لخّصلي

The photovoltaic effect is the basic physical process through which a PV cell converts
sunlight into electricity. Sunlight is composed of photons (packets of solar energy). These
photons contain different amounts of energy that correspond to the different wavelengths
of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed,
or they may pass right through. The absorbed photons generate electricity.The energy of a photon is transferred to an electron in an atom of the semiconductor
device. With its newfound energy, the electron is able to escape from its normal position
associated with a single atom in the semiconductor to become part of the current in an
electrical circuit. Special electrical properties of the PV cell (a built
-in electric field)
provide the voltage needed to drive the current through an external load.The sun's energy is vital to life on Earth. It determines the Earth's surface temperature,
and supplies virtually all the energy that drives natural global systems and cycles.
Although some other stars are enormous sources of energy in the form of x
-rays and
radio signals, our sun releases 95% of its energy as visible light. Visible light represents
only a fraction of the total radiation spectrum; infrared and ultraviolet rays are also
significant parts of the solar spectrum.
Each portion of the solar spectrum is associated with a different level of energy. Within
the visible portion of the spectrum, red is at the low
-energy end and violet is at the high

energy end. In the invisible portions of the spectrum, photons in the ultraviolet region,
which cause the skin to tan, have more energy than those in the visible region.Photons in the infrared region, which we feel as heat, have less energy than the photons
in the visible region.
The movement of light from one location to another can best be described as a wave
motion, and different types of radiation are characterized by their individual wavelengths.
These wavelengths
-the distance from the peak of one wave to the peak of the next

