Webinar devoted to the World Environment Day

1. Microelectronic technologies for alternative energy sources
Assoc. Prof. Dr. Mariya Aleksandrova
Technical University of Sofia, Bulgaria
Department of Microelectronics – Materials Science
and Thin Films Division
E-mail: m_aleksandrova@tu-sofia.bg
05 June 2020
Government V.Y.T.PG.Autonomous College Durg’s webinar
devoted to the World Environment Day
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2. Outline:
• Introduction to Energy Harvesting (EH)
• Driving forces for development of Energy Harvesting-based power
• How does EH work?
• Sources of energy and energy conversion
• Applications
• Microelectronic technologies for thermoelectric convertors. Thin film
thermoconvertors
• Microelectronic technologies for piezoelectric convertors. Thin film
piezoconvertors – sharing our Lab experience.
• Microelectronic technologies for solar cells fabrication. Thin film solar cells –
joint project with Prof. Dr. Ajaya K. Singh and his team under the program for
cooperation in the field of science and technology between India and
Bulgaria (2019-2021).
2

3. Introduction to Energy Harvesting (EH)
• Energy Harvesting (EH) is also known as Power Harvesting or Energy Scavenging and it is
the process in which energy is captured from a variety of ambient energy sources of
non-electric energy to be converted into usable electric energy for power supply (green
energy).
• Energy harvesters provide small amount of power, suitable only for low-power
consuming electronics, because they capture small amount of solar, thermal of
vibrational energy, which is typically dissipated and wasted in ambient.
• EH allows low-power portable electronic devices to operate if conventional power
supply cannot be used, thus eliminating the need for wires or batteries.
https://www.akm.com
3

4. Driving forces for development of Energy Harvesting-based power
1) Transducing elements, converting the energy from one form to another improve their
performance. New developed materials and technologies (nanomaterials and
nanotechnologies) significantly increased the efficiency of all of them (solar,
thermoelectric and piezoelectric, shown below. The areas are few square centimeters).
Solar panel Thermoarray Vibrational beam
2) Low-power circuits require less power. While the power available from transducers has
been increasing, the power needed to run electronic circuits has been continuously
decreasing.
3) Electronics costs are getting down faster than battery costs. Soon, if a battery is
needed, it can represent a major fraction of the total device cost. In addition, the size of
the battery cannot be further decreased, so the electronic modules would be too large.
4) Devices reliability depends on the power source reliability. Batteries are based on
chemical processes that have many different failure modes, according to the working mode
(temperature, voltage and their variation).
4

5. How does EH work?
• An energy harvester consists of one or more transducers, power conditioning (impedance
matching circuit), and energy storage. These technologies work together to collect energy
and deliver power to the device. On the other hand, the device which uses the energy needs
to be designed to work with energy harvesting as the power source.
Light RF/Electromagnetic Vibration Thermal
Processor Sensor Actuator
Power conditioning is necessary because the natural output of the transducer may show
unsuitable parameters (frequency, voltage and current) to be directly used for device power
supply. A specialized DC-DC and AC-DC converter microchips are used, depending on the
output voltage type (DC, AC low-frequency, AC-high frequency, unipolar, bipolar, etc.).
Energy storage is needed when the harvesting device is not active to balance the energy
supply and energy demand (rechargeable battery, capacitor, or supercapacitor is used).
Tan Nguyen, Introduction to Energy Harvesting
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6. Sources of energy
Light Energy: This source can be divided into two
types: room light and sunlight energy. Light
energy can be captured via solar photovoltaic
(PV) panels (cells)
Mechanical Energy: Vibrations and pressure from
machines, mechanical stress, strain from high-
pressure motors, manufacturing machines,
cars, acoustic waves and weather conditions
(wind/rain) can be source of kinetical energy
and can be captured via piezoelectric
generators.
Thermal Energy: Waste heat energy variations
from furnaces, heaters, weather conditions,
engines can be captured via thermoelectric
convertors.
Electromagnetic Energy: Inductors, coils, and
transformers can be considered as RF energy
sources and can be captured via antenna.
Human Body: Mechanical and
thermal (heat variations)
energy can be generated
from a human or animal
body by actions such as
walking and running;
6

