LETTERS
A large-area wireless power-transmission
sheet using printed organic transistors and
plastic MEMS switches
TSUYOSHI SEKITANI1 , MAKOTO TAKAMIYA2 , YOSHIAKI NOGUCHI1 , SHINTARO NAKANO1 , YUSAKU KATO1 ,
TAKAYASU SAKURAI3 AND TAKAO SOMEYA1 *
1
Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
VLSI Design and Education Center, The University of Tokyo, Tokyo 153-8505, Japan
3
Center for Collaborative Research, The University of Tokyo, Tokyo 153-8904, Japan
* e-mail: someya@ap.t.u-tokyo.ac.jp
2
Published online: 29 April 2007; doi:10.1038/nmat1903
The electronics fields face serious problems associated with
electric power; these include the development of ecologically
friendly power-generation systems and ultralow-powerconsuming circuits. Moreover, there is a demand for developing
new power-transmission methods in the imminent era of
ambient electronics, in which a multitude of electronic devices
such as sensor networks will be used in our daily life to enhance
security, safety and convenience. We constructed a sheet-type
wireless power-transmission system by using state-of-the-art
printing technologies using advanced electronic functional
inks. This became possible owing to recent progress in organic
semiconductor technologies; the diversity of chemical syntheses
and processes on organic materials has led to a new class
of organic semiconductors, dielectric layers and metals with
excellent electronic functionalities1–5 . The new system directly
drives electronic devices by transmitting power of the order of
tens of watts without connectors, thereby providing an easyto-use and reliable power source. As all of the components are
manufactured on plastic films, it is easy to place the wireless
power-transmission sheet over desks, floors, walls and any other
location imaginable.
Organic semiconductor technology relies on carbon-based
materials, and it is known as one of the promising key technologies
for realizing large-area and low-cost electronics. Owing to recent
intensive studies on organic electronic materials, the mobility
of p-type organic field-effect transistors exceeds 1 cm2 V−1 s−1
(refs 6,7), besides pentacene; moreover, their high performance in
integrated circuits has attracted considerable attention because of
attributes that complement silicon-based devices8 . Furthermore,
owing to recent intensive research efforts with regards to the
development of n-type organic transistors with high mobility
and air stability1–5,9–12 , high-performance organic complementary
metal–oxide–semiconductor circuits and ambipolar transistors
have been reported10–12 . Organic semiconductor devices cannot
attain the high-speed performance exhibited by their silicon
counterparts, but their fabrication cost is considerably lower
and they are better suited for fabrication on large-area flexible
plastic substrates12–14 . They can also be conveniently manufactured
at ambient temperature on plastic films using printing and/or
roll-to-roll processes8,15–19 . Their application in flexible displays19–23
nature materials VOL 6 JUNE 2007 www.nature.com/naturematerials
and printable wireless tags24–26 has been the main motivation
behind recent efforts for the development of organic transistors.
Our group has demonstrated promising applications of largearea flexible sensors and actuators; these comprise an electronic
artificial skin for future generations of robots27,28 , a sheet-type
image scanner29 and a sheet-type Braille display for blind people30 .
However, so far all of the above-mentioned devices have been
limited to low-power applications, where organic transistors
are used as scanning and reading-out circuits. This is because
low mobility of organic transistors causes high resistance; this
situation prevents organic transistors from being applied in highpower applications.
In this work, we have constructed a large-area sheet-type
wireless power-transmission system on a plastic film by combining
state-of-the-art printing and plastic microelectromechanicalsystem (MEMS) technologies. Complementary circuits integrating
plastic MEMS switches and organic transistors allow us to realize
large-area, flexible, high-power applications. When an electronic
object that possesses a receiver coil at the bottom is approaching
or placed on the sheet, the position of the object is contactlessly
sensed by using electromagnetic coupling using a printed organictransistor active matrix. Subsequently, one of the elements of the
two-dimensional array of sender coils is selected by a printed
plastic MEMS switching matrix. In this way, power is selectively
transmitted from one of the coils in the sender coil array to the
receiver coil of the object by electromagnetic induction. As the
device is manufactured using printing technologies, the large-area
system is potentially inexpensive. The effective area of the powertransmission sheet is 21 × 21 cm2 . The thickness and weight of the
entire sheet are 1 mm and 50 g, respectively. Owing to the selective
power transmission, we achieve a coupling efficiency of 81.4% for
the power transmission and receive a power of 40.5 W.
