Photovoltaic Devices

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Best Material for Photovoltaic Devices

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Question # 1:
Explain best materials
photovoltaic devices?
Answer:

for

Photovoltaic (PV), or solar cells, can be made of many
semiconductor materials. Each material has unique strengths
and characteristics that influence its suitability for specific
applications. For example, PV cell materials may differ based on
their crystallinity, bandgap, absorbtion, and manufacturing
complexity.
Some solar cell materials are listed below :



Silicon :
(Si)—including single-crystalline Si, multicrystalline Si, and
amorphous Si

Polycrystalline Thin Films
including copper indium diselenide (CIS), cadmium telluride
(CdTe), and thin-film silicon

Single-Crystalline Thin Films

including high-efficiency material such as gallium arsenide
(GaAs).
CRYSTALLINITY
The crystallinity of a material indicates how perfectly ordered the
atoms are in the crystal structure. Silicon, as well as other solar
cell semiconductor materials, comes in various forms, including
single-crystalline,
multicrystalline,
polycrystalline,
and
amorphous these all are explained below.
Monocrystalline
Silicon
Photovoltaic
(PV)
Cells
Monocrystalline silicon PV cells are made from silicon wafers that
are cut from cylindrical single-crystal silicon ingots. Thus,
refined silicon is wasted in the cell production process.

Monocrystalline silicon shows predictable and uniform behaviour,
but, due to the careful and slow manufacturing processes
required, it is also the most expensive type of silicon.
Modules consisting of monocrystalline silicon PV cells reach
commercial efficiencies between 15 and 18

A solar cell made from a monocrystalline silicon wafer

Polycrystalline

Silicon

Photovoltaic

(PV)

Cells:

Polycrystalline or multicrystalline silicon PV cells are made from
large blocks of molten silicon, carefully cooled and solidified.
They are less expensive to produce than monocrystalline silicon
PV cells, but are marginally less efficient, with module
conversion efficiencies between 13 and 16 %.

Polycrystaline PV cells laminated to backing material in a PV
module

Thin Film Photovoltaic (PV) Cells
The various thin film technologies currently being developed
reduce the amount (or mass) of light absorbing material
required in creating a solar cell. This can lead to reduced
processing costs from that of bulk materials (in the case of
silicon thin films) but also tends to reduce energy conversion
efficiency (an average 6 to 12 % module efficiency).
Third

Generation

Solar

Cells

Currently there are solar cells based in different new technologies
in the way to market maturity, for example the high efficiency
cells:
Comparison of Different Types of PV Modules:
Cell material

Module

Surface area

efficiency

needed for 1

Advantages

Disadvantages

- most efficient PV modules

- most expensive

- easily available on the market

- waste of silicon in the

- highly standardised

production process

- less energy and time needed for

- slightly less efficient than

production than for monocrystalline cells

monocrystalline silicon

(= lower costs)

modules

kWp
Monocrystalline

15-18 %

7-9 m²

silicon

Polycrystalline

13-16 %

8-9 m²

silicon

- easily available on the market
- highly standardised
Micromorph tandem

6-9 %

9-12 m²

- more space for the same

(aµ-Si)
Thin film:

output needed
10-12 %

9-11 m²

- higher temperatures and shading have

- more space for the same

Copper indium

lower impact on performance

output needed

diselenide (CIS)

- lower production costs

Thin film:

- higher temperatures and shading have

- more space for the same

Cadmium telluride

lower impact on performance

output needed

(CdTe)

- highest cost-cutting potential

Thin film:

9-11 %

6-8 %

11-13 m²

13-20 m²

- higher temperatures and shading have

- more space for the same

Amorphus silicon (a-

lower impact on performance

output needed

Si)

- less silicon needed for production

Due to their high efficiency, market penetration and
standardisation mono- and polycrystalline silicon
modules are the most common modules used in solar
home systems (SHS). However, the market share of thin
film modules is growing, especially in free-standing
and building integrated PV systems.

