Solar Cells
By: Edward • Essay • 844 Words • January 19, 2010 • 945 Views
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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,