How does a solar cell work? Is it possible to create one using simple lab apparatus?
Solar cells (photovoltaics), use the energy from light photons to create electrical
potential between two layers of silicon crystal. The atomic nature of silicon,
with some added impurities, is what makes it all possible. The outer orbital
electron shell of a silicon atom contains four electrons. Since it takes eight
electrons to fill the electron shell, a silicon atom is continually looking for
four electrons to bond with. This it finds by bonding covalently with other atoms
of silicon forming a characteristic crystalline structure. Silicon atoms thusly
joined are very stable and are not electrically conductive, but this is where the
impurities come in. By 'doping' the silicon with substances such as phosphorus and
boron, entirely different electrical properties are introduced into the silicon
creating semi-conductive material.
For instance, when phosphorus joins with silicon, it creates an N-type
semi-conductive material because phosphorus has five electrons in its outer shell.
The silicon wants four of them but that leaves one electron hanging out by its
lonesome and giving the molecule a negative charge. If boron joins with silicon,
it creates P-type semi-conductive material (positive charge), as boron has three
electrons in its outer shell. Even though silicon bonds with it, it leaves an
electron 'hole,' where the molecule is positively charged and is still seeking an
If layers of phosphorus impregnated silicon and boron-impregnated silicon are
joined together with metal leads or conduits, an electrical potential can be
created with some help from light. When light photons strike the phosphorus layer
containing the extra electrons, those electrons can be sheered off and freed. When
they are, they immediately recognize the potential in the boron layer and head that
way. If a load (some work that you want to have done with electricity), happens to
be connected in between these two layers where the potential has been created, then
the migrating electrons are useful electrical current.
Solar cells are a wonderful alternative energy sources but have definite
limitations. Since not all visible light is useful for this process, most of the
sunlight energy can not be used to free electrons in the solar cells. Much of it
is reflected or passes through not hitting the desired electron target. In
addition, the electrical potential is very small and even with the most efficient
solar cells; they must be chained together in large arrays to produce enough
electricity to be useful. Because of the nature in which they produce their
electricity, solar cells do experience a slight drop in effectiveness but they
essentially never wear out. Then of course the most obvious problem: what do you
do if the sun is not shining?
The nature work required to fabricate semi-conductive materials is probably beyond
the realm of simple lab equipment. But many solar cell companies will give away
broken cell fragments for the asking if you are looking for something to play with.
Stephen Portz, Technology Teacher, Space Coast Middle School, FL
'There comes a time when the mind takes a higher plane of knowledge but can never prove how it got there. All great discoveries have involved such a leap. The important thing is not to stop questioning.'