BANDGAP
The bandgap of a semiconductor material is the minimum energy
needed to move an electron from its bound state within an
atom to a free state. This free state is where the electron can
be involved in conduction. The lower energy level of a
semiconductor is called the valence band, and the higher
energy level where an electron is free to roam is called
the conduction band. The bandgap (often symbolized by Eg) is
the energy difference between the conduction and valence
bands.
ABSORPTION
The absorption coefficient of a material indicates how far light with
a specific wavelength (or energy) can penetrate the material
before being absorbed. A small absorption coefficient means
that light is not readily absorbed by the material.
The absorption coefficient of a solar cell depends on two factors:
the material of the cell and the wavelength or energy of the
light being absorbed. Solar cell material has an abrupt edge in
its absorption coefficient because light with energy below the
material's bandgap cannot free an electron.

MANUFACTURING COMPLEXITY
The most important parts of a solar cell are the semiconductor
layers because this is where electrons are freed and electric
current is created. Several semiconductor materials can be
used to make the layers in solar cells, and each material has its
benefits and drawbacks.
The cost and complexity of manufacturing varies across materials
and device structuresbased on many factors, including
deposition in a vacuum environment, amount and type of
material used, number of steps involved, and the need to move
cells into different deposition chambers.
There are some other factors by concedering then we can get a
best material for photovoltaic devices
FACTORS AFFECTING EFFICIENCY
Much of the energy from sunlight reaching a PV cell is lost before it
can be converted into electricity. But certain characteristics of
solar cell materials also limit a cell's efficiency to convert the
sunlight it receives.
WAVELENGTH OF LIGHT
. Photons with energy less than the material's bandgap are not
absorbed, which wastes about 25% of incoming energy. The
energy content of photons above the bandgap is wasted
surplus—re-emitted as heat or light—and accounts for an
additional loss of about 30%. Thus, the inefficient interactions
of sunlight with cell material waste about 55% of the original
energy.so we have to chose a material which response to the
suitable wavelength of light .
RECOMBINATION
Charge carriers—which are electrons and holes—in a solar cell
may inadvertently recombine before they make it into the
electrical circuit and contribute to the cell's current. Direct
recombination, in which light-generated electrons and holes
randomly encounter each other and recombine, is a major
problem for some materials. Other materials experience

indirect recombination, in which electrons or holes encounter
an impurity, defect in the crystal structure, or interface or
surface that makes it easier for them to recombine.
NATURAL RESISTANCE
The natural resistance to electron flow in a cell decreases cell
efficiency. These losses predominantly occur in three places: in
the bulk of the primary solar material, in the thin top layer
typical of many devices, and at the interface between the cell
and the electrical contacts leading to an external circuit.
TEMPERATURE
Solar cells work best at low temperatures, as determined by their
material properties. All cell materials lose efficiency as the
operating temperature rises. Much of the light energy shining
on cells becomes heat, so it is good to either match the cell
material to the operation temperature or continually cool the
cell.
REFLECTION
A cell's efficiency can be increased by minimizing the amount of
light reflected away from the cell's surface. For example,
untreated silicon reflects more than 30% of incident light. Most
commonly, a special coating is applied to the top layer of the
cell. A single antireflective layer will effectively reduce
reflection only at one wavelength. Better results, over a wider
range of wavelengths, are possible with multiple antireflective
layers.
ELECTRICAL RESISTANCE
Larger electrical contacts can minimize electrical resistance, but
covering a cell with large, opaque metallic contacts would block
too much incident light. Therefore, a trade off must be made
between loss due to resistance and loss due to shading effects.
Typically, top-surface contacts are designed as grids, with many
thin, conductive fingers spread over the cell's surface.

