Built-In Electric Field
Light shining on crystalline silicon may free electrons within the crystal lattice. But for these electrons to do useful work—as in providing electricity to light a light bulb—they must be separated and directed into an electrical circuit. To separate the electrical charges, the silicon solar cell must have a built-in electric field.
Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a p/n junction at their interface, thereby creating an electric field.
To create this electric field within a photovoltaic (PV) cell, two separate semiconductors are sandwiched together. P-type (or "positive") semiconductors have an abundance of positively charged holes, whereas n-type (or "negative") semiconductors have an abundance of negatively charged electrons.
When n- and p-type silicon come into contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.
Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet—what we call the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, making them available for the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.
See these pages for more information on crystalline silicon solar cells:
- Atomic Description
- Bandgap Energies
- Doping Silicon
- Absorption and Conduction
- Electrical Contacts
- Antireflective Coating