Depletion of the energy resources
https://www.theneweconomy.com/energy/fossil-fuels-are-filling-the-gap-in-rising-energy-demand
https://www.goodenergy.co.uk/blog/2020/02/04/swapping-coal-for-gas-is-no-way-to-fight-climate-change/
https://notenoughgood.com/2013/03/water http://energy.mit.edu/news/the-future-of-solar-energy-a-summary-and-recommendations-for-policymakers
Renewable and clean energy
source
Environment friendly
Low maintenance
Cost effective and resilient
power
Why there is a need of solar energy?

Applications of solar cell
https://www.emworks.com/application/solar-cell,
https://www.eqmagpro.com/lightyear-one-is-a-solar-car-with-a-range-of-450-miles/, https://www.sepco-solarlighting.com/solar-street-lighting
https://www.greensunpv.com/deep-well-submersible-solar-powered-water-pump-for-home_p40.html
https://www.solarreviews.com/blog/best-solar-powered-phone-chargers
Solar-powered car Solar-powered satellite Solar-powered buildings
Solar-powered street
lights
Solar-powered water
pumps
Solar-powered mobile
charger

What is a solar cell?
• A solar cell is a type of an electronic device which is able to harvest the energy from the sunlight
and produce electricity.
• It is also widely called as photovoltaic cell as it is based upon the principle of photovoltaic effect.
Components
of Solar cell
Light
absorber
Carrier
collector
Metal
contact
Typical solar cell device architecture

History of solar cell
• 1839- Antoine Cesar Becquerel- reported first photovoltaic effect with platinum electrode by using a silver salt
• 1873- Willoughby Smith- photoconductivity of selenium
• 1883- Charles Edgar Fritts- copper plates with selenium coating and gold thin layer to develop light sensitive
junction
• 1887- Heinrich Hertz and Wilhelm Hallwachs- independently studied UV light photoconductivity which led to
photoelectric effect discovery
• 1888- Aleksandr Stoletov- assembled a cell to demonstrate photoelectric effect
• 1905- Albert Einstein- postulated the photoelectric effect
• 1941- Russell Ohl- single junction Si semiconductor solar cell
• 1954- Pearson, Fuller and Chapin- reported first working PV solar cell
• 1970- Zhores Alferov- first GaAs heterostructure solar cells
• 1972- Hovel and Woodall- 18% efficiency with AlGaAs and GaAs solar cell
• 1976- Carlson and Wronski- 2.4% efficiency solar cell with amorphous silicon
• 1991- O’ Regan and Gratzel- photovoltaic cell call as dye sensitized solar cells with 12% efficiency
• The subsequent years saw the rise of various generation of solar cells at a very rapid pace.

Type/Classification of solar cells
First generation
cells
Monocrystalline and
Multi-crystalline
Silicon
Second generation
cells
Thin film solar cells (a-Si,
CdSe, CdTe CuInSe,
CuInGaSe, CZTS )
Third generation
cells
DSSC
Organic solar cells
Perovskite solar cells
Quantum dot solar cells
Alaaeddin et al., Renewable and Sustainable
Energy Reviews, 2019, 102, 318-332
https://www.hessolar.com/blog/2016/september/c
omparing-solar-cell-types/
https://infinitypv.com/technology/opv

First generation solar cells
• These are based upon either monocrystalline silicon or polycrystalline silicon.
Advantages:
1. Long lasting solar cell devices (device can last for up to 25 years).
2. Monocrystalline solar cell is the most efficient solar cell material in the market.
3. Stable cell in terms of humidity, heat and environmental atmospheres.
4. No toxic waste production throughout its operational conditions.
Disadvantages:
1. Initial cost of solar cell production is high.
2. At higher temperatures, significant reduction in efficiency
3. Difficult to fabricate on flexible substrates
4. High manufacturing cost as well as energy requirement.
Polycrystalline
silicon solar cell
Monocrystalline
silicon solar cell

Second generation solar cells
• Second generation solar cells are based upon thin film technologies.
• Amorphous silicon, CIGS, CdTe are commonly used solar cell materials.
Advantages:
1. Less hindrance on output with change in temperature.
2. Very efficient in areas with low sunlight.
3. Can be fabricated on flexible substrates.
Disadvantages:
1. Amorphous silicon has low efficiency.
2. The use of toxic Cd element is hazardous to the environment.
3. Proper disposal and recycling of cell is also an issue.
Flexible thin film
solar cell device

Third generation solar cells
• These are currently researched and future solar cell technologies.
• Multijunction solar cells, DSSC, organic solar cell, Perovskite solar cell, Quantum dot solar cells
typically form the third generation solar cell technologies.
Multijunction or
tandem solar cell
Dye sensitized
solar cell
Perovskite solar
cell

