The Composition of a Cell and a Module
Photovoltaic cells or panels are only one way of generating electricity from solar energy. They are not the most efficient, but they are the most convient to use on a small to medium scale.
PV cells are mostly made from silicon (though a number of non silicon based methods are being developed), similar to that used in computer "chips". While silicon itself is a very abaundant mineral, the manufacture of solar cells (as with computer chips) has to be in a very clean environment. This causes production costs to be high.
A PV cell is constructed from two types of silicon, which when hit by solar energy, produce a voltage difference accross them, and, if connected to an electrical circuit, a current will flow.
A number of photovoltaic cells will be connected together in an "Module", and usually encapsulated in glass held a frame which can then be mounted as required. The cells in a module will be wired in series or parallel to produce a spacified voltage. What may be refered to as a 12 volt panel may produce around 16 volts in full sun to charge a 12 volt battery.
In most cases, a number of panels (modules) will be connected together to form an "Array". Panels of a similar type may be connected in series to give a higher voltage (two 12 volt panels may be connected in series to produce 24 volts).
Usually a number of panels will be connected in parallel to give an increased current.
History of Photovoltaic Cells
It may surprise you to know, that the ability of certain treated substances to generate electricity when light falls on them was discovered as far back as 1839. That is 40 years before Thomas Eddison was credited with inventing the first workable electric light bulb.
However something that could be described as a solar cell was not created until the late 1800's, using selenium.
It wasn't until the 1950's that it was discovered that silicon performs much better and the way was paved to create viable solar cells
So How does a Solar Cell Work?
In a crystal of pure silicon, the atoms form a lattice. These atoms, like any others, have nucleus which includes positive charged protons, while around the nucleus, are negatively charged electrons in layers or shells. The outer shell of electrons is not "full", so neighbouring atoms share electrons and hold each other together in the crystal. These electrons are held quite firmly in place and do not readily move around.
However, the pure silicon crystal can be "doped" with a different element, ie small amounts of an "impurity" are added. If the doping is done with an element that has more electrons in it's outer shell than silicon, there will be negatively charged electrons that are free to move around, and this is called "n-type" silicon. This material will conduct electricity much better than pure silicon as these spare electrons are more free to move, and we have created a semiconductor.
The crystal does not have an overall negative charge however as the negative electrons are still balanced by positive protons in the nucleus.
If instead, the silicon is doped with an element having fewer electrons in it's outer shell, there will be an overall shortage of electrons, and the material will be a p-type silicon. The minute areas where electrons are effectively missing are called holes, and these holes can also freely move around.
In a solar cell, there will be both n-type and p-type silicon in contact with each other. Electrons will move across from the n-type to the p-type at their junction as they will be attracted to the nearby holes. Once this has happened at the junction, this area acts a barrier, stopping further electrons moving across and an electric field exists across the junction.
If light energy is absorbed by the cell, the energy will push electrons across the junction and, if an electrical circuit is made between the two silicon types, the electrons will flow through it, back to where they came from, and continue to do so.
Luckily for us, the flow of electrons (in other words, the electric current) can be made do work on the way round, ie charging batteries.
This type of cell may be 15-20% efficient, partly due to the silicon wafers not absorbing all the light energy.
A more sophisticated type of cell, known as a Multi-Junction Cell, may have further wafer pairs above or below, using different doping chemicals, each able to absorb different wavelengths of light.