The use of silicon in solar cells is all to do with energy levels. Conduction occurs in a material due to free
electrons moving through the solid. In order to do this the electrons must gain energy and
move into a higher energy state.
In a semiconductor the difference between the
valence band and the conduction band is very small. This means that electrons can 'jump'
the gap with the addition of only a small amount of energy. This is the case in pure silicon – a
so-called intrinsic semiconductor.
If a little aluminium is added there are fewer an P
type silicon is produced. However if a little phosphorus is added N type silicon results. The
energy bands for these impurity atoms (aluminium or phosphors) lie between the conduction
and valence bands for silicon and so it even easier for electrons to either leave these and
jump into the conduction band (N type) or accept electrons from the valence band (P type).
Such a semiconductor is known as an extrinsic semiconductor.
The conductivity of
impure silicon is much higher than that of the pure material as the movement of electrons
can be made possible by the input of very small amounts of energy – such as shining light on
the surface. This energy is provided by a quantum of light radiation (photon) falling on the
surface. Many more 'free' electrons are liberated.
A join between a P type and N
type semiconductor is known as a p-n junction diode. At the junction between the p and n
types electrons initially move across the junction and an electric field is set up with more free
electrons on one side than on the other.
Addition of energy, as in incident light,
frees an electron-hole pair, these move under the action of the field and a current is
produced.
Silicon is a good material for such a system because it is:
(a)
cheap
(b) plentiful
(c) has the right energy level structure
(d) can be made to
accept impurity atoms easily and so that the conductivity of the material can be changed by
the addition of radiant energy