One of the major components of solar cells is silicon, which has several interesting properties allowing it to be a very useful material in solar cells. Pure silicon forms a crystalline structure with each atom bonding to its four nearest neighbours.
Image taken from http://library.thinkquest.org without permission
Pure silicon is a relatively poor conductor because all the electrons are locked up in the crystalline structure. For a solar cell, the silicon has some impurities present such as some phosphorus atoms. Phosphorus has five electrons in it's outer shell so it is able to form bonds with four neighbouring phosphorus atoms, but it also has one electron which is not used in bonding.
When energy is added to pure silicon, such as in the form of heat, some electrons can break away from their bonds and leave a hole behind. These electrons, known as free carriers, will then move through the crystal looking for another hole to fill. In pure silicon, there are so few of them however that they are of very little use.
However, when energy is added to silicon with some phosphorus impurities, many extra phosphorus electrons can become free carriers, as they are not used in bonding. The process of adding impurities is called doping and if phosphorus is used, the resulting material is called N-type silicon (N for negative). This N-type silicon is a much better conductor than pure silicon.
The other part of a solar cell is P-type silicon (P for positive). This silicon has been doped with boron. Boron only has three electrons in it's outer shell so this silicon has extra holes instead of extra electrons. Holes are just the absence of electrons so they have a positive charge and can also move around.
When the N-type and the P-type layers are in contact with each other, all the free electrons on the N-side try and fill all the holes on the P-side. When the holes and the electrons mix at the junction between the two layers, the neutrality of the silicon is disrupted. At the junction, the holes and electrons mix and form a barrier. This makes it harder for the electrons to cross to the P-side and an equilibrium is reached where an electric field separates the two sides.
Image taken from http://science.howstuffworks.com without permission
When light (as photons) hits the cell, each photon with sufficient energy will free one electron and also result in one hole being formed. If this occurs close to the electric field, the field will cause the electron to go to the N-side and the hole to go to the P-side. This will cause a disruption of electrical neutrality and if a current path is present, electrons will flow along this to fill the holes which were sent to the P-side. This flow of electrons provides the current. The cell's electric field provides the voltage so together we have power (the product of current and voltage).
Image taken from http://acre.murdoch.edu.au/refiles/pv/text.html
