Solar CellsSolar cells today are mostly made of silicon, one of the most common elements on Earth. The crystalline silicon solar cell was one of the first types to be developed and is still the most common type in use today. They do not pollute the atmosphere and leave no harmful waste products. Photovoltaic cells work effectively even in cloudy weather and, unlike solar heaters, are more efficient at low temperatures. They carry out their work silently and there are no moving parts subject to wear. It's no wonder we marvel at how such a device would work. To understand how a solar cell works, you need to go back to some basic atomic concepts. In the simplest model of the atom, electrons orbit around a central nucleus, composed of protons and neutrons. each electron carries a negative charge and each proton a positive charge. Neutrons do not carry charge. Each atom has the same number of electrons as protons, so overall it is electrically neutral. Electrons have discrete kinetic energy levels, which increase with orbital radius. When atoms bond together to form a solid, the energy levels of the electrons blend into bands. In electrical conductors, these bands are continuous but in insulators and semiconductors there is an "energy gap", in which no electronic orbits can exist, between the inner valence band and the outer conduction band [Book 1]. Valence electrons help bind atoms together in a solid by orbiting 2 adjacent nuclei, while conduction electrons, being less tightly bound to the nuclei, are free to move in response to an applied voltage or electric field. The fewer conduction electrons, the higher the electrical resistivity of the material. In semiconductors, the materials that solar sales are made of, the energy gap Eg is quite small. For this reason, electrons in the valence band can easily be made to transition to the conduction band by the injection of energy, in the form of heat or light [Book 4]. This explains why the high resistivity of semiconductors decreases as the temperature or illumination of the material increases. Excitation of valence electrons in the conduction band is best achieved when the semiconductor is in the crystalline state, that is, when the atoms are arranged in a precise geometric formation or "lattice". At room temperature and low illumination, pure or so-called" intrinsic semiconductors have high resistivity. But the resistivity can be significantly reduced by "doping", that is, by introducing a very small amount of impurity, of the order of one atom in a million There are 2 types