A background in chemistry is helpful for an understanding of the photovoltaic phenomena. If your high school chemistry is a distant blurry spot in your childhood recollections I'll attempt to add a little clarity.
The following animation illustrates how a solar cell cut from a single crystal of Silicone is able to convert sunlight into electricity. The bottom layer is doped with a P type material such as Aluminum, Gallium or Indium to produce holes (the green circles). The N type layer is doped with Phosphorous, Arsenic or Antimony to create mobile electrons (the blue dots). The animation has two phases. The dark phase in which no light is necessary and the light phase in which light is necessary.
DARK PHASE: Initially no charge difference exists between the barrier junction. However because of the crystalline nature of Silicone electrons unbounded by a stable octet in the N layer have a tendency to migrate across the junction barrier to form a stable octet in the P layer. When this happens a difference in potential is set up between the two layers.
LIGHT PHASE: Electrons become excited when light quanta penetrates into the P layer. The excited electrons have two choices of movement. They may migrate through the external circuit or short circuit there way across the barrier junction. If the photoelectric circuit is constructed correctly they will find their way back to the N junction through the external circuit.
Animation of a Photovoltaic Circuit
To better understand the workings of a solar cell
it is necessary to understand the nature of a chemical bond. I have chosen
Hydrogen and Helium to illustrate bonding.
Hydrogen atomic number one, the simplest atom is the one responsible for all this sunlight stuff anyhow so I guess we should say something about it. Helium is of course the byproduct of hydrogen fusion.
How big is a hydrogen atom?
Their are 62,800,000,000,000,000,000,000 Hydrogen atoms in a gram of Hydrogen.
So a Hydrogen atom is very small. What does it
Well all atoms are defined by the number of protons in the nucleus. At earthlike temperatures atoms have electrons associated with them. Hydrogen has one electron. If we use an orange to represent a positive nucleus an electron would be the size of a dot and have a spherical orbit a mile from the orange. For practical purposes the electronic configuration of atoms shall be represented with the Lewis electron dot symbolization.
For example : Oxygen, atomic number 8, with an electronic configuration of 1s2,2s,2p4 would be represented like this:
: O :
The electrons in the outermost orbit of an atom
are the ones that a chemist is concerned with. These are valence electrons that
interact with other atoms and are responsible for bonding. Nonmetals like Oxygen
react by taking or sharing electrons to form a stable octet configuration. Lets
take a quick look at a stable water molecule H2O.
Using the Lewis dot method of representation the water molecule would look like this:
H : O :
Notice that Oxygen now has a stable octet of electrons. Hydrogen is also happy with 2 electrons in the 1s shell. Electrons like to travel in pairs. You might imagine the two electrons traveling in opposite directions canceling out their electromagnetic fields as they move through space. Anyhow I don't want to get too far off the beaten path. The important thing for you to remember is that electrons like to travel in pairs and also form a stable octet.
Could you give me a few
more simple examples ?
Certainly. In the example below you can see pictorial and Lewis dot representations of the Hydrogen atom, the Hydrogen molecule, and the Helium atom/molecule. Notice the Hydrogen atom with one electron . It can not exist very long by itself and readily combines with another hydrogen atom to form a stable hydrogen molecule. The Helium atom has two electrons in orbit and is very happy to keep things that way. It is non reactive and belongs to a family of elements known as inert gasses.
I now understand
the importance of electron pairs and stable octets, but what does that have to
do with getting electricity out of a solar cell?
Be patient we are getting there, but first you must understand the difference between metals, nonmetals and semiconductors.
Lay it on me.
METALS are elements that have few, loosely bound electrons in their outermost orbit. You might visualize the surface of a copper wire as a sea of unbound electrons. This sea of electrons gives metals it's shinny luster and is also responsible for conductivity.
NONMETALS like Oxygen or Nitrogen have plenty of electrons in their outermost orbit but these electrons are tightly bound to their nuclei so nonmetals are not good conductors.
SEMICONDUCTORS like Silicon are neither a metals nor nonmetals. They exist is in the twilight zone of elements. To act like a metal Silicone must give up four electrons. To be a nonmetal it must take on four electrons. Either option is not feasible for Silicone so it gains stability by sharing. When it shares electrons with other silicone atoms the crystalline structure that it forms is quite remarkable. Carbon, in the same family of semiconductors as Silicone, is also known to form a special type of crystalline structure known as diamond. Anyhow notice how the electrons pair up and form a stable octet in the crystal of Silicone. The central Silicone atom in the crystal is surrounded by an octet of electrons. This atom represents the internal structure of a Silicone crystal matrix.
OK now I understand what a
crystal of silicone looks like from the inside, but how do I get electricity out
Squeeze real hard.
Only kidding. Actually in a way we do squeeze electrons out of one end of the Silicon and return it to the other end through a circuit. A better analogy is to think of the solar cell as an electron pump powered by the energy from sunlight. There are plenty of electrons in Silicone. All we have to do is find a way to get them to move around a circuit in an orderly fashion.
Good Question. Let's think about what we've learned so far about stable electronic configuration an see if we can answer this question. Silicon has a very stable crystal structure as has been demonstrated above. What would happen if we disrupted this perfect crystal with a few atoms that were almost but not quite the same as silicone?
To one side of our silicon crystal lets sprinkle a little Arsenic. Arsenic is very similar to Silicon except that it has 5 electrons in its outermost orbit. This means that it has more of a tendency to gain three electrons than it has to loose five electrons. This would make it more of a nonmetal or N type material.
On the other side of our Silicone crystal let's deposit a tiny amount of Aluminum. Aluminum has 3 electrons in it's outermost orbit which means that it has a tendency to give up three electrons rather than gain 5 electrons. This makes it a metal and a P type material. Let's take a look at the kind of monster that you want me to create.
What should I be looking
Notice the extra electrons floating around near the N material and the missing electrons around the P material?
Yes I see what you mean. Why is this so?
Well remember the N material has 5 electrons in it's outermost orbit instead of four?
Oh yeah now I get it. The extra electron is from the N material.
That's right, and this electron is without a home since the other Silicon atoms, with their stable octets, want nothing to do with it.
I see. So where will this electron go?
How about: It hopped the fence into the P doped layer to fill the hole caused by the metal with only three electrons in its outermost orbit?
Could it do that?
Why not? The drive to find a home is very strong among electrons. Let's take a look.
Do You see what happened?
Yes. The electrons hopped the fence to find a home and form a stable octet. So What?
So everything. We have just constructed a solar cell. The octet rule is satisfied in both the N and P layers and we have also created an imbalance in electronic charge. The P layer appears negative with respect to the N layer. Now all we have to do is coax the electrons to migrate through a circuit rather than take a shortcut across the barrier junction. When light penetrates into the P layer the negative electrons are knocked loose and encouraged to travel through a circuit toward the positively charged N layer. Melissa Woods does an excellent job of explaining this phenomena.
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