PHOTOVOLTAIC
The term "photovoltaic" comes from the Greek φῶς (phōs) meaning "light", and "voltaic",
meaning electric, from the name of the Italian physicist Volta. A solar cell is a device that
converts the energy of sunlight directly into electricity by the photovoltaic effect.
PHOTOVOLTAIC MATERIAL:
Based on the electrical conductivity behavior, the solid materials are classified into three
categories: conductors, insulators and semiconductors. Conductors contain many electrons in its
conduction band at room temperature and does not have band gap between valance and
conduction band. Insulators do not conduct electricity because of large band gap, and no transfer
of electron takes place from valance band to empty conduction band. Semiconductors are able to
transfer the electron from valance bond to conduction band after acquiring some energy.
SEMOCONDUCTORS
Semiconductors are classified into two category:
1. Intrinsic: Pure non-metallic materials such as Germanium and silicon
2. Extrinsic: Intrinsic material added with impurity is called extrinsic. The extrinsic
semiconductors are ‘n’ type or ‘p’ type.
Semiconductors ‘n’ type: Crystalline silicon when doped with pentavalent impurity
(Arsenic, antimony or phosphorous), covalent bond is formed with 4 electron and remainings
surplus electron enters in the conduction band.
Semiconductors ‘p’ type: Crystalline silicon when doped with trivalent impurity
(aluminium, gallium or boron) during its crystallization, they form covalent bond having one
less electron i.e. creating a hole in the valance bond.
1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials,
such as silicon. The photon can be absorbed by the silicon, if the photon energy is higher
than the silicon band gap value. When a photon is absorbed, its energy is given to an
electron in the crystal lattice. Usually this electron is in the valence band, and is tightly
bound in covalent bonds between neighboring atoms, and hence unable to move far. The
energy given to it by the photon "excites" it into the conduction band, where it is free to
move around within the semiconductor. Thus Electrons (negatively charged) are knocked
loose from their atoms, allowing them to flow through the material to produce electricity.
Due to the special composition of solar cells, the electrons are only allowed to move in a
single direction.
2. The covalent bond that the electron was previously a part of now has one fewer electron
— this is known as a hole. The presence of a missing covalent bond allows the bonded
electrons of neighboring atoms to move into the "hole," leaving another hole behind, and
in this way a hole can move through the lattice. Thus complementary positive charges,
called holes, are also created and flow in the opposite direction to the electrons.
Silicon by itself is not a very good conductor of electricity. In order to be used in a solar
cell, silicon must be modified, or "doped," with other elements. Phosphorous and boron
are the elements of choice for this purpose. A layer of silicon doped with phosphorous
can take advantage of the fact that the latter element only has one electron in its
outermost electron shell. This is the electron that can be knocked off by light energy.
In the other layer, the boron atoms have only three electrons in their outer shell, instead
of four, which silicon has. This creates a spot for the extra electrons to go to, and this
movement of electrons is what makes the electrical current. The two silicon layers also
have opposite charges, which is what provides the voltage.
An array of solar cells converts solar energy into a usable amount of direct current (DC)
electricity.
As per Plank’s law, energy of photon is proportional to the frequency of radiation.
E = hν =
ℎ𝑐
𝜆
Where h = plank’s constant = 6.62 x 10-27, c is speed and ν is frequency of light wave
Where h = plank’s constant = 6.62 x 10-27 erg sec = 6.62 x 10-34J sec = 4.13576 x 10-15eVs
…[As 1eV = 1.6 x 10-12 erg]
c is speed of light wave = 2.997925 x 108 m/sec
and ν is frequency of light wave
𝜆 = Wave length of radiation in meter (m)
Example: For mono energetic radiation beam having a wave length of 0.5 micrometer 𝜆 =0.5
Energy of a photon E =
ℎ𝑐
𝜆
3 ×108
= 4.13576 x 10-15 x 0.5×10−6 = 2.48 eV
 PHOTON FLUX: Number of photons crossing a unit area perpendicular to the beam
radiation per unit line (фp)
Solar energy flux E” = фphνav
 Terrestrial insolation and average photon energy decreases as air mass (m) and angle of
latitude ( ф) increase
Example: Extraterrestrial ( Outside the earth’s atmosphere) photon flux = 1359 W/m2
= 0.1359W/cm2
Average photon energy hνav = 1.48eV
When photons strike a PV cell, they may be reflected or absorbed, or they may pass
right through. Only the absorbed photons generate electricity. When this happens, the
energy of the photon is transferred to an electron in an atom of the cell (which is actually a
semiconductor). With its newfound energy, the electron is able to escape from its normal
position associated with that atom to become part of the current in an electrical circuit. By
leaving this position, the electron causes a "hole" to form.
