Large-Area Flexible Electronics
with Organic Transistors
Koichi Ishida, Tsung-Ching Huang, Tsuyoshi Sekitani, Makoto Takamiya, Takao Someya, and Takayasu Sakurai
The University of TokyoTokyo, Japanishida@iis.u-tokyo.ac.jp
Abstract—Organic transistors are promising candidates forrealizing large-area flexible electronics such as smart displays,
power transmission sheets, and electronic skin for robots. In thispaper, recent progress of organic transistors and novel
interconnects are briefly surveyed. Example applications oflarge-area flexible organic electronics in particular EMImeasurement sheet and User Customizable Logic Paper (UCLP)
are introduced.

I.

INTRODUCTION

With the advantages of flexibility, printable/printed
technology, and low cost in the future, organic transistors arepromising candidates for realizing large-area flexible
electronics including smart displays, power transmission sheets,
and electronic skin for robots. In this paper, recent progress oforganic transistors, novel interconnects, and some examples oflarge-area flexible organic electronics are introduced. In
Section II, resent progress of organic transistors and theircircuit blocks are briefly surveyed. Section III introduces twonovel stretchable/flexible interconnects. Section IV introducessome examples of large-area flexible organic electronics inparticular EMI measurement sheet, User Customizable LogicPaper. Finally some conclusions are provided in Section V.
II.

RECENT PROGRESS OF ORGANIC TRANSITORS

Organic PMOS transistors have been getting good yieldand time dependent reliability, and therefore, highly integratedorganic PMOS circuits were presented such as an 8-bit
microprocessor [1] and a first-order delta-sigma ADC [2]. Interms of low-voltage operation, self assembled monolayer(SAM) [3] realized 1.5 to 3V operation enabling direct siliconorganic circuit interface [4] was presented. In terms of printedelectronics, some printed CMOS circuits were presented [5].
This section introduces resent progress of organic transistors
and their circuit blocks briefly.
A. An 8-bit Microprocessor on Plastic Foil
The improvement of yield of organic PMOS transistor
realized an 8-bit organic microprocessor on a plastic foil for thefirst time [1]. The processor consists of only NAND gates andinverters in 5µm organic PMOS process, and operates with
6Hz clock at 10V VDD. Numbers of transistors of the processor
foil and an instruction code foil are 3381 and 612, respectively.

B. Fully Integrated ΔΣ ADC with OrganicPMOS Transistors
A continuous time ΔΣ ADC with 5µm organic PMOS
transistors was presented [2]. It is a fully integrated organic
ADC for the first time. In order to compensate VTH mismatchof input differential pair transistors in an amplifier and improve
the gain, the circuitry employs backgate biasing. Measured
SNR and bandwidth are 26.5dB and 15.6Hz, respectively.
Clock frequency is 500Hz at 15V VDD. The circuit consists of129 transistors.
C. Low-Voltage Organic CMOS Circuit
A low-voltage organic CMOS circuit using SAM gate
oxide was presented [3]. A nanometer order gate oxide
thickness with SAM realizes 1.5 to 3V operation which isaround 10-time lower than that of conventional organic
transistors. This improvement enables direct silicon-organic
circuit interface.
D. Fully Printed Organic CMOS Circuit
A complete printed organic CMOS technology on a plastic
foil was proposed [5]. The devices are fabricated by 5µmline/space resolution screen printing process and annealing at100°C. Oscillation frequency of a 7-stage ring oscillator thatconsists of W/L=1000/20µm CMOS transistors varies from70Hz at 40V to 16Hz at 20V. An NMOS differential amplifier
was also provided in [5]. The maximum gain achieves 17 at
10V VDD.
TABLE I.

SUMMARY OF RECENT PROGRESS OF ORGANIC TRANSISTORS
[1]

[2]

[3]

[5]

Key features

8-bit microprocessoer

Fully
integrated
ADC

Transistor type

PMOS

PMOS

CMOS

CMOS

Gate length

5µm5µm5µm20µm

Supply voltage

10V15V

1.5 to 3V10 to 40V

Number oftransistors

3381

1292

≈20

Low-voltage
Fully printed
operation

978-1-61284-857-0/11/$26.00 ⓒ2011 IEEEIII.