Thus We Concluded That On
Concedering All These We Can
Construct A Best Material For
Phltovoltaic Devices

Question # 2:
Explain different types of
photovoltaic devices and
how they work ?
Answer :
Hundreds of solar cells (also called photovoltaic cells)
make up a solar photovoltaic (PV) array. Solar cells are
the components of solar arrays that convert radiant
light from the sun into electricity . Solar cells contain
materials with semiconducting properties in which their
electrons become excited and turned into an electrical
current when struck by sunlight. While there are dozens
of variations of solar cells.The are 3 basic types of
construction of PV panels

Monocrystalline cells are cut from a single crystal of
silicon- they are effectively a slice from a crystal.
In appearance, it will have a smooth texture and you
will be able to see the thickness of the slice.
These are the most efficient and the most expensive
to produce. They are also rigid and must be mounted
in a rigid frame to protect them.

Polycrystalline (or Multicrystalline)
cells are effectively a slice cut from a block of silicon,
consisting of a large number of crystals.
They have a speckled reflective appearance and again
you can you see the thickness of the slice.
These cells are slightly less efficient and slightly less
expensive than monocrystalline cells and again need
to be mounted in a rigid frame.

Amorphous
cells are munufactured by placing a thin film of
amorphous (non crystalline) silicon onto a wide choice
of surfaces. These are the least efficent and least
expensive to produce of the three types. Due to the
amorphous nature of the thin layer, it is flexible, and if
manufactured on a flexible surface, the whole solar
panel can be flexible.
One characteristic of amorphous solar cells is that
their power output reduces over time, particularly
during the first few months, after which time they are
basically stable. The quoted output of an amorphous
panel should be that
.
There are some othertypes of photovoltaic materials
Concentrator photovoltaics (CPV)
is a photovoltaic technology that generates electricity
from sunlight. Contrary to conventional photovoltaic
systems, it uses lenses and curved mirrors to focus
sunlight onto small, but highly efficient, multijunction (MJ) solar cells. In addition, CPV systems often
use solar trackers and sometimes a cooling system to

further increase their efficiency. Ongoing research and
development is rapidly improving their competitiveness
in the utility-scale segment and in areas of high
solar insulation. This sort of solar technology can be
thus used in smaller areas.
Especially systems using high concentrator
photovoltaic (HCPV), have the potential to become
competitive in the near future. They possess the
highest efficiency of all existing PV technologies, and a
smaller photovoltaic array also reduces the balance of
system cost
Dye-sensitized solar cell
(DSSC, DSC or DYSC) is a low-cost solar cell belonging
to the group of thin film solar cells. It is based on
a semiconductor formed between a photo-sensitized
anode and an electrolyte, a photo
electrochemical system. The modern version of a dye
solar cell, also known as the Grätzel cell.
Hybrid solar cells
combine advantages of both organic and
inorganic semiconductors. Hybrid photovoltaics have
organic materials that consist of conjugated
polymers that absorb light as the donor and
transport holes.[1] Inorganic materials in hybrid cells are
used as the acceptor and electron transporter in the
structure. The hybrid photovoltaic devices have a
potential for not only low-cost by roll-to-roll processing
but also for scalable solar power conversion.

Cadmium Telluride Solar Cells

Cadmium Telluride is the only of the thin-film materials
that have been cost-competitive with crystalline silicon
models. In fact, in recent years, some cadmium models
have surpassed them in terms of their costeffectiveness. Efficiency levels result in a range of 911%.
Copper Indium Gallium Selenide Solar Cells
Copper Indium Gallium Selenide cells have
demonstrated the most promise with respect to their
efficiency levels that range from 10-12%, somewhat
comparable to crystalline technologies. However, these
cells are still in the nascent stages of research and
have been commercial deployed on any wide scale.
That said, the technology is most used in larger or
commercial applications.
luminescent solar concentrator (LSC)
is a device for concentrating radiation, non-ionizing
solar radiation in particular, to produce electricity.
Luminescent solar concentrators operate on the
principle of collecting radiation over a large area,
converting it by luminescence (commonly specifically
by fluorescence) and directing the generated radiation
into a relatively small output target.
Multi-junction (MJ) solar cells
are solar cells with multiple p–n junctions made
of different semiconductor materials. Each material's pn junction will produce electric current in response to
different wavelengths of light. The use of
multiple semiconducting materials allows the
absorbance of a broader range of wavelengths,
improving the cell's sunlight to electrical energy
conversion efficiency.

