The UN's Intergovernmental Panel on Climate Change
predicts that average global temperatures will rise as much as 5.8
by the end of the century, almost double their previous forecast. The
ice caps are melting faster than expected, raising global water
levels; natural disasters are predicted to continue to become more
frequent and severe as the planet warms; coral reefs are struggling to
survive; polar bears are under pressure; asthma caused by air
pollution in our cities has dramatically increased; many people
predict the coming of an environmentally catalyzed economic collapse.
2002 was the second warmest year on record, beaten only by 1998. It
was the 24th warmest year in a row.
The solar energy cell (photovoltaic) is one, among
many, of the progressive technologies that nations must invest in, and
engineers must take interest in moving the industry forward if we are
ever to kick our lethal fossil fuel habit. Even though the
environmental reason is enough to necessitate an article on this
subject, I do have other reasons-I'm doing a physics independent
studies course on photovoltaic cells. So this week I get to rant about
my physics independent studies topic while we learn about a technology
that is generally accepted to be an integral part of what has to be
done to prevent a global catastrophe.
The most common type of photovoltaic cell is made
primarily from silicon, an element with an outer shell containing 4
electrons. Since 8 electrons fit snugly into the outer shell, when you
stick a lot of silicon atoms together they will arrange themselves so
that they share each other's outer electrons and have full outer
orbitals. When an electron is in the outer orbital of an atom in a
solid, it is said to be in a valence band. But, given enough energy,
electrons can leave the restrictive valence band and move into the
conduction band, where they are able to move around and carry charge.
This moving charge is called electric current - the stuff that we're
after. Conduction band electrons are either extra electrons that don't
fit into an already full valence band or have been coaxed into moving
from the valence band to the higher energy conduction band.
There are a number of ways to make it easier to coax
electrons into the conduction band. One way is to dope the silicon.
Doping a pure material is rather similar to doping a person. In both
cases a foreign substance must be injected into the lump of the
material at hand. In the case of silicon we can add phosphate, which
has 5 outer electrons. Remember that the silicon atoms have arranged
themselves in such a way so that each atom has a full valence orbital.
Now where there is a phosphate atom, there will be an extra electron.
This electron is then easily moved to the conduction band and becomes
a little charge carrying slave. This is called an n-type semiconductor
(the n refers to the extra negative charge). The reverse is also true:
we can add a atom with 3 electrons in its outer orbital creating a
positive charge carrying "hole." A silicon material doped in
this way is called a p-type semiconductor.
When n-type and p-type semiconductors are put
together, an "intrinsic electric field" is created in a thin
layer at the junction between the two materials. This electric field
acts to make the semiconductor a diode - a device in which the current
can only move in one direction. At equilibrium, the currents
associated with these conduction band electrons and holes will be
balanced. However, if you orient the p-type material towards the
sunlight, a photon (light quanta) may transfer its energy to an
electron in the valence band and cause that electron to move to the
conduction band. Since the internal electric field in the p-n junction
acts to make the material a diode, all the electrons that move to the
conduction band can travel in only one direction. When this happens, a
useful electrical current is created and the imaginations of those who
see a technologically advanced but environmentally sound future for
the world, are set alight.