How Silicon Makes a Solar Cel

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Silicon has some special chemical properties, especially in
its crystalline form. An atom of sili­con has 14 electrons,
arranged in three different shells. The first two shells,
those closest to the center, are completely full. The outer
shell, however, is only half full, having only four electrons.
A silicon atom will always look for ways to fill up its last
shell (which would like to have eight electrons). To do this,
it will share electrons with four of its neighbor silicon
atoms. It's like every atom holds hands with its neighbors,
except that in this case, each atom has four hands joined to
four neighbors. That's what forms the crystalline structure,
and that structure turns out to be important to this type of
PV cell.

We've now described pure, crystalline silicon. Pure silicon
is a poor conductor of electricity because none of its
electrons are free to move about, as electrons are in good
conductors such as copper. Instead, the electrons are all
locked in the crystalline structure. The silicon in a solar
cell is modified slightly so that it will work as a solar
cell.

A solar cell has silicon with impurities -- other atoms mixed
in with the silicon atoms, changing the way things work
a bit. We usually think of impurities as something
undesirable, but in our case, our cell wouldn't work without
them. These impurities are actually put there on purpose.
Consider silicon with an atom of phosphorous here and there,
maybe one for every million silicon atoms. Phosphorous has
five electrons in its outer shell, not four. It still bonds
with its silicon neighbor atoms, but in a sense, the
phosphorous has one electron that doesn't have anyone to hold
hands with. It doesn't form part of a bond, but there is
a positive proton in the phosphorous nucleus holding it in
place.

When energy is added to pure silicon, for example in the
form of heat, it can cause a few electrons to break free of
their bonds and leave their atoms. A hole is left behind in
each case. These electrons then wander randomly around the
crystalline lattice looking for another hole to fall into.
These electrons are called free carriers, and can carry
electrical current. There are so few of them in pure silicon,
however, that they aren't very useful. Our impure silicon
with phosphorous atoms mixed in is a different story. It
turns out that it takes a lot less energy to knock loose one
of our "extra" phosphorous electrons because they aren't tied
up in a bond -- their neighbors aren't holding them back. As
a result, most of these electrons do break free, and we have
a lot more free carriers than we would have in pure silicon.
The process of adding impurities on purpose is called doping,
and when doped with phosphorous, the resulting silicon is
called N-type ("n" for negative) because of the prevalence of
free electrons. N-type doped silicon is a much better
conductor than pure silicon is.

Actually, only part of our solar cell is N-type. The other
part is doped with boron, which has only three electrons in
its outer shell instead of four, to become P-type silicon.
Instead of having free electrons, P-type silicon ("p" for
positive) has free holes. Holes really are just the absence
of electrons, so they carry the opposite (positive) charge.
They move around just like electrons do.

The interesting part starts when you put N-type silicon
together with P-type silicon. Remember that every PV cell has
at least one electric field. Without an electric field, the
cell wouldn't work, and this field forms when the N-type and
P-type silicon are in contact. Suddenly, the free electrons
in the N side, which have been looking all over for holes to
fall into, see all the free holes on the P side, and there's
a mad rush to fill them in.