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PHOTOVOLTAIC CELL Abstract Background Working principle Fabrication Arrays and Systems Potential

Photovoltaic cell1

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introduction to photovoltic cell

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PHOTOVOLTAIC CELL

AbstractBackgroundWorking principleFabricationArrays and Systems Potential

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FEW

APPLI

CATION O

F PHOTO

CELL

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ABSTRACT

Solar photovoltaic energy conversion is a one-step conversion process which

generates electrical energy from light energy.

Light is made up of packets of energy called Photons. When they hit a solid

surface they excite the electrons, bound into solid, up to a higher energy level in which

they are more free to move. But these electrons relax and come back to the ground

state within no time.

In a photovoltaic device, however, there is some built-in asymmetry which pulls the

excited electrons away before they can relax, and feeds them to an external circuit.

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BACKGROUND

most commonly manufactured PV cells are made of crystalline silicon and have energy conversion efficiency of 12%.

The cost of these cells is $3 per Watt of power generated under solar AM 1.5G conditions

these costs need to be reduced by an order of magnitude to around $0.3 per Watt for PV cells to be competitive with other energy generation system

reducing the costs of PV cells may be achieved if the semiconductor

were deposited from solution onto large flexible substrates in reel-to-reel coating

reducing the costs of PV cells may be achieved if the semiconductor

were deposited from solution onto large flexible substrates in reel-to-reel coating

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WORKING PRINCIPLE PHOTOCURRENT

The photo current generated by a solar cell under illumination at short circuit

is dependent on the incident light.

The photocurrent density Jsc is

QE(E) is the probability that an incident photon of energy ‘E’ will deliver one electron

to the external circuit.

bs(E) is the incident spectral photon flux density, the number of photons of energy in

the range E.

QE and spectrum can be given as functions of either photon energy or

wavelength, λ

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DARK CURRENT AND OPEN CIRCUIT VOLTAGE

When a load is present, a potential difference develops between the terminals of the

cell. This potential difference generates a current which acts in the opposite direction

to the photocurrent, and the net current is reduced from its short circuit value. This

reverse current is usually called the dark current.

Where Jo is a constant, kB is Boltzmann's constant and T is temperature in degrees

Kelvin.

When the contacts are isolated, the potential difference has its maximum

value, the open circuit voltage Voc. This is equivalent to the condition when

the dark current and short circuit photocurrent exactly cancel out. For the

ideal diode, from ideal diode equation

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EFFICIENCY

The cell power density is given by P=JV

P reaches a maximum at the cell's operating point or maximum power point. This

occurs at some voltage Vm with a corresponding current density Jm.

The fill factor is defined as the ratio FF = (JmVm) / (JscVoc)

The efficiency of the cell is the power density delivered at operating point as a

fraction of the incident light power density, Ps

Efficiency is related to Jsc and Voc using FF.

These four quantities: Jsc, Voc, FF and η are the key performance characteristics

of a solar cell.

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PERFORMANCE OF SOME TYPES OF PV CELL

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Non-ideal diode behaviour

The ideal diode behaviour is seldom seen. It is common for the dark current to

depend more weakly on bias. The actual dependence on V is quantified by an ideality

factor, m and the current-voltage characteristic given by the non-ideal diode equation,

 

 

 

m typically lies between 1 and 2.

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ILLUMINATION OF PN JUNCTION

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SOME CHARACTERISTIC OF DIODE

Due to doped element gradient electron and hole get drifted to other side that cause built in potential at junction

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For positive voltage current will exponential and for negative voltage it will constant negative exponential

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When circuit is working in 4’th quadrant power driven to circuit will be positive and in there two case it will be negative so photovoltaic cell do work in fourth quadrant

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DESIGN PARAMETER OF PHOTO VOLTAIC CELL

Material

Impurity as adaptor and donor

Impurity quantity

Impurity injection method

Resistance of cell

Efficiency

Band gap

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EFFICIENCY

To increase efficiency we use material which has proper band gap

To ensure full absorption of photo we use anti reflective material on cell

large mirrors or lenses to concentrate and focus the sunlight onto a string of cell can be used to improve efficiency by reduction in no. of cell

Efficiency is inversely proportional to temperature so hight efficiency can be achieved by keep cooling the panel

To get maximum photon flux panel should facing to sun

Efficiency can be maximize by multiple carrier generation by single photon

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RESISTANCE OF CELL

Series resistance of only few ohm can seriously cause In reduction in power loss

Resistance could be minimize by increasing cell aria

Resistance can be further minimize by distributing the contact over n region so current would distributed over the surface

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DIMENSION OF CELL

Dimension of cell should be such that generated electron-hole pair could reach the surface before recombination take place

So there should be proper match between diffusion length and thickness of p region and penetration depth 1/diff. coff.

life time of carrier is inversely proportional to concentration of doping

Contact potential is directly propositional to doping

So there is trade-off between lifetime of Carrier and contact potential

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WORKING PRINCIPLE

Solar cell is simple diode with special desgin

Enough energetic photon cause generation of electron-hole pair

Excited electron and hole get drifted by built-in potential in depletion region

The drift current cause current in circuit.

Voltage across individual cell is equal to built in potential

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TYPE OF SOLAR CELL

Single Crystal solar cells in panel•Silicon solar cells are made using either single crystal wafers, polycrystalline wafers or thin films•approx. 1/3 to 1/2 of a millimeter thick•The silicon must be of a very high purity and have a near perfect crystal structure

Polycrystalline solar panel•Polycrystalline wafers are made by a casting process

Amorphous-Si solar panel•Amorphous silicon, one of the thin film technologies

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FABRICATION

Single Crystal solar cells Single crystal wafers are sliced from a large single crystal ingot

It is a very expensive process

The silicon must be of a very high purity and have a near perfect crystal structure

Polycrystalline solar Polycrystalline wafers are made by a casting process

molten silicon is poured into a mould and allowed to set

Then it is sliced into wafers 

 it is not as efficient as monocrystalline cells

The lower efficiency is due to imperfections in the crystal structure resulting from the casting process

Amorphous-Si solar

Amorphous silicon is one of the thin film technologies

It is made by depositing silicon onto a glass substrate from a reactive gas such as silane (SiH4) 

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PN JUNCTION FORMATION

dopant atoms introduced to create a p-type and an n-type region

doping can be done by high temperature diffusion

where the wafers are placed in a furnace with the dopant introduced as a vapour

Once a p-n junction is created, electrical contacts are made to the front and the back of the cell

evaporating or screen printing metal on to the wafer to form contact

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Arrays and Systems

 PV cells have a working voltage of about 0.5  they are usually connected together in series (positive

to negative) to provide larger voltages low power panels are made by connecting between 3

and 12 small segments of amorphous silicon PV larger systems can be made by linking a number of

panels together PV panel array, ranging from two to many hundreds of

panels the output voltage is limited to between 12 and 50

volts, but with higher amperage  This is both for safety and to minimize power losses Arrays of panels are being increasingly used in

building construction

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POTENTIAL

The photovoltaic industry is growing rapidly as concern increases about global warming

For most of the eighties and early nineties the major markets for solar panels were remote area power supplies and consumer products

However in the mid nineties a major effort was launched to develop building integrated solar panels for grid connected applications

energy output from PV panels will vary depending on the orientation, location, daily weather and season

On a clear sunny day, the power density of is approximately 1kW/m2

The solar energy received by Earth is more than 10,000 times the current use of fossil fuels and nuclear energy combined

harnessing such a large potential energy source has the potential to replace a significant amount of carbon based fuels