7. Transducer (energy
convertor)
Challenges Impedance Typical
generated
voltage
Typical output
power
Solar generator To achieve large
yields from a small
area and to generate
a wide range of
voltages.
Varies according
to the light input
between 1-90 кΩ
DC 0,5-5 V
according to the
cell size
5-15 mW
Piezoelectric generator To achieve broad
range of operation
(vibrational)
frequencies and
higher currents to
gain power.
Constant
impedance > 100
кΩ
AC 0,1-1 V
according to the
size
1-200 µW
Thermoelectric
generator
To achieve large yield
at small temperature
difference.
Constant
impedance, 100
Ω
DC 0,1-5 V
according to the
size
0,5 mW-10 mW
(20oC gradient)
RF generator To achieve effective
rectification at ultra-
high frequencies.
Constant
impedance , 1
кΩ
AC 0,5-5V
according to the
distance
0,1 mW – 0,1 W
Energy conversion per cm sq.
7

8. 8

9. Adapted from Design News, Accessed May 2019
Applications of microelectronic energy harvesters for power supply
of low consuming electronics
- Remote patient monitoring - Home automation
- Tracking systems - Implantable sensors
- Machinery/equipment monitoring - Devices for personal use
9

10. 10
Microelectronic thin film growth technologies are involved
materials for
evaporation
copper or graphite
pocket evaporator
water cooling
system
filament
10kV
accelerating
aperture
electron
beam
magnetic field
for e-beam
bending
substrate
melt
vaporized flux
vapor flux
substrate holder
substrate
current
controller
vacuum
chamber
to the vacuum
pump
water cooling
system
evaporator
Vacuum thermal evaporation of Al,
Ag, Au, Ni, or Cu electrodes
Photocondverting, piezoconverting, or
thermoconverting compounds like CdTe,
ZnSe, InP (solar cell), Bi2Te3, SeSb
(thermoelectric) etc. grow by vacuum e-
beam evaporation.
Metal-oxide coatings BaSrTiO3, ZnO
(piezoelectric), ITO, TiO2, SiO2 (front electrode
and filters) grow by vacuum reactive
sputtering.
-DC (or ~RF)
+DC (or ground)
substrate holder
(anode) substrate
thin film
plasma
cathode
water cooling
system
shield
Ar+ Ar+
inert gas
(Ar) ions
ejected
particle
target
Ar
Ar
Ar Ar
---
electron
-DC (or ~RF)
+DC (or ground)
substrate holder
(anode) substrate
thin film
plasma
cathode
water cooling
system
shield
Ar+Ar+ Ar+Ar+
inert gas
(Ar) ions
ejected
particle
target
Ar
Ar
Ar Ar
------
electron

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Microelectronic technologies for thermoelectric convertors.
Thin film thermoconvertors
Seebeck effect in thermoelectric energy
harvesting
• At electrical contact between two semiconductors of different dopant type, respectively of
different thermal and electrical conductivity, if the mutual connecting point is exposed to
one temperature and their free ends to another, this temperature difference causes
voltage generation.
• Thermoelectric conversion depends on the dopant concentration, mobility of major charge
carriers, electric and thermal conductivity along the thermolegs and Schottky barrier at
the electrodes.
• Semiconductor with suitable properties
for this purpose is bismuth telluride,
doped with antimony (Sb) and selenium
(Se) to obtain n-type and p-type domains.
• It is characterized by a high Seebeck
coefficient ≥ (180-280) .10-6 V/K at room
temperature.DOI: 10.1155/2013/232438

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• Challenges at thin film thermoconvertor
fabrication – cold and hot side are too close
so their thermal fields cannot be effectively
separated and affect to each other, thus
decreasing the temperature difference
across the element.
Bulk vs. thin film thermoelectric convertors
• When selecting a substrate for the thermo-couples growth, thermal coefficients of linear
expansion have to be close to each other, in order to avoid mechanical stress induced by
the different degrees of change in the geometric dimensions when changing the
temperature.
Applications of thermoelectric generators for autonomous power supply of GPS system
or systems for monitoring the human body's performance in biomedical electronics.

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Microelectronic technologies for piezoelectric convertors. Thin film piezoconvertors.
• Piezoelectric generators arise mainly because of the
idea ​human body movements to serve as a source of
mechanical loading for piezoelectric materials and thus to
produce amount of charge, which could power supply
portable low power consumers that a persons bring.
• Due to non-ideality of the devices and energy losses at mechanical to electrical energy
conversions, and because the devices have to be small and light-weight, only few micro-
Watts or even few hundred of nano-Watts could be produced – that’s why
nanogenerators. In addition, the films integrated in the device are nano-sized.

