Essay/Term paper: Solar cells

Essay, term paper, research paper:  Biology

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 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. 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