Flexible Semitransparent Energy Harvester with High Pressure

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Flexible Semitransparent Energy Harvester with High Pressure Sensitivity and Power Density Based on Laterally Aligned PZT SingleCrystal Nanowires Quan-Liang Zhao,*,† Guang-Ping He,*,† Jie-Jian Di,† Wei-Li Song,*,‡ Zhi-Ling Hou,§ Pei-Pei Tan,† Da-Wei Wang,† and Mao-Sheng Cao∥ †

School of Mechanical and Materials Engineering, North China University of Technology, Beijing 100144, PR China Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, PR China § School of Science, Beijing University of Chemical Technology, Beijing 100029, PR China ∥ School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡

S Supporting Information *

ABSTRACT: A flexible semitransparent energy harvester is assembled based on laterally aligned Pb(Zr0.52Ti0.48)O3 (PZT) single-crystal nanowires (NWs). Such a harvester presents the highest open-circuit voltage and a stable area power density of up to 10 V and 0.27 μW/ cm2, respectively. A high pressure sensitivity of 0.14 V/kPa is obtained in the dynamic pressure sensing, much larger than the values reported in other energy harvesters based on piezoelectric single-crystal NWs. Furthermore, theoretical and finite element analyses also confirm that the piezoelectric voltage constant g33 of PZT NWs is competitive to the leadbased bulk single crystals and ceramics, and the enhanced pressure sensitivity and power density are substantially linked to the flexible structure with laterally aligned PZT NWs. The energy harvester in this work holds great potential in flexible and transparent sensing and selfpowered systems. KEYWORDS: flexible electronics, energy harvester, PZT nanowires, piezoelectricity, hydrothermal growth

1. INTRODUCTION Piezoelectric single-crystal nanowires (NWs) are a typical onedimensional (1D) nanostructure that possesses high deformability1 and electromechanical conversion performance because of the size effect.2 In the past few years, a variety of piezoelectric NWs, such as ZnO,3−6 Pb(Zr,Ti)O3 (PZT),7−9 BaTiO3,10,11 and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT),12 have been synthesized and used in the mechanical energy-harvesting devices. Among these materials, lead-based piezoelectric NWs have offered high output voltages because of their large piezoelectric coupling coefficients, enabling them to have dramatic potential applications in the high-performance energy harvesting systems.13 Flexible energy harvesters based on piezoelectric NWs, usually with polymers or metal films as substrates, have attracted growing attention owing to their small elastic modulus, large bending curvature, and facile fabrication of large-scale films. In the conventional sandwich configuration, these energy harvesters consist of a piezoelectric composite layer (piezoelectric NWs and insulating polymer matrices) and two conductive electrodes on both sides, as shown in Figure 1a.14,15 Unfortunately, insulating polymers, such as polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS), are suppressed by the low dielectric constants, and the effective dielectric constants of the composite layers would be small in © XXXX American Chemical Society

comparison with those of piezoelectric materials. The induced charge density in the electrodes can decrease and can lead to a decline in the current density and power density, even if the effect on the output voltage is negligible.16 For avoiding the critical problems, in this work, planar interdigitated Pt/Ti electrodes were designed to assemble a flexible semitransparent PZT NW-based energy harvester, as shown in Figure 1b. With a single-crystal structure, the asprepared PZT NWs are laterally aligned on the electrodes, which is favorable for avoiding the decrease in induced charge density caused by the insulating polymers. Moreover, the gaps among the planar interdigitated Pt/Ti electrodes increase the light transmission and avoid electrical short circuit, which is vulnerable to direct contact between the two electrodes in the conventional sandwich structure upon pressing.17Hence, such a flexible transparency piezoelectric energy harvester (FTPH) can be used in the touch-sensing screen for portable displays and self-powered lighting systems. Received: March 19, 2017 Accepted: July 7, 2017

A

DOI: 10.1021/acsami.7b03929 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

electromagnetic vibration system (JZK-2, Sinocera Piezotronics Inc., China), and the electrical output signals were measured by an oscilloscope (Agilent DSO-X 3024A). The internal capacitance of the FTPH was measured by an LCR meter (Victor 4080, Victor Electronics Co., Ltd, Hongkong).

3. RESULTS AND DISCUSSION The single-crystal PZT NWs were prepared on the Ti foil substrate via the hydrothermal method; Figure 2a shows the

Figure 1. Schematic diagrams of energy harvesters with (a) the conventional sandwich structure and (b) planar interdigitated Pt/Ti electrodes. (c) Fabrication process of the FTPH.

