Jeff Muliowney, age 11, of Tyler, Texas, for his question:
How does a solar cell work?
Solar cells in some of our orbiting satellites have been helping to send back signals for several years. They are powered by energy from the sun and the simple gadgets have no moving parts.
A solar cell usually consists of a thin wafer of silicon blended with a trace of boron. It works because the atoms of these two elements have the right quota of charged particles arranged in the right fashion. The slightly negative silicon atom has an outer shell of four electrons. The slightly positive boron atom has an outer shell of three. Opposite charges have a natural attraction for each other. The electrons in a solar cell use the energy of sunlight to build up high voltage that we can tap and use like any other electrical power.
That is the bald outline of the operation. The fascinating details behind the scenes are more complex. Electricity, as we know, is the motion of electrons. A big power system needs conductors materials with foot loose electrons, to help the current along. At key places it also needs insulator materials to stop the electricity from escaping. Certain materials do nothing to help or hinder the current. These semi conductors, you would think, are useless in a power system. But we need one in a solar cell. The semi conductor used to make a solar cell is often a blend of silicon and boron.
The triggering energy comes from infinitesimal packages of light called photons. When the cell is in sunlight, photons strike it constantly. Each atom of the cell has a quota of electrons orbiting its nucleus in orderly shells. An electron uses the energy of a colliding photon to hop to an outer shell. It takes its negative charge to the outer shell, leaving a positive charge in its normal shell. In a split second it drops back where it belongs. The bombardment of photons causes zillions of these positive negative switches and each switch changes photon energy into electrical energy.
Most of the solar cell is silicon, and the boron is impregnated into the surface of the wafer. The negative silicon and positive boron have a natural attraction for each other. When photons strike, their electrons build up a barrier of high voltage between them. Then no more electrons can cross this neutral no man's land. The silicon and boron become separated by a junction of high voltage. This junction is about 1/10,000th of an inch below the surface of the wafer. Its electrical power is drawn off and used. In a solar cell as long as the sun shines, hopping electrons will replace the power used by converting more photon energy into electrical energy.
A satellite may orbit every 90 minutes. Half of this period it is in darkness and its solar cells are useless. We can, however, store some of the electricity they generate during the sunlight hours. This is done by attaching a storage battery to the wafers. In sunlight, the battery uses power produced by the wafers to recharge itself. The power in the battery can then be used in darkness when the solar cells are not producing. While the battery lasts, perhaps for five years, a satellite's instruments have day and night power supplies. When it fails, the instruments become silent at sundown.