.

displaying the measured energy, and 5) an AC connector
inserted between the power plug and the AC outlet and discretepassive components (capacitors and resistors for circuit tuning)which are fully integrated on a 200 x 200 mm2 flexible film.
We name this type of system implementation an SoF. Theenergy meter based on the SoF is flexible and therefore can beinstalled to monitor each AC outlet simultaneously. The entire
sheet of the film can be folded as shown in Fig. 6 and the total
size of the proposed 100-V AC energy meter can be reduced to
70 x 70 mm2.

D. Surface Electromyogram Measurement Sheet
A surface electromyogram (EMG), which measures a
voltage waveform produced by skeletal muscles on a skin, is an
important tool for applications detecting the human will of
motion, such as for prosthetic hand control. In order toprecisely control the hand, a multipoint EMG measurement isrequired [14, 15]. Conventional EMG measurements with
passive electrodes, however, have the following two problems;
1) a long time measurement is annoying, and 2) the signal

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Arm

Organic
amplifierBand elimination filter(50Hz)
OUT

3.5cm
Electrodes

Open hand

Closed Hand

5mV
5mV

10ms

10ms

Fig. 9. Measured waveforms of the surface EMG with theorganic amplifier.
integrity of EMG is degraded. To address these challenges, wedeveloped a surface EMG measurement sheet (SEMS) [16].
Fig. 8 shows a photograph of the developed 45 x 40 mm264-channel SEMS. In the SEMS, an 8 x 8 EMG electrode array
sheet and an 8 x 2 amplifier array sheet with 2V organic
transistors on a 1μm-thick ultra-flexible PEN film are stacked.
The pitch of the EMG electrode is 0.7 mm and the area of the 8x 8 EMG electrode array is 3.5 mm2. Since the PEN film isultra-flexible thanks to its thickness of 1μm and the distributedorganic amplifiers improve the signal integrity of measured
EMG signals, the developed SEMS enables a comfortablelong-time measurement without signal integrity degradation.
In the developed SEMS, gain mismatch of the organic
amplifier array is a problem. A post-fabrication tuning
technique using inkjet-printed interconnects was proposed toreduce mismatch in organic circuits. Such technique is too
costly and not practical in silicon VLSI technology, whereas it
is feasible in printable/printed technology.
Fig. 9 shows the measured waveforms of the surface EMG
with the organic amplifier. The difference between the
waveforms with open and closed hands was clearly observed.
III.

SUMMARY

In this paper, four examples of large area and flexible
sensors with organic transistor, such as electronic artificial skin(E-skin), sheet-type image scanner, AC energy meter, andsurface EMG measurement sheet, were explained. Organic
transistors are suitable for such sensors for human interface
applications, because human interface applications require highmechanical flexibility and large area, and organic transistors
are able to meet the demand. Therefore, large area and flexible
sensors are one of promising applications for organic
electronics.

REFERENCES[1]

