Mariya Aleksandrova et al. - Conference ELMA2017

Piezoelectric Energy Harvesting Device with Nanobranched
ZnO on Polymer/Metal/Polymer Coated Flexible Substrate
Georgi Kolev1, Mariya Aleksandrova1, Georgi Dobrikov1, Habib Pathan2,
Mileti Fartunkov1, Krassimir Denishev1
1 Technical University of Sofia, Department of Microelectronics (Faculty of
Electronic Engineering and Technologies), Sofia, Bulgaria,
2 Savitribai Phule Pune University, Department of Physics, Pune, India
Corresponding author: m_aleksandrova@tu-sofia.bg
XV-th International Conference on Electrical Machines, Drives and Power Systems ELMA2017 1 - 3 June 2017, Sofia

Outline
• Brief introduction in the activity of the technology division at Department of
Microelectronics, Technical University of Sofia.
• Brief information about the project “Study of the piezoelectric response of
layered microgenerators on flexible substrates” - DH 07/13 BNSF
• Current trends in the piezoelectric microgenerators – materials,
technologies, main parameters.
• Aim of the study “Piezoelectric Energy Harvesting Device with Nanobranched
ZnO on Polymer/Metal/Polymer Coated Flexible Substrate “
• Results achieved by our group
• Future work
2

Integrated circuits fabrication
(K. Denishev)
Rigid electroluminescent devices
(G. Dobrikov, M. Aleksandrova)
Transparent electrodes for flexible optoelectronic devices
and flexible organic light-emitting diodes (M. Aleksandrova, G. Dobrikov) 3

Ammonia sensor
(G. Kolev, K. Denishev)
„Electronic ink“
(Aleksandrova, Dobrikov)
Piezoelectric pressure sensor and
transformer (Kolev, Denishev,
Aleksandrova)
Laboratory of vacuum deposition processes and
thin films at Department of Microelectronics,
TU-Sofia.
2V light blue
4V dark blue
4

5
• Brief information about the project “Study of the piezoelectric response of layered
microgenerators on flexible substrates” - DH 07/13 BNSF
Participants:
- Department of Microelectronics - Technical University of Sofia, Bulgaria
- Institute of Solid State Physics – Bulgarian Academy of Sciences (BAS), Bulgaria
- Central Laboratory of Solar Energy and New Energy Sources – BAS, Bulgaria
- Technical Faculty - Bielefeld University of Applied Sciences, Germany
- Department of Materials Engineering, Defence Institute of Advanced Technology , India
- Department of Physics, Savitribai Phule Pune University, India

6
• Brief information about the project “Study of the piezoelectric response of layered
microgenerators on flexible substrates” - DH 07/13 BNSF
Project aim:
• To acquire new knowledge about the mechanisms of charge generation in portable
piezoelectric elements on flexible substrate and the dependence of the piezoelectric
response on the position of piezoelectric areas over the flexible surface, on the
material for the active piezoelectric layer, technological modes for its deposition and
the applied mechanical stress.
• To design, fabricate and test piezoelectric generator elements with a potential use as
alternative energy sources (energy harvesting devices), made of layers with nano-
and / or micrometer thickness.
• To apply a variety of technologies for deposition of layers from organic and inorganic
materials for the active piezoelectric layers, such as screen printing, vacuum
sputtering, spray deposition and atomic layer deposition, in order to find the most
suitable one in term of throughput and cost efficiency.

