3. solar cell

Theory of Solar Cell
 Before we move towards solar energy collector or solar cell we
have to focus towards following points:
 Solar constant
◦ Sun diameter = 1.39 x 106 km
◦ Earth diameter = 1.27 x 104 km
◦ Mean distance = 1.5 x 108 km
◦ The rate at which solar energy arrives at the top of the atmosphere is called solar
constant. This is the amount of energy received in unit time on a unit area
perpendicular to the sun’s direction at the mean distance of the earth from the
sun.
◦ NASA standard value for solar constant is 1353 watt/ m2
 Solar Radiation
◦ Solar radiation that penetrate the earth’s atmosphere and reaches the surface
differs in both amount and character.
◦ Radiation reflected back by the clouds, then absorbed by the molecules in air.

◦ O2 and O3 absorb all UV radiation.
◦ Water vapors and CO2 absorb infra red radiation.
◦ Solar radiation which not absorbed and reaches to the ground is known as direct
radiation or beam radiation.
 Solar radiation geometry

PH 0101 Unit-5 Lecture-2 5
Introduction
Photovoltaic effect
Electron-hole formation
A solar panel (or) solar array
Types of Solar cell
Principle, construction and working of
Solar cell
Advantage, disadvantage and
application
UNIT-1 LECTURE 2

*Photo means light in Greek and Volt is the name of a
pioneer in the study of electricity .
Solar cell: Solar cell is a photovoltaic device that converts
the light energy into electrical energy based on
the principles of photovoltaic effect
Albert Einstein was awarded the 1921 Nobel Prize in physics
for his research on the photoelectric effect—a phenomenon
central to the generation of electricity through solar cells.
In the early stages, the solar cell was developed only with 4 to
6 % efficiency( because of inadequate materials and problems
in focusing the solar radiations). But, after 1989, the solar cells
with more than 50% efficiency was developed.
1. Introduction

Three generations of solar cells
First Generation
First generation cells consist of large-area, high quality
and single junction devices.
First Generation technologies involve high energy and
labour inputs which prevent any significant progress in
reducing production costs.

Second Generation
Second generation materials have been developed to
address energy requirements and production costs of
solar cells.
Alternative manufacturing techniques such as vapour
deposition and electroplating are advantageous as they
reduce high temperature processing significantly

Materials for Solar cell
Solar cells are composed of various semiconducting materials
1. Crystalline silicon
2. Cadmium telluride
3. Copper indium diselenide
4. Gallium arsenide
5. Indium phosphide
6. Zinc sulphide
Note: Semiconductors are materials, which become
electrically conductive when supplied with light or heat, but
which operate as insulators at low temperatures

• Over 95% of all the solar cells produced worldwide are
composed of the semiconductor material Silicon (Si). As the
second most abundant element in earth`s crust, silicon has
the advantage, of being available in sufficient quantities.
• To produce a solar cell, the semiconductor is contaminated
or "doped".
• "Doping" is the intentional introduction of chemical
elements into the semiconductor.
• By doing this, depending upon the type of dopant, one can
obtain a surplus of either positive charge carriers (called
p-conducting semiconductor layer) or negative charge
carriers (called n-conducting semiconductor layer).

• If two differently contaminated semiconductor layers are
combined, then a so-called p-n-junction results on the
boundary of the layers.
• By doping trivalent element, we get p-type semiconductor.
(with excess amount of hole)
• By doping pentavalent element, we get n-type
semiconductor ( with excess amount of electron)
n-type semiconductor
p- type semiconductor
p-n junction layer

2. Photovoltaic effect
Definition:
The generation
of voltage across the
PN junction in a
semiconductor due
to the absorption of
light radiation is
called photovoltaic
effect. The Devices
based on this effect
is called photovoltaic
device.
Light
energy
n-type semiconductor
p- type semiconductor
Electrical
Power
p-n junction

3. electron-hole formation
• Photovoltaic energy conversion relies on the number
of photons strikes on the earth. (photon is a flux of
light particles)
• On a clear day, about 4.4 x 1017 photons strike a square
centimeter of the Earth's surface every second.
• Only some of these photons - those with energy in
excess of the band gap - can be converted into
electricity by the solar cell.
• When such photon enters the semiconductor, it may
be absorbed and promote an electron from the
valence band to the conduction band.

