Ultra-low frequency P(VDF-TrFE) piezoelectric energy harvester on flexible substrate Zhaoyang Pi1, Lun Zhu1, Jingwei Zhang1, Dongping Wu1*, David Wei Zhang1, Zhi-Bin Zhang2, Shi-Li Zhang2 1 Department of Microelectronics, Fudan University, Shanghai 200433, China 2 Solid-State Electronics, The Ångström Laboratory, Uppsala University, P.O. Box 534, 75121 Uppsala, Sweden * Email:
[email protected] Abstract This paper proposes a flexible piezoelectric energy
harvester using the piezoelectric copolymer P(VDF-TrFE) films as active elements to convert mechanical inputs to electrical energy. The repeatedly spin-coated P(VDF-TrFE) films were thermally poled and then characterized by SEM, ferroelectric hysteresis and FTIR. The piezoelectric performance of the fabricated harvester was measured under periodical mechanical inputs at ultra-low frequencies. The electrical output of average Vpp was as high as 0.96 V at 0.65 Hz. Keywords: P(VDF-TrFE) films; low frequency; flexible substrate; piezoelectric energy harvester 1. Introduction Self-powered sources by scavenging energy from
piezoelectric coefficients. PVDF and P(VDF-TrFE) have been utilized in low-frequency, large deflection energy scavenging applications in the form of nanofibres [9-11]. These nanofibres, often made by electrostatic spinning technology, require precise distance and speed control and are easy to cancel the polarity of each other because of the random orientation [1, 9-11]. Recently, devices with PVDF and P(VDF-TrFE) in the form of thin film have also been studied to generate electric power based on the mechanical-to-electrical energy conversion [10, 12]. These devices have relatively thicker films, leading to lower surface area to volume ratio for effective working area and lower energy conversion efficiency, or non-flexible substrate, causing limited strain level and restricted application environments. In this paper, a P(VDF-TrFE) thin film piezoelectric energy harvester
ambient environments beyond rechargeable batteries for
produced on flexible Kapton substrate is fabricated and
portable and wireless devices have been studied
then measured under sub-1Hz periodic mechanical
extensively for current and future stand-alone systems
inputs.
[1-3]. In particular, mechanical energy harvesting using inorganic semiconductive piezoelectric nanowires (ZnO
2. Experiment
[2], InN [4], GaN [5], AlN [6]) to convert the body
P(VDF-TrFE) copolymer (75/25) (Piezotech, France),
movement, muscle stretching and acoustic/ultrasonic
was dissolved in DEC (diethyl carbonate) solution with
waves to electrical energy, has attracted lots of attention
concentration of 2.0 wt% and 15 wt%. In order to ensure
[7]. However, these inorganic nanowire-based energy
good adhesion between the bottom electrode and the
harvesters suffer from fabrication condition complexity,
substrate, a 100 nm SiO2 layer was first deposited on the
limited strain level and low flexibility. Attributed to their
Kapton substrate. Then the bottom Al electrode was
capability of being processed into thin, light, tough and
evaporated on the Kapton through shadow mask. The
flexible films [8], piezoelectric polymer and copolymer,
P(VDF-TrFE) copolymer films were subsequently
such
and
fabricated by spin-coating technology and annealed at
trifluoroethylene)
140 °C for 2 hours to enhance the crystallinity. The
[P(VDF-TrFE)], have recently emerged in energy
spin-coating and annealing process were repeated to
harvesting applications despite of their relatively low
prepare films with thicknesses of 200 nm and 10 ȝm.
as
poly(vinylidene
poly(vinylidene
fluoride)
fluoride
978-1-4673-6417-1/13/$31.00 ©2013 IEEE
(PVDF)
Top Al electrode was finally evaporated on the
P(VDF-TrFE) thin film with thickness of 10 ȝm.
P(VDF-TrFE) copolymer films. The overlapping areas of
Figure 1 (a) shows SEM of dried and gold coated
-4
cross-sections of the energy harvester, which had the
cm and 1×10 cm DŽThe P(VDF-TrFE) copolymer films
Al/P(VDF-TrFE)/Al/SiO2/Kapton composite structure,
were poled by connecting the evaporated electrodes to a
while (b) displays the cross-section of the P(VDF-TrFE)
high-voltage source supplying a field of 0.3-0.5 MV/cm
film with thickness of 10 ȝm with fine flatness, which is
at 90 °C for about 30 min and cooled to ambient room
crucial for obtaining stable piezoelectricity.
temperature while keeping the electric field constant.
