lated piezoelectric copolymer-based highly transparent and flexible self-poweredsensors†
Chang Kyu Jeong, ‡ab Dong Yeol Hyeon,‡c Geon-Tae Hwang, d
Gyoung-Ja Lee,e Min-Ku Lee,e Jin-Ju Parke and Kwi-Il Park *c
With the expansion of Internet of Things (IoT) and sensor network systems, transparent and flexible energy
supply devices are becoming more vital for ultra-connected and highly convenient human interfaces. In
particular, the mechanical energy harvesting technology using piezoelectric materials is very attractive
due to the ability of direct energy conversion from wasted mechanical energy to useful electrical energy.
In this work, we demonstrate a highly transparent and flexible piezoelectric energy harvester (f-PEH)
using a metallic nanowire-percolated piezoelectric copolymer on a flexible plastic substrate. The silver
nanowire (Ag NWs)-based conductor has been considered as a powerful future electrode material with
high transparency and flexibility, while poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) is
a representative high-performance piezoelectric polymer material. Based on these two attractive
materials, the proposed transparent f-PEH generated an output voltage, current, and power of �17 V,
�2.5 mA, and �12 mW, respectively, which are a record-high performance compared to previously
reported transparent f-PEHs. Besides material and device characterizations, a multiphysics simulation
was firmly investigated to clarify the properties of the transparent f-PEH devices. Finally, the transparent
f-PEH devices were directly modified and used as a self-powered pressure sensor array (5 � 5), which
well detected the pattern images of the external pressure input without serious cross-talk. This work can
guide the field of transparent and flexible piezoelectric devices to the way to accomplish transparent
self-powered electronics for high-performance applications.
Introduction
With the dramatic increase of the Internet of Things (IoT) inmodern convenient systems, various sensors have been utilizedin our daily life as intimately connected networks;1,2 in addition,lot of sensor devices are required to be exible and wearable forhuman-interfaced applications.3,4 However, conventionalbattery-based systems are still preferred for themyriad of sensordevices in the widely spread network system despite the
frequent discharging issues as well as the mechanical inexi-bility with large sizes.5 Flexible mechanical energy harvestingdevices have drawn much attention because they can generateelectrical energy from external mechanical energy sources inisolated, indoor, and biomechanical environments.6–9 More-over, energy harvesters can be directly used as self-poweredsensors because their harvested signals can be considered asthe sensor signals detecting mechanical input such as bending,pulse, pressure, and touch.10–12
The currently representative energy harvesting principlesinclude both piezoelectricity and triboelectricity.13,14 Althoughtriboelectric devices can produce very high-voltage signals withsimple materials and processes, most of them suffer frommechanical abrasion, environmental humidity, and air gap orsliding-based structural limitations.15 In contrast, piezoelectricenergy harvesters are not signicantly affected by environ-mental factors and can be fabricated to exhibit mechanicalexibility using various methods.16–22 Until now, there havebeen various improvements in the eld of self-powereddevices.23–27 Unlike triboelectric devices, however, piezoelectricdevices with transparent congurations are difficult to makedue to the restricted choice of materials having piezoelectricproperties. Transparent electronics have been used in
augmented reality (AR) devices as well as in touch and imagingsensors for important applications.28 Furthermore, the elds ofinterest in transparent exible devices are being expanded fromdisplay devices to diverse energy applications, such as batteriesand supercapacitors on exible plastic lms for both aestheticalpreference and multifunctional usefulness.28–30
Some transparent exible piezoelectric energy harvesters (f-PEHs) have been developed using ceramic-polymer compos-ites.31,32 However, piezoelectric composites cannot exhibit highpiezoelectric performance because their transparency should beestablished by low-loaded piezoelectric inorganic componentswith large volumes of non-piezoelectric polymers; furthermore,composite-based energy harvesters are generally thick and ndit difficult to form uniform contacts with electrode parts. ZnO-based transparent piezoelectric devices have also shown poorenergy harvesting performance so far.33
Limited material selection for the electrodes of piezoelectricenergy harvesters is another critical obstacle to achieve faciletransparent piezoelectric devices. Indium tin oxide (ITO) isa widely commercialized transparent conducting material, butits potential for exible applications is reduced by the highstiffness, potential mechanical damages, and indium scar-city.28,34 Although the applicability of carbon nanotubes (CNTs)and graphene electrodes for transparent piezoelectric genera-tors has been demonstrated,33,35 they have the disadvantages oflow optical transparency, high sheet resistance, or complicatedtransfer processes.34,36
Silver nanowire (Ag NW) network-based conductors havebeen a dominant research topic because they show hightransparency, excellent conductivity, notable exibility, andeasy fabrication processing.37,38 Moreover, it was already provedthat Ag NW-based conductors are compatible with electrodes ofpiezoelectric energy harvesters through the percolation networkof metallic nanowires.28,39 Although a transparent f-PHE hasbeen previously reported using a single-crystalline piezoelectricceramic thick lm with decent performance,28 single-crystallineceramics show critical problems such as a brittle and fragilenature in exible congurations, complicated processing, andthe high cost.