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- Degree of displacement of the symmetric centers for positive and negative charges.
This can be controlled by the growth conditions and doping concentration of certain
dopants like Ti, Ba, Sr, Pb, Zr.
- Pb(ZrxTi1-xO3) – PZT
- Quartz
- BaSrTiO3
- ZnO
- Element geometry - beveled trapezoidal beams generate more electrical energy than
the strip shaped. The membrane type occupies an intermediate place.
Factors affecting the energy conversion efficiency
https://csclub.uwaterloo.ca/
Sensors 2014, 14

16. 16
Factors affecting the energy conversion efficiency
- Piezoelectric material thickness: 200
μm - 2 mm - matter of compromise
between efficiency and mechanical
durability, i.e. it depends on the
application and the maximum applied
force. There are piezo-harvesting
elements with a piezofilm thickness of
200-500 nm.
- The location and patterning of the
electrodes relative to the mechanical
deformation is also important for the
extraction of the generated charges.
Interdigitated electrodes are more
suitable for d31/g31 mode, than for
d33/g33 mode i.e. according to the
direction of the force applied with
respect to the the crystal orientation
and charge accumulation sides).

17. 17
Applications of this type of harvesters
Piezoelectric micro-generator converting kinetic
energy from falling rain drops into electricity –
front glass on the cars. Piezoelectric polymer PVDF
is pulverized on glass substrate and has thickness
of 2.5 micrometers to produce 450 mV voltage per
square centimeter.
MEMS microphones in GSMs - the "diaphragm"
design is very susceptible to a mechanical wave
with sound frequency. Since the frequency of the
signal to be transformed covers a wide range of
20Hz-20kHz, the moving part must be light enough
to follow the quick changes, so it is made from a
membrane with certain elastic and electrical
properties – most often PVDF.
Alternative keyboard under the
conventional, shoe activated by
walking, and many others.

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Sharing our Lab’s experience – project “Study of flexible piezoelectric layered
nanogenerators” DN07/13 BNSF
SEM image of the surface morphology of 3D nanobranched ZnO sputtered on a PEDOT:PSS-coated flexible
substrate and oscillogram of the generated piezoelectric voltage at low mass loading and low frequency.
The critical limits for the device functioning in terms of mechanical loading are 11,000 bends at dynamic
loading (350 gr/cm2 mass loading and 10 Hz cyclic repeating of the loading), and 3.6 kg at static loading.
The function of PEDOT:PSS is to enhance the mechanical stability of the entire microdevice, and to serve
as an amorphous sublayer for non-ordered ZnO growth. It is believed that in this way, the obtained laterally
aligned nanobranches on a bendable substrate will be subjected to maximal deformation without incurring
a fast degradation process
Characterization of Piezoelectric Microgenerator with Nanobranched ZnO Grown on a Polymer Coated Flexible
Substrate, M Aleksandrova, G Kolev, Y Vucheva, H Pathan, K Denishev, Applied Sciences 7 (9), 890, 2017.

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Sharing our Lab’s experience
Cross section and top view of nanowires grown by template-assisted
filling of sputtered piezoelectric KNbO3 in an anodic aluminum oxide,
as well as the generated voltage at mass load of 50 g/cm2, 50 Hz. It
exhibits excellent piezoelectric response due to the increased specific
area as compared to non-structured films and it can be used as multi-
sensor (pressure and pyroelectric) due to linear response and
sensitivity.
Sensing Ability of Ferroelectric Oxide Nanowires Grown in Templates
of NanoporesM Aleksandrova, T Tsanev, A Gupta, AK Singh,
G Dobrikov, V Videkov, Materials 13 (7), 1777, 2020

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Sharing our Lab’s experience – project “Study of flexible piezoelectric layered
nanogenerators” DN07/13 BNSF
Screen printed poly[(vinylidenefluoride-co-trifluoroethylene] - P(VDF-TrFE) ink on flexible
1 2 3
0
200
400
600
800
AlAuAg
piezoelectricvoltage,mV
types of metal electrodes
rectangular
meander
side comb
0 200 400 600 800 100012001400
300
400
500
600
700
800
side comb
meander
rectangular
piezoelectricvoltage,mV
number of bends
Yield and stability of the piezoelectric voltage produced from PVDF-TrFe samples with different metal
electrodes and different patterns of the electrodes at maximum mass load of 100 g and low frequency of 20 Hz.

