The most common material used in solar cells is
single crystal silicon. Solar cells made from
single crystal silicon are currently limited to
about 25% efficiency because they are most
sensitive to infrared light, and radiation in this
region of the electromagnetic spectrum is
relatively low in energy.
Figure 3 -- Single crystal solar
cells (image courtesy ACRE
But single crystal silicon isn't the only
material used to build solar cells.
Figure 4 -- Polycrystalline
solar cells (image courtesy ACRE
Polycrystalline ("many crystals") solar cells
are made by a casting process in which molten
silicon is poured into a mould and allowed to cool,
then sliced into wafers. This process results in
cells that are significantly cheaper to produce
than single crystal cells, but whose efficiency is
limited to less than 20% due to internal resistance
at the boundaries of the silicon crystals.
Figure 5 -- Amorphous solar
cells (image courtesy ACRE
Amorphous cells are made by depositing silicon
onto a glass substrate from a reactive gas such as
silane (SiH4). This type of solar cell
can be applied as a thin film to low cost
substrates such as glass or plastic. Thin film
cells have a number of advantages, including easier
deposition and assembly, the ability to be
deposited on inexpensive substrates, the ease of
mass production, and the high suitability to large
applications. Since amorphous silicon cells have no
crystal structure at all, their efficiencies are
presently only about 10% due to significant
internal energy losses.
Aside from the various forms of silicon, a
number of other materials can also be used to make
solar cells -- gallium arsenide, copper indium
diselenide and cadmium telluride to name a few.
Note that solar cells are sensitive to different
wavelengths of light (i.e., photons of different
energies) as a function of the materials they are
built from. Accordingly, some cells are better
performers outdoors (i.e., optimized for sunlight),
while others are better performers indoors
(optimized for fluorescent light).
Newer, high-tech solar cells have yielded
improved energy conversion efficiency by
incorporating two or more layers of different
materials with different wavelength sensitivities.
Top layers are designed to absorb higher energy
photons while allowing lower energy photons through
to be absorbed by the layers beneath.
Double-junction cells are commercially available to
on occasion (surplus shops often call these
"spacecraft cells"). Spacecraft and other
mass-sensitive applications have now started to
make use of triple-junction cells (but don't expect
to find these in surplus shops any time soon).
Solar cells also are available in a variety of
packages. Most common are "raw cells," often with
some cover sheet attached. One popular line of
cells, the Panasonic Suncerams, consist of
amorphous silicon cells, deposited on the back of a
glass substrate (in this case, the glass functions
as both substrate and cover sheet). These are
durable and cost-effective cells, if a bit heavy
due to the thickness of the glass. Encapsulated
solar cells are also sold -- as the name implies,
an enclosure (often plastic, often with some sort
of concentrator lenses built into the cover sheet)
contains a regular (generally multicellular) solar
cell or cells. These are extremely durable, if
heavy and none too efficient. Recently, flexible
solar cells have become available. These are
amorphous cells on a thin plastic substrate -- low
efficiency, fairly high cost, but light and a very
useful package for some applications.
Now let's move on to the practical issues
involved in using