M. Takamiya, K. Ishida, T. Sekitani, T. Someya, and T. Sakurai,“Printable and Flexible Electronics with Organic Transistors,” IEEE
International Conference on Computer-Aided Design, 2012.
[2] T. Someya and T. Sakurai, “Integration of Organic Field-EffectTransistors and Rubbery Pressure Sensors for Artificial Skin
Applications,” IEEE International Electron Devices Meeting, pp. 203 –
206, 2003.
[3] T. Someya, H. Kawaguchi, and T. Sakurai, “Cut-and-paste organic FET
customized ICs for application to artificial skin,” IEEE International
Solid-State Circuits Conference, pp. 288-289, 2004.
[4] H. Kawaguchi, T. Someya, T. Sekitani, and T. Sakurai, “Cut-and-Paste
Customization of Organic FET Integrated Circuit and Its Application toElectronic Artificial Skin,” IEEE Journal of Solid-State Circuits, Vol.
40, No. 1, pp. 177 – 185, 2005.
[5] T. Someya, S. Iba, Y. Kato, T. Sekitani, Y. Noguchi, Y. Murase, H.
Kawaguchi, and T. Sakurai, “A Large-Area, Flexible, and Lightweight
Sheet Image Scanner Integrated with Organic Field-Effect Transistors
and Organic Photodiodes,” IEEE International Electron Devices
Meeting, pp. 365 - 368, 2004.
[6] H. Kawaguchi, S. Iba, Y. Kato, T. Sekitani, T. Someya, and T. Sakurai,“A sheet-type scanner based on a 3D stacked organic-transistor circuitwith double word-line and dou-ble bit-line structure,” IEEE International
Solid-State Circuits Conference, pp. 580-581, 2005.
[7] T. Someya, Y. Kato, S. Iba, Y. Noguchi, T. Sekitani, H. Kawaguchi, and
T. Sakurai, “Integration of Organic FETs with Organic Photodiodes fora Large Area, Flexible, and Lightweight Sheet Image Scanners,” IEEE
Transactions on Electron Devices, Vol. 52, No. 11, pp. 2502 – 2511,
2005.
[8] H. Kawaguchi, S. Iba, Y. Kato, T. Sekitani, T. Someya, and T. Sakurai,“A 3-D-Stack Organic Sheet-Type Scanner with Double-Wordline andDouble-Bitline Structure,” IEEE Sensors Journal, Vol. 5, No. 5, pp.
1209 – 1217, 2006.
[9] Z. Bao, A. Lovinger, and A. Dodabalapur, “Organic field ‐ effecttransistors with high mobility based on copper phthalocyanine,” Applied
Physics Letters, Vol. 69, No. 20, pp. 3066 – 3068, 1996.
[10] J. Shinar, “Organic Light-Emitting Devices: A Survey,” Springer, 2003.
[11] K. Ishida, T. -C. Huang, K. Honda, T. Sekitani, H. Nakajima, H. Maeda,
M. Takamiya, T. Someya, and T. Sakurai, “100-V AC Power Meter
System-on-a-Film (SoF) Integrating 20-V Organic CMOS Digital andAnalog Circuits with Floating Gate for Process Variation Compensationand 100-V Organic PMOS Rectifier,” IEEE International Solid-State
Circuits Conference, pp. 218-219, 2011.
[12] K. Ishida, T. -C. Huang, K. Honda, T. Sekitani, H. Nakajima, H. Maeda,
M. Takamiya, T. Someya, and T. Sakurai, “A 100-V AC Energy Meter
Integrating 20-V Organic CMOS Digital and Analog Circuits With a
Floating Gate for Process Variation Compensation and a 100-V Organic
pMOS Rectifier,” IEEE Journal of Solid-State Circuits, Vol.47, No.1,
pp. 301-309, 2012.
[13] H. Nakajima, S. Morito, H. Nakajima, T. Takeda, M. Kadowaki, K.
Kuba, S. Hanada, and D. Aoki, “Flexible OLEDs poster with gravure
printing method,” in Soc. Inf. Display Dig., vol. XXXVI, pp. 1196–1199,
2005.
[14] P. Liu, D. Brown, F. Martel, D. Rancourt, and E. Clancy, “EMG-toForce Modeling for Multiple Fingers,” IEEE Annual Northeast
Bioengineering Conference, pp. 1-2, 2011.
[15] B. Lapatki, J. Dijk, I. Jonas, M. Zwarts, and D. Stegeman, “A Thin,
Flexible Multielectrode Grid for High-Density Surface EMG,”
American Physiological Society Journal of Applied Physiology, vol. 96,
no. 1, pp. 327-336, 2004[16] H. Fuketa, K. Yoshioka, Y. Shinozuka, K. Ishida, T. Yokota, N.
Matsuhisa, Y. Inoue, M. Sekino, T. Sekitani, M. Takamiya, T. Someya,
and T. Sakurai, “1μm Thickness 64 Channel Surface Electromyogram
Measurement Sheet with 2V Organic Transistors for Prosthetic Hand
Control,” IEEE International Solid-State Circuits Conference, pp. 104105, 2013.

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