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High-Performance Piezoelectric, Pyroelectric, and Triboelectric Nanogenerators Based on P(VDF-TrFE) with Controlled Crystallinity and Dipole Alignment

Jihye Kim, Jeong Hwan Lee, Hanjun Ryu, Ju-Hyuck Lee, Usman Khan, Han Kim, Sung Soo Kwak, and Sang-Woo Kim*

Poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)), as a ferroelectric polymer, offers great promise for energy harvesting for flexible and wear-able applications. Here, this paper shows that the choice of solvent used to dissolve the polymer significantly influences its properties in terms of energy harvesting. Indeed, the P(VDF-TrFE) prepared using a high dipole moment solvent has higher piezoelectric and pyroelectric coefficients and triboelectric property. Such improvements are the result of higher crystallinity and better dipole alignment of the polymer prepared using a higher dipole moment solvent. Finite element method simulations confirm that the higher dipole moment results in higher piezoelectric, pyroelectric, and triboelectric poten-tial distributions. Furthermore, P(VDF-TrFE)-based piezoelectric, pyroelectric, and triboelectric nanogenerators (NGs) experimentally validate that the higher dipole moment solvent significantly enhances the power output performance of the NGs; the improvement is about 24% and 82% in output voltage and current, respectively, for piezoelectric NG; about 40% and 35% in output voltage and current, respectively, for pyroelectric NG; and about 65% and 75% in output voltage and current for triboelectric NG. In brief, the approach of using a high dipole moment solvent is very promising for high output P(VDF-TrFE)-based wearable NGs.

DOI: 10.1002/adfm.201700702

J. Kim, J. H. Lee, H. Ryu, Dr. J.-H. Lee, Dr. U. Khan, H. Kim, S. S. Kwak, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/adfm.201700702.

they have a limited lifetime. Indeed, the number of required battery replacements could be unbelievably high.[4–6] In order to circumvent this challenge, nanogenerators (NGs), as a kind of energy harvester that harvests the energy from the working envi-ronment of a sensor through piezoelectric, pyroelectric, and triboelectric phenomena, are proposed.[7–15] Very recently, NGs have received enormous attention in order to provide a self-powered operation of sen-sors for WSNs and internet of things (IoT) applications. In addition, flexible NGs carry crucial importance because most of the objects in our daily life are not rigid, but are flexible and bendable, such as textiles.[15,16] The ferroelectric polymer of P(VDF-TrFE) is flexible, and is a strong candidate for realizing flexible NGs using piezoelectric, pyroelectric, and triboelec-tric phenomena.[8,10,13]

Polymers, both ferroelectric and non-ferroelectric, are often utilized as the gate-insulating layers of field effect transistor (FET) devices.[17–20] Recent studies have

shown that the dipole moment of the solvent that is used to dissolve the gate insulating polymers strongly influences the performance of FET devices, in terms of their ON/OFF ratio, charge carrier mobility, etc.[17] A high dipole moment solvent enhances the end-to-end chain length, and results in better chain orientation and therefore better dipole alignment.[17] However, in the context of energy harvesting, and considering the importance of P(VDF-TrFE) for flexible NGs, the solvent effect on the performance of NGs based on the ferroelectric polymer has, to the best of our knowledge, not yet been carried out.

In this paper, we thoroughly investigate the influence of the role of the dipole moment of the solvent on the performance of P(VDF-TrFE)-based NGs. Tetrahydrofuran (THF), methyl ethyl ketone (MEK), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), at 20 °C with dipole moments of 1.75, 2.7, 3.8, and 4.1 D, respectively, were considered as solvents. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were carried out to investigate the crystallinity of the polymer due to the different solvents. Gel permeation chromatography (GPC) measurements were utilized to measure

Energy Harvesting

1. Introduction

Wireless sensor networks (WSNs) in the form of spatially distributed sensors monitor various environmental activities such as chemical, biological, tactile, acoustic, navigational, and thermal data, and communicate the information to a central sta-tion to perform corrective measures. They can therefore greatly benefit human society in terms of healthcare, environmental sensing, and industrial monitoring.[1–3] Powering the enormous number of sensor nodes in a network is a considerable chal-lenge. However, batteries do not offer a viable solution because

