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 isstill the most common type in use today. They do not pollute the atmosphere and theyleave behind no harmful waste products. Photovoltaic cells work effectively even in cloudyweather and unlike solar heaters, are more efficient at low temperatures. They do their jobsilently and there are no moving parts to wear out. It is no wonder that one marvels onhow such a device would function. To understand how a solar cell works, it is necessary to go back to some basicatomic concepts. In the simplest model of the atom, electrons orbit a central nucleus,composed of protons and neutrons. each electron carries one negative charge and eachproton one positive charge. Neutrons carry no charge. Every atom has the same number ofelectrons as there are protons, so, on the whole, it is electrically neutral. The electronshave discrete kinetic energy levels, which increase with the orbital radius. When atomsbond together to form a solid, the electron energy levels merge into bands. In electricalconductors, these bands are continuous but in insulators and semiconductors there is an"energy gap", in which no electron orbits can exist, between the inner valence band andouter conduction band . Valence electrons help to bind together the atoms in a solid byorbiting 2 adjacent nucleii, while conduction electrons, being less closely bound to thenucleii, are free to move in response to an applied voltage or electric field. The fewerconduction electrons there are, the higher the electrical resistivity of the material. In semiconductors, the materials from which solar sells are made, the energy gapEg is fairly small. Because of this, electrons in the valence band can easily be made tojump to the conduction band by the injection of energy, either in the form of heat or light.This explains why th...
