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Solar Cells


Solar cells today are mostly made of silicon, one of the
most common elements on Earth. The crystalline silicon
solar cell was one of the first types to be developed and
it is still the most common type in use today. They do not
pollute the atmosphere and they leave behind no harmful
waste products. Photovoltaic cells work effectively even in
cloudy weather and unlike solar heaters, are more efficient
at low temperatures. They do their job silently and there
are no moving parts to wear out. 

To understand how a solar cell works, it is necessary to go
back to some basic atomic concepts. In the simplest model
of the atom, electrons orbit a central nucleus, composed of
protons and neutrons. Each electron carries one negative
charge and each proton one positive charge. Neutrons carry
no charge. Every atom has the same number of electrons as
there are protons, so, on the whole, it is electrically
neutral. The electrons have discrete kinetic energy levels,
which increase with the orbital radius. 

When atoms bond together to form a solid, the electron
energy levels merge into bands. In electrical conductors,
these bands are continuous but in insulators and
semiconductors there is an "energy gap", in which no
electron orbits can exist, between the inner valence band
and outer conduction band [Book 1]. Valence electrons help
to bind together the atoms in a solid by orbiting 2
adjacent nucleii, while conduction electrons, being less
closely bound to the nucleii, are free to move in response
to an applied voltage or electric field. The fewer
conduction electrons there are, the higher the electrical
resistivity of the material.
In semiconductors, the materials from which solar sells are
made, the energy gap Eg is fairly small. Because of this,
electrons in the valence band can easily be made to jump to
the conduction band by the injection of energy, either in
the form of heat or light [Book 4]. This explains why the
high resistivity of semiconductors decreases as the
temperature is raised or the material illuminated. The
excitation of valence electrons to the conduction band is
best accomplished when the semiconductor is in the
crystalline state, i.e. when the atoms are arranged in a
precise geometrical formation or "lattice".
At room temperature and low illumination, pure or so-called
"intrinsic" semiconductors have a high resistivity. But the
resistivity can be greatly reduced by "doping", i.e.
introducing a very small amount of impurity, of the order
of one in a million atoms. There are 2 kinds of dopant.
Those which have more valence electrons that the
semiconductor itself are called "donors" and those which
have fewer are termed "acceptors" [Book 2].
In a silicon crystal, each atom has 4 valence electrons,
which are shared with a neighboring atom to form a stable
tetrahedral structure. Phosphorus, which has 5 valence
electrons, is a donor and causes extra electrons to appear
in the conduction band. Silicon so doped is called "n-type"
[Book 5]. On the other hand, boron, with a valence of 3, is
an acceptor, leaving so-called "holes" in the lattice,
which act like positive charges and render the silicon
"p-type" [Book 5]. Holes, like electrons, will remove under
the influence of an applied voltage but, as the mechanism
of their movement is valence electron substitution from
atom to atom, they are less mobile than the free conduction
electrons [Book 2].
In a n-on-p crystalline silicon solar cell, a shadow
junction is formed by diffusing phosphorus into a
boron-based base. At the junction, conduction electrons
from donor atoms in the n-region diffuse into the p-region
and combine with holes in acceptor atoms, producing a layer
of negatively-charged impurity atoms. The opposite action
also takes place, holes from acceptor atoms in the p-region
crossing into the n-region, combining with electrons and
producing positively-charged impurity atoms [Book 4]. The
net result of these movements is the disappearance of
conduction electrons and holes from the vicinity of the
junction and the establishment there of a reverse electric
field, which is positive on the n-side and negative on the
p-side. This reverse field plays a vital part in the
functioning of the device. The area in which it is set up
is called the "depletion area" or "barrier layer" [Book 4].
When light falls on the front surface, photons with energy
in excess of the energy gap (1.1 eV in crystalline silicon)
interact with valence electrons and lift them to the
conduction band. This movement leaves behind holes, so each
photon is said to generate an "electron-hole pair" [Book
2]. In the crystalline silicon, electron-hole generation
takes place throughout the thickness of the cell, in
concentrations depending on the irradiance and the spectral
composition of the light. Photon energy is inversely
proportional to wavelength. The highly energetic photons in
the ultra-violet and blue part of the spectrum are absorbed
very near the surface, while the less energetic longer wave
photons in the red and infrared are absorbed deeper in the
crystal and further from the junction [Book 4]. Most are
absorbed within a thickness of 100 æm.
The electrons and holes diffuse through the crystal in an
effort to produce an even distribution. Some recombine
after a lifetime of the order of one millisecond,
neutralizing their charges and giving up energy in the form
of heat. Others reach the junction before their lifetime
has expired. There they are separated by the reverse field,
the electrons being accelerated towards the negative
contact and the holes towards the positive [Book 5]. If the
cell is connected to a load, electrons will be pushed from
the negative contact through the load to the positive
contact, where they will recombine with holes. This
constitutes an electric current. In crystalline silicon
cells, the current generated by radiation of a particular
spectral composition is directly proportional to the
irradiance [Book 2]. Some types of solar cell, however, do
not exhibit this linear relationship.
The silicon solar cell has many advantages such as high
reliability, photovoltaic power plants can be put up easily
and quickly, photovoltaic power plants are quite modular
and can respond to sudden changes in solar input which
occur when clouds pass by. However there are still some
major problems with them. They still cost too much for mass
use and are relatively inefficient with conversion
efficiencies of 20% to 30%. With time, both of these
problems will be solved through mass production and new
technological advances in semiconductors. 
1) Green, Martin. Solar Cells, Operating Principles,
Technology and System Applications. New Jersey,
Prentice-Hall, 1989, pg. 104-106.
2) Hovel, Howard. Solar Cells, Semiconductors and
Semimetals. New York, Academic Press, 1990, pg. 334-339.
3) Newham, Michael ,"Photovoltaics, The Sunrise Industry",
Solar Energy, October 1, 1989, pp. 253-256.
4) Pulfrey, Donald. Photovoltaic Power Generation. Oxford,
Van Norstrand Co., 1988, pg. 56-61.
5) Treble, Fredrick. "Generating Electricity from the Sun".
New York, Pergamon Press, 1991, pg. 192-195.



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