c3ee42311e

Energy &Environmental Science

COMMUNICATION

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National Key Lab of Nano/Micro Fabricatio

100871, China. E-mail: [email protected]

† Electronic supplementary informationadditional videos showing the applicatiotouch sensor based on the STEG. See DOI

Cite this: Energy Environ. Sci., 2013, 6,3235

Received 10th July 2013Accepted 14th August 2013

DOI: 10.1039/c3ee42311e

www.rsc.org/ees

This journal is ª The Royal Society of

A transparent single-friction-surface triboelectricgenerator and self-powered touch sensor†

Bo Meng, Wei Tang, Zhi-han Too, Xiaosheng Zhang, Mengdi Han, Wen Liuand Haixia Zhang*

Broader context

Harvesting mechanical energy from the environment is widely consideredan attractive approach to provide a green energy source for self-poweredsystems. Novel energy harvesting devices based on contact electricationand electrostatic induction have been developed and shown to achievehigh output power. In this work, a single-friction-surface triboelectricgenerator (STEG) has been developed and characterized. The STEG can beproduced using a very simple fabrication process and makes possible theuse of TENGs in an extended range of applications. As the STEG device istransparent and exible, they may prove an attractive power source forexible electronics and portable devices. As a demonstration, a STEG wasapplied as a transparent cover on the screen of a smartphone to generateelectrical energy during normal use of the smartphone touchscreen. A self-powered touch sensor was developed using 4 STEGs as touch pads. Thisself-powered device indicates which pad was touched on an LCD screen,

We present a single-friction-surface triboelectric generator (STEG).

The STEG is transparent and flexible, making possible the use of

triboelectric generators in an extended range of applications. This

device is fabricated in a simple and very low-cost way. When tapped

with a finger, the STEGwith micro-patterned PDMS surface achieved

an open-circuit voltage over 130 Vwith a short-circuit current density

of about 1 mA cm�2. A STEG with a flat PET surface is employed as a

transparent cover on the screen of a smartphone to generate electric

energy from the control motion of the users. The STEG can directly

power 3 LEDswhen the phone screen is tapped during normal use. In

addition, based on the STEG, we have developed a self-powered

visualized touch sensor with 4 STEGs serving as the touch pads. The

STEG shows promise for applications in systems such as self-powered

touch panels and artificial skins.
demonstrating the possibility that an array of STEG devices could poten-tially be used to develop self-powered supersensitive touch panels andarticial skins.
Introduction

Harvesting mechanical energy from the environment is widelyconsidered an attractive approach to provide a green energysource for self-powered systems,1,2 such as wireless sensornetworks, implanted medical devices, and other electronics.Energy harvesters which convert mechanical energy into elec-trical energy based on piezoelectric,3–6 electromagnetic7,8 andelectrostatic9,10 operating principles have been developed.

Contact electrication is a well-known phenomenon thatoccurs all aroundus indaily life. It has been studied for centuriesand applied in various ways.11–14 In recent research, novel energyharvesting devices termed triboelectric nanogenerators (TENGs)have beendeveloped.15–28These devices operate based on contactelectrication and electrostatic induction. TENGs can achievehigh output power density, which has made possible

n Technology, Peking University, Beijing,

u.cn

(ESI) available: The ESI contains 4ns of the STEG and the self-powered: 10.1039/c3ee42311e

Chemistry 2013

applications in wireless systems,18,19 portable electronics,18

biomedical microsystems20 and self-powered nanosensors.27

Previously reported TENGs employed a typical structureconsisting of a pair of friction surfaces and two inductionelectrodes, one for each friction surface. For this design, themost effective way to improve the output power is to signi-cantly separate the two friction surfaces either vertically orlaterally aer they are charged by contact electrication. Outputvoltages of over 1000 V were reported in the recent research byWang's group,21,24 a signicant increase in comparison to theopen-circuit voltage of only 18 V achieved in TENGs using aninseparable structure.16

However, implementing a large vertical or lateral separationrequires additional space, which will limit the applications ofseparating structure TENGs.

