|
|
Silicon Solar Cells
| Silicon—used to make some the
earliest photovoltaic (PV) devices—is still the most popular
material for solar cells. Outranked only by oxygen, silicon is
also the second-most abundant element in the Earth's crust.
However, to be useful as a semiconductor material in solar
cells, silicon must be refined to a purity of 99.9999%.
In single-crystal silicon, the molecular structure—which is
the arrangement of atoms in the material—is uniform, because
the entire structure is grown from the same crystal. This
uniformity is ideal for transferring electrons efficiently
through the material. To make an effective PV cell, however,
silicon has to be "doped" with other elements to make it
n-type and p-type.
Semicrystalline silicon, in contrast, consists of several
smaller crystals or grains, which introduce boundaries. These
boundaries impede the flow of electrons and encourage them to
recombine with holes to reduce the power output of the solar
cell. However, semicrystalline silicon is much less expensive
to produce than single-crystalline silicon. So researchers are
working on other ways to minimize the effects of grain
boundaries. |
| To create silicon in a single-crystal state,
we must first melt high-purity silicon. We then cause it to
reform or solidify very slowly in contact with a single
crystal "seed." The silicon adapts to the pattern of the
single-crystal seed as it cools and gradually solidifies. Not
surprisingly, because we start from a seed, we say that this
process is "growing" a new rod (often called a "boule") of
single-crystal silicon out of molten silicon. Several
different processes can be used to grow a boule of
single-crystal silicon. The most established and dependable
processes are the Czochralski (Cz)method and the float-zone (FZ)
technique. We also discuss "ribbon-growth" techniques.
Czochralski Silicon
In the Czochralski process, a seed crystal is dipped into a
crucible of molten silicon and withdrawn slowly, pulling a
cylindrical single crystal as the silicon crystallizes on the
seed.
Float-Zone Silicon
The float-zone process produces purer crystals than the
Czochralski method, because they are not contaminated by the
crucible used in growing Czochralski crystals. In the
float-zone process, a silicon rod is set atop a seed crystal
and then lowered through an electromagnetic coil. The coil's
magnetic field induces an electric field in the rod, heating
and melting the interface between the rod and the seed.
Single-crystal silicon forms at the interface, growing upward
as the coils are slowly raised.
Once the single-crystal rods are produced, by either the Cz
or FZ method, they must be sliced or sawn to form thin wafers.
Such sawing, however, wastes as much as 20% of the valuable
silicon as sawdust, known as "kerf." The resulting thin wafers
are then doped to produce the necessary electric field. They
are then treated with a coating to reduce reflection, and
coated with electrical contacts to form functioning PV cells.
Ribbon Silicon
Although single-crystal silicon technology is well
developed, the Czochralski and float-zone processes are
complex and expensive (as are the ingot-casting processes
discussed under multicrystalline silicon). Another group of
crystal-producing processes, however, goes by the general name
of "ribbon growth." These single crystals may cost less than
other processes, because they form the silicon directly into
thin, usable wafers of single-crystal silicon. These methods
involve forming thin crystalline sheets directly, thus
avoiding the slicing step required of cylindrical rods.