NOVEL FLEXBLE INTERCONNECTS

In order to realize flexible electronics, interconnects should
also be stretchable or flexible. In this section, two novel
flexible interconnects, stretchable Interconnects with carbon
nanotubes (CNTs) and printed interconnects with at-home inkjet printers, are introduced.
A. Stretchable Interconnects with Carbon Nanotubes
A rubberlike stretchable interconnection was presented [6].
The conductive material for the stretchable interconnects is
CNTs. CNTs are an inherently stretchable material with a very
low resistivity of 0.02µΩcm [6], however, CNTs should bedisentangled prior to being combined with a rubberlike
material. The process flow of the stretchable interconnects isshown in Fig. 1. Single-walled CNTs are used as a conductive
material. An ionic liquid (1-butyl-3-methyl imidazoliumbis(trifluoromethanesulfonyl) imide, BMITFSI [6]) and CNTs arecombined and ground to disentangle the CNTs. A rubberlike
material, fluorinated copolymer, and CNT dispersed in gel are
combined and stirred by sonication. The stretchable
interconnects film is formed by air-drying. Fig. 2 demonstrates
its stretchability. The flexible interconnects can be stretched by
40% thanks to the sliding CNTs embedded in the rubberlike
material, fluorinated copolymer.
B. Printed Interconnects with At-Home Ink-Jet Printers
A number of ink-jet printed interconnects technologies
Carbon nanotubes(CNTs)> 4 mmIonic liquid(BMITFSI)
Grinding todisentangle CNTs

Fluorinatedcopolymer

CNTs dispersed gel
Stirring / Sonication

pre-coatednanoconductive baseInk-jet printing

conductive
Ag nanoparticle inkNozzle

Paper

Conductive Ag
nanoparticle ink
Interconnects
formed underroom temperature
Pre-coatednanoconductive baseFig. 3. Materials and process flow of the ink-jet printed interconnects.

have been developed to date. However, either a hightemperature (130-150ºC) sintering or chemical process withorganic solvent is required after the printing process, whichcauses damage to organic transistors. In contrast, recentlyproposed ink-jet printing technology [7] does not require suchprocesses. The technology consists of pre-coatednanoconductive base and silver nanoparticle ink as shown inFig. 3. The pre-coated nanoconductive base is a type of microporous layer made of Polyethylene, Polyvinyl alcohol, boracic
acid, and other materials to absorb the solvent in the silver
nanoparticle ink. Its thickness is around 40um. The ink
includes 15% silver particles, alcohol, surface active agent, anddispersant. The diameter of silver nanoparticles in the ink istypically 20nm. The ink chemically reacts with the pre-coatednanoconductive base on the paper at room temperature, that is,a sintering-free process. This does not cause any damage toorganic transistors. When the printed interconnect is stored in alaminated bag, the life time is five months or longer. In air, theresistance of the interconnects increases by 10% after the onemonth.

Ink-jet printerAir-drying
Stretchable interconnect film
Fig. 1. Process flow of the stretchable interconnects.

UCLP
30mm, 12ΩPull

Stretch (+40%)
42mm, 17Ω

L/S = 200μm/200μm

φ200µm

200μm
Printed interconnects

Via hole

Sheet resistance: 0.2Ω/squareVia resistance: 2.7Ω/via
Fig. 2. Demonstration of stretchable interconnect.

Fig. 4. Photograph of the printing process of the interconnects on UCLP
using ink-jet printer, printed interconnects, and a via hole.

Fig. 4 shows a photograph of the printing process of theinterconnects using an off-the-shelf ink-jet (piezo-jet) printerand printed interconnects and a via hole [8]. The thickness ofthe Ag interconnect is 1µm (typ.) and the measured sheet
resistance is 0.2Ω/square (typ.). The via holes of 200µm in
diameter are prefabricated by punching. The measured via
resistance is 2.7Ω/via.
IV.

LARGE-AREA FLEXIBLE ELECTRONICS WITH ORGANICTRANSISTORSThis section introduces some examples of large-area
flexible organic electronics such as EMI measurement sheet
and User Customizable Logic Paper (UCLP).
A. A Stretchable EMI Measurement Sheet
Fig. 5 shows an example of an intrasystem EMC issue in acell phone. EMI largely depends on the circuit board layout.
Localizing either an EMI source or a critical wiring is difficult
by simulation. EMI measurement is, therefore, important forthe development of electronic systems. However, there is nomethod of measuring EMI on the surface of 3D structures. Inthe conventional method, a pencil-like magnetic field probewith X-Y scanning equipment and spectrum analyzers are used
for the EMI measurement [9]. In the method, the surface of theelectronic device should be scanned repeatedly with the probe.
However, the scanning equipment can only move in a flat

Conventional

ProposedProbe
EMI

Wrap

Scan
AnalogDC to DC converter

Magnetic field probe

The EMI source isdifficult to localize.

EMI measurement sheet

The EMI source iseasy to localize.