Solar cell device parameters
• Open circuit voltage (Voc)- The maximum voltage of solar cell, when current is zero.
• Short circuit current (Isc)- It is the current across the solar cell, when voltage is zero.
• Fill factor (FF)- It determines the maximum power of a solar cell device.
• Efficiency ()- It tells about the percentage of efficiency of a typical solar cell device.
• Short circuit current (Isc) For V = 0, I = Isc
• Open circuit voltage (Voc) For I = 0, V = Voc
• Fill factor (FF) =
I𝑚𝑎𝑥
×V𝑚𝑎𝑥
Isc
×Voc
=
Pmax
Isc
×V𝑜𝑐
• Power conversion efficiency (η):
=
Pmax
Pin
=
I𝑚𝑎𝑥
×V𝑚𝑎𝑥
Pin
=
FF ×Isc
× V𝑜𝑐
Pin
* Pin = incident light intensity 1000 W/ m2
J-V characteristic of a
typical solar cell device
Zhou et al., Energy Environ Sci, 2010, 3, 1851-1864

Efficiency trend of all solar cell technologies
NREL, http://www.nrel.gov/ncpv/images/efficiency_chart.2020

• Piezoelectricity or piezoelectric effect is ability of materials to generate voltage in response to
some mechanical stress applied on material.
• The origin of phenomenon is specific distribution of electric charges in crystal.
Schematic of Piezoelectric effect
• Piezoelectric effect can be either direct or reverse
piezoelectric effect
Kokkinopoulos et al. Energy Procedia50 (2014) Lee, Hyuck et al. Journal of Materials
Chemistry A 4.21 (2016)
Piezoelectricity

History of piezoelectric materials
Zhang et al., Journal of Korean Chemical Society, 55 (5), 2018, 419-439

Types of piezoelectric materials
Piezoelectric
materials
Piezoelectric
polymers
Piezoelectric
ceramics
PVDF
PET
PVDF-TrFE
Quartz
PZT
BaTiO3, ZnO

How piezoelectric effect occurs?
(A) (B) (C)
• In the initial stage, without any stress applied, molecule is in a neutral state (Fig. (a)).
• Under the application of stress, there is some change in crystal and negative and positive charges are separated and electric
dipoles are created (Fig. (b)).
• The opposite facing poles inside the material cancel each other and fixed charges appear on the surface. The material becomes
polarized (Fig. (c)).
Dahiya and Valle., Robotic tactile sensing: technologies and system, Springer Science & Business media, 2012

Why piezoelectric material are poled?
• In a macroscopic crystalline structure that comprises many unit cells, the dipoles are randomly oriented.
• When the material is subjected to a mechanical stress, each dipole rotates from its original orientation toward a direction that
minimizes the overall electrical and mechanical energy stored in the dipole.
• If all the dipoles are initially randomly oriented, the piezoelectric effect exhibited will be negligible.
• So, it is important to align the dipoles more or less in an oriented manner. This is done by process called poling.
• The direction along which the dipoles align is called as the poling direction.
• The application of electric field to material imparts poling and dipoles orient in the direction of field.
• After poling is removed, most of the dipoles are arranged in similar kind of orientation.
• By applying reverse electric field to the poling material or temperature change more than Curie temperature, material can be de-
poled.

Application of piezoelectric materials
• Piezoelectric nanogenerators- It is a type of energy harvesting device in which with the
application of mechanical energy on a nanostructured piezoelectric material, we can generate
electrical energy.
• Piezoelectric sensor- It is a type of device, in which we use piezoelectric phenomenon to
measure change in temperature, strain, pressure or force by converting them to electricity.
• Piezoelectric actuator- It is a type of device which contracts or expands when electrical
charge is applied, these in turn, generate motion and force.
• Piezoelectric medical devices- measure pulse rate, lung pressure, eye and brain pressure,
heartbeat and respiration detection, voice detection, hand and walk motion.
• Other applications- Ultrasonic cleaning device, sonar technology, transmitter and receiver,
microphones and sound transducers.

