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ENERGY CONVERSION
Lecture 2
Physic of Solar Cell
Content
▪Band gap energy
▪Solar spectrum
▪PN junction
▪Modeling of solar cell
▪I-V characteristic of solar cell
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Material for producing solar cell
Valence electron
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Classification of material
The band gap energy
▪The energy that an electron must acquire to jump across the
forbidden band to the conduction band is called the band-gap
energy, designated Eg.
▪The units for band-gap energy are usually electron-volts
(eV), where one electron-volt is the energy that an electron
acquires when its voltage is increased by 1 V (1 eV = 1.6 ×
10−19 J).
▪The band-gap Eg for silicon is 1.12 eV, which means an
electron needs to acquire that much energy to free itself from
the electrostatic force that ties it to its own nucleus—that is,
to jump into the conduction band.
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Electron-hole pair
Photon
▪Photons with enough energy create hole–electron pairs in a
semiconductor.
▪Photons can be characterized by their wavelengths or their
frequency as well as by their energy; the three are related by
the following:
c = λν
where c is the speed of light (3 × 108 m/s), v is the frequency
(hertz), λ is the wavelength (m), and
E = hν = hc/λ
Where E is the energy of a photon (J) and h is Planck’s
constant (6.626 × 10−34 J-s).
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Band gap of solar cell material
Solar Spectrum
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Solar constant
▪Just outside of the earth’s atmosphere, the average radiant
flux is about 1.377 kW/m2, an amount known as the solar
constant.
▪As solar radiation passes through the atmosphere, some is
absorbed by various constituents in the atmosphere, so that
by the time it reaches the earth’s surface the spectrum is
significantly distorted.
▪The amount of solar energy reaching the ground, as well as
its spectral distribution, depends very much on how much
atmosphere it has had to pass through to get there.
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Air mass (AM)
▪Air Mass: the length of the path taken by the
sun’s rays through the atmosphere to reach a
spot on the ground, divided by the path length
corresponding to the sun directly overhead.
▪Thus, an air mass ratio of 1 (designated
“AM1”) means that the sun is directly overhead.
▪By convention, AM0 means no atmosphere;
that is, it is the extraterrestrial solar spectrum.
▪For most photovoltaic work, an air mass ratio
of 1.5, corresponding to the sun being 42
degrees above the horizon, is assumed to be the
standard.
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Solar Spectrum
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Losses in solar cell
Other factors that contribute to the drop in theoretical
efficiency include:
▪ Recombination of holes and electrons before they can
contribute to current flow.
▪ Photons that are not absorbed in the cell either because
they are reflected off the face of the cell, or because they
pass right through the cell, or because they are blocked
by the metal conductors that collect current from the top
of the cell.
▪ Internal resistance within the cell, which dissipates
power.
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Semi-conductor dopping
▪As long as a solar cell is exposed to photons with energies
above the band gap energy, hole–electron pairs will be
created. The problem is, of course, that those electrons can
fall right back into a hole, causing both charge carriers to
disappear – this is call “recombination”.
▪To avoid that recombination, electrons in the conduction
band must continuously be swept away from holes. In PVs
this is accomplished by creating a built-in electric field
within the semiconductor itself that pushes electrons in one
direction and holes in the other → Doping of semiconductor
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PN Junction
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PN Junction
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PN junction model
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Solar cell modeling
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Solar cell modeling
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4-parameter model of solar cell
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5-parameter model of solar cell
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I-V characteristic of solar cell
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Fill factor of solar cell
▪Fill Factor (FF) is a quantity that is often used to
characterize module performance.
▪The fill factor is the ratio of the power at the maximum
power point to the product of VOC and ISC.
▪Fill factors around 70–75% for crystalline silicon solar
modules are typical, while for multi-junction amorphous-Si
modules, it is closer to 50–60%.
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Effect of temperature and
irradiance
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Cell temperature
Cell temperature varies depending on ambient temperature and solar
insolation
◦ Tcell: Cell temperature
◦ Tamb: Ambient temperature
◦ NOCT: Normal Operating Cell Temperature (usually provided by
manufacturer)
◦ S: Solar insolation [kW/m2]
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