2. EXPERIMENTAL SECTION

Figure 2. (a) SEM image and (b) XRD pattern of the as-prepared PZT NWs. (c) TEM image and electron diffraction of the PZT NW. (d) HRTEM image of the PZT NW.

2.1. Preparation of PZT NWs. The PZT NWs were synthesized on a 100 μm-thick Ti foil substrate by the hydrothermal method. Before the process, the Ti foil was oxidized at 650 °C for 30 min. ZrOCl2·8H2O and C16H36O4Ti were dissolved in deionized water and ethanol, respectively. Then, a white ZrxTi1−xO(OH)2 (ZTOH) precipitate was prepared by adding 0.25 mol/L NH3·H2O into Zr4+ and Ti4+ mixing solutions. After filtering and rinsing several times, the ZTOH precipitate, including 0.1 mol/L Pb(NO3)2, 2 mol/L KOH, 0.4 g/L poly(vinyl alcohol) (PVA), and 7.2 g/L poly(acrylic acid) (PAA), were dispersed into deionized water. The ratio of Pb/Zr/Ti was 1.1:0.52:0.48. The PZT precursor and the Ti foil were placed into a 25 mL Teflon-lined stainless steel autoclave with a fill factor of 0.7, with subsequent heating at 200 °C for 72 h. The final product was peeled off from the Ti foil and washed with ethanol and deionized water. Finally, the sample was dried at 60 °C for 12 h and dispersed in ethanol to form a PZT NW suspension. 2.2. Fabrication of the Energy Harvester. The fabrication process of the PZT NW-based FTPH is shown in Figure 1c. First, 1 cm × 1 cm interdigitated Pt/Ti (100 nm/20 nm) electrodes (20 μm spacing and electrode width) were fabricated on the muscovite mica substrate by lift-off technology. The bottom mica sheets were eliminated by physical delamination using sticky tapes; the mica layers were peeled off piece by piece without any contamination, until the mica substrate was sufficiently flexible18 (Figure S1, Supporting Information). The thickness of the mica substrate was about 20 μm. A uniform PZT NW film was prepared on the mica sheet through the low-speed spin-coating method with a speed rate of 200 rpm for 30 s19−21 and then dried at room temperature for 5 min. After solidifying the conductive silver paste, the mica sheet with Pt/Ti electrodes and PZT NWs was sealed by two pieces of PDMS films (1:10 mixing ratio). The total thickness of the FTPH was about 0.6 mm. 2.3. Characterization. The crystal structures of the PZT NWs were analyzed by X-ray diffraction (XRD) with Cu Kα radiation (Bruker AXS/D8, λ = 0.15405 nm). The morphologies were observed by scanning electron microscopy (SEM, Hitachi-4800) and highresolution transmission electron microscopy (HRTEM, FEI Tecnai G2 20). The piezoelectric strain coefficient d33 of the PZT NWs was measured with a piezoelectric force microscope (PFM, Bruker MultiMode 8). The visible and near-infrared transmission of the FTPH and the PDMS film were measured by a spectrometer (Shimadzu UV-3600). The physical performance was carried out on an

SEM image of the as-prepared PZT NWs. The length of the PZT NWs is on the order of 40−60 microns, with a diameter of 400−600 nm (Figure 2a,c). Note that the length of the nominal NW in this work is much larger than that of the previously reported PZT NW (∼10 μm),7−9,22 and it is more favorable to bridge the interdigitated Pt/Ti electrodes. For a single piezoelectric NW, the electric potential is directly related to the length/diameter aspect ratio of the NW instead of its dimension.23 The length/diameter aspect ratio of the asfabricated PZT NWs is about 100, which is close to that of the previously reported PZT NWs (∼50−100)22 used in the energy harvester. Thus, the size effect of the as-fabricated PZT NWs in this work should be competitive to that of the reported PZT NWs. Figure 2b shows the XRD pattern of the PZT NWs, which demonstrates that the PZT NWs are typical perovskite structures as expected. Transmission electron microscopy (TEM) images exhibit the representative feature of the typical PZT NW, as shown in Figure 2c. According to the observation, the diameter is about 500 nm, and the selected area electron diffraction indicates a single-crystal structure pattern. The measured lattice spacing of the PZT NW shown in Figure 2d is about 0.28 nm, which is close to the distance between (110) planes in the tetragonal perovskite structure of the PZT NW (PDF card no. 33-0784), indicating that the PZT NW preferentially grows along the [001] direction. The implication of the TEM observations suggests good agreement with the results achieved in SEM and XRD characterizations. The effective piezoelectric strain coefficient d33 of the PZT NW measured with a PFM is about 67 pm/V (Figure S2, Supporting Information), which is in the range of the reported d33 values of PZT NWs (50,24 80 pm/V22), much larger than B