Application of piezoelectric materials as nanogenerators
(B) (D)
(A) (C) (A) Schematic of types of PZT based nanogenerator (a) crystalline
PZT ribbons on flexible PDMS substrate (b) optical microscopy
image of nanogenerator (c) Wavy/buckled PZT nanogenerator
(d) SEM images of wavy/buckled piezoelectric PZT ribbons via
a prestretched method. (e) Fabrication process of flexible PZT
thin film based nanogenerator using the laser lift-off method. (f)
Image of flexible PZT Nanogenerator. (Qi et al., Nano Letters,
11(3), 2011, 1331-1336).
(B) Fabrication of BaTiO3 - carbon based nanogenerator on plastic
substrate. (Park et al., Advanced Materials, 24(22), 2012, 2999-
3004).
(C) Flexible PENGs using piezoelectric monolayer MoS2 (a-c)
Operation scheme of piezoelectric NG based on MoS2
monolayer.(b-d) Morphological, performance characterization
of flexible single-layer PENG (e-g) Probing piezoelectric
property of free-standing monolayer MoS2. (Wu et al., Nature,
514(7523), 2014, 470-474).
(D) Piezoelectric PVDF nanogenerator (a) Near-field
electrospinning (NFES) combining direct-write, mechanical
stretching, and in-situ electrical poling to create and place
piezoelectric nanogenerator comprised of single PVDF
nanofiber, two electrodes and plastic substrate (c) Output
voltage measured with time under 2Hz strain (d) Output current
measured with respect to time under 2Hz strain. (Chang et al.,
Nano Letters, 10(2), 2010, 726-731).

Thermoelectricity
• A thermoelectric device is a type of an electronic device which is able to harvest the heat energy and produce
electricity.
• It is based upon two key effects:
1. Seeback effect- When conductive material is subjected to thermal gradient, charge carrier migrate along the
gradient and move from hot to cold, resulting in electric potential difference in the material. A thermocouple
measures the changes in potential.
2. Peltier effect- When electric current is passed through thermocouple, heat is released at one junction and
absorbed at the other junction.
Seeback
effect
Peltier
effect
https://www.scienceabc.com/pure-sciences/what-are-the-seebeck-effect-and-peltier-effect.html

History of Thermoelectric
• 1821- Seeback effect was first observed.
• 1834- Peltier effect was observed.
• 1851- Magnus stated that potential difference at two junctions is proportional to applied temperature difference.
• 1856- Thomson effect was observed.
• 1909 and 1911- Altenkirch introduced the term thermoelectric figure of merit.
• 1949- Ioffe proposed that thermal conductivity of semiconductors is function of atomic weight of material.
• 1954- Goldsmid studied variation of electrical conductivity with crystal structure and mobility of electron.
• 1993- Dresselhaus and Hicks proposed that superlattice structure can improve figure of merit.
• 2008- Bulusu and Walker proposed non-equilibrium green function method to find thermoelectric operformance of
silicon nanowires and its superlattices.

Thermoelectric materials
• Bismuth chalcogenides and their nanostructures-Bi2Te3, Bi2Se3 are best room temperature thermoelectric materials
with zT between 0.8-1.
• Lead telluride-PbTe achieves zT around 1.5 at 773K.
• Inorganic clathrates-these achieve zT~0.5 at room temperature and ~1.7 at 800K.
• Mg based compound and group 14 element- MgSn, MgSi alloys doped with Sn, Ga, Ag or Li can also be used as
thermoelectric material with zT~1.4.
• Oxide thermoelectric-Layered Ca3Co4O9 exhibit zT in range 1.4 to 2.7 at 900K.
• Conductive polymers such as Poly (3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI), polyacetylene,
polypyrrole, polycarbazole can also be used as thermoelectric material.
• 2D materials such as graphene oxide, transition metal dichalcogenides (MoS2, WSe2, MoSe2), phosphorene,
silicene, boron nitride, MXene, layered materials (SnSe, BiCuSeO), superlattice structures (Bi2Te3/Sb2Te3), black
phosphorous, silicene and germanene can also be used as thermoelectric materials.

Application of Thermoelectric materials
Fitriani et al., Renewable and Sustainable Energy Reviews, 64, 2016, 635-659

Parameters affecting performance of a thermoelectric device

Figure of merit and Power conversion efficiency
1. Figure of merit (zT)- It decides the overall heat to electricity conversion of a material. It is a dimensionless
quantity expressed as:
𝑍𝑇 =
𝑆2.𝜎
κ
𝑇 , where σ denotes electrical conductivity, S denotes the Seebeck coefficient, T denotes absolute
temperature and κ denotes thermal conductivity, respectively.
• To achieve maximum value of ZT, a large Seebeck coefficient (S), high electrical conductivity (σ), and low thermal
conductivity (к) are essential for a material.
2. Power conversion efficiency- The power conversion efficiency is denoted as
∆𝑇
𝑇ℎ𝑜𝑡
1−𝑧𝑇 −1
1−𝑧𝑇+
𝑇ℎ𝑜𝑡
𝑇𝑐𝑜𝑙𝑑
where T/Thot and zT dependent quantity give efficiency of device, Thot and Tcold denote the hot-
end and cold-end temperatures.
• Figure depicts the plot of thermoelectric efficiency
as function of temperature and zT
• 10-15% efficiency for zT=1
• 20-30% efficiency for zT=2
• Ideal case- zT  1 for achieving efficiency of more
than 10%
Fleurial, JOM, 2009, 61, 79-85