Herein, we developed a highly exible, large-area, andtransparent piezoelectric energy harvesting device by using anAg NW-percolation structured electrode and a poly(vinylideneuoride-co-triuoroethylene) (P(VDF-TrFE)) piezoelectriccopolymer lm on a exible plastic substrate. P(VDF-TrFE) isa well-known ferroelectric copolymer that exhibits high piezo-electricity, compared to PVDF homopolymer, via the facilepolarization and crystallization of the polar b-phase. On theP(VDF-TrFE)-coated exible plastic substrate, the Ag NW-percolated electrode was established for the transparent ex-ible piezoelectric energy harvester (f-PEH). The conductivity ofthe Ag NW-percolated electrode, the polarization behavior ofNW-percolated P(VDF-TrFE) lm, and the optical transparencyof the overall f-PEH were deeply investigated to guarantee thehigh performance of the copolymer-based transparent f-PEH.An open-circuit voltage and a short-circuit current of �17 Vand �2.5 mA, respectively, were generated by the bendingdeformation of the fabricated f-PEH. Furthermore, this device
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shows the best instantaneous power output (�12 mW) comparedto previously reported exible PVDF-based piezoelectric energyharvesters. Aer the device characterizations and numericalsimulations, we nally realized a transparent energy harvestingsensor array (5 � 5) for a self-powered pressure detectionsystem. Using the transparent self-powered sensor, the applieddynamic pressure was well recognized for literal visualizationwithout external voltage sources. This study may turn into thehighly sought-aer concept in exible, large-area, and fullytransparent piezoelectric devices for high-performance self-powered electronics.
Experimental procedureFabrication steps of transparent f-PEH based on piezoelectricpolymer lm
Poly(vinylidene uoride-co-triuoroethylene) (P(VDF-TrFE))powders (Quintess Co.) with a copolymeric molecular ratio ofPVDF/PTrFE ¼ 70/30 mol% which shows high piezoelectricitywere added to an acetone and dimethylacetamide (DMAc) (v/vratio of 70 : 30) mixture solvent. A 15 wt% P(VDF-TrFE) insolvent was stirred at 30 �C for preparing the piezoelectricpolymer. The resulting solution was spin-coated at 3000 rpmonto a 5 � 4 cm2 sized transparent 100 nm indium tin oxide(ITO)-deposited polyethylene terephthalate (PET) sheet (thick-ness of 127 mm, Sigma-Aldrich Co.) and then was dried at 70 �C.The transparent piezoelectric polymer layer (�20 mm thick) washeated at 145 �C, followed by rapid quenching on a chilledmetal pad for forming the b-phase. Next, to form the trans-parent top electrode, the Ag NW (length of 22 mm and diameterof 27 nm) dispersed solution (NANOPYXIS Co., 0.5 wt%) wasspin-coated on the piezoelectric polymer layer. A fabricatedpiezoelectric polymer lm-based transparent f-PEH waspassivated by depositing a thin poly(methyl methacrylate)(PMMA) layer (MicroChem Co., molecular weight of 950k) witha thickness of �1.7 mm. PMMA was selected as the passivationlayer due to its sufficient stiffness capable of protecting thedevice stably. Note that the PMMA thickness of �1.7 mm issufficient to be highly exible. Aer wiring the copper (Cu) wiresto the top and bottom electrodes with conductive epoxy(Chemtronics Co.), the nal transparent f-PEH was subjected toa poling process with a high electric eld from 200 to 600kV cm�1 for 2 h to improve its energy conversion performance.
