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Assignment No. 1 (CBCT Energy)

Q. Write about the working principle of solar cell. What are the different

types of solar cell are available. Write their characteristics and properties.

Write your own argument what kind of solar cell technology you would

suggest for North-East India.

A solar cell is an electronic device which directly converts sunlight into

electricity. Light shining on the solar cell produces both a current and a voltage

to generate electric power.

Operating Principle

Solar cell operation is based on the principle of photovoltaic effect.

Photovoltaic effect is he generation of a voltage difference at the junction of

two different materials in response to visible or other radiation.

The photovoltaic energy conversion process follows following steps-

1. Absorption of light –Generation of charge carriers

2. Separation of charge carriers

3. Collection of the carriers at the electrodes

A variety of materials and processes can potentially satisfy the requirements

for photovoltaic energy conversion, but in practice nearly all photovoltaic

energy conversion uses semiconductor materials in the form of a p-n junction.

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Assignment No. 1 (CBCT Energy)

• 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.

The mechanism of electricity production-

Different stages

The 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.

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Assignment No. 1 (CBCT Energy)

When light falls on the

semiconductor surface, the electron

from valence band promoted to

conduction band.

In the stage 2, the electron and

holes are diffuse across the p-

n junction and there is a

formation of electron-hole pair.

In the stage 3, As electron continuous to

diffuse, the negative charge build on emitter

side and positive charge build on the base

side.

When the PN junction is

connected with external circuit,

the current flows.

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Assignment No. 1 (CBCT Energy)

Different types of solar cells

1. Amorphous silicon cells

2. Cadmium telluride solar cell (CdTe)

3. Crystalline silicon solar cell (c-Si)

4. Monocrystalline silicon cells

5. Polycrystalline silicon cells

6. Dye-sensitized solar cell (DSSC)

7. Plasmonic solar cell

8. Polymer solar cell

9. Thin film solar cell (TFSC)

10. Quantum dot solar cell

1. Amorphous silicon is 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.

2. Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV)

technology that is based on the use of cadmium telluride, a thin

semiconductor layer designed to absorb and convert sunlight into

electricity.[1] Cadmium telluride PV is the only thin film technology with

lower costs than conventional solar cells made of crystalline silicon in

multi-kilowatt systems.

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Assignment No. 1 (CBCT Energy)

3. Crystalline silicon (c-Si) is an umbrella term for the crystalline forms of

silicon encompassing multicrystalline silicon (multi-Si) and

monocrystalline silicon (mono-Si), the two dominant semiconducting

materials used in photovoltaic technology for the production of solar

cells, that are assembled into a solar panel and part of a photovoltaic

system to generate solar power from sunlight.

4. 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.

5. Polycrystalline silicon, also called polysilicon or poly-Si, is a high purity,

polycrystalline form of silicon, used as a raw material by the solar

photovoltaic and electronics industry.

6. A 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 photoelectrochemical system.

7. Plasmonic solar cells are a class of photovoltaic devices that convert

light into electricity by using plasmons. Plasmonic solar cells are a type

of thin film solar cell which are typically 1-2μm thick. They can use

substrates which are cheaper than silicon, such as glass, plastic or steel.

The biggest problem for thin film solar cells is that they don’t absorb as

much light as thicker solar cells. Methods for trapping light are crucial in

order to make thin film solar cells viable. Plasmonic cells improve

absorption by scattering light using metal nanoparticles excited at their

surface plasmon resonance. [1] This allows light to be absorbed more

directly without the relatively thick additional layer required in other

types of thin-film solar cells.

8. A polymer solar cell is a type of flexible solar cell made with polymers,

large molecules with repeating structural units, that produce electricity

from sunlight by the photovoltaic effect. Polymer solar cells include

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Assignment No. 1 (CBCT Energy)

organic solar cells (also called "plastic solar cells"). They are one type of

thin film solar cell, others include the more stable amorphous silicon

solar cell.

9. A thin-film solar cell (TFSC), also called a thin-film photovoltaic cell

(TFPV), is a second generation solar cell that is made by depositing one

or more thin layers, or thin film (TF) of photovoltaic material on a

substrate, such as glass, plastic or metal.

10. A quantum dot solar cell is a solar cell design that uses quantum dots as

the absorbing photovoltaic material. It attempts to replace bulk materials

such as silicon, copper indium gallium selenide (CIGS) or CdTe.

Quantum dots have bandgaps that are tunable across a wide range of

energy levels by changing the dots' size. In bulk materials the bandgap is

fixed by the choice of material(s). This property makes quantum dots

attractive for multi-junction solar cells, where a variety of materials are

used to improve efficiency by harvesting multiple portions of the solar

spectrum. As of 2014 efficiency ranges from 7.0 to 8.7%.

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Assignment No. 1 (CBCT Energy)

I think, the solar cell technology that is most suitable for North Eastern

region is the hybrid solar cell technology because of its following benefits-

1. Regulated Power supply - The Hybrid Solar PV system provides power to

a dedicated load thereby avoiding use of DG sets which cannot be

regulated for a fixed power supply according to the requirement.

2. It senses the availability of solar power, grid power and gives charging

preference to the solar power charge and only switches to the grid when

the solar power is not available.

3. Drastic reduction of dependence of these sites on intermittent electricity

supplied from the grid.

4. Easy installation.

5. Affordable pricing.

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Assignment No. 1 (CBCT Energy)