You've
probably seen calculators with solar cells -- devices that never need batteries and in some cases, don't even have an
off button. As long as there's enough light,
they seem to work forever. You may also have seen larger solar panels, perhaps
on emergency road signs, call boxes, buoys and even in parking lots to power
the lights.
Although
these larger panels aren't as common as solar-powered calculators, they're out
there and not that hard to spot if you know where to look. In fact, photovoltaics -- which were once used almost exclusively
in space, powering satellites' electrical systems as far back as 1958 -- are
being used more and more in less exotic ways. The technology continues to pop
up in new devices all the time, from sunglasses to electric vehicle charging
stations.
The
hope for a "solar revolution" has been floating around for decades --
the idea that one day we'll all use free electricity from the sun. This is a seductive promise,
because on a bright, sunny day, the sun's rays give off approximately 1,000
watts of energy per square meter of the planet's surface. If we could collect
all of that energy, we could easily power our homes and offices for free.
In this
article, we will examine solar cells to learn how they convert the sun's
energy directly into electricity. In the process, you will learn why we're
getting closer to using the sun's energy on a daily basis, and why we still
have more research to do before the process becomes cost-effective.
Photovoltaic
Cells: Converting Photons to Electrons
The
solar cells that you see on calculators and satellites are also called photovoltaic (PV)
cells, which as the name implies (photo meaning "light" and voltaic
meaning "electricity"), convert sunlight directly into electricity. A
module is a group of cells connected electrically and packaged into a frame
(more commonly known as a solar panel), which can then be grouped into larger
solar arrays, like the one operating at Nellis Air Force Base in Nevada.
Photovoltaic
cells are made of special materials called semiconductors such as silicon,
which is currently used most commonly. 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 field that acts 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 for external use, say, to 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. On the next page, let's
take a deeper look into one example of a PV cell: the single-crystal silicon
cell.
How
Silicon Makes a Solar Cell
Silicon
has some special chemical properties, especially in its crystalline form. An atom of
silicon has 14 electrons, arranged in three different shells. The first two
shells -- which hold two and eight electrons respectively -- are completely
full. The outer shell, however, is only half full with just four electrons. A
silicon atom will always look for ways to fill up its last shell, and to do
this, it will share electrons with four nearby atoms. It's like each atom holds
hands with its neighbors, except that in this case, each atom has four hands
joined to four neighbors. That's what forms the crystalline structure, and that structure
turns out to be important to this type of PV cell.
The
only problem is that pure crystalline silicon is a poor conductor of
electricity because none of its electrons are free to move about, unlike the
electrons in more optimum conductors like copper. To address this issue, the
silicon in a solar cell has impurities -- other atoms purposefully mixed in
with the silicon atoms -- which changes the way things work a bit. We usually
think of impurities as something undesirable, but in this case, our cell
wouldn't work without them. Consider silicon with an atom of phosphorous here
and there, maybe one for every million silicon atoms. Phosphorous has five electrons
in its outer shell, not four. It still bonds with its silicon neighbor atoms,
but in a sense, the phosphorous has one electron that doesn't have anyone to
hold hands with. It doesn't form part of a bond, but there is a positive proton
in the phosphorous nucleus holding it in place.
When energy is added to pure silicon, in the form
of heat for example, it can cause a few electrons to break free of their bonds
and leave their atoms. A hole is left behind in each case. These electrons,
called free carriers,
then wander randomly around the crystalline lattice looking for another hole to
fall into and carrying an electrical current. However, there are so few of them
in pure silicon, that they aren't very useful.
But our
impure silicon with phosphorous atoms mixed in is a different story. It takes a
lot less energy to knock loose one of our "extra" phosphorous
electrons because they aren't tied up in a bond with any neighboring atoms. As
a result, most of these electrons do break free, and we have a lot more free
carriers than we would have in pure silicon. The process of adding impurities
on purpose is called doping,
and when doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because
of the prevalence of free electrons. N-type doped silicon is a much better
conductor than pure silicon.
The
other part of a typical solar cell is doped with the element boron, which has
only three electrons in its outer shell instead of four, to become P-type
silicon. Instead of having free electrons, P-type ("p" for positive) has free
openings and carries the opposite (positive) charge.
Anatomy
of a Solar Cell
Before
now, our two separate pieces of silicon were electrically neutral; the
interesting part begins when you put them together. That's because without an electric field, the
cell wouldn't work; the field forms when the N-type and P-type silicon come
into contact. Suddenly, the free electrons on the N side see all the openings
on the P side, and there's a mad rush to fill them. Do all the free electrons
fill all the free holes? No. If they did, then the whole arrangement wouldn't
be very useful. However, right at the junction,
they do mix and form something of a barrier, making it harder and harder for
electrons on the N side to cross over to the P side. Eventually, equilibrium is
reached, and we have an electric field separating the two sides.
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).
When
light, in the form of photons,
hits our solar cell, its energy breaks apart electron-hole pairs. Each photon
with enough energy will normally free exactly one electron, resulting 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 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.
There
are a few more components left before we can really use our cell. Silicon
happens to be a very shiny material, which can send photons bouncing away
before they've done their job, so
an antireflective coating is applied to reduce those losses. The
final step is to install something that will protect the cell from the elements
-- often a glass
cover plate. PV modules are generally made by connecting
several individual cells together to achieve useful levels of voltage and
current, and putting them in a sturdy frame complete with positive and negative
terminals.
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