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Sensors and Materials, Vol. 32, No. 7 (2020) 2503–2515
MYU Tokyo
S & M 2278

Piezoelectric Polymer Multilayer Coating Method
for Vibration Energy Harvester
Nobuaki Kuriyama,1 Takashi Nakajima,2,3 Ryo Ichige,1 and Takaaki Suzuki1*
1

2

Gunma University, 1-5-1 Tenjin cho, Kiryu, Gunma 376-8515, JapanTokyo University of Science, 6-3-1 Niijuku, Katsushika, Tokyo 125-8585, Japan
3
JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan(Received March 19, 2020; accepted May 7, 2020)

Keywords: piezoelectric, polymer, multilayer coating, P(VDF/TrFE), vibration energy harvester, MEMS

In this research, we propose a piezoelectric polymer multilayer coating method by solution
casting and spin coating to improve the fabrication yield and piezoelectricity of the vibrationenergy harvester (VEH). The proposed multilayer coating method improves the smoothness ofthe surface of the piezoelectric layer and the device fabrication yield. To confirm the validityof the proposed coating method, we fabricated and compared four types of VEHs with different
numbers of piezoelectric layers coated on a Cu/polyimide film. As a result of the evaluation,
the piezoelectric characteristics and the fabrication yield of the coated film were found to be
improved with increasing number of coating layers. In addition, because of the decrease in thefilm thickness per coating, the amount of output power generated by the fabricated piezoelectric
VEH (PVEH) was increased.

1.

Introduction

In recent years, energy harvesters have been attracting attention as sensor node power
supplies for the Internet of Things (IoT) society.(1) An energy harvester is a power-generation
device that captures a small amount of power from the surrounding environment and is used
in a self-sustained sensor node in combination with a sensor. The electric power generation
methods include that using heat,(2) light,(3) electromagnetism,(4) and vibration.(5,6) In particular,
the vibration energy harvester (VEH) is being actively researched because vibration is
ubiquitous in the environment and has relatively high energy density.(7) There are several typesof VEH, i.e., electrostatic,(8) electromagnetic,(9) friction,(10) ionic liquid,(11) and piezoelectric,(12)in the micro-electromechanical systems (MEMS) field. The electrostatic type uses thephenomenon that when a charged object approaches a conductor, a charge of the opposite
polarity is generated. The electromagnetic type obtains current from a change in the magnetic
flux of a coil caused by a magnet. These types have high flexibility in material selection anddevice design but require complex microstructures combined with moving and fixed parts. On
the other hand, piezoelectric power generation has a simple device structure because electric*Corresponding author: e-mail: suzuki.taka@gunma-u.ac.jp
https://doi.org/10.18494/SAM.2020.2875
ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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power is generated by strain inside a single material. In addition, the VEH can achieve bothdevice miniaturization and low-frequency vibration response through the use of a highly
flexible polymer piezoelectric material.(13) In particular, methods to reduce the overall rigidityof the VEH have been proposed by combining a flexible substrate with an inorganic thinpiezoelectric film(14) or by using polymer materials for both the substrate and the piezoelectricelement.(15)
For a piezoelectric polymer VEH, there are two methods of forming a piezoelectric layerusing a commercially available piezoelectric film and a solution casting method.(16–18) In thecase of using a film, a device is manufactured by attaching a uniaxially stretched piezoelectric
film, which has high piezoelectric performance due to stretching to an elastic layer.(19)However, since a prefabricated film is used, the flexibility of the thickness of the piezoelectric
layer is limited in the process. In addition, since the direction of the strain with a high power
generation is limited, the characteristics cannot be utilized in a device having a complicatedstrain-direction distribution. On the other hand, spin coating using a solution in which a
piezoelectric material was dissolved with organic solvent in the solution casting method isuseful for forming a piezoelectric layer of a device constructed by photolithography. However,
the yield of device fabrication is reduced by pinholes that impair insulation and prevent power
generation. Furthermore, since the piezoelectricity of the spin-coated layer is inferior to that of
the uniaxially stretched film, a sufficient amount of power generation may not be attained.
In this study, we propose a piezoelectric polymer multilayer coating method using solution
casting and spin coating to form a piezoelectric polymer VEH. To verify the usefulness of the
proposed method, we evaluate the fabrication yield and morphology by atomic force microscopy(AFM) and crystallinity by X-ray diffraction (XRD), and power generation by increasing thenumber of deposited layers.

