Solar cells convert sunlight directly into electrical power through the photovoltaic effect. They have several advantages such as being clean, renewable, and producing no pollution or greenhouse gases. Solar cells work by using semiconducting materials, usually silicon, to create a p-n junction. When sunlight hits the junction, electrons are knocked loose, creating an electrical current.

1. SOLAR CELL

2. Introduction
Power crisis is one of the major concerns in today’s world. Due to drive of the huge
number of modern technology, the demand of fuel is increasing day by day. At the
same time the natural sources are reducing. Another concern of using this fossil
fuel is to its’ detrimental effect on environment because burning of this fuel
generates carbon-di-oxide that ultimately increase the global warming due to
greenhouse effect. The renewable energy sources would be the promising
candidate for resolving the power crisis and environmental concern.
Renewable energy is the energy that come from natural resources like sunlight,
wind, tides, rain, biomass, geometrical, hydro etc that generates energy with
renewability. Among all the available renewable energy sources, solar energy
gained much interest because of the limitless availability and non-toxicity. Solar
cells are used for generating power from solar energy.
Solar cells are photovoltaic (PV) device that converts the energy of sunlight into
electrical power without any chemical reaction. Solar energy is abundant,
inexhaustible and clean. It is free from any kind of pollution, needs no moving parts,
consumes no fuel and requires little maintenance.

4. Solar Radiation Pattern
ALLAH HAS made the sun as origin of huge energy needed for driving various systems on
earth. The process by which this solar heat energy travels through space in the form of
electromagnetic waves is known as radiation. The sun can be considered as a blackbody
having a temperature of 5800K on its surface. The total energy emitted from the sun’s
surface is about 63,000,000 W/m2. It emits 1.1 x 10e20 Kw-hr/sec

5. Solar irradiance:
The solar energy varies because of the relative motion of the sun. This variations depend
on the time of day and the season. The amounts of solar energy arriving at the earth's
surface vary over the year, from an average of less than 0,8 kWh/m2 per day during winter
in the North of Europe to more than 4 kWh/m2 per day during summer in this region. The
difference is decreasing for the regions closer to the equator. Irradiance is defined as the
amount of electromagnetic energy incident on a surface per unit time per unit area. There
are different ways to measure light, however in terms of Solar and PV (photo voltaic)
technology the equation is shown below:
Irradiance = Power / Area

7. The sun also produces Gamma rays on its surface during the nuclear fusion process which is
converted into a lower energy before it reaches the earth’s surface. It also emits ultraviolet,
X-rays, infrared, visible light, radio waves. The spectrum of the solar radiation is in a range of
100 nm to 1 mm.
Ultraviolet C (UVC): it has a range of 100 to 280 nm wavelength. Ultraviolet means it has a
radiation at higher frequency than the violet light which also invisible to the human eye. The
most of it is absorbed by the atmosphere. Thus a very little portion of it reaches the surface
of the earth.
Ultraviolet B(UVB): The range spans from 280 to 315 nm which is also absorbed by the
atmosphere in a huge amount. It is responsible for the photochemical reaction leading to
the production of the ozone layer. It is very harmful for the health. it causes sunburn and
damages DNA.
Ultraviolet A(UVA):The range of its wavelength spans from 315 to 400 nm. This can also
damage DNA and is able to cause cancer via indirect routes.
Visible light: Spans from 380 to 780nm. It is visible to human eye as its name suggests. It is
the strongest output range of the sun’s total irradiance spectrum.
Infrared: spans from 700 nm to 1,000,000 nm. It is a type of electromagnetic radiation
which is invisible to human eye. On the basis of wavelength it can be divided into three
parts:
Infrared-A: 700 nm to 1,400 nm
Infrared-B: 1,400 nm to 3,000 nm
Infrared-C: 3,000 nm to 1 mm.
Solar radiation:

