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8/21/2019 Les 1 Struct Operat Solar Cell
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F.SpertinoF.Spertino
STRUCTURE ANDOPERATION
F. Spertino
.
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BACKGROUND ON SEMICONDUCTORS
The solar or photovoltaic (PV) cell is
the basic element of the PV systems, Si
sunlight into electricity occurs inside.
The conversion efficienc is achieved
by exploiting the properties ofsemiconductors. In Silicon (Si) the
4 valence electrons, available for
chemical bonding with surroundingatoms.
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BACKGROUND ON SEMICONDUCTORS
To understand the conversion process, it is advisable toremember some h sical remarks: the ener bandstructure of the atom and the generation of electron-
hole couples. The energy band is the set of energy levels owned by
the electron. It is composed of:
- valence band, that is composed of electron set involvedin chemical bonding;
- con uc on an , a s compose o e ec rons w anenergy level (higher than the previous) to permit their
- band gap (energy gap), which represents the energy
band to the conduction band.
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BACKGROUND ON SEMICONDUCTORS
Insulators
Conduction band
ConductorsSemiconductors
EcConduction band
Ec
E
Ec
Eg
Valence bandEv
gValence band
Valence band
Ev
Crystalline Silicon (c-Si) 1,12
-
nergy gap e o ma or sem con uc ors
,
Germanium (Ge) 0,67
Gallium Arsenide (GaAs) 1,42
Indium Phosphide (InP) 1,34
Copper Indium Diselenide (CuInSe) 1,05
Cadmium Telluride (CdTe) 1,45
Cadmium Sulfide (CdS) 2,4
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BACKGROUND ON SEMICONDUCTORS
In order to obtain the diodeSihole
have to realize the P-N junction in a
semiconductor, putting together two B SiSicrystals: one is "doped" withtrivalent atoms (boron) and the
r w vatoms (phosphorus).Si electron
-atoms and holes in the lattice, while
the N-t e cr stal has free electrons P Sifrom the bond.
Holes and electrons move freely in
the lattice due to diffusion.
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THE PHOTOVOLTAIC EFFECT
At the interface, electrons of the N-type crystal spread to theP-type crystal (vice versa for holes), forming a space charge
region ("+" on N-type and "-" on the P-type), in which there.
When the electric field generated counter-balances the, .
The radiation, with sufficient energy (Eph= hf = hc/> Eg),
In the presence of the junction electric field, electrons are
attracted to the N area ositivel char ed and holes to theP area (negatively charged).
This charge motion is the source of the photovoltaic current.
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THE PHOTOVOLTAIC EFFECT
In a single-junction cell, the doped N layer is placed abovethe P layer in a thickness ranging from 200 to 300 m.
Junction
field1V/m
The radiation is able to reach the junction (depletion region)because the N layer is very thin (1m).
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THE PHOTOVOLTAIC EFFECT
In the solar spectrum, onlyp o ons w su c en
energy are able to generate- .
Photons (visible and above
surplus of energy that is lostin the creation of electron/hole pairs: itimplies a limit in the
.
Limits conversion for Silicon
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THE PHOTOVOLTAIC EFFECT
Additional disadvantages are:
not all incident photons penetrate into the, r r rvanished by the frontal electrode;
some electron-hole pairs recombinetheirselves before they can be separatedby the electric field inside the junction;
the frontal electrode of the cell has anamount of resistance.
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THE PHOTOVOLTAIC EFFECT
N layer There are also other types of junction...
Electric
N layer
insic
Electric
field intr
fieldEg2
g
P-N junction
Eg
P-I-N junction
Eg1
Hetero- unction
Note the different erformance of the field in the unction
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THE PHOTOVOLTAIC EFFECT
Look at energy bands of a P-N junction with forward or
IforrevNote that the
direction of current
- + the flow of positiveorwar as
(+ on the P layer)
-
Reverse bias
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thus the diffusion current in the diode is growing
considerably
The reverse bias increases the potential barrier: asmall reverse saturation current runs
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THE STRUCTURE OF THE SOLAR CELL
The material of widespread PV cells is
single- and multi- (orpo y- crysta ine si icon.
Each cell is obtained by aw r thickness and 10 20 cm
On the frontal side (often
"textured" a la er ofantireflective coating isapplied.
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THE STRUCTURE OF THE SOLAR CELL
-from the sawing of a round cylinder
in ot wafer .
Poly-crystalline cells are derived fromthe fusion and solidification of Silicon
in a quartz crucible (square ingot). On the back side (P-type) it is applied
a contact as arge as t e sur ace othe cell.
r - ycontact is formed by thin Ag
.
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