Variation in electrical properties
of a thermoelectric material as
function of carrier concentration
3. Charge carrier concentration (n)
• The number of charge carriers determine electronic conductivity and Seebeck coefficient.
• Carrier concentration increases with temperature, which improves electrical conductivity.
• Carrier concentration is affected by doping, alloying, grain boundaries, impurity, density, addition of dispersed
secondary phases.
Effect of carrier concentration on thermoelectric performance
• From plot, we infer, charge carrier concentration
effects both Seebeck coefficient and electrical
conductivity.
• Increased carrier concentration, increases
electrical conductivity and decreases Seebeck
coefficient.
Gayner and Kar, Progress in Materials Science 2016, 83, 330-382

4. Seebeck coefficient ()
• It depends on band gap, crystal structure, carrier concentration and varies with temperature in a non-linear manner.
• Typical values of Seebeck coefficient for metal, semiconductors and insulators are ~10, ~200 and >200µV/K.
• The Seebeck coefficient should be measured when temperature and voltage are in a steady state, voltage response
to temperature gradient varies in a linear manner.
Effect of Seebeck coefficient on thermoelectric performance
Effect of carrier concentration
on Seebeck effect of doped PbSe
and other materials
• In Na doped PbSe, Seebeck coefficient reduces
with carrier concentration. At 270C, Seebeck
coefficient for system is ~17µV/K with carrier
concentration of 17x1019 cm-3.
• Tl dopants have high Seebeck coefficient that
shows less variation with carrier concentration due
to tuning of resonant energy levels by Tl atoms in
Tl- PbTe system.
• K doped dopants have varied Seebeck coefficient.
• Nature of dopants and concentration affects the
Seebeck coefficient.
Gayner and Kar, Progress in Materials Science 2016, 83, 330-382

5. Effective mass (m*)
• It reflects the band curvature and doping is major parameter which affects effective mass.
• Parabolic band models are used for evaluation of electronic properties of thermoelectric materials, in which effective mass
determines the electronic structure and electronic transport.
• For non parabolic infinite-gap semiconductors, energy and temperature dependent mass affects thermoelectric material.
• In heavily doped semiconductors, at a given carrier concentration and temperature, thermopower increases with effective mass.
• Seebeck coefficient can be expressed using constant scattering time approximation as:
 =
4𝜋2𝑘𝑏
2
𝑒ℎ2 𝑚 ∗ 𝑇
4𝜋
3𝑛
2/3 ,for parabolic bands, thermopower  m*.
• The For isotropic parabolic bands, conductivity, carrier concentration and m* are also related by equation:
 =ne2τ/m*, where n is carrier concentration and τ is the scattering time.
• A high Seebeck coefficient can be attained by generating a large DOS effective mass, which decreases the mobility (i.e.,
conductivity).
• A lower effective mass leads to high ZT values and a greater effective mass to low ZT values; doping and temperature can be used
to tune the effective mass.
Effect of effective mass on thermoelectric performance

Effect of electrical & thermal conductivity on thermoelectric performance
6. Electrical conductivity ()
• Electrical conductivity of semiconductor depends on carrier mobility and concentration , denoted as:
 =e (µe. n + µh. p) where µe, µh , n, p denote the electron mobility, hole mobility, number density of electrons
and the number density of holes.
• Lattice and impurity scattering determine mobility of electrons and holes.
• With increase in temperature, lattice vibrations increase, causing decrease in mobility.
• Impurity scattering increases with a decrease in temperature, causing decrease in mobility.
• The type of material, doping, impurities, temperature affect electrical conductivity.
• Structural parameters such as grain size, strain and lattice constant also affect electrical conductivity.
7. Thermal conductivity (κ)
• The total thermal conductivity is sum of lattice thermal conductivity(κlattice) and charge-carrier thermal
conductivity(kc).
• A lower thermal conductivity can be achieved by maximizing the κc/κlattice ratio, which is achieved by lowering the
lattice thermal conductivity.
• Measurement of thermal conductivity is a good tool for investigating lattice defects or imperfections.
• The charge carrier thermal conductivity is changed by bipolar diffusion, when carrier concentration and mobility of
electrons and holes are comparable.
• The lattice thermal conductivity depends on the crystal structure and lattice parameters of material.
• Factors such as lattice parameter, density of material, anharmonic lattice vibration, temperature, all cause effect on
thermal conductivity in one way or other.
• For materials with small particle size, thermal conductivity is low as small grains cause increased phonon scattering.