Solar system

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Solar system

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
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
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
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