Solar Cells

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