Forming the Electric Field

Photovoltaic cells contain an electric field that is created when semiconductors with different electrical characteristics come into contact. The electric field drives positive and negative charges in opposite directions. The movement of charge carriers (through an external circuit) is what defines electricity.

There are several ways to form the electric field in a crystalline silicon PV cell. The most common technique is to slightly modify the structure of the silicon crystal. This technique, known as "doping," introduces an atom of another element (called the "dopant") into the silicon crystal to alter its electrical properties. The dopant has either three or five valence electrons, as opposed to silicon's four.

Phosphorus atoms, which have five valence electrons, are used for doping n-type silicon (so called because of the presence of free negative charges or electrons). A phosphorus atom occupies the same place in the crystal lattice that was occupied formerly by the silicon atom it replaced. Four of its valence electrons take over the bonding responsibilities of the four silicon valence electrons that they replaced. But the fifth valence electron remains free, without bonding responsibilities. This unbonded valence electron behaves like a permanent member of the crystal's conduction band.

When numerous phosphorus atoms are substituted for silicon in a crystal, many free, conduction-band electrons become available. The most common method of substitution is to coat the top of a layer of silicon with phosphorus and then heat the surface.

This allows the phosphorus atoms to diffuse into the silicon. The temperature is then lowered so that the rate of diffusion drops to zero. Other methods of introducing phosphorus into silicon include gaseous diffusion, a liquid dopant spray-on process, and a technique in which phosphorus ions are driven precisely into the surface of the silicon.

This n-type silicon cannot form the electric field by itself; it is also necessary to have some silicon altered to have the opposite electrical properties. Boron, which has three valence electrons, is used for doping p-type (positive-type) silicon. Boron is introduced during silicon processing, where silicon is purified for use in PV devices (see Chapter 3). When a boron atom assumes a position in the crystal lattice formerly occupied by a silicon atom, there is a bond missing an electron--in other words, an extra hole. In p-type material, there are many more positive charges (holes) than free electrons.

Holes are much more numerous than free electrons in a p-type material and are therefore called the majority charge carriers. The few electrons in the conduction band of p-type material are referred to as minority charge carriers. In n-type material, electrons are the majority carriers, and holes are the minority carriers.

Both p-type and n-type silicon are by themselves electrically neutral; that is, each material contains an equal number of negatively charged electrons and positively charged protons.

The majority charge carriers, however, have excess energy that is not bound up in valence bonding with neighboring atoms. This higher energy allows them to traverse the crystal lattice. The majority carriers--electrons in n-type and holes in p-type silicon--are the ones that physically respond to an electric field. Electrons are attracted to and holes are repelled by an electric field.

Where n-type and p-type silicon come into contact, an electric field forms at the junction (referred to as the p-n interface, or p-n junction). Like floodwaters breaking through a dam, some majority charge carriers on each side rush over to the other side. There are two forces at work in this process. The majority charge carriers are more energetic and more mobile than the minority carriers. They are therefore able to move from where they are highly concentrated across the junction to a lower concentration. This is called diffusion. In addition, they are attracted (electrically) by the opposite charge of the majority carriers across the junction. In the immediate area of the junction, the "extra" electron from the phosphorus fills the hole across the junction in the boron atom. Holes then overpopulate the immediate vicinity of the interface on the n-type side; electrons overpopulate the p-type side. This overabundance is true only in the immediate vicinity of the junction, however. The bulk of the n-type silicon is still populated with negative charges; holes remain the majority charge carriers in the bulk of the p-type silicon.

At equilibrium, when all the charge carriers have settled down again, a net charge concentration exists on each side of the junction. This overpopulation of opposite charges creates an electric field across the interface. The strength of the electric force field depends upon the amount of dopant in the silicon-the more dopant we have, the greater will be the difference in electrical properties on each side and the greater the strength of the built-in electric field.

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