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Introduction to Photovoltaic (PV) Technology

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Eng. Firas Alawneh

Outline

History

Semiconductors

From Sand to Solar Cells

Semiconductors & Photovoltaic phenomenon

Silicon PV Cell Operation

Properties of the PV Cells

Standard Test Conditions (STC) of PV Cells &

performance parameters

Types of PV Cells

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History

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1839 Photovoltaic effect discovered by Becquerel.

1870s Hertz developed solid selenium PV (2%).

1905 Photoelectric effect explained by A. Einstein.

1930s Light meters for photography commonly employed cells of copper oxide or selenium.

1954 Bell Laboratories developed the first crystalline silicon cell (4%).

1958 PV cells on the space satellite U.S. Vanguard (better than expected).

Semiconductors

Solar cells are fabricated using semiconductors.

Semiconductors are made from crystal and can act as conductors or insulators in different circumstances, according to the amount of energy that is given to the material.

Silicon is the most common semiconductor crystal.

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Silicon

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semiconductor silicon

(hyper pure)

reduction

Solar cell processing slicing

purification(several steps)

Cast ingot

wafers

From Sand to Silicon Solar Cells

SiO2Quartz Sand MetallurgicalSilicon

Solar cell

1500-2000 C 300 C

1100C for ~200 300 hours

Photovoltaic Technology

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Photovoltaic (PV) is the technology of converting light directly

to electrical energy (photo = light, voltaic = electricity).

Commonly known as solar cells.

The simplest systems power the small calculators we use every

day. More complicated systems will provide a large portion of

the electricity in the near future.

PV represents one of the most promising means of maintaining

our energy intensive standard of living while not contributing to

global warming and pollution.

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Photon Energy

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Silicon Chemical Properties

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Melting Point:

1410 C

Boiling Point:

2355 C

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Energy Bands for materials

Conduction

Band

Valence

Band

Metal

Conduction

Band

Valence

Band

Semiconductor

Eg

Conduction

Band

Valence

Band

Insulator

Eg

The semiconductors in general lies between metal and insulator properties, it needs a

small energy related to insulator to be in conduction band.

E

Eg(eV)

Element

1.14Silicon0.67Germanium0.1Tin0Copper

All at 20C

Photon

E = h.c/

e -

The response of the silicon due to the

incident Photons

e-e- e+e+

e- e-e+ e+

e-e- e+e+

e- e-e+ e+

Conclusion: we have to reengineer the material, so that we can separate the electrons (e) from the holes(e) to prevent the recombination inside the material.

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e+e-

e+e-

e+e-

e+e-

Photosensitivity?

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2. Doping of Silicon : positive (p)

and negative (n) layers

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What is Doping?

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Answer: Adding foreign atoms to the silicon crystal to produce

negative or positive free charge carries (electrons or holes).

Why Doping?

Answer: As mentioned before, electrons freed and energized by

photons will wander for a short time and then recombine with a

wandering hole. The energy originally transferred to the electron

from the photon is simply lost as heat. The key to producing

usable output current is to sweep the freed electrons out of the

material before they recombine with holes.

Doping the silicon

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Pure silicon wafer is doped with a small amount of

another atoms at temperature (1000-1200)C, which

creates a valence bond between it and the silicon.

The most common impurity atoms are the Boron (B5)

and the Phosphorus (P15).

The Boron has three electrons in its outer level (less

than the silicon by one electron).

The Phosphorus has a five electrons in its outer level

(more than the silicon by one electron).

The Boron is doped by one atom for every 10,000,000

silicon atoms to form the P-type silicon.

The Phosphorus is doped by one atom for 1000 silicon

atoms, to form the N-type silicon.

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The P-type silicon

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The silicon atom creates four

covalent bonds with other

neighboring atoms in the pure silicon

crystal.

When the crystal is doped with

Boron atoms, the silicon will make

three covalent bonds with it with the

forth bond missing, which represents

a hole (e+), so this type of

semiconductor is called P-type.

This hole is waiting for a free

electron to fill its location to create

the forth bond, so the impurity

atoms then is referred to it as

acceptor atoms.

Hole

The N-type silicon

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Silicon is doped with Phosphorus

which has five electrons in its

outer orbit. So one electron (e-)

will be free. This type of

semiconductor is called N-type.

Phosphorous atoms (P) can donate

this electron to another bond that

needs it, so it is referred to as

donor atoms.

Electron

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Doping in 3D view

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P-type N-type

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Doping in 2D view

N-type semiconductorP-type semiconductor

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3. Photovoltaic Effect: p-n

junction operation and its parts

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e-e- e+e+

e- e-e+ e+

Voltage Difference

Depletion region Built-in electric field

e- e+

E

e-e+e-e+

The p-n junction

Conclusion: The goal of doping is to create the depletion region to

create the electric field that separates the electrons from the holes to

produce the potential difference.

