Solar Photovoltaics

A talk on

“physics of solar cell ”

Organized by

Sameh,Viet,and He

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Solar Photovoltaics

Solar Cells

The heart of a PV system, are meant to absorb sunlight and convert it directly to electricity, they rely on the photoelectric effect( the ability of matter to emit electrons when a light incident on it).

Physics of solar cells

For best solar energy conversion the optimum band gap is ~ 1.0 - 1.5 eV.

Some of the best solar cell materials are: Silicon (1.12 eV), GaAs (1.42 eV), CdTe (~1.44 eV) and CuInSe2 (~1.0 eV).

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Band StructureConsider a semiconductor at T = 0 K, then there is no electrons in the conduction band.At T > 0 K a small fraction of electrons are thermally excited into the conduction band, “leaving” the same number of holes in the valence band.

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Extrinsic SemiconductorsElectrical Properties of Semiconductors:

It can be changed drastically by adding a small amounts of suitable impurities to the pure crystals(doping).Types of impurities atoms:– Interstitial: “foreign” atoms “squeezed”

between regular crystal sites.– Substitutional: “foreign” atoms occupying the

sites of host atoms

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DopingDoping

The addition of some impurities into a semiconductor according to our requirements.

In other words, impurities are introduced in a controlled manner to change the conductivity of the material.

Silicon – Doping (n-type & p-type)

• N-type semiconductors (for ex. doping with Phosphorous)

• P-type semiconductors.(for ex. doping with Boron)

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Donors We use Silicon (Si) as an

example– Si atoms have four valence

electrons that participate in covalent bonding

– When a Group V atom replaces a Si atom, it will use four of its electrons to form the covalent bonding.

The remaining electron will not be very tightly bound, and can be easily ionized at T > 0K

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Donors: Energy LevelsThe Band Structure View Such impurities “create” an

energy level within the band gap, close to the conduction band

They create so-called “shallow” levels , the levels that are very close to the conduction band, so the energy required to ionize the atom is small and a sizable fraction of donor atoms will be ionized at room temperature.

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The Single Crystalline Solar Cell

Pure silicon is a poor conductor of electricity.“Doping” of silicon with phosphorus and boron is necessary to create n-type and p-type regions ,this allows presence of free electrons and holes.The p-n junction generates an electric field that acts as a diode, pushing electrons to flow from the P side to the N side.

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The N-type material is keep thin to allow light to pass through to the PN junction.

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QUESTIONS ????

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The solar spectrum

Sunlight consists of a broad range of spectrumThe photon energy depends on the photon wavelength: Ephot = hc/λHarnessing the great amount of sunlight to energy

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Theoretical efficiency limits of single junction solar cells made out of various semiconductors

Energy Band Gaps in solar cell materials

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AcceptorsSubstitute one Group III atom (e.g. Al or In) with a Si (Group IV) atom

At T > 0 K, electron from the neighboring Si atom can jump into this hole – the hole starts to migrate, contributing to the current

At T > 0 K this hole can be ionized

Such semiconductors are called p-type semiconductors since they contribute positive charge carriers

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Acceptor: Energy LevelsFrom the Band Structure View– Such impurities “create” energy levels within the band gap,

close to the valence band

– They are similar to “negative” hydrogen atoms

– Such impurities are called hydrogenic acceptors

– They create “shallow” levels - levels that are very close to the valence band, so the energy required to ionize the atom (accept the electron that fills the hole and creates another hole further from the substituted atom) is small

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Types of silicon used for making solar cells

Type of Silicon Abbreviation Crystal Size Range Method for Production

Single-crystal silicon

c-Si >10cm Crystal growth by Czochralski (CZ),Float zone (FZ)

Multicrystalline silicon

mc-Si 1mm-10cm Cast : Sheet & ribbon

Micro crystalline silicon

c-Si 0.1 m – 1 m Plasma Enhanced Chemical-vapor deposition (PECVD)

Nano crystalline silicon

nc-Si <<1 m PECVD

Amorphous silicon

a-Si Crystallites absent PECVD

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