The entire system comprising an 8 × 8 array of cells is
manufactured by integrating the position-sensing and powertransmission sheets, as shown in Fig. 1a. The spacing of the cells
is 1 inch. The manufacturing process is described in detail in
Supplementary Information.
The contactless position-sensing system comprises a positionsensing coil array sheet and an organic-transistor active-matrix
sheet. A cross-sectional diagram and the circuit diagram are
413
LETTERS
a
Organic transistor
Plastic MEMS switch
Position-sensing coil
Sender coil
b
Column selector
Position-sensing coil
Cu
BL 1
BL
Cu (BL)
Output
10 mm
Organic transistor
G
Au (S)
WL 2
Sensing
coil
Transistor
Polyimide
Pentacene
1 mm
vs
Ag
(WL)
Ag
BL 8
WL 8
Pentacene
Polyimide
Ag (G)
S
D
Cu
Agpaste
Ag
Parylene
Au (D)
BL 2
WL 1
Polyimide
Row decoder
Via
Frequency
vs
vs
a.c. source
Control chip
Column selector
c
Sender coil
Cu
BL 1
BL 2
BL 8
Polyimide
Cu
Cu
10 mm
Agpaste
Plastic MEMS switch
BL
Via
WL 2
WL 3
Sender coil
Ag
Polyimide
Ag (to BL)
WL 8
Ag
Polyimide
WL
Row decoder
WL 1
Via
Ag
MEMS switch
Ag (to WL)
Polyimide
5 mm
Parylene
Input
Ag
Power generator
Figure 1 Configuration of the wireless power-transmission system and its components. a, Photograph showing an exploded view of the wireless power-transmission
sheet embedded in the floor and comprising a wireless power-transmission system and contactless position-sensing system. Part of the cover layer is peeling off. The size of
all of the sheets is 21× 21 cm2 (64 power-transmission units and 64 position-sensing units). b, Pictures, cross-sectional diagrams and circuit diagram of the
position-sensing coil and organic transistor. S, D and G represent the source, drain and gate electrodes, respectively. c, Pictures, cross-sectional diagrams and circuit
diagram of the sender coil and plastic MEMS switch. Electrodes for electrostatic attraction are connected to the word line (WL) and bit line (BL) of the MEMS switch.
Electrodes for power transmission are connected to the sender coils and a power generator. WLs and BLs of a position-sensing system and power-transmission system are
connected to row decoders, column selectors and control chips for coordination, for example, for addressing and reading-out to each other.
shown in Fig. 1b. The organic-transistor active-matrix sheet with
pentacene channel layers is fabricated on a polyimide film. The
channel length and width are 13 μm and 48 mm, respectively.
414
The manufactured organic transistors exhibit a mobility of
1 cm2 V−1 s−1 and an on/off ratio of 105 . A position-sensing coil
array sheet is manufactured by screen printing. The inner diameter
nature materials VOL 6 JUNE 2007 www.nature.com/naturematerials
LETTERS
Receiver coil
a
10
b
Position-sensing coil array
No receiver coil
VGS = –60 V
5
Vs (V)
d
0
VS
–5
Detector
–10
2.95 MHz
VGS
10
VGS = –60 V
d=∞
5
Vs (V)
0
0.5
1.0
Organic transistor array
1.5
2.0
Time (μs)
2.5
3.0
7
d
d=∞
6
5
0
d = 1 mm
–5
Vs (V)
c
VGS = 0 V
4
Transistor-on
(VGS = –60 V)
3
2
–10
0
0.5
1.0
1.5
2.0
2.5
3.0
Time (μs)
1
0
0
5
10
15
20
25
30
d (mm)
Figure 2 Diagram of circuit waveforms and characteristics of the contactless position-sensing system. a, Schematic diagram of the position-sensing unit. b, Sense
voltage (VS ) in the unit for VGS = 0 (grey line) and −60 V (black line), where the vertical distance between the position-sensing coil and receiver coil is infinite. VS is less than
10 mV in the off state of the organic transistor even with the application of resonance frequency, fR = 2.95 MHz (grey line). The on/off ratio of the transistors at 2.95 MHz
exceeds 103 . c, VS at the resonance frequency of 2.95 MHz at which the vertical distance between the position-sensing coil and receiver coil is infinite (black line) and 1 mm
(grey line). A voltage VGS = −60 V is applied to the transistor. d, VS as a function of the vertical distance, d, between the position-sensing coil and the receiver coil. As the
latter approaches the former, VS decreases and this decrease amounts to 91% at a distance of 1 mm.
of the copper coils is 10 mm and their outer diameter is 25 mm.
