1. Solar Photovoltaics
A talk on
“physics of solar cell ”
Organized by
Sameh,Viet,and He
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2. Solar Photovoltaics
SolarCells
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 solarcells
 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 22

3. Solar Photovoltaics
Band Structure
Consider 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 33

4. Solar Photovoltaics
Extrinsic Semiconductors
Electrical 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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5. Solar Photovoltaics
Doping
Doping
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 55

6. Solar Photovoltaics
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 66

7. Solar Photovoltaics
Donors: Energy Levels
The 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
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8. Solar Photovoltaics
The Single Crystalline SolarCell
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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9. Solar Photovoltaics
The N-type material is keep thin to allow
light to pass through to the PN junction. 99

10. 1010

11. Solar Photovoltaics 1111

12. Solar Photovoltaics 1212

13. Solar Photovoltaics 1313

14. Solar Photovoltaics
QUESTIONS ????
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15. Solar Photovoltaics
The solar spectrum
Sunlight consists of a broad range of spectrum
The photon energy depends on the photon wavelength: Ephot = hc/λ
Harnessing the great amount of sunlight to energy
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16. Solar Photovoltaics
Theoretical efficiency limits of single junction solar cells
made out of various semiconductors
Energy Band Gaps in solar cell materials
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17. Solar Photovoltaics
Acceptors
Substitute 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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18. Solar Photovoltaics
Acceptor: Energy Levels
From 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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19. Solar Photovoltaics
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
µc-Si 0.1 µm – 1 µm Plasma Enhanced
Chemical-vapor
deposition (PECVD)
Nano crystalline
silicon
nc-Si <<1 µm PECVD
Amorphous
silicon
a-Si Crystallites absent PECVD
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