2.

Materials and Methods

2.1

Fundamental concept of piezoelectric polymer multilayer coating

The problem with the piezoelectric polymer poly(vinylidene fluoride/trifluoroethylene)[P(VDF/TrFE)] formed by the solution casting method is that pinholes are generated in the film.
They are caused by particles on the surface of the substrate on which the film is formed andundissolved powder remaining in the solution for dropping, for example. As a result, the top
and bottom electrodes for charge acquisition are in contact with each other, and electric power
is not generated owing to impaired insulation, as shown in Fig. 1(a). In this study, as shown in
Fig. 1(b), to form the piezoelectric layer with insulation, the single coating of film is thinned
to reduce the surface roughness, and a multilayer coating is applied to cover and smooth theunevenness. Furthermore, if the thickness of a single layer of P(VDF/TrFE) is more than acertain thickness, the molecular chains stand in the direction perpendicular to the substrate, andthe piezoelectricity decreases owing to the inability to invert the polarization.(20,21) Therefore,
the piezoelectric performance is improved by the crystallinity of each piezoelectric layer upon
reducing the thickness of each piezoelectric layer.

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Fig. 1. (Color online) Principle of piezoelectric polymer multicoating. (a) Thick single layer coating. (b) Thin
multilayer coating.

2.2

Design and fabrication of piezoelectric VEH (PVEH)

As shown in Fig. 2, the evaluation device was composed of an elastic layer, a piezoelectric
layer, two electrodes, and a proof mass on a typical unimorph cantilever-type(22,23) PVEH.
Electrodes for obtaining charge were deposited on the top and bottom of the piezoelectric
layer. The device size was set to 10 × 20 mm2 as the representative dimension similar tothat of a button battery. A tungsten proof mass of 0.2 g was installed on the free end of thecantilever, and the other end was the fixed end attached to the vibration source. When thedevice is deformed up and down by the acceleration of proof mass and strain is generated in
the piezoelectric layer, the spontaneous polarization inside the piezoelectric material changes
and charges are generated on the material surface. Since the main purpose of this study was toevaluate the performance of the multilayer-coated piezoelectric film, the device geometry was
not optimized to improve the power generation.
As shown in Fig. 3, to simplify the fabrication process of a large number of devices, a
Cu/PI film (Sumitomo Metal Mining Co., Ltd.: S’PERFLEX) serving as an elastic layer witha thickness of 35 μm and a bottom electrode with a thickness of 8 μm were used for device
fabrication. The surface roughness of the Cu/PI film is Rq = 59 nm, which is sufficiently
small compared with the total thickness of the coated piezoelectric layer, so the effect on
the surface roughness of the Cu/PI film is considered to be small for the piezoelectric layer.
The film was cut to a size of 10 × 30 mm2, and a piezoelectric layer and top electrode were
deposited on the Cu/PI film to fabricate the device. The Cu/PI film was attached and fixed topolydimethylsiloxane (PDMS) (Dow Corning Toray Co., Ltd.: SILPOT 184) formed on a glass
substrate (Matsunami Glass Ind., Ltd.: Micro Cover Glass 30 × 40 mm2 Thickness No. 5).
Since the organic material is damaged in the plasma process,(24) the top electrode was coated by
vacuum heating vapor deposition (ULVAC KIKO, Inc.: VPC-1100).

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Fig. 2. (Color online) Schematic of fabrication process of solidified ionic liquid. The piezoelectric layer is theflexible organic material P(VDF/TrFE) and the elastic layer is made of polyimide.

Fig. 3.

(Color online) Process flow for PVEH formation. All processes were performed on a single glass substrate.