Fig. 5. Intrasystem EMC issue in a cell phone.

plane. In another conventional method, a measurement systemwith an integrated array of magnetic field loop antennas is used[10]. Although the method captures the distribution of amagnetic field, it is not applicable to 3D structures since theantenna array is implemented on a flat and rigid printed circuitboard.
To solve this problem, an EMI measurement sheet [4] wasproposed, which enables the measurement of EMI distribution
on the surface of 3D structures by wrapping the devices with asheet like “furoshiki”. Once EMI noise is roughly localizedwith the measurement sheet, one can easily scan EMI noiseusing the probe method for precise localization or better
quantification of the EM field. The sheet can measure not onlya magnetic field but also an electric field suitably by simply
changing its antenna connection [4].
Fig. 6 shows a prototype of the stretchable EMI
measurement sheet. Each printed circuit board (PCB) includes
2×2 antenna coils and a silicon EMI measurement LSI. The
sheet consists of 4×4 PCB’s, and therefore, 8×8 antennas arelocated in 12×12cm2 area. The antennas and LSI’s arecontrolled using 2V organic CMOS decoder and selector. Each
module is electrically connected with a stretchable interconnect
made of CNTs. The overall system is sealed with a rubber
sheet made of silicone elastomer. The sheet is, therefore,
flexible and stretchable. The sheet detects the total power of anelectric field in the band up to 700MHz and that of a magnetic
field up to 1GHz. The minimum detectable power of theelectric and magnetic fields are -60 and -70dBm, respectively.
B. User Custamizable Logic Paper (UCLP)UCLP is proposed for both prototyping of larger-area
electronics and educational applications [8]. In particular, as
shown in Fig. 7, learners can study and experience theoperation of integrated circuits by fabricating custom integrated
circuits, using at-home ink-jet printers to print conducting
interconnects on paper that contains prefabricated arrays oforganic transistors. The feasibility of UCLP is demonstrated
with the newly proposed Sea-of Transmission-Gates (SOTG)
of organic CMOS transistors, providing field customizability
through the use of the printable electronics technology. UCLP
is applicable to a wide range of products of printableelectronics including flexible displays and electronic paper, as

Rubber sheet
Antenna coil
12cm

Si CMOS LSIEMI
measurement
circuitStretchable
Interconnect(CNTs)

2V organic CMOS decoder circuits
Fig. 6. Stretchable EMI measurement sheet.

Fig. 7.User Customizable Logic Paper (UCLP) for educational purpose.
Learners can study and experience operation of integrated circuits byfabricating custom integrated circuits, using at-home ink-jet printers toprint conducting interconnects on paper that contains prefabricated arraysof organic transistors.

a protective layer of parylene. Connection pads to the paper areformed with gold on top of this protective layer. Theinterconnects are ink-jet printed onto the paper by users.

Paper with via array.
Interconnects will beformed by Ink-jet printer.

V.

In this paper, resent progress of organic transistors and theircircuit blocks are briefly surveyed. Highly integrated circuit
blocks such as microprocessors and ADC can be implemented
with organic transistors. Low-voltage organic CMOS
transistors enables direct silicon-organic circuit interface.
Printed electronics can be fabricated at a low cost. The progressof organic transistors will realize large-area, flexible
electronics with low-cost process in the near future.

8x8 organic sea-oftransmission-gate(SOTG) film

ACKNOWLEDGMENT

Fig. 8. Prototype of UCLP.

This study was partially supported by JST/CREST and theSpecial Coordination Funds for Promoting and Technology.

Ag, Ink-jet printed interconnects (t=1µm)
Ag, Via(φ200µm)
Au, PadSAM*

CONCLUSIONSREFERENCES

Paper (t=150µm)

Au, Pad Au, Pad
Au (Source/ Drain)
Au, Via[1]Parylene
AlOx

Al (Gate, interconnects)
Polyimide substrate
2V Organic semiconductor
Pentacene (pMOS), F16CuPc (nMOS)

SOTG* SAM:Phosphonic acid-based self-assembled monolayer**F16CuPc:Hexadecafluorocopperphthalocyanine

Fig. 9. Cross sectional view of UCLP.

well as for educational purposes. This technology provides a
new means to add programmability for integrated circuits usedin large-area electronics. Fig. 8 shows a prototype of UCLP [8].
In UCLP, paper that contains an array of vias and an organic
SOTG film are stacked. Although it would be ideal to fabricate
the whole UCLP including the organic transistors using ink-jet
printers, this proves to be very difficult since both hightemperature (130-150ºC) and an organic solvent are required tofabricate the organic transistors. Since high temperatures andorganic solvents cannot be handled by ink-jet printers, inUCLP the organic transistors are prefabricated and the fieldcustomizability is provided by the printed interconnects.
Fig. 9 shows the cross sectional view of UCLP. The 2-V
organic CMOS transistors [8] are fabricated on polyimide film.
Organic semiconductors for NMOS and PMOS are F16CuPc
and pentacene, respectively [8]. Self-assembled mono-layers(SAM) realize 2-V organic transistors, which are covered with

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