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the relative chain length of P(VDF-TrFE). In addition, the dipole alignment of P(VDF-TrFE) with various solvents was measured using piezoelectric force microscopy (PFM). In order to inves-tigate the solvent effect on the performance of the NGs, we measured the piezoelectric and pyroelectric coefficients, and tri-boelectric property in terms of the contact potential difference (CPD) of the P(VDF-TrFE), using PFM and Kelvin probe force microscopy (KPFM) techniques, respectively. Furthermore, in order to study the piezoelectric, pyroelectric, and triboelectric potential distributions of the various solvent-based P(VDF-TrFE)-based NGs, finite element method (FEM) simulations were car-ried out in COMSOL. Finally, P(VDF-TrFE)-based piezoelectric, pyroelectric, and triboelectric NGs were realized, and the perfor-mance enhancements due to the solvents were determined.

2. Results and Discussion

Figure 1a shows a schematic of P(VDF-TrFE) dissolved using a low dipole moment solvent, while Figure 1b describes P(VDF-TrFE) dissolved using a high dipole moment solvent; P(VDF-TrFE) prepared using the high dipole moment shows longer chain length, better orientation of chains, and higher crystallinity.[17,21] In order to investigate the solvent effect, we prepared 20 wt% P(VDF-TrFE) solutions in THF, MEK, DMF, and DMSO solvents, and fabricated P(VDF-TrFE) films; Figure S1 (Supporting Information) shows a schematic of the fabrication process. Figure 1c shows cross-sectional field emis-sion scanning electron microscope (FE-SEM) images of the P(VDF-TrFE) films dissolved in the four solvents; the films had

a thickness of about 8.5 µm. Figure S2 (Supporting Informa-tion) shows the fFurier transform infrared spectroscopy (FT-IR) measurement, which was used to confirm the β phase of P(VDF-TrFE) dissolved in the four solvents.[10]

XRD and DSC measurements were performed to investigate the crystallinity of P(VDF-TrFE) dissolved in the four solvents. Figure 2a shows the results of XRD measurements of P(VDF-TrFE) dissolved in the four solvents that were carried out at a scanning rate of 1° min−1 over an angle range of 15° to 25°. The single peak at a diffraction angle of about 2θ = 19.67° confirms the β phase of the polymer.[10] A smaller value of full width at half maximum (FWHM) of the β phase peak means a higher crystallinity of P(VDF-TrFE).[22] FWHM values of 0.79, 0.78, 0.74, and 0.70, for THF, MEK, DMF, and DMSO, respectively, demonstrate that a higher dipole moment solvent (i.e., DMSO) results in higher crystallinity of the polymer. DSC characteri-zations were carried out in a sealed sample pan at a heating rate of 10 °C min−1 over a temperature range of 50–180 °C, and Figure 2b shows the corresponding results. The percentage crystallinity of P(VDF-TrFE) can be determined by[23]

100%cm

m0

XH

H= ∆

∆

×

(1)

where Xc is the percentage crystallinity of P(VDF-TrFE), ΔHm is the melting enthalpy of the P(VDF-TrFE), and m

0H∆ is the melting enthalpy value of a 100% crystalline P(VDF-TrFE) (i.e., 91.45 mJ mg−1).[24] The ΔHm values of P(VDF-TrFE) dis-solved in THF, MEK, DMF, and DMSO extracted from the DSC heating curves are 16.9, 21.1, 23.0, and 23.4 mJ mg−1, respectively.


Figure 1. a) A schematic of P(VDF-TrFE) dissolved in a low dipole moment solvent (left), and the corresponding P(VDF-TrFE) film (right); and b) a schematic of P(VDF-TrFE) dissolved in a high dipole moment solvent (left), and the corresponding P(VDF-TrFE) film (right); P(VDF-TrFE) prepared using high dipole moment has longer chain length and better chain orientation. c) Cross-sectional FE-SEM images of P(VDF-TrFE) film prepared using the solvents; THF, MEK, DMF, and DMSO.



