|Solar Power | The Components of a Solar Cell | How Silicon Produces Electricity | Relationship between N & P-Type Silicon | Photons | Limits Of Solar Cells|
Once restricted to exotic applications such as satellites,
solar panels have found their way into our everyday
A solar cell or module is constructed of several different layers, all
serving a different purpose light to bounce off. And on top of all of
this is a sheet of glass to protect the delicate silicon and other layers.
Below all of the silicon, conductive material, antireflective coating
and glass, is another conductive material layer. By attaching wires to
the top and bottom of your cell, you get full-fledged, electricity-producing,
If someone were trying to sell you an impure water, or impure gold you would rightly deny the transaction. But if someone tries to give you impure silicon, take it! That's the key to your solar cell. (Why else would you buy silicon?)
Silicon is a semiconductor. You may have heard the term "semiconductor" before, but do you know what that means? It's very simple when you break the word down. "Semi" means "partial" and "conductor" can either be someone that drives a train or something that carries an electric current. And since something as dangerous as a train being "partially driven" doesn't create electricity, we can assume that we're referring to the latter definition. Silicon is something that partially carries electricity. That means it's a poor conductor of electricity, unlike metals or your finger.
Silicon atoms have 14 atoms in three electron levels. The third level of any atom usually holds 8 electrons. But poor deprived silicon, has but four. This means silicon atoms are on a constant search for four more atoms. Luckily, every silicon atom has four electrons in its outer level also! This means that wherever you see silicon, four more atoms are sure to follow, all arranged in a crystalline structure, with full electron levels. This means that all of the electrons are happy and won't budge from their cozy orbital levels. If electricity is directed through pure silicon crystals, it will heat up (as any semiconductor does) and some electrons will break free. These "free carriers" as they're called, will wander about until they find an opening in another crystalline structure.
But when we remove some silicon atoms (say one per million) and replace them with phosphorous atoms, we get a whole new situation. Now there's an extra electron that does not participate in the crystalline structure. This left out electron is held in place only by the force of the atom's positively charged nucleus (an electron is negatively charged; and thus attracted.) When electricity is added to this setup, there are much more free carriers. This process of adding impurities is called doping. And a silicon sample doped with phosphorous is called N-type silicon (N for negative.)
N-type silicon is the one of the two layers of silicon in a solar cell. The other layer is called P-Type silicon (P for Positive). P-type silicon is doped with boron, an atom with only three valence (outer) electrons. This creates perfect little holes for our N-type silicon electrons to fall into.
When N and P-type silicon come into contact, they create their own electric
field, (this isn't where the electricity comes from, we haven't gotten
to the sun's role in all of this, be patient.) This electric field is
created when the electrons on the N-type silicon close-by fall into the
holes in the P-type silicon. The result is a barrier between the positive
and negative sides that allows electrons to travel one way. The barrier
is called a diode. The diode allows electrons to travel from the P-type
silicon to the N-type silicon, but not the other way around. When light
gets into the fray, the whole puzzle starts to fit together.
When light strikes the solar cell's silicon sheets,
one photon will bump exactly one electron away to the N-type silicon.
A photon will also rearrange the electrons on the N-type silicon and
will effectively move a "hole" over to the P-type silicon.
The result is a electron pump, that moves electrons from one side to
the other. When a wire is attached on either type of silicon, the electrons
follow the path of least resistance and travel along the wires. This
flow of electrons creates a current, while the cell's electric field
creates voltage. The two forces, voltage and current create power.
The power that give you the answer to the math question on the SAT,
and allows the electronic sides to tell you there's an accident at
the next exit.
Even though the sun produces about 1,000 watts of energy per square foot on a sunny day, a solar cell can use a maximum of about 25 percent of that. The problem isn't in the solar cell's construction, but in the light itself. Light that the sun produces, spans an entire spectrum of frequencies and strengths. Thus, the photons that can bump the electrons are a select few. Most are either too powerful, or too weak. The only exception is if a photon can displace exactly two electrons. But this isn't enough to compensate for the loss.
Another problem with solar cells, are the collection difficulties. As you expand the contact points of the wires, you block the sun. The result is a careful balance of electrode surface area and light collection surface area. Some solar cells overcome this problem with transparent conductors, but this is expensive and few employ such a technique.