There are a number of different technologies that can be used to
produce devices which convert light into electricity, and we are going
to explore these in turn. There is always a balance to be struck between
how well something works, and how much it costs to produce, and the
same can be said for solar energy.
We take solar cells, and we
combine them into larger units known as "modules," these modules," these
modules can again be connected together to form arrays. Thus we can see
that there is a hierarchy, where the solar cell is the smallest part.
Let
us look into the structure and properties of solar "cells," but bear in
mind, when combined into modules and arrays, the solar "cells" here are
mechanically supported by other materials-aluminum, glass, and plastic.
One
of the materials that solar cells can be made from is silicon-this is
the material that you find inside integrated circuits and transistors.
There are good reasons for using silicon; it is the next most abundant
element on earth after oxygen. When you consider that sand is silicon
dioxide (SiO2), you realize that there is a lot of it out there!
Silicon
can be used in several different ways to produce photovoltaic cells.
The most efficient solar technology is that of "monocrystalline solar
cells," these are slices of silicon taken from a single, large silicon
crystal. As it is a single crystal it has a very regular structure and
no boundaries between crystal grains and so it performs very well. You
can generally identity a monocrystalline solar cell, as it appears to be
round or a square with rounded corners.
One of the caveats with
this type of method, as you will see later, is that when a silicon
crystal is "grown," it produces a round cross-section solar cell, which
does not fit well with making solar panels, as round cells are hard to
arrange efficiently. The next type of solar cell we will be looking at
also made from silicon, is slightly different, it is a "polycrystalline"
solar cell. Polycrystalline cells are still made from solid silicon;
however, the process used to produce the silicon from which the cells
are cut is slightly different. This results in "square" solar cells.
However, there are many "crystals" in a polycrystalline cell, so they
perform slightly less efficiently, although they are cheaper to produce
with less wastage.
Now, the problem with silicon solar cells, as
we will see in the next experiment, is that they are all effectively
"batch produced" which means they are produced in small quantities, and
are fairly expensive to manufacture. Also, as all of these cells are
formed from "slices" of silicon, they use quite a lot of material, which
means they are quite expensive.
See video below how solar panels work
Now, there is another type of
solar cells, so-called "thin-film" solar cells. The difference between
these and crystalline cells is that rather than using crystalline
silicon, these use chemical compounds to semiconduct. The chemical
compounds are deposited on top of a "substrate," that is to say a base
for the solar cell. There are some formulations that do not require
silicon at all, such as Copper indium diselenide (CIS) and cadmium
telluride. However, there is also a process called "amorphous silicon,"
where silicon is deposited on a substrate, although not in a uniform
crystal structure, but as a thin film. In addition, rather than being
slow to produce, thin-film solar cells can be produced using a
continuous process, which makes them much cheaper.
However, the
disadvantage is that while they are cheaper, thin-film solar cells are
less efficient than their crystalline counterparts.
When looking
at the merits of crystalline cells and thin-film cells, we can see that
crystalline cells produce the most power for a given area. However, the
problem with them is that they are expensive to produce and quite
inflexible (as you are limited to constructing panels from standard cell
sizes and cannot change or vary their shape).
Efficiency of different cell types:
Cell material EfficiencyArea required to generate 1 KW peak power
Monocrystalline silicon 15-18% 7-9 m2
polycrystalline silicon 13-16% 8-11 m2
Thin-film copper indium diselenide (CIS) 7.5-9.5% 11-13 m2
Cadmium telluride 6-9% 14-18 m2
Amorphous silicon 5-8% 16-20 m2
By
contrast, thin-film cells are cheap to produce, and the only factor
limiting their shape is the substrate they are mounted on.This means
that you can create large cells, and cells of different shapes and
sizes, all of which can be useful in certain applications.
We are
now going to take a detailed look at making two different types of solar
cell, one will be a crystalline solar cell, and the other a thin-film
solar cell. Both of the experiments are designed to be "illustrative,"
rather than to actually make shape is the substrate they are mounted on.
The technology required to make silicon solar cells is out of the reach
of the home experimenter, so we are going to "illustrate" the process
of how a solar cell is made, using things you can find in your kitchen.
For thin-film solar cells, we are going to make an actual solar cell,
which responds to light with changing electrical properties; however,
the efficiency of our cell will be very poor, and it will not be able to
generate a useful amount of electricity.