B. Piezoelectric and ferroelectric characteristics
top and bottom Al electrodes were designed to be 5×10 2
-2
2
The fabricated P(VDF-TrFE) copolymer films and
Piezoelectricity of P(VDF-TrFE) films arise from
devices were characterized by Scanning Electron
higher remnant polarization after polarizing treatment,
Micrograph (SEM), ferroelectric hysteresis (Radiant
i.e., higher net dipole moment, which is greater in polar
Technologies Precision Analyzer, Premium II) and
regions of copolymer compared to nonpolar crystalline
Fourier
(FTIR).
regions [8, 13, 15, 16]. Figure 2 shows the ferroelectric
Self-made electrical measurement setup was able to
hysteresis for thermally poled film with thickness of 200
generate cycled mechanical stretch and release as inputs
nm and electrode area of 5×10-4 cm2. The corresponding
for energy harvester.
polarization Pr value increased from 6.3 ȝC/cm2 to 9.1
transform
infrared
spectroscopy
ȝC/cm2 as external voltage swept from 10 V to 14 V, which indicates a high degree of crystalline dipole
3. Discussion A. Structural characteristics
alignment.
Figure 2. Hysteresis loops of PVDF-TrFE thin film C. Crystalline characteristics The chains of Į phase are packed in the unit cell such that the molecular dipoles are anti-parallel and no net dipole is present, resulting in paraelectric behavior. The ȕ phase is the main polar phase and presents net dipole, and its chains stack in the unit cell such that their respective polarizations are aligned in the same direction. The ȕ phase can be obtained directly by stretching Figure 1. (a) Cross-section micrograph of the electrical
melting films [14] or electrical poling, i.e., contact
harvester
poling and corona poling, under high voltages [8, 15-18].
structure.
(b)
Cross-section
graph
of
The crystalline structure was determined by using
When two devices were connected in series, the
FTIR to establish the conformation of ȕ phase [14, 19].
voltage output would be the sum of the two as illustrated
Figure 3 shows the FTIR spectrum of the poled
in Figure 5. The devices were tested under the same
-1
P(VDF-TrFE) thin film. The bands seen at 1174 cm , -1
-1
1402 cm and 1430 cm were attributed to the Į phase -1
while the bands seen at 840 and 1281cm were indexed
strain (2.5%) and frequency (0.65 Hz). The total electrical output was as high as 0.96 V, which is an inspiring and reasonable value of output voltage.
to the ȕ phase [11, 14, 16, 19]. Thus the hysteresis loops and FTIR spectra both demonstrate that the thermal poling process has enhanced ȕ-crystalline phase.
Figure 5. Output voltage vs. time of two devices and their connection in series. The two devices were tested under the same strain (2.5%) and frequency (0.65 Hz). Figure 3. FTIR spectra of poled P(VDF-TrFE) films D. Electrical performance Electrical voltage outputs generated by mechanical
Both mechanical strain and frequency had shown significant influence on the electrical output, which is consistent to our previous simulation results [21].
inputs were measured. To confirm the validity of the recorded piezoelectric responses [20, 22], polarity switch test was performed as illustrated in Figure 4.
Figure 4. Forward and reverse connection for switching polarity test. (a) forward connection (b) reverse connection.
Figure 6. Output voltage vs. time for a device stretched at constant frequency of 0.65 Hz and various strain level. At constant frequency of 0.65 Hz, when the strain increased from 0.5% to 2.5% [22], the average Vpp was increased dramatically from 0.14 V to 0.28 V as
illustrated in Figure 6. The obtained large strain was attributed to the flexibility of P(VDF-TrFE) copolymer films and Kapton substrate. As illustrated in Figure 7, under constant strain of 2.5%, as the frequency increased from 0.32 Hz to 0.91 Hz, the average Vpp was boosted from 0.13 V to 0.51 V. Such a high voltage generated by a low mechanical frequency was beneficial in harvesting energy from movements of human body, such as heartbeat and respiration [23].
Figure 7. Output voltage vs. time for a device stretched at constant strain of 2.5% and various frequencies. 4. Summary A flexible piezoelectric P(VDF-TrFE) thin film energy harvester, capable of converting sub-1Hz ultra-low frequency mechanical motion to electrical signal, has been successfully fabricated. The output voltage is found to increase with of the strain and frequency. Output voltage as high as 0.96 V has been achieved at 0.65 Hz. The proposed energy harvesting device in this paper provides a promising solution to harvest ultra-low frequency mechanical energy such as movements of human body. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 61176090), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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