Therefore, the percentage crystallinity values, determined using Equation (1) of P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO, are 18.48%, 23.07%, 25.15%, and 25.59%, respec-tively, as shown in Table 1. It is evident that DMSO, with the highest dipole moment of 4.1 D (at 20 °C), has resulted in a P(VDF-TrFE) film with the highest crystallinity of 25.59%. In brief, both XRD and DSC results show that a higher dipole moment solvent enhances the crystallinity of P(VDF-TrFE).

The enhanced crystallinity of the P(VDF-TrFE) dissolved in a high dipole moment solvent is mainly due to the increase of the end-to-end chain length of the polymer.[25] Since the rela-tive chain length of P(VDF-TrFE) can be expressed in terms of the weight average molecular weight (Mw), we determined the Mw values by using GPC measurements. Table 2 and Figure S3 (Supporting Information) show that the results of the GPC measurements for P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO are 2.37 × 105, 2.44 × 105, 2.75 × 105, and 2.76 × 105 g mol−1, respectively. Therefore, the GPC results confirm that P(VDF-TrFE) dissolved in higher dipole moment solvents (i.e., DMSO) has a higher Mw of P(VDF-TrFE), and so has longer chain length.

The degree of dipole alignment is of principle importance for a ferroelectric-polymer-based NG. Higher dipole alignment of the polymer can potentially result in higher output from the NG.[13] In order to study the degree of dipole alignment of P(VDF-TrFE) dissolved in the four solvents, PFM measure-ments were performed. First, we confirmed the surface mor-phology and roughness of P(VDF-TrFE) dissolved in the four solvents by using atomic force microscope (AFM). Figure S4 (Supporting Information) and Table 3 show that P(VDF-TrFE) samples dissolved in THF, MEK, DMF, and DMSO have sim-ilar surface morphology and roughness (Rq), which values are 5.88, 5.38, 5.79, and 5.42 nm, respectively. Figure 3 shows the PFM results; the P(VDF-TrFE) film was first poled. The phase mean values of P(VDF-TrFE) dissolved in THF, MEK, DMF,

and DMSO are −99.66°, −103.92°, −105.40°, and −106.64°, respectively. The PFM results demonstrate that the P(VDF-TrFE) dissolved in a higher dipole moment solvent is more neg-atively poled, and so has a higher degree of dipole alignment. Higher dipole alignment is attributed to the enhanced end-to-end chain length, and better chain orientation.

Higher dipole moment solvent results in P(VDF-TrFE) poly mer with higher crystallinity, longer chain length, and higher dipole alignment. In order to investigate the corre-sponding improvement in energy harvesting, we first deter-mined the piezoelectric and pyroelectric coefficients and the triboelectric property of the P(VDF-TrFE) dissolved in the four solvents. Figure 4 shows the results; the P(VDF-TrFE) films were first poled. Figure 4a shows the piezoresponse of the P(VDF-TrFE) and α-quartz as a function of the voltage; the α-quartz’s response serves as a reference. The piezoelectric coefficient is determined from the slope of the piezo response-applied voltage curve.[26] Since the piezoelectric coefficient of d11 for the α-quartz is about 2.3 pm V−1, the relative piezoelec-tric coefficients of d33 for P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO are 2.49, 6.00, 19.58, and 24.54 pm V−1, respectively; Table 4 also lists the results. The results show that the P(VDF-TrFE) polymer dissolved in higher dipole moment solvent has a higher piezoelectric coefficient.