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ACS Applied Materials & Interfaces that of ZnO NWs (∼7.5 pm/V25) widely used for energy harvesters. With the employment of PZT NWs along with Pt/Ti electrodes and PDMS films, the FTPH was integrated; Figure 3a shows the photograph of the as-fabricated PZT NW-based

FTPH. The typical PZT NW distribution is shown in Figure 3b. The laterally aligned PZT NW density is about 6200/mm2, which is counted from the SEM images with a total area of 0.72 mm2. Despite the fact that PZT NWs are randomly distributed, the ultralong feature would enable the NWs to serve as bridges for connecting the adjacent electrodes because the NW length is substantially larger than the electrode spacing. According to the transmittance measurement, the transparency of a single PDMS layer is about 95%, whereas that of the FTPH is approximately 55% in visible and near-infrared regions, as shown in Figure 3c. It is known that both the electrode width and the spacing are 20 μm, and thus the total area of opaque Pt/Ti electrodes is about 50% of the entire device area. The semitransparency of the FTPH should be attributed to void spaces from the Pt/Ti electrodes, and PZT NWs almost have no meaningful impact on the transparency. Moreover, the mechanical features of the materials and device structures could endow the FTPH with high flexibility, as exhibited in Figure 3d. The performance of the as-fabricated FTPH was characterized by the dynamic pressures generated from an electromagnetic vibration system, whose moving coil was driven by the input signal voltage. The applied pressures along the z axis with different frequencies were controlled by the input signal frequencies, and the electrical output signals were measured by an oscilloscope, as shown in Figure 4a. The frequency response of the FTPH with a pressure of 40 kPa is shown in Figure 4b. The open-circuit voltage increases with the increase in the elevating pressure frequency from 25 to 50 Hz, and then, no

Figure 3. (a) Photograph of the as-fabricated FTPH. (b) SEM image of the PZT NWs laterally aligned on the electrodes. (c) Visible and near-infrared transmission of the FTPH and the PDMS film. (d) Photograph of the FTPH with large bending.

Figure 4. (a) Schematic diagram of the FTPH measurement. (b) Frequency response of the FTPH with an applied pressure of 40 kPa. (c) Output AC voltage of the FTPH and the energy harvester without PZT NWs. (d) Pressure dependence of the output AC voltage. (e) Linear fit line. C


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where g33 is the piezoelectric voltage constant, EPZT is the modulus of the PZT NW, l is the length of the PZT NW between two adjacent electrodes, ε(l) is the strain function along the axial direction of the PZT NW, E11 and υ are the longitudinal modulus and Poisson’s ratio of the composites, respectively, and σxx, σyy, and σzz are the stresses along the three directions. In this work, only stress σzz is applied on the FTPH, and thus, Voc and g33 could be written as

obvious change was observed up to 90 Hz. Figure 4c shows the detail of the output voltage waves of the FTPH and the energy harvester without any PZT NWs under a pressure of 40 kPa at 90 Hz. The energy harvester without PZT NWs was fabricated using the same method as the FTPH. Both the frequencies of the output voltages agree well with those of the applied pressure during pressing and releasing, and the frequency response below 100 Hz implies that the FTPH could operate in the region of the mechanical vibration frequency of the surrounding environment. Moreover, the open-circuit voltage of the energy harvester without PZT NWs is about 0.4 V, which is smaller than one-sixteenth of that of the FTPH. Thus, it is believed that the open-circuit voltage is generated primarily by the piezoelectric effect of the PZT NWs in the FTPH. The stability in the output voltage over a prolonged period was measured by the cyclic test, and the FTPH could work more than 105 cycles without obvious decrease in the output voltage (Figure S3, Supporting Information). The output AC currents with different load resistances and pressures are shown in Figure S4 of Supporting Information. Figure 4d,e presents the open-circuit voltages with different applied pressures at 90 Hz. The open-circuit voltage increased linearly with increasing pressure, and the maximum output voltage was about 10 V under the pressure of 70 kPa. This value is considerably larger than those of the reported flexible energy harvesters that were assembled with conventional sandwich structures based on PZT,22 PMN-PT,26 ZnO,27 and BaTiO328 single-crystal NWs, as shown in Table 1. In addition, the pressure sensitivity S of