Fabrication steps of the piezoelectric polymer lm-basedsensor array
The transparent ITO layer on the PET substrate was patternedby using a 5% HF diluted solution to form the bottom elec-trode lines. P(VDF-TrFE) solution was spin-coated onto thispatterned ITO/PET substrate and heat-treated. Then, to formthe sensor arrays, an Ag NW dispersion was coated to obtaintransparent top electrode lines and patterned perpendicularlyto the ITO bottom electrode lines. Next, 25 cells of the self-powered sensor array (5 � 5) were fabricated on a single ex-ible substrate. Aer the piezoelectric polymer lm-basedtransparent sensor array was passivated with epoxy (PMMA),
Fig. 1 (a) Schematics describing the fabrication steps of the trans-parent f-PEH device. (b) Cross-sectional SEM image and top-viewsurface (inset) of the P(VDF-TrFE) layer. (c) SEM image of the Ag NW-percolated conductor formed on the P(VDF-TrFE) layer as a topelectrode. (d) Digital photograph of the transparent f-PEH device. (e)XRD diffraction pattern of the P(VDF-TrFE) layer. (f) UV-Vis-NIRspectrophotometer results of the transparent f-PEH device accordingto the fabrication steps. (g) P–E hysteresis curve of the transparent f-PEH device.
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the Cu wires were connected to each bottom and top electrodeline. Finally, the poling process was conducted with an electriceld of 600 kV cm�1 for improving the piezoelectricity ofP(VDF-TrFE).
Material characterization
The surface and cross-section of the piezoelectric P(VDF-TrFE) layer on an ITO-coated PET substrate were observedby eld-emission scanning electron microscopy (SEM, Hita-chi, SU8220). An X-ray diffractometer (XRD, Rigaku D/Max-2500) with Cu Ka radiation (l ¼ 1.5406 A) operated at 40 kVand 30 mA was used to investigate the crystal structures of thedeposited piezoelectric polymer lm. The polarization-electric eld hysteresis behavior of the P(VDF-TrFE) layerwas characterized by means of a Precision materials analyzer(Radiant Technologies Inc.) with top electrodes of Au dotsand an electric eld ranging from 3 MV cm�1 to 9 MV cm�1.The transmittances of the laminated structures correspond-ing to each fabrication step were investigated by using a UV-Vis-NIR spectrophotometer (Agilent Technologies Inc.,Cary 5000).
Measurement of converted output signals from bending of f-PEH
A customized bending machine (SnM, Bending MachineSystem), which was precisely controlled by the LabVIEWprogram, was used to periodically deform a transparent f-PEHbased piezoelectric P(VDF-TrFE) polymer with bending radiusfrom 1.168 cm to 3.678 cm and strain rate from 1.42% s�1 to7.1% s�1. The alternate electric signals converted from therepeated bending of transparent f-PEH were detected by anelectrometer (Keithley, Electrometer 6514/E) and recorded inreal-time using a computer.
Evaluation of the P(VDF-TrFE) lm-based transparent sensorarray
A programmable pushing machine (SnM, Pushing MachineSystem) was used to constantly and repeatedly press with anexternal load of about 102 N to the transparent P(VDF-TrFE)lm-based pressure sensor array. The applied load wasmeasured using a load cell (CAS, BCA-10L) and in real-timemonitored using an indicator (CAS, NT-505A). When the loadwas applied to a piezoelectric pressure sensor, a multi-channelmeasurement system (Keithley, 3706A) connected with eachunit measured the electric signals generated from the unitsand simultaneously mapped the color code according to theoutput voltage on the screen. T, K, N, and U-shaped poly-dimethylsiloxane (PDMS) plates were placed on the sensorarray and pushed with the customized pushing machine toconrm the sensing ability of the piezo-polymer based tactilesensor. To prevent the triboelectric effects by the frictionbetween each layer during the evaluation of the transparenttactile sensor, the PDMS plates were well xed onto the sensorarray. Moreover, close working distances between the platesand the pushing arm have been established for minimizingthe gap.