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Depletion Region

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e-e- e-

e+

e+

e+

Solar Cell Operation

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Solar Cell Operation

Solar Cell Parts

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(n+) & (p+) diffusions (heavily

doped silicon) used to

connect the layers with the

metal to decrease the series

resistance.

The top metal gridN layer

P layer

Top view of

the cellBottom view

of the cell

Bottom

metal

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Silicon Solar Cell Packaging

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26CZ Crystallization Method

Monoc-Si

Si liquid

seed

Mono-crystalline vs. Poly-crystalline SiliconThere are two types of crystalline silicon depending on its purity and crystalsorientation obtained during the crystal growth process: Poly-crystalline: Non-uniform crystals orientation Mono-crystalline: Uniform crystals orientation (purer and more expensive andefficient)The mono-crystalline silicon ingots are prepared by the exacting Czochralski(CZ) crystal growth process (crystal pulling). While the poly-crystalline siliconingots are prepared by a simpler casting (or, more generally, directionalsolidification).

Simple Crystallization

Insulation

Electric Heaters

Poly c-Si

Si liquid

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How to distinguish between polycrystalline and monocrystalline silicon

solar cells by visual inspection?

Poly-crystalline Mono-crystalline

4. Equivalent circuit of the solar cell

and characteristic curve

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Equivalent Circuit for Solar Cell

29Real Solar cell

Standard Solar cell

Equivalent Circuit for Solar Cell

Where:

Iss : Reverse saturation current (depends on: Material, Geometry, & temperature)

q : Electron charge (1.6*10-19 C)

n : Diode quality factor

(1 for ideal diodes and >1 up to 2 for real diodes)

k : Boltzmann constant (1.38*10-23 J/K)

T: Absolute cell temperature in Kelvin degrees

For real solar cells with finite values for RS and Rsh:

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Characteristics and Power for Solar Cell

0

0.5

1

1.5

2

2.5

3

3.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Cell Voltage (V)

Cell

Curr

en

t (A

)Iph

Id

Power

IV-Curve

Isc

Voc

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I = IPH - ID

Operating Point & Maximum Power Point

0

0.5

1

1.5

2

2.5

3

3.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Cell Voltage (V)

Cell

Curr

en

t (A)

MPP

Vmp

ImpRL

Operating Point

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5. Standard test conditions (STC)

and main performance parameters

and factors

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Standard Test Conditions (STC)

Global Solar Irradiance (G): 1000 W/m2

Cell Temperature (T): 25 C

Air Mass (AM): 1.5

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PV Performance Parameters

Open-circuit voltage (Voc)

Short-circuit current (Isc -(Iph))

Maximum power voltage (Vmp)

Maximum power current (Imp)

Maximum power (Pmp)

Maximum Power Efficiency (max)

Fill factor ( FF )

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Solar Cell Fill Factor

0

0.5

1

1.5

2

2.5

3

3.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Cell Voltage (V)

Cell

Curr

en

t (A)

(Vmp*Imp) Square

(Voc*Isc) Square

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Solar Cell Efficiency

The electrical output depends on the operating point of the solar cell and the incident radiant power depends on the solar radiation (perpendicular to the surface of the solar cell) and cell surface area.

The maximum efficiency of the solar cell is calculated at MPP, which is:

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G Global Solar Irradiance

Area

Efficiency of Solar Cell at MPP

Input Power = G [W/m2] x Area [m2]

Output Power = Vmp [V] x Imp [A]

+

-

V

I

Resistor

Solar Cell

The efficiency of the solar cell is the ratio of electrical power output to the incident radiant power :

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PV Efficiency Losses

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Optical losses: Not all of the light is absorbed because of finite reflectivity.

Use antireflective coating.

Use multilayer coating with different indices of refraction.

Further reduction is caused by light blocked by the metal grid which is

needed for electrical contacts.

Recombination losses: Many charge carriers recombine before they candiffuse to the device terminals.

Series and Shunt resistance: The bulk resistance of the semiconductorcontributes some series resistance. The shunt resistance can be caused bycrystal lattice defects in the depletion region and/or leakage currentsaround the edges of the cell.

Temperature Effect on Solar Cells

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The parameter most affected by an increase in temperature is the open-circuit voltage (Voc). Accordingly, the power of the solar cell at theMaximum Power Point (MPP) decreases by increasing the cellstemperature.

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Temperature Effect on Solar Cells

6. Solar cells types

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44www.nrel.gov/pv/thin_film/docs/kaz_best_research_cells.ppt

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Thanks

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