Doping Silicon to Create n-Type and p-Type Silicon
In a crystalline silicon cell, we need to contact p-type silicon with n-type silicon to create the built-in electrical field. The process of doping, which creates these materials, introduces an atom of another element into the silicon crystal to alter its electrical properties. The "dopant," which is the introduced element, has either three or five valence electrons—which is one less or one more that silicon's four.
Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal.
Phosphorus atoms, which have five valence electrons, are used in doping n-type silicon, because phosphorus provides its fifth free electron. A phosphorus atom occupies the same place in the crystal lattice formerly occupied 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, having no bonding responsibilities. When phosphorus atoms are substituted for silicon in a crystal, many free electrons become available.
Substituting a boron atom (with three valence electrons) for a silicon atom in a silicon crystal leaves a hole (a bond missing an electron) that is relatively free to move around the crystal.
The most common method of doping is to coat a layer of silicon material with phosphorus and then heat the surface. This allows the phosphorus atoms to diffuse into the silicon. The temperature is then reduced so 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 where phosphorus ions are precisely driven into the surface of the silicon.
The n-type silicon doped with phosphorus cannot form an electric field by itself. We also need p-type silicon. Boron, which has only three valence electrons, is used for doping p-type silicon. Boron is introduced during silicon processing when the silicon is purified for use in PV devices. When a boron atom takes a position in the crystal lattice formerly occupied by a silicon atom, a bond will be missing an electron. In other words, there is an extra positively charged hole.
See these pages for more information on crystalline silicon solar cells:
- Atomic Description
- Bandgap Energies
- Built-In Electric Field
- Absorption and Conduction
- Electrical Contacts
- Antireflective Coating