In this work, we present a single-friction-surface triboelectricgenerator (STEG). This device incorporates only one singlemicro-structured PDMS or at PET friction surface. When anactive object such as nger, glove, pen, clothes, or similarcontacts the xed friction surface, the surface of the contacting

Energy Environ. Sci., 2013, 6, 3235–3240 | 3235

http://dx.doi.org/10.1039/c3ee42311e

http://pubs.rsc.org/en/journals/journal/EE

http://pubs.rsc.org/en/journals/journal/EE?issueid=EE006011

Energy & Environmental Science Communication

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object serves as the second friction surface in the friction pair.The motion of the contacting object will thus stimulate theSTEG. This STEG can be fabricated on transparent and exiblematerials using a very simple process and is low cost. Thesingle-friction-surface device achieves a signicantly higheroutput power density than a previously reported inseparable-structure transparent TENG.16 Since this TENG is exible andtotally transparent, it could be a promising power source forexible electronic devices and portable electronic products withdisplays such as smartphones and tablets.

Experimental

Fig. 1a shows a schematic diagram of the STEG. The device isfabricated on a 125 mm thick PET substrate. A PDMS lmpatterned with an array of micro-pyramids serves as the frictionsurface. A transparent ITO induction electrode is coated on theback side of the PET substrate. A 100 mm thick copper foil wasemployed as the reference electrode, and was grounded (as analternative, a large-size reference electrode could be used as theequivalent ground). Thus, the location of reference electrode isirrelevant to the operation of the device. As required for differentapplications, the reference electrode can be placed beside(Fig. 1ai) or beneath (Fig. 1aii) the induction electrode. Forcomparison, we also developed a simplied STEG that employs aat PET substrate as the single friction surface. When a nger orother object is brought into contactwith thePDMSor PETsurfaceof the STEG, the surface of this object will serve as an active

Fig. 1 (a) Schematic of the STEG using a micro-structured PDMS friction surface witelectrode. (b) SEM image of the micro-patterned PDMS film. (c) Photographs of theflexibility.

3236 | Energy Environ. Sci., 2013, 6, 3235–3240

friction surface and compose a friction pair with thexed frictionsurface. The effective friction area of the STEG is about 1.5 cm�2.5 cm, which is similar in size to the contact area of a humannger. Fig. 1b shows the SEM image of the micro-patternedPDMS lm. The side length of the micro-pyramids is 10 mm, andthe period is 20 mm. The photographs of the fabricated STEG inFig. 1c illustrate the exibility and transparency of the device.

Preparing of the micro-structured PDMS lm

A 4 inch (100) silicon wafer with an LPCVD SiO2 lm on top waspatterned with an array of square windows by photolithographyand BHF etching. The wafer was then etched in 30% KOHsolution at 80 �C to fabricate the inverted-pyramids array. A thinlayer of chromium was deposited on the Si mold to facilitatepeeling off of the PDMS lm. Mixed PDMS elastomer and crosslinking agent (in the ratio of 10 : 1) was coated on the Si mold.Aer a thermal curing process at 90 �C for 30 minutes thesolidied PDMS lm was bonded to the PET–ITO substrate andthen peeled off from the Si mold.

Results and discussion

The energy harvesting mechanism of the STEG is described inFig. 2. This device works based on the effects of contact elec-trication and electrostatic induction. When an active object(such as a nger, glove or pen) contacts the friction surface(PDMS or PET) by touching, tapping or sliding, the surface ofthe active object and the xed friction surface compose a

h the grounded reference electrode placed (i) beside or (ii) beneath the inductionSTEGs with a PDMS surface and PET surface showing their high transparency and

This journal is ª The Royal Society of Chemistry 2013


Fig. 2 Energy harvesting mechanism of the STEG (a) when the friction surface shows a tendency to attract electrons in contact electrification. (b) The schematic andequivalent circuit of the device when the friction surface is stimulated by a human finger.

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friction pair. Owing to the difference in electron-attractingabilities between the two surfaces, electrons will be transferredfrom the surface that attracts electrons less to the surface thatattracts electrons more, thus making the friction pair electro-statically charged. When the charged active object separatesfrom the friction surface, a potential difference forms betweenthe induction electrode and the grounded reference electrode.Charge will transfer via the external load from one electrode tothe other in order to reach an electrostatic equilibrium state.When the active object contacts the friction surface again, aninverse charge transfer occurs. Fig. 2a shows the conditionwhen the friction surface shows a tendency to attract electronsin contact electrication. Electrons transfer from the referenceelectrode to the induction electrode during contact and moveback to the reference electrode during separation.

