Solar cell

Solar cell:

The only problem is that pure crystalline silicon is a poor conductor of electricity because
none of its electrons are free to move about, unlike the electrons in more optimum conductors
like copper. To address this issue, the silicon in a solar cell has impurities other atoms
purposefully mixed in with the silicon atoms which changes the way things work a bit. We
usually think of impurities as something undesirable, but in this case, our cell wouldn't work
without them. 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, in the form of heat for example, 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, called free carriers, then wander randomly around the crystalline
lattice looking for another hole to fall into and carrying an electrical current. However, there
are so few of them in pure silicon, that they aren't very useful.
But our impure silicon with phosphorous atoms mixed in is a different story. It
takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they
aren't tied up in a bond with any neighboring atoms. 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.
The other part of a typical solar cell is doped with the element 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 ("p" for positive) has free openings and carries the opposite
(positive) charge.



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