There are two main modes for charge carrier separation in a solar cell:
1. drift of carriers, driven by an electrostatic field established across the device
2. diffusion of carriers from zones of high carrier concentration to zones of low carrier
concentration (following a gradient of electrochemical potential).
In the widely used p-n junction solar cells, the dominant mode of charge carrier separation is by
drift. However, in non-p-n-junction solar cells (typical of the third generation solar cell research
such as dye and polymer solar cells), a general electrostatic field has been confirmed to be
absent, and the dominant mode of separation is via charge carrier diffusion. When the electrons
diffuse across the p-n junction, they recombine with holes on the p-type side. The diffusion of
carriers does not happen indefinitely, however, because charges build up on either side of the
junction and create an electric field. The electric field creates a diode that promotes charge flow,
known as drift current that opposes and eventually balances out the diffusion of electron and
holes. This region where electrons and holes have diffused across the junction is called the
depletion region because it no longer contains any mobile charge carriers. It is also known as the
space charge region.
The diffusion of free electrons leaves a net positive charge in the n-side while diffusion of free
holes leaves a net negative charge in the p-side. This results in disappearance of electrons and
holes from the vicinity for the junction called depletion area. This charge distribution gives rise
to an electric field
Electrical Contacts
Electrical contacts are essential to PV cells because they bridge the connection between the
semiconductor material and the external electrical load, such as a light bulb.
A typical solar cell consists of a glass or plastic cover, an antireflective coating, a front contact to
allow electrons to enter a circuit, a back contact to allow them to complete the circuit, and the
semiconductor layers where the electrons begin and complete their journey.
The back contact of a cell—the side away from the incoming sunlight—is relatively simple. It
usually consists of a layer of aluminum or molybdenum metal.
But the front contact—the side facing the sun—is more complicated. When sunlight shines on a
PV cell, a current of electrons flows over the surface. To collect the most current, contacts must
be placed across the surface of the cell. This is normally done with a grid of metal strips or
"fingers." However, placing a large grid, which is opaque, on top of the cell shades active parts
of the cell from the sun and reduces the cell's conversion efficiency. To improve conversion
efficiency, shading effects must be minimized.
Another challenge in cell design is to minimize the electrical resistance losses when applying
grid contacts to the solar cell material. These losses are related to the solar cell material's
property of opposing the flow of an electric current, which results in heating the material.
Therefore, shading effects must be balanced against electrical resistance losses. The usual
approach is to design grids with many thin, conductive fingers that spread to every part of the
cell's surface. The fingers of the grid must be thick enough to conduct well (with low resistance)
but thin enough not to block too much incoming light.
Grid contacts on the top surface of a typical cell are designed to have many thin, conductive
fingers spreading to every part of the cell's surface.
To make top-surface grids, metallic vapors are deposited on a cell through a mask or painted on
via a screen-printing method. An alternative to metallic grid contacts is a transparent conducting
oxide (TCO) layer made of, for example, tin oxide (SnO2). The advantages o TCOs are that they
are nearly invisible to incoming light and they form a good bridge from the semiconductor
material to the external electrical circuit. TCOs are very useful in manufacturing processes
involving a glass superstrate, which is the covering on the sun-facing side of a PV module. In
this process, the TCO is generally deposited as a thin film on the glass superstrate before the
semiconducting layers are deposited. The semiconducting layers are then followed by a metallic
contact that is actually the bottom of the cell. The cell is therefore constructed "upside down,"
from the top to the bottom.
The sheet resistance of the semiconductor is also an important consideration in grid design. In
crystalline silicon, for example, the semiconductor carries electrons well enough to reach a finger
of a metallic grid. Because the metal conducts electricity better than a TCO, shading losses are
less than losses associated with a TCO. Other semiconductors, such as amorphous silicon,
conduct very poorly in the horizontal direction. Therefore, they benefit from having a TCO over
the entire surface.