In order to obtain the pyroelectric coefficient, we fabricated the pyroelectric NGs based on the P(VDF-TrFE) dissolved in the four solvents; Figure S5 (Supporting Information) shows the structure of the NG. A resistive heater was used to supply the thermal energy; the probe of thermocouple, which senses the temperature, was located on the pyroelectric NG based on P(VDF-TrFE). The temperature was varied from 23 to 36 °C in a period of 5 s, while the pyroelectric current was simultaneously measured. Figure S6 (Supporting Informa-tion) shows the temperature, temperature variation rate, and pyroelectric current. The pyroelectric coefficient of P(VDF-TrFE) can be calculated by[27–29]


Figure 2. a) XRD measurements of the P(VDF-TrFE) dissolved in the four different dipole moment solvents (THF, MEK, DMF, and DMSO). b) DSC measurements of the P(VDF-TrFE) dissolved in the four solvents.

Table 1. DSC results of P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO).

THF MEK DMF DMSO

ΔHm [mJ mg−1] 16.9 21.1 23.0 23.4

Xc [%] 18.48 23.07 25.15 25.59

Table 2. GPC results of P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO).

THF MEK DMF DMSO

Mw [105 g mol−1] 2.37 2.44 2.75 2.76



/

d /dp

pI A

T t

( )( )

=

(2)

where p is the pyroelectric coefficient, A is the effective elec-trode area, (dT/dt) is the temperature variation rate, and Ip is the pyroelectric current. The effective electrode area of the pyroelectric NG is 1 × 1 cm2. At a temperature of 25.5 °C (i.e., at 0.5 s), the temperature variation rates of P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO are 4.31, 3.97, 3.84, and 3.87 °C s−1, respectively, while the pyroelectric currents of P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO are 13.57, 15.40, 16.33, and 17.02 nA, respectively. The pyroelectric coefficients calculated using Equation (2) of the P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO are 3.14 × 10−5, 3.87 × 10−5, 4.24 × 10−5, and 4.39 × 10−5 C m−2 °C−1, respec-tively. Figure 4b shows the pyroelectric coefficient of P(VDF-TrFE) dissolved in the four solvents; it is evident that a higher dipole moment solvent has a higher pyroelectric coefficient.

In order to investigate the solvent effect on the triboelectric property of the polymer, the CPDs of P(VDF-TrFE) dissolved in the four solvents were measured using KPFM; Figure 4c shows the results.[30] The central area of P(VDF-TrFE) was first poled by using AFM, as shown in Figure S7 (Supporting Infor-mation). Then, the CPDs between the P(VDF-TrFE) and pt tip were measured by KPFM, which values of P(VDF-TrFE) dis-solved in THF, MEK, DMF, and DMSO are −0.87, −1.10, −1.63,

and −2.37 V, respectively. Using the results of the CPDs, we calculated the work function of P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO, which are 5.91, 6.14, 6.67, and 7.41 eV, respectively, as shown in Figure 4d. High dipole moment sol-vent results in P(VDF-TrFE) polymer with higher crystallinity and better dipole alignment, so it makes the functional groups of P(VDF-TrFE) more aligned on the surface. Therefore, the P(VDF-TrFE) polymer dissolved in a higher dipole moment sol-vent has more negative CPD and larger work function, which shows that it has a higher ability to attract electrons during tri-boelectrification, and a higher triboelectric property.[31–33]

In order to study the solvent effect on energy harvesting, we performed FEM simulations, using COMSOL multiphysics, for piezoelectric, pyroelectric, and triboelectric potential distri-butions of the P(VDF-TrFE)-based NGs. Figure 5a shows the results for the piezoelectric potential distribution; the piezoe-lectric coefficients that were already determined (see Figure 4a and corresponding discussion) were utilized in the simulation. Upon upward bending due to a force of 59 N, the peak poten-tials of P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO are 1.00, 1.16, 2.28, and 2.68 V, respectively, which show that a higher dipole moment solvent results in higher piezoelectric potential.[34]

Since for a pyroelectric nanogenerator, the change of surface charge density by applying heat is given by[35]