Results and discussionFabrication and performance of the transparent f-PEH device
Fig. 1a shows schematic illustrations presenting the fabricationprocess of the proposed transparent f-PEH device, which is asfollows. P(VDF-TrFE), as the raw material, was dissolved in anacetone and dimethylacetamide (DMAc) mixture solvent. A pre-cleaned ITO-coated polyethylene terephthalate (PET) sheet (5 �4 cm2) was covered with the resulting viscous P(VDF-TrFE)solution by the spin-coating method. To solidify and crystal-lize the P(VDF-TrFE) layer, heat treatment at 145 �C was appliedand subsequently quenched rapidly to x the dense ferroelectricb-phase.40–42 To achieve the appropriate thickness of the P(VDF-TrFE) layer, this procedure was repeated about ve times sincea too thin P(VDF-TrFE) layer would be very easily broken duringthe high-eld poling process and inefficient for voltagegeneration.
As shown by the cross-sectional scanning electron micros-copy (SEM) image in Fig. 1b, the thickness of the P(VDF-TrFE)layer was about 20 mm aer complete solidication. ITO actsas the bottom electrode in the fabricated f-PEH device; it shouldbe mentioned that we did not use the Ag NWs as the bottomelectrode because its percolation network and conductivitywould have been damaged by the subsequent thermal anneal-ing of the P(VDF-TrFE) piezo-polymer layer.43,44 The inset ofFig. 1b shows the top-view SEM image of the P(VDF-TrFE) layer,revealing a dense and pinhole-free surface structure. This can
Fig. 2 (a) Photographs of the transparent f-PEH device under original/releasing (left panel) and bending (right panel) deformations bya customized bending machine. (b and c) Open-circuit voltage (b) andshort-circuit current (c) generated from the bending and releasing ofthe transparent f-PEH. (d and e) Energy harvesting performance of thef-PEH according to the level of poling electric field (d) and the thick-ness of the P(VDF-TrFE) layer (e).
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be ascribed to the annealing conducted near the meltingtemperature of P(VDF-TrFE) and the fast quenching process.40,41
As revisited in Fig. 1a, the top electrode of the f-PEH device wasformed by coating the Ag NW dispersed solution on the P(VDF-TrFE) piezo-polymer layer. The spin-cast Ag NWs were very wellpercolated and entangled on the surface, ensuring the currentpath throughout the whole area (Fig. 1c). The average sheetresistance of the Ag NW-percolated top electrode was about 26.5Usq�1 which is an appropriate value for general electronics.28
Although the distribution of Ag NWs is random in nanoscale, thetotal contact of electrodes is uniform, as shown in the macro-scopic measurement (Table S1†). There is a slight deviation insheet resistance, but all values are within few tens of U sq�1
indicating the sufficiently high conductivity of metal electrodes.To prevent mechanical scratch and chemical oxidation, the topelectrode was covered with a poly(methyl methacrylate) (PMMA)layer, followed by connecting wires with bottom and top elec-trodes, respectively. The device was nally subjected to a polingprocess at few hundreds of kV cm�1 for the alignment of ferro-electric dipoles in P(VDF-TrFE). The fabrication process isdescribed in more detail in the Experimental section.