8. Photo Voltaic or PV Mechanism
Photovoltaics is the direct conversion of light(photon) into electricity at the atomic
level. Some materials exhibit a property known as the photoelectric effect that
causes them to absorb photons of light and release electrons. When these free
electrons are captured, an electric current results that can be used as electricity.
The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in
1839, who found that certain materials would produce small amounts of electric
current when exposed to light. In 1905, Albert Einstein described the nature of light
and the photoelectric effect on which photovoltaic technology is based, for which
he later won a Nobel prize in physics. The first photovoltaic module was built by Bell
Laboratories in 1954

9. Bandgap diagram of conductor, semiconductor and Insulator

10. Conductors: At absolute zero temperature a large amount of electrons remain on the
conduction band. The valance band and conduction overlaps each other. There is no
forbidden energy gap. As the resistance being very low, a huge number of charge carriers are
available here. So, electricity can pass easily through the conductors.
Insulators: Electricity cannot pass through these materials. The valance band remains full of
electrons and the conduction band remains empty. The forbidden energy gap between the
conduction band and the valence band is widest. The difference is more than 10ev. A large
amount of energy is needed to cross the forbidden energy gap.
Semiconductors: The electrical conductivity is between conductors and insulators. The
forbidden energy gap of a semiconductor is nearly same as insulator and is narrower. The
value of Eg=1.1eV for silicon crystal and Eg=0.7eV for germanium at 0K. A semiconductor
remains partially full valence band and partially full conduction band at the room
temperature. So, silicon and germanium are insulators at absolute zero temperature. On the
other hand with the increasing of temperature the electrical conductivity of semiconductors
increases.

11. Working Principle of Solar Cell
P-type and N-type Materials
Atomic distributions of Si(2+8+4); B(2+3), and P(2+8+5) .Each silicon atom has four
electrons in its valance band and these electrons make bonds with other Silicon atom
Doping process is needed to obtain n-type or p-type Si.
Si- atom N-type Si- atom P-type Si- atom

12. Working Principle of Solar Cell
P-N Junction:
When we bring p-type and n-type material together, a diffusion occurs on the surface between
them. Electrons starts to diffuse from n-type to p-type. Similarly, holes diffuses from p-type
region to n-type region. This diffusion creates aelectron-hole free region in a very short
distance at the interface region. This thin layer is called depletion region.

13. Working Principle of Solar Cell
The solar cell is considered as a major candidate for obtaining energy from the sun as it can
convert sunlight directly to electricity with high conversion efficiency. It can provide nearly
permanent power at low operating cost, and is almost free of pollution.

14. Solar Cell Device Structure:
Solar cells are mainly formed by creating p-n junctions region in semiconductors. Figure
shows a cross-sectional schematic of a typical solar cell.

15. The first layer is called emitter layer which is usually more heavily doped and thinner than
the base layer. The base layer is thicker and lightly doped and most of the light absorption
happens in this layer. Lower doping causes higher minority carrier lifetime and higher
diffusion coefficients, which in turn improves the carrier diffusion length. Lower doping in
the base increases the depletion region in the base, which can aid in carrier collection due to
the electric field. If the doping is too low it may increase the dark current, which may
degrade the performance.
Solar Cell Device Structure:

16. Silicon solar cells are of different types:
i) Mono crystalline silicon solar cell,
ii) Polycrystalline solar cell,
iii) Amorphous silicon solar cell.
Types of Solar Cell
Monocrystalline solar cells, also called “single crystalline” cells are considered to be made
from a very pure type of silicon. Polycrystalline solar cells, also known as polysilicon and
multi-silicon cells . Amorphous silicon solar cells belong to the category of silicon thin-film.
The word “amorphous” literally means shapeless. The silicon material is not structured or
crystallized on a molecular level, as many other types of silicon-based solar cells are. It is
made by layering several photovoltaic materials deposited onto a substrate.

19. Appealing Characteristics
• Consumes no fuel
• No pollution
• Wide power-handling capabilities
• Solar power helps to slow/stop global warming.
• Solar power saves money.

21. Solar cell – Working Principle