Table 1 shows the piezoelectric layer coating conditions. The total thickness of thepiezoelectric layer was kept constant at 5 μm to evaluate the effect of the film thickness per
layer and the total number of layers on the piezoelectric performance. The solution used forcoating the piezoelectric layer was prepared by dissolving P(VDF/TrFE) powder (75/25 mol%)(Kureha Corp.: KF Polymer W#2000) in diethyl carbonate (DEC) (Wako Pure Chemical Corp.:
Diethyl Carbonate Wako Special Grade). Before preparing the solution, the powder was driedat 130 °C for 6 h. The powder and the solvent were mixed in a nitrogen-purged glove box toprevent impurities and moisture. In the multilayer coating method, each layer was baked at
130 °C for 90 s for drying and crystallization. After coating the piezoelectric layer and the top
electrode, the piezoelectric layer protruding from the Cu/PI film was cut off with a cutter andpeeled from the glass substrate.
We applied a voltage to the formed piezoelectric layer using a high-voltage DC power supply(Matsusada Precision Inc.: HJPQ-10P3) in polarization processing. The polarization treatment
was carried out in vacuum to prevent discharge to the atmosphere and the effect of the external
environment.

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Table 1
Deposition conditions of piezoelectric layers.
Device type
A
Piezoelectric layer thickness (μm)
5Number of layers
1
Solution concentration (wt%)
20
Rotation speed (rpm)
7000

2.3

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B
5
3
10
3000

C
5
5
10
5000

D
5
10
5
1000

Experimental setup

We evaluated the fabrication yield by verifying the insulating properties of the fabricatedpiezoelectric layer. The percentage of devices completing the polarization process among thedevices manufactured under the same conditions is the fabrication yield and was calculated as
Fabrication yield =

NS
,
NT

(1)

where NS is the number of successfully fabricated devices, and NT is the total number offabricated devices. A successfully fabricated device is a device for which polarization
has been completed. The completion of polarization was judged from the decrease in therelative dielectric constant after applying the electric field. The alignment of the spontaneouspolarization suppresses the polarization fluctuation on applying an AC electric field, anddecreases the dielectric constant.(25) The relative dielectric constant εr was calculated using Eq. (2)with the representative dimensions of the device and the capacitance of the piezoelectric layer
measured using an LCR meter (NF Corp.: ZM2372).

εr =

Cd

ε0S

(2)

Here, C is the electrostatic capacity of the piezoelectric layer, d is the thickness of thepiezoelectric layer, ε 0 is the dielectric constant of vacuum, and S is the area of the top electrode.
When the electric field during polarization was 100 MV/m for 10 min or more, a piezoelectricperformance of 95% or more was obtained.(26) Devices that became conductive before reaching
the target electric field were judged to have failed. The thickness of the piezoelectric layer wasmeasured using a surface roughness measuring device (Tokyo Seimitsu Co., Ltd.: SURFCOM
130A), and the voltage was applied taking into consideration the thickness error of each sample.
To evaluate the film characteristics of the fabricated piezoelectric layer, surface morphology
was measured by AFM (Oxford Instruments: MFP-3D) and crystallinity was measured by
XRD (Shimadzu Corp.: XRD-6100). The effect of the number of layers on the smoothness ofthe piezoelectric layer surface was evaluated by AFM. The effects of the film thickness of one
layer and the number of film layers on crystallinity were evaluated by XRD.

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The function of the coated piezoelectric layer and the effect of the change in crystallinity
on the output power of the device were evaluated by the excitation experiment. For all typesof devices used in the test, the total thickness of the coated film was kept constant at 5 μm toevaluate the effect of the multilayered piezoelectric layer on power generation with the samestrain induced inside the piezoelectric layer. The applied voltage for excitation during thepolarization of the device was 60 MV/m. Figure 4 shows the excitation experimental system.
The shaker (IMV Corp.: m60/MA1) that applies excitation force to the device, a function
generator (NF Corp.: WF1974) that inputs the excitation waveform to the shaker, an acceleration
sensor (Micro Stone Corp.: MVP-RF8-HC-2000) for measuring the acceleration on the shaker,
an oscilloscope (Teledyne LeCroy: Waverunner 44xi-A) for measuring the output voltage, anda load resistance constitute the experiment system. First, the resonance frequency of the device
was specified by gradually changing the excitation frequency of the sine wave input to theshaker. Next, the impedance was matched by changing the external load resistance, and the
maximum output power Pmax was calculated from the external load resistance and the output
voltage as
Pmax =(Vmax ) 2,
R

(3)

where Vmax is the maximum output voltage, and R is the synthetic external load resistance.
The internal resistance of the oscilloscope used in the measurement circuit was 1 MΩ. The
combined resistance of the connected load resistance and the internal resistance of theoscilloscope was treated as the synthetic external load resistance.(27)

(a)

(b)

Fig. 4. (Color online) Excitation experiment system. (a) Photograph of device set up on the shaker. The device
electrodes are directly connected to the oscilloscope. (b) Measuring circuit. All elements are connected in series.

