Photovoltaic (Solar Cells) - pin mat troi

Photovoltaic (Solar Cells) - pin mat troi

The history of photovoltaics goes back to the year 1839,

when Becquere discovered the photovoltaic effect, but no

technology was available in the 19t century to exploit this

discovery The semiconductor age only began about 100

years later After Shockley had developed a model for the

pn junction, Bell Laboratories produced the first solar cell

in 1954; the efficiency of this, in converting light into

electricity, was about 5%

Photovoltaics offers the highest versatility among

renewable energy technologies

Theoretically, PV systems could cover the whole

electricity demand of most countries in the world

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Worldwide, the installed photovoltaics capacity and the

share of electricity generated by PV are still low, despite

impressive market growth The political environment and

magnitude of market introduction programmes will

determine the future of this technology

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@ The photo effect I

Light, with its photon energy, can provide the energy to lift

an electron to a higher orbit The photon energy is given

by: E= 1 ` h:c

L

The energy sufficient to lift the electron to orbit E is also

called the ionization energy (external photoelectric effect)

photovoltaic cells mainly convert to electricity photons of

visible, ultraviolet and infrared light, therefore, the internal

photo effect determines the effect of light in a solar cell

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€ The photo effect I

The highest fully occupied band is called the valence band The next highest band, which can be partially occupied or totally empty, is called the conduction band

The space between VB and CB is called the forbidden band The energy gap between the band is called the band gap

The photo effect I Photovoltaic (PV) cells are made of

special materials called

semiconductors such as silicon, which

is currently the most commonly used

Basically, when light strikes the cell, a

band

the semiconductor material This

means that the energy of the absorbed

light is transferred to the

electrons loose, allowing them to flow

freely

@ The photo effect I

Every PV cell has at least one electric field Without an

electric field, the cell wouldn't work, and this field forms

when the N-type and P-type silicon are in contact

Right at the junction, electrons and holes mix and form a

barrier, equilibrium is reached, and an electric field

separating the two sides is formed This electric field acts

as a diode

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The photo effect _

'When light, in the form of photons, hits solar cell, its energy frees

electron-hole pairs Each photon with enough energy will normally

free exactly one electron, and result in a free hole as well If this

happens close enough to the electric field, or if free electron and hole

happen to wander into its range of influence, the field will send the

electron to the N side and the hole to the P side This causes further

disruption of electrical neutrality, and if we provide an external current

path, electrons will flow through the path to their original side (the P

side) to unite with holes that the electric field sent there, doing work

for us along the way The electron flow provides the current, and the

cell's electric field causes a voltage

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@ Principle of solar cells I

Not all the energy of photons with wavelengths near the

band gap is converted to electricity The solar cell surface

reflects a part of the incoming light, and some is

transmitted through the solar cell Further more, electrons

can recombine with holes

The spectral response is given by :

SÙ)= Hd)

€ Solar Cell Materials I

Solar cells can be made from a wide range of

semiconductor materials They are:

Silicon (Si)—including single-crystalline Si, multicrystalline Si, and amorphous Si

Polycrystalline thin films—including copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film silicon

Single-crystalline thin films—including high-efficiency material such as gallium arsenide (GaAs)

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€ Solar Cell Materials _

The crystallinity of a material indicates how perfectly

ordered the atoms are in the crystal structure Silicon, as

well as other solar cell semiconductor materials, can come

in various forms: single-crystalline, multicrystalline,

polycrystalline, or amorphous In a single-crystal material,

the atoms making up the framework of the crystal are

repeated in a very regular, orderly manner from layer to

layer In contrast, in a material composed of numerous

smaller crystals, the orderly arrangement is disrupted

moving from one crystal to another One classification

scheme for silicon uses approximate crystal size and also

includes the methods typically used to grow or deposit

such material

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Multicrystalline silicon | mc-Si 1mm-10cm _| Cast, sheet, ribbon

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€ Solar Cell Materials I

Absorption

The absorption coefficient of a material indicates how far light having a specific wavelength (or energy) can penetrate the material before being absorbed A small absorption coefficient means that light is not readily absorbed by the material Again, the absorption coefficient of a solar cell depends on two factors: the material making up the cell, and the wavelength or energy of the light being absorbed Solar cell material has an abrupt edge in its absorption coefficient The reason is that light whose energy is below the material's bandgap cannot free an electron And so, it isn't

absorbed

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Solar Cell Materials I Bandgap

The bandgap of a semiconductor material is an amount of energy

Specifically, it's the minimum energy needed to move an electron

from its bound state within an atom to a free state This free state is

where the electron can be involved in conduction The lower

energy level of a semiconductor is called the "valence band.“ And

the higher energy level where an electron is free to roam is called

the "conduction band." The bandgap (often symbolized by Es) is

the energy difference between the conduction band and valence

band

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€ Solar Cell Materials I

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Solar Cell Types (Silicon) I Single-Crystal Silicon

To create silicon in a single-crystal state, we must first melt high-

purity silicon We then cause it to reform or solidify very slowly in

contact with a single crystal "seed.“ The silicon adapts to the

pattern of the single-crystal seed as it cools and gradually solidifies

Not surprisingly, because we start from a seed, we say that this

process is "growing" a new rod (often called a "boule") of single

crystal silicon out of molten silicon

Several different processes can be used to grow a boule of single-

crystal silicon The most established and dependable processes are

the Czochralski (Cz)method and the float-zone (FZ) technique We

also discuss "ribbon-growth" techniques

€ Solar Cell Types (Silicon) I

Single-Crystal Silicon

The most widely used technique for

pattern of the seed and extend the single- cols a

crystal structure

Crucible

SS — com

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Solar Cell Types (Silicon)

