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 thesilicon
"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,