Both the width and spacing of the copper lines are 100 μm. The
number of turns is 38. The inductance and resistance are 20 μH and
17 , respectively.
The wireless power-transmission system comprises a printed
plastic MEMS switch sheet and a sender coil array sheet. A crosssectional diagram and the circuit diagram are shown in Fig. 1c.
The plastic MEMS switch sheet is formed using inkjet printing.
The electrodes for power transmission and those for electrostatic
attraction are patterned on a 25-μm-thick polyimide membrane.
The sender coil array sheet is also manufactured by screen printing,
which comprises copper coils with an inner diameter of 10 mm and
an outer diameter of 25 mm. Both the width and spacing of the
copper lines are 300 μm. The number of turns is 13. The inductance
and resistance are 3 μH and 2 , respectively.
When voltage biases are applied to the electrodes for
electrostatic attraction, the top electrodes for power transmission
are mechanically connected to the power-transmission electrodes
on the bottom sheet owing to the electrostatic attraction,
thereby leading to a very low on-resistance. The electrical
characteristics of the MEMS switch are described in detail in
Supplementary Information.
The characteristics of the contactless position-sensing unit are
investigated (Fig. 2a). First, we examine the on/off ratio of the
unit at the resonant frequency, (fR ), which is 2.95 MHz. Figure 2b
shows the sense voltage, (VS ), in the position-sensing unit for
VGS = 0 and −60 V. VS is less than 10 mV in the off state of the
organic transistor. The on/off ratio of the transistors at 2.95 MHz
exceeds 103 .
Consider the receiver coil approaching the position-sensing
coil. Figure 2c,d shows VS as a function of the vertical distance
between the position-sensing coil and the receiver coil. When
the distance is less than 25 mm, VS starts decreasing and this
nature materials VOL 6 JUNE 2007 www.nature.com/naturematerials
decrease amounts to 91% at a distance of 1 mm. VS decreases
because the resonance frequency deviates from the original value
of 2.95 MHz owing to the changes in the impedance of the
position-sensing unit as the receiver coil approaches. The sensitivity
(reduction in VS ) is determined by the inductance of the positionsensing coil, which increases with the number of turns in the
receiver coils.
Electric power is fed to the receiver coils wirelessly by
electromagnetic induction. The bottom sides of all of the powertransmission electrodes of the plastic MEMS switch are connected
to the a.c. power source operating at a frequency of 13.56 MHz
(Fig. 1c). Once the position of the object is determined, one of
the sender coils on which the object is placed is selected by one of
the MEMS switches and current starts flowing through the sender
coil, thus generating a magnetic field. This magnetic field induces
a current in the receiver coil. To measure the electromagnetic
coupling efficiency and the on/off ratio, the receiver coil is placed
above one of the sender coils at a distance of 100 μm and is made
to share the same centre axis. A power of 2 W is transmitted
from the sender coil and the received power is measured using
a spectrum analyser. The received power is 1.6 W when the
MEMS switch is turned on, whereas it is 0.86 mW when the
MEMS switch is turned off (Fig. 3a). The on/off ratio of the
MEMS switch exceeds 103 , demonstrating its excellent switching
characteristics at high frequencies and for high power transmission.
Figure 3b shows the transmission efficiency and received power
at the receiver coils as a function of the transmitting power. The
received power increases linearly with the transmitting power,
and the maximum transmitted power is as high as 40.5 W. The
transmission efficiency is almost constant (81.4%). A further
increase in the transmitting power results in the disconnection
of the power-transmission electrodes patterned by inkjet printing
using Ag nanoparticles.
415
LETTERS
2.0
a
a
No light emission
1.5
1.6 W
MEMS on
(Vop = 70 V)
Received power (W)
1.0
0.5
5 cm
0
0.0008
0.0006
MEMS off
(Vop = 0 V)
0.86 mW
0.0004
5 mm
0.0002
0
10
b
12
14
16
Frequency (MHz)
18
20
100
b
Efficiency (%)
80
60
81.4%
40
20
Received power (W)
0
40.5 W
40
30
c
20
10
0
0
10
20
30
40
Transmitting power (W)
50
Figure 3 Characteristics of the wireless power-transmission system. a, Electric
power in the receiver coil for MEMS switch operation voltage, (Vop ), values of 70 and
0 V. At the sender coil, the transmitting power and frequency are 2 W and
13.56 MHz, respectively. Measurements are carried out using a spectrum analyser,
which is connected to the receiver coil. The distance between the sender coil and
the receiver coil is 100 μm. b, The transmission efficiency and received power at the
receiver coils as functions of the transmitting power. The dashed line represents the
power at which the MEMS switch is rendered non-functional.