Voc =

type of energy harvesters PZT NW/PDMS composite22 PMN-PT NW/PDMS composite26 vertically aligned ZnO NW27 BaTiO3 NW/PDMS composite28 this work a

pressure sensitivity (mV/kPa)

power density (μW/cm2) 0.04a

7 7.8 0.7 7

g33 =

10

140

55%

where the subscripts denote the corresponding PZT NWs, mica, and PDMS layers and V is the cross-sectional area ratio that is approximately equal to the ratio of the thickness of the corresponding layer to the total thickness of the composite (Table S1, Supporting Information). Figure 5b shows the strains of the PZT NW between the two adjacent electrodes calculated by eqs 3−45 and the finite element method (FEM) under different applied pressures. The inset in Figure 5b is the strain contour image of the PZT NW under a pressure of 40 kPa, analyzed by the FEM (Figures S5−S8, Supporting Information). Note that the maximum and minimum strains are located at both the ends of the PZT NW, which is close to the area in contact with the Pt electrodes. However, the strains of the PZT NW uniformly distribute between the two adjacent Pt electrodes, and the average strains along the axial direction of the PZT NW are around the central values of the maximum and minimum strains. Compared with the theoretical results, the slightly smaller central values in the FEM may be caused by the effect of planar Pt electrodes that is not considered in the theoretical model. On the basis of the output voltages in Figure 4e, the values of g33 can be calculated by the strains shown in Figure 5b. Figure 5c shows that both the g33 values estimated by the theoretical and FEM models are almost consistent with different applied pressures, and the values are about 0.022 and 0.0317 Vm/N at 70 kPa, respectively. They are as high as those of lead-based films and bulk ceramics,36,37,40,41 and the FEM result is even competitive to those of lead-based bulk single crystals;38 however, it is much smaller than those of PZT nanofibers31−33 and P(VDF-TrFE) nanotubes,34 as shown in Figure 5d. For a piezoelectric material, g33 can also be expressed as42

=

∫0

g33 ·E PZT ·ε(l) dl ⎞ ⎛σ σyy σ g33 ·E PZT⎜ xx − ·υ − zz ·υ⎟ dl E11 E11 ⎠ ⎝ E11

(3)

(5)

l

l

)

υ = υPZTVPZT + υmicaVmica + υPDMS(1 − VPZT − Vmica)

the FTPH is calculated to be about 0.14 V/kPa, which is competitive to that of the flexible ferroelectret field-effect transistor (0.1 V/kPa29), much greater than that of the vertically aligned ZnO NW-based flexible energy harvester27 and the P(VDF-TrFE)/BaTiO3-based flexible electronic skin (2.6 mV/kPa30). In the theoretical model of the laterally aligned PZT NWbased FTPH, all PZT NWs, mica, and PDMS layers could be assumed to be elastic and unidirectional continuous fiber laminas, as shown in Figure 5a. The electric potential, that is, the output open-circuit voltage Voc, generated by the applied stress is given by31

∫0

11

and

The values are calculated from the corresponding references.

Voc =

σ

(4)

no

0.27

(

E PZT − Ezz ·υ ·l

E11 = E PZTVPZT + EmicaVmica + E PDMS(1 − VPZT − Vmica)

no

no

Voc

According to the mechanics theory of composite materials, E11 and υ could be estimated by

transparency

0.1a

⎞ ⎛ σ ⎞ ⎛ σ g33 ·E PZT⎜ − zz ·υ⎟ dl = g33 ·E PZT⎜ − zz ·υ⎟ ·l ⎝ E11 ⎠ ⎝ E11 ⎠

and

no 0.019a

l

(2)

Table 1. Comparison of Various Types of Flexible Energy Harvesters Based on Piezoelectric Single-Crystal NWs opencircuit voltage (V)

∫0

g33 =

d33 ε0K

(6)

where ε0 is the permittivity of free space and K is the relative dielectric constant of the piezoelectric material. It is suggested