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3.

Results and Discussion

3.1

Result of fabrication process

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Figure 5 shows the photographs of the fabricated devices. The metallic luster of the surfaceof the top electrode above the piezoelectric layer improves as the number of deposited layers
increases. Since metallic luster reflects the smoothness, the multilayer coating method of
the piezoelectric layer reduced the coating nonuniformity, unevenness of solute generated
during solvent drying, and roughness due to particles on the substrate. On the surface of thepiezoelectric layer observed by AFM, as shown in Fig. 6, the scale of the unevenness of thesurface became smaller with increasing number of deposited layers. In addition, Fig. 7 shows
the relationship between the root-mean-square (RMS) roughness of the piezoelectric surface
obtained from the AFM measurement and the number of deposited layers. Thus, the multilayer
coating method is effective for smoothing the surface of the piezoelectric layer. As the numberof layers is further increased, a smoother piezoelectric layer is obtained. However, since thenumber of steps increases with the number of layers in this study, ten coating layers is the mosteffective from a practical viewpoint.
Figure 8 shows the results measured by XRD. Since the diffraction intensity increased as thenumber of layers increased, the crystallinity of the film was improved. Although the diffraction
intensity per layer decreased as the film became thinner, the total diffraction intensity inthe multilayer becomes higher than that of the single layer. In addition, a piezoelectriccharacteristic of the multilayer was expected owing to the improvement in crystallinity.
Table 2 shows the mean values of the measured electric properties of the piezoelectric layers
before and after the polarization treatment. For all device types, the capacitance decreases
after polarization treatment, indicating that the relative permittivity decreases from Eq. (2). In
addition, the dielectric tangent value is also small, indicating that the loss is low. Thus, thepolarization process is complete and the device can be used as a power generation device.

(a)

(b)

(c)

(d)

Fig. 5. (Color online) Photographs of the fabricated devices. The piezoelectric layer near the fixed end side wasremoved to expose the bottom electrode.

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(a)

(b)

(c)

(d)

Fig. 6. (Color online) Results of surface property measurement of the piezoelectric film resurface by AFM. The
measurement was performed at a point where no electrode was coated.

Fig. 7. Relationship between number of layers and RMS of piezoelectric layer surface. The measurement was
performed at a point where no electrode was coated.

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(a)

(b)

(c)

(d)

Fig. 8. (Color online) Crystallinity of piezoelectric layer measured by XRD. The samples for XRD were directly
formed on the glass substrate. The total peak of intensity increases with the number of layers.

Table 2
Changes in electric properties after polarization treatment.
Before polarization
Device type
Capacitance (nF)
Dissipation factor
A
3.97
0.022
B
4.05
0.022
C
2.90
0.019
D
3.63
0.019

3.2

After polarization
Capacitance (nF)
Dissipation factor
3.16
0.029
3.27
0.022
2.18
0.021
2.83
0.024

Fabrication yield

Table 3 shows the evaluation results of fabrication yield. The fabrication yield increasedby 52.4% when the number of layers was increased from one to ten layers with a constant total
film thickness. As the number of the deposited layers increased, the smoothness of the surfaceof the piezoelectric layer was improved, unevenness and pinholes of the piezoelectric layer were
suppressed, and device failure due to conduction was reduced. In addition, in this evaluation,

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the device fabrication process was simplified by using a Cu/PI film precoated with metal.
When a metal is coated directly onto a flat substrate, the piezoelectric layer polarization can be
expected to be further improved.

3.3

Power generation

Figure 9 shows the time history response of the output voltage and the shaker acceleration.
Since the output voltage of a device has a waveform synchronized with the input vibration
acceleration, the fabricated PVEH responds in accordance with the vibration acceleration.
Figure 10 shows the changes in power generation with the number of layers. The output
power from the device increased by 12% with an increase in the number of film layers, fromdevice A having a single layer to device D having ten layers. Reducing the film thickness per
coating layer led to improvements in the piezoelectric property and power generation. The
thinning of the piezoelectric layer enabled the polarization reversal of a larger number ofmolecular chains. The output voltage of the fabricated device was less than 1 V. However, alarger amount of power can be generated by adopting a device shape that makes the distributionof strain uniform or a structure that shows a special deformation behavior, as well as optimizing
the thickness of the piezoelectric layer suitable for the strain distribution of each device.