Fig. 1d shows a photograph of the fabricated transparent f-PEH device based on the Ag NW percolation-coated P(VDF-TrFE) layer. It is visually transparent enough to see the back-ground landscape with the mechanical exibility. The X-raydiffraction (XRD) pattern of the P(VDF-TrFE) lm showsa sharp peak at �19.5� which is reminiscent of the sum ofdiffraction from (110) and (200) planes of ferroelectric b phasecrystallites (Fig. 1e). Fig. 1f presents the quantitative trans-mittance spectra of f-PEH measured by the ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy, according to eachfabrication step of f-PEH; the transmittance of both pristineITO-PEH and the P(VDF-TrFE)-coated substrate was �80% but,aer the top electrode formation, it decreased to �63% due tothe haze effect of the Ag NWs. However, this nal value is nottoo critical for high-performance transparent piezoelectricdevices, as mentioned in Fig. 1d. The transmittance of theoptimized P(VDF-TrFE) thickness (20 mm) is almost same asthose of the thinner P(VDF-TrFE) layers both before and aerthe formation of the Ag NW-based electrode (Fig. S1†). It isbecause this level of microscale thickness difference can beignored for the transparency of P(VDF-TrFE) lms. We alsocharacterized the ferroelectric polarization behavior of theP(VDF-TrFE) layer in this f-PEH device to deduce its piezoelec-tric effect (Fig. 1g). The polarization-electric eld (P–E) curveindicates the well-dened hysteresis loops, conrming the clearferroelectric behavior of P(VDF-TrFE). The slightly contortedpart on the loops, with a severely high maximum sweeping eld(>4 MV cm�1), was presumably caused by the damage of the AgNW-percolated top electrode. This phenomenon naturallyoccurs because high electric elds can become extremely hugeat the nanoscale level (i.e., within each NW), but our deviceperformance was completely impervious to this phenomenonsince exible energy harvesting and sensor devices are notusually operated in a harsh electrical environment.
The energy harvesting performance was evaluated by themechanical deformation of the transparent f-PEH device
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mounted on a bending machine, in the original (releasing) andbending states (Fig. 2a). As shown in Fig. 2b and c, the fabri-cated f-PEH device generated an open-circuit voltage of �17 Vand a short-circuit current of �2.5 mA by virtue of the bending/unbending stimulations (strain of �0.473% at a strain rate of�7.1% s�1); these values are a record-high output performancewith regard to the bending motion, among the various PVDF-based energy harvesters previously reported, even with hightransparency.40,41,45–49 The polarities of the voltage and currentsignals were denitely switched when the forward connectionwas changed to the reverse connection to a measurementinstrument, which claries that the output signals originatefrom the piezoelectric effect of the P(VDF-TrFE) layer. Note thatthe square-shape of the voltage signals with plateaus wasprobably due to the self-charged capacitive effect resulting fromthe typical capacitor structure (metal–insulator–metal sandwichstructure) with the thick ferroelectric layer.50 The relativelyirregular peak shape of current signals is the generalphenomena in the piezoelectric energy harvesting devices dueto the time resolution of the measuring equipment.18,51–53
Fig. 2d presents the voltage and current output signalsgenerated from the transparent f-PEH device aer being pro-cessed under different external poling elds (0–600 kV cm�1);the energy harvesting output was low under the non-polingcondition and dramatically increased with the poling electriceld. Above the maximum poling electric eld, the device waselectrically destroyed by dielectric breakdown and discharge
due to the extremely high eld. The voltage output generationwas also affected by the thickness of the piezoelectric P(VDF-TrFE) layer, as shown in Fig. 2e. As generally expected inpiezoelectric devices, the thicker piezoelectric layer resulted inthe higher voltage generation;50 moreover, the thinner P(VDF-TrFE) layer was easily destroyed by the poling process. Thus,we optimized the thickness of P(VDF-TrFE) to 20 mm (obtainedfor ve spin-coating cycles) since a thicker P(VDF-TrFE) layerwould make the device too stiff to be well exible. Unlike thevoltage output, the current signal was not affected by thethickness of the piezoelectric layer (the distance between theelectrodes).50 Although there are some previous reports fortransparent energy harvesters, they have shown limitedperformance due to some disadvantages such as materialrestriction, complicated processing, inefficient structures,dependence on triboelectrication, etc.54–56 For example, theregular PDMS matrix of piezoelectric composites can be inef-fective mechanically or electrically for energy harvesting appli-cations.22,41,57 In contrast, our device presents much higherpiezoelectric energy harvesting performance than the previousdevices, including the output of mA-level charge ow withsimple and effective fabrication as well as optimum materialsand structures.