Multicrystalline Silicon

Multicrystalline silicon devices are generally less

efficient than those of single-crystal silicon, but they can

be less expensive to produce The multicrystalline silicon

can be produced in a variety of ways The most popular

commercial methods involve a casting process in which

molten silicon is directly cast into a mold and allowed to

solidify into an ingot The starting material can be a

refined lower-grade silicon, rather that the higher-grade

semiconductor grade required for single-crystal material

The cooling rate is one factor that determines the final

size of crystals in the ingot and the distribution of

impurities The mold is usually square, producing an

ingot that can be cut and sliced into square cells that fit

more compactly into a PV module

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in solar electric, or photovoltaic (PV), systems Today, amorphous silicon is common in solar-powered consumer devices that have low power requirements, such as wristwatches and calculators

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Solar Cell Types (Silicon) I

Amorphous Silicon

Amorphous silicon absorbs solar radiation 40 times more

efficiently than does single-crystal silicon, so a film only about 1

micrometer—or one one-millionth of a meter— thick can absorb

90% of the usable light energy shining on it This is one of the

chief reasons that amorphous silicon could reduce the cost of

photovoltaics Other economic advantages are that it can be

produced at lower temperatures and can be deposited on low-cost

substrates such as plastic, glass, and metal This makes amorphous

silicon ideal for building-integrated PV products like the one

shown in the photo And these characteristics make amorphous

silicon the leading thinfilm PV material

(Polycrystalline thin films) Solar Cell Types

One scientific discovery of the computer semiconductor industry also has great potential in the photovoltaic (PV) industry: thin-film

technology The "thin film" term comes from the method used to deposit the film, not from the thinness of the film: thin-film cells are deposited in very thin, consecutive layers of atoms, molecules, or ions

Thin-film cells have many advantages over their "thick- film“ counterparts For example, they use much less materia—the cell's active area is usually only 1 to 10 micrometers thick, whereas thick films typically are 100 to 300 micrometers thick Also, thin-film cells can usually be manufactured in a large-area process, which can

be an automated, continuous production process Finally, they can be deposited on flexible substrate materials

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Solar Cell Types (Polycrystalline thin films) Thin-Film Deposition

Several different deposition techniques can be used, and all of

them are potentially less expensive than the ingot growth

techniques required for crystalline silicon We can broadly classify

deposition techniques into physical vapor deposition, chemical

vapor deposition, electrochemical deposition, or a combination

And like amorphous silicon, the layers can be deposited on various

low-cost substrates (or "superstrates") such as glass, stainless steel,

or plastic in virtually any shape

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(Polycrystalline thin films) Solar Cell Types Thin-Film Deposition

In addition, these deposition processes can be scaled up easily,

which means that the same technique used to make a 2-inch x 2-

inch laboratory cell can be used to make a 2-foot x 5-foot PV Thin

films are unlike single-crystal silicon cells, which must be

individually interconnected into a module In contrast, thin-film

devices can be made as a single unit—that is, monolithically—

with layer upon layer being deposited sequentially on some

substrate, including deposition of an antireflection coating and

transparent conducting oxide

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(Polycrystalline thin films) Solar Cell Types

Thin-Film Cell Structure

Unlike most single-crystal cells, a typical thin-film device doesn't have a metal grid for the top electrical contact Instead, it uses a thin layer of a transparent conducting oxide, such as tin oxide These oxides are highly transparent and conduct electricity very well A separate antireflection coating might top off the device, unless the transparent conducting oxide serves that function Polycrystalline thin-film cells are made of many tiny crystalline grains of semiconductor materials The materials used in these polycrystalline thin-film cells have properties that are different from those

of silicon So, it seems to work better to create the electric field with an

interface between two different semiconductor materials This type of interface is called a heterojunction ("hetero" because it is formed from two different materials, in comparison to the "homojunction" formed by two doped layers of the same material, such as the one in silicon solar cells)

(Polycrystalline thin films) Solar Cell Types Thin-Film Cell Structure

The typical polycrystalline thin film has a very thin (less than 0.1 micron) layer on top

called the "window" layer The window Transparent sight

bandgap (2.8 eV or more) to let all available °#®°#*

a high absorptivity (ability to absorb photons) for high current and a suitable band gap to provide a good voltage Still, it is typically just 1 to 2 microns thick

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Solar Cell Types (Polycrystalline thin films) Copper Indium Diselenide (CIS)

Copper indium diselenide (CulnSez or "CIS") has an extremely high absorptivity, which means that 99% of the light shining on CIS will be absorbed in the first micrometer of the material The most common material for the top or window layer in CIS devices is cadmium sulfide (CdS), although zinc is sometimes added to improve transparency

Adding small amounts of gallium to the lower absorbing CIS layer boosts its bandgap from its normal 1.0 electron-volts (eV), which improves the voltage and therefore the efficiency of the device

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(Polycrystalline thin films) Solar Cell Types Cadmium Telluride (CdTe)

Cadmium telluride is another prominent polycrystalline thin-film material

With a nearly ideal bandgap of 1.44 eV, CdTe also has a very high absorptivity Like CIS, films of CdTe can be manufactured using low-cost techniques Also like CIS, the best CdTe cells employ a heterojunction interface, with cadmium sulfide (CdS) acting as a thin window layer Tin oxide is used as a transparent conducting oxide and antireflection coating

One problem with CdTe is that p-type CdTe films tend to be highly resistive electrically, which leads to large internal resistance losses A solution is to allow the CdTe layer to be intrinsic (that is, neither p-type nor n-type, but natural), and add a layer of p-type zinc telluride (ZnTe) between the CdTe and the back electrical contact When it comes to making CdTe cells, a wide variety of methods are possible, including closed-space sublimation, electrodeposition, and chemical vapor deposition

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