Nanocrystal solar cells
are solar cells based on a substrate with
a coating of nanocrystals. The nanocrystals are
typically based on silicon, CdTe or CIGS and the
substrates are generally silicon or various organic
conductors. Quantum dot solar cells are a variant of
this approach, but take advantage of quantum
mechanical effects to extract further performance

Question # 3
Define and explain
absorption coefficient of
different photovoltaic
devices?
Answer :
The absorption coefficient of a material indicates how far light
with a specific wavelength (or energy) can penetrate the
material before being absorbed. A small absorption coefficient
means that light is not readily absorbed by the material. The
absorption coefficient of a solar cell depends on two factors:
1. The material of the cell and the
2. Wavelength or energy of the light being absorbed .

Solar cell material has an abrupt edge in its absorption
coefficient because light with energy below the material's
bandgap cannot free an electron. The absorption coefficient
determines how far into a material light of a particular wavelength
can penetrate before it is absorbed. In a material with a low
absorption coefficient, light is only poorly absorbed, and if the
material is thin enough, it will appear transparent to that
wavelength. The absorption coefficient depends on the material
and also on the wavelength of light which is being absorbed.
Semiconductor materials have a sharp edge in their absorption
coefficient, since light which has energy below the band gap does
not have sufficient energy to excite an electron into the
conduction band from the valence band. Consequently this light is
not
absorbed.
The
absorption
coefficient
for
several
semiconductor materials is shown below.

The absorption coefficient, α, in a variety of
semiconductor materials at 300K as a function of
the vacuum wavelength of light.

The above graph shows that even for those photons which have
an energy above the band gap, the absorption coefficient is not
constant, but still depends strongly on wavelength. The
probability of absorbing a photon depends on the likelihood of
having a photon and an electron interact in such a way as to
move from one energy band to another. For photons which have
an energy very close to that of the band gap, the absorption is

relatively low since only those electrons directly at the valence
band edge can interact with the photon to cause absorption. As
the photon energy increases, not just the electrons already having
energy close to that of the band gap can interact with the photon.
Therefore, a larger number of electrons can interact with the
photon and result in the photon being absorbed.
The absorption coefficient, α, is related
coefficient, k, by the following formula:

to

the

extinction

where λ is the wavlength. If λ is in nm, multiply by 107 to get
the absorption coefficient in the the units of cm -1.
Absorbtion depth:
The
relationship
between
absorption
coefficient
and
wavelength makes it so that different wavelengths penetrate
different distances into a semiconductor before most of the
light is absorbed. The absorption depth is given by the inverse
of the absorption coefficient, or α-1. The absorption depth is a
useful parameter which gives the distance into the material at
which the light drops to about 36% of its original intensity, or
alternately has dropped by a factor of 1/e. Since high energy
light (short wavelength), such as blue light, has a large
absorption coefficient, it is absorbed in a short distance (for
silicon solar cells within a few microns) of the surface, while red
light (lower energy, longer wavelength) is absorbed less
strongly. Even after a few hundred microns, not all red light is
absorbed in silicon. The variation in the absorption depth for
"blue" and "red" photons is shown below.
Optical properties of silicon:
The optical properties of silicon measure at 300K . While a wide
range of wavelengths is given here, silicon solar cells typical
only operate from 400 to 1100 nm.

Question #4
Explain photoconductivity and
photogeneration rate for
photovoltaic cell?
Photogeneration :
The generation rate gives the number of electrons
generated at each point in the device due to the
absorption of photons. Generation is an important
parameter in solar cell operation.
Neglecting reflection, the amount of light which is
absorbed by a material depends on the absorption
coefficient (α in cm-1) and the thickness of the
absorbing material. The intensity of light at any point in
the device can be calculated according to the equation:

where α is the absorption coefficient typically in cm -1;
x is the distance into the material at which the light
intensity
is
being
calculated;
and
I0 is the light intensity at the top surface.
The above equation can be used to calculate the
number of electron-hole pairs being generated in a
solar cell. Assuming that the loss in light intensity (i.e.,
the absorption of photons) directly causes the
generation of an electron-hole pair, then the