Multiphysics simulation of the transparent f-PEH device
To theoretically conrm the effective piezoelectric potential ofthe fabricated transparent f-PEH device, we carried out a niteelement analysis (FEA) simulation using the COMSOL 5.4Multiphysics soware package. Fig. 3a illustrates the modelingof the f-PEH device deformed with a bending radius (Rc) of1.168 cm, to match the experiment. As shown in the le panel ofFig. 3b, we used a simplied simulation model consisting of thetop and bottom electrodes (Ag NWs and ITO, respectively, both100 nm thick) and the 20 mm thick piezoelectric layer (P(VDF-TrFE)). The material parameters for the simulation, such asYoung's modulus, mass density, and dielectric constant, weretaken from the default material browser of the COMSOL 5.4package. The effective strain generated inside the piezoelectric
Fig. 3 (a) Schematic illustration describing the transparent f-PEHdeformed with a bending radius (Rc). (b) FEA simulation model withboundary conditions (left panel) and calculated result (right panel) ofthe transparent f-PEH device under the corresponding mechanicalbending deformation. The produced piezoelectric potential differenceis indicated by color-codes.
polymer layer was derived from Rc and the distance from the topsurface of the device to the mechanical neutral plane. Aer theeffective Young's modulus (�Ei ¼ E/(1�n2)) was calculated byusing the Young's moduli (EPMMA ¼ 3 GPa, EP(VDF-TrFE) ¼ 5 GPa,and EPET ¼ 2.35 GPa) and Poisson's ratios (nPMMA ¼ 0.4, nP(VDF-TrFE) ¼ 0.3, and nPET ¼ 0.4) corresponding to each materialconstituting the actual device, the mechanical neutral planehneutral was obtained from eqn (1),
hneutral ¼
PNi¼1
Ei � ti
Pij¼1
tj � ti
2
!
PNi¼1
Ei � ti
(1)
where the thicknesses t of the various materials are tPMMA ¼ 1.7mm, tP(VDF-TrFE) ¼ 20 mm and tPET ¼ 127 mm, and N is the totalnumber of layers. The distance (d ¼ 55.27 mm) between thehneutral and the middle of the piezoelectric layer was used tocalculate the effective strain (3¼ d/Rc); thus, the strain introducedinside the actual f-PEH is 0.473%. It should be noted that theP(VDF-TrFE) layer is fully subjected to the tensile strain becausethe mechanical neutral plane is located within the PET substrate,completely out of the P(VDF-TrFE) layer. The right panel ofFig. 3b shows the calculated simulation results with the piezo-potential inside the piezoelectric layer indicated by a colorlegend. The fully crystallized piezoelectric layer between top andbottom electrodes could generate a piezopotential of �40 Vunder a bending deformation with Rc of 1.168 cm, correspondingto a lateral displacement of 1 cm from the original 5 cm-longsample. The difference between the experimental and the theo-retical values was presumably due to the inevitable discrepancyin the amount of lamellar crystallization and b-phase formation.
Characterization and stability of the transparent f-PEH device
Fig. 4a presents the load voltage and current output of thetransparent f-PEH device according to the external circuitalresistance from 100 kU to 1 GU. Increasing the external resis-tance resulted in the rise of voltage output whereas the decreaseof current output. From the product of voltage and current ateach load resistance, the instantaneous power was calculated(Fig. 4b), which shows that the matching resistance for thehighest power output (�12 mW) is about 10 MU. This high-output performance is a marvelous record, compared to thepreviously reported PVDF-based exible piezoelectric genera-tors, even with denite transparency.31–33 Moreover, the har-vested signals are comparable to that of the best transparentenergy harvester made with single-crystalline piezoceramics.50
As shown in the top panel of Fig. 4c, the larger strain inducedthe higher current output, probably due to the more abundantpiezoelectric charges generated in the P(VDF-TrFE) layer thatconsequently pushed more electron ows through the circuit.The largest strain applied by the bending machine was 0.473%(Rc of 1.168 cm). Because the Young's modulus of P(VDF-TrFE)is 5 GPa, the applied stresses are regarded as 0.0075 GPa,0.0115 GPa, 0.01535 GPa, 0.0193 GPa, and 0.02365 GPa at eachstrain level, respectively. Hence, the relationship between theapplied stress and the generated piezoelectric signals can be
Fig. 4 (a) Generated voltage, current output and (b) correspondinginstantaneous power by the transparent f-PEH according to externalcircuit load resistance. (c) Current signal performance produced by thetransparent f-PEH device under various bending radii and speeds. (d)Durability test result of the f-PEH device with 12 000 times repeatedbending cycles.