• Current trends in the piezoelectric microgenerators – materials, technologies, main
parameters, applications.
- Energy harvesting concept - Power generation from the environment.
- Piezoelectric energy harvesting - vibrations, pressure, forces are converted into
electrical charges due to direct piezoelectric effect that some materials exhibit.
- By using appropriate electronics, this effect can be used for making a self-sufficient
energy supply – great advantage, where cable is not possible and the use of batteries is
not desired.
Energy harvesting marketing up-to-date
Source: www.tyndall.ie, accessed 21.05.2017
Numbers of papers on piezoelectric harvesting
Source: H. Maiwa, "Piezoelectric Materials“, InTech, 2016
7

Source: IDTechEx Report, Energy Harvesting and Storage for Electronic Devices 2009-2019
Power needed to supply the modern electronics
8

Sources: www.ataelektroteknik.com, elektrobaukasten.com
Examples for rigid piezoelectric transducers –
piezoelectric converting part is usually piezo-
ceramic with thickness of 0.5mm in order to
produce 10mW-5W at 40-800 gr (500Hz).
Macro- to millimetric piezogenerators Micro- to nano- piezogenerators
Sources: Georgia Institute of Technology Research News. "Energy
harvesting: Nanogenerators grow strong enough to power small
conventional electronic devices." ScienceDaily, accessed
22.05.2017; Flexible and Nanobio Device Lab
http://fand.kaist.ac.kr/index.htm
Examples for next-generation bendable,
implantable, and wearable power supplies –
piezoelectric films are 200nm -2µm thick in
order to generate 100nW-150µW.
9

Vibrations from human
everyday’s activity
http://www.nanoscience.gatech.edu
Medical applications – vibrations from
moving organs
www.medgadget.com
Piezoelectric energy assisted car
www.slideshare.net
Some applications of the flexible piezoelectric microgenerators
10

- Lead Zirconium Titanate (PZT) – effective, but lead-containing;
- Zinc Oxide (ZnO) – lead free and can be effective after suitable growth conditions set;
- Polyvynyliden difluorid (PVDF ) – piezo-polymer, flexible and reliable at multiple bends,
but has weak piezoelectric response.
11
Typical materials studied as a potential candidates for thin film piezoelectric
generators
PZT ZnO PVDF
d31, pC/N 180 5 0,6-1
d33, pC/N 360 12,4 1,3-2,2
g31, Vm/N 0,011 0,36 0,14
g33, Vm/N 0,025 1,57 0,22
Y, GPa 49-63 30-200 8,3
εr 170 10-11 10-12
These parameters are not strictly fixed and can vary with the films structure at
nanoscale, which can be controlled by the deposition technology, modes and/or
composites formations.

Scanning electron micrograph of PZT/PVP
nanowires – up to 6 V open-circuit voltage
45 nA short-circuit current, with an average
output power of 0.12 mW across a 100MΩ
load.
Flexible nanocomposite-based generator made
of BaTiO3 nanoparticles and carbons.
Source: Nano Convergence (2016) 3:12
Scanning electron micrograph of
PZT-coated ZnO nanorods.
Source: Nano Convergence (2016) 3:12
Sources: Nano Energy(2015) 14, 15–29
Composites and specific nanostructures for combining the positive features of the
separated materials.
12

13
Fabrication technologies for piezoelectric coatings deposition
Schematic illustrations of the fabrication of
PVDF nanofibers onto the interdigitated
electrode by electrospinning.
Source: IEEE Transactions On Nanotechnology, Vol. 13, No. 4,
2014
Growth of oxides nanostructures by
the solid–vapor phase process.
Source: J. Phys.: Condens. Matter 16 (2004) R829–R858
(+) Simple, fast and cheap method.
(-) No vacuum, therefore poor control of
the contamination.
(+) Controlled atmosphere in a chamber
with inert gas (argon).
(-) High temperatures of the chemical
reactions, requiring high melting point
substrates (>300oC).

• Aims of the study
- Deposition of nanostructured ZnO films by conventional microfabrication
technology (RF sputtering) on new type polymer/metal/polymer coated flexible
substrate for improved mechanical stability.
- Fabrication of flexible environmental friendly (lead-free) piezoelectric device
with nanobranched ZnO film for enhanced piezoelectric response.
- Structural characterization of the ZnO films and electromechanical study of
the ZnO-based harvesting devices, in order to estimate their ability to generate
electricity when subjected to mechanical vibrations.
14