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Sharing our Lab’s experience – project “Study of flexible piezoelectric layered
nanogenerators” DN07/13 BNSF
1D
1C
pI
 pC
PEH equivalent
electronic circuit
– (+)
+ (–)
2D
2C
3D
3C
4D
in
)(tvp
BV CAPV
Super capacitor
+
–
out
4C
+
–
0
0.18
1
)(hoursTime
0 2 3 4 5 6 7
Outputvoltage(V)
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
Quadrupler circuit, experimentally measured voltage from single piezoelectric
harvesting on a micro supercapacitor for charging and a prototype of a real charging
system that can serve as alternative power supply.

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Microelectronic technologies for solar cells fabrication.
Thin film solar cells
• Converting light energy into electrical voltage is a photovoltaic effect. The
generated electrical signal is DC in nature. To produce the generator's electric
poles, P and N type semiconductors are needed (most commonly silicon).
Sunlight spectrum (intensity vs. wavelength)
– UV+visible+infrared
Silicon solar cell general structure
• Silicon has a bandgap of 1.14 eV i.e. covers a small part of the spectrum, from
infrared to red-orange band of the visible range, or about 1/3 of the total
spectrum of white light.
https://redlightman.com/

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Joint project between the Government V.Y.T.PG.Autonomous College Durg
and the Technical University of Sofia, Bulgaria
Title of the Project:‘Ultrahigh-Efficiency lead free Perovskite solar Cells’
Project coordinator for India: Prof. Dr. Ajaya Kumar Singh
Project coordinator for Bulgaria: Assoc. Prof. Dr. Mariya Aleksandrova
Duration: September 2019- September 2021
Thin film solar cells
AIMS:
- To fabricate a very high quality lead free organic-inorganic perovskite solar cell.
- To increase the efficiency of photo-voltaic solar cell to absorb solar light with high
efficiency.
- To find perovskite material’s stability and addressing environmental concerns are aspects
requiring attention.
- Perovskite solar cell exhibits low stability, so we will attempt to find a perfect method to
increase its stability.
- Band gap is also an important factor which mainly depends on choice of materials. This
band gap can be tune by exchanging halide ions.

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Optical losses in the solar cells
Optical losses – 30% of the incident light is lost due to:
• Multiple refraction and
reflection during traveling
through different layers.
• Polycrystalline nature of
some semiconductors
with multiple internal
reflections at the grain
boundaries.
• Surface roughness of
the layers (microscopic
image of layer’s cross-
section).
• Front electrode - transparent (indium-tin oxide ITO, 300-400 nm). It incorporates thin
and narrow metal strips, mostly silver (10-15 nm), forming a metal grid that does not
interfere the transparency, but help to collect the generated charge.
• Anti-reflection coating - optical bandwidth filter transmitting the visible light range. At
the same time, it must be rejecter or reflective for the infrared (thermal) component of
the sun and UV protective. Suitable materials are SiO2 and TiO2, alternating in a certain
sequence and with a thickness (20-100nm), according to their refractive index.

25. 25
Optical filtering in the perovskite solar cells – our results
Sputtering of ZnO-doped by Ga (GZO) and co-sputtering with indium-tin oxide (ITO)
Atomic Force Microscopic (AFM) images of sputtered on glass a) single GZO film;
b) bi-layer ITO/GZO without additional oxidation; c) with 10 % and d) with 20% of
additional oxidation.
GZO – average roughness of 13 nm ITO(70 nm)/GZO(40 nm)- avg. roughness 16 nm
ITO/GZO1 – roughness 9.5 nm ITO/GZO2 - roughness 7.1nm

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Optical filtering in the perovskite solar cells – our results
Sputtering of ZnO-doped by Ga (GZO) and co-sputtering with indium-tin oxide (ITO)
200 300 400 500 600 700
0
20
40
60
80
100
GZO
ITO/GZO
ITO/GZO1
ITO/GZO2
Transmittance,%
Wavelength, nm
0.8 1.2 1.6 2.0 2.4
10
20
30
40
50
60
70
80
90
100
ITO/GZO2
ITO/GZO1
ITO/GZO
GZO
Reflectance,%
Wavelength, m
Optical transmittance in the UV-VIS range and reflection in the NIR range of single layer
of GZO and bi-layer coatings ITO/GZO without and with additional oxidation during
sputtering.
Transmission of visible light enhanced from 90.9 % to 93.3%, rejection of the infrared
component greater than 65 % for ITO/GZO system with additional oxidation of GZO
during sputtering.