Fig. 5 (a) Six LED bulbs turned on by the generated power from thetransparent f-PEH device. (b) Voltage and current output signalsgenerated by the f-PEH device attached on a moving finger withvarious bending speeds.
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calculated as about 0.1 mA MPa�1. In addition, the fast bendingmotion caused the increment of the current peak, as presentedin the bottom panel of Fig. 4c. At the same strain (bendingradius), the generated piezoelectric charges should be equiva-lent; therefore, the total amount of electrons owing throughthe circuit should also be the same. To maintain the totalamount of electron charge ows, the height of the current peakshould be higher by the shorter time interval (the faster strainrate). Fig. 4d reveals the robust durability of the transparent f-PEH device in the repeated bending deformation of �12 000cycles, without performance degradation, which means theproposed device is very stable for practical energy harvestingand self-powered devices.
We operated six white light-emitting diodes (LEDs) using thepower generated by the transparent f-PEH device to verify itsapplicability for self-powered electronics (Fig. 5a). Since theturn-on voltage of each LED was �2.5 V, the six bulbs in a seriesconnection were well turned-on by the bending and releasingdeformation of the transparent f-PEH. To demonstrate biome-chanical energy harvesting in a wearable conguration, thesmall-sized transparent f-PEH made in a patch design wasattached on a knuckle, as presented in the top panel of Fig. 5b.By bending and releasing the knuckle, the f-PEH device couldeasily generate an output voltage and a current pulse of �8 Vand �0.2 mA, respectively, as shown in the bottom panel ofFig. 5b. In the current signals, the peaks gradually increasedwith the bending speed, as expected from the experimentaloutcomes in Fig. 4c. In contrast, low and moderate bendingspeeds of the nger resulted in similar levels of voltage outputbecause the voltage output is not affected by the speed gener-ally. However, note that fast speed led to a slight increase in thevoltage signal; this behavior is presumably due to the additionof triboelectric charges. Although the recorded output of thebody-attached transparent f-PEH was lower and more irregularthan that of the freestanding f-PEH, this demonstration
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highlights the feasibility of our new transparent energyharvester as a exible energy source in wearable applications.
Transparent f-PEH sensor array for self-powered pressuredetection
To facilitate self-powered mechanical sensors, we can capitalizeupon energy harvesting devices directly because their signalsgenerated by external mechanical stimulation are immediatelymeasurable and useable for detecting the mechanical inputwithout other complex systems. To investigate this potentialapplication for a transparent self-powered pressure sensorusing the P(VDF-TrFE) lm-based f-PEH device, we builta transparent self-powered sensor array made of piezo-copolymer lm sandwiched between the transparent Ag NW-based top electrode lines and the patterned ITO bottom elec-trode lines on a exible plastic substrate.