• Therefore, a vacant is created in the valence band and it is
called hole.
• Now, the electron in the conduction band and hole in
valence band combine together and forms electron-hole pairs.
hole
Valence band
Conduction band
electron
Photons

4. A solar panel (or) Solar array
Single solar cell
• The single solar cell constitute the n-typpe layer
sandwiched with p-type layer.
• The most commonly known solar cell is configured as a
large-area p-n junction made from silicon wafer.
• A single cell can produce only very tiny amounts of electricity
• It can be used only to light up a small light bulb or power a
calculator.
• Single photovoltaic cells are used in many small electronic
appliances such as watches and calculators

N-type
P-type
Single Solar cell

Solar panel (or) solar array (or) Solar module
The solar panel (or) solar array is the interconnection of
number of solar module to get efficient power.
• A solar module consists of number of interconnected
solar cells.
• These interconnected cells embedded between two
glass plate to protect from the bad whether.
• Since absorption area of module is high, more energy
can be produced.

Based on the types of crystal used, soar cells can be classified as,
1. Monocrystalline silicon cells
2. Polycrystalline silicon cells
3. Amorphous silicon cells
1. The Monocrystalline silicon cell is produced from
pure silicon (single crystal). Since the Monocrystalline
silicon is pure and defect free, the efficiency of cell will be
higher.
2. In polycrystalline solar cell, liquid silicon is used as raw material
and polycrystalline silicon was obtained followed by solidification
process. The materials contain various crystalline sizes. Hence,
the efficiency of this type of cell is less than Monocrystalline cell.
5. Types of Solar cell

3. Amorphous silicon was obtained by depositing silicon
film on the substrate like glass plate.
•The layer thickness amounts to less than 1µm – the
thickness of a human hair for comparison is 50-100 µm.
•The efficiency of amorphous cells is much lower than that
of the other two cell types.
• As a result, they are used mainly in low power
equipment, such as watches and pocket calculators,
or as facade elements.

Comparison of Types of solar cell
Material Efficiency (%)
Monocrystalline silicon 14-17
Polycrystalline silicon 13-15
Amorphous silicon 5-7

6. Principle, construction and working of Solar cell
Principle: The solar cells are based on the principles
of photovoltaic effect.The photovoltaic effect is the
photogeneration of charge carriers in a light absorbing
materials as a result of absorption of light radiation.
Construction
• Solar cell (crystalline Silicon) consists of a n-type
semiconductor (emitter) layer and p-type semiconductor
layer (base). The two layers are sandwiched and hence
there is formation of p-n junction.
• The surface is coated with anti-refection coating to avoid the
loss of incident light energy due to reflection.

Material for solar coating
 silicon nitride, silica (SiO2), Titania
and zinc oxide.
 There several techniques to fabricate
them , which mainly depend on the
type of application.

• A proper metal contacts are made on the n-type and p-
type side of the semiconductor for electrical connection
Working:
• When a solar panel exposed to sunlight , the light energies
are absorbed by a semiconduction materials.
• Due to this absorded enrgy, the electrons are libereted
and produce the external DC current.
• The DC current is converted into 240-volt AC current using
an inverter for different applications.

Mechanism:
• First, the sunlight is absorbed by a solar cell in a solar
panel.
• The absorbed light causes electrons in the material to
increase in energy. At the same time making them free to
move around in the material.
• However, the electrons remain at this higher energy for
only a short time before returning to their original lower
energy position.
• Therefore, to collect the carriers before they lose the
energy gained from the light, a PN junction is typically
used.

• A PN junction consists of two different regions of a
semiconductor material (usually silicon), with one side
called the p type region and the other the n-type region.
• During the incident of light energy, in p-type material,
electrons can gain energy and move into the n-type region.
• Then they can no longer go back to their original low
energy position and remain at a higher energy.
• The process of moving a light- generated carrier from
p-type region to n-type region is called collection.
• These collections of carriers (electrons) can be either
extracted from the device to give a current, or it can remain in
the device and gives rise to a voltage.

• The electrons that leave the solar cell as current give
up their energy to whatever is connected to the solar
cell, and then re-enter the solar cell. Once back in the
solar cell, the process begins again:

The mechanism of electricity production- Different stages
Conduction band High density
Valence band Low density
E
The above diagram shows the formation of p-n junction in a solar
cell. The valence band is a low-density band and conduction
band is high-density band.