New type polymer/metal/polymer coated polyethyleneterephthalate substrate (PET)
15
Problems with the conventional electrodes on flexible substrate – cracks at low number of
bending cycles (<500). Electromechanical devices should stand at least 1000 cycles. Therefore,
the electrodes should be as flexible as the substrate is. What about the conductive polymers (for
example polyethylene dioxithiophene doped with polystyrene sulfonate (PEDOT:PSS)?
Metal vacuum deposited
electrode (Cu) on flexible
substrate degrades after
400 bending cycles.
Left: Sheet and specific resistance of PEDOT:PSS/Au/PEDOT:PSS electrodes as a function of the Au
thickness; Right: relative change of the sheet resistance of PEDOT:PSS/Au/PEDOT:PSS, PEDOT:PSS
and indium-tin oxide (ITO) films on PET substrate, as a function of the number of bending cycles N.

16
New type polymer/metal/polymer coated polyethylene terephthalate substrate (PET)
Benefits of using polymeric based electrode, compared to conventional ones:
- Enhanced mechanical stability of the electrode film and films interfaces, therefore stable
electric processes are also ensured;
- Lower temperature of electrode films deposition, thus avoided thermal stress in the
flexible substrate;
- Faster, simpler process (spray deposition/spin coating) for the polymer films, therefore low
cost.
- Precondition for amorphous or slight crystalline ZnO growth on amorphous electrode with
possibility for easier tuning of the order’s degree;
Atomic force microscopy of the a) flexible
substrate; b) substrate/ multilayer electrode
surface before bending; c) after 100 cycles
and d) after 1000 cycles of bending.
Optical micrographs of polymeric based electrode
PEDOT:PSS/Au(15nm)/PEDOT:PSS (a) before and (b) after
2000 bends; as well PEDOT:PSS/Au(25nm)/PEDOT:PSS (c)
before and (d) after 2000 bends.

17
New type nanobranched ZnO film grown on polymeric based flexible electrode –
deposition technology and structural characterization.
to the vacuum
pump
~RF
ground
substrate
holder
(anode)
substrate
thin film
plasma
cathode
water
cooling
system
shield
Ar+ Ar+
inert gas
(Ar) ions
ejected
particle
ZnO target
Ar
Ar
Ar Ar
---
electron vacuum
chamber
oxygen flow
Vacuum RF sputtering of 550nm thin ZnO film with
additional modification of the oxide content and
oxygen deficit at the end of the process.
Scanning Electron Microscopy (SEM) image of ZnO
grown on PET/PEDOT:PSS/Au/ PEDOT:PSS substrate.
X-Ray Diffraction of PET/PEDOT:PSS/Au/
PEDOT:PSS before (a) and after deposition of
ZnO (b). The insets show the sample in front
and bent state.

18
Electrical characterization of the piezoelectric energy harvesting device with
nanobranched ZnO
0.1 1 10
1E-8
1E-7
1E-6
forward direction
CurrentI(A)
Voltage U (V)
Current-voltage characteristic of the piezoelectric
sample with nanobranched ZnO.
Frequency dependence of piezoelectric sample‘s
impedance Z and capacity C.
0.1 1 10 100
0
1
2
3
4
Dielectriclossestgx10
-2
Frequency, kHz
Frequency dependence of the dielectric losses.

19
Electromechanical characterization of the piezoelectric energy harvesting device with
nanobranched ZnO
Experimental setup for dynamic mode measurement
of the piezoelectric response.
Piezoelectric response (voltage vs time) of sample with nanobranched ZnO at
frequencies of the mechanical stimulus 30 Hz, 400 Hz and 5 kHz at minimum weight
load of 3gr. The voltage scale is 100 mV/div.
Piezoelectric response (voltage vs time) of dense ZnO
compared to nanobranched ZnO at maximum mechanical
load of 4.8kg. The scales are 500 mV/div and 20ms/div.
2.2V vs 3.1 V
240 mV 200 mV 160 mV