generation G in a thin slice of material is determined by
finding the change in light intensity across this slice.
Consequently, differentiating the above equation will
give the generation at any point in the device. Hence:
where N0 = photon flux at the surface (photons/unitarea/sec.);
α =absorption coefficient and
x = distance into the material.
The above equations show that the light intensity
exponentially decreases throughout the material and
further that the generation is highest at the surface of
the material.
For photovoltaic applications, the incident light consists
of a combination of many different wavelengths, and
therefore the generation rate at each wavelength is
different. The generation rate at different wavelengths
in silicon is shown below.
The wavelength is 800
coefficient of 850 cm-1

nm

with

an

absorption

Changing the slider in the graph above
changes the wavelength of the
incoming
light.
The
changing
absorption coefficient causes the light
to be absorbed at different depths. The
generation rate has been normalized.
To calculate the generation for a collection of different
wavelengths, the net generation is the sum of the
generation for each wavelength. The generation as a
function of distance for a standard solar spectrum
(AM 1.5) incident on a piece of silicon is shown below.
The y-axis scale is logarithmic showing that there is an

enormously greater generation of electron-hole pairs
near the front surface of the cell, while further into the
solar cell the generation rate becomes nearly constant.

Generation rate of electron-hole pairs
in a piece of silicon as a function of
distance into the cell. The cell front
surface is at 0 µ m and is where most
of the high energy blue light is
absorbed

Photoconductivity:
Photoconductivity is an optical and electrical
phenomenon in which a material becomes
more electrically conductive due to the absorption
of electromagnetic radiation such as visible
light, ultraviolet light, infrared light, or gamma
radiation.
Explanation :

When light is absorbed by a material such as
a semiconductor, the number of free electrons
and electron holes increases and raises its electrical
conductivity. To cause excitation, the light that strikes
the semiconductor must have enough energy to raise
electrons across the band gap, or to excite the
impurities within the band gap. When
a bias voltage and a load resistor are used in series
with the semiconductor, a voltage drop across the load
resistors can be measured when the change in
electrical conductivity of the material varies the current
through the circuit.
Photoconductivity of organic solar cells :
Experimental and theoretical studies of the electronic
structure of bulk heterojunction (BHJ) organic solar cells
are reported. The photoconductivity spectral response
of the solar cells has a weak absorption band extending
from the band-gap energy down to <1 eV due to
charge-transfer optical excitation at the interface
between the polymer and the fullerene. The low-energy
absorption indicates an exponential band tail of
localized states and an absorption model based on the
one-electron joint density of electronic states accounts
for the data. Transient photoconductivity
measurements of the carrier mobility exhibit a
temperature-dependent carrier dispersion. Data
analysis for the particular case of transport in the BHJ
structure is developed. A multiple trapping model of the
dispersive transport is consistent with localized band
tail states having a comparable density-of-states
distribution to those observed by optical absorption.
Photo conductivity of Indium selenide:
Indium selenide layers have been formed by successive
evaporations of indium and selenium and by direct

evaporation of indium selenide. The films are
photoconductive with a long wavelength cutoff at about
2 microns and peaks in their spectral response at 0.6
and 1.4 microns. A different time constant is associated
with each peak. In the visible region the time constant
is greater than 1 millisecond, while in the infrared it is
generally less than 100 microseconds.

Question #5
Write down practical /
commercial application of
photovoltaic devices?
1.Concentrating Solar Power (CSP):
Concentrating solar power (CSP) plants are utility-scale
generators that produce electricity using mirrors or
lenses to efficiently concentrate the sun’s energy. The
four principal CSP technologies are parabolic troughs,
dish-Stirling engine systems, central receivers, and
concentrating photovoltaic systems (CPV).
2.Solar Thermal Electric Power Plants:
Solar thermal energy involves harnessing solar power
for practical applications from solar heating to electrical
power generation. Solar thermal collectors, such as
solar hot water panels, are commonly used to generate
solar hot water for domestic and light industrial
applications. This energy system is also used in
architecture and building design to control heating and
ventilation in both active solar and passive solar
designs.

