The oldest and
most efficient type of photovoltaic cell is made from
single-crystalline Silicon. It
is called single-crystalline because the atoms form a nearly
perfect, regular lattice – if you could see into the cell, it
would look exactly the same in almost every spot.
In these cells, electrons released during the photovoltaic
effect have clear, unobstructed paths on which to travel. Most silicon
comes from ordinary sand and several steps are required to turn it
into a crystalline solar cell. The silicon must first be separated from the oxygen with
which it is chemically bound. Then
it must be purified to a point where the material includes less than
one non-silicon atom per billion.
The resulting semiconductor grade silicon is one of the
world’s purest commercial materials and has a price tag of $40 to
$50 per kilogram. The process of
growing crystalline silicon begins with a vat of extremely hot,
liquid silicon. A
“seed” of single-crystal silicon on a long wire is placed inside
the vat. Then, over the
course of many hours, the liquid silicon is cooled while the seed is
slowly rotated and withdrawn. As
they cool, silicon atoms inside the vat bond with silicon atoms of
the seed. The slower
and smoother the process, the more likely the atoms are to bond in
the perfect lattice structure. When the wire
in fully removed, it holds a crystal about 8 inches in diameter and
3 feet long – the size of long salami.
It is cut into wafers, 8/1000 to 10/1000 of an inch thick
with a diamond-edge blade and much of the silicon crystal, now worth
hundreds of dollars per kilogram, is turned into dust in the
process. The wafers are
polished, processed into cells, and mounted in modules. More than a
hundred industry and university research teams have worked to
upgrade and automate the manufacture of crystalline silicon solar
cells. They try to
further reduce the cost of purified silicon, to develop high-speed
crystal pullers and water-slicing techniques, and to improve the
overall design of modules. One of the main objectives of PV research, however, has been to increase the efficiency with which photovoltaic modules convert sunlight into electricity. Commercial solar modules typically turn 10 to 14 percent of the sunlight that strikes them into electricity. In the laboratory, module efficiencies of more than 20 percent have been achieved. NOTE:
Photovoltaic conversion efficiency is generally based on module
output rather than cell output. Modules include many connections and
tiny wires in which electricity is lost.
Consequently, they give lower efficiencies than individual
cells.
Polycrystalline photovoltaic cells are exactly what the name implies – a patchwork quilt of single-crystalline silicon molecules. Connections between these molecules are random and do not form a perfect lattice structure. Polycrystalline cells are less efficient than single-crystalline cells because released electrons cannot follow clear paths. These cells are
produced by pouring hot, liquid silicon into square molds or casts.
The silicon is cooled to form solid blocks, which are sliced
like single-crystalline silicon. These cells are
less expensive to produce than single-crystalline cells because
their manufacturing process does not require many careful hours of
cooling and rotating silicon material. The main
challenge of polycrystalline cells is attaining a sufficiently high
efficiency. Typically,
the boundaries between crystals impede the flow of electrons,
resulting in module efficiencies of only 7 to 10 percent.
Concentrator
cells employ lenses and mirrors to focus the sun’s light onto a
high-efficiency, single-crystalline cell. Concentrators help gather
sunlight so that a smaller-than-normal cell can produce the same
amount of electricity as a standard module.
Efficiencies range from 15 to 20 percent with efficiencies as
high as 26 percent for a single cell. Although they
use less of the costly photovoltaic material, other elements
increase their cost. Because
of their lenses and mirrors, for example, concentrator cells must
air directly at the sun. A
tracking system is crucial for effective operation.
In the past
decade, much progress has been made in developing and refining
thin-film photographic cells. These cells are created by depositing hot, liquid silicon or
other semi-conductor materials onto glass, metal or plastic. One thin-film
technology, which is already employed in many PV modules, is called
“amorphous silicon”. It is composed or randomly arranged atoms,
forming a dense, noncrystalline material resembling glass.
The silicon layer is less than a millionth of a meter (a
micron) thick requiring considerably less pure silicon then other
cell types. Researchers are
working to obtain higher efficiency from this material, which lacks
the ordered structure and inherent photovoltaic properties of
crystalline silicon. Today’s
commercial efficiency average 5 to 6 percent but efficiencies as
high as 14.5 percent have been exhibited in laboratories.
These cells are
still in the developmental stage but offer great potential for the
future of photovoltaics. Tandem, or multiple-junction cells, are actually several
cells stacked on top of each other.
Each cell layer is able to convert a different wavelength, or
color, of the light spectrum into electricity. Tandem cells
have displayed efficiencies higher than 14 percent in the laboratory
and theorist predict efficiencies as 35 to 40 percent.
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