20
Determination of the basic piezoelectric coefficients
Coupling factor − 𝑘33 =
𝜋
2
.
𝑓 𝑚𝑖𝑛
𝑓 𝑚𝑎𝑥
tan
𝜋
2
𝑓 𝑚𝑎𝑥−𝑓 𝑚𝑖𝑛
𝑓 𝑚𝑎𝑥
= 0.023,
where fmin and fmax are the frequencies at which the impedance reaches minimum and
maximum value, respectively.
Piezoelectric coefficients:
𝑑33 = 𝑘33 𝜀 𝑟 𝑠33
𝐸
8.85𝑥10−12 = 120 pC/N,
𝑔33=
𝑑33
𝜀 𝑟
1
8.85𝑥10−12 = 1.96 V.m/N,
Elastic constant sD
33 = 1/[4.ρ.fmin2.l2] was determined to be 0.445m2/N, taking into
account the average length of a main dendrite l = 2 µm and ZnO density ρ = 5,61 g/cm³.
At max. load of 4.8kg the peak-to-peak output voltage of the fabricated thin film
piezoelectric generator is 3.1V and the current is 300 nA, therefore the maximal useful
power is 1.14µW.
There is measurable reaction at min. load of 3gr, where the average peak to peak
voltage (depends on the frequency) is ~200mV and the current is ~25nA, therefore the
min. power is ~ 5nW.

Main parameters of the piezoelectric samples – comparison with the available
results for other thin film piezoelectric harvesting.
21
Piezoelectric material Film
thickness,
nm
Deposition method Maximum
voltage, V
Maximum
power, W
d33, pC/N Reference
PZT with vertically
aligned nanocolumnar
grains
2000 pulsed laser deposition
on
Pt(111)/Ti/SiO2/Si(100)
substrates
Not provided Not provided 305 Nguyen et al., Appl
Mater Interfaces.
2017; 9(11): 9849–
9861.
PZT nanorods Not
provided
Hydrothermal growth
on Pt electrode
1V 2.4 µW 80 Kang et al.,
Actuators 2016, 5,
5
ZnO nanobelts 300 Hydrothermal method
on Si/Pd
Not provided Not provided 26.7 Wang, J. Phys.:
Condens. Matter
16 (2004) R829–
R858
ZnO nanowires 1500 Ion Beam deposition
system+
hydrothermal growth
2V Not provided Not provided Cardoso,
Dissertation
University of Porto
in
Enginnering
Physics, 2015
ZnO nanobranched 550 RF reactive sputtering
on
PEDOT:PSS/Au/PEDOT:P
SS/PET substrate
3.1V 1.4 µW 120 This work

- The biggest challenge in the vibration piezoelectric energy harvesting is the
scavenging of low input frequencies. Most of the vibrational energy harvesters are
designed to operate in resonance mode and the half-power bandwidth is usually
small. Here, this obstacle has been overcame due to the specific nanostructure of the
ZnO, capturing variety of frequencies and broadening the working frequency range of
the harvesting device (including the low frequencies of the vibrations coming from the
everyday life).
- The voltage generated exceeds the typical results available for similar design of
harvesting device.
- High elasticity of the element due to the polymeric based electrode and porous ZnO
nanocoating is precondition for enhanced mechanical stability at multiple bandings.
- We used conventional technologies for microfabrication compatible with these for the
integrated circuits.
Conclusions:
22

23
• Future work
- To increase the current through the device by modification of the nanobranched
ZnO with suitable nanoparticles, making conductive nanonetwork in the hollows.
- To measure the characteristics with electrical load attached (this was open circuit
voltage).
- To design an electronic circuit for further processing of the signal (AC-DC
conversion or filtration/amplification).
Any collaborations are welcome!!! If you are interested, please, write us at
m_aleksandrova@tu-sofia.bg

24
Acknowledgement:
This work is supported by the Bulgarian National Science Fund, grant No DH 07/13.
THANK YOU FOR YOUR ATTENTION!
Corresponding author’s e-mail: m_aleksandrova@tu-sofia.bg

Mariya Aleksandrova et al. - Conference ELMA2017

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