Photovoltaic Cells: Converting Photons to Electrons
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The
solar cells that you see on calculators and
satellites are
photovoltaic cells or
modules (modules
are simply a group of cells electrically connected and
packaged in one frame). Photovoltaics, as the word implies
(photo = light, voltaic =
electricity),
convert sunlight directly into electricity. Once used
almost exclusively in space, photovoltaics are used more
and more in less exotic ways. They could even power your
house. How do these devices work?
Photovoltaic (PV)
cells are made of special materials called
semiconductors such as
silicon, which is currently the most commonly used.
Basically, when light strikes the cell, a certain portion
of it is absorbed within the
semiconductor material.
This means that the energy of the absorbed light is
transferred to the semiconductor. The energy knocks
electrons loose, allowing them to flow freely. PV cells
also all have one or more electric fields that act to
force electrons freed by light absorption to flow in a
certain direction. This flow of electrons is a current,
and by placing metal contacts on the top and bottom of
the PV cell, we can draw that current off to use
externally. For example, the current can power a
calculator. This current, together with the cell's
voltage (which is a result of its built-in electric
field or fields), defines the power (or wattage) that
the solar cell can produce.
That's the basic process, but there's really much more to
it. Let's take a deeper look into one example of a PV cell:
the single-crystal silicon cell.
Anatomy
of a Solar Cell
Before
now, our silicon was all electrically neutral. Our extra
electrons were balanced out by the extra protons in the
phosphorous. Our missing electrons (holes) were balanced
out by the missing protons in the boron. When the holes and
electrons mix at the
junction between
N-type and P-type silicon, however, that neutrality is
disrupted. Do all the free electrons fill all the free
holes? No. If they did, then the whole arrangement wouldn't
be very useful. Right at the junction, however, they do mix
and form a barrier, making it harder and harder for
electrons on the N side to cross to the P side. Eventually,
equilibrium is reached, and we have an electric field
separating the two sides.
|
The effect of the electric field in a PV cell |
This electric field acts as a
diode,
allowing (and even pushing) electrons to flow from the P
side to the N side, but not the other way around. It's like
a hill -- electrons can easily go down the hill (to the N
side), but can't climb it (to the P side).
So we've got an electric field acting as a diode in which
electrons can only move in one direction.
When light, in the form of
photons,
hits our solar cell, its energy frees electron-hole pairs.
Each photon with enough energy will normally free exactly
one electron, and result in a free hole as well. If this
happens close enough to the electric field, or if free
electron and free hole happen to wander into its range of
influence, the field will send the electron to the N side
and the hole to the P side. This causes further disruption
of electrical neutrality, and if we provide an external
current path, electrons will flow through the path to their
original side (the P side) to unite with holes that the
electric field sent there, doing work for us along the way.
The electron flow provides the
current, and
the cell's electric field causes a
voltage. With
both current and voltage, we have
power, which
is the product of the two.
|
Operation of a PV cell |
There are a few more steps left before we can really use
our cell. Silicon happens to be a very shiny material,
which means that it is very reflective. Photons that are
reflected can't be used by the cell. For that reason,
an
antireflective coating is
applied to the top of the cell to reduce reflection losses
to less than 5 percent.
The final step is the
glass cover plate that
protects the cell from the elements. PV modules are made by
connecting several cells (usually 36) in series and
parallel to achieve useful levels of voltage and current,
and putting them in a sturdy frame complete with a glass
cover and positive and negative terminals on the back.
|
Basic structure of a generic silicon PV cell |