P p Tσ∆ = ∆ = ∆ (3)

where Δσ is the change of surface charge density, ΔP is the change of polarization, p is the pyroelectric coefficient, and ΔT is the change of temperature. The pyroelectric coefficients of P(VDF-TrFE) dissolved in the four solvents are already deter-mined (see Figure 4b and corresponding discussion). By substituting the pyroelectric coefficients and the change of tem-perature at 0.5 s (ΔT is 2.5 °C, as the temperature is increased from 23 to 25.5 °C) in Equation (3), the change of surface charge density by applying heat can be calculated. Thereafter, we applied the change of surface charge density to the top (+Δσ) and bottom (−Δσ) electrodes of the P(VDF-TrFE)-based pyroelectric NG in COMSOL software. Figure 5b shows the pyroelectric potential distributions of P(VDF-TrFE) dissolved in the four solvents; the values of pyroelectric potential difference between the top and bottom for THF, MEK, DMF, and DMSO are 5.94, 7.32, 8.03, and 8.26 V, respectively, which shows that higher dipole moment solvent results in higher pyroelectric potential.

The triboelectric NG utilized in COMSOL simulation to investigate the triboelectric potential distribution consisted of P(VDF-TrFE) (on an Indium Tin Oxide/Poly(ethylene naph-thalate) (ITO/PEN) substrate) and Al as the friction layers. The surface charge density can be expressed as[36]

0 rV

dσ ε ε=

(4)

where σ is the surface charge density, ε0 is the permittivity of free space, εr is the relative dielectric constant of P(VDF-TrFE), d is the thickness of P(VDF-TrFE), and V is the dif-ference of CPDs between P(VDF-TrFE) and Al. The CPDs of


Table 3. The values of surface roughness (Rq) of P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO), as measured by AFM.

THF MEK DMF DMSO

Rq [nm] 5.88 5.38 5.79 5.42

Figure 3. PFM phase images (after poling) for the analysis of the dipole alignment of P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO).



P(VDF-TrFE) dissolved in four solvents awerealready deter-mined (see Figure 4c and corresponding discussion); the CPD of Al determined using KPFM is 0.909 V, as shown in Figure S8 (Supporting Information). The surface charge densities of P(VDF-TrFE) and Al were first calculated using Equation (4), and were then utilized in COMSOL simula-tions. Figure 5c shows the corresponding results for the tri-boelectric potential distributions of triboelectric NG based on the P(VDF-TrFE) dissolved in the four solvents; the peak values of the triboelectric potential for THF, MEK, DMF, and DMSO are 187.47, 212.29, 267.63, and 345.80 V, respectively. In sum-mary, the COMSOL simulation results in Figure 5 show that P(VDF-TrFE) dissolved in higher dipole moment solvent has higher piezoelectric, pyroelectric, and triboelectric potential distributions.

Since the solvent effect on the piezoelectric, pyroelectric, and triboelectric properties of the P(VDF-TrFE) has become eminent, we realized P(VDF-TrFE)-based piezoelectric, pyro-electric, and triboelectric NGs,in order to study the corre-sponding influence on the output performance of the energy harvesters. Figure S5 (Supporting Information) shows a sche-matic of the design of the piezoelectric, pyroelectric, and tribo-electric NGs; the piezoelectric and pyroelectric NGs have a size of 5 × 2 cm2 with an effective electrode area of 1 × 1 cm2, while the triboelectric NG has an active area of 4 × 4 cm2; P(VDF-TrFE) was first poled. Figure 6a shows the output voltage (top)

and current (bottom) for the piezoelectric NGs based on the four solvents; the piezoelectric NG was bent upward with a curvature of 5 mm, such that the tensile strain was applied for 0.1 s, and the NG was relaxed for 1 s. Peak values of the output voltage and current for THF, MEK, DMF, and DMSO are 12.18 V and 1.48 µA, 12.89 V and 1.55 µA, 13.65 V and 1.97 µA, and 15.70 V and 2.69 µA, respectively. With increases of about 24% in the output voltage and 82% in the output cur-rent, a higher dipole moment solvent significantly enhances the output performance of the P(VDF-TrFE)-based piezoelec-tric NG.

