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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.
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
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
 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
 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.
PERFORMANCE OF SOME TYPES OF PV CELL
 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.
Due to doped element gradient electron and
hole get drifted to other side that cause built in
potential at junction
For positive voltage current will exponential and for negative voltage it will constant negative
exponential
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
DESIGN PARAMETER OF PHOTO VOLTAIC CELL
 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
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
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
 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
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
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)
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
 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
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