Essay About N-Type And Solar Cells

Solar Cells
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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. 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