Fig. 6a shows a schematic illustration of the fabrication stepsfor the transparent f-PEH sensor arrays, which is detailed in theExperimental Section. Fig. 6b–d show photographs for thefollowing sequential steps: the patterned transparent ITO elec-trode lines on a PET exible substrate, the P(VDF-TrFE) lmcovered on the ITO/PET lm, and the patterned transparent AgNW-based electrode lines formed on the P(VDF-TrFE), respec-tively. The red squares in Fig. 6d represent the f-PEH sensorunits in the self-powered pressure sensor array. Fig. 6e showsthe fabricated transparent self-powered pressure sensor arraywith 5� 5 units with the nal size of 6� 6 cm2. The area of eachsensor unit is about 7.5 � 7.5 mm2, and the spatial resolutionbetween the sensor units is 2.5 mm. Because piezoelectric-based pressure sensors can directly convert to electric signalsfrom external pressure, vibration, and bending without externalenergy sources, it is benecial to easily integrate them to self-powered sensor modules to monitor human motions or detectcracks on pipes. As shown in Fig. 6f and g, a T-shaped poly-dimethylsiloxane (PDMS) frame placed onto the transparent f-PEH sensor array was stressed by using a programmablepushing machine under an external load input of �102 N;simultaneously, the produced output signals were measuredusing a multi-channel measurement system (Keithley Co.,Switch/Multimeter 3706A) and recorded in real-time ona computer. Fig. 6g shows the photograph of the f-PEH sensor
Fig. 6 (a) Illustrated fabrication steps of the transparent f-PEH basedself-powered pressure sensor array. Digital photographs showing eachstep: (b) patterning the ITO bottom electrodes for five lines, (c) coatingthe P(VDF-TrFE) piezoelectric polymer, (d) coating the Ag NW-percolated top electrodes with patterning for five lines perpendicularto ITO bottom electrodes and (e) the final transparent f-PEH sensorarray (5 � 5) for self-powered pressure detection. Red square markingin (d) indicates each cell of the f-PEH sensor array. (f) Testing systemfor the f-PEH pressure sensor array with a given letter frame. (g and h)Pressure detection results of the f-PEH pressure sensor array underthe letter frame, T-shaped frame. The right panel of (g) is a mappingimage visualized by the imaging program from the measured realvoltage signals (h) through multiple channel output.
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array with the T-shaped frame and the mapping image in termsof generated voltage modulation. The output voltage signalstransduced from each sensor unit depended on the level ofapplied pressure and are represented by the color code ina mapping image. The graphs of Fig. 6h display the outputvoltage signals simultaneously measured from each unit of thetransparent f-PEH pressure sensor array. Figs. S2a to S2c† showthe mapping images and the measured signals when PDMSframes of various shapes (K, N, and U) were used. The cross-talkphenomena in the mapping images were probably due to thelateral piezoelectric effect by the non-patterned P(VDF-TrFE)piezoelectric layer. From these results, we can conrm thatthe proposed f-PEH devices fabricated by simple and low-costfabrication processes can be used as transparent self-poweredpressure or tactile sensors to distinguish external pressuresand monitor human activities in real-time. We should discusswhy we need piezoelectric devices for self-powered sensor
systems although there is another powerful principle – tribo-electric self-powered sensors. Because the contact and separa-tion between external objects and the sensors may always existin practical applications, the triboelectric device looks like thesimplest conguration for self-powered pressure sensors.However, triboelectric devices are much weaker for mechanicalabrasion, are affected by the humidity in the environment andexternal contamination, and so on.58 Moreover, the ability of thequantied sensor is much more conrmed in the eld ofpiezoelectric devices due to the denite physical and materialsmechanism of piezoelectric behavior. In contrast, triboelectricdevices could suffer from unpredictable electrostaticcharges.59,60 Perhaps, the convergence research between piezo-electric and triboelectric devices can provide an innovativeadvancement toward the future self-powered sensors.
Conclusions
In summary, we have developed a transparent and exiblepiezoelectric energy harvester and self-powered sensor devices,with high-performance and large-area, by using an Ag NW-percolated P(VDF-TrFE) copolymer lm. The piezoelectriccopolymer lm and the Ag NWs were easily assembled ona exible plastic substrate. The generated voltage and currentoutput were �17 V and �2.5 mA by bending deformation,respectively, which is a record-high performance among theexisting similar transparent f-PEHs. The piezoelectric effect wasalso evaluated via a nite element analysis simulation with theCOMSOL Multiphysics soware to investigate theoretically theoutput performance. Finally, the proposed transparent f-PEHwas directly applied as a self-powered pressure sensor array (5� 5), which well detected the external pressure at each sensorunit and mapped successfully the pressure patterns. This studycould be a cornerstone for fully transparent and exible self-powered electronics as well as for sensor systems.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (NRF-2019R1I1A2A01057073) and the Ministry of Science and ICT(NRF-2019R1C1C1002571, NRF-2018R1A4A1022260). This studywas also supported by the Korean Nuclear R&D program orga-nized by the National Research Foundation of Korea (NRF)grant funded by the Korea government (MSIT) (NRF-2017M2A8A4017220).
Notes and references
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