27. 27
E, eV
3
4
5
6
4.3
ITO
4.4
4.23
GZO
GZO2 CdS/ZnSQD
perovskite
3.9
6
4.1
Al
3.8
5.3
Optical filtering in the perovskite solar cells – our results
Sputtering of ZnO-doped by Ga (GZO) and co-sputtering with indium-tin oxide (ITO)
Energy band diagram of CdS/ZnS core-
shell quantum dots/perovskite solar cell
with optimal ITO/GZO2 film as
transparent conductive electrode.
Results are from ultraviolet
photoelectron spectroscopy (UPS).
Fabrication of Transparent ITO/Ga-Doped ZnO Coating as a Front Panel Electrode
toward Efficient Thin Film Solar Cells, Mariya Aleksandrova, Tsvetozar Tsanev, Tatyana
Ivanova, Kostadinka Gesheva, Velichka Strijkova, Jai Singh, Ajaya Kumar Singh ,
Published: 13 May 2020 by MDPI AG in 2nd Coatings and Interfaces Web Conference
session Advances in Coatings and Surface Characterization , submitted for Coatings
MDPI.

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Alternative Energy Sources, Materials & Technologies (AESMT’20) – Monday 8th of
June virtual session, Role of the absorber layer in the thin film solar cells with
perovskites, M. P. Aleksandrova, G. D. Kolev, R.Tomov, A. K. Singh, K. C. Mohite,
G.H.Dobrikov.
ZnS/CdSe core-shell /(CH3NH3)3Sb2ClxI9-x
0.1 0.2 0.3 0.4 0.5
0
4
8
12
16
with sulphide layer
without sulphide layer
Internalefficiency,%
PI
, W/cm
2
Dependence of the current density on
the thickness of the absorbing layer.
Comparison between the efficiency of the
solar cell without and with buffer
absorbing layer with optimal thickness.

29. 29
Future work related to comparison of bulk vs. thin film solar cells
Advantages of thin film technology for photovoltaic cell production over bulk cells:
- a small thickness that results in a small diffusion length of the charge carriers;
- high speed of extraction of the charges through the contacts;
- material savings and low weight – cost-efficiency and portability;
- possibility of using several types of microelectronic technologies for obtaining the
thin films with a precise adjustment of the microstructure and composition, and
thus fin tuning of the electro-optical parameters;
- possibility to grow the cell films on flexible substrates.
Disadvantage:
- possible parasitic recombination of generated electron-hole pairs due to their
difficult separation into the thin nano-sized film.
- if vacuum-free deposition processes are used, impurities and defects could be
incorporated in the film. It has low density and the cell exhibit lower efficiency.

30. 30
Option for low-cost deposition of photoconductive film from solution (precursor).
The equipment is simple, the process is fast and the parameters for control are
small number (temperature, stream pressure, aerosol size (nozzle size)).
The technology to be used for solutions – spray deposition

31. 31
https://shakepeers.org
• An approach to increase the efficiency is to build
multilayer structure, in which every coating is
responsible for absorption of target wavelength
range. The order of deposition should be in line
with the semiconductor bandgap and its ability to
absorb preferentially shorter or longer
wavelengths.
Future works: Alternative to the multilayer technology is the core-shell technology
Keeping the structure simple, single
layer of dispersed core-shell quantum
dots with different absorbing abilities
can eventually replace the left structure.

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Volt-ampere characteristics at
different operational temperatures
and sun irradiation intensities
η – solar cell efficiency, %;
А – irradiated area, m2;
Popt – irradiance, W/m2
FF-fill factor;
PMPP – nominal power over
the load, W;
UOC – open circuit voltage;
ISC – short circuit current.
Integrated circuits developed to
process the signal from solar cells
BQ25504 - Texas Instruments Inc.
MAX17710 - Maxim Integrated Prod.
MAS6011 - Micro Analog Systems.
LTC4071 - Linear Technology Corp.
Basic parameters and characteristics of solar cells and implementation

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Thank you for your attention!