indicate radiation with different amounts of energy; the longer the wavelength, the less
the energy. Red light, for example, has a longer wavelength and thus has less energy than
violet light.
Each second, the sun releases an enormous amount of radiant energy into the solar
system. The Earth receives a tiny fraction of this energy; still, an average of 1367 W
reaches each square meter (m
2
) of the outer edge of the Earth's atmosphere. The
atmosphere absorbs and reflects some of this radiation, including most x
-rays and
ultraviolet rays.Yet, the amount of sunshine energy that hits the surface of the Earth every minute is
greater than the total amount of energy that the world's human population consumes in a
year.
When sunlight reaches Earth, it is distributed unevenly in different regions. Not
surprisingly, the areas near the equator receive more solar radiation than anywhere else
on Earth. Sunlight varies with the seasons, as the rotational axis of the Earth shifts to
lengthen and shorten days as the seasons change. The quantity of sunlight reaching any
region is also affected by the time of day, the climate (especially the cloud cover, which
scatters the sun's rays), and the air pollution in that region. These climatic factors all
affect the amount of solar energy that is available to PV systems.
The amount of energy produced by a PV device depends not only on available solar
energy but on how well the device, or solar cell, converts sunlight to useful electrical
energy. This is called the device or solar cell efficiency. It is defined as the amount of
electricity produced divided by the sunlight energy striking the PV device. Scientists
have concentrated their efforts over the last several years on improving the efficiency of
solar cells to make them more competitive with conventional power
-generation
technologies.We use crystalline silicon to explain the photovoltaic effect for several reasons. First,
crystalline silicon was the semiconductor material used in the earliest successful PV
device. Second, crystalline silicon is still the most widely used PV material. And third,
although other PV materials and designs exploit the PV effect in slightly different ways,
knowing how the effect works in crystalline silicon gives us a basic understanding of how
it works in all devices.
As you know, all matter is composed of atoms. Atoms, in turn, are composed of
positively charged protons, negatively charged electrons, and neutral neutrons. The
protons and neutrons, which are of approximately equal size, comprise the close
-packed
central nucleus of the atom, where almost all of the mass of the atom is located. The
much lighter electrons orbit the nucleus at very high velocities. Although the atom is built
from oppositely charged particles, its overall charge is neutral because it contains an
equal number of positive protons and negative electrons.
The electrons orbit the nucleus at different distances, depending on their energy level; an
electron of lesser energy orbits close to the nucleus, while one of greater energy orbits
farther away. The electrons farthest from the nucleus interact with those of neighboring
atoms to determine the way solid structures are formed.The silicon atom has 14 electrons. Their natural orbital arrangement allows the outer four
of these to be given to, accepted from, or shared with other atoms. These outer four
electrons, called valence electrons, play an important role in the photovoltaic effect.
Large numbers of silicon atoms, through their valence electrons, can bond together to
form a crystal. In a crystalline solid, each silicon atom normally shares one of its four
valence electrons in a covalent bond with each of four neighboring silicon The solid, then, consists of basic units of five silicon atoms: the original atom plus the
four other atoms with which it shares its valence electrons.
The solid silicon crystal is composed of a regular series of units of five silicon atoms.
This regular, fixed arrangement of silicon atoms is known as the crystal lattice.When a photon of sufficient energy strikes a valence electron, it may impart enough
energy to free it from its connection to the atom. This leaves a space in the crystal
structure where an electron once resided (and bonded), called a "hole." The electron is
now free to travel about the crystal lattice. The electron is now a part of the conduction
band, so called because these free electrons are the means by which the crystal conducts
electricity. Meanwhile, the atom left behind by the freed electron contains a net positive charge in
the form of the generated hole. This positive hole can move almost as freely about the
crystal lattice as a free electron in the conduction band, as electrons from neighboring
atoms switch partners. These light
-generated charges, both positive and negative, are the
constituents of electricity. Photovoltaic cells contain an electric field that is created when semiconductors with
different electrical characteristics come into contact. The electric field drives positive and
negative charges in opposite directions. The movement of charge carriers (through an
external circuit) is what defines electricity.
There are several ways to form the electric field in a crystalline silicon PV cell. The most
common technique is to slightly modify the structure of the silicon crystal. This
technique, known as "doping," introduces an atom of another element (called the
"dopant") into the silicon crystal to alter its electrical properties. The dopant has either
three or five valence electrons, as opposed to silicon's four.
Phosphorus atoms, which have five valence electrons, are used for doping n
-type silicon
(so called because of the presence of free negative charges or electrons). A phosphorus
atom occupies the same place in the crystal lattice that was occupied formerly by the
silicon atom it replaced. Four of its valence electrons take over the bonding
responsibilities of the four silicon valence electrons that they replaced. But the fifth
valence electron remains free, without bonding responsibilities. This unbonded valence
electron behaves like a permanent member of the crystal's conduction band.
When numerous phosphorus atoms are substituted for silicon in a crystal, many free,
conduction
-band electrons become available. The most common method of substitution is
to coat the top of a layer of silicon with phosphorus and then heat the surface.
This allows the phosphorus atoms to diffuse into the silicon. The temperature is then
lowered so that the rate of diffusion drops to zero. Other methods of introducing phosphorus into silicon include gaseous diffusion, a liquid dopant spray
-on process, and a

technique in which phosphorus ions are driven precisely into the surface of the silicon.
This n
-type silicon cannot form the electric field by itself; it is also necessary to have
some silicon altered to have the opposite electrical properties. Boron, which has three
valence electrons, is used for doping p
-type (positive
-type) silicon. Boron is introduced
during silicon processing, where silicon is purified for use in PV devices. When a boron
atom assumes a position in the crystal lattice formerly occupied by a silicon atom, there is
a bond missing an electron, in other words, an extra hole. In p
-type material, there are
many more positive charges (holes) than free electrons.
Holes are much more numerous than free electrons in a p
-type material and are therefore
called the majority charge carriers. The few electrons in the conduction band of p
-type
material are referred to as minority charge carriers. In n
-type material, electrons are the
majority carriers, and holes are the minority carriers.
The majority charge carriers, however, have excess energy that is not bound up in
valence bonding with neighboring atoms. This higher energy allows them to traverse the
crystal lattice. The majority carriers
-electrons in n
-type and holes in p
-type silicon