(1) D


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Figure 5. (a) Schematic diagram of the FTPH theoretical model. (b) Strains and (c) g33 of the laterally aligned PZT NW between two adjacent electrodes under different applied pressures. The blue central line is the average values of the maximum and minimum analyzed by the FEM. The strains of PDMS and mica layers are hidden in the contour image of the inset. (d) Histogram of g33 for the as-prepared PZT NW, PZT nanofiber,31−33 P(VDF-TrFE) nanotube,34 (K0.5Na0.5)NbO3 (KNN),35 PZT,36 and Pb(Zr,Ti)O3-Pb(Mn1/3Nb2/3)O3 (PZT-PMnN)37 films, PMNPT bulk single crystals,38 BaTiO3-BiFeO3 (BT-BFO),39 PZT-5A,40 and Sm, Mn-modified PbTiO3 (SM-PT)41 bulk ceramics. The gray area is estimated by the FEM. For the PZT nanofiber, the red and yellow areas are measured by the nanomanipulator and dynamic mechanical analyzer (DMA), respectively.

Figure 6. (a) Output DC voltage and power characteristics of the FTPH with different load resistances. The rectifying bridge circuit in the schematic diagram consists of four small-signal Schottky diodes (1N5711) with about 0.2 V forward voltage drop and a 470 nF storage capacitor Ce. (b) Equivalent circuit of FTPH. (c) Pressure dependence of the output DC voltage and power. (d) LCD powered by striking the FTPH with the handle of a gripper.

that g33 increases with decreasing dielectric constant. According to the above results, the relative dielectric constant K of the PZT NW used in this work could be estimated to be about 344, which is much larger than that of the P(VDF-TrFE) nanotube even with low d33 (d33 = 35 pm/V, K = 7.7).34 For the reported polycrystalline PZT nanofiber,31−33 it is prepared by the

electrospinning process, and there should be some remnant organics and pores with low dielectric constants in the nanofiber because of the solvent evaporation during heat treatment. The effective dielectric constant of the PZT nanofiber will be smaller than that of the dense single-crystal PZT NW. Thus, the low effective dielectric constants introduce E


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EPZT is the Young’s modulus of the PZT NW, and A is the effective contact area. When the applied pressure frequency increases, the strain rate dε/dt of the PZT NWs increases, and Voc is also enlarged. However, PDMS has been demonstrated as a typical viscoelastic material by the dynamic mechanical analysis test, and it behaves as a stiff or glassy solid under high frequency loadings. Its elastic modulus increases as the applied strain rate increases.46 As a result, the strain of the PZT NWs decreases under high frequency pressures and compensates for the increase in dε/dt of the PZT NWs. Therefore, Voc will not increase obviously under the relative high frequency pressures, as shown in Figure 4a. The asymmetric amplitudes of the open-circuit voltage in Figure 4a could be caused by the different Young’s modulus of each layer in the FTPH. The composite film of the FTPH mainly consists of PZT NW, mica, and PDMS layers, and the Young’s modulus of the PZT NW is the largest, as shown in Table S1 of Supporting Information. When pressure is applied, the PZT NWs would be stretched along with the strain of mica and PDMS layers. As the pressure is released, all of the layers would be retracted by the elastic recovery stress. However, the recovery strain rate of PZT NWs is the fastest because of the large Young’s modulus and will be reduced by the slow recovery strain rate of mica and PDMS layers. According to eq 10, the open-circuit voltage of PZT NWs is proportional to the strain rate, and thus, the lower recovery strain rate of PZT NWs leads to a smaller open-circuit voltage than that of PZT NWs during pressing. With increasing pressure, the output DC voltage appears to increase linearly while the DC power increases quadratically, as shown in Figure 6c. The maximum values of about 1.60 V and 0.27 μW/cm2 are achieved at 70 kPa. This area power density is much larger than those of the previously reported PZT and BaTiO3 NW-based flexible energy harvesters that were assembled with conventional sandwich structures, as shown in Table 1. The reason should be mainly attributed to the larger capacitance and lower resistance of such FTPH configurations, in comparison with those of conventional sandwich structures with the extra capacitance CI and resistance RI of the insulating polymer (Figure 6b). The capacitance CI in series with Ci decreases the total capacitance of the conventional harvester and leads to a smaller current density and power density even if the open-circuit voltage equals to that of the FTPH. In a representative demonstration, the FTPH can power on a monochrome liquid crystal display (LCD), as shown in Figure 6d (Video S1).