The P(VDF-TrFE)-based pyroelectric NGs harvested the heat that was applied by a resistive heater. Figure 6b shows the output voltage (top) and current (bottom) of the pyroelectric NGs when the temperature was increased from 23 to 36 °C. Peak values of the output voltage and current for THF, MEK, DMF, and DMSO are 253.0 mV and 27.06 nA, 294.6 mV and 31.10 nA, 315.8 mV and 33.59 nA, and 353.2 mV and 36.29 nA, respectively. The increases of about 40% in the output voltage and 35% in the output current show that the higher dipole moment solvent significantly enhances the output performance of the P(VDF-TrFE)-based pyroelectric NG.

The triboelectric NG comprised the P(VDF-TrFE) and Al as the pair of friction materials, and operated in con-tact separation mode. Figure 6c shows the output voltage (top) and current (bottom) of the triboelectric NG when the two friction materials contact and separate with a force of 1 kgf at a frequency of 3 Hz. The peak values of the output voltage and current for THF, MEK, DMF, and DMSO are 206.0 V and 125.9 µA, 234.0 V and 147.8 µA, 251.7 V and 180.5 µA, and 340.4 V and 220.4 µA, respectively. The increases of about 65% in the output voltage and 75% in the output current show that the higher dipole moment solvent


Figure 4. a) Piezoresponses as a function of the electric field for the P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO). b) Pyroelectric coefficients as a function of heating time for the P(VDF-TrFE) dissolved in the four solvents. c) CPDs for P(VDF-TrFE) dissolved in the four solvents; bias was applied to the central area of P(VDF-TrFE) for poling. d) Work functions of P(VDF-TrFE) dissolved in the four solvents.

Table 4. d33 values of P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO), as measured by PFM.

THF MEK DMF DMSO

d33 [pm V−1] 2.49 6.00 19.58 24.54



significantly enhances the output performance of the P(VDF-TrFE)-based triboelectric NG. And also Figure S9 (Supporting Information) shows the long-time output voltage data of the piezoelectric, pyroelectric, and triboelectric NG based on P(VDF-TrFE).

3. Conclusion

We have thoroughly investigated the solvent effect on the per-formance enhancement of P(VDF-TrFE)-based NGs; solvents of THF, MEK, DMF, and DMSO that have dipole moments


Figure 5. COMSOL simulation results for a) the piezoelectric potential distribution, b) pyroelectric potential distribution, and c) triboelectric potential distribution of the P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO).

Figure 6. Output voltage (top) and current (bottom) of the a) piezoelectric NG, b) pyroelectric NG, and c) triboelectric NG, based on P(VDF-TrFE) dissolved in the four solvents (THF, MEK, DMF, and DMSO).


1700702 (7 of 8) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2017, 1700702

of 1.75, 2.7, 3.8, and 4.1 D (at 20 °C), respectively, were uti-lized for dissolving the polymer. Initially, we have shown through XRD, DSC, and GPC measurements that a higher dipole moment solvent increases the crystallinity and relative chain length of P(VDF-TrFE). Thereafter, we found through PFM characterizations that the higher crystallinity and longer relative chain length result in better dipole alignment of the polymer. In order to evaluate the corresponding improve-ment in energy harvesting performance, we first studied the piezoelectric coefficient, pyroelectric coefficient, and the tribo-electric property of the P(VDF-TrFE) dissolved in THF, MEK, DMF, and DMSO; we found that the P(VDF-TrFE) dissolved in a higher dipole moment solvent has higher piezoelectric and pyroelectric coefficients, and offers higher triboelectric property, in terms of more negative CPD and larger work function. Furthermore, FEM simulations in COMSOL have shown that ha igher dipole moment solvent results in higher piezoelectric, pyroelectric, and triboelectric potential distribu-tions. Finally, we realized P(VDF-TrFE)-based piezoelectric, pyroelectric, and triboelectric NGs, and found that a solvent with higher dipole moment significantly enhances the power output performance of the NGs. In brief, the strategy of uti-lizing higher dipole moment solvents offers great promise for realizing high output P(VDF-TrFE)-based wearable NGs for self-powered small electronics such as low -ower consuming wireless sensors and electronics.