• are
  the ones that physically respond to an electric field. Electrons are attracted to and holes
  are repelled by an electric field.
  Where n
  -type and p
  -type silicon come into contact, an electric field forms at the junction
  (referred to as the p
  -n interface, or p
  -n junction). Like floodwaters breaking through a
  dam, some majority charge carriers on each side rush over to the other side. There are
  two forces at work in this process. The majority charge carriers are more energetic and
  more mobile than the minority carriers. They are therefore able to move from where they
  are highly concentrated across the junction to a lower concentration. This is called
  diffusion. In addition, they are attracted (electrically) by the opposite charge of the
  majority carriers across the junction. In the immediate area of the junction, the "extra"
  electron from the phosphorus fills the hole across the junction in the boron atom. Holes
  then overpopulate the immediate vicinity of the interface on the n
  -type side; electrons
  overpopulate the p
  -type side. This overabundance is true only in the immediate vicinity
  of the junction, however, The bulk of the n
  -type silicon is still populated with negative
  charges; holes remain the majority charge carriers in the bulk of the p
  -type silicon. At equilibrium, when all the charge carriers have settled down again, a net charge
  concentration exists on each side of the junction. This overpopulation of opposite charges
  creates an electric field across the interface. The strength of the electric force field
  depends upon the amount of dopant in the silicon
  -the more dopant we have, the greater
  will be the difference in electrical properties on each side and the greater the strength of
  the built
  -in electric field. When a photon of light energy is absorbed by a silicon atom, an electron
  -hole pair is
  created, and both the electron and the hole begin moving through the material. If nature
  were left to take its random course, they would recombine in about a millionth of a
  second and contribute nothing to an electrical current. But PV cells are so constructed
  that minority carriers have a good chance of reaching the electric field before
  recombining. When a minority carrier (on either side of the junction) comes close enough to feel the
  force of the electric field, it is attracted to the interface; if the carrier has sufficient
  energy, it is propelled over to the other side. Majority carriers, on the other hand, are
  repelled by this same electric field.
  By acting this way, the field sorts out the photo
  -generated electrons and holes, pushing
  new electrons to one side of the barrier and new holes to the other. This sorting


• out
  process is what gives the push to the charge carriers in an electrical circuit. Without the
  electric field, charge carriers generated by the absorption of light would go nowhere
  except back into the lattice. Attaching an external circuit allows the electrons to flow from the n
  -layer through the
  circuit and back to the p
  -layer, where the electrons combine with the holes to repeat the
  process. If there is no external circuit, the charge carriers collect at the ends of the cell.
  This buildup continues until equilibrium voltage (called the open circuit voltage) is
  reached. When photons of sun light strike a PV cell, only the photons with a certain level of
  energy are able to free electrons from their atomic bonds to produce an electric current.
  This level of energy, known as the band
  -gap energy, is the amount of energy required to
  move an outer
  -shell electron from the valence band (or level) to the conduction band (or
  level). It is different for each material and for different atomic structures of the same
  material.
  For crystalline silicon, the band
  -gap energy is 1.1 electron
  -volts (eV). An electron
  -volt is
  equal to the energy an electron acquires when it passes through a potential of 1 volt in a
  vacuum. Other PV cell materials have band
  -gap energies ranging from 1 to 3.3 eV. The
  energy of individual photons in light is also measured in eV. Photons with different
  energies correspond to distinct wavelengths of light. The entire spectrum of sunlight,
  from infrared to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. Red light has
  an energy of about 1.7 eV; blue light has an energy of about 2.7 eV.
  One key to obtaining an efficient PV cell is to convert as much sunlight into electricity as
  possible. Choosing the best absorbing material is a very important step, because the band

gap energy of the material determines how much of the sun's spectrum can be absorbed.
Silicon, for example, requires photons to have an energy of at least 1.1 eV to be absorbed
and create pairs of charge carriers. Photons with less energy either pass right through the
silicon or are absorbed as heat. Photons with too much energy are absorbed and
contribute free charge carriers, but they also heat up the cell. About 55% of the energy of
sunlight cannot be used by most PV cells because this energy either is below the band gap or carries excess energy. Researchers are working on advanced cell designs that can
reduce those losses.
Materials with lower band
-gap energies can exploit a broader range of the sun's spectrum
of energies, creating greater numbers of charge carriers (greater current). We might
conclude that material with the lowest band gap would thus make the best PV cell. But it
isn't quite that simple. The band
-gap energy also influences the strength of the electric field, which determines
the maximum voltage the cell can produce. The higher the band