Cell Coatings
Silicon is a shiny gray material that can act as a mirror by reflecting more than 30% of the light
that shines on it. To improve the conversion efficiency of a solar cell, the amount of light
reflected must be minimized.
Two techniques are commonly used to reduce reflection. The first technique is to coat the top
surface with a thin layer of silicon monoxide (SiO). A single layer reduces surface reflection to
about 10%, and a second layer can lower the reflection to less than 4%. The second technique is
to texture the top surface. Chemical etching creates a pattern of cones and pyramids, which
captures light rays that might otherwise be deflected away from the cell. Reflected light is
redirected into the cell, where it has another chance to be absorbed.
Silicon crystals are a naturally reflective material. When they are used in photovoltaic cells, they
must have an anti-reflective coating placed on them, otherwise most of the solar energy that
reaches the panel will be reflected without producing electricity.
Photovoltaic cells are composed of layered materials which include two types of silicon, an antireflective coating, and a glass cover. By far, the most prevalent bulk material for solar cells is
crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk
silicon is separated into multiple categories according to crystallinity and crystal size in the
resulting ingot, ribbon, or wafer.
1. Monocrystalline silicon : Single-crystal wafer cells tend to be expensive, and because they are
cut from cylindrical ingots, do not completely cover a square solar cell module without a
substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four
corners of the cells. Band gap = 1.12 eV, Max efficiency = 24%
2. Poly- or multicrystalline silicon: large blocks of molten silicon carefully cooled and solidified.
Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less efficient.
Ribbon silicon[30] is a type of multicrystalline silicon: it is formed by drawing flat thin films from
molten silicon and results in a multicrystalline structure. Band gap = 1.12 eV, Max efficiency =
17.8%
3. Amorphous silicon (Thin film): These are non crystalline silicon. The various thin-film
technologies currently being developed reduce the amount (or mass) of light absorbing
material required in creating a solar cell. This can lead to reduced processing costs from that of
bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion
efficiency (an average 7 to 10% efficiency), although many multi-layer thin films have efficiencies
above those of bulk silicon wafers. Band gap = 1.75 eV, Max efficiency = 13%
4. The gallium arsenide (GaAs) is also a crystalline material like silicon and is used for making PV
cell. These are used where high efficiency is desired. These cells have high light absorption and
need thin layer of material but have wider band gap. The polycrystalline GaAs cells can operate
at higher temperature without loosing efficiency. Thin film cells made of Cadmium telluride
(CdTe), copper indium gallium diselenide (CuInSe2)have the efficiency of 17%. CIS is an
abbreviation for general chalcopyrite films of copper indium selenide (CuInSe2). The perception
of the toxicity of CdTe is based on the toxicity of elemental cadmium.
5. Concentrated solar power (CSP) systems use lenses or mirrors to focus a large area of sunlight
onto a small area. Electrical power is produced when the concentrated light is directed onto
photovoltaic surfaces.
 At present silicon solar cells occupy 60% of world market.
 For single crystal silicon,
-p is obtained by doping silicon with boron and typically 1 µm thick
-n is obtained by doping silicon with arsenic and typically 800 µm thick
 Thin film cells are composed of
Copper sulphide for p typically 0.12 µm thick
Cadmium sulphide for n, typically 200 µm thick
PERFORMANCE OF PV CELL
An ideal solar cell may be modeled by a current source in parallel with a diode; in practice no
solar cell is ideal, so a shunt resistance and a series resistance component are added to the model.
The equivalent circuit of a solar cell is given below. Manufacturer supply the parameters like
open circuit conditions, short circuit conditions, maximum power conditions and temperature
coefficients. Other parameters are obtained by measuring the current and voltage characteristics.
From the equivalent circuit it is evident that the current produced by the solar cell is equal to that
produced by the current source, minus that which flows through the diode, minus that which
flows through the shunt resistor:[15][16]
I = IL − ID − ISH
where



I = output current (amperes)
IL = photogenerated current (amperes)
ID = diode current (amperes)

ISH = shunt current (amperes).
The current through these elements is governed by the voltage across them:
Vj = V + IRS
where




Vj = voltage across both diode and resistor RSH (volts)
V = voltage across the output terminals (volts)
I = output current (amperes)
RS = series resistance (Ω).
By the Shockley diode equation, the current diverted through the diode is:
[17]
where





I0 = reverse saturation current (amperes)
n = diode ideality factor (1 for an ideal diode)
q = elementary charge
k = Boltzmann's constant
T = absolute temperature

At 25°C,
volts.