Stage-1
Therefore, the hole
(vacancy position left
by the electron in the
valence band) is
generates. Hence, there
is a formation of
electron-hole pair on
the sides of p-n
junction.
When light falls on the semiconductor surface, the electron
from valence band promoted to conduction band.
E

Stage-2
In the stage 2, the electron and holes are diffuse across the
p-n junction and there is a formation of electron-hole pair.
E
junction

Stage-3
In the stage 3, As electron continuous to diffuse, the negative
charge build on emitter side and positive charge build on the
base side.
E
junction

Stage-4
When the PN junction is connected with external circuit, the
current flows.
E
junction
Power

7. Advantage, disadvantage and application of Solar cell
Advantage
1. It is clean and non-polluting
2. It is a renewable energy
3. Solar cells do not produce noise and they are totally
silent.
4. They require very little maintenance
5. They are long lasting sources of energy which can be
used almost anywhere
6. They have long life time
7. There are no fuel costs or fuel supply problems

Disadvantage
1. Soar power can be obtained in night time
2. Soar cells (or) solar panels are very expensive
3. Energy has not be stored in batteries
4. Air pollution and whether can affect the production
of electricity
5. They need large are of land to produce more
efficient power supply

Applications
1.Soar pumps are used for water supply.
1.Domestic power supply for appliances include
refrigeration, washing machine, television and lighting
1.Ocean navigation aids: Number of lighthouses and
most buoys are powered by solar cells
1.Telecommunication systems: radio transceivers on
mountain tops, or telephone boxes in the country can
often be solar powered
1.Electric power generation in space: To providing
electrical power to satellites in an orbit around the Earth

Solar Energy collector
 Flat plate collectors
 When temp. below about 90 ◦C are adequate and for water heating.
 They are made in rectangular panels from about 1.7 to 2.9 m2 in
area.
 Flat plate can collect and absorb both direct and diffuse solar
radiation.
 They are effective even on cloudy days when there is no direct
radiation.
 Flat plate solar collectors may be divided into two main
classification based on the type of heat transfer fluid used.
◦ Liquid heating collectors
◦ Solar air heater or air collectors
 The difference between the two types is the design of the
passages for the heat for the transfer fluid.

Major components
 A transparent cover which may be one or more sheets of glass or
radiation transmitting plastic film or sheet.
 Tubes, fins, or passages or channels are integral with the collector
absorber plate or connected to it, which carry the water, air or other
fluid.
 The absorber plate, normally metallic or with a black, surface,
although a wide variety of other materials can be used with air
heaters.
 Insulation, which should be provided at the back and sides to
minimize the heat losses. Standard insulating materials such as fiber
glass or styro foam are used for this purpose.
 the casing or container which enclose the other components and
protects them from the weather.

Typical liquid collector
 There are many flat plate collector design, but most are based on
principle as shown in diagram:
 It is the plate and tube type collector.
 It basically consists of a flat surface with high absorptive for solar
radiation, called the absorbing surface.
 A metal plate usually of copper, steel or aluminum materials with
tubing of copper in thermal contact with the plate, are the most
commonly used materials.

Fluid (water and ehtylene
glycol)
 The absorber plate is usually made from a metal sheet 1 to 2 mm in
thickness, while the tube range in diameter 1 to 1.5 cm.
 they are soldered to the bottom or top of the absorber plate with the
pitch ranging from 5 to 15 cm.
 Heat is transferred from the absorber plate to a point of use by
circulation of fluid (usually water) across the solar heated surface.
 Thermal insulation of 5 to 10 cm thickness is usually placed behind
the absorber plate to prevent the heat losses from the rear surface.
 Insulation materials is generally mineral wool or glass wool or fibre
wool.
 The front cover is generally glass and opaque to the infra red re-
radiation from the absorber. (act like convection shield).
 Thickness of glass i.e. 3 to 4 mm.
 Typical dimensions of collector = 2m x 1m x 15 cm

Concentrating Collector
 They are also known as focusing collector.
 The concentrating collector are used for medium (100-300◦C),
application such as steam production for generation of electricity.
 The high temperature is achieved at absorber because of reflecting
arrangement provided for concentrating the radiation required
location using mirror and lenses.
 These collector are best suited at place having more numbers of
clean days in a year.
 Concentrating collector employ systems in the form of reflectors to
concentrate the energy of direct solar radiation on the absorbing
surface.
 Application demanding temperature of 250◦F or higher usually
requires a concentrating collector.

 They are also called as focusing type collectors.
 The surface of a concentrating collector must be highly reflective,
enabling concentration of the sun’s rays on the heat absorption
device.
 Such collectors normally use optical system in the form of
reflectors.
 In these collectors, the solar radiation falling on a relatively large
area is concentrated on to receiver or absorber of significantly lesser
area.
 as a result of the energy concentration, fluids can be heated to
temperatures up to 500◦C or more.
 The heat transfer fluid can be a liquid or gas.