• gap energy of the
  material, the higher the open
  -circuit voltage.
  The power from an electrical device such as a PV cell is equal to the product of the
  voltage (V) and the current (I). Low
  -band
  -gap cells have high current but low voltage;
  high
  -band
  -gap cells have high voltage and low current. A compromise is necessary in the
  design of PV cells. Cells made of materials with band gaps between 1 eV and 1.8 eV can
  be used efficiently in PV devices. One of the scientific discoveries that has shown great potential for the PV industry is
  thin
  -film technology. Thin films are exceedingly fine layers of semiconductors placed on
  top of each other. Thin
  -film cells can be made from a variety of materials. Today, the
  most widely used commercial thin
  -film cells are made from amorphous silicon. Two
  other materials that are showing great promise for low


• cost production, are
  polycrystalline copper indium di
  -selenide and cadmium telluride. Thin
  -film devices require very little material and have the added advantage of being easy
  to manufacture. They are made them by sequentially depositing thin layers of the
  required materials The PV cell is the basic unit in a PV system. An individual PV cell typically produces
  between 1 and 2 W, hardly enough power for the great majority of applications. But you
  can increase the power by connecting cells together to form larger units called modules.
  Modules, in turn, can be connected to form even larger units known as arrays, which can
  be interconnected for more power, and so on. In this way, you can build a PV system to
  meet almost any power need, no matter how small or great.
  Modules or arrays, by themselves, do not constitute a PV system. You must also have
  structures on which to put them and point them toward the sun, and components that take
  the direct
  -current (dc) electricity produced by the modules or arrays and conduct the
  electricity so that it may be utilized in the application. These structures and components
  are referred to as the balance
  -of-system (BOS) Solar cells have many applications. They have long been used in situations where
  electrical power from the grid is unavailable, such as in remote area power systems,
  Earth
  -orbiting satellites and space probes, consumer systems, e.g. handheld calculators or
  wrist watches, remote radiotelephones and water pumping applications. 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, that is, the
  load for which the cell can deliver maximum electrical power at that level of irradiation. Vm x Im = Pm in watts.
  The maximum power point of a photovoltaic varies with incident illumination. For
  systems large enough to justify the extra expense (say, ~1 kiloWatt ), a power point
  tracker tracks the instantaneous power by continually measuring the voltage and current
  (and hence, power transfer), and uses this information to dynamically adjust the load so
  the maximum power is always transferred, regardless of the variation in lighting. A
  photovolatic optimally runs at 50% electrical efficiency (the point of maximum power
  transfer, not to be confused with energy conversion efficiency, as it is a now
  -or
  -never
  energy source.
  A solar cell's energy conversion efficiency (
  η )is the percentage of power converted
  (from absorbed light to electrical energy) and collected, when a solar cell is connected to
  an electrical circuit. This term is calculated using the ratio of
  P
  m, divided by the input
  light irradiance under "standard" test conditions (
  E, in W/m
  2
  ) and the surface area of the
  solar cell (A
  c in m²). At solar noon on a clear March or September equinox day, the solar radiation at the
  equator is about 1000 W/m
  2
  . Hence, the "standard" solar radiation (known as the "air
  mass 1.5 spectrum") has a power density of 1000 watts per square meter. Thus, a 12%
  efficiency solar cell having 1 m² of surface area in full sunlight at solar noon at the
  equator during either the March or September equinox will produce approximately 120
  watts of peak power.
  Another defining term in the overall behavior of a solar cell is the fill factor
  (FF). This is
  the ratio of the maximum power point divided by the open circuit voltage
  (
  Voc) and the
  short circuit current
  (
  Isc):