4. Experimental SectionFabrication of the P(VDF-TrFE)-Based NGs: Figure S1 (Supporting

Information) shows a schematic of the fabrication process of P(VDF-TrFE) thin film. First, P(VDF-TrFE) solution was prepared by dissolving P(VDF-TrFE) 70/30 vol% copolymer powder (from Piezotech) in four different dipole moment (D) solvents, by stirring for 12 h at room temperature (20 wt%); the solvents were tetrahydrofuran (1.75 D at 20 °C), methyl ethyl ketone (2.7 D at 20 °C), dimethylformamide (3.8 D at 20 °C), and dimethyl sulfoxide (4.1 D at 20 °C). Thereafter, the P(VDF-TrFE) solutions were coated on flexible ITO/PEN substrate using automatic bar coater, followed by drying at 60 °C for 30 min on a hot plate. P(VDF-TrFE) films were then annealed at 140 °C for 2 h in an oven to convert the phase of P(VDF-TrFE) from α to β; the thin films were naturally cooled to prevent damage. For the piezoelectric and pyroelectric NGs, Ag electrode was deposited on the P(VDF-TrFE) surface, and an external electric field (0.6 kV, 10 min) was applied to the P(VDF-TrFE) for the poling. For the triboelectric NG, an external electric field (2 kV, 10 min) was applied to the P(VDF-TrFE) for corona poling; a nylon film was utilized on the surface in order to prevent dielectric breakdown of P(VDF-TrFE). Figure S5 (Supporting Information) shows the structures of piezoelectric, pyroelectric, and triboelectric NGs based on P(VDF-TrFE).

Measurements: For FE-SEM (Jeol Ltd., JSM-6701F), FT-IR (Bruker IFS-66/S, TENSOR27), DSC (SEICO INST., DSC 7020), and GPC (Agilent 1100 S) measurements, the P(VDF-TrFE) film was prepared on glass substrate. For XRD (Bruker D8 DISCOVER) measurements, the P(VDF-TrFE) film was prepared on silicon substrate. For PFM analysis, the P(VDF-TrFE) film was prepared on ITO/glass substrate, and was corona poled by an external electric field (2 kV, 10 min). For KPFM analysis, the P(VDF-TrFE) film of 100 nm thickness was prepared by spin coating 10 wt% P(VDF-TrFE) solution on ITO/glass substrate (4000 rpm, 60 s), followed by drying and annealing processes. During KPFM analysis, tip bias (−10 V) and sample bias (+10 V) were applied to the central area of P(VDF-TrFE), by using the conductive tip of AFM, in order to achieve the poling of P(VDF-TrFE).

PFM studies of piezoresponse were carried out (Park Systems, XE-100) with a Pt/Cr-coated silicon tip and lock-in amplifier (Stanford Research, SR830); a 2 Vac signal with frequency of 17 kHz was applied to the sample by the lock-in amplifier. KPFM measurements were carried out in noncontact mode with a 2 Vac signal at a frequency of 17 kHz. PFM and KPFM measurements were performed under the same measuring condition (temperature = 21 °C, humidity = 17%, and contact force = 30 nN). A bending tester (Z-Tec, ZBT-200) was utilized to apply the bending strain to the piezoelectric NG. A resistive heater (JW-22) was utilized for heating, a thermocouple (type K, Chromel/Alumel) and a multifunctional nanovoltameter (Keithlley 2182A) were utilized for measuring temperature during the characterization of the pyroelectric NG. For the triboelectric nanogenerator, a vibration test system (Labworks Inc, pa-151) was utilized for contact and separation of the friction pair. A digital phosphor oscilloscope (Tektronix, DPO 3052 Digital Phosphor) and low-noise current preamplifier (Stanford Research Systems Inc., SR570) were used to characterize the output voltage and current of piezoelectric, pyroelectric, and triboelectric NGs.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsJ.K. and J.H.L. contributed equally to this work. This work was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CRC-15-05-ETRI).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsdipole alignment, P(VDF-TrFE), piezoelectric, pyroelectric, triboelectric, nanogenerators

Received: February 8, 2017Revised: March 3, 2017

Published online:

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