By Ohm's law, the current diverted through the shunt resistor is:
where

RSH = shunt resistance (Ω).
Substituting these into the first equation produces the characteristic equation of a solar cell,
which relates solar cell parameters to the output current and voltage:
Open-circuit voltage and short-circuit current
When the cell is operated at open circuit, I = 0 and the voltage across the output terminals is
defined as the open-circuit voltage. Voltage is highest but no current flows. Assuming the shunt
resistance is high enough to neglect the final term of the characteristic equation, the open-circuit
voltage VOC is:
Similarly, when the cell is operated at short circuit, V = 0 and the current I through the terminals
is defined as the short-circuit current. It can be shown that for a high-quality solar cell (low RS
and I0, and high RSH) the short-circuit current ISC is:
The schematic symbol of a solar cell
When the circuit is open, the flowing current is zero, and known as open circuit voltage (Voc)
valuing around 0.6V. When the resistance is zero, current will be maximum and termed as short
circuit current (Isc). Horizontal straight line indicates nearly constant short circuit current through
out till the voltage reaches to 0,5 V and then drops sharply because of large resistance. This point
of maximum power is called maximum power point of the cell.
The drop in radiation intensity lowers the short circuit current generated by the cell and
proportional to radiation intensity while the voltage drop.
With the increase in temperature, the open circuit voltage and the power drops down
significantly. However increases in short circit current is insignificant because of increase in e- hole pair with the rise in temperature.
Effect of temperature: The voltage of the cell decreases by 2.2mV for 1°C rise in temperature
because of decrease in forbidden energy gap.
The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic
efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency
is the product of each of these individual efficiencies.
Due to the difficulty in measuring these parameters directly, other parameters are measured
instead: thermodynamic efficiency, quantum efficiency, integrated quantum efficiency, VOC
ratio, and fill factor. Reflectance losses are a portion of the quantum efficiency under "external
quantum efficiency". Recombination losses make up a portion of the quantum efficiency, VOC
ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also
make up minor portions of the quantum efficiency, VOC ratio.
Typical commercial solar cells have a fill factor > 0.70. Grade B cells have a fill factor usually
between 0.4 to 0.7. The fill factor is, besides efficiency, one of the most significant parameters
for the energy yield of a photovoltaic cell.[13] Cells with a high fill factor have a low equivalent
series resistance and a high equivalent shunt resistance, so less of the current produced by light is
dissipated in internal losses.
Maximum-power point
A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the
resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value
(an open circuit) one can determine the maximum-power point, the point that maximizes V×I;
that is, the load for which the cell can deliver maximum electrical power at that level of
irradiation. (The output power is zero in both the short circuit and open circuit extremes).
A high quality, mono-crystalline silicon solar cell, at 25 °C cell temperature, may produce 0.60
volts open-circuit (VOC). The cell temperature in full sunlight, even with 25 °C air temperature,
will probably be close to 45 °C, reducing the open-circuit voltage to 0.55 volts per cell. The
voltage drops modestly, with this type of cell, until the short-circuit current is approached (Isc).
Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the
open-circuit voltage (0.43 volts in this case) and 90% of the short-circuit current. This output can
be up to 70% of the VOC x ISC product. The short-circuit current (Isc) from a cell is nearly
proportional to the illumination, while the open-circuit voltage (VOC) may drop only 10% with a
80% drop in illumination.
Fill factor
Another defining term in the overall behavior of a solar cell is the fill factor (FF).
The fill factor is defined as the ratio of the actual maximum obtainable power, to the product of
the open circuit voltage and short circuit current. This is a key parameter in evaluating the
performance of solar cells. This is the ratio of the maximum power point divided by the open
circuit voltage (Voc) and the short circuit current (Isc):
The fill factor is directly affected by the values of the cells series and shunt resistance. Increasing
the shunt resistance (Rsh) and decreasing the series resistance (Rs) will lead to higher fill factor,
thus resulting in greater efficiency, and pushing the cells output power closer towards its
theoretical maximum.