One "ribbon growth" technique—edge-defined film-fed
growth—starts with two crystal seeds that grow and capture a
sheet of material between them as they are pulled from a
source of molten silicon. A frame entrains a thin sheet of
material when drawn from a melt. This technique does not waste
much material, but the quality of the material is not as high
as Cz and FZ silicon. |
Single-Crystal Silicon
The most widely used technique for making
single-crystal silicon is the Czochralski process, in which a
seed of single-crystal silicon contacts the top of molten
silicon. As the seed is slowly raised, atoms of the molten
silicon solidify in the pattern of the seed and extend the
single-crystal structure
After growing the silicon ingot, we must saw it into thin
wafers for further processing into PV cells. |
Multicrystalline Silicon
Multicrystalline silicon devices are generally less
efficient than those of single-crystal silicon, but they can
be less expensive to produce. The multicrystalline silicon can
be produced in a variety of ways. The most popular commercial
methods involve a casting process in which molten silicon is
directly cast into a mold and allowed to solidify into an
ingot. The starting material can be a refined lower-grade
silicon, rather that the higher-grade semiconductor grade
required for single-crystal material. The cooling rate is one
factor that determines the final size of crystals in the ingot
and the distribution of impurities. The mold is usually
square, producing an ingot that can be cut and sliced into
square cells that fit more compactly into a PV module. (Round
cells have spaces between them in modules, but square cells
fit together better with a minimum of wasted space). |
Amorphous Silicon
Amorphous solids, like common glass, are materials whose
atoms are not arranged in any particular order. They don't
form crystalline structures at all, and they contain large
numbers of structural and bonding defects. But they have some
economic advantages over other materials that make them
appealing for use in solar electric, or photovoltaic (PV),
systems.
In 1974, researchers began to realize that they could use
amorphous silicon in PV devices by properly controlling the
conditions under which it is deposited and by carefully
modifying its composition. Today, amorphous silicon is common
in solar-powered consumer devices that have low power
requirements, such as wristwatches and calculators.
Amorphous silicon absorbs solar radiation 40 times more
efficiently than does single-crystal silicon, so a film only
about 1 micrometer—or one one-millionth of a meter—thick can
absorb 90% of the usable light energy shining on it. This is
one of the chief reasons that amorphous silicon could reduce
the cost of photovoltaics. Other economic advantages are that
it can be produced at lower temperatures and can be deposited
on low-cost substrates such as plastic, glass, and metal. This
makes amorphous silicon ideal for building-integrated PV
products like the one shown in the photo. And these
characteristics make amorphous silicon the leading thin-film
PV material. |
Amorphous silicon's random
structural characteristics cause deviations like "dangling
bonds." Dangling bonds provide places for electrons to
recombine with holes, but they may be neutralized somewhat
with hydrogen |
A Closer Look
Amorphous silicon does not have the structural uniformity of
single- or multicrystalline silicon. Small deviations in this
material result in defects such as "dangling bonds," where atoms
lack a neighbor to which they can bond. These defects provide
places for electrons to recombine with holes, rather than
contributing to the electrical circuit. Ordinarily, this kind of
material would be unacceptable for electronic devices, because
defects limit the flow of current. But amorphous silicon can be
deposited so that it contains a small amount of hydrogen, in a
process called "hydrogenation." The result is that the hydrogen
atoms combine chemically with many of the dangling bonds,
essentially removing them and permitting electrons to move through
the material.
Staebler-Wronski Effect
Instability is the greatest stumbling block for amorphous
silicon. These cells experience the Staebler-Wronski effect, where
their electrical output decreases over a period of time when first
exposed to sunlight. Eventually, however, the electrical output
stabilizes. This effect can result in up to a 20% loss in output
before the material stabilizes. Exactly why this effect occurs is
not fully understood, but part of the reason is likely related to
the amorphous hydrogenated nature of the material. One way to
mitigate—though not eliminate—this effect is to make amorphous
silicon cells that have a multijunction design (discussed in
another section).
Cell Design
Because of amorphous silicon's unique properties, solar cells
are designed to have an ultrathin (0.008 micrometer) p-type top
layer, a thicker (0.5 to 1 micrometer) intrinsic middle layer, and
a very thin (0.02 micrometer) n-type bottom layer. This design is
called a "p-i-n" structure, being named for the types of the three
layers. The top layer is made so thin and relatively transparent
that most light passes right through it, to generate free
electrons in the intrinsic layer. The p- and n-layers produced by
doping the amorphous silicon create an electric field across the
entire intrinsic region, thus inducing electron movement in that i-layer.
|
© 1999-2006 GenPro Energy Solutions, LLC - All Rights Reserved
Toll Free: 866.593.0777
info@genproenergy.com
|
|
|