high values of g33 in the PZT nanofiber and in the P(VDFTrFE) nanotube. However, the open-circuit voltage of the harvester based on the PZT nanofiber is only about 1.6 V when the strains of the PZT nanofiber and harvester are 7.5 × 10−5% and 13%, respectively.32 In this work, the open-circuit voltage of the FTPH is about 5.6 V under a pressure of 40 kPa, which corresponds to strains of about 1.6 × 10−4 and 5% of the PZT NW and the FTPH, respectively, as shown in Figures 5b and S5 of Supporting Information. It is suggested that the strain of the PZT NW is much larger than that of the PZT nanofiber even if the strain of the FTPH is lower. Therefore, the higher pressure sensitivity of the FTPH should be attributed to the flexible matrices, which have high transfer efficiency of the strain from the matrices to the PZT NWs. Additionally, the load characteristics of the DC electrical outputs are given in Figure 6a, and the inset is the schematic diagram of the DC measurement circuit. It indicates that the output DC voltage increases with the increasing load resistance RL, and there is a peak in the DC power located at the load resistance of 9 MΩ. For the FTPH, the DC power PL can be written as 2 ⎛ Voc ′ ⎞ PL = ⎜ ⎟ RL = ⎝ R i + RL ⎠

′2 Voc

(

Ri RL

2

+

RL

)

′2 Voc

=

(

RL −

Ri RL

2

)

+ 4R i

(7)

where Voc′ and Ri are the output open-circuit voltage of the DC measurement circuit and internal resistance of the FTPH, respectively. It indicates that PL approaches the maximum value when RL is equal to Ri. Therefore, the internal resistance Ri of the FTPH is about 9 MΩ, which could be used to explain the results in Figure 4a through analyzing the equivalent circuit model. The equivalent circuit of the FTPH, as shown in Figure 6b, consists of a charge source q, an internal capacitance Ci, and an internal resistance Ri.43 The total impedance Zi and the resonant frequency f 0 can be written as |Z i | =

f0 =

Ri 1 + (2πfR iC i)2 1 2πR iC i

(8)

(9)

4. CONCLUSIONS In summary, a flexible and semitransparent PZT NW-based FTPH was presented based on planar interdigitated Pt/Ti electrodes. Owing to the unique material and device design, the flexible FTPH held a transparency of around 55% in visible and near-infrared light regions. The maximum open-circuit voltage and stable output power density were measured to be about 10 V and 0.27 μW/cm2, respectively, coupled with good linearity and sensitivity of up to about 0.14 V/kPa. Compared with the previously reported piezoelectric NW-based energy harvesters with conventional sandwich structures, the FTPH presented remarkable advantages in electrical output efficiency, pressure sensitivity, and optical transparency. Therefore, the results promise a novel prototype of flexible and transparent devices that are able to serve for sensing and self-powered systems.

where f is the electrical frequency in the circuit. Ri is about 9 MΩ based on the results in Figure 6a, and Ci is about 2pF@100 Hz measured by an LCR meter. Then, f 0 can be calculated to be about 17.6 kHz. Because the electric frequency f (25−90 Hz) generated by the FTPH shown in Figure 4a is much lower than 17.6 kHz, the total impedance Zi ≈ Ri = 9 MΩ. Therefore, the effect of Ci on the frequency response of the open-circuit voltage is not pronounced in this work. On the basis of our experiment, the output open-circuit voltage Voc of the FTPH could be written as44,45 Voc = IscR i =

dQ dε Ri = E PZTAd33R i dt dt

(10)

where Isc is the short-circuit current, Q is the magnitude of the generated charge, t is the time, ε is the strain of the PZT NW, F


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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03929. Photographs of the planar Pt/Ti interdigitated electrodes on a flexible mica substrate; displacement of a PZT NW measured by a piezoelectric force microscope; stability in the output performance of the FTPH; output AC currents with different load resistances and pressures; properties of the materials in the FTPH for theoretical calculation; strain of the FTPH calculated by the FEM; electric potential of the FTPH calculated by the FEM; effect of the PZT NW number on the open circuit voltage, calculated by the FEM; and effect of the aligned direction of the PZT NW on the open-circuit voltage, calculated by the FEM (PDF) FTPH powering on an LCD (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.-L.Z.). *E-mail: [email protected] (G.-P.H.). *E-mail: [email protected] (W.-L.S.). ORCID

Quan-Liang Zhao: 0000-0001-7295-2594 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 51305005, 51375016, and 51402005) and the Beijing Natural Science Foundation (grant nos. 3172009 and L160001).

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REFERENCES

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Flexible Semitransparent Energy Harvester with High Pressure

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