| Photovoltaic cells have been used for solar powered | | | | phosphorous is used. It has five electrons in its outer |
| calculators for years. Even before that, they were | | | | ring, so that when it mates up with the silicon atoms |
| used in space. Now they are being used in many | | | | surrounding it, there is one free electron for every |
| ways, including powering houses. The way | | | | phosphorous atom. |
| photovoltaic cells work is interesting. | | | | P-type silicon is made by doping with boron. It has |
| The photovoltaic cells themselves are made out of a | | | | only three electrons in its outer shell. This leaves a |
| special kind of material called a semiconductor. Silicon | | | | hole, which is a place where an electron can jump in, |
| is most often used. When light strikes this substance, | | | | for every atom of boron. The key to using silicon in |
| the semiconductor takes in some of the energy. | | | | photovoltaic cells is to use both P-type and N-type |
| The incoming energy is enough to push some | | | | silicon. |
| electrons loose. These free-floating electrons flow in | | | | The two types of silicon are set up side by side. At |
| the same direction in the semiconductor. This | | | | first, the electrons and holes near the shared border |
| produces an electrical current. The next step is to | | | | fill in rapidly. Then, equilibrium is reached where the |
| use contacts that have been placed on the | | | | electrons can no longer interact in that way. |
| photovoltaic cells and run a circuit from the top | | | | An electric field is created separating the two halves. |
| contacts, through the item to be powered, and back | | | | In this field, it is easy for electrons to flow from the |
| to the bottom contacts. | | | | P-side to the N-side. Electrons on the N-side cannot |
| The silicon is, surprisingly, impure. This is because | | | | make it over to the P-side. This creates a one-way |
| silicon in its natural crystalline state does not allow for | | | | flow of electrons. |
| much movement of electrons. Each of the silicon | | | | Photons are units of energy from the sun. Photons |
| atoms is unstable on its own because there are four | | | | keep the energy flowing by bumping into the atoms |
| places for electrons that can be filled. Yet, every | | | | of the solar cell and freeing electrons and holes. This |
| silicon atom is usually locked up with the other silicon | | | | perpetuates the movement of electrons from hole |
| atoms by sharing electrons on the outer valance. | | | | to hole and establishes current. The field created at |
| In order to make the silicon material more | | | | the site of the N-type and P-type silicon junction |
| conductive, it is necessary to dope it, or mix in some | | | | makes the voltage that completes the power picture. |
| other substance. To make N-type silicon, | | | | |