ELG 4126 – DGD

Sustainable Electrical Power Systems

Winter 2015

Outline of this DGD

• Quiz 2 will be on March 16th

– 15-25 min long

– 5% of your grade

– Based on the last two DGDs

• Review from the last DGD

• Solar cells theory

– Semiconductors

– Bandgap impact on solar cell efficiency

– p-n junction

– p-n junction as a solar cell

– Current-voltage characteristics of a solar cell

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The effect of the atmosphere

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About 33% of AM0 irradiance is absorbed by the earth’s atmosphere.

ELG 4126 – V. Tatsiankou

The effect of the atmosphere

4March 2, 2015 ELG 4126 – V. Tatsiankou

Semiconductors

• Solar cells use semiconductors to convert light into electricity.

• The most popular semiconductor is silicon (Si).

• Si has 14 electrons orbiting its nucleus, which consists of 14 protons.

• Si has 4 electrons invalence band.

• Si atom forms 4 covalentbonds with other Si atoms to form a molecule.

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Semiconductors

• If the electrons in the valence band are promoted to the conduction band – current flow is possible.

• At room temperature the thermal energy can promoted an electron to the conduction band.

• For Si only 1 in 1010 electrons are located in the

conduction band at room temperature (poor conductor).

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Bandgap = Conduction band energy –

Valence band energy

Bandgap for Si = 1.12 eV

Bandgap impact on solar cells

• In solar cells, the photon gives its energy to an electron which is promoted to the conduction band (current flow).

• The photon must have the energy greater than a bandgap to excite the electron.

• The difference in bandgaps between the semiconductor materials determine what photons or wavelengths that material can absorb!

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Bandgap impact on solar cells

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Bandgap impact on solar cells

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For a blackbody spectrum at 6000 K

A p-n junction

• p-n junction is a basic building block of a solar cell.

• It is formed by joining p-type and n-type materials together.

• A p-type semiconductor is created by introducing dopants or impurities to a semiconductor, typically from group III elements, such as boron atoms (holes are the majority carriers).

• A n-type semiconductor is created by introducing dopants or impurities to a semiconductor, typically from group V elements, such as phosphorus atoms (electrons are the majority carriers).

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Unbiased p-n junction

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A p-n junction at equilibrium

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A p-n junction as solar cell

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Solar cell IV curves

• Diode equation

• Ideal solar cell IV curve

• Non-ideal solar cell IV curve

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Solar cell IV curves

• Solar cell equivalent circuit

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Solar cell IV curves

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Temperature effect

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Irradiance effect

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Parallel resistance effect

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Series resistance effect

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Questions

DGD - ELG 4126