Table 3
Number of layers and device fabrication yield.
Device typeA
B
C
Fabrication yield (%)
33.3
60
70
Number of samples
6
20
10

Fig. 9. (Color online) Output voltage of fabricateddevice D under the following conditions: 0.2 G and
13.8 Hz frequency.

D
85.7
14

Fig. 10. Power generation plotted against the numberof piezoelectric layers in impedance matchingat resonance frequency. Proof mass was 0.1 g.
Acceleration was 0.2 G.

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Conclusions

We proposed a piezoelectric polymer multilayer coating method using solution casting andspin coating to improve the fabrication yield and piezoelectricity of the piezoelectric layer
for VEH. When the piezoelectric layers having a constant total thickness were multilayered,
the yield of a 10-layer piezoelectric film was improved by 52% compared with that of
the single-layer piezoelectric film. In addition, the power generation increased by 12% owing
to the decrease in the film thickness per coating layer. By the proposed process, the thicknessof the piezoelectric layer is changed in-process, and the manufacturing accuracy of the designed
device is improved. The proposed multilayer coating method for the piezoelectric polymer
has high compatibility with the MEMS process, and contributes to improving the performanceof not only PVEH but also piezoelectric polymer MEMS, such as piezoelectric sensors andactuators.

Acknowledgments
This research was partially supported by JSPS Science Research Grant JP17H03196,
JP20H02095, JST CREST Grant Number JPMJCR19Q2, JST PRESTO Grant Number
JPMJPR19R4, Gunma University Regulatory Science Research Grant 2019, and MEXT
Nanotechnology Platform Project (The University of Tokyo Microfabrication Platform).

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About the Authors
Nobuaki Kuriyama received his B.S. degree from Gunma University, Japan,
in 2019. He is now pursuing his M.S degree in the Division of Mechanical
Science and Technology at the same university. His research interests are in
MEMS and energy harvesting for IoT. (t191b029@gunma-u.ac.jp)

Takashi Nakajima received his B.Sci., M.Sci., and Ph.D. degrees from Tokyo
University of Science, Tokyo, Japan, in 2003, 2005, and 2008, respectively.
From 2008 to 2012, he worked at Tokyo University of Science as a research
associate. From 2012 to 2014, he was at the Institute of Materials Research,
Tohoku University, as a research associate. Currently, he is an associate
professor in the Department of Applied Physics, Faculty of Science, Tokyo
University of Science. He concurrently holds a post of PRESTO researcher of
the Japan Science and Technology Agency. His research has been focused on
the functional properties of ferroelectric materials and on ferroelectric devices
including piezoelectric energy harvesters. (nakajima@rs.tus.ac.jp)

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Ryo Ichige received his B.S. degree from Gunma University, Japan, in 2019.
He is now pursuing his M.S. degree in the Division of Mechanical Scienceand Technology at the same university. His research interests are in MEMS
and energy harvesting for IoT. (t191b009@gunma-u.ac.jp)

Takaaki Suzuki received his B.E. and M.E. degrees from Gunma University,
Japan, in 1998 and 2000, respectively. He received his Ph.D. degree from
Kyoto University, Japan, in 2003. From 2004 to 2008, he was an assistantprofessor at Kyoto University. From 2008 to 2015, he was an associate
professor at Kagawa University. From 2015 to 2018, he was an associate
professor at Gunma University. Since 2018, he has been a professor at theDivision of Mechanical Science and Technology, Gunma University. From2015 to 2019, he was also a PRESTO researcher at the Japan Science andTechnology Agency. His research interests include the development of novel
microfabrication technology, microfluidic systems for gene and cell analyses,
and energy harvesters for IoT. He is a member of IEEE, the Japan Societyof Mechanical Engineers, and the Institute of Electrical Engineers of Japan.
(suzuki.taka@gunma-u.ac.jp)