les 1 struct operat solar cell

8/21/2019 Les 1 Struct Operat Solar Cell

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F.SpertinoF.Spertino

STRUCTURE ANDOPERATION

F. Spertino

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F.Spertino

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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F.Spertino


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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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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F.Spertino


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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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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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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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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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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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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les 1 struct operat solar cell

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