Essay by Anonymous User May 1996

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Solar cells today are mostly made of silicon, one of the most common elements on


The crystalline silicon solar cell was one of the first types to be developed and it is still the


common type in use today. They do not pollute the atmosphere and they leave behind no


waste products. Photovoltaic cells work effectively even in cloudy weather and unlike


heaters, are more efficient at low temperatures. They do their job silently and there are no

moving parts to wear out. It is no wonder that one marvels on how such a device would


To understand how a solar cell works, it is necessary to go back to some basic atomic

concepts. In the simplest model of the atom, electrons orbit a central nucleus, composed


protons and neutrons. each electron carries one negative charge and each proton one


charge. Neutrons carry no charge.

Every atom has the same number of electrons as there


protons, so, on the whole, it is electrically neutral. The electrons have discrete kinetic


levels, which increase with the orbital radius. When atoms bond together to form a solid,


electron energy levels merge into bands. In electrical conductors, these bands are

continuous but

in insulators and semiconductors there is an 'energy gap', in which no electron orbits can


between the inner valence band and outer conduction band [Book 1]. Valence electrons

help to

bind together the atoms in a solid by orbiting 2 adjacent nucleii, while conduction


being less closely bound to the nucleii, are free to move in response to an applied voltage


electric field. The fewer conduction electrons there are, the higher the electrical resistivity


the material.

In semiconductors, the materials from which solar sells are made, the energy...