 Focusing type collectors concentrate only direct radiation coming
from a specific direction.
 Therefore, concentrating collector is usually a tracing collector in
order to keep the sun’s rays focused on a small surface.
 Concentrating collectors that accurately track the suns position are
more efficient that the non tracking one.
 Other type of concentrating, tracking collectors use movable
mirrors that can concentrate the solar energy on a small surface
that remains fixed.
 The angle of the individual mirrors are such that they reflect solar
radiation from a specific direction on to the same focal point.
 These fully tracking mirrors are usually computer controlled to
concentrate the maximum amount of solar energy.
 Application are mainly for steam generation used to produce
electric power.

Material for concentrating
collector
 The reflector should have high reflectivity. Therefore mirror glass
may be used.
 Glass is the most durable with low iron content and are used as
transmitting material.
 Now a days plastic are also in use. Acrylic is found to be good
material for fresnel lenses.
 Poly methyl methacrylate
is generally used.
 Aluminum and silver are
very good reflecting
surfaces.

 Glass and transparent plastic films are generally used as cover
material for receivers.
 Coating are required to have strong solar absorptivity, weather
resistance, stability at high temperature. Black paint is good.
 It can be electroplated on steel, copper, aluminum etc.

Advantages and disadvantages
of concentrating collector
 Advantage
 These collectors are used for high temperature application.
 Thermal losses are less.
 The efficiency increases at high temp.
 The size of the absorber can be reduced that gives high
concentration ratio.
 Disadvantages
 Non uniform flux on absorber.
 It collects only beam radiation because diffuse radiation
components can not be reflected, hence these are lost.
 Need costly tracking device.
 High initial cost.
 Need maintenance to retain the quality of reflecting surface against
dirt and oxidation.

Performance of concentrating
collectors
 The useful heat output Qc of a concentrating collector is given by:
 The optical efficiency of a concentrating collector is defined as the
ratio of the solar radiation absorbed by the absorber to the beam
radiation on the concentrator and is given by :

Solar Cell power plant
 As we discussed semiconductor materials such as silicon are used in
photovoltaic solar cell.
 In the cell incoming photons separate positive and negative charge
carriers.
 This produces and electrical voltage and electric current and can
drive a load.
 The solar cell can be connected in series and parallel and
incorporated in a module.
 Several modules may be interconnected to comprise a solar array,
but a large land area is required.
 In solar cell power plant we generally convert solar energy into
electrical energy.

 There are two types of solar power plants namely:
◦ Grid independent solar power plant
◦ Grid connected solar power plant
 Grid independent solar power plant
 Grid independent solar power plant are also known as stand alone
PV power plant or Autonomous solar power plant for supplying
the current and having no connection with grid.
 These power plants are used for local network such as solar water
pump, telecommunications, home power supply in rural areas.
 The main components of Autonomous solar power plant are as
follows:
◦ Photovoltaic array
◦ Storage battery
◦ Power conditioner

 (a) Photovoltaic array
 It consists of the required number of modules interconnected in
series and parallel to give desired system voltage and current.
 (b) Storage battery
 The battery supplies energy to the load during periods of little or no
solar irradiance and solar energy from the array during periods of
high irradiance.
 This enables the system to meet momentary peak power demands
and to maintain stable voltage to load.
 (c) Power conditioner
 Because the voltage output of the photovoltaic array varies with
insolation and temperature.

 systems with battery storage require voltage or shunt regulator to
prevent excessive overcharging of the battery.
 Further controls are used to prevent discharge or to ensure that the
array is operating at its maximum power point.
Figure shows the combined solar/wind/diesel power plant

 Grid connected solar power plant
 In block diagram photovoltaic array produces DC power and this
must be converted into AC power for local use and feeding into
grid.
 An inverter converts DC voltage to AC and feeds the solar power to
grid or supply to consumer.
 In case of low power availability from photovoltaic generation, the
local load can be fed from the grid.

s
 Photovoltaic arrays supplies the current only when sunlight falling
on it, the magnitude of direct current depends upon the intensity of
solar radiation.
 In grid connected system the power is fed into the grid during day
time and taking power from grid during night.
 Grid connected system require additional components to regulate
voltage, frequency and waveform to meet the requirement of
feeding the power into the grid.

Solar cell materials
 The solar cell are made of various materials.
 Silicon is the most commonly used material for the solar cell.
 The electrical properties of silicon depends on the type and amount
of dopants.
 Phosphorous and Boron are most widely used donor and acceptor
dopant in silicon.
 The choice of the material depend upon the energy band gap,
efficiency and cost.
 There are four important types of material which is used in solar
cells:
◦ Single crystal silicon
◦ Poly crystalline and Amorphous silicon
◦ Cadmium sulphide cadmium telluride
◦ Copper indium diselenide

3. solar cell

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