3. Photovoltaics:
Photovoltaic or PV technology employs solar cells or
solar photovoltaic arrays to convert energy from the
sun into electricity. Solar cells produce direct current
electricity from the sun’s rays, which can be used to
power equipment or to recharge batteries. Many pocket
calculators incorporate a single solar cell, but for larger
applications, cells are generally grouped together to
form PV modules that are in turn arranged in solar
arrays. Solar arrays can be used to power orbiting
satellites and other spacecraft, and in remote areas as
a source of power for roadside emergency telephones,
remote sensing, and cathodic protection of pipelines.
4.Solar Heating Systems:
Solar hot water systems use sunlight to heat water.
The systems are composed of solar thermal collectors
and a storage tank, and they may be active, passive or
batch systems.

5.Passive Solar Energy
It concerns building design to maintain its environment
at a comfortable temperature through the sun’s daily
and annual cycles. It can be done by (1) Direct gain or
the positioning of windows, skylights, and shutters to
control the amount of direct solar radiation reaching
the interior and warming the air and surfaces within a
building; (2) Indirect gain in which solar radiation is
captured by a part of the building envelope and then
transmitted indirectly to the building through
conduction and convection; and (3) Isolated gain which
involves passively capturing solar heat and then
moving it passively into or out of the building via a
liquid or air directly or using a thermal store.

Sunspaces, greenhouses, and solar closets are
alternative ways of capturing isolated heat gain from
which warmed air can be taken.
6.Solar Lighting:
Also known as daylighting, this is the use of natural
light to provide illumination to offset energy use in
electric lighting systems and reduce the cooling load on
HVAC systems. Daylighting features include building
orientation, window orientation, exterior shading, saw
tooth roofs, clerestory windows, light shelves, skylights,
and light tubes. Architectural trends increasingly
recognize daylighting as a cornerstone of sustainable
design.

7.Solar Cars:
A solar car is an electric vehicle powered by energy
obtained from solar panels on the surface of the car
which convert the sun’s energy directly into electrical
energy. Solar cars are not currently a practical form of
transportation. Although they can operate for limited
distances without sun, the solar cells are generally very
fragile. Development teams have focused their efforts
on optimizing the efficiency of the vehicle, but many
have only enough room for one or two people.
8.Solar Power Satellite:
A solar power satellite (SPS) is a proposed satellite
built in high Earth orbit that uses microwave power
transmission to beam solar power to a very large

antenna on Earth where it can be used in place of
conventional power sources. The advantage of placing
the solar collectors in space is the unobstructed view of
the sun, unaffected by the day/night cycle, weather, or
seasons. However, the costs of construction are very
high, and SPSs will not be able to compete with
conventional sources unless low launch costs can be
achieved or unless a space-based manufacturing
industry develops and they can be built in orbit from
off-earth materials.

9. Solar Updraft Tower:
A solar updraft tower is a proposed type of
renewable-energy power plant. Air is heated in a
very large circular greenhouse like structure, and the
resulting convection causes the air to rise and
escape through a tall tower. The moving air drives
turbines, which produce electricity. There are no
solar updraft towers in operation at present. A
research prototype operated in Spain in the 1980s,
and EnviroMission is proposing to construct a fullscale power station using this technology in
Australia.
10.
Renewable Solar Power Systems with
Regenerative Fuel Cell Systems:
NASA has long recognized the unique advantages of
regenerative fuel cell (RFC) systems to provide energy
storage for solar power systems in space. RFC systems
are uniquely qualified to provide the necessary energy
storage for solar surface power systems on the moon or
Mars during long periods of darkness, i.e. during the 14day lunar night or the12-hour Martian night. The nature
of the RFC and its inherent design flexibility enables it

to effectively meet the requirements of space missions.
And in the course of implementing the NASA RFC
Program, researchers recognized that there are
numerous applications in government, industry,
transportation, and the military for RFC systems as
well.

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