Figure 4a shows power transmission to a Christmas tree
decorated with 21 light-emitting diodes that require a power of 2 W.
The distance between the sender coil and the receiver coil is 5 mm.
Although some existing systems already use wireless power
transmission, it is difficult to transmit high power over large areas.
For example, low power can be fed to integrated circuit cards
over a relatively large area, whereas a fairly high power can be
fed to electric tooth brushes at the exact mount position. In the
present method, the transmission loss is minimized because power
is supplied to the position where the electronic objects are placed.
In fact, the transmission efficiency is less than 0.1% when power is
transmitted from a single sender coil with a size of 300 × 300 mm2
to a receiver coil of size 25 × 25 mm2 . On the other hand, the
transmission efficiency is 7% when all of the 64 sender coils, which
416
Figure 4 Demonstration of power transmission. a, Electric power of 2 W is
transmitted to a Christmas tree decorated with 21 light-emitting diodes. b, Power
transmission to a light-emitting diode in water. c, Power transmission to miniature
electronic objects such as home-care robots, automatic cleaners, mobile phones,
mobile personal computers, wall-hung televisions and ambient illuminations in the
room. Power-transmission sheets to power them are embedded in the wall, floor
and table.
are activated simultaneously, transmit power to a single receiver
coil. These results imply that a position-sensing system that can
detect the position of electronic objects in real time is crucial to
realize high power transmission over a large area.
Note that the operation speed of the present MEMS switch
is very low (∼4 Hz), and therefore it is unsuitable for scanning
purposes. On the other hand, organic transistors are capable of
nature materials VOL 6 JUNE 2007 www.nature.com/naturematerials
LETTERS
fast scanning to sense the position of electronic objects (∼1 kHz).
Thus, high power transmission relies on MEMS switches that
have a low on-resistance of the order of 10 , whereas position
sensing that does not require high-power switches relies on organic
transistors. The present unique sheet-type system can be realized
by the combination of printed plastic MEMS switches and organic
transistors, with the attributes of one complementing those of
the other.
Readers may be apprehensive about electric shock or leakage
problems. Figure 4b demonstrates power transmission to a lightemitting diode in water. Power cables, sockets and plugs are not
required, and all of the metallic parts (including sender coils,
receiver coils and electrodes) are coated with insulating materials.
Therefore, these devices can even be used in water, thereby
providing a new class of electronic devices.
As all of the components are manufactured on plastic films
using printing technologies, the system is thin, lightweight and
mechanically flexible. As shown in Fig. 4c, it is easy to place
the wireless power-transmission sheet on walls, ceilings and any
other location imaginable, opening up new ways to interact with
electronic products. Therefore, this study suggests the first step
towards building an infrastructure for ambient electronics.
Received 21 February 2007; accepted 27 March 2007; published 29 April 2007.
References
1. Katz, H. E. et al. A soluble and air-stable organic semiconductor with high electron mobility. Nature
404, 478–481 (2000).
2. Chua, L. L. et al. General observation of n-type field-effect behaviour in organic semiconductors.
Nature 434, 194–199 (2005).
3. Bao, Z. N. Materials and fabrication needs for low-cost organic transistor circuits. Adv. Mater. 12,
227–230 (2000).
4. Yoon, M. H., Facchetti, A. & Marks, T. J. σ -π molecular dielectric multilayers for low-voltage organic
thin-film transistors. Proc. Natl Acad. Sci. USA 102, 4678–4682 (2005).
5. Kobayashi, S. et al. Control of carrier density by self-assembled monolayers in organic field-effect
transistors. Nature Mater. 3, 317–322 (2004).
6. Briseno, A. L. et al. Patterning organic single-crystal transistor arrays. Nature 444, 913–917 (2006).
7. Payne, M. M., Parkin, S. R., Anthony, J. E., Kuo, C. C. & Jackson, T. N. Organic field-effect transistors
from solution-deposited functionalized acenes with mobilities as high as 1 cm2 /Vs. J. Am. Chem. Soc.
127, 4986–4987 (2005).
8. Loo, Y. L. et al. Soft, conformable electrical contacts for organic semiconductors: High-resolution
plastic circuits by lamination. Proc. Natl Acad. Sci. USA 99, 10252–10256 (2002).