Solar cell efficiency =
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑤𝑒𝑟 𝑜𝑢𝑡𝑝𝑢𝑡 (𝑉𝑚 ×𝐼𝑚 )
𝐼𝑛𝑠𝑜𝑙𝑎𝑡𝑖𝑜𝑛 ×𝐴𝑟𝑒𝑎 𝑜𝑓 𝑑𝑒𝑣𝑖𝑐𝑒
𝐹𝐹×𝑉 ×𝐼
𝑜𝑐 𝑠𝑐
= 𝑇𝑜𝑡𝑎𝑙 𝐼𝑛𝑠𝑜𝑙𝑎𝑡𝑖𝑜𝑛
Quantum efficiency =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 ℎ𝑜𝑙𝑒 𝑝𝑎𝑖𝑟𝑠 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑝𝑒𝑟 𝑢𝑛𝑖𝑡 𝑎𝑟𝑒𝑎
Spectral responsivity =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑠𝑡𝑟𝑖𝑘𝑖𝑛𝑔 𝑡ℎ𝑒 𝑑𝑒𝑣𝑖𝑐𝑒
𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝑓𝑙𝑢𝑥 𝑞
𝑃ℎ𝑜𝑡𝑜𝑛 𝑒𝑛𝑒𝑟𝑔𝑦
ℎ𝑐
𝜆
𝑞𝜆
= ℎ𝑐
Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 40.7% with
multiple-junction research lab cells and 42.8% with multiple dies assembled into a hybrid
package.[24] Solar cell energy conversion efficiencies for commercially available multicrystalline
Si solar cells are around 14-19%. However, there is a way to "boost" solar power. By increasing
the light intensity, typically photogenerated carriers are increased, resulting in increased
efficiency by up to 15%. These so-called "concentrator systems" have only begun to become
cost-competitive as a result of the development of high efficiency GaAs cells. The increase in
intensity is typically accomplished by using concentrating optics. A typical concentrator system
may use a light intensity 6-400 times the sun, and increase the efficiency of a one sun GaAs cell
from 31% at AM 1.5 to 35%.
SOLAR ARRAYS
A photovoltaic array consists of a small or large group of connected PV panels, depending on the
amount of power desired. Solar cells are connected in series to form a string and a number of
such strings are connected in parallel to form a solar array.
Net voltage output of n cells of V volt each connected in series = nV
But the current will remain the same.
When cells are connected in parallel, voltage remains constant, same as that of one sell but
current gets multiplied. Cells or strings can be connected in parallel only if their voltages are the
same. If ni cells of V voltage and i current each are connected in parallel,
The net current I = ni x I and voltage will remain V only.
The nominal voltage of a solar module is 12V and a PV module for charging batteries usually
have 33 to 36 cells. These cells are mounted together under an air tight, mechanically rigid,
transparent cover
SupposeV = Operating voltage of a solar generator
Vn, In = Nominal voltage and current of a module
Number of modules connected in series NS = V/ Vn
Number of modules connected in parallel NP = I/In
Total number of modules N = NS x Np
SOLAR CELL POWER PLANT
SPV can either be grid interactive or it can act in stand-alone and self reliant mode. Larger
central power stations are designed like conventional one.
SOLAR PHOTOVOLTAIC SYSTEM
The major components of the system are:
- Photovoltaic solar array
- Inverter
- Energy storage: Lead acid battery or Nickel Cadmium battery
- System charge control (Battery controllers)
- Balance of system such as: 1. Blocking diode current thru which charge the battery
during sunshine hours. But when there is no sunshine, the diode will prevent the flow of
current in reverse direction i.e. from battery to arrays. 2. Voltage regulator to prevent
fluctuation of current due to variable intensity of sunlight.
With about 300 clear, sunny days in a year, India's theoretical solar power reception, on only its
land area, is about 5 Petawatt-hours per year (PWh/yr) (i.e. 5 trillion kWh/yr or about 600 TW).
The daily average solar energy incident over India varies from 4 to 7 kWh/m2 with about 1500–
2000 sunshine hours per year (depending upon location), which is far more than current total
energy consumption. For example, assuming the efficiency of PV modules were as low as 10%,
this would still be a thousand times greater than the domestic electricity demand projected for
2015.
Installed capacity
The amount of solar energy produced in India is less than 1% of the total energy demand. The
grid-interactive solar power as of December 2010 was merely 10 MW. Government-funded solar
energy in India only accounted for approximately 6.4 MW-yr of power as of 2005. However, as
of October 2009, India is currently ranked number one along with the United States in terms of
solar energy production per watt installed