Solar Cells

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. It is no wonder that one marvels on how such a device would
function.
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 neighbouring 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]. The drawings in Figure
1.2 are 2-dimensional representations of n-and p-type silicon
crystals, in which the atomic nucleii in the lattice are
indicated by circles and the bonding valence electrons are shown
as lines between the atoms. 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 'm.
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.



Bibliography

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