9. Newman, C. R. et al. Introduction to organic thin film transistors and design of n-channel organic
semiconductors. Chem. Mater. 16, 4436–4451 (2004).
nature materials VOL 6 JUNE 2007 www.nature.com/naturematerials
10. Crone, B. et al. Large-scale complementary integrated circuits based on organic transistors. Nature
403, 521–523 (2000).
11. Meijer, E. J. et al. Solution-processed ambipolar organic field-effect transistors and inverters. Nature
Mater. 2, 678–682 (2003).
12. Klauk, H., Zschieschang, U., Pflaum, J. & Halik, M. Ultralow-power organic complementary circuits.
Nature 445, 745–748 (2007).
13. Dimitrakopoulos, C. D. & Malenfant, P. R. L. Organic thin film transistors for large area electronics.
Adv. Mater. 14, 99–117 (2002).
14. Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature
428, 911–918 (2004).
15. Mcculloch, I. Thin films: Rolling out organic electronics. Nature Mater. 4, 583–584 (2005).
16. Sirringhaus, H. et al. High-resolution inkjet printing of all-polymer transistor circuits. Science 290,
2123–2126 (2000).
17. Garnier, F., Hajlaoui, R., Yassar, A. & Srivastava, P. All-polymer field-effect transistor realized by
printing techniques. Science 265, 1684–1686 (1994).
18. Suo, Z., Ma, E. Y., Gleskova, H. & Wagner, S. Mechanics of rollable and foldable film-on-foil
electronics. Appl. Phys. Lett. 74, 1177–1179 (1999).
19. Rogers, J. A. et al. Paper-like electronic displays: Large-area rubber-stamped plastic sheets of
electronics and microencapsulated electrophoretic inks. Proc. Natl Acad. Sci. USA 98,
4835–4840 (2001).
20. Gelinck, G. H. et al. Flexible active-matrix displays and shift registers based on solution-processed
organic transistors. Nature Mater. 3, 106–110 (2004).
21. Andersson, P. et al. Active matrix displays based on all-organic electrochemical smart pixels printed
on paper. Adv. Mater. 14, 1460–1464 (2002).
22. Klauk, H., Gundlach, D. J., Nichols, J. A. & Jackson, T. N. Pentacene organic thin-film transistors for
circuit and display applications. IEEE Trans. Electron Devices 46, 1258–1263 (1999).
23. Zhou, L. S. et al. All-organic active matrix flexible display. Appl. Phys. Lett. 88, 083502 (2006).
24. Baude, P. F. et al. Pentacene-based radio-frequency identification circuitry. Appl. Phys. Lett. 82,
3964–3966 (2003).
25. Rotzoll, R. et al. Radio frequency rectifiers based on organic thin-film transistors. Appl. Phys. Lett. 88,
123502 (2006).
26. Huang, D., Liao, F., Molesa, S., Redinger, D. & Subramanian, V. Plastic-compatible low resistance
printable gold nanoparticle conductors for flexible electronics. J. Electrochem. Soc. 150,
G412–G417 (2003).
27. Someya, T. et al. A large-area, flexible pressure sensor matrix with organic field-effect transistors for
artificial skin applications. Proc. Natl Acad. Sci. USA 101, 9966–9970 (2004).
28. Someya, T. et al. Conformable, flexible, large-area networks of pressure and thermal sensors with
organic transistor active matrixes. Proc. Natl Acad. Sci. USA 102, 12321–12325 (2005).
29. Someya, T. et al. Integration of organic FETs with organic photodiodes for a large area, flexible, and
lightweight sheet image scanners. IEEE Trans. Electron Devices 52, 2502–2511 (2005).
30. Kato, Y. et al. Sheet-type braille displays by integrating organic field-effect transistors and polymeric
actuators. IEEE Trans. Electron Devices 54, 202–209 (2007).
Acknowledgements
This study was partially supported by Special Coordination Funds for Promoting and Technology, the
Ministry of Education, Culture, Sports, Science and Technology and JST/CREST. We also thank
Kyocera Chemical Cooperation for providing high-purity polyimide precursors (KEMITITE CT4112),
Daisankasei for a high-purity parylene (diX-SR) and H. Kawaguchi and K. Hizu for technical support.
Correspondence and requests for materials should be addressed to T. Someya.
Supplementary Information accompanies this paper on www.nature.com/naturematerials.
Competing financial interests
The authors declare no competing financial interests.
Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/
417