Specially, when the friction surface is touched by a humannger, the fact that the human body is a conductor should beconsidered. Here, we employ the simplest human body model(HBM)29 which adopts a series RC circuit (Fig. 2bi). In which, RB

represents the body resistance and CB is the body capacitance.Since the lateral dimension of the friction surface is much

larger than the thickness, within a small range of the gap


between nger and the friction surface, the two contactedsurfaces and the induction electrode can be assumed to beinnitely large planes. Thus, the device is simplied into theequivalent circuit described in Fig. 2bii, in which CG(t) corre-sponds to the capacitance between nger surface and the fric-tion surface, C1 is the corresponding capacitance between thefriction surface and the induction electrode, and RL is the loadresistance. According to Kirchhoff's law, the equivalent tran-sient equation of the circuit is dened by:

Q�QiðtÞCGðtÞ ¼ QiðtÞ

CB

þQiðtÞC1

þ ðRB þ RLÞdQiðtÞdt

(1)

in which, Q is the charge on the friction surface, Qi(t) is thecharge on the induction electrode. Taking the leak of chargefrom the human body into consideration, the transient equa-tion is modied into:

Q�QiðtÞCGðtÞ ¼ QiðtÞ � DQ

CB

þQiðtÞC1

þ ðRB þ RLÞ dQiðtÞdt

(2)

in which DQ is the charge that is leaked. The correspondingsteady-state equation is given by:



Fig. 3 Characterization of the STEG. The open-circuit voltage, short-circuit current and charge transferred in a half cycle of the STEG with the micro-patterned PDMSsurface (a) when tapped with a bare finger and (b) when tapped with a finger covered in a PE glove and the same measurements for the STEG with the flat PET surface(c) when tapped with a bare finger and (d) when tapped with a finger covered in a PE glove.


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Q�Qi

CG

¼ Qi � DQ

CB

þ Qi

C1

(3)

Based on the steady-state equation, we can easily gure outhow Qi varies with the gap between the two contacted surfaces:


Qi ¼ QCBC1 þ DQC1CG

CBC1 þ C1CG þ CBCG

(4)

and the charge that is transferred via the load in a half cycleis:



Fig. 4 A STEG applied as a transparent cover on the screen of a smartphone (a)Photograph of the STEG sized to fit the screen of the phone. (b) A monochromeLCDwas powered and (c) 3 LEDs connected in series were illuminated by the STEGwhen the smartphone touchscreen was operated in the normal way.

Fig. 5 Self-powered visualized touch sensor based on the STEG. (a) The logiccircuit diagram and (b) photograph of the self-powered touch sensor. (c) The LCDdisplays which touch pad was touched when a STEG touch pad is tapped with afinger.

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QT ¼ Qi max�Qi min ¼ Q� DQC1

C1 þ CB

(5)

As a conclusion of the above discussion, with the ngermotion of repeated contacting and separating, charge movesforward and back between the induction electrode and thegrounded reference electrode via the external load. Therefore,the applied mechanical energy is transformed into electricenergy. The leak of charge from the human body would partlyweaken the performance of the device.

The performance of this STEG was characterized by tappinga bare nger and a nger covered in a PE glove on the frictionsurface. The open-circuit voltage was measured by a commer-cial RIGOL oscilloscope (RIGOL DS1102E) with an inputimpedance of 100 MU. To measure the short-circuit current, a10 KU sampling resistor was employed. The charge transferred


in a half cycle was measured by charging a 200 nF capacitorthrough a full wave rectier bridge. In each measurement, thereference electrode was grounded by the probe.

Fig. 3 shows the open-circuit voltage, short-circuit currentand the amount of charge transferred in a half cycle of the STEGwith the micro-patterned PDMS surface when tapped with abare nger (Fig. 3a) and tapped with a nger covered in a PEglove (Fig. 3b) and the same measurements for the STEG withthe at PET surface when tapped with a bare nger (Fig. 3c) andtapped with a nger covered in a PE glove (Fig. 3d). It is clearthat considerable contact electrication occurred under all 4conditions tested. The PDMS surface shows a tendency toattract electrons, while the PET surface shows a tendency todonate electrons. With transferred charge of about 4 nC to 5 nCin a half cycle, the STEG with the micro-patterned PDMS surfaceshows similar performance in charging regardless of whether itis tapped by a bare nger or by the PE glove. Though PDMS andPE exhibit similar abilities in attracting electrons,18 the micro-structure on the PDMS surface signicantly enhances thecontact electrication. When tapped with a bare nger, theSTEG with themicro-patterned PDMS surface achieved an open-circuit voltage of over 130 V with a short-circuit current densityof about 1 mA cm�2. Owing to the big difference between PETand PE in their ability to attract electrons and the rough surfaceof the PE glove, the STEG with the at PET surface can achieve ahigher output than the STEG with a micro-structured PDMSsurface with an increase of about 40% in the amount of trans-ferred charge when tapped with PE glove. In comparison, whentapped by a bare nger, the open-circuit output voltage of theSTEG with the at PET surface was less than 100 V, andthe charge transferred was less than 36% of that transferred inthe case of the STEG using micro-patterned PDMS.

Since the STEG is totally transparent, we employed a STEGusing a at PET friction surface as a transparent cover on thescreen of a smartphone. The reference electrode with a size ofabout 10 cm � 5 cm was placed on the back of the smartphone.The STEG can generate electric energy from the control motionof the users while not disturbing the normal operation of thephone touchscreen. As shown in Fig. 4a, the STEG was sized tot the screen of the phone, with approximate dimension 7 cm�4 cm. When a user operates the smartphone touchscreen in thenormal way (by tapping, sliding, etc.), the STEG can power amonochrome LCD display (Fig. 4b and Video S1 in ESI†) andilluminate 3 LEDs connected in series (Fig. 4c and Video S2 inESI†) via a full wave rectier bridge. In another demonstration,when the screen was tapped with 3 ngers, 50 LEDs connectedin series could be simultaneously illuminated (Video S3 inESI†).

Based on the STEG device, we demonstrated a visualizedtouch sensor with a self-powered display. In this device, 4 STEGsserve as touch pads. These 4 STEGs used at PET for the frictionsurface. Fig. 5 shows the logic circuit diagram (Fig. 5a) and aphotograph (Fig. 5b) of this self-powered touch sensor. Severalordinary diodes (IN4007) were used to create a logic circuit tocontrol the LCD. These components were integrated on a ex-ible printed circuit board. When the STEG touch pad was tap-ped with a nger, the LCD displays which touch pad was




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touched (Fig. 5c and Video S4 in ESI†). As contact electricationoccurs for a large range of contacting materials, the STEG deviceis a promising technology to develop self-powered supersensi-tive touch panels and articial skins for consumer electronicsand robotic applications.

Conclusions

In summary, a novel design of triboelectric generator has beendeveloped and characterized. This STEG incorporates only onefriction surface and can be produced using a simpler fabrica-tion process than that used for traditional TENG designs. TheSTEG makes possible the use of TENGs in an extended range ofapplications. Under excitation by nger tapping, a STEG using amicro-patterned PDMS surface achieved an output voltage ofover 130 V with a short-circuit current density of about 1 mAcm�2. As the STEG devices are transparent and exible, theymay prove an attractive power source for exible electronics andportable devices. As a demonstration, a STEG with a at PETsurface was applied as a transparent cover on the screen of asmartphone to generate electrical energy during normal use ofthe smartphone touchscreen. A self-powered touch sensor wasalso developed using 4 STEGs as touch pads. This self-powereddevice indicates which pad was touched on an LCD screen,demonstrating the possibility that an array of STEG devicespresented here could potentially be used to develop self-pow-ered supersensitive touch panels and articial skins.

Acknowledgements

This work is supported by the National Natural Science Foun-dation of China (Grand no. 91023045 and no. 61176103), 863project (no. 2013AA041102) and